1349116 - Whole-Heart Cardiovascular Magnetic Resonance Angiography (CMRA) with iNAV-based Non-Rigid Motion-Corrected Reconstruction at 0.55T

Background: The potential clinical value of free-breathing whole-heart magnetic resonance angiography (CMRA) with image navigator (iNAV)-based non-rigid motion-corrected reconstruction and 100% respiratory scan efficiency has been recently demonstrated at 1.5T¹. The feasibility of this approach has also been shown at 3T² y 7T³. However, it is unknown if 3D whole-heart CMRA is feasible at 0.55T due to the lower signal-to-noise ratio (SNR). Compared to 1.5T, the SNR at 0.55T is 4.5 times smaller, as the SNR is proportional to B0^3/2. In this study, we sought to investigate the feasibility of iNAV-based4 non-rigid motion-corrected 3D CMRA on the latest 0.55T MR scanner generation, which could make CMRA potentially more accessible and affordable.

Methods: The proposed accelerated whole-heart CMRA framework was performed in a healthy subject on a 0.55T scanner (MAGNETOM Free.Max, Siemens) using a 3D bSSFP prototype sequence with an undersampled variable-density cartesian trajectory with spiral-like order (VD-CASPR). T2 preparation (T2-Prep) and Fat Saturation (FAT SAT) pre-pulses were performed before the acquisition at each heartbeat. A 2D iNAV⁴ preceded each spiral-like arm acquisition to enable beat-to-beat 2D translational and bin-to-bin 3D non-rigid respiratory motion correction of the heart with 100% respiratory scan efficiency (Fig.1). Estimated bin-to-bin non-rigid 3D motion fields are incorporated into a non-rigid motion-compensated reconstruction. Motion-compensated reconstruction was performed with iterative SENSE (itSENSE) with and without patch-based low-rank regularization (PROST)⁵. Acquisitions parameters included: FOV = 312 x 312 x 44 mm, resolution = 1.5 mm³, flip-angle = 90 deg, TR = 5.48 ms, TE = 2.74 ms, T2-Prep time = 50 ms, FAT SAT flip-angle = 110 deg, 3x undersampled, total scan time ~ 6min. Shimming box, REST slab location, and trigger delay were set following the steps described in Fig.2. Free-breathing 2D CINE was performed to determine the subject-specific trigger delay (mid-diastole) and acquisition window (~120ms).

Results: itSENSE reconstruction without and with non-rigid motion corrected reconstruction are shown in Fig.3 together with the proposed non-rigid motion corrected PROST reconstruction. Both itSENSE reconstructions show low SNR at low field. Non-rigid motion correction reduces respiratory motion artifacts (itSENSE non-rigid), whereas the proposed approach greatly increases image quality due to the further incorporation of PROST regularization, which partially compensates for low SNR at low field. Reformatted images along the right (RCA) and circumflex (LCX) coronary arteries are shown in Fig.3, showing a good depiction despite the relatively low resolution used in this preliminary study.

Conclusion: In this work, we showed a first proof of concept of iNAV-based whole-heart CMRA at 0.55T. Future work will focus on further parameter optimization, increasing spatial resolution, and evaluation in healthy subjects and patients with cardiovascular disease.