CAT SCAN

Findings: Since its introduction in the 1970s, CT has become a widely used technology, particularly in the ED. With the advent of spiral CT, the studies can be performed in less time than before with less patient discomfort, greater accuracy, and lower iodine load. Spiral CT permits patient translation and data acquisition simultaneously. A major advantage of this technology is in the evaluation of thoracic trauma, which enables the rapid diagnosis of thoracic injury. Multislice or multidetector CT can be used for faster imaging or to acquire thinner slices that can be reconstructed in multiple planes

A typical helical scanning protocol for aortic dissection includes the following parameters: 5-mm collimation, 1.5 pitch, and 7.5-mm imaging spacing. Multidetector CT can be performed with 1-2.5 mm collimation. Initial nonenhanced CT is used for the diagnosis of acute hemorrhage and aortic rupture. This is followed by helical CT performed approximately 25-30 seconds after the injection of contrast material. Nonionic contrast material (120-135 mL) is power injected via a peripheral intravenous site at a rate of 3-4 mL/s. Because cardiac output is quite variable in these sick patients, use a test injection of contrast to determine circulation time or an automated bolus detection scheme. One advantage of the test injection method is that one may visually differentiate the true and false lumen based on contrast arrival time.

Usually, scanning is performed from the thoracic inlet to the common femoral arteries. When a dissection is identified, repeat scanning can be performed to obtain delayed images of the false lumen and aortic branches. Multiplanar reformation images are obtained in sagittal, coronal, oblique sagittal, and curved projections generated with an independent workstation. The use of volume rendering can be helpful for planning surgery

Typical CT findings in acute dissection or intramural hematoma include the following:

• Aortic intramural hematoma: Crescentic high-attenuating clot within the media, with internally displaced calcification (see Image 3)

• Intimal flap separating the two aortic channels (see Images 4-5)

Hemorrhagic pleural and pericardial effusions and mediastinal hemorrhage may be seen.

CT is also helpful in postoperative follow-up. It can accurately depict associated complications, including the following:

• Thrombosis

• Hemorrhage

• Infection

• Pseudoaneurysms

• Aortoenteric fistula

• Ureteral obstruction

Degree of Confidence: The sensitivity of CT is 87-94%, and the specificity is 92-100%.

False Positives/Negatives: Inadequate contrast opacification can lead to false-negative findings. Aortic intramural hematoma can be misinterpreted as an aneurysm with thrombus or arteritis.

Spiral CT artifacts include perivenous streaks and motion artifacts. The perivenous streaks are caused by beam hardening and motion due to transmitted pulsation in a vein that carries undiluted contrast medium into the heart. Some authors recommend injecting the contrast agent at a rate of 2 mL/s via a peripheral intravenous site in the right arm. Aortic motion artifact is produced by the aortic wall motion from the end of diastole to the end of systole. Typically, this artifact is seen in the left anterior and right posterior margins of the aortic circumference.

In some patients, especially those with cystic medial necrosis, the intimal flap may be subtle.

MRI

Findings: MRI is an accurate tool for use in diagnosis but it may not be readily available in the acute setting. In addition, unstable patients with Swan-Ganz catheters should not be studied with MR.

MRI findings of aortic dissection include the following:

• MRI shows an intimal flap of medium–signal intensity surrounded by a signal void of fast-flowing blood on "black blood" echocardiogram (ECG)-gated spin-echo or double inversion recovery single shot fast spin-echo (see Image 6).

• With cine gradient echo imaging, the intimal flap is a dark line against the high -signal intensity of the flowing blood and may change configuration during the cardiac cycle. Careful examination of the aortic flap during the cardiac cycle on cine MR imaging is important to detect the presence of "true lumen collapse," which may be associated with end-organ ischemia. When the intima is stripped 360° from the media and is essentially "free floating," this may result in catastrophic intimo-intimo intussusception.

Newer pulse sequences such as True Fisp or Fiesta offer very fast cine imaging.

Basic MRI sequences to evaluate for aortic dissection include spin-echo T1-weighted or breath-hold double inversion recovery sequences, cardiac-gated gradient-echo sequences, and three-dimensional (3D) thin-section MR angiography with a bolus injection of a single or double dose of gadolinium-based contrast agent

MRI findings of AIH include a crescent of blood surrounding but not compressing the aorta. The signal intensity of the crescent varies with age on T1-weighted imaging: it is isointense to muscle in the acute setting and markedly hyperintense after 3-7 days.

MRI is also helpful in postoperative follow-up. It can accurately depict associated complications, including the following:

• Thrombosis

• Hemorrhage

• Infection

• Pseudoaneurysms

• Aortoenteric fistula

• Ureteral obstruction

Degree of Confidence: The sensitivity and specificity are both more than 90%.

