Novel gradient designs for robust MR flow imaging and low-power selective excitation [electronic resource]
Keywords: Electrical Engineering

Real-time magnetic resonance imaging (MRI) provides superior imaging environments for diagnosing and assessing heart disease due to its imaging speed and interactivity. This dissertation presents novel gradient design methods for robust MR flow imaging and low-power selective excitation where the interactivity permits the use of more efficient gradient waveforms. The peak velocity of stenotic blood flow can be measured more reliably by adopting more robust readout k-space trajectories against the artifacts from non-constant velocity and off-resonance. Peak RF power of selective excitation can be reduced by controlling the traversing speed in excitation k space. The peak velocity of transvalvular blood flow is a widely used metric in evaluating the severity of valvular stenosis. Doppler ultrasound (US) has been routinely used clinically due to the ability to provide velocity spectrum in real time. MR Doppler is the MR equivalent of Doppler US. It has been developed to provide a real-time velocity distribution of valvular blood flow. When compared to US, MRI has the advantage of providing unrestricted access to structures throughout the chest where US performance is degraded by poor acoustic window conditions caused by air or bone in the pathway. Moreover, MRI can assess various types of heart disease such as coronary disease, heart muscle abnormalities, tumors, and valve disease. In the context of comprehensive cardiac MRI, MR Doppler has been developed to provide a real-time velocity distribution of valvular blood flow. To better assess stenotic flow, both spatial excitation and data acquisition methods of the MR Doppler are improved through efficient k-space schemes. In particular, peak velocity detection capability can be improved by adopting an echo-shifted interleaved readout with a variable-density and circular k-space trajectory. The artifacts from non-constant velocity and off-resonance are reduced by the shorter echo and readout times of the echo-shifted interleaved acquisitions and temporal and spatial resolutions are improved through the variable-density and circular k-space sampling approach. A novel multipoint-traversing algorithm is introduced to achieve flexible gradient-waveform design. MR Doppler uses 2-D RF pulses. These are limited by the gradient and RF systems. A pulse needs to be either reshaped or redesigned when the peak RF power exceeds hardware or safety limits. Such RF power adjustment needs to be done online when RF waveforms are designed to reflect subject dependent main and RF field inhomogeneities as in parallel transmit. Variable-rate selective excitation (VERSE) technique can be used to limit the peak RF power without disrupting the on-resonance profile while minimizing the amount of reshaping via a local-only RF and gradient scaling. A simple and robust VERSE-guided RF pulse design framework is developed as an online RF reshaping tool in controlling peak RF power. VERSE principle is first generalized to a broader domain of multidimensional and multichannel excitation, where the conditions of identical spin rotation are formulated in excitation k space. Then, a noniterative time-optimal design method for VERSE is developed by translating peak RF limits into a gradient upper bounds in s domain, where the gradient upper bounds are used for a time-optimal gradient waveform designs. Implementation considerations are discussed to improve the fidelity of VERSE operation and an iterative approach is introduced to resolve the potential deviation of peak RF magnitude from the target value.