Pulmonary hypertension (PH) is highly heterogeneous and despite treatmentadvances it remains a life-shortening condition. There have been significantadvances in imaging technologies, but despite evidence of their potentialclinical utility, practice remains variable, dependent in part on imagingavailability and expertise. This statement summarizes current and emergingimaging modalities and their potential role in the diagnosis and assessment ofsuspected PH. It also includes a review of commonly encountered clinical andradiological scenarios, and imaging and modeling-based biomarkers. An expertpanel was formed including clinicians, radiologists, imaging scientists, andcomputational modelers. Section editors generated a series of summary statementsbased on a review of the literature and professional experience and, followingconsensus review, a diagnostic algorithm and 55 statements were agreed. Thediagnostic algorithm and summary statements emphasize the key role and addedvalue of imaging in the diagnosis and assessment of PH and highlight areasrequiring further research. Phenotyping is based on a carefulhistory, blood testing for associated conditions, detailed physiological, andimaging investigations.Pulmonary hypertension (PH) is highly heterogeneous, is challenging to diagnose andtreat, and has a survival worse than many forms of common cancer.613 However, PAH remains alife-limiting and debilitating condition. In chronic thromboembolic pulmonaryhypertension (CTEPH), pulmonary endarterectomy is an established technique withexcellent long-term outcomes1418 and, more recently, drugtherapy1921 and balloonpulmonary angioplasty (BPA)23 have shown benefit in selected groups of patients with CTEPH.For patients with other forms of PH, such as in association with cardiac andrespiratory disease, trials of PAH therapies have thus far beendisappointing.2426 Accurateclassification is key, not only as it defines treatment but also prognosis2729 and careful assessment istherefore crucial in the assessment of patients with suspected PH.The treatment of PH is dependent on the underlying cause. For patients with PAH, drugtherapy targeting imbalances in vasoconstrictor and vasodilator mediators have beenshown to improve exercise capacity, quality of life, and event-freesurvival.3036 Importantly, over the last 20years, there has been major advances in imaging techniques and their applicationincluding the use of echocardiography, nuclear medicine, computed tomography (CT)scanning, magnetic resonance imaging (MRI), and molecular imaging. There is growingevidence demonstrating the value of various imaging modalities in theclassification, risk stratification, and follow-up of patients with PH. Imagingstudies have also provided insights into pathophysiological mechanisms.3745Current assessment tools in the PH clinic and endpoints used in clinical trials maybe limited by a number of factors. These include insensitivity to change, lack ofrepeatability, and the invasive nature of tests. There is a need to identify newtools and endpoints to aid the physician both in the clinical environment and instudies of new interventions.The use of imaging varies across the globe due to a variety of factors includingpersonal preference, availability, and cost. Given the complex nature of certainimaging investigations, there are also differences in methods of scan acquisitionand post-processing within a given modality. Consequently, the Pulmonary VascularResearch Institute (PVRI) have identified imaging as an important area forinternational collaboration, with the aim of developing evidence-based statementsand sharing best practice, while recognizing that approaches need to be tailored toimaging availability. This statement on imaging in PH is aimed at physicians(including cardiologists and pulmonologists), PH specialists, radiologists, andimaging scientists.The PVRI Imaging Task force met for the first time in Rome in 2016 with an aim ofimproving imaging practice globally in PH. A summary statement from the PVRI wasidentified as an important first step in achieving this goal. Participants wereinvited from the existing PVRI membership in addition to international imagingexperts. The group included representatives from wide-ranging professionalbackgrounds and different geographical areas with varied access to imaging.Groups of authors were assigned to specific sections to review current literature,identify summary statements, and develop a diagnostic algorithm. An editorial boardmet to preview and refine summary statements and ensure uniformity of style . To be included in the final document, summary statements requiredagreement of 80% of authors. Statements not meeting this requirement were rewordeduntil this threshold was reached or the statement rejected. Given the rapiddevelopment of imaging technologies, the recommendations reflect a combination ofpublished evidence, current practice, and expert opinion.All authors read and approved the manuscript before submission.A chest radiograph is recommended as the initial imaging test inthe assessment of unexplained breathlessness.A normal chest radiograph does not exclude the diagnosis ofPHFeatures of PH include pulmonary artery (PA) enlargement andcardiomegaly.46 In addition, cardiomegaly and features suggestive of right atrialenlargement may be observed. However, a normal CXR cannot exclude thediagnosis and the CXR may be normal, where PAP elevation is modest.Radiographic features may also suggest the cause of PH, such as upper lobevenous diversion and left atrial enlargement in patients with left heartdisease, vascular plethora, and peripheral pruning in patients withEisenmenger physiology, and interstitial opacities in patients with diffuseparenchymal lung disease.Patients with PH frequently present with breathlessness. The principal roleof the chest radiograph (CXR) is to identify other common causes ofbreathlessness . The findings of PH on CXR vary. Theremay be enlargement of the central pulmonary arteries with pruning of theperipheral vessels, features that were observed in the majority of patientsin a registry of patients with idiopathic PAH.Echocardiography is the test of choice in the initial evaluationof suspected PH.Echocardiography should include an assessment of PAP, cardiac,and valvular function.A probability of PH should be generated using estimated PAP andadditional echocardiographic features.Doppler and two-dimensional (2D) echocardiography remains thescreening test of choice in the evaluation of suspected PH.4750 While rightventricular systolic pressure (RVSP) from the continuous wave Doppler of thepeak tricuspid regurgitant velocity (TRV) is the most well-known tool forassessing the presence or absence of PH, this metric can be subject toseveral limitations resulting in over or under estimation of the truepulmonary artery systolic pressure (PASP).51 Over the last decade, there have been many studies evaluating therole of clinically useful and readily available echo-Doppler parameters thatallow one to move beyond the PASP and assess not just the likelihood of PH,but also the hemodynamic underpinnings of the disease (i.e. left heartdisease vs. PAH).53 As noted in the most recent European Society ofCardiology (ESC)/European Respiratory Society (ERS) PH guidelines,“echocardiography should always be performed when pulmonary hypertension issuspected and may be used to infer a diagnosis of pulmonary hypertension inpatients in whom multiple different echocardiographic measurements areconsistent with this diagnosis,” even in the absence of an elevated TRV.54 This can then be used to generate a probability of PH which willinform the diagnostic strategy and “decide the need for cardiaccatheterization in individual patients.”54 Furthermore, several echo-Doppler parameters have been shown to haveprognostic value in the setting of established PH.5557 systolicnotching,59 and pulmonary insufficiency velocity to estimatemean PAPs (mPAP), as well as interventricular septal flattening (ascharacterized by the eccentricity index),60 increased right-to-left ventricular ratio ,54 RV hypertrophy andright atrial dilation61 and measures of RV function including tricuspid annular planesystolic excursion (TAPSE),63 and RV fractional areachange (RVFAC).6365 While RVFAC is often limited in the setting of severeRV enlargement,66 TAPSE is a reproducible measure of RV functionwhich measures the total displacement of the heart from the RV base towardthe apex in systole, and correlates with radionuclide-derived RV ejectionfraction (RVEF).63 TAPSE has also been shown to be prognostic of poor outcome in PH in all-comers,62 as well as in follow-up assessment in a PAH population afterinitiation of PH therapy.68 However, TAPSE has notbeen shown as an effective marker of RV function in pediatric PH.69 Other echocardiographic measures include the myocardial performance(Tei) index70 and S’ obtained from tissue Doppler imaging of the tricuspid annulus.71 Markers of adverse outcomes include pericardial effusion and enlargedright atrium.72 More recently, RV longitudinal strain using 2D speckle tracking hasbeen employed in the quantification of RV function in patients with PH andhas been shown to be impaired in patients with PH, a predictor ofmortality7376 anduseful for assessment of therapy response.78 Lastly, given thecomplex anatomy of the right ventricle, recent investigation has focused onthe use of three-dimensional (3D) echocardiography79 and strain to assess global and regional RV structure and functionand predict outcomes in PH.80Clinically useful measures of PH (i.e. elevated RV afterload) onechocardiography include characteristics of the RV outflow tract (RVOT)pulse wave Doppler envelope such as a reduced acceleration time(<100 ms),A normal perfusion single photon emission computed tomography(SPECT) excludes chronic thromboembolic disease that willbenefit from pulmonary endarterectomy and BPA.Ventilation and perfusion SPECT or SPECT CT is superior to planarscintigraphy.In unexplained hypoxemia, a nuclear medicine shunt assessment canbe used to identify the presence of a right-to-left shunt.In patients with suspected PA sarcoma, positron-emissiontomography (PET) is recommended.Ventilation/perfusion (V/Q) SPECT is recommended by the ESC as thefirst line screening test for patients