MSCT techniques have evolved rapidly, as scanners with additional detector rows have become available. Nonetheless, certain variables must be optimized to yield the highest image quality: collimation, pitch, field-of-view, reconstruction increment, amount, rate and timing of contrast administration, distance to be scanned, mA, and tube rotation time. Occasionally, a compromise among these parameters is necessary to optimize the scan.

MSCT of the thoracic aorta usually begins with a noncontrast series. Slice thickness of 5 mm is sufficient for the unenhanced scan, which is followed by the contrast-enhanced scan.

Optimal postcontrast imaging requires a slice thickness of 0.6 to 2.5 mm, which provides excellent spatial resolution and allows for high quality reconstructed images. A pitch of 1.375 to 2 allows a large volume of coverage to be obtained rapidly. The field of view should include at least the outer rib to outer rib at the widest portion of the thorax.

There is no consensus regarding the iodine concentration that should be used for MSCT. Undiluted (300 or 360 mg I/mL) contrast agent is usually appropriate. Approximately 80 to 140 mL of contrast agent is injected at a rate of 3 to 5 mL/s. These rates can be achieved using a 20-gauge intravenous catheter. A standard scan delay of 25 seconds after start of injection will usually suffice for MSCT. However, patients with diminished cardiac function or an unsuspected stenosis of the injected vein may require longer delays to achieve optimal opacification of the vessel. Therefore, either a test
bolus to determine contrast transit time, or bolus triggering software allow for synchronization of CT acquisition with optimal contrast opacification in nearly all patients.

The contrast-enhanced MSCT should extend from the thoracic inlet to below the aortic bifurcation in the pelvis, to assess the possibility of pathology extending into the arch vessels and iliac arteries.

The mA used is critical for obtaining high quality studies. Values of 120 to 150 mA per tube rotation will usually suffice for most patients, but heavier individuals may need an upward adjustment in the mA. Automated exposure algorithms that permit axial and longitudinal (XYZ) dose modulation exist for most scanners with nonelectrocardiographic (ECG) synchronized helical acquisitions. The use of prospective ECG gating will reduce motion artifact and should be used when available. MSCT of the aorta is best achieved with the shortest available tube rotation time, as low as 330 to 350 ms with currently available machines.

Several MRI techniques are useful for imaging the thoracic aorta. The most important sequences are ECG-gated black blood double inversion recovery and white blood steadystate free precession (SSFP) cine sequences as well as contrast-enhanced three-dimensional (3D) MR angiography (MRA). With respect to the long axis of the aorta, images can be acquired parallel (sagittal oblique or sagittal) or perpendicular (transverse). Use of orthogonal planes helps to distinguish among intraluminal, mural, and extramural pathologies and reduces partial volume averaging. ECG gating is used for black blood and SSFP cine acquisitions but not for MRA.

A typical protocol for imaging of the thoracic aorta consists of black blood, SSFP cine, and MRA sequences. Transverse images are acquired from a level several centimeters above the aortic arch to the level of the diaphragm, using a slice thickness of approximately 5 mm. Transverse breath hold SSFP cine MRI is performed in the same anatomic region. A slice thickness of 4 to 6 mm is used for the cine sequence. A black blood inversion recovery sequence is acquired in the oblique sagittal or sagittal plane (slice thickness 3 to 5 mm) to display the entire length of the aortic arch, along with the ascending aorta and proximal descending aorta, on the same image. This sequence may be useful to visualize the origins of the arch vessels, which is important when aortic dissection is suspected. The oblique sagittal imaging sequence is prescribed from the transverse image at the level of the aortic arch. Contrast-enhanced 3D MRA is performed in the sagittal or oblique sagittal plane to reduce the number of phase encodes required, resulting in shorter acquisition times and/or better spatial resolution.

For black blood imaging, double inversion recovery technique is used to suppress the signal of the intraluminal blood. The aortic lumen is therefore homogeneously black. Although this technique can be valuable, the luminal signal is not completely black when flow is inordinately slow.

SSFP sequences render the blood pool homogeneously bright. These sequences are especially useful for aortic evaluation because they demonstrate intraluminal pathology as a “filling defect” in the bright blood pool. The bright signal of the blood on SSFP images is secondary to “flow-related” enhancement. With SSFP imaging radiofrequency (RF) pulses are applied to saturate a volume of tissue. Because the TR is short, there is little time for signal recovery from stationary tissue. Therefore the maximum signal is emitted by blood flowing into the volume of tissue, because this blood contains the only protons that have not been saturated by the RF pulses. Slow blood flow, which results in decreased flow enhancement, is the primary factor that causes signal depression on these images. Signal loss also can occur as a consequence of disturbed flow caused by intravoxel phase dispersion, such as occurs with turbulence from a flow jet.

In the thorax, SSFP sequences are frequently performed as breath hold cine acquisitions. SSFP cine images are acquired at multiple contiguous slice locations. An ECG signal is acquired along with the imaging data so that images can be reconstructed at multiple phases (frames) of the cardiac cycle. Generally at least 16 to 32 frames are acquired at each anatomic level.