False Positives/Negatives: Potential drawbacks of MRI include reported artifacts on cardiac-triggered thoracic spin-echo phase images. These can appear as an artifactual borderlike feature across the aorta (due to helical flow in the aorta) that can be interpreted as a dissection. Other potential causes of misinterpretation include an atypical configuration of the intimal flap seen in short dissections and multiple false channels where the flaps are complex (see Images 7-8). Aortic anomalies also can cause confusion. False positives seen in gadolinium-enhanced MRA include a central line or "maki" artifact. This occurs when the acquisition is performed too early as intraortic gadolinium concentration is rising. This artifact can be readily differentiated from an aortic dissection as it does not take a spiral course like a true intimal flap.

ULTRASOUND

Findings: ECG is helpful in the diagnosis of aortic dissections. It is particularly helpful with ascending thoracic dissections, cardiac tamponade, and aortic regurgitation where TEE has a greater sensitivity and specificity than CT or MR in detecting coronary arterial occlusion, aortic insufficiency, and cardiac tamponade. The sensitivity is 97-99%. The specificity is in the range of 97-100%.

TEE was the favored study for the evaluation of aortic dissection according to a study by Mastrogiovanni et al from Salerno, Italy. In their report of 54 patients, TEE findings confirmed the diagnostic dissection in all patients but one. The site of the intimal tear; the extension of the dissection, pericardial effusion, aortic incompetence; and left ventricular function was noted. Because of the high level of correspondence between the diagnosis made at TEE and the surgical anatomic findings, the authors favor the use of TEE, often as the sole diagnostic modality.

False Positives/Negatives: A tortuous aorta may result in a false-positive diagnosis of dissection. In massive dilated ascending aortas (usually due to cystic medial necrosis) it may be difficult to identify a small intimal flap.

NUCLEAR MEDICINE

Findings: Nuclear medicine has no role in the diagnostic evaluation of acute aortic dissection. In a stable patient with chronic dissection and severe renal insufficiency, a renal scan may demonstrate the presence or absence of renal perfusion (usually the left kidney).

ANGIOGRAPHY

Findings: Aortography was the reference standard for the preoperative evaluation and diagnosis of aortic dissection. With the advent of TEE, CT, and MRI, its role has become important only if nonsurgical interventional procedures are indicated. It is quite controversial whether coronary angiography should be performed prior to sternotomy in a stable patient with aortic dissection, as concomitant coronary bypass grafting can be performed if diseased vessels are present.

Diagnostic criteria include visualization of a lucent flap and delayed filling and washout of the false lumen. The expanding false lumen may compress the true lumen and cause it to become narrowed. A dual lumen aorta is noted when both the true and false lumens are opacified (see Image 9). AIH is almost impossible to diagnose with aortography as no compression of the lumen exists

During aortography, overinjection of the false lumen should be avoided if it is entered during the procedure. The operator should be suspicious if he or she has difficulty advancing the guidewire into the aortic valve. Abdominal and pelvic aortography should be included in the diagnostic study to assess the level of the reentry site. Obstruction of the aortic branches may be noted (most commonly in the left renal artery in approximately 25-30% of patients). Visceral and extremity ischemia can occur when the superior or inferior mesenteric arteries and the iliac arteries are compromised.

Degree of Confidence: Aortographic findings are less sensitive than those of newer noninvasive techniques, especially for aortic intramural hematoma.

False Positives/Negatives: Pitfalls of angiography include a lack of visualization of the false lumen because of thrombosis or inadequate opacification with contrast material. Streak artifacts secondary to aortic or cardiac motion or opacification of the sinus of Valsalva can be confused with thrombus. Pitfalls also include missing the diagnosis of an intramural hematoma (frequently associated with progression to frank dissection) and misdiagnosis when the false lumen is thrombosed.

INTERVENTION

Intervention: Aortic fenestration is an alternative in treating the complications of aortic dissection, including renal and/or mesenteric ischemia in type B dissections. Surgical repair of type A or B dissection usually redirects blood to the true lumen and cures branch vessel compromise. However, because operative mortality is 35% in acute type B dissection, interventional techniques are a viable option. The mortality rate of patients with aortic dissection and renal or mesenteric ischemia is extremely high: 50-70% in renal ischemia and as high as 87% in mesenteric ischemia. Aortic fenestration, with or without stenting an underperfused vessel, appears to have a role in the treatment of static or dynamic obstruction of aortic branches.

The goal of treatment is to create a tear in the dissection flap that separates the true and the false lumens. This procedure allows local blood flow across the flap and reperfuses the organs supplied by the true lumen or those that are underperfused. The following steps are required:

• If CT or MRI is available, digital subtraction angiography with a power injection into the true lumen suffices in the assessment of the orientation of the true and false lumens. Intravascular or transesophageal ultrasound (TEUS) may be performed to help localize the needle and differentiate the true from false lumen as well.

