Intravoxel incoherent motion (IVIM) imaging at different field strengths – what is feasible ?

Introduction Since the introduction of the intravoxel incoherent motion (IVIM) concept by Le Bihan [1], the cerebral applications of this technique have been sparse due to the limited SNR. However, MRI hardware development in general and increased main magnetic field strength specifically, could result in improved SNR and may justify a reassessment of IVIM imaging for non-invasive quantification of the cerebral blood volume (CBV) and cerebral blood flow (CBF) [2,3]. In this study, results from both simulations and experimental measurements are presented, and signalversus-b curves at three different field strengths (1.5, 3 and 7 T) are shown as a first step towards determining the optimal field strength for IVIM imaging. Subjects and Methods Simulated signal-versus-b curves, S(b), were obtained for a synthetic voxel consisting of four compartments, i.e. cerebral gray matter (80%), cerebrospinal fluid (2%), arterial blood (3%, [4]) and venous blood (7%, [4]). Simulations were based on conventional signal equations for spin echo sequences including the IVIMeffect (Eq. 1). [ ] *) ( ) 1 ( ) 0 ( ) ( D D b bD fe e f S b S + − − + − = [Eq. 1] where D is the diffusion coefficient, D* is the pseudodiffusion coefficient and f is the perfusion fraction . Values for T1, T2, D and pseudo-diffusion coefficient D* for the three magnetic field strengths were obtained from the literature. To confirm the results of the simulations, one volunteer was examined at three different field strengths within two days, using 1.5 T (Philips Intera), 3T (Philips Achieva) and 7T (Philips Achieva) whole-body MRI scanners. To facilitate comparison of the experiments, the imaging parameters were selected to be as equal as possible for all three field strengths. A spin-echo sequence with diffusion encoding in the phase-encoding direction was employed with b-values ranging between 0 and 900 s/mm. Imaging parameters were TR=3500 ms, FOV 220×220 mm, matrix size 80×80 and slice thickness 5 mm. The TE was 87, 70 and 62 ms at 1.5, 3 and 7T, respectively, due to different hardware requirements. The experimental signal-versus-b data, obtained as the mean value of two ROIs placed in thalamus (left and right hemisphere), were normalized to the signal at b=3 mm/s to correct for differences in signal intensity between the different field strengths. Results The results from the simulations of signal-versus-b data for the three different field strengths are given in Figs 1a-c. The most prominent finding is that for higher field strengths the relative contribution from venous blood decreases. The experimental signal-versus-b curves for the thalamus ROIs, measured in the same volunteer at three different field strengths, are shown in Fig 1d.