Imaging of intracranial plaques and thrombi is important, as they play a role in e.g. formation, development and rupture risk of cerebral aneurysms.¹ 7T MRI provides the potential to increase spatial resolution due to its increased SNR.

Hybrid of opposite-contrast MR angiography (HOP-MRA)¹ was developed to increase the visibility of small vessels in MRA. However, it was recently also applied for vessel wall imaging at 3T.² An advantage, compared to recently published TSE-based vessel wall imaging at 7T,³ is that no contrast agent is necessary. The principle of HOP-MRA is a gradient-echo-based time-of-flight (TOF) MRA with a 2nd echo providing black-blood (BB) contrast by preceding flow dephasing gradients (FDG). Plaques/thrombi appear hypointense in both echoes. As the blood lumen is bright in both images, TOF and inverted BB contrast, only the signal of the plaques remains bright after subtraction.

A second echo was implemented in a custom TOF MRA sequence⁴ that is routinely used in various 7T MRA studies.⁵ Similar to ¹, a bipolar gradient pair was introduced right before the 2nd echo to dephase the spins of the flowing blood (Fig. 1), while stationary spins are rephased after both gradients.

In an initial volunteer, the FDG were successively increased, 14.3/20.8/33.8/46.8/59.8/114.4/140.4 ms*mT/m (=moment of one single FDG), for three different image resolutions (Fig. 2) until sufficient vessel suppression was reached. TE1 was kept constant while TE2 and TR increased accordingly with gradient area (Fig. 2). In all protocols, the shortest possible TR and echo times were used. For lower resolutions, shorter 2nd echo times can be chosen, as lower readout gradients are needed and thus the acquisition bandwidth can be increased. To confirm the observations, protocols with sufficient FDG were applied in two additional volunteers.

Imaging was performed on a 7T whole-body MRI system (MAGNETOM 7T; Siemens). The sequences used asymmetric tilt-optimized non-saturated excitation (TONE) RF pulses and asymmetric echoes. A venous saturation pulse with 35° flip angle was used to suppress the veins in the TOF contrast.⁴ To ameliorate SAR restrictions, the variable-rate selective excitation (VERSE) algorithm was used for both excitation and saturation RF pulses.⁴ Protocol parameters are given in Fig. 2.

In post-processing, a N3 Bias Field Correction was applied to correct for B1 field inhomogeneities, then the TOF images were subtracted from the inverted BB images (FMRIB Software Library; FSL version 5.0.1; Jenkinson et al., 2012).

Vessel-background contrast was measured in TOF, BB, subtraction, and summation images with circular ROIs placed in the middle cerebral artery (MCA) and adjacent brain tissue (2mm/5mm diameter, respectively).

Gradient stimulation thresholds were almost at the limits for these protocols (87% - 98%). Highest SAR was 76% and decreased with increasing dephasing gradients (respectively TR). There is a trade-off between FDG area and TE2 in this sequence, as higher FDG increases T2* weighting in the BB contrast. A FDG moment of 114.4 ms*mT/m (diffusion b-value = 0.1 s/mm²) provided sufficient suppression of blood signals (Fig. 3) for all resolutions, corresponding to TE2 of 16.49/14.66/13.59 ms and measurement times of 9:29/6:06/3:38 min:s (Fig. 4). Smaller vessels were in general more difficult to suppress; direction of flow (through-plane or in-plane) also influenced the effect of the FDG.

In Figure 5, TOF, BB, and subtraction images are shown. Summation of the contrasts (Fig. 5d) showed fewer flow artifacts, as no flow artifacts were visible in the 2nd echo. However, contrast is reduced.

In the volunteer shown in Fig. 4 and 5, a Michelson contrast for TOF of 0.62/0.63/0.49 (±0.1; highest to lowest resolution), BB 0.19/0.3/0.61, subtracted 0.44/0.54/0.82, and summed images 0.49/0.50/0.31 was determined. No measureable difference in vessel diameter was found comparing TOF and subtraction images.

The subtracted images lead to sufficient contrast (better contrast than BB images alone). Due to gradient system/nerve stimulation limitations, there is not much potential to further reduce TE2 while maintaining sufficient FDG moment. Masking (part of) the artery of interest should relieve effects (background signal difference/veins) introduced in the 2nd echo by the relatively high TE2 values.

Expanding a given TOF protocol^4,5 facilitates integration into current MRA patient studies. Also, no additional BB sequence and no contrast agent are necessary while saving measurement time, decreasing SAR, and minimizing misregistrations. Based on these results, a patient study is now planned to evaluate imaging of intracranial plaques/thrombi.

Patient studies evaluating imaging of plaques/thrombi are now feasible with a HOP-MRA double contrast sequence using the tested protocol parameters. Spatial resolution could be increased in this 7T UHF application compared to recent methods (0.93x0.93x1/0.8x0.8x0.8mm³)^2,3 while maintaining reasonable scan times.