The basic principle behind CE MRA is that imaging is performed during the arterial first pass of a paramagnetic contrast agent in the vessel of interest after intravenous injection . The delay between arterial and venous enhancement provides a time-window for preferential arterial imaging. This arterial-venous time-window depends on the rate of contrast agent injection, but also on the anatomical region being imaged. In the upper extremity, venous return is much faster than in the lower limbs, thereby conferring a markedly reduced period of time for preferential arterial enhancement (short arterial-venous time window). Since the technique depends on imaging of the first pass of the contrast agent, accurate timing of the start of image acquisition after contrast agent injection is essential.Various techniques have been developed to calculate this so-called scan-delay (see below).
The quality of the MRA images with regard to selective arterial visualization, resolution and volume of interest depends both on the sequence parameters used and on the geometry of the contrast agent bolus. Likewise, arterial enhancement depends not only on individual physiologic parameters such as cardiac output and blood volume, but also on contrast agent application parameters such as flow rate, dose and volume of saline flush, all of which can be manipulated. The T1-shortening of blood depends on the intravascular concentration of the contrast agent, which in turn depends on the rate of injection. In general, the faster the injection rate, the higher the arterial concentration of gadolinium contrast agent. However, injection rates exceeding 5 mL/sec do not result in any further increase in signal intensity. In fact, intravascu-lar signal may become lower at rates higher than 6 mL/sec .
As with other CE MRA applications, one must always attain a balance between imaging time, resolution and arterial-venous time-window. The scan duration for imaging of the relatively large subclavian and brachial arteries can be relatively short since the spatial resolution does not have to be particularly high and the scan volume can be reasonably small. Therefore, in the upper arms, a flow rate of 2-3 mL/sec is usually adequate.
A heavily T1 weighted spoiled gradient echo sequence (short TR, short TE, flip angle 25-50°) is usually sufficient. The achievable field-of-view, matrix size, scan volume and number of partitions are determined by the arterial/venous time-interval. Subtraction of pre- and postcontrast images is performed to permit selective visualization of just the arteries. This facilitates a greater ease of image interpretation and reduces image artifacts . The use of parallel imaging techniques (e.g. SMASH [simultaneous acquisition of spatial harmonics] or SENSE [sensitivity encoding]) permits a significant reduction in overall examination time, which can be used to improve spatial resolution.
The volumetric data set obtained (containing high signal intensity voxels corresponding to arteries) can be post processed using maximum intensity projection (MIP) or volume rendering algorithms. In many cases improved diagnostic performance can be achieved by reviewing the individual source images or by reformatting images in the transverse plane.
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