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Variable Flip Angle Balanced Steady State Free Precession MRI

  • Author(s): Srinivasan, Subashini;
  • Advisor(s): Ennis, Daniel B;
  • et al.

Cardiac cine imaging for conscious pediatric patients and patients with respiratory and/or cardiac dysfunction is challenging due to the need for repeated, reproducible, and sometimes prolonged breath holds. Hence a free-breathing cardiac cine exam is essential for improved patient comfort. Cardiac cine exams are routinely performed using balanced steady-state free precession (bSSFP) imaging due to its high signal-to-noise ratio (SNR) efficiency. However, bSSFP imaging requires the use of a high flip angle for increased signal and a short repetition time to reduce image artifacts. These two requirements result in increased heat energy deposition, measured in specific absorption rate (SAR), in units of Watts per unit mass of the patient's weight (W/kg). FDA regulates SAR in order to ensure patient safety during MRI exams. As such the increased SAR that accords with using bSSFP can limit its application at higher field strengths (≥3T).

High SAR also poses the highest risk for RF induced lead-tip heating and potential tissue damage and may inhibit pacing and/or ICD shocks in patients with implanted pacemakers or ICDs. Hence, performing cardiac cine imaging with lower SAR bSSFP is essential for safe imaging of patients with devices as well as to minimize patient discomfort.

3D T2 weighted spin echo imaging is another SAR intensive pulse sequence due to the use of large flip angles. Extending the acquisition duration can reduce the sequence's SAR. As a consequence, 3D T2 weighted prostate imaging takes ~7 minutes to acquire, which leads to patient discomfort as well as increased sensitivity to motion artifacts.

In this dissertation, variable flip angle (VFA) bSSFP techniques have been developed to lower the 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. However, if through-plane flow is limited, as may occur for patients with low ejection fraction or low heart rates or four-chamber and three-chamber imaging planes, then the FA should be limited to ~75°.

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 novel asynchronous VFA-bSSFP technique can lower the SAR by at least 36% with similar blood-myocardium CNR compared to the conventional bSSFP cardiac cine imaging. Asynchronous VFA-bSSFP can also be used to increase the CNR by at least 28%, with similar SAR compared to conventional bSSFP.

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 SAR was reduced by 25% in FB-VFA compared to BH-CFA with similar blood-myocardium contrast and with image quality sufficient to perform global and regional cardiac function analysis.

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 T2 weighted imaging; and 3) modifying the acquisition of the 3D ky-kz plane using spiral-out phase encode ordering for efficient and faster filling of the 3D k-space. Bloch simulations were performed to evaluate the T2 weighting, contrast between normal prostate tissue and tumor, and the point-spread function of the 3D T2-TIDE sequence. In vivo experiments were performed in ten healthy subjects. 3D T2-TIDE images were acquired in 2:54 minutes compared to 7:02 minutes for 3D FSE with identical imaging parameters with SNR efficiency that exceeds 3D FSE.

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.

Overall, VFA bSSFP techniques can be used to lower the SAR or improve the signal for 2D free-breathing cardiac cine imaging, 3D T2 weighted prostate imaging, and non-contrast MR arteriography of the lower leg.

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