with CTEPH.1 The technique is well established and has excellent diagnostic valueparticularly in the absence of lung disease. The perfusion image involvesexposure to ionizing radiation and requires injection of 99mTc labelledmacroaggregated human albumin (10–90 µm in diameter).20 The macro-aggregated albumin becomes trapped within the smallpulmonary arterioles and a 3D image of pulmonary perfusion is acquired. InCTEPH, typically peripheral wedge-shaped defects of varying size are shown.Mismatch to ventilation can be confirmed by comparing ventilation andperfusion images. The added value of performing ventilation imaging isdebated and in many centers perfusion imaging alone is performed andcompared to CT which better demonstrates parenchymal lung disease. CTEPH maybe missed on CT as attenuated distal vessels, subsegmental stenosis, andwebs may not be appreciated. Early studies demonstrated that scintigraphywas more sensitive than CT for the detection of CTEPH. However, givenadvances in technology, more recent studies have shown that CT pulmonaryangiography and CT SPECT techniques are equally sensitive.2123Dual-energy CT (DECT) or CT with iodine mapping allow construction ofrelative perfusion maps in addition to providing angiographicimages.8187 Given that 99mTclabelled macroaggregates are trapped by small pulmonary arterioles, thepresence of radioactive uptake in organs supplied by the systemiccirculation (e.g. the kidneys) can be used to identify the presence of aright-to-left shunt in hypoxemic patients with PH.1928 In patients with suspected PA sarcoma, PET will show high uptakeallowing differentiation from chronic clot.30 However, in acute clotFDG, uptake may also be elevated, relative to unaffected vessels.88 In PAH, uptake within the lungs and right ventricle have beendemonstrated2427although the clinical utility is uncertain.Positron-emission tomography (PET) allows for observation of metabolicactivity in the body which can be reconstructed to produce 3D images.Fluorodeoxyglucose (FDG), a glucose analogue, can be used to assess glucoseuptake in tissues. PET scanning is commonly performed with simultaneousacquisition of CT and more recently with MRI for both functional andstructural correlation.CT provides a non-invasive evaluation of vascular, cardiac, lungparenchymal, and mediastinal structures in patients with knownor suspected PH.Significant parenchymal abnormalities may be seen on CTevaluation in the presence of normal spirometry, particularlywhen there is a significant reduction in gas transferfactor.CT pulmonary angiography to assess the pulmonary vasculatureshould be considered in patients presenting with PH.Imaging biomarkers from CT in patients with suspected PH shouldinclude measurement of PA size, right-to-left ventricular ratio,and left atrial size.CT aids the classification of PH.CT is increasingly recognized as a valuable imaging modality for theevaluation of known or suspected PH. Advantages of CT include its widespreadavailability and accessibility, high spatial resolution, multi-planarimaging capabilities, and the ability to evaluate the pulmonary vasculature,lung parenchyma, cardiac, and mediastinal structures simultaneously.89 The reliability of measuring the main PA diameter and the ratio ofthe PA to aorta (Ao) ratio has also been studied in suspected PH.Investigators found that a PA:Ao ratio > 1 was 92% specific for a mPAP > 20 mmHg.90 Other reports also support the use of PA size in the clinicalassessment of patients with PH.92 However, it has beenshown that an increase in PA size also reflects disease duration andcorrelates only moderately with PAP.93 It has previously been postulated that the presence of interstitiallung disease independently influences PA size;94 however, in a large cohort of patients with suspected PH inassociation with interstitial lung disease, the presence and severity ofinterstitial lung disease did not influence PA size which was found to be auseful diagnostic marker in patients with and without interstitial lung disease.95PA size can be easily measured and enlargement may suggest the diagnosis ofPH. Routine measurement is recommended particularly in patients at risk ofPH. For the diagnosis of PH in lung disease, a main PA diameter >29 mmhad 84% sensitivity, 75% specificity, and 97% positive predictive value(PPV) for PH defined as a mPAP ≥ 25 mmHg.96 Mosaic perfusion of the lung parenchyma and enlarged bronchialarteries are also commonly seen.9799While contrast-enhanced CT angiography is the method of choice for theevaluation of suspected acute pulmonary embolism, its role in the evaluationof the pulmonary vasculature in the setting of CTEPH has been a more recentdevelopment. Multiple findings are associated with CTEPH, includingintravascular organizing thrombi, webs, and regions of vascular narrowing or occlusion.85 DECT showed 100% sensitivity on a per-patient basis compared to V/Qscintigraphy. However, there was imperfect agreement on a per-vessel basis.CT perfusion imaging may demonstrate residual perfusion abnormalitiesfollowing therapy for acute pulmonary embolism, even in the absence ofvisualized thrombus on the angiographic portions of the study.100 Perfusion imaging can also estimate cardiac output and in a smallpilot study could detect PH with high sensitivity and specificity.102Advanced CT capabilities have been studied in the evaluation of PH. DECTprovides an assessment of relative perfusion and improves the detection ofperipheral vascular occlusion. In one study,103 However, changes in the fractal dimension were found only in childrenwith PAH, but not in adults.104 Loss of distal vascular volume has also been described in patientswith severe emphysema105 and in patients with CTEPH.106PAH is associated with vascular remodeling, including loss of arterialbranching and increased vessel tortuosity. CT angiography can quantify thesefeatures, using fractal dimension and the ratio of actual vessel length toshortest linear distance to estimate tortuosity. Studies have shown these tocorrelate with hemodynamic measures in PAH.38 Centrilobular ground-glass opacities are frequently seen in PAH andtheir presence on a CT performed for unexplained breathlessness should raisethe possibility of this diagnosis.108 Features of cardiacdecompensation, pleural effusion/septal lines, and inferior vena cava sizepredict outcome.38 The presence of emphysema or interstitial lung disease orbronchiectasis makes the diagnosis of PH in association with lung diseaselikely. The addition of expiratory imaging is helpful to assess for smallairways disease. CT may also identify features associated with rareconditions such as pulmonary veno-occlusive disease (PVOD) and pulmonarycapillary hemangiomatosis (PCH) (section 2.2). This is an importantdifferential diagnosis to idiopathic PAH (IPAH) because PAH therapy may beindicated but has a much higher risk of severe adverse effects thanIPAH.CT is the gold standard for the evaluation of the lung parenchyma. In a largeregistry series, CT measures have proven useful clinically in the assessmentof patients with PAH.109111 A study has shownthat dynamic contrast-enhanced CT can measure the transit of contrast,correlated with cardiac output.102 and is associated with the presence of PH.101CT has historically not been used for the evaluation of cardiac structuralabnormalities as MRI and echocardiography are the current modalities ofchoice. Nonetheless, many cardiac findings associated with PH can beidentified with non-gated contrast-enhanced CT, including enlargement ofcardiac chambers, thickening of the RV free wall, and leftward deviation ofthe interventricular septum. CT can also identify structural abnormalitiesassociated with congenital heart disease, such as partial anomalouspulmonary venous return and intracardiac shunts. Electrocardiogram-gated CTcan be used to quantitatively assess RV and left ventricular (LV) function.Additionally, a decrease in distensibility of the main PA is highlycorrelated with the presence of PAH.112 A dilated esophagusin the setting of PH suggests the diagnosis of systemic sclerosis. Othermediastinal findings, while not specific to a given etiology, may suggest apoor prognosis. These include the presence of pericardial effusion,lymphadenopathy, and reflux of contrast into the hepatic veins.38CT provides detailed imaging within the mediastinum and may demonstratefindings that give information as to the etiology or severity of PH.Dilatation of bronchial arteries is a common finding in CTEPH, but lesscommon in other forms of PH.MRI enables comprehensive cardiac evaluation in patients withsuspected PH.MRI is the gold standard technique for the assessment ofbiventricular morphology and function and is highly suitable formonitoring patients with PH.MRI provides prognostic value in PAH.MRI aids the classification of PH particularly for left heartdisease and chronic thromboembolic disease.Cardiac MRI is the gold standard for quantification of RV volumes,mass, function, and flow hemodynamics in the pulmonarycirculation.114 Cardiac MRItechniques allow for non-invasive assessment of RV function and structureusing high spatiotemporal resolution imaging sequences with high accuracyand reproducibility without exposure to radiation.113 Furthermore, cardiac MRI can be used for assessment of myocardialtissue deformation properties (strain), global structural evaluation, andperfusion.1151175 In particular, in advanced stages of PH, a severely dilated andfunctionally compromised right ventricle has a negative effect on LVdiastolic function by means of leftward septal shift and reduced LV fillingassociated with decreased RV stroke volume. Indeed, both RV and LVdimensional metrics have been shown to have diagnostic potential intreatment-naïve patients with PH and prognostic value in both adult andchild populations.118120 MRI-derived bi-ventricular functional and volumetricindices have been shown to have independent prognostic potential anddifferentiated incidental treatment-naïve and prevalent patients in a largegroup of patients (n = 576).121 Cine MRI derived indices including interventricular septal bowing, LVeccentricity, and ventricular dyssynchrony due to prolonged RV contractiontime have been shown to correlate with invasive hemodynamics and arereflective of the overall