Findings:
Axial and sagittal oblique T1 spin echo images through the mediastinum demonstrate marked fusiform enlargement of the ascending aorta (blue arrow 0001a and 0002a) measuring 5.8 cm in the greatest diameter, without evidence a dissection flap, aneurysmal leak or thrombosis. The left atrium (green arrow 0001a) and descending thoracic aorta (red arrow 0001a) are normal. The aortic arch (orange arrow 0002a), innominate (red arrow 0002a), and left common carotid (green arrow 0002a) arteries are normal.

Impression:
5.8 cm ascending aortic aneurysm.

Discussion:
The walls of the aorta are made up of three different layers of tissue: a thin inner layer (intima); a thick, elastic middle layer (media); and a thin outer layer (adventitia). The aorta is divided into four major sections: aortic root, ascending aorta, aortic arch, descending aorta. The root is the beginning of the aorta, starting from the aortic valve (annulus) and becoming slightly wider in diameter (sinuses of Valsalva), it gives rise to two coronary arteries and ends at the beginning of the ascending aorta (sinotubular junction). The two coronary arteries are responsible for carrying oxygen-rich blood to the heart muscle itself. The ascending aorta segment extends upward from the aortic root to the point where the innominate artery branches off the aorta, and the aorta begins to form an arch. The ascending aorta is within the pericardium by itself and no arteries branch from it. There is little support from surrounding tissue and it must face the entire cardiac output volume (minus the coronary arteries), making the ascending segment the most vulnerable part of the aorta. The arch represents the curved portion at the top of the aorta. The innominate, left common carotid, and left subclavian arteries, which supply blood to the head and upper body, branch from the arch. It is outside the pericardial sac and generally has better support from surrounding structures. The descending aorta begins just beyond the arch as the aorta bends down into the body. The descending aorta ends at the diaphragm. It gives rise to the intercostal arteries. The beginning portion of the descending aorta at the level of the ligamentum arteriosus is vulnerable to intimal tear during deceleration conditions. The thoracoabdominal aorta begins at the diaphragm and ends at the renal arteries. The abdominal aorta begins below the renal arteries and ends at the aortic bifurcation which gives rise to the common iliac arteries.

Diseased aortic tissue is characterized by degeneration of the cells composing the aortic wall, weakening the tissue and having insufficient elastic components to stretch and contract well. The first indication of this abnormality may be a localized enlargement in the area of weakness resulting in aneurysm formation. Aortic tissue may also tear, even if the aorta is not enlarged, resulting in aortic dissection. Traditionally, for the ascending aorta, any permanently dilated section measuring 4.0 cm or greater in diameter has been called an aneurysm. The definition of an aneurysm may also be based on comparison with the normal blood vessel size for an individual. When the permanent enlargement of some part of a blood vessel is at least 1.5 times greater than normal size, it may be termed an aneurysm. Applying this to the aorta, if an individual's normal aorta is 2.5 cm, then dilation of 3.75 cm or greater represents an aneurysm in that person. A variation of this defines an aneurysm when the enlarged aorta is at least twice its normal size. Aortic enlargement, although perhaps not yet qualified as an aneurysm, should be monitored, and the lifestyle and diet of the patient addressed. Identifying dilation of the aorta implies the ability to determine the aorta's normal size for an individual. It is understood that the aorta's size will vary across any given population based on age, gender and body size. (Table 1) A broad range of aortic diameter, sometimes listed as the normal size of the aorta, necessarily spans a large variation of body sizes in the population and may be misleading regarding a given individual. It is important that every effort is made to determine the normal aortic diameter for each individual in order to detect the early stages of aortic expansion due to underlying aortic disease. Generally, in the majority of patients, that part of the aorta that is not enlarged may be used as an indicator of what is normal for that individual. Thoracic aortic aneurysms are described according to their location, size and shape. There are generally two different shapes for aneurysms: fusiform and saccular. A pseudoaneurysm is an aneurysm that does not have some or all of the aortic wall layers, often due to some injury to the inner aortic wall. These aneurysms are more unpredictable, with a higher tendency to rupture at smaller sizes. Usually there is an inflammatory process involved in the surrounding tissue, which potentially can complicate a redo operation. Some pseudoaneurysms are the result of infection, which increases the risk of embolization and stroke. Risk factors for aneurysm formation include: bicuspid aortic valve, Marfan’s syndrome, Ehlers-Danlos syndrome, other connective tissue disorders (Marfanoid), atherosclerosis, infectious and inflammatory etiologies, hypertension, smoking, and trauma. Aortic dissection is tearing of the inner layer of the aortic wall, allowing blood to leak into the wall itself and cause separation of the inner and outer layers. It is usually associated with severe chest pain radiating to the back.

Table 1

Remarkable advances in noninvasive imaging methods particularly magnetic resonance imaging, have replaced many invasive angiographic procedures, lowering the cost and morbidity of diagnosis Several different MR angiography techniques are used to image the arteries. These include black-blood imaging, phase-contrast imaging, time-of-flight (TOF) imaging, and contrast enhanced MR angiography. Contrast-enhanced MR angiography is the most widely used method because it is rapid and robust.

Black blood technique (conventional spin echo or fast spin echo) is optimal for depiction of an intraluminal or mural abnormality, such as intimal flaps, ASVD plaques, wall thickening or intramural hematoma. Phase contrast imaging is valuable due to its ability to quantify flow velocity and its clinical application to evaluate physiological properties of blood flow. Time of flight technique has been generally replaced by contrast-enhanced MR angiography. Dynamic contrast MR angiography produces higher spatial resolution and fewer artifacts than any other MR angiography technique.

Medical treatment and lifestyle changes are specified for each individual and include blood pressure optimization and lifestyle recommendations. Blood pressure medications, such as beta-blockers, ACE inhibitors, and calcium channel blockers, are commonly prescribed. Generally, an optimal systolic blood pressure range prior to surgery is between 105 and 110 during normal activity. Lifestyle recommendations address diet, exercise and smoking cessation. Surgery is indicated for aneurysms over 5 cm or rapidly changing in size (> 0.5 cm/ year) or symptomatic. For aneurysms less than 5cm the incidence of rupture is 2%; for aneurysms greater than 10 cm the incidence is greater than 50%.

References:
Servet Tatli, Martin J. Lipton, Brian D. Davison, Ronald B. Skorstad, and E. Kent Yucel From the RSNA Refresher Courses: MR Imaging of Aortic and Peripheral Vascular Disease RadioGraphics 2003; 23: 59-78.

Yuji Watanabe, Masako Dohke, Akira Okumura, Yoshiki Amoh, Takayoshi Ishimori, Kazushige Oda, Takafumi Hayashi, Atsuto Hiyama, and Yoshihiro Dodo
Dynamic Subtraction Contrast-enhanced MR Angiography: Technique, Clinical Applications, and Pitfalls RadioGraphics 2000; 20: 135-152.