Over the subsequent years TOF and PC-MRA techniques were refined but essentially remained unchanged. Since typical MRA cquisitions originally took several minutes to acquire, they found their primary application in the cranial and extracranial carotid circulation. Whilst thoracic, abdominal and peripheral MRA were attempted, image quality was not sufficient for routine clinical use, primarily due to artefact caused by cardiac and/or respiratory motion or pulsatile flow. It was not until Prince et al. introduced the concept of (exogenous) contrast-enhanced MRA (CE-MRA) in 1993 that it became possible to robustly image the systemic vasculature. CE-MRA involves rapidly imaging the desired vascular territory during the first pass of a gadolinium-based MR contrast agent, which has the effect of transiently decreasing the T1 of blood, thereby increasing its signal intensity. This transient increase in signal is achieved by administering the contrast agent as a compact bolus using a power injector. Careful determination of the circulation time from the point of injection to the time the contrast agent arrives at the anatomy of interest is crucial to obtaining high-quality images. Subsequent improvements in MR gradient performance as well as receiver coil technology, e.g. multi-element coil arrays, have further reduced CE-MRA acquisition times, to the point of allowing time-resolved 3D angiograms to be acquired with high temporal and spatial resolution, making the need for determination of the circulation time less necessary. The development of moving table technology allows extended FOV coverage by moving the patient during image acquisition. Such multi-station images allow the operator to chase the contrast agent bolus down the legs, producing high-quality peripheral angiograms.
CE-MRA has now become the standard method for thoracic, abdominal and peripheral angiography, as well as carotid MRA in some centres. However, in 2008 concerns concerning gadolinium-based contrast agents and nephrogenic systemic fibrosis (NSF) started to arise and have caused renewed interest in the development of so-called non-contrast-enhanced MRA (NCE-MRA) techniques. NCE-MRA has developed into a catch-all term that covers all MRA techniques without exogenous contrast agents, including both PC and TOF. However, the term has primarily been used to characterise a new class of methods that attempts to leverage more recent pulse sequence developments in order to address the deficiencies of TOF and PC in imaging the systemic vasculature.
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