The influence of acquisition parameters on the metrics of the bi - exponential IVIM model

(maximum: 200 words, word count: 200 words) Purpose: The IntraVoxel Incoherent Motion (IVIM) MRI signal, typically described as a mono-exponential decay, can sometimes be better modeled as a bi-exponential function accounting for two vascular pools, capillaries and medium-size vessels. The goal of this work is to define precisely in which conditions the IVIM signal shape becomes bi-exponential and to understand the evolution of the IVIM outputs with different acquisition parameters. Methods: Rats were scanned at 7T and 11.7T using diffusion-weighted pulsed-gradient spin-echo (SE) and stimulated-echo (STE) sequences with different repetition times (TR) and diffusion encoding times. The obtained IVIM signals were fit to the mono-and bi-exponential models and the output parameters compared. Results: The bi-exponential and mono-exponential models converge at long diffusion encoding times and long TRs. The STE is less sensitive to inflow effects present at short TRs, leading to a smaller volume fraction for the fast pool when compared to the SE sequence. Conclusion: The two vascular components are more easily separated at short diffusion encoding times, short TRs and when using SE sequences. The volume fractions of the two blood pools depend on the pulse sequence, TR and diffusion encoding time while the pseudo-diffusion coefficients are only affected by the diffusion encoding time.