Highly efficient image navigator based 3D whole-heart cardiac MRA at 0.55T

Purpose: To develop and evaluate a highly efficient free-breathing and contrast-agent-free three-dimensional (3D) whole-heart Cardiac Magnetic Resonance Angiography (CMRA) sequence at 0.55T.

Methods: Free-breathing whole-heart CMRA has been previously proposed at 1.5 and 3T. Direct application of this sequence to 0.55T is not possible due to changes in the magnetic properties of the tissues. To enable free-breathing CMRA at 0.55T, pulse sequence design and acquisition parameters of a previously proposed whole-heart CMRA framework are optimized via Bloch simulations. Image navigators (iNAVs) are used to enable nonrigid respiratory motion-correction and 100% respiratory scan efficiency. Patch-based low-rank denoising is employed to accelerate the scan and account for the reduced signal-to-noise ratio at 0.55T. The proposed approach was evaluated on 11 healthy subjects. Image quality was assessed by a clinical expert (1: poor to 5: excellent) for all intrapericardiac structures. Quantitative evaluation was performed by assessing the vessel sharpness of the proximal right coronary artery (RCA).

Results: Optimization resulted in an imaging flip angle of $110^{\circ }$ , fat saturation flip angle of $180^{\circ }$ , and six k-space lines for iNAV encoding. The relevant cardiac structures and main coronary arteries were visible in all subjects, with excellent image quality (mean $4.9/5.0$ ) and minimal artifacts (mean $4.9/5.0$ ), with RCA vessel sharpness ( $50.3\%\pm 9.8\%$ ) comparable to previous studies at 1.5T.

Conclusion: The proposed approach enables 3D whole-heart CMRA at 0.55T in a 6-min scan ( $5.9\pm 0.7\min$ ), providing excellent image quality, minimal artifacts, and comparable vessel sharpness to previous 1.5T studies. Future work will include the evaluation of the proposed approach in patients with cardiovascular disease.