Intravoxel Incoherent Motion at 7 Tesla to quantify human spinal cord perfusion: limitations and promises

Abstract

Purpose:To develop a noninvasive technique to map human spinal cord (SC) perfusion in vivo. More specifically, to implement an intravoxel incoherent motion (IVIM) protocol at ultrahigh field for the human SC and assess parameters estimation errors. Methods: Monte-Carlo simulations were conducted to assess estimation errors of 2 standard IVIM fitting approaches (two-step versus one-step fit) over the range of IVIM values reported for the human brain and for typical SC diffusivities. Required signal-to-noise ratio (SNR) was inferred for estimation of the parameters product, f IVIM D* (microvascular fraction times pseudo-diffusion coefficient), within 10% error margins. In-vivo IVIM imaging of the SC was performed at 7T in 6 volunteers.An image processing pipeline is proposed to generate IVIM maps and register them for an atlas-based region-wise analysis. Results: Required b = 0 SNRs for 10% error estimation on f IVIM D* with the one-step fit were 159 and 185 for diffusion-encoding perpendicular and parallel to the SC axis, respectively. Average in vivo b = 0 SNR within cord was 141 ± 79, corresponding to estimation errors of 12.7% and 14.7% according to numerical simulations.Slice-and group-averaging reduced noise in IVIM maps, highlighting the difference in perfusion between gray and white matter. Mean ± standard deviation f IVIM and D* values across subjects within gray (respectively white) matter were 16.0 ± 1.7 (15.0 ± 1.6)% and 11.4 ± 2.9 (11.5 ± 2.4) × 10 −3 mm 2 /s. Conclusion: Single-subject data SNR at 7T was insufficient for reliable perfusion estimation. However, atlas-averaged IVIM maps highlighted the higher microvascular fraction of gray matter compared to white matter, providing first results of healthy human SC perfusion mapping with MRI. K E Y W O R D S7T MRI, intravoxel incoherent motion, IVIM, perfusion, spinal cord, ultrahigh field MRI | 1199 LÉVY et aL.

Feasibility of human spinal cord perfusion mapping using dynamic susceptibility contrast imaging at 7T: Preliminary results and identified guidelines

“…Intravoxel incoherent motion was evaluated at 7T, but sensitivity at single-subject and single-slice scale was limited, and averaging across multiple subjects (≥ 6) was necessary to map the perfusion difference between GM and WM. 17 Very little literature exists regarding contrast agent-based perfusion MRI techniques in the human SC. Vascular space occupancy MRI has been used to map the absolute SC blood volume (BV) in healthy subjects.…”

“…However, only poor sensitivity and reliability were obtained. Intravoxel incoherent motion was evaluated at 7T, but sensitivity at single‐subject and single‐slice scale was limited, and averaging across multiple subjects (≥ 6) was necessary to map the perfusion difference between GM and WM 17 …”

Quantitative MR Markers in Non-Myelopathic Spinal Cord Compression: A Narrative Review

“…Intravoxel incoherent motion (IVIM) imaging measures microscopic movement of water molecules caused by capillary perfusion using dMRI sequence with low b-values (≤300 mm 2 /s) to assess flowing blood fraction and pseudo-diffusion coefficient [89]. Pilot IVIM imaging studies in the human SC at 7T in 6 HC [90], and at 3T in 2 DCM patients along with 11 HC [91] depicted higher perfusion in GM compared to WM in HC and impaired perfusion in DCM patients at compression levels. However, interpretation is limited due to the small sample size and possible influence of CSF pulsation [91].…”

Combined diffusion‐relaxometry microstructure imaging: Current status and future prospects

“…In particular, the reported increase of micro‐FA at ultra‐high‐field deserves further investigation. 85 , 86 Another promising avenue is the B0 dependency of the intravoxel incoherent motion (IVIM) signal curve at ultra‐high‐field, 87 , 88 where the weight of the arterial pool should further increase, combining this with additional relaxometry information can potentially make “arterial‐pool‐weighted” IVIM feasible. 89 …”