Variable Flip Angle Balanced Steady State Free Precession MRI

In this dissertation, variable flip angle (VFA) balanced steady-state free precession (bSSFP) techniques have been developed to lower the specific absorption rate (SAR) or improve the signal and contrast of bSSFP for free-breathing cardiac cine applications, 3D T2 weighted prostate imaging and non-contrast MR arteriography of the lower leg all with the aim of reducing patient discomfort while maintaining image quality.;The first objective was to determine the optimal bSSFP FA for cardiac cine imaging. Detailed Bloch simulations of stationary myocardium, flowing blood in the presence of imperfect slice profile and off-resonance effects were simulated and compared with in vivo imaging experiments of ten healthy subjects and seven patients with a range of ejection fraction and heart rate. The in vivo imaging in healthy subjects and clinical patients show that high blood-myocardium contrast can be obtained with a FA ~105°, when imaging a plane with predominantly through-plane flow such as the short-axis plane.;The second objective was to develop a low SAR 2D breath-hold, cardiac cine imaging technique with blood-myocardium contrast similar to conventional bSSFP imaging using VFA scheme, which was implemented by using an asynchronous k-space acquisition. Bloch simulations were performed to determine if multiple VFA acquisitions of stationary myocardium and flowing blood produces a steady state signal between acquisitions and was verified by phantom experiments. In vivo experiments were also performed in ten healthy subjects with several VFA-bSSFP schemes, and were compared to the conventional segmented bSSFP acquisitions.;The third objective was to apply the asynchronous VFA-bSSFP cardiac cine imaging technique for low SAR, 2D free-breathing, bellows gated acquisitions at 3T. Bloch simulations of stationary myocardium and flowing blood were performed to determine the optimal VFA scheme that maximizes the SAR reduction while maintaining blood-myocardium contrast similar to conventional bSSFP imaging. In vivo experiments were performed in ten healthy subjects in both short-axis and long-axis view and the image quality of the bellows gated cardiac cine images (FB-VFA) were compared to standard breath-hold imaging (BH-CFA) using both quantitative metrics and evaluation by two radiological experts.;The fourth objective was to develop a fast, 3D T2 weighted VFA-bSSFP technique (3D T2-TIDE) for prostate imaging at 3T. Conventional 3D fast spin echo techniques take longer acquisition duration for T2 weighted imaging due to the long recovery time between multi-shot acquisitions. This was overcome in 3D T2-TIDE imaging by: 1) designing a VFA scheme for lowering the SAR and hence improving the signal for imaging at 3T; 2) using the transient bSSFP signal for T 2 weighted imaging; and 3) modifying the acquisition of the 3D k y-kz plane using spiral-out phase encode ordering for efficient and faster filling of the 3D k-space.;The fifth objective was to develop a VFA scheme to maintain constant transverse magnetization of bSSFP without RF phase alternation (VUSEnoalt). This novel VUSEnoalt sequence has high SNR transient signal but is very off-resonance sensitive. This property of VUSEnoalt sequence is used to differentiate the arterial and the venous blood vessels due to their inherent off-resonance differences and was evaluated for non-contrast enhanced MR arteriography of the lower leg. The VFA scheme of VUSEnoalt was determined using Bloch simulations and their off-resonance effects were evaluated. In vivo imaging experiments were performed in three healthy subjects and the SNR of arterial and venous blood was compared between VUSEnoalt, VUSE and conventional bSSFP. VUSEnoalt reduces the venous blood signal by a factor of ~3 compared to the arterial blood with reduced fat signal. (Abstract shortened by UMI.).