hemodynamic condition and diseaseseverity.123 Septal deviation measured by MRI is useful for thediagnosis of PH, but also in patients with left heart disease septaldeviation > 160° can identify patients with elevated diastolic pulmonary gradient.124 In addition, MRI has proven useful in the diagnosis of PH in patientswith chronic obstructive pulmonary disease (COPD), typically a challengingcohort for echocardiography.125RV hypertrophy and dilation reflect an increased afterload.126130 Specifically, LGEwas correlated with the reduced RV function, dilation, mass, and regionallyspecific LGE was also inversely associated with reduced longitudinalstrain.131 Additionally, the presence of delayed enhancementat the RV insertion points has been associated with clinical worsening,127 though in a study using mortality as the endpoint, RV insertion pointLGE was not of independent prognostic significance. Extension of the LGEinto the interventricular septum was of prognostic significance atunivariate analysis but was not significant at multivariate analysis.45 A limited number of studies have explored the role of coronaryarterial flow in PH.132 Features may be seenin patients with PH with LGE imaging; however, its utility in the routineassessment of suspected PH is not proven. LGE imaging can be consideredwhere intrinsic cardiac disease is suspected.Late gadolinium enhancement (LGE) imaging is used to identify focalmyocardial pathology but has also been applied to investigate regionalmyocardial disease in the right ventricle as a response to elevatedmechanical stress. The predominant focus has been on the RV free wallinsertion sites to septum and how the extent of LGE corresponds to RVmorphological and dynamic changes.133 T1 has been shown to be elevated at the insertion points in PH in animal134 and human studies.135137 T1 correlates withmarkers of RV remodeling135 and septal position;138 however, no clear diagnostic or prognostic role has been identifiedin PH.138T1 mapping is a quantitative method of assessing myocardial health. Native T1mapping is performed without the use of contrast agents and has been shownto be an excellent differentiator between a healthy and diseased myocardium.139141 RV morphology limits myocardial tagging and featuretracking to global longitudinal and circumferential deformation analysis.MRI-based feature tracking has been shown to have prognostic potential andwas associated with the severity of PH.142 Measuring LV strain and torsion using tag MRI as part of ventricularinterdependency and dyssynchrony investigations in CTEPH revealed left–rightventricular resynchronization post endarterectomy.143 However, the clinical utility of deformation analysis is yet to bedetermined.Myocardial tissue deformation analysis has been assessed in patients withPH.p:Qs assessment,necessary in patients with suspected congenital lesions.114 Phase-contrast MRI of pulmonary flow is recommended for assessment ofRV stroke volume due to variable tricuspid regurgitation and the challengesof contouring the right ventricle.144 Relative area change of the PA has been shown to be of clinicalvalue,146 and recently has been shown to be independent ofRV measurements and clinical data.37 Black blood slow flow has been shown to be a strong diagnostic markeras the flow characteristics of the main and branch vessels arevisualized.148 Four-dimensional flow MRI (4D-Flow MRI) is anemerging technique allowing evaluation of flow, vorticity, and kineticenergy in any region of interest. Vortices have been noted in the main PA ofpatients with PH. The lifetime of the existence of a vortex has been shownto correlate with mPAP and may have utility in the identification ofPH.149152 4Dflow also has the additional benefit that it allows retrospective flowevaluation by selecting a 2D slice in any plane of the 4D dataset,153 whereas current techniques rely on the slice positioning at the timeof the scan.Phase-contrast MRI (PC-MRI) enables assessment of flow waveform in majorvessels and allows for accurate Q113 MRA is useful for the assessment of chronic embolus in the lobar andsegmental PA vessels. Beyond the segmental level, assessment of the PAs withMRA is very challenging.96 In addition, a central embolus, particularly a wall adherent clot,can be missed if MRA is reviewed in isolation; standard white blood MRIsequences can assist with visualization of a central clot.96MR angiography (MRA) can show characteristic vessel patterns in subtypes ofPH, including pruning in IPAH, thromboembolic obstruction and stenosis inCTEPH, and splayed vessels in COPD/emphysema.39 with the advantages of higher spatial resolution and lack of ionizingradiation. Time-resolved MRA or dynamic contrast-enhanced (DCE) imaging canbe used to measure passage of contrast bolus through the heart and lungs toassess pulmonary perfusion.155 This can be used tomeasure mean transit time, time to peak, and blood volume.157Dynamic contrast-enhanced MRI perfusion is a promising technique for theassessment of chronic thromboembolic disease allowing visualization ofpulmonary perfusion defects with sensitivity and specificity similar to thatachieved with SPECT,Catheter-based angiography is used primarily to assess patientswith CTEPH considered as potential candidates for pulmonaryendarterectomy or BPA.Performance of catheter-based angiography requires skilledoperators and should generally be performed in a PH referralcenter.Non-invasive imaging approaches can be used to select patientsfor pulmonary endarterectomy.Imaging measurements combined with catheter measurements may beused to study RV pressure and volume relationships.159 This used to be the primary method for evaluationof the pulmonary vasculature. However, given the development of CT and MRmethods, these modalities can also be used.160 Catheter-basedangiography may be used at expert institutions for the evaluation of chronicthromboembolism before pulmonary endarterectomy and is required for BPA.Catheter-based angiography involves rapid imaging of the PAs during theinjection of contrast material through a catheter placed into the pulmonaryarterial system.161163 One method for determining RV contractility involvescomputation of the pressure-volume loop while using balloon occlusion of theinferior vena cava, permitting a preload independent assessment ofventricular contractility.164 In practice, this method requires use of conductance catheters or themeasurement of pressure and flow at the same time in order to construct thepressure volume loops with different degrees of preload modulated by theocclusion of the inferior vena cava. Conductance catheters typically requirecalibration of the volume signal from imaging, typically a baseline cardiacMRI. The “Single Beat” method using a cardiac catheter can be used toestimate Pmax. This has been used in conjunction with MRI to measureventricular volumes and can serve as a surrogate for Ees/Ea,165 the relative utility of information from these methods remains anarea of research.166 Cardiac MRI has been used to measure stroke volume and RV volumes andin conjunction with pressure measurements to construct pressure-volume loopsand estimate RV contractility.167170 An entirelyMRI-based non-invasive method of measuring RV to PA coupling has beenproposed, defined by RV stroke volume/RV end-systolic volume; however, thisholds similar information to RVEF (right ventricular stroke volume / rightventricular end-diastolic volume).171 Studies have suggested added prognostic value of couplingmeasurements,173 although a recentlarge study suggested it did not add additional prognostic significance overRV volume alone in patients with PAH.121Imaging can be incorporated with invasive catheter-based methods forcharacterization of the mechanics of the right ventricle and the pulmonaryarteries. Flow and volumetric MRI measurements in conjunction with pressuresobtained from RHC can be used for assessment of ventricular-arterialcoupling.A number of CT and MRI findings are characteristic of PH.Current qualitative approaches to imaging cannot be used toconfidently exclude the presence of PH.Quantitative data obtained from imaging can be used to diagnosePH and estimate pulmonary hemodynamics.CT imaging is widely available and measurement of the PA size hasbeen shown to correlate with mPAP;174 however, in established PH, there are progressive increases in PAsize over time.175 Pulmonary artery enlargement may be seen in interstitial lung diseasein the absence of PH,94 although a recent publication has shown equivalent diagnosticaccuracy in patients with and without interstitial lung disease. In patientswith systemic sclerosis in the absence of interstitial lung disease, a ratioof main PA to Ao diameter of at least 1 was highly predictive of thepresence of PAH although a normal ratio did not exclude PAH.176177 Studies have shown a high correlation between RV mass and ventricularmass index (VMI), the ratio of right-to-left ventricular mass, and mPAPpressure measured at cardiac catheterization.179 Recentlyinvestigators have shown that combining VMI and septal curvature improvesthe accuracy of estimating mPAP.180 By using MRI to calculate left atrial volume, pulmonary arterialwedge pressure (PAWP) can be estimated allowing calculation of thetrans-pulmonary gradient.180 Cardiac output can be calculated from LV volumetric measurements orphase contrast of flow in the PA or Ao allowing an entirely non-invasiveestimate of pulmonary vascular resistance (PVR) based on individuallyderived MRI measurements. Models using RVEF and average PA velocity havealso demonstrated accuracy for estimating catheter-derived PVR.181 Studies comparing cardiac magnetic resonance cardiac MRI and RHC inpatients suspected of PH have shown that an elevated VMI, reduced PAvelocity, and the presence of increased gadolinium at the hinge points couldpredict the presence of PH with a positive predictive value of >0.9although no cardiac MRI measure could confidently exclude PH.179 In summary, although able to estimate pulmonary hemodynamics andidentify PH with high accuracy in certain groups, imaging is currentlyunable to exclude PH.MRI is non-invasive, reproducible, and is considered the gold standard forassessing RV function.Different forms of PH and their subtypes may exhibitcharacteristic imaging features.Echocardiography