Three-dimensional MRA with intravenous gadolinium chelate contrast agent relies on the T1-shortening effect of gadolinium to produce high-signal intensity in blood vessels, thereby avoiding flow-related artifacts. The protocol for 3D MRA varies from institution to institution. One of the most effective prescribes MRA in the sagittal or oblique sagittal planes. ECG gating is not required to perform this sequence. Thirty to sixty images are typically obtained using a slice thickness of 1 to 2 mm.

The MRA sequence is performed during a breath hold of approximately 15 to 25 seconds. The patient can be provided with oxygen by nasal cannula, at a rate of 2 L/min, to aid in breath holding. The contrast agent is administered via an antecubital vein with use of a power injector (2 mL/s, 0.1 mmol/kg total). Timing of the acquisition can be optimized by using real-time imaging for bolus detection or by performing a preliminary acquisition employing a test bolus to estimate circulation time.

Image postprocessing of the CT and MRI data sets can include:

In the imaging evaluation of thoracic aortic aneurysm, the primary goals are measurement of the maximum diameter of the aorta, demonstration of the extent of the aneurysm, identification of branch vessel involvement, compression of adjacent structures, and detection of periaortic hematoma. The demonstration of mural thrombus is important in patients who present with peripheral embolization.

MSCT findings of aortic aneurysm rupture include high-density fluid in the wall of the aorta, pleural space, or pericardium
(Fig. 29-7). Mediastinal hematoma also suggests the possibility of aneurysm rupture. The “draped aorta” sign may indicate an early, contained rupture. This sign manifests as indistinctness of the posterior wall or close alignment of the posterior wall with the contour of the adjacent vertebral bodies.

MRI also readily depicts the size and extent of an aneurysm of the ascending or descending thoracic aorta. The outer dimension of the aneurysm and the size of the patent lumen can be seen. Because it can demonstrate the outer wall of the aorta, unlike angiography, MRI allows accurate measurement of the aneurysm diameter even when there is wall thickening or mural thrombus. Mural thrombus and atherosclerotic plaque are depicted as eccentric or concentric thickening of the aortic wall. The patent lumen can usually be determined
by the flow void on black blood MRI. However, slow flow can produce intraluminal signal on black blood images. Slow flow can be differentiated from thrombus with SSFP cine MRI or contrast-enhanced MRA. MRA is especially effective for demonstrating the extent of thoracic aortic aneurysm, the relationship to the aortic branches, and the precise dimensions—especially in the aortic arch.

Identification of periaortic or mediastinal hematoma is vital in the evaluation of aneurysm rupture. On black blood images hematoma appears as an area of high-signal intensity, although this appearance may not be seen early. It may produce intermediate signal intensity during the first few hours after bleeding. MRI can delineate the extent of the hemorrhage, which may spread through the mediastinum or be localized to the periaortic region, pleural space, subpleural space, or pericardium.

MRI is an effective noninvasive means of monitoring the progression of thoracic aortic aneurysms. If the aortic diameter is greater than 6.0 cm or if there is a rapid increase in diameter, surgical repair is generally necessary. Because it is noninvasive and accurate, MRI is also the optimal method for sequential follow-up of the aorta in patients with a high risk for developing dissection, such as those with Marfan or Ehlers-Danlos syndromes (see Fig. 29-3).

False aneurysms can be revealed by MSCT, contrast-enhanced MRA, or a combination of transverse and sagittal or sagittal oblique MR images. MRI is useful for depiction of
the longitudinal extent of the false aneurysm. Post-traumatic false aneurysms seen on MSCT and MRI are most frequently located at the site of ligamentum arteriosum and involve either the entire aortic circumference or only the anterior wall (see Fig. 29-5).

Aortic dissection is a separation of the aortic wall that results from a tear in the intima. Blood can enter the aortic wall through an intimal disruption and extend proximally and distally in the media, displacing the intima inward. The dissection may reenter the lumen at one or more sites of fenestration. Typically blood flows in both the true and false channels, although the false channel is sometimes thrombosed. The dissection may disrupt the adventitia and cause pericardial tamponade, exsanguination, pleural hemorrhage, or periaortic hematoma. If the dissection involves or occludes branches of the aorta, cerebral ischemia, myocardial infarction, renal insufficiency, or mesenteric infarction can ensue.

Aortic dissection can result from an intramural hematoma, which is caused by rupture of the vasa vasorum, the arteries that supply the aortic wall. The ruptured vasa vasorum bleed
into the aortic wall, which can result in an intimal tear and separation of the wall. If the intima does not rupture, causing a frank dissection, then an intramural hematoma remains. The intramural hematoma may be localized, or it can extend along the wall in antegrade or retrograde direction or sometimes rupture through the adventitia. An intramural hematoma can be considered to be a form of dissection, and management is similar to that of a frank dissection.