Medicine and Radiology - Theses
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ItemClinical application of non-contrast MRA techniques in the era of nephrogenic systemic fibrosis and gadolinium deposition( 2019)Magnetic resonance angiography (MRA) is clinically used for both arterial and venous assessment. It is an attractive alternative to CTA in younger patients, where risks of ionising radiation are greatest. Beyond neurovascular imaging, contrast enhanced MRA (CE-MRA) is most commonly performed, following injection of a gadolinium chelate. Non-contrast MRA (NC-MRA) refers to a range of techniques performed without exogenous contrast. NC-MRA removes risks of injected contrast and need for renal function testing and intravenous cannulation. Time of flight MRA (TOF) is a well-established technique that remains the mainstay of cerebrovascular MRA. More NC-MRA techniques have been developed recently in reaction to the association of gadolinium chelates with Nephrogenic Systemic Fibrosis (NSF), and gadolinium retention in previously injected subjects. This dissertation explores the clinical applicability of non-contrast magnetic resonance angiography (MRA) techniques. Vascular pathology and currently available angiographic imaging modalities are reviewed. Subsequently, NC-MRA is reviewed in detail, including potential benefits, challenges, mechanisms used to obtain vascular signal and applications. Data from five prior prospective trials by the investigator are then presented. The first study reports on a breath hold flow-independent NC-MRA balanced steady state free precession (bSSFP) technique, describing its performance in 30 patients presenting for assessment of the thoracic aorta, with comparison to contrast-enhanced MRA. Diagnostic image quality was found for all aortic segments, and comparable aortic dimensions to CE-MRA, with a statistically significant but clinically insignificant (<1mm) difference in measured dimension for the ascending aortic segment. The second study evaluates the performance of a Non-contrast Hybrid Arterial Spin Labelling NC-MRA technique (NoHASL) for imaging the circle of Willis in 30 patients presenting with known or suspected cerebrovascular ischaemia. Whilst shown to be feasible, with diagnostic quality imaging obtained, the NoHASL technique was found to have inferior image quality compared to both TOF and CE-MRA. Low prevalence of haemodynamically significant disease limited assessment of technique accuracy. The third study reports the accuracy of three electrocardiographically (ECG)-gated subtractive non-contrast MRA techniques, flow-sensitising dephasing gradient prepared MRA (FSD-MRA), variable flip angle fast spin echo MRA (VFA-MRA) and constant flip angle fast spin echo MRA (CFA-MRA), in the below knee arteries in 21 patients, compared to CE-MRA. FSD-MRA demonstrated highest sensitivity and specificity of 80.3% and 81.7% for haemodynamically significant stenosis respectively, with no significant difference in accuracy identified between the techniques. The fourth study reports feasibility of VFA-MRA for imaging the hand arteries in 9 healthy volunteers and 4 scleroderma patients. The technique showed clear separation of arteries from veins and depiction of stenoses. Vascular reactivity following subject warming was also assessed, highlighting an advantage of NC-MRA, repeatability. The final study applies the CFA-MRA technique to assessment of calibre of the upper extremity veins of healthy volunteers, as preliminary work to evaluate potential feasibility of subsequently applying the technique to patients being considered for haemodialysis. Similar venous calibre was obtained in comparison to CE MRA, however, MRA overestimated calibre in comparison to ultrasound. Finally, a summary of the current state of NC-MRA is presented and potential future advances discussed.