Patient Preparation

As with any MR scan, proper patient preparation will minimize exam duration and improve clinical

Care Bolus Mra

Fig. 3. Vascular Image Contrast on CE MRA. On CE MRA, the repetitive application of radiofrequency pulses generates the necessary image contrast to distinguish Gd-enhanced blood from unenhanced structures. Proper timing of imaging to occur during the phase of preferential arterial enhancement will thus result in the selective visualization of Gd-enhanced arteries over adjacent un-enhanced veins and background tissue (e.g. muscle)

Fig. 2. Contrast bolus timing for 3D CE MRA. At the top of this figure, representative contrast enhancement curves for arteries and veins are shown. Arteries are best visualized if the critical center of k-space data is acquired during the peak and preferential arterial enhancement period. This will minimize possible venous contamination of the data set that may hamper proper separation of arterial structures. There are several methods for acquiring central k-space data. With the traditional sequential or linear method, the center of k-space is sampled during the middle of the acquisition ("A"). With centric phase ordered k-space acquisition "schemes" ("B"), which include elliptical centric and reverse sequential partial Fourier acquisitions (see Fig. 8), the center of k-space is sampled earlier during the beginning of the imaging period. Thus, proper timing of CE MRA depends not only on the arrival time of contrast in the target vessels but also on knowledge of the type of k-space sampling that is being used for the 3D imaging sequence (Reprinted and adapted with permission from [18])

Fig. 3. Vascular Image Contrast on CE MRA. On CE MRA, the repetitive application of radiofrequency pulses generates the necessary image contrast to distinguish Gd-enhanced blood from unenhanced structures. Proper timing of imaging to occur during the phase of preferential arterial enhancement will thus result in the selective visualization of Gd-enhanced arteries over adjacent un-enhanced veins and background tissue (e.g. muscle)

efficiency. In addition to screening for the usual contraindications for MR scanning (e.g. pacemakers) and for the use of Gd-chelate contrast agents (e.g. pregnancy), patients scheduled for a CE MRA examination should also be asked about underlying pulmonary disease and their ability to hold their breath. Intra-abdominal and thoracic CE MRA image quality is markedly improved when performed during a breath hold. Even patients with diminished breath-hold capacity, however, can typically hold their breath for 20-25 seconds if proper coaching is performed in advance and breath holding is optimized by the use of supplemental oxygen and hyperventilation [41]. For multi-station CE MRA examinations (i.e. bolus chase CE MRA) it is also important to know if the patient has any underlying condition that may prevent them from staying still for even short periods of time (e.g. history of Huntington disease or severe back pain), as image subtraction is usually necessary for these exams [42]. Patient motion between pre- and post-contrast data sets can result in spatial mis-registration leading to degraded image subtraction. Patients should also be asked about prior interventions, especially vascular or en-dovascular procedures. Knowledge of extra-anatomic bypass grafts or stent grafts will ensure proper scan prescription and planning.

During the physical examination, patients should also be assessed for venous access. Ideally, for CE MRA, the intravenous catheter is placed in the antecubital fossa and should be sufficiently large (i.e. at least 22 gauge) to support a bolus rate of at least 2 mL/sec. When imaging the aortic arch and great vessel origins, it is preferable to place the intravenous catheter in the right arm, as left sided venous contrast administration can cause T2* artifacts due to the high concentration of Gd within the left bracheocephalic vein en route to the right heart. This can often be mistaken for a proximal great vessel stenosis (Fig. 4) [43]. For multi-station bolus chase exams, care must be taken to ensure that the intravenous catheter is stabilized and that the tubing is sufficiently long to allow free movement of the patient and table during the bolus chase table translation. It is frequently advisable to firmly tape the tubing to the patient and to cover potential areas that may snare the tubing. Snaring of the tubing during table translation may not only pull the intravenous catheter out, but also stop table translation as some scanners will automatically stop if any resistance or hindrance to table translation is detected.

For most CE MRA examinations, patients are positioned feet first in the supine position in the bore of the magnet. The one exception is neu-rovascular imaging which requires the use of a head or neurovascular head and neck coil and thus requires patients to be placed head first into the scanner.Vascular signal-to-noise (SNR) can be improved by the use of phased array coils and, additionally, by the proper centering of the coil elements about the region(s) of interest. For this reason it is well worth the time to ensure proper coil

Mra Upper ExtremityMra Upper Extremity

Fig. 4a, b. T2* artifact from left antecubital injection. a On the initial arterial-phase, 3D CE MRA of the aortic arch vessels (coronal MIP), there is suggestion of two stenoses (arrows) of the left subclavian artery. bOn delayed-phase images (sub-volume MIP), however, both regions are noted to have a normal caliber (proximal left subclavian artery not shown). The apparent narrowing of the arch vessels on arterial-phase images results from T2* susceptibility artifact resulting from the high concentration of Gd within adjacent veins and is more commonly seen following a left upper extremity injection. As shown in this case in which a left antecubital vein was used for contrast administration, this T2* artifact can occur in any arterial segment close to the left subclavian vein and left bracheocephalic vein. As a practice, dual phase acquisitions will greatly assist in the recognition of this artifact, as delayed phase images will invariably demonstrate normal arterial caliber (Reprinted and adapted with permission from [18])

positioning. A growing number of third party vendors now offer dedicated coils for vascular imaging, primarily for peripheral and neurovascular imaging. With the advent of new scanners with increased numbers of reception channels, coverage of the entire body of the patient with multiple coils is now possible allowing whole body imaging without moving the patient. The use of phased array coils provides the additional benefit of markedly shortening image acquisition times or, with the use of parallel imaging schemes (see below), of acquiring higher spatial resolution image sets in the same time period.

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