and MRI are useful for differentiation of preand post capillary PH.CT provides an accurate evaluation of lung structuralabnormalities.Nuclear medicine, CT and MR perfusion imaging can be used toexclude chronic thromboembolic disease.Different combinations of imaging modalities can be employedtailored to local expertise and availability.Many of the imaging findings associated with PH are common to most orall disease processes leading to PH. These findings may include enlargementof the central PAs, right-sided cardiac enlargement, and abnormalities oflung attenuation. Some imaging findings, however, are more specific and canhelp to distinguish among the various causes of PH. These are discussed inthe following section.38 Patients with Eisenmenger physiology may also have laminated proximalthrombus and calcification within the wall of the PA. Bronchial arteryenlargement is most frequently observed in CTEPH but is also observed inpatients with congenital heart disease182 and patients with IPAH and a BMPR-2 mutation.183 PVOD is rare and is characterized by mediastinal lymphadenopathy andinterlobular septal thickening,185 with or withoutassociated findings of alveolar edema, mediastinal lymphadenopathy in thesetting of a normal sized left atrium. PCH is frequently associated withnodular foci of parenchymal infiltration. The triad of peripheralinterlobular septal thickening, centrilobular ground-glass opacities, andmediastinal lymphadenopathy has a good association with PVOD andPCH.187Within group 1 PAH, imaging findings may suggest a specific etiology.Patients with systemic sclerosis typically have a dilated esophagus, acentral ground glass pattern, and often associated interstitial lungdisease. Drug and toxin exposures may also be associated with mildparenchymal fibrosis or mosaic lung attenuation. In patients withportopulmonary hypertension, the presence of varices, features of livercirrhosis, and splenomegaly are frequent. Anomalous pulmonary venousdrainage should be sought as this is frequently associated with congenitalheart disease, in particular a sinus venosus atrial septal defect which isdifficult to detect by transthoracic echocardiography. An interatrial shuntmay also be suggested by contrast visualized entering the left atrium viathe interatrial septum. While enlargement of the PAs is common to all formsof PAH, it is greatest in patients with Eisenmenger physiology.188190 absence of posteriordisplacement of the interventricular septum,179 and relatively normal RV volumes, although RV enlargement is seen inmore severe disease particularly in the setting of severe tricuspidregurgitation. The presence of valvular and coronary artery calcificationand evidence of previous cardiac surgery are more common in left heartdisease although may also be present in patients with other forms of PH.Echocardiography and MRI allow a comprehensive functional assessment.Patients with left heart disease have less RV hypertrophy and compared topre-capillary forms of PH have better preserved RV function. The absence ofparadoxical septal motion/septal displacement in the setting of highright-sided pressures infers an increase in left-sided pressures.Echocardiography allows assessment of both systolic and diastolicdysfunction and the identification of valvular heart disease, in addition toidentifying features suggestive of combined post- and pre-capillary disease.191Within these groups, imaging may be useful to identify either left-sidedcardiac enlargement (group 2) or diffuse lung disease (groups 3 or 5). Manylung diseases will have a distinctive radiographic appearance allowing forspecific diagnosis and grading of severity. Characteristic structuralfeatures of patients with left heart disease which can be visualized on bothCT and MRI include left atrial enlargement,192 Imaging plays a critical role in the evaluation of suspected CTEPH,although the exact role of each imaging modality is debated. Some of thisuncertainty reflects the rapid development of imaging technologies. Chestradiographs may suggest the diagnosis of CTEPH – with cardiomegaly,asymmetrical pulmonary artery enlargement, pruning of the vasculature andsubpleural scarring; however, they are not diagnostic. Historicallydecisions on diagnosis and surgical management were based on V/Qscintigraphy and conventional pulmonary angiography with RHC. The role ofV/Q scintigraphy (either planar or SPECT imaging) has changed. SPECT isrecommended over planar imaging as it has higher diagnostic accuracy.193A large prospective study estimated the risk of developing CTEPH after apulmonary embolism at 3.8% at two years.194 These recommendations for V/Q scanning are in part based on clinicalexperience, previous recommendations, and on older data.195197These data demonstrated a significant improvement in the detection of CTEPHwith scintigraphy compared to CTPA. However, rapid developments in CTtechnology have led to marked improvement in disease detection andcharacterization. More recent studies in experienced centers demonstratethat CTPA has high diagnostic accuracy for CTEPH.39 Key imaging findings include identification of eccentric thrombus,intravascular webs, stenoses with or without post-stenotic dilatation, andocclusions. Bronchial artery dilatation (commonly described as a diameter of>2 mm) is more commonly seen than in other forms of PH. The presence ofbronchial artery dilatation is associated with a better outcome followingpulmonary endarterectomy. A mosaic perfusion pattern is seen in the vastmajority of patients with CTEPH. Its presence should alert the observer tothe possibility of CTEPH but should be differentiated from the mosaicpattern seen in small airways disease where often single or small clustersof lobules are involved in contrast to larger geographical areas typicallyseen in CTEPH. Performance of expiratory CT may be helpful in this setting.Peripheral areas of sub-pleural scarring and cavitation representing healedinfarcts may also be seen in approximately 10% of patients with CTEPH.198 DECT imaging generates maps of regional iodine density in the lungparenchyma as a surrogate for perfusion. This may further improve theevaluation of suspected CTEPH by better demonstrating regions of decreasedor absent blood flow199 and has been shown to have excellent agreement with SPECT.200 Although the availability of this technology is relatively limited,iodine mapping using CTPA with an unenhanced pre-scan is an emergingtechnique, which generates a lung perfusion map. In contrast to DECT, itdoes not require specialized hardware but involves subtraction of unenhancedimages from the contrast-enhanced study. This has the advantage that subtleabnormalities that may be missed on the angiography are unlikely to beoverlooked on a perfusion map thus improving detection of subtle webs anddistal disease.81The current ESC/ERS guidelines recommend that V/Q scintigraphy be performedin all patients with suspected CTEPH.By providing an assessment of the lung parenchyma and the mediastinum CT canbe helpful in identifying the presence of diffuse lung diseases andemphysema (groups 3 and 5) and excluding pulmonary vascular obstruction froma central mass. Parenchymal evaluation may also be valuable in identifyingfeatures suggestive of interstitial edema, PVOD, or vasculopathy thoughthere is overlap in the imaging features. CT may also identify featuressuggestive of large vessel vasculitis or PA sarcoma which may not be readilyappreciated on projectional angiography.201 are widely available on current MRI systems and have shown excellenttest performance in diagnosing CTEPH in a single center registry setting.39 In addition, a non-contrast, free-breathing ventilation perfusion MRItechnique, known as the Fourier Decomposition (FD)-MRI method203 hasrecently shown initial encouraging results in diagnosing chronic pulmonary embolism;204 however, this technique needs to be confirmed in larger multicenterstudies. Combined cardiac MRI and time-resolved MRA exam is suitable fordetailed treatment response evaluation before and after pulmonaryendarterectomy as well as BPA in CTEPH patients.206MRI can also provide an evaluation of the pulmonary vasculature and isincreasingly being used in select centers for the evaluation of known orsuspected CTEPH. 4D DCE lung perfusion MRI techniques207 Additionally, some expert centers prefer catheter-based angiographyfor evaluation of the extent of thrombotic disease before PEA.208 Again, CT performance relative to catheter-based angiography isgenerally poorer in older studies159 compared to more recent data.210 This differenceagain likely reflects significant advances in CT technique and technology aswell as greater awareness of CTEPH and its imaging features in imagingcircles. When there is diagnostic uncertainty regarding the extent anddistribution of chronic thromboembolic changes on the basis of a singleimaging modality , the use of a second morphologic imagingmodality (MRA or catheter-based angiography) may prove complementary byimproving confidence in subtle lesions or identifying others. This canaugment surgical decision-making.CTPA has largely replaced invasive catheter-based angiography as the initialmorphological test for PA evaluation in most centers. However,catheter-based angiography may still have a role in the evaluation of CTEPH.Older data suggest that catheter-based angiography may provide a betterassessment of the extent of pulmonary vascular obstruction than V/Q scintigraphy.211In recent years, BPA has emerged as an effective therapeutic modality inselected patients with CTEPH. This has put greater emphasis on theevaluation of distal segmental and subsegmental vasculature. It is generallyconsidered that at the subsegmental level, catheter-based angiographyoutperforms non-invasive morphological techniques (CT/MRI) which hasresulted in increased utilization. Clinical criteria and imaging algorithmsfor the selection of patients for BPA vary between centers butcatheter-based diagnostic angiography for potentially suitable candidatespermits confirmation of suitable extent and distribution of disease as wellas the patients ability to tolerate a BPA procedure (ability to maintainbreath-hold and tolerate the required period on the catheter table).Echocardiography is more widely available, lower in