Approach to Congenital and Acquired Disease of Aorta



Approach to Congenital and Acquired Disease of Aorta



Introduction


Imaging of the adult aorta requires the recognition that at any given time point, the aorta represents an intersection of 4 potential phenomena:




Knowledge of this complex interplay allows the cardiovascular imager to adequately assess potential abnormalities encountered on imaging and help determine whether there is a variant or significant finding.


In the current era, radiography may incidentally detect an aortic abnormality but is rarely used for first-line imaging for suspected aortic pathology. That role has been assumed by echocardiography in the asymptomatic patient, largely owing to its lack of ionizing radiation, and lower cost than CT or MR. In symptomatic patients, or in patients with high risk for aortic disease, CT, MR, and echocardiography have all been shown to be nearly equal in sensitivity and detection of disease, especially AAS. In many emergency departments (ED), CT has gained in popularity owing to its proximity to the ED and speed of image acquisition. CT is also very easily understood by all members of the caring team. Increasingly, PET/CT is being used for problem solving with an abnormal aortic finding such as a potential thrombus vs. tumor or vasculitis vs. atypical atherosclerosis.



Imaging Approach


In the approach to the adult aorta, a few points can be helpful to keep in mind.


The normal adult aorta can be divided into distinct zones: the sinuses of Valsalva (the root); the ascending aorta (separated by a sinotubular junction from the root); the arch; the isthmus; the descending thoracic aorta and the abdominal aorta. Normal sizes depend on the location within the aorta and vary based on size of the patient. In children, measurements are usually obtained by echocardiogram and compared to body surface area to obtain a population-derived Z-score. In adults, measurements also tend to vary by gender and body surface area. In normal patients without cardiovascular disease, the mean size of the aorta at the sinuses and ascending aorta hovers ~ 3 cm but should not exceed 4 cm or fall below 2 cm. The isthmus and descending aorta tend to be closer to 2.5 cm and should not exceed 3.2 cm or fall below 1.8 cm. Measurements should be performed orthogonal to the long axis of the vessel (not on transverse images alone).


Isolated ascending aortic enlargement is unusual in hypertension alone and tends to be indicative of other conditions, most notably bicuspid aortopathy, Marfan syndrome, Loeys-Dietz Syndrome, inherited aortopathies, conotruncal abnormalities, and large vessel vasculitis.


Atherosclerosis-related aneurysms tend to be fusiform. A saccular outpouching should invoke consideration of a posttraumatic or infected pseudoaneurysm. Penetrating atherosclerotic ulcers (PAU) may result in saccular aneurysm. At the level of the aortic isthmus, ductus aneurysms can be saccular.


AAS [aortic dissections, intramural hematomas (IMH), and PAU] occurs within the media. Normally, the three layers of the aortic wall are seen as a thin structure (< 2 mm). Any aortic wall thickening must be critically assessed for the potential of an AAS. These conditions tend to present with chest pain and have the potential to be lethal. The blood in the wall (especially with IMH and PAU) is high attenuating on NECT or high signal on black blood MR. It should be somewhat eccentric and not involve all 360° of the aorta on a transverse plane.


Aortic dissection represents the most common AAS. Imaging must center on delineating the extension of the flap and the location of the true and false lumina. In the era of endovascular repair, the presence of fenestrations is also quite important.


Vasculitis, conversely, often presents with circumferential thickening of the aortic wall and may have concomitant lymphadenopathy. Fluorodeoxyglucose (FDG) PET will show uptake in vasculitis, which may help distinguish vasculitis from AAS. MR may be useful as vasculitis will show enhancement, whereby AAS will not. This enhancement is often best appreciated on delayed (venous phase) imaging post intravenous contrast.


Occasionally, periaortitis can mimic vasculitis. These conditions (which include IgG4 disease and Erdheim-Chester disease) often have involvement of other organs, which can help suggest the diagnosis. On MR, a fat plane between the thickening and the aorta may be seen.


The fat surrounding the aorta should be seen as homogeneously low attenuating or uniform in signal intensity. When it is effaced, the stranding may be indicative of a ruptured aneurysm, infection, or traumatic injury. Adjacent processes may also result in periaortic fat effacement, but any stranding should prompt a second look at the aorta.


The most common aortic mass is an intraluminal thrombus. Increasingly, the thoracic aortic mobile thrombus (TAMT) is being recognized as an embolic source in patients without any predisposing condition. These lesions are often seen near the isthmus. Although the etiology of the TAMT is unknown, some authors have postulated that they come from developmental scars related to ductal resorption. TAMTs should not enhance.


An aortic mass that enhances should raise suspicion for neoplasm. Aortic neoplasms are unusual. Most often, they are metastases from a lung or breast adenocarcinoma. Primary aortic sarcomas are rare, with the most common being the intimal sarcoma. Extraluminal extension and adjacent lymphadenopathy can be helpful in distinguishing neoplasm from TAMT.


Congenital aortic conditions can be symptomatic, especially when associated with a double aortic arch or its variants. The double arch and its variants tend to present with respiratory syndromes from the vascular ring, especially when one part of the ring is atretic. The atretic cords cannot be visualized on CT or MR but can be inferred when there are signs of a vascular ring with compression on the trachea and esophagus.


Another relatively common variant is the right aortic arch. The two main variants include the right arch with mirror image branching and the right arch with an aberrant left subclavian artery that passes posterior to the esophagus. The former tends to be seen with congenital heart disease and is not a vascular ring. The latter tends to be seen in isolation and is usually asymptomatic. Both CT and MR are useful in the delineation of these entities.


Acute aortic occlusion is rare but can be seen in hypercoagulable states with clot expanding and obstructing the aorta. More chronic occlusions (either congenital as in a coarctation or acquired from atherosclerosis) will manifest with narrowing and collateral vessel formation. On MR, velocity-encoded sequences can be used to estimate gradients of the stenosis based on using the peak velocity and a modified Bernoulli equation.


Rare congenital lesions can result in unusual aortic locations, which may simulate a mass on physical examination (e.g., cervical arch) or on radiography (e.g., pseudocoarctation). Knowledge of these variants will help prevent needless work-up or, worse, biopsy.


Imaging findings of traumatic aortic injury rest on the indirect and direct signs. CT is the preferred modality for traumatic aortic evaluation. Effacement of the periaortic fat is the only indirect sign. When encountered in isolation, follow-up CT or MR is usually performed in 24-36 hours. Direct signs include flaps, contrast extravasation, eccentric filling defects, abrupt change of aortic caliber, and lobulated or saccular configuration. These findings are sufficient to result in treatment.

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Apr 6, 2020 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on Approach to Congenital and Acquired Disease of Aorta

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