Towards Plaque and Thrombus Imaging with Hybrid of Opposite-Contrast (HOP) MR Angiography at 7T
Sören Johst1, Karsten H Wrede1,2, Harald H Quick1,3, and Mark E Ladd1,4

1Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Essen, Germany, 2Department of Neurosurgery, University Hospital Essen, Essen, Germany, 3High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, 4Medical Physics in Radiology, German Cancer Research Center (dkfz), Heidelberg, Germany

Synopsis

Imaging of intracranial plaques and thrombi is important, as they play a role e.g. in rupture risk of cerebral aneurysms. 7T provides the potential to increase spatial resolution due to increased SNR. A time-of-flight MR angiography (TOF MRA) sequence was extended to a hybrid of opposite-contrast MRA sequence providing two contrasts, bright and black blood. Inverting and subtracting both contrasts makes thrombi/plaques visible. Sufficient flow dephasing moments for black-blood contrast were determined for three resolutions, image post-processing was implemented, and contrast evaluated. This approach potentially enables patient studies with higher resolution than previously achievable while maintaining reasonable acquisition times.

Purpose

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.

Methods

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).

Results

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.

Discussion

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 protocol4,5 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.

Conclusion

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.

Acknowledgements

The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 291903 MRexcite.

References

1 Kimura T, Ikedo M, Takemoto S. Hybrid of Opposite-Contrast MR Angiography (HOP-MRA) Combining Time-of-Flight and Flow-Sensitive Black-Blood Contrasts. MRM. 2019;62:450–458.

2 Matsushige T, Akiyama Y, Okazaki T, et al. Vascular Wall Imaging of Unruptured Cerebral Aneurysms with a Hybrid of Opposite-Contrast MR Angiography. Am J Neuroradiol. 2015;36:1507-1511.

3 van der Kolk AG, Zwanenburg JJM, Brundel M, et al. Intracranial Vessel Wall Imaging at 7.0-T MRI. Stroke. 2011;42:2478-2484.

4 Johst S, Wrede KH, Ladd ME, et al. Time-of-Flight Magnetic Resonance Angiography at 7 T Using Venous Saturation Pulses With Reduced Flip Angles. Invest Radiol. 2012;47:445-450.

5 Wrede KH, Dammann P, Mönninghoff C, et al. Non-Enhanced MR Imaging of Cerebral Aneurysms: 7 Tesla versus 1.5 Tesla. PLoS ONE. 2014;9(1): e84562.

Figures

Sequence diagram showing venous saturation (Sat) and acquisition of two echoes (ADC 1/2). A pair of flow dephasing gradients (FDG) precede the 2nd echo. Readout pre-dephasing gradient for the 2nd echo and 2nd FDG are overlapped. Phase-encode rewinders are moved after the 2nd acquisition. Only the first echo is flow-compensated.

Protocol parameters for the three different resolutions. 2nd echo time TE2 and TR increased accordingly as the flow dephasing gradient moments were increased. For both echoes the same bandwidth was used.

Black-blood contrast source images (resolution 0.35x0.35x0.41 mm3) showing that sufficient flow dephasing gradient moment (a) is achieved for a 114.4 ms*mT/m gradient moment but not for the next smaller moment (b). One artery is exemplarily marked by a white arrow (zoomed in, bottom right).

Subtraction images for 0.22x0.22x0.41 mm3 (a), 0.35x0.35x0.41 mm3 (b), and 0.53x0.53x0.53 mm3 (c) resolution. Signal inside arteries is nulled for all resolutions with clear vessel delineation (e.g. middle cerebral artery, white arrow).

Figure showing 1st (a) and 2nd (b) echo (TOF and black-blood contrast) for a resolution of 0.22x0.22x0.41 mm3, the corresponding subtraction image (TOF is subtracted from the inverted black-blood image) (c), and a summation of TOF and black blood (d).



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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