cost, andmore portable than MRI.Echocardiography is superior to MRI for the evaluation ofvalvular heart disease and is more established in the assessmentof diastolic function.MRI provides more accurate quantitative assessment of RVmorphology and function.MRI is more suited to serial assessment than echocardiography dueto higher reproducibility.Echocardiography and cardiac MRI provide value in the assessment ofpatients with PH. Echocardiography is well established in the initialassessment of patients with suspected PH. It has also been evaluated in theserial assessment of patients with PH66,67 and has been found tobe prognostic and is recommended in current guidelines at follow-up andfollowing treatment change.212 has helped to counter this limitation at the cost of lower spatialresolution. Echocardiography is more operator-dependent and is lessreproducible. MRI has the advantage that it has better contrast resolutionand is able to image in any plane making it more suited to accuratelyquantify RV morphology and function. Furthermore, contrast imaging allowsassessment of focal abnormalities of the myocardium and an assessment ofmyocardial perfusion.116Echocardiography has high temporal resolution, is widely available, low incost, portable, and less affected by arrhythmia than MRI although real-time imaging213 Empiric approaches have been used to estimate pulmonary hemodynamicsusing MRI.180 In a recent study, mPAP was accurately estimatedusing multivariate regression analysis of MRI indices, with ventricular massindex and interventricular septal angle having additive value in model forestimation of mPAP.180 It has recently been shown that with the inclusion of black bloodpulmonary arterial slow flow in addition to ventricular mass index andinterventricular septal angle high diagnostic accuracy can be improved further.147In a systematic review of the literature, the sensitivity and specificity forechocardiography for diagnosing PH was 83%  and 72% (95% CI = 53–85), respectively. However,echocardiography is less accurate, in lung disease with sensitivity of 60%and specificity of 74%.214 and is considered the gold standard for serial assessment of RVvolume and function.215 Inaccuracies in segmentation of the right ventricle at the base ofthe heart and segmentation of ventricular trabeculations may be improvedusing threshold-based approaches.Complete visualization of the right ventricle with echocardiography can bechallenging, particularly in lung disease, preventing a complete volumetricevaluation of the right ventricle; this may be partially negated by the useof 3D echocardiography. MRI has the advantage of complete volumetricevaluation of the cardiac chambers with good reproducibility of volume,mass, and ejection fraction216 and in a small study imaging of the mitral valve annulus, incombination with mitral valve and pulmonary venous flow, was as accurate as echocardiography.217 Other techniques such as myocardial SPAMM tagging218 and 4D three-directional velocity encoded MRI219 have been used to evaluate LV diastolic function. Long analysis timesmay reduce the applicability for routine clinical use.Echocardiography, using pulsed wave and tissue Doppler, is an establishedtechnique for the assessment of diastolic function. MRI phase contrastevaluation of the mitral annulus is feasible220 upwards of 30% in patients with severe mitral regurgitation;221 in asymptomatic patients, the presence of PH serves as an indicationfor valve surgery.222 With the emergence of transcatheter aortic valve replacement, PH hasbeen noted in up to 75% of patients with severe aortic stenosis.223 Currently, echocardiography is the recommended non-invasive techniquefor the assessment of the presence and severity of valvular disease givenits advantages over MRI including availability, low cost, high temporalresolution, and widespread experience over many years. Despite this, thereare instances in which echo is limited due to poor acoustic windows, lack ofagreement across quantitative methods, and significant inter- andintra-observer variability, which has led to interest in the emerging roleof MRI in the assessment of valvular heart disease.225PH may occur in the setting of valvular heart disease. Echocardiography isthe most useful non-invasive technique and has the advantage over MRI inthat it has high temporal resolution and allows accurate quantification ofthe severity of valvular heart disease. Left-sided valvular heart disease isa common cause of group 2 PH. Contemporary studies suggest a prevalence ofPH of 30–40% in patients with mitral stenosisCardiac catheterization is the gold standard for the measurementof PAP.CT and MRI may suggest the diagnosis of PH and quantitative MRImetrics can be used to confirm the presence of PH with highaccuracy.At diagnosis MRI and cardiac catheterization provide equivalentlevels of prognostic information.Cardiac catheterization is currently the only way to identifypatients with IPAH likely to benefit from calcium antagonisttherapy.Current ESC/ERS guidelines226 recommend RHC for the definitive diagnosis of PH, assessment ofintra-cardiac shunting, and vasodilator testing in selected patients toidentify the 10% of patients with IPAH who may benefit from high-dosecalcium antagonist therapy. Measurements such as right atrial pressure,cardiac index, and mixed venous oxygen saturation have prognosticvalue,227229 and serial measurements are currently recommended toassess the response to therapy.226 The procedure requires meticulous attention to detail but in experthands is safe in adults with a morbidity of approximately 1% and mortalityof 0.055%,230 although the risk of complications is significantly higher inchildren. Well established criteria at right heart catheter exist to assesspatients with IPAH likely to benefit from calcium antagonist therapy.However, no such criteria exist for imaging metrics.Although current guidelines discuss the role of CT and MRI in theclassification of PH, they are currently considered an adjunct and not yetconsidered a replacement for RHC.Echocardiography allows assessment of RV function andmeasurements such as right atrial area, RV fractional areachange, and tricuspid annular plane systolic excursion haveprognostic value.CT imaging provides prognostic information in PAH but its role infollow-up is currently limited by exposure to radiation.A number of cardiac MRI metrics have prognostic value and changesin cardiac MRI parameters at follow-up reflect changes infunctional capacity and survival.Adjustment of RV functional measurements for age and sex improvesprognostication.Changes in RV function measured by MRI have prognostic value.Echocardiography is widely available and a number of metrics havebeen shown to have prognostic value including right atrial area,231 RV fractional area change, TAPSE,232234 and the presence ofa pericardial effusion.235 Echocardiography iswidely used, in many centers to assess the response to treatment,acknowledging issues of operator-dependency and reproducibility.38 Although this may be helpful at presentation to guide urgentassessment and introduction of emergency therapies, the availability ofother imaging modalities and associated radiation exposure necessitates thatCT is not recommended in the assessment of treatment response.Several measurements made at CTPA—including RV to LV ratio, right atrialsize, and a posteriorly deviated inter-ventricular septum—predict prognosisin PAH subgroups while the presence of pleural effusions, septal lines, andincreased inferior vena cava area were independent predictors of a worseoutcome in PAH at presentation regardless of subgroup.236 changes in MRI measured cardiac index and RV RVEF correlated withchanges in World Health Organization functional class and survival. A studyexamining changes in cardiac MRI parameters demonstrated that this was abetter predictor of outcome than PVR measured invasively at cardiac catheterization.215 MRI metrics including increased RV volumes, reduced LV volumes,stroke volume, cardiac output, and pulsatility of the vasculature predict aworse outcome in PAH although there is currently no large study comparingthe prognostic value of cardiac MRI and right heart catheter measures.In contrast, the highly reproducible nature and non-invasive and non-ionizingnature of MRI makes it an ideal modality to assess treatment response and inthe EURO-MR studyMinimizing radiation dose by the use of non-ionizing techniqueswhere possible and implementation of dose reduction protocolswill reduce the risks to patients.Involvement of patients in investigative decision-making willallow a more tailored approach to investigation.While plain chest radiographs and echocardiography are generallywell-tolerated and acceptable examinations, cross-sectional techniques dohave some specific limitations and issues, in particular radiation doses forCT and claustrophobia for MRI.237 In fact, large reductions in dose to less than one-third the dose ofstandard acquisitions have been achieved without loss of image quality usingiterative reconstruction,238 and the use of dynamic Z axis collimation has been shown to furtherreduce dose.239The diagnostic information provided by CT needs to be balanced with theradiation dose. In a life-shortening illness, concerns regarding radiationexposure rarely impact on the decision to perform CT imaging at the time ofdiagnosis. Advances in dynamic imaging, with improved temporal resolution,has provided hope that gated CT can challenge MRI in the assessment ofcardiac function; however, such examinations typically involve higher dosesof radiation. Using iterative reconstruction, dose reduction can be achieved.Limitations of MRI include long scanning times and scanner noise, andfrequently patients feel claustrophobic (5%) leading to incompleteexaminations. Developments to reduce scanning time, e.g. novel rapid imagingtechniques, and limiting the study to the most clinically relevant sequencesmay help. The more widespread installation of wide bore scanners may help toreduce claustrophobia and make MRI a more acceptable imaging modality forall patients. The use of media entertainment may improve acceptability topatients. More patient involvement in discussions around the implications ofthe tests are needed. Patient participation is advised to determine