The Abdominal Aorta

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CHAPTER 102 The Abdominal Aorta

TECHNIQUES

The choice of imaging technique used to evaluate the aorta is determined by patient and disease-specific indications. Plain radiography has a limited role except following endovascular stent graft placement. Ultrasound has been shown to be an effective screening tool and has the advantages of portability, price, absence of ionizing radiation, or potentially nephrotoxic contrast. MDCT has much greater spatial resolution than any other cross-sectional modality and offers the added advantage of clear demonstration of visceral organs and extremely rapid image acquisition. Magnetic resonance imaging (MRI) techniques can provide additional physiologic information such as flow quantification. There is increasing evidence that PET-CT may be useful in the evaluation of the inflammatory and the noninflammatory aortic aneurysm.

CT Techniques

With recent advances in MDCT technology, routine acquisition of a high resolution isotropic volumetric dataset at the peak of contrast enhancement is easily achieved. There are several advantages to MDCT, but perhaps the greatest is the ability to analyze and display the aorta and branch vessels in any plane and to perform three-dimensional volume rendering for improved visualization and vascular diagnosis.

Using MDCT, a helical volume is acquired with a detector collimation of between 0.5 and 1.5 mm, and reconstructed with a similar thickness using a matrix size of 512 × 512 or greater. The use of tube current modulation techniques (e.g., automated exposure control) reduces radiation exposure compared to fixed-tube current techniques. For MDCT angiography, increased vascular enhancement can be achieved by reducing the kVp to 100 keV (or 80 keV in smaller patients) because this increases photon attenuation by iodinated contrast and moves the mean energy closer to the k-edge of iodine (33.2).1

Prior to acquisition of the arterial phase images, a noncontrast CT image set can be obtained. This is particularly useful where significant vascular calcification is expected, or in instances in which intramural hematoma is a differential consideration. Proper timing of the CTA volume acquisition with peak arterial contrast enhancement is critical for optimal arterial illustrations. For detection of associated parenchymal abnormalities, scar tissue, and endoleaks, the addition of delayed phase images can also be helpful. If multiple phases are used routinely, acquisition of those additional phases using lower radiation dose techniques should be considered to minimize patient radiation exposure doses.

After the patient is scanned, the dataset can be sent to a postprocessing workstation for both two-dimensional and three-dimensional image reconstruction, and for evaluation of the aorta and branch vessels in a double-oblique true axial plane.

MRI Techniques

Coils and Patient Position

Patients are imaged supine, ideally in a 1.5T or 3.0T MR scanner. Arms should be placed above the patient’s head, or folded across the chest to prevent wrap artifact (also known as image aliasing artifact).

MR Pulse Sequences

Precontrast Sequences

Axial T1 Weighted Sequences: To survey the intra-abdominal structures, axial T1 weighted images are useful. These can be performed as breath-hold in and out of phase sequences, or using a double inversion fast spin echo technique to eliminate signal from slowly flowing blood. Precontrast T1-weighted images are particularly important for evaluation of the aortic wall for possible pathologies, such as intramural hematoma, which may have high T1 signal related to the presence of hemorrhage. Axial T2 fast spin echo sequences (usually with fat suppression) can also be performed from the diaphragm to the iliac crests to evaluate mass lesions and fluid collections around grafts.

Steady-State Free Precession: Steady-state free precession sequences (also termed balanced-FFE, true-FISP, and FIESTA) are rapid, have high intrinsic contrast resolution, and have been shown to be accurate in evaluating renal artery stenosis,2 aneurysm sac contents (Figure 102-1),3 thoracic aortic dissection, and aneurysm.4 These sequences can be performed with breath-hold techniques or free breathing with navigator gating, and they provide an overview of the aneurysm sac and surrounding contents.

Velocity Encoded Cine MR Techniques: Unlike the thoracic aorta, velocity encoded cine MR techniques (also known as cine phase contrast MR) are not widely used in the abdominal aorta. However, in cases of abdominal aortic coarctation or stenosis, these techniques may assist in quantifying the hemodynamic severity of the obstruction.

Postcontrast MRA: Following contrast injection, the abdominal aorta is scanned using a 3D spoiled gradient echo pulse sequence. The sequence should be optimized to acquire a near isotropic voxel size of 1 to 1.5 mm, in approximately 10 to 15 seconds.

CONTRAST MEDIA

A bolus of intravenous gadolinium-chelate contrast agent for MRA, or of iodinated contrast agent for CTA, is typically injected through a peripheral vein followed by a saline flush. The aim of MRA or CTA of the abdominal aorta, as with MRA and CTA elsewhere in the body, is to achieve peak arterial enhancement during the critical imaging periods, which for CTA is during the whole helical acquisition, and for MRA, is during the acquisition of the central lines of k-space. Proper synchronization of imaging for the arterial transit of the bolus through the target vascular bed will ensure optimal arterial illustration.