A Novel Non-contrast MR Angiography Technique using Triggered Non-Selective Refocused SPACE for Improved Spatial Resolution and Speed

Introduction: A non-contrast-enhanced MRA technique using gated 3D Turbo Spin Echo (TSE) has been reported for various clinical applications [1] [2], whereby systolic images are subtracted from diastolic images to provide high resolution MRA without exogenous contrast agents. Half-Fourier single-shot TSE results in a relatively long readout duration which causes vessel blurring due to T2 filtration effects and susceptibility to motion artifacts and arrhythmias. To reduce the long acquisition window for this cardiac imaging, one can use parallel imaging to reduce the acquired lines [3] and higher receiver bandwidth to reduce echo spacing time (ETS) [4],another potential solution to this limitation include multi-shot approach with the trade-off of increased scan time is less clinically acceptable especially as the technique is subtraction based and long scan times result in higher potential for motion between the corresponding systolic and diastolic scans. As an approach to alleviating the spatial resolution limitations and to improve the reliability of this technique in fast heart rates we have investigated the use of a variable flip angle method for non contrast MRA using triggered 3D SPACE (Sampling Perfection with Application optimized Contrasts by using different flip angle Evolutions), which uses selective RF excitation, non selective RF refocusing pulses and variable flip angles for contrast manipulation [5]. To determine times of peak and minimal flow within the cardiac cycle, triggered (pulse or ECG) phase contrast flow measurements were performed. SPACE helps achieve lower SAR and shorter ETS of 2 ms, resulting in total acquisition windows of around 100ms, a slice turbo factor in 3D direction can be used to speed up the acquisition, because two or more partitions can be acquired in one cardiac cycle. Figure 1 shows the intrinsic contrasts mechanisms between the two techniques, TSE based approach uses C-A, SPACE based approach use B-A. We performed a comparison between the SPACE and the TSE based approaches in terms of spatial resolution and reduced blurring and small vessel conspicuity.