theissues most relevant to patients to help develop imaging services.2 Echocardiography is recommended as a first-line non-invasive diagnosticinvestigation in case of suspicion of PH. Chest X-ray and high-resolution CT arerecommended in patients with high or intermediate probability of PH followingechocardiography and high-resolution CT should be considered in all patientswith PH. Ventilation/perfusion or perfusion lung scan is recommended in patientswith unexplained PH to exclude CTEPH. Contrast CT angiography of the PAcirculation is recommended and pulmonary (catheter-based) angiography should beconsidered in the work-up of patients with CTEPH. There are no recommendationsfor the use of MRI as part of the diagnostic strategy or algorithm and nodiscussion of emerging techniques.241 Current American Collegeof Chest Physicians guidelines on PH provide no specific recommendations onemploying imaging as part of a diagnostic strategy in patients with suspected PH.242The ESC/ERS guideline on diagnostics and therapy of PH review current imagingmodalities and make a number of recommendations for incorporation in adiagnostic strategy.The Cologne Consensus Conference 2016 provides no additional imagingguidelines.194 or increase the probability of PH. Further investigation may not berequired when a confident alternative diagnosis can be made. In patients presenting with symptoms and/or signs suggestive of PH, adetailed history and the results of basic tests are key in determining thediagnostic strategy . Risk fa194Where patients have rapidly progressive symptoms and a highprobability of PH from echocardiography, physicians should not delayreferral to expert centers until the above investigations arecompleted.Echocardiography is the recommended first-line imaging modality in theassessment of suspected PH and allows evaluation of cardiac structure andfunction and an estimate of PAP. Following echocardiography, patients shouldbe stratified into those at low, high, or intermediate probability of PHaccording to ESC/ERS guidelines.147 A number of metrics on CTPA have been shown to reflect elevated PAPsin addition to providing information on other potential causes forbreathlessness; although evidence is limited, this may be considered inselected patients.95For patients with a sub-optimal echocardiogram, imaging with cardiac MRI canbe used to identify patients at increased risk of PH although currently usedmetrics cannot confidently exclude mild PH.243 If risk factors forPAH exist, the diagnostic strategy will be dependent on the risk factor; insystemic sclerosis, given the high prevalence of PAH in symptomaticpatients, further evaluation is advised and a number of screening regimensexist such as DETECT.244 For other at-risk patients, an interval echocardiographic examinationmay be appropriate.194For symptomatic patients identified as low probability from echocardiography,further assessment is dependent on the presence or absence of risk factors.For those with risk factors for CTEPH, perfusion lung imaging is recommendedusing CT imaging (CT-LSIM of DECTA), nuclear medicine techniques , or MRI perfusion imaging.26 If no features of left heart disease exist, patients should undergoCT pulmonary angiography if no contraindications exist.For patients with intermediate or high probability of PH, the echocardiogramshould be evaluated for evidence of left heart disease such as significantvalvular heart disease, LV systolic or diastolic dysfunction. If present,the history should be re-reviewed to assess for risk factors for left heartdisease . Where risk factors for PAH or CTEPH are absent andrisk factors for left heart disease present, PAP modestly elevated, leftatrial size increased, no paradoxical septal motion, and/or significant RVdysfunction present, then no further investigation to assess for PH may berequired. However, where risk factors for PAH or CTEPH are present, RVfunction is severely impaired, systolic PAP is severely elevated (≥70 mmHg),and/or paradoxical septal motion exists, then further investigation toexclude other causes of PH should be considered.38 If features of CTEPH are identified at this stage, patients should bereferred directly to a center experienced in the management of PH forfurther evaluation.Where left heart disease is excluded, or if present and other causes of PHcannot be confidently excluded, then cross-sectional imaging with CTincluding CT pulmonary angiography should be considered as it can aid in:(1) assessment of the likelihood of PH; (2) classification of disease(identifying features of co-existing lung disease or left heart disease);and (3) identification of patients with CTEPH.245If CTPA is sub-optimal, indeterminate, or is performed by a center notexperienced in the assessment of PH and CTEPH is not identified, in theabsence of significant parenchymal lung disease, perfusion lung imaging(Q-SPECT of 3D MR perfusion) is advised at this stage. CT lung subtractioniodine mapping (CT-LISM) or DECT in addition to directly visualizingabnormalities in the pulmonary arterial tree also allows construction ofperfusion lung maps, preventing the need for other forms of perfusion lungimaging to exclude CTEPH.194Supplementary investigations including tests to assess for conditionsassociated with PAH such as connective tissue disease and HIV infectionshould be considered.247 these patients may have other forms of PH such asundiagnosed connective tissue disease or CTEPH.247 Where uncertainty exists, discussion or referral to a PH center isrecommended.Following imaging, the results should be integrated with the patient’sclinical characteristics. CT imaging may identify unexpected findings suchas thromboembolic disease or parenchymal lung disease. Given the currentlack of evidence for specific interventions targeting the pulmonaryvasculature for patients with PH in the context of respiratory diseasecurrent therapies should be aimed at the underlying condition, recognizingthat the presence of PH identifies patients at increased risk of death;where appropriate options such as transplantation should be explored. Inpatients with respiratory disease with risk factors for PAH or CTEPH,significant RV dysfunction or severe elevation in systolic PAP (≥70 mmHg),referral to a PH center should be considered; selected patients may beentered into studies or receive a trial of therapy. In addition to pulmonaryvascular phentoypes increasingly recognized in respiratorydisease,PH referral centers provide an environment where specialists are experiencedin the assessment of patients with suspected PH. They also provide specifictherapies and support for people affected by PH. Imaging investigations willbe reviewed and, where sub-optimal, may be repeated. At this stage, furtherinvestigation will usually be dependent on the pre-test probability ofdifferent forms of PH . For pat249For those with evidence of CTEPH on CTPA or with risk factors such asprevious pulmonary embolus, deep venous thrombosis, splenectomy, orpacemakers, further evaluation of (1) the pulmonary vasculature with DSA orMRA, (2) lung perfusion, with SPECT or 3D MR perfusion, DECTA/CT-LISM, (3)lung ventilation, (4) biventricular function, with cardiac MRI or 3D echo,and (5) coronary or further cardiac valvular assessment, if risk factors forischemic heart disease exist or co-existant valvular heart disease is noted,and the patient is considered a candidate for pulmonary endarterectomy, maybe performed. The choice of investigations is also dependent on thepreference of PH referral center, where pulmonary endarterectomy or BPA, isbeing considered. RHC with measurement of PAWP and PVR will aid decisionsregarding appropriateness of the intervention and to confirm the presence ofPH. Where filling defects extend into the proximal PA and or RV outflowtract, other conditions such as sarcoma should be considered and FDG-PET-CTmay be helpful.251 In patients with PH,where the cause is felt to primarily related to left heart disease orrespiratory disease, RHC may be required to assess disease severityparticularly if a trial of treatment is contemplated. Further assessment ofRV function in these patients may be helpful, particularly where theechocardiographic assessment of RV function was challenging; findings atcardiac MRI of preserved or mildly impaired RV function or a normal septalangle in PH-LHD (suggesting the absence of a pre-capillary component) maynegate the need for RHC.124In patients with risk factors for specific forms of PAH, furtherinvestigation should be tailored; ultrasound examination of the liver withportal Doppler ultrasound should be performed in patients suspected ofunderlying liver disease/portal hypertension. In cases where congenitalheart disease is suspected , transesophageal echocardiography and orcardiac MRI to estimate Qp:Qs ratio and cardiac catheterization with asaturation run should be considered.194Where no obvious cause of PH exists, following review of imaging andintegration with other clinical characteristics, cardiac catheterizationwith vasodilator testing should be performed to identify the 10% of patientswith IPAH who have a fall in mPAP or at least 10 mmHg to <40 mmHG with noreduction in cardiac output, who may respond to treatment with high-dosecalcium channel blockers.Hypoxemia in PH is uncommon at the time of diagnosis in the absence ofrespiratory disease, a right-to-left shunt, or a severely reduced gastransfer factor. If a right-to-left shunt is suspected, a bubbleechocardiogram, renal perfusion SPECT, or MRI (time-resolved imaging orQp:Qs) should be considered.215 in combination witha clinical assessment and a measurement of exercise capacity. Cardiac MRI orechocardiography can be used to assess RV function. In selected cases,follow-up cardiac catheterization may be performed.Following diagnosis, follow-up assessments of RV function are recommended toaid risk stratificationEchocardiography is recommended as the initial imaging investigationin children with suspected PH.Performance of cardiac catheterization in children frequentlyrequires general anesthesia and is associated with a higher risk ofcomplications than in the adult population.Diagnostic imaging strategies differ in children compared to adultsreflecting significant differences in disease etiology.MRI is of additional value in the