Introduction

Magnetic resonance angiography (MRA), including the inflow time-of-flight (TOF) technique^1,2, has proven to be valuable in the evaluation of carotid stenosis and occlusion, as well as the diagnosis of stroke and aneurysms^3-5. The recent resurgence of ultra-low-field (ULF) MRI (i.e., below 0.1 T) is showing great promise for future clinical applications due to its low cost, portability, and accessibility^6-14. ULF MRI holds the possibility to advance neck MRA for the evaluation and diagnosis of carotid diseases in point-of-care scenarios and low/mid-income countries. In this study, we explore the feasibility of neck MRA using the TOF technique on a low-cost and shielding-free 0.05 Tesla MRI scanner.

Method

In vivo experiments
All in vivo experiments were conducted on a permanent magnet based 0.05 Tesla MRI scanner using neck coil for signal receiving. The scanner had a similar design to that reported in our previous work⁶, which was free from any magnetic and RF shielding. Healthy subjects were enrolled in this study under institutional review board approval. Written informed consents were obtained.
Multi-slice 2D GRE sequences with and without flow saturation were implemented to visualize neck vessels. Sequence parameters were: FA = 90°, TR/TE = 38/10ms, BW = 5kHz, 75% partial echo in readout (RO) direction and 80% undersampling in phase encoding (PE) direction, acquisition resolution = 2×2×4mm³, matrix size = 80×86×34, NEX = 4, and total acquisition time = 7.4mins. To reduce the signal loss due to flowing blood spin dephasing, first-order flow compensation was applied in all three directions.
For carotid artery and jugular vein imaging, a selective flow saturation pulse followed by spoiler gradients in all three directions was applied before each excitation pulse, to saturate the signal of downstream venous blood and upstream artery blood, respectively.
Image reconstruction and processing
K-space data were first processed for electromagnetic interference (EMI) signal removal by utilizing active EMI sensing and deep learning EMI signal prediction and cancellation^6,15, without any EMI characterization data acquisition. Then partial Fourier reconstruction was applied in both RO and PE directions. Afterward, 4× in-plane and 8× through-plane zero-padding was conducted before image reconstruction and maximum intensity projection (MIP).

Results

Multi-slice 2D TOF MRA achieved strong vessel-to-background contrast. In general, 2D GRE with a large FA of 90° and short TR effectively suppressed the background signal and enhanced the vessels through a strong inflow effect. Note that the same slices and MIP rotation angles were selected and shown in the following figures. As shown in Fig. 1, without venous contamination, the left and right common carotid arteries, vertebral arteries, internal carotid arteries, and external carotid arteries (Fig. 1, red arrows) could be readily identified and visualized in both the raw 2D images and MIPs. Moreover, the left and right carotid bifurcations were clearly visible in the MIPs (Fig. 1, red arrows). Note that the arteries were not smooth or incomplete in some regions. Fig. 2 shows the jugular veins results obtained with carotid artery saturation. The jugular veins, including the left and right internal and external jugular veins (Fig. 2, red arrows), could be observed. The observed signal intensity differences between the left and right jugular veins might be attributed to different in-plane saturation of the slow flowing venous blood. Fig. 3 shows the neck vessel results obtained without flow saturation. The morphology of carotid arteries and jugular veins is consistent with those in Figs. 1 and 2, demonstrating the robustness of neck MRA in different scans at ULF.

Discussion and Conclusions

Neck MRA is a key protocol for the assessment of carotid diseases^3-5. In this preliminary study, we demonstrate the possibility of non-contrast enhanced neck MRA at ULF for the first time. Both carotid arteries and jugular veins could be visualized at 0.05 Tesla. The preliminary results clearly indicated the potential of ULF MRI for neck MRA despite the low SNR.
To improve neck TOF MRA quality at ULF, several steps can be taken in future development: (i) sequence optimizations of slice overlapping and thinner slice thickness to reduce the non-smooth vessel visualization¹⁶; (ii) utilization of large power flow saturation pulses or extra strong magnetization transfer preparation to suppress stationary muscle tissues, especially given the extremely low RF SAR at ULF; (iii) use of compressed sensing acquisition and reconstruction to reduce scan time, increase the spatial resolution, or/and suppress noise; and (iv) use of intravascular blood pool contrast agent to shorten blood T1, then use 3D acquisition to increase SNR or/and spatial resolution. These strategies may also advance the potential use of ULF MRI for abdominal and peripheral vessel imaging in the future.

Acknowledgements

This work was supported in part by Hong Kong Research Grant Council (R7003-19F, HKU17112120, HKU17127121, HKU17127022 and HKU17127523 to E.X.W).

References

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