Technique

A standard body coil generally provides sufficient signal for imaging of the thoracic aorta. However, a phased array torso-coil may significantly increase the signal-to-noise ratio (SNR) resulting in superior image quality and better depiction of smaller vessels. If a phased-array coil is used and the supra-aortic vessels are of clinical interest, then the neck should be included in the evaluation as well. In such cases it is important to fix the phased array-coil adequately in order to prevent movement and malpositioning of the coil in response to repetitive breath-holding of the patient.

Since the anatomy of the thoracic aorta can be variable and complex due to the underlying disease, extensive localizer imaging should be performed prior to positioning the slab for the final contrast enhanced MRA scan. In some cases, for example in patients with extensive kinking of the descending aorta, this may even necessitate positioning the slab sagittal rather than coronal in order to include the whole aorta in the slab.

In patients with sudden onset of thoracic pain for whom ischemic heart disease or acute heart in-farct can be excluded, additional dark blood cross-sectional imaging prior to the contrast enhanced study should be performed to detect possible intramural hematoma of the aorta. This condition may sometimes be missed on contrast enhanced T1w images since the hematoma may have a similar high signal to the contrast enhanced lumen [2].

In general breath-holding significantly increases the quality of images. However, if only the descending aorta is of interest even non-breath-hold CE MRA may provide good image quality.

As with all contrast enhanced MRA examinations, it is essential to accurately time the contrast bolus either with an automatic system, interactively by applying MR fluoroscopy or by means of a test bolus. However, automatic triggering systems may not always give optimal results for imaging of the thoracic aorta since the need to perform breath-hold after the arrival of the bolus may result not only in the thoracic aorta being missed, but also in an overlay of venous structures due to early filling of the jugular veins.

At our institution we typically use a test bolus for examinations of the thoracic aorta. The slice level for the test bolus examination is positioned so as to permit visualization of sections of both the ascending aorta and the descending aorta. In this way information is obtained about the delay of filling of the descending versus the ascending aorta. This may be particularly important in cases of coarctation of the aorta or in large aneurysms which may take some time to fill completely. Another clinical scenario in which it may be important to acquire additional information on the dynamics of enhancement is in cases of aortic dissection.

On new state-of-the-art equipment it is now possible to perform two separate MRA studies, the first with a slightly decreased spatial but highly increased temporal resolution and the second with a greatly increased spatial resolution. The first study is aimed at depicting the aorta and obtaining information on dynamics of enhancement and bolus timing, while the second is performed for diagnostic purposes.

As mentioned above, the 3D MRA volume acquisition may be performed in the coronal, sagittal or in an oblique orientation, depending on the anatomy of the patient and his ability to position the arms above the head.

In patients with an enlarged sagittal extension of the aorta, coronal plane imaging requires more sections and thus may be too long to complete in a single breath-hold. In these cases a sagittal acquisition should be performed in order to evaluate the entire thoracic aorta with fewer slices. However, the final choice of whether to perform imaging in the sagittal, coronal or oblique plane depends on the aortic anatomy as displayed on the localizer images.

To ensure complete coverage of the thoracic anatomy an unenhanced scan should be performed first. This scan should later be subtracted from the contrast enhanced scan but should be reviewed prior to the enhanced study in order to establish that all important structures are included and to ensure that no aliasing artifacts are present.

In general, more than one dataset should be acquired after contrast agent injection since a delay of bolus arrival may occur in spite of optimally planned bolus timing. Moreover, additional acquisitions may reveal important additional information concerning venous structures. Typically, the first scan should be timed for optimal enhancement of the thoracic aorta. Thereafter, a deep breath by the patient should be followed by a second breath-hold and the acquisition of a second scan immediately afterwards.

As in imaging of the supra-aortic vessels, injections should be performed into the right arm in order to avoid the T2* effects of residual highly concentrated contrast agent in the venous structures. Injections into the left arm are known to frequently result in artificial depiction of a stenosis of the supra-aortic vessels.

The injection rate should be approximately 3-4 ml/sec if a dose of 0.2 mmol/kg of an extracellular contrast agent is applied. If a weakly protein interacting agent with higher relaxivity (e.g. Gd-BOP-TA) is used, a dose of 0.1 mmol/kg is usually sufficient at an injection rate of 2-2.5 ml/sec.

If additional information is required beyond the luminal images derived from CE MRA (e.g. information on anatomic structures), complimentary sequences should be performed, which in the thorax often necessitates ECG- or pulse-gating. Structures outside of the aortic lumen which frequently require further characterization include the vessel wall and the perivascular tissue. The latter may be of particular interest during evaluations of inflammatory changes surrounding the aorta, such as in aortitis [6]. As mentioned above, the same holds true for imaging of intramural hemorrhage which is increasingly of interest due to its recognition as an early form of aortic dissection [7].

Many of the above questions can be answered adequately with conventional ECG-gated unen-hanced T1- and T2-weighted SE, TSE or GRE imaging, performed before and in cases of inflammatory changes, immediately after the acquisition of the contrast enhanced MRA. However, additional functional information is often needed, for example, in cases of dilatation of the ascending aorta. In this scenario it is necessary to ascertain whether the dilatation is due to a congenital variation such as a bicuspid aortic valve, or to an acquired disease such as aortic stenosis, aortic insufficiency or a combination of both. In these situations CINE-type imaging can give very good results. The application of CINE phase-contrast imaging additionally permits the absolute quantification of blood flow. This may be applicable for the quantification of regurgitation fraction in aortic valve insufficiency and for the quantification of pressure gradients based on velocity measurements in aortic coarctation [3].

Imaging planes are either perpendicular or along the long axis of the aorta depending on the desired information. Other interesting applications of CINE imaging would be in patients with aortic dissection, in whom diagnosis is unclear based on CE MRA findings due to the tremendous movement of the dissection membrane during the heart cycle and a resulting misregistration of the membrane on CE MRA images. For example, the false lumen may be depicted only during late diastole in some patients and may thus be missed on non ECG-gated contrast enhanced MRA [7].

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