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Excerpt
Fortunately, MR can now produce these contrast arteriograms without requiring arterial catheterization or the risks of iodinated contrast.8,9 This is done by using gadolinium contrast, injected intravenously and timed so as to image the bolus during the arterial phase. The T1-shortening effect of the paramagnetic gadolinium contrast agent enormously increases the image signal-to-noise ratio. By imaging in the plane of the vessels of interest, a relatively small number of slices can cover an extensive region of vascular anatomy, typically in the coronal plane. At one time, it seemed as though the maximal field-of-view of the magnet limited anatomic coverage. Now we know that by moving the table during the examination, a contrast bolus can be chased down the legs (Meaney JFM 1996, Wang Y 1997, personal communication).10 With this bolus-chase technique, even the imager field-of-view no longer limits the extent of arterial anatomy that can then be imaged with a single contrast injection.
Safety is the most important advantage of contrast MRA. Gadolinium is free of clinically detectable nephrotoxicity, even at the higher doses required for MRA.12 Gadolinium also has a low incidence of allergic reactions.13 Serious reactions are exceedingly rare. Contrast MRA also eliminates the risks of ionizing radiation. Repeating an examination multiple times does not accumulate radiation exposure. Indeed, high temporal resolution MR scanning is done without concern for the number of exposures or "fluoroscopy time." By imaging pre-contrast to acquire a mask, it is possible to perform digital subtraction angiography.14 High-speed, high-resolution MRA that exceeds the performance of what we might ordinarily expect from a given magnet gradient performance, is possible by using clever k-space mapping tricks.15,16 This Fourier nature of MR data collection allows acquisition of arterial phase images, even if contrast is within the arteries only during collection of central k-space data. Similarly, most of the benefits of breath-holding can be achieved with a short breath-hold during just the central k-space acquisition, thereby obviating the need for a patient to suspend breathing for an entire acquisition.17
Although gadolinium is expensive, MRA with gadolinium is considerably cheaper than the arteriograms it replaces. As MR scanners and gadolinium contrast prices continue to plummet, contrast MRA will become increasingly affordable. As ongoing research establishes the accuracy of MRA in the evaluation of vascular pathology throughout the body, it will likely become the screening examination of choice for many cardiovascular diseases.
