Optimization of combination of voxel size and flip angle by visual evaluation for renal magnetic resonance angiography without a contrast agent

To improve spatial resolution of renal magnetic resonance (MR) angiography, we attempted to use MR signals before reaching steady state in balanced turbo field echo (b-TFE) sequence. An isotropic protocol (isotropic voxel size; 0.78 mm) was constructed, but the loss of contrast to noise ratio (CNR) was not avoided though we employed the dedicated k-space scheme. The trade-off of the spatial resolution and CNR is difficult to decide by numerical value only, and it is necessary to introduce evaluation of clinical images for the determination of a flip angle. In this study, we investigated the optimal combination of voxel size and flip angle using visual evaluation in order to obtain a good balance of the spatial resolution and CNR. A paired comparison method was employed as visual evaluation. As a result, the highest scale-value of available combination of isotropic voxel size and flip angle was 0.67 mm and 60 degrees. This isotropic protocol of the highest resolution for MR angiography of renal arteries using contrast behavior during the transient phase of b-TFE sequence will be useful in the clinical field.