Rapid phase-contrast magnetic resonance imaging using spiral trajectories and parallel imaging

Phase contrast (PC) MRI is a proven method of measuring blood flow in the clinical environment. Traditionally, PCMR data is acquired using cardiac gated Cartesian sequences. However, these sequences are time consuming and difficult to perform in patients with irregular heart rates. The work of my thesis covers three alternative PC sequences, all using undersampled spiral sequences with SENSE reconstruction algorithms. The first piece of work investigates real-time spiral PCMR. The spiral flow sequence was validated at rest by comparing stroke volumes in the aorta of healthy volunteers, against a retrospectively gated Cartesian sequence. By combining flow data with simultaneous blood pressure measurements during exercise, this sequence was used to quantify the hemodynamic response to physical stress. The second piece of work investigates improvements in spatial or temporal resolution for real-time PCMR, by splitting the acquisition of flow-compensated and flow-encoded data into separate short blocks. The data is then retrospectively matched in cardio-respiratory space, to remove background phase offsets. This sequence was validated (at rest) in an adult population. The improved spatial resolution was shown to provide more accurate flow measurements than standard real-time flow measurements, in a paediatric population. The third piece of work investigates prospectively triggered spiral PCMR to achieve high spatio-temporal resolution, within a short breath-hold. Flow volumes, regurgitation fraction and shunt ratios were compared from a high spatial-resolution, free breathing retrospectively gated Cartesian sequence with 3 averages (~2.5 minute scan time), a low spatial-resolution breath-hold retrospectively gated Cartesian sequence (~20 second scan time), and the (high spatial-resolution) prospectively triggered spiral sequence (~6 second scan time). It was shown that accurate flow measurements can be made in the aorta, pulmonary artery and pulmonary branches, within manageable breath-hold times for children and sick adults. This sequence may improve patient compliance and increase patient throughput