initial evaluation and follow-up ofPH in conjunction with other non-invasive techniques such asechocardiography.Imaging techniques should be modified where possible to provideadequate diagnostic information while avoiding anesthesia.Imaging provides valuable information that aids the management of patients withPH, from diagnosis and accurate phenotyping, through to monitoring disease andassessing response to therapy. The majority of imaging modalities in medicineand their implementation in hospital environments have been developed with theneeds of adult patients in mind. While the underlying philosophy and principlesof imaging in children are the same as in adult PH, there are some challengeswhich may be more pertinent to imaging children. With appropriate modificationthese can be overcome.252 Furthermore, approximately 30% of children with PH have more than onepotentially causal association.253 Finally, in a large proportion of children with PH, the PH isassociated with other rare conditions. Taken together, this means that thepre-test probabilities of different PH etiologies differ from that inadults. This, in turn, affects the overall diagnostic imaging strategy. Adiagnostic algorithm for children has recently been published following theWorld Symposium of Pulmonary Hypertension and reflects differences betweenadults and children.254While pulmonary vascular disease pathophysiology is very similar, the contextin which it occurs in children is often very different to adult cohorts ofPH. Most notably, children are much more likely to have PH in the context ofcongenital or developmental abnormalities, whereas in adult populationsco-morbid diseases of aging may be more important. Large cohorts ofpediatric patients with PH demonstrate that PH related to congenital heartdisease and PH related to developmental lung disorders predominate with IPAHresponsible for approximately 20% of published cohorts and CTEPH responsiblefor <1% of cases.The pediatric period covers a period from birth to adulthood. The bodyundergoes enormous growth and development during this period of time andbody size can increase by almost two orders of magnitude. Organ structure,function, maturity, and complexity continue to develop through childhood andagain enormously during puberty, profoundly affecting physiology.256 Normative values inCMR in large populations of healthy children is challenging and normativedata are sometimes lacking. A second approach is to adopt ratio–metricrelationships, i.e. the parameter in question is simply divided by a measureof body size, e.g. body surface area (BSA), or is expressed as a ratioagainst another cardiovascular parameter in the same patient, e.g. PA to Aosize ratio; however, these approaches have significant limitations. A moreappropriate and physiologically sound approach to scaling may be to adoptallometric scaling relationships. This approach divides the cardiovascularvariable of interest by the body size variable raised to a scalar exponentin the form x/yb. There are data across a huge range of scalesand species which show empirically that this approach eliminates the effectof body size on cardiovascular structure and function. This approach toscaling or normalization has not been widely adopted and therefore therelevant scaling exponents are not well established or accepted.The most obvious change through childhood is in size. This produceschallenges when aiming to distinguish normal organ size from abnormal, e.g.ventricular volume. A number of approaches have been adopted to address thischallenge. The first is to establish normative data for children throughoutchildhood and express these in terms of centile charts or standard deviation(Z) scores. Normative values in echocardiography in large populations ofhealthy children are increasing with normative echocardiographic valuespublished in pediatrics with Boston z-scores.By definition, in children spatial scales of structures are smaller  and temporal scales are typically shorter (i.e.children have higher respiratory rates and higher heart rates). Thistherefore affects imaging quality at any given spatiotemporal resolution.While some adaptations are possible for example higher frequency ultrasoundprobes for echocardiography, other imaging modalities suffer fromfundamental physical and engineering limits to their spatial temporalresolution. Many imaging modalities require the cooperation of the patient to achieve optimalimaging results. Children are often emotionally and intellectually less maturethan adults; thus, securing their cooperation can be more challenging andtime-consuming. One approach to this which is widely used is to sedate oranesthetize children; however, sedation and anesthesia in children with PH isassociated with a substantial risk of morbidity and mortality and it isdesirable to avoid this wherever possible. Imaging in an appropriate environmentwith adequate time and support, e.g. play therapist involvement and time tofamiliarize patients with the environment, can substantially improve cooperationand imaging quality. Distraction techniques and allowing parents into theimaging room are extremely helpful.257259 It isused for continued follow-up and medication management.260 Developments in echocardiography in the past two decades have led to newinsights into the structure and function of the right ventricle and its role invarious diseases including PH.262 Conventional imagingincludes assessment of anatomy in two dimensions (2D), hemodynamics via Dopplerechocardiography, and qualitative and quantitative evaluation of RV and LVfunction.263265 Advancedechocardiography includes evaluation of right heart size and function,myocardial mechanics, and estimated RV to PA coupling ratio.266270271274Echocardiography is used as the initial screening diagnostic imaging for thediagnosis of pediatric PH and the most important non-invasive tool that is usedfor routine assessment.252 MRI-derived functional and volumetric indices predict morbidity andmortality in pediatric PH and may provide additional information with respect tointer-ventricular interactions.275 In addition,phase-contrast MRI remains the state-of-the-art flow imaging technique enablingprecise flow volume quantification and consequently provides valuable assessmentof Qp/Qs ratios in children with PH associated withcongenital heart disease and intracardiac shunts. Furthermore, parallel orsequential phase-contrast MRI and pressure evaluation in the catheterization labhas been proposed as a novel and potentially more reliable method for thecalculation of PVR in comparison to standard Fick principle orthermodilution.277 Recent studies suggest that pulsatile pulmonaryvascular stiffness indices derived by phase-contrast MRI and ultrasound may havea strong prognostic potential to predict both hard and soft outcomes inpediatric PH.278280 Lastly,MRA can aid with differential diagnosis by fine characterization of thepulmonary vasculature and exclusion of thrombi. Unlike echocardiography, cardiacMRI is currently not suitable for frequent serial assessment due to its clinicalavailability, longer post-processing time, and also due to the necessaryanesthesia required for younger children and neonates.Cardiac MRI remains the gold standard imaging modality for assessment ofbi-ventricular function and volumes in pediatric PH.In summary, children pose a different set of challenges to adult populations whenit comes to imaging. Given the proven benefits of imaging adults with PH, it isappropriate that all patients including children are allowed to experience thesebenefits. Appropriate adaptations to imaging strategy and environment canachieve high-quality results in the vast majority of pediatric patients.Physiological modeling can be used to characterize the behaviorof the cardiopulmonary system.Computational models assessing PA flow have high diagnosticaccuracy in suspected PH.Machine learning approaches may assist image segmentation andimprove diagnostic and prognostic assessments.Further work to assess computational approaches versus currentdiagnostic approaches in PH is recommended.Imaging modalities and markers derived based on images alone havebeen shown to have clinical potential. Their interpretation could beenriched by introducing additional knowledge from the application ofmathematical models, which can bring insights into the hemodynamic systembehavior in health and disease.282 to characterize globally the pulmonary circulationin terms of vascular resistance and compliance in healthy and PH patients.They have also shown promising results for quantifying the changes of theelectric parameters in patients with PAH, at baseline and follow-up.283 Wave transmission  models are particularlypowerful, as “waves carry information,” and the energy285contained in the backward reflected wave can be used as an indicator betweenthe right ventricle and its afterload mismatch. Using a 1D model of astraight elastic tube and temporal MRI flow and area waveforms, it has beenshown that, on average, >40% of the total wave power was contained in thebackward wave measured in patients with PH, whereas <20% wascharacteristic to the healthy volunteers group.282 Different PH heterogeneities can be mimicked by modifying thestructural and elastic parameters of a 1D pulmonary tree structure. Severalauthors285287 investigated the pressure and flow waveforms inhealthy and several simulated PH conditions using numerical solutions of 1Dmodels of the major vessel network. Notably, Qureshi et al.285 changed the configuration of the structured tree and altering, inturn, the compliance of the large and small vessels to predict thehemodynamic changes induced by PAH, CTEPH, and hypoxic lung disease. Theauthors showed an increase in the reflected waves under these simulatedconditions, with the potential to differentiate between different PHphenotypes.Based on mathematical and physical principles, models of the pulmonarycirculation are currently being evaluated in the translational/clinicalresearch. Electrical analogue  modelssupplied by patient-specific 2D phase-contrast MRI data have beenproposed289 Full3D CFD simulations can resolve the physiological flow field in all threedirections and time. Further post-processing of the CFD results can providecomputed metrics , which give additionalinsights on disease progression. It has been argued290 that pathological flows in the PA alter cell behavior favoringvasoconstriction. 