Detecting sub-voxel microvasculature with USPIO-enhanced susceptibility-weighted MRI at 7 T

BACKGROUND: Susceptibility weighted imaging (SWI) combines phase with magnitude information to better image sub-voxel veins. Recently, it has been extended to image very small sub-voxel arteries and veins by injecting intravenously the ultra-small superparamagnetic iron oxide, Ferumoxytol. OBJECTIVE: To determine practical experimental imaging parameters for sub-voxel cerebral vessels at 7 T. METHODS: Six Wistar-Kyoto rats aged 7-13 weeks were imaged. For a given spatial resolution, SWI was acquired pre- and post- Ferumoxytol with doses of 2, 4, 6 and 8 mg/kg and echo times (TEs) of 5, 10 and 15 ms at each dose. The spatial resolutions of 62.5 × 125 × 250 μm³ (acquisition time of 7.5 min) and 62.5 × 62.5 × 125 μm³ (30 min) were used. Both SWI and quantitative susceptibility mapping (QSM) data were analyzed. Contrast-to-noise ratio (CNR) was measured and used to determine the optimal practical imaging parameters for detection of small cortical penetrating arteries. RESULTS: For a given spatial resolution with an aspect ratio (frequency: phase: slice) of 2:4:8 relative to the vessel size, we found the TE-dose index (TE x dose) must be at least 40 ms·mg/kg for both SWI and QSM to reveal the most vessels. The higher the TE-dose index, the better the image quality for both SWI and QSM up to 60 ms·mg/kg. CONCLUSIONS: There is an optimal TE-dose index for improved visualization of sub-voxel vessels. Choosing the smallest TE and the largest allowed dose made it possible to run the sequence efficiently. In practice, the aspect ratio of 2:4:8 and the TE-dose index ranging from 40 to 60 ms·mg/kg provided the optimal and most practical solution.