To have an understanding of aortic diseases, it is imperative to have a good knowledge base in anatomy, physiology, and function of the normal aorta. The aorta is a large conductance blood vessel, often called the greatest artery, normally reaching 0.7 m in length and 3.5 cm in diameter. It arises from the heart, arches anterocranially and then posterocaudally, and descends caudally, terminating as it bifurcates into the right and left common iliac arteries. The aorta is divided into six segments: (1) the aortic root, (2) the sinotubular junction, (3) the ascending aorta, (4) the aortic arch, (5) the isthmus and descending thoracic aorta, and (6) the abdominal aorta (Fig. 22–1).
Segmental division of the aorta: the aortic root (light blue), the sinotubular junction (green), the ascending aorta (yellow), the aortic arch (dark blue), the isthmus and descending (thoracic) aorta (red), and the abdominal aorta (purple). This figure was published in Aortic Diseases: Clinical Diagnostic Imaging Atlas, Hutchinson SJ, copyright Elsevier 2009.
The aortic root originates at the aortic valve annulus level and extends to the sinotubular junction. Normal diameter of the aortic root (at the widest proximal portion, known as bulbous) is up to 3.5 cm. The bulbous portion of the aorta is subdivided into the sinuses of Valsalva. The site where the bulbous portion of the aorta meets the narrower tubular-shaped aorta is termed sinotubular junction. Effacement of this junction suggests annuloaortic ectasia and often is seen in patients with Marfan syndrome. The ascending aorta follows the sinotubular junction and extends to the right brachiocephalic artery. The ascending aorta is an intrapericardial structure, measuring approximately 5 cm in length and up to 3 cm in diameter. The aortic arch is extrapericardial and courses leftward anterior to the trachea and continues posterior to the left of the trachea and esophagus and gives rise to all three great vessels: the innominate artery (brachiocephalic trunk), left common carotid artery, and left subclavian artery (Fig. 22–2). The normal diameter of the aortic arch is up to 2.9 cm. Arch branch vessel variants and right-sided arches can sometimes be seen. The isthmus is the narrower portion of the aorta (by approximately 3 mm) between the left subclavian artery and ligamentum arteriosus, a remnant of the ductus arteriosus. Blunt traumatic deceleration injury, resulting in transection to the aorta often occurs at this site. The descending thoracic aorta begins at the ligamentum arteriosus and continues to the level of the diaphragm. The descending thoracic aorta gives rise to intercostal, spinal, and bronchial arteries. The abdominal aorta starts at the hiatus of the diaphragm and courses retroperitoneal to its bifurcation. Major branch vessels include celiac arteries, superior and inferior mesenteric arteries, and middle suprarenal, renal, and testicular or ovarian arteries. The diameter of the suprarenal abdominal aorta is usually up to 2 cm. The infrarenal aorta diameter should never exceed that of the suprarenal aorta.
The most common (normal) aortic arch branching pattern found in humans has separate origins for the innominate, left common carotid, and left subclavian arteries (a). Reproduced with permission from Layton KF, et al. Bovine aortic arch variant in humans: clarification of a common misnomer. Am J Neuroradiol. 2006;27:1541-1542, © by American Society of Neuroradiology.
The aorta has a trilaminar wall comprised of tunica intima, tunica media, and tunica adventitia. The tunica intima consists of an endothelium, subendothelial tissue, and an internal elastic membrane. The tunica media is composed of elastic fibers arranged as circumferential lamellae, elastin, collagen, smooth muscle cells, and ground substance. Adventitia is a thin, outermost layer of the aorta consisting of collagen; vasa vasorum, which is the vascular supply to the aorta; and nervi vascularis, which is the network of primarily adrenergic fibers.
Physiology and Function of the Aorta
Elastin-to-collagen ratio in the tunica media progressively decreases from proximal aorta to distal aorta and to peripheral arteries. This architecture ensures accommodation of stroke volume (cushioning function) and storage of potential energy in systole (capacitor function) and release of kinetic energy in diastole, propelling blood forward (Windkessel phenomenon). In addition, this is responsible for progressive increase in pulse wave velocity of propagation from the proximal aorta to peripheral vessels. In the normal aorta, the pulse wave is transmitted down the aorta to its bifurcation and peripheral vasculature where it is reflected back toward the heart during diastole, contributing to diastolic pressure. These properties usually decline with aging or diseased aorta due to disruption of elastin and decreased elastin-to-collagen ratio. In the diseased aorta, the propagation wave is faster, which causes an earlier return of the reflected waves (during systole) and a summation with the systolic wave. This leads to self-perpetuation of hypertension. In addition, increased aortic stiffness has been associated with increased risk of myocardial infarction, stroke, congestive heart failure, and both cardiovascular and overall mortality.
