Modeling an equivalent b-value in diffusion-weighted steady-state free precession.
PURPOSE
Diffusion-weighted steady-state free precession (DW-SSFP) is shown to provide a means to probe non-Gaussian diffusion through manipulation of the flip angle. A framework is presented to define an effective b-value in DW-SSFP.


THEORY
The DW-SSFP signal is a summation of coherence pathways with different b-values. The relative contribution of each pathway is dictated by the flip angle. This leads to an apparent diffusion coefficient (ADC) estimate that depends on the flip angle in non-Gaussian diffusion regimes. By acquiring DW-SSFP data at multiple flip angles and modeling the variation in ADC for a given form of non-Gaussianity, the ADC can be estimated at a well-defined effective b-value.


METHODS
A gamma distribution is used to model non-Gaussian diffusion, embedded in the Buxton signal model for DW-SSFP. Monte-Carlo simulations of non-Gaussian diffusion in DW-SSFP and diffusion-weighted spin-echo sequences are used to verify the proposed framework. Dependence of ADC on flip angle in DW-SSFP is verified with experimental measurements in a whole, human postmortem brain.


RESULTS
Monte-Carlo simulations reveal excellent agreement between ADCs estimated with diffusion-weighted spin-echo and the proposed framework. Experimental ADC estimates vary as a function of flip angle over the corpus callosum of the postmortem brain, estimating the mean and standard deviation of the gamma distribution as   mm2 /s and   mm2 /s.


CONCLUSION
DW-SSFP can be used to investigate non-Gaussian diffusion by varying the flip angle. By fitting a model of non-Gaussian diffusion, the ADC in DW-SSFP can be estimated at an effective b-value, comparable to more conventional diffusion sequences.
['Tendler|Benjamin C|BC|0000-0003-2095-8665', 'Foxley|Sean|S|', 'Cottaar|Michiel|M|', 'Jbabdi|Saad|S|', 'Miller|Karla L|KL|']
D001344:Autopsy D001921:Brain D003337:Corpus Callosum D004058:Diffusion D038524:Diffusion Magnetic Resonance Imaging D006801:Humans
b-value diffusion-weighted steady-state free precession non-Gaussian diffusion Monte-Carlo diffusion-weighted spin-echo postmortem MRI