Angle reduction for fast scans in gradient-coil-free portable MRI using rotational spatial encoding magnetic fields...

The goal of this study is to reduce rotation angles for faster scans in a gradient-coil-free portable MRI system, employing rotational spatial encoding magnetic fields (rSEM) with nonlinear gradients. This is achieved by utilizing a novel evaluation metric for encoding capability, thereby making optimization feasible.

A local k-space-based evaluation metric for the encoding capability of rSEM is proposed. The evaluation metric for each subregion of the image is calculated by summing the area covered by the spokes in the local k-space when the SEM rotates; each spoke corresponds to one angle. This evaluation metric is expected to correlate closely with the image quality of the reconstructed images and is applied for angle selection to accelerate scans in rSEM systems. The greedy algorithm is used for optimization. Both simulations and experiments were conducted to validate the effectiveness of the evaluation method and the selected angle set.

The results demonstrate that the proposed evaluation metric for the encoding capability of rSEM is highly correlated with image quality metrics of the corresponding reconstructed images (correlation coefficient greater than 0.90). Using the angle set selected by the greedy algorithm with the proposed method, higher image quality with reduced angles is achieved, thereby shortening scan time. This further validates the accuracy of the proposed evaluation metric and underscores the necessity of rapid assessment for feasible optimization.

Significant reduction of rotation angles is achieved by selecting rotation angles using the proposed evaluation method. This facilitates fast scans in gradient-coil-free portable MRI systems with rotating SEM while maintaining high image quality. This advancement is crucial for the practical application of this type of portable MRI.