• The actual fenestration is performed by advancing a Rosch-Uchida needle (Cook, Bloomington, IN) or any equivalent device from the true lumen (usually the smaller one) to the false lumen (usually the larger one).

• The needle is rotated perpendicular to the dissection flap and advanced to make the puncture. The needle causes the wall to become tented during this step. The fenestration site is chosen as close as possible to the arteries that are compromised.

• Using a sheath to measure pressures on both sides (ie, in the true and false lumens) is convenient.

• Once the needle is advanced into the false lumen, a working wire can be advanced into the false lumen. A 12- to 15-mm balloon is inflated across the flap to create a transverse tear. The flap is frequently soft, and a waist in the balloon may not be seen. The IVUS device can be used to assess correct placement of the balloon.

• Successful puncturing of the flap can be confirmed by injecting saline into the fenestration catheter. Microbubbles in the false lumen confirm successful flap puncture. The IVUS device can be swept up and down the aorta. If fenestration is successful, the IVUS device switches from one lumen to the other at the site of the fenestration.

• Aortic stents are used to buttress open the true lumen. The stents are deployed between the fenestration and the compromised artery but not across the fenestration. When the dissection propagates into a branch artery, it can be treated with stent placement after the gradient is measured across the branch vessel.

Recently, some centers treat patients with type B dissections by placing a covered stent-graft across the entry tear. This results in the same physiology as surgical repair (ie, it redirects blood flow to the true lumen). However, the long-term results of this technique are not known as often multiple reentry tears are present, which may keep the false lumen patent and result in aneurysmal dilatation.

Medical/Legal Pitfalls:

• Failure to identify the imaging features of a dissection

• Delaying treatment by performing unnecessary imaging studies

• Death during an aortic fenestration performed without appropriate informed consent

PICTURES

Aorta in aortic dissection

 

Caption: Picture 1. Aortic dissection. Image A represents a Stanford A or a Debakey type 1 dissection. Image B represents a Stanford A or Debakey type II dissection. Image C represents a Stanford type B or a Debakey type III dissection. Image D is classified similar to A but contains an additional entry tear in the descending thoracic aorta. Note that a primary arch dissection does not fit neatly into either classification.
Picture Type: Image

Caption: Picture 2. Aortic dissection. Plain anteroposterior view of the chest demonstrates a wide mediastinum (Courtesy of Jorge J. Guerra, Jr., MD)
Picture Type: X-RAY

Caption: Picture 3. Aortic dissection. Nonenhanced CT scan of the chest demonstrates a type B acute aortic intramural hematoma with displacement of intimal calcification and a crescentic high-attenuating clot without mass effect on the aortic lumen (Image Courtesy of Joel L. Fishman, MD)
Picture Type: CT

Caption: Picture 4. Aortic dissection. Contrast-enhanced axial CT image demonstrates an intimal flap that separates the two channels in the ascending and descending aorta diagnostic of a Stanford A type dissection (Image Courtesy of Joel L. Fishman, MD)
Picture Type: CT

Caption: Picture 5. Aortic dissection. Contrast-enhanced axial CT image demonstrates an intimal flap that separates the two channels in the ascending aorta and descending aorta and begins at the level of the aortic root (Image Courtesy of Joel L. Fishman, MD)
Picture Type: CT

Caption: Picture 6. Aortic dissection. Sagittal gradient-echo MRI image obtained in early systole shows a jet of blood flowing through the intimal tear from the smaller anterior true lumen into the larger posterior false lumen. The intimal flap is recognized as the medium signal intensity linear structure that divides the true and false lumens. An aortic stent-graft could be placed across the intimal/entry tear in the descending thoracic aorta to redirect blood into the anterior true lumen (Image Courtesy of Joel L. Fishman, MD)
Picture Type: MRI

Caption: Picture 7. Aortic dissection. Unenhanced CT scan of the chest depicts a complex dissection with multiple false channels in an aneurysmal descending aorta. Aortic rupture could not be excluded as there is a large aorta with hyperintense areas of fresh hematoma and a pleural effusion. Left lower lobe lung disease is present (Image Courtesy of Joel L. Fishman, MD)
Picture Type: CT

Caption: Picture 8. Aortic dissection. Contrast-enhanced CT scan obtained at the level of the aortic arch demonstrates an aortic dissection with almost complete separation of the aortic intima. The slight prolapse may be the beginning of a configuration at risk for intimo-intimo intussusception, a potentially fatal event (Image Courtesy of Joel L. Fishman, MD)
Picture Type: CT

Caption: Picture 9. Aortic dissection. Oblique arteriogram of the thoracic aorta demonstrates the double-barrel aorta sign of aortic dissection. Both the true and false lumina are opacified (Image Courtesy of Joel L. Fishman, MD)
Picture Type: Image