3D CFD292 studies of thepulmonary circulation showed that shear stress has an impact on endothelialhealth and dysfunction, and reduced WSS in the proximal PAs were shown to becharacteristic to PH patients.The 0D and 1D models have the main advantage that they are relatively simpleto implement and do not require significant computing resources. However, 3Dcomputational fluid dynamics (CFD) models are more complex, being able toprovide patient-specific characterization of hemodynamics.293 Such approaches are of great potential, minimizing user input/errorand potentially providing a more complete prognostic assessment. The addedvalue versus measurements adjusted for age, sex, and BSA,294 orstandard CMR strain parameters in PAH is an area for further work. Machinelearning approaches may significantly improve the automation andquantification of parameters from imaging. For example, deep learningapproaches have already been utilized for automation of arteriovenoussegmentation of the pulmonary circulation.295Machine learning of RV contours to derive tissue motion has shown to be ofprognostic value in PAH and of greater significance than standard cardiacvolumetric metrics.297 the computationalmodels and imaging modalities play together an essential role in the processof non-invasive PH diagnostic, prognostic, and understanding of the diseasemechanisms.The success of the computational models is closely related to the availabledata from the imaging modalities. Regardless of using the data only tosupply the models’ boundary conditions, or integrate it within artificialintelligence algorithms,In this statement, we have discussed recent advances in imaging techniques in PH andtheir clinical application. This is summarized in the PVRI Imaging Task Forcediagnostic algorithm. Rapidly evolving technologies also provide opportunities toimprove our understanding of PH and assess the impact of much-needed new therapies.This section identifies areas requiring further research in addition to highlightinga number of ongoing and planned studies.298304 However, given thesignificant impact of age, sex, and ethnicity on morphological characteristics,more research in this area is warranted to understand variation acrosspopulations.Despite the widespread use of imaging techniques in clinical practice, there areonly limited data on the impact of age, sex, and ethnicity on commonly usedmetrics. The studies conducted to develop normative equations to allow forcorrection have been performed.There is increasing interest in using imaging endpoints in clinical trials toassess treatment response in the clinic setting. However, there are limited dataon the repeatability of cardiac MRI measurements. The RESPIRE study, which should report in the third quarter of 2019, willprovide information on the sensitivity to change relative to measurementrepeatability of cardiac MRI morphological and functional data. This will aidthe design of studies considering MRI as a primary endpoint. Even in the absenceof such data, studies such as REPAIR  are now using imagingto assess the impact of pharmacological interventions. The results of thesestudies are awaited with interest.There are pros and cons of different imaging modalities with respect todiagnostic performance, repeatability, availability, exposure to ionizingradiation, acceptance to patients, and cost. Often new imaging modalities and/orapproaches are introduced into clinical practice with limited data. The cost ofconducting large comparative studies and the rapid advances in the underlyingtechnology that may occur during the conduct of a study may impact negatively ondecisions to conduct such comparative studies. However, there is a pressing needto perform such technology appraisals. An important area is the diagnosis ofCTEPH, where there have been significant advances in the imaging of thepulmonary vasculature. Additional data are required to critically evaluate thesenew techniques and challenge current guideline approaches. Importantly, a numberof studies are planned, including the prospective, multicenter, comparativephase III diagnostic trial CHANGE-MRI, a European multicenter study comparingfunctional MRI and VQ-SPECT. This study aims to recruit 1000 patients. The INSPIRE study is a pilot non-inferiority studycomparing iodine subtraction mapping with VQ-SPECT in patients with suspectedCTEPH . A prospective study comparing the cost and utilityof follow-up approaches using echocardiography and cardiac MRI in patients withPAH is highly desirable.An additional important area of research is the optimization of initialdiagnostic testing and follow-up based on imaging availability, particularlywhere imaging and invasive testing are limited185 and have been associated with worse RV function on cardiac MRI.305 Bi-allelic mutations in the eukaryotic translation initiation factor 2alpha 5 kinase 4 gene (EIF2AK4) are described in PVOD and PCH,307 whichare important to diagnose given their worse prognosis and poorer response to PAHtherapies. In a large international cohort study,185 patients with PVOD were diagnosed based on radiological criteria;however, a number of patients who were carriers of EIF2AK4 were not identifiedby imaging alone. These patients were younger and had a lower gas transferfactor and greater interlobular septal thickening and mediastinallymphadenopathy. This suggests that combining imaging with genetic testing inat-risk patients has the potential to improve diagnostics and prognostication.Integration of genetics, clinical data, and imaging is an exciting area forfurther research.With advances in genetics and imaging, there is an opportunity to betterunderstand genotype–phenotype associations that have the potential to aidclinical decision-making. Heterozygous mutations in the gene-encoding bonemorphogentic protein receptor type 2 (BMPR2) are the most common genetic causeof PAH, occurring in ∼15% of cases,309 It combines simultaneousinvasive hemodynamic and MRI morphological and functional assessment in a singleradiation-free procedure, providing detailed physiological insights. Furtherwork to investigate the potential advantages in terms of clinical utility,improved understanding of disease, patient acceptability, and cost isrecommended.Diagnosis and accurate prognostication in patients with pulmonary vasculardisease remains challenging. Hemodynamic data from RHC provides importantinformation with direct pressure measurement. However, it does not provide acomplete morphological or functional assessment of the right ventricle andpulmonary circulation. MRI-augmented catheterization involves invasive RHCperformed inside the MRI system.A limitation of current imaging modalities in PH is the insensitivity tovisualize and interrogate the distal pulmonary arterial vasculature, the primarysite of disease in patients with PAH. Assessing pulmonary perfusion directlyusing novel MRI or CT methods may provide a significant insight to theunderlying small vessel vasculopathy. Temporal and spatial heterogeneity of theblood flow in patients at baseline and during PAH therapy is an area for furtherresearch.129 hyperpolarized gas imaging is an emerging technique. Reports ofhyperpolarized gases in CTEPH have suggested a potential role in follow-up of patients310 and abnormalities in IPAH have also been reported.311 A patient inhales Xe129 gas and ventilation images are acquired. Xe129 possesses relatively high solubility in tissues and blood. The transit ofthe gas from gas to dissolved phase can be probed using MRI spectroscopy. Thisallows the acquisition of functional parameters characterizing the gas exchangeas well as gas uptake by imaging the dissolved phase. By studying these Xe129 diffusion properties, various microstructure parameters includingalveolar-volume ratio, blood–gas barrier thickness, and surface-to-volume ratiocan be determined.Inhaled Xe312315 in the assessment ofpatients with PH, but currently its use is not widespread. There is increasinginterest in using CMR and augmented MRI RHC to assess cardiopulmonary system onexercise to determine the differential response of RV morphology and function inhealth and disease. Further evaluation of the effect of acute interventions suchas fluid and acute vasodilator challenges would also be of value.Imaging in patients with suspected PH is usually undertaken at rest but there isincreasing interest in evaluating changes in PAP and RV function on exercise andfollowing other acute interventions. Exercise echocardiography is welldescribed297 Notableareas for further research include acceleration of MRI acquisition, improvedsegmentation of the cardiac chambers on CT and MRI and the pulmonary vasculatureon CT, and diagnostic and prognostic classification and interpretation.With the rapid development in machine learning technologies, there is hugepotential to improve imaging assessments. Image acquisition, analysis, andinterpretation are areas that may benefit from integration of AI technologies.There are a number of recent publications in this area.316 This study illustrated that images can fulfil a mediating role by aidingacquisition of knowledge and facilitating communication in addition toillustrating the highly social aspects of interactions with patients and withinmultidisciplinary meetings. Involvement of patients and their families and usersof imaging is required if imaging is going to fulfil its potential.Much of the focus of imaging research is based on the knowledge that can obtainedby the images themselves. However, an important area of investigation is theimpact of imaging: on patients and their families, physicians and healthcareprofessionals, researchers who interpret the images, and healthcare systems thatfund imaging. The acceptability, emotional impact, and cost of clinical pathwaysto diagnose and serially assess patients requires further research, includingthe relative tolerability, risks, and benefits of different imaging approaches.A study evaluated the social and technological epistemology of clinicaldecision-making as mediated by imaging.The author(s) declare that there is no conflict of interest.This study was supported by the Wellcome Trust [R/148654-11-1].