Perfusion Measurement with Low Field NMR

Blood perfusion is a fundamental parameter of tissue health and function. Measurements of blood perfusion are clinically used to assess the severity of strokes, tumors and other diseases. Nuclear Magnetic Resonance (NMR) is a common modality for these measurements, due to its flexibility, sensitivity and lack of ionising radiation.

This thesis focuses on exploring the use of low-field NMR systems to measure perfusion. Previous literature on NMR-based perfusion measurement focuses exclusively on high-field clinical imaging systems — the potential benefits offered by low field systems have thus far been ignored. Both contrast agent based and contrast-free methods were implemented, refined and evaluated in low field conditions.

A novel 9 MHz permanent magnet NMR system was the core instrument of this project. This system possessed a highly inhomogeneous B0 field, with a relatively homogeneous sweet spot at the centre of an open cavity — no imaging capability was available. The prototype system was designed to enable low cost, portable NMR-based assessment of brain health, such as the diagnosis and active monitoring of strokes. Validation of contrast-based perfusion measurements was achieved using a custom-built two compartment DCE flow phantom. T1 and T2 weighted methods of analysis were trialed; T1-weighting performed better, and realistic concentration-time curves were recorded in biologically-adjacent relaxivity conditions.

A second, simpler flow circuit was used to develop and validate a contrast-free measurement technique — tentatively named IFAIR. This new technique, adapted from the FAIR ASL protocol, was designed to be suitable for inhomogeneous field conditions and to function without active gradient fields. The IFAIR signal response displayed a consistent proportional relationship to flow velocity, with potential to allow for quantitative flow-dependent measurements.

A preliminary in-vivo experiment was carried out, detecting changes in local blood perfusion related to forearm muscle activation. This experiment showed the IFAIR technique can be successfully applied to in-vivo conditions.