Deep Vein Thrombosis is a common complication in bed-ridden patients, described as the main cause of preventable hospital deaths in the UK (NICE 2010). Mechanical prophylaxis aims to promote venous flow, either statically with compression stockings, or dynamically with intermittent pneumatic compression or electrical muscle stimulation. Previous studies used ultrasound for venous flow measurements, limited to a single deep vein at a time, and some anatomical MRI for investigating the mechanisms behind these prophylaxes.
MRI velocity mapping is used clinically in the arterial system where gating enables data accumulation over multiple cardiac cycles. This thesis describes the development of two real-time MRI spiral velocity mapping sequences for imaging venous blood flow in the leg, where venous flow variability is largely unrelated to the cardiac cycle. Real-time imaging with spiral gradient readouts minimised image duration. A phase-image fitting technique requiring only a velocity-encoded phase image was implemented for acceleration.
For in vivo comparison, conventional flow imaging required metronome-guided breathing for a regular venous flow waveform. The long spiral readouts were sensitive to off-resonance and flow artefacts, where some unpublished effects were investigated. The off-resonance associated with deoxygenation of venous blood did not cause notable spiral artefacts, but disrupted the phase-image fitting technique and required correction with a pre-scan.
The spiral flow methods demonstrated increased venous blood velocity and flow during application of mechanical compression. Metronome-guided breathing was also applied to vein wall imaging, where it detected wall thickening in patients with Behçet’s disease compared with normal subjects.
For the first time, this thesis evaluated real-time MRI spiral velocity mapping of venous blood velocity and flow. The high resolution (1mm) and short image time required caused challenging off-resonance and flow artefacts. With some limitations, real-time spiral flow MRI during operation of compression devices may assist in their optimisation.