Aortic pressure waveform can be distinguished by its four components: (1) the upslope, (2) the peak, (3) the dicrotic notch, also known as incisura, and (4) the pressure decay. The slope of the upslope and the peak are dependent on the elasticity of the vessel and the volume of blood entering the aorta. Dicrotic notch is only seen in the proximal aorta and results from the aortic valve closure. The pressure decay corresponds to diastolic pressure and is affected by peripheral resistance and reflected pressure waves (in the healthy aorta). There is a progressive decrease in mean aortic and diastolic pressures and an increase in systolic aortic pressure with increasing distance from the heart.
The aortic "knob" may be seen in the superior mediastinum, on the left side, just lateral to spinal column on the chest roentgenogram. The aortic root and proximal ascending aorta are visible as they arise from the base of the heart on the lateral chest roentgenogram. Left anterior oblique projection provides the best view of the ascending aorta and the arch. Intimal calcifications and aneurysmal dilation may be seen.
Advantages of chest radiography include wide availability, no contrast exposure, low cost, minimal radiation exposure, and the ability to be performed quickly at the bedside. Disadvantages include low spatial resolution, two-dimensional (2D) static image, and relatively poor target acquisition for aortic diseases (Table 22–1).
Table 22–1. Comparison of Technical Specifications for Various Imaging Modalities ||Download (.pdf)
Table 22–1. Comparison of Technical Specifications for Various Imaging Modalities
|Variables||Chest Radiography||Echocardiography||Angiography||CT Angiography||MR Angiography|
|Spatial resolution||1-2 mm||~0.5-2 mm||0.16 mm||0.5-0.625 mm||1-2 mm|
|Temporal resolution||N/A||20-30 ms (M-mode <5 ms)||1-10 ms||83-135 ms||20-50 ms|
|Radiation||0.1 mSv||No||5-8 mSv||5.3-20 mSv||No|
|Time||<1 min||5-30 min||30-60 min||<1 min||30-40 min|
|Dimension||2D||2D or 3D||2D||3D||3D|
Echocardiography and Ultrasonography
The aortic root, sinuses of Valsalva, sinotubular junction, and proximal ascending aorta are usually well seen on a parasternal long axis view by transthoracic echocardiography (TTE).1 However, mechanical and stented bioprosthetic aortic valves produce acoustic shadows and reverberations. Distal ascending aorta and the arch are less well visualized by TTE. Suprasternal notch and supraclavicular windows offer the best views of the aorta in long and short axes. TTE allows visualization of the retrocardiac aorta in short and long axis (with clockwise rotation and lateral angulation of the transducer) on parasternal long axis view. Lateral angulation of the image plane in the apical two-chamber view can also be used to image the descending thoracic aorta. A posterior chest wall approach is particularly useful for imaging of the descending aorta in patients with large left pleural effusion. In the subcostal window, a lateral angulation of the transducer in the inferior vena cava plane allows visualization of the proximal abdominal aorta.
Transesophageal echocardiography (TEE) at 30° to 45° and 120° to 140° rotation at mid to high esophageal level is particularly helpful and less artifactual in evaluation of the aortic root, sinuses of Valsalva, sinotubular junction, and proximal ascending aorta. Withdrawing the transducer in the esophagus in the esophageal long axis plane produces more cephalad images of the ascending aorta. Medial turning and inferior angulation of transesophageal transducer at the arch level provide superior detail of the aortic arch in the long axis plane. Transesophageal transducer maybe turned around (either direction) at 0° to obtain a short axis view of the descending aorta. Rotation of the image plane to 90° allows depiction of the aorta in the long axis view, and the left subclavian artery may be located in this plane.
Ultrasonography (US) is a useful modality for abdominal aorta examination. It is commonly used for aortic disease screening, sequential monitoring for the assessment of aortic size, and evaluation of branch vessel involvement.
Advantages of echocardiography and ultrasonography include superb temporal resolution (especially with M-mode imaging), three-dimensional (3D) imaging, ability to assess functional cardiac complications of aortic diseases, wide availability, no contrast or ionizing radiation exposure, relatively low cost, and ability to be performed quickly at the bedside. Disadvantages include suboptimal target acquisition, inability to image the entire aorta, and can be time consuming.
The aortic root, sinotubular junction, and ascending aorta have significant motion throughout the cardiac cycle, and thus are prone to significant motion artifact with non–electrocardiography (ECG)-gated imaging modalities. ECG gating is recommended when imaging these proximal structures by either computed tomography (CT) or magnetic resonance angiography (MRA). The aortic arch, isthmus, and descending aorta are subject to little motion artifact.
