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descending aorta

descending aorta

descending aorta

test bolus

test bolus

Abbreviations: MDCT = multidetector CT; FOV = field of view; KV = kilovolt; MA = milliampere x seconds; ROI = region of interest: if automated bolus detection is used, the scan is automatically started when contrast appears within the region of interest.

Abbreviations: MDCT = multidetector CT; FOV = field of view; KV = kilovolt; MA = milliampere x seconds; ROI = region of interest: if automated bolus detection is used, the scan is automatically started when contrast appears within the region of interest.

dominal aorta and branch vessels into which the dissection may extend or occlude. Even with fast helical CT scanning that reduces scan times considerably, thereby reducing patient motion generated artifacts, there are areas of the thoracic aorta that may be compromised due to cardiovascular motion. In particular, there is considerable motion at the aortic root and ascending aorta. For the thoracic aorta, ECG gating may be used to obtain motion-free imaging of the aortic root and thoracic aorta, improving evaluation of the dissection flap due to reduction of motion artifact and also allowing visualization of the coronary arteries2-4. In addition to providing higher-quality axial images, ECG gating improves the quality of multiplanar and three-dimensional reconstructed images.

Iodinated contrast material is administered with a power injector through a 20 gauge or larger venous cannula, usually located in an antecubital vein. The time delay between injection of contrast material and start of the scan is optimized to obtain maximum concentration of contrast within the aorta. This is achieved either with a timing bolus or with automated bolus detection; automated bolus detection is used for nongated studies. A timing bolus is used for all gated studies, with 15-20 ml of contrast material injected at a rate of 4-5 ml/s, followed by 20 ml saline, with serial imaging through the aortic root to determine peak arterial enhancement.

For the contrast enhanced series, 80-120 ml of nonionic contrast material is injected at a rate of 4-6 ml/s, followed by 50 ml saline. The amount of contrast material depends on scanner generation and may be reduced with higher scanner speed. Injection into the right arm is preferred to avoid streak artifact caused by concentrated contrast material in the innominate vein, which may obscure the proximal aspects of the great vessels. Specific parameters for the contrast enhanced series using single slice or multidetector helical CT are listed in Table 5.1. In selected patients, a delayed scan of the aorta is performed (see below).

Although assessment of cardiac pathology may not be needed in every patient, it might be useful in selected patients, particularly patients with type A dissection or chest pain. To facilitate evaluation of aortic valve, coronary arteries, and cardiac morphology and function, image data obtained with retrospective cardiac gating is postprocessed to reconstruct an additional series with temporal resolution at 5% increments of the cardiac cycle. Coverage for this series of image reconstructions is from the aortic root to the apex of the heart, at a slice thickness of 1.25 mm or less. Note that this is not an additional image acquisition that would require additional radiation exposure; this is a reconstruction of the dataset already acquired with intravenous contrast to evaluate the thoracic aorta.

Images are reviewed on a computer workstation, which allows generation of multiplanar reformatted images (MPR). This is necessary due to the large

Figure 5.1. Intravenous, contrast-enhanced CT examination of a normal aorta with a 16-row multidetector CT; no ECG gating was used: (a) axial image of the aortic arch (black arrow); (b) axial image of ascending (white arrow) and descending aorta (black arrow) at the level of the right pulmonary artery (black arrowhead); (c) abdominal aorta at the level of the renal artery origins (right renal artery indicated by white and left renal artery indicated by black arrow); (d-f) multiplanar reformatted images of ascending aorta and aortic arch (white arrow in d), descending aorta (black arrow in e), and abdominal aorta with renal artery origins (arrows in f indicate renal arteries) (note stair step artifact through the aortic root on this non-ECG gated examination).

Figure 5.1. Intravenous, contrast-enhanced CT examination of a normal aorta with a 16-row multidetector CT; no ECG gating was used: (a) axial image of the aortic arch (black arrow); (b) axial image of ascending (white arrow) and descending aorta (black arrow) at the level of the right pulmonary artery (black arrowhead); (c) abdominal aorta at the level of the renal artery origins (right renal artery indicated by white and left renal artery indicated by black arrow); (d-f) multiplanar reformatted images of ascending aorta and aortic arch (white arrow in d), descending aorta (black arrow in e), and abdominal aorta with renal artery origins (arrows in f indicate renal arteries) (note stair step artifact through the aortic root on this non-ECG gated examination).

amount of thin-section images generated. The MPR images are sometimes helpful to better delineate the extent of the dissection flap and are either better than or as helpful as 3D reformatted images5. Figure 5.1 illustrates an intravenous contrast-enhanced CT examination of a normal aorta obtained on a 16 row multidetector CT scanner, without ECG gating.

Figure 5.1. (Continued)

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