CT allows various postprocessing displays and generates very detailed images of the arterial wall and lumen with high spatial resolution. Pacemaker wires in the superior vena cava (SVC) and brachiocephalic vein, mechanical aortic valves, pericardial structures, and contrast dye concentrated in SVC may produce artifacts when the aortic root, ascending aorta, and aortic arch are imaged with CT. The prevalence of motion artifact on a 1-second scanning time CT is reported to be as high as 57%.2 Multidetector CT (MDCT) technology significantly reduces respiratory artifacts and improves temporal resolution due to reduction of scanning time to 0.5 seconds. However, non–ECG-gated MDCT was reported to produce motion artifacts in 91.9% of cases.3 Fujioka et al4 reported a prevalence of diagnosis nonhampering motion artifact in 6.7% of cases with ECG-gated MDCT. In the absence of spinal prosthesis, CT of descending aorta produces very few artifacts.
Advantages of CT include 3D imaging, superb spatial resolution, excellent temporal resolution, very fast scanning time (<1 minute), high sensitivity and specificity for the assessment of aortic disease, postprocessing reconstruction, and wide availability. Disadvantages include ionizing radiation and contrast exposure.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) and MRA can accurately depict the aortic lumen and the wall with minimal artifacts. The artifacts that one can encounter, however, include signal dropout from metallic clips and mechanical valves; ghosting artifacts (encountered when images are acquired during systolic phases of the cardiac cycle); insufficient dose of contrast agent (results in low signal-to-noise ratio); wraparound artifact, usually in coronal plane (aliasing due to data sampled in a discrete rather than continuous fashion); and susceptibility artifacts from concentrated contrast agent in the nearby venous structures.
MRI and MRA offer the ability to assess the aorta in both a 2D and 3D format. The number of different pulse sequences can be used to assess function and anatomy. A typical evaluation for the aorta would begin with anatomic assessment with axial, sagittal, and coronal dark blood turbo spin-echo (TSE) pulse sequence covering the entire thoracic or abdominal aorta. The dark blood TSE sequence obtains all the data for the particular slice within one heartbeat and thus allows for multiple slices without requiring the patient to hold their breath. Once the anatomic imaging is performed, cine images with a gradient echo pulse sequence are performed to assess the function of the aorta as well as the aortic valve (AV) when necessary. This image is generally performed in an oblique plane by scouting off of the dark blood TSE images. The next step in the evaluation includes typically a contrast-enhanced angiogram, which is gated to the heart (this avoids motion artifact within the proximal ascending aorta). The angiogram is a breath-hold sequence, which typically takes approximately 15 to 20 seconds to complete. If necessary, when blood flow needs to be quantified (ie, coarctation of the aorta), velocity-encoded pulse sequence is performed to assess the velocity of blood through the aorta at the level in question. This is the standard exam for the assessment of the aorta and can be tailored for each specific disease. The TSE dark blood sequence is useful in assessing the aortic wall, aortic dissection (AD), and other anatomic structures/abnormalities. If higher resolution is needed, then a single slice breath-hold TSE could be performed. This is often necessary when one wants to assess the aortic wall as in intramural hematoma or plaque assessment. The cine imaging is useful for more functional aspects of the aorta as in coarctation, AV insufficiency, and aortic distensibility. The MRA allows for a very high-resolution image of the aorta and is excellent in AD assessment and the 3D assessment of aneurysms. The benefit of 3D imaging is that the true cross-sectional diameter of an aneurysm can be easily assessed via postprocessing computer software.
Advantages of MRI include excellent temporal and spatial resolution, the wide spectrum of pulse sequences allowing for a 3D comprehensive assessment of the aorta, excellent target acquisition, and no ionizing radiation or contrast exposure. Disadvantages include lack of access at some centers, longer scanning time (can be prohibitive in patients who are hemodynamically unstable), artifacts due to respiratory motion in patients who are unable to breath-hold), inability of patients with large body habitus to fit inside the scanner, contraindication in patients with metal implants (eg, pacemakers/defibrillators, cerebral aneurysm clips), and the possibility that some patients may experience claustrophobia.
Aortography is an invasive test during which a multiholed catheter (usually pigtail) is passed percutaneously via radial, brachial, axillary, or femoral approach into the aorta; contrast material is injected via a power injector; and multiple radiographic views (or cine) are recorded. The ascending aortogram is often performed in a moderately steep, 30° to 60° left anterior oblique view. In the past, aortography was a first-line diagnostic test for imaging the aorta; presently, it is typically reserved for guiding aortic interventions.
Advantages of aortography include superb spatial resolution, excellent target acquisition, and good temporal resolution. Disadvantages include invasiveness, exposure to ionizing radiation and contrast, lack of access in some centers, production of 2D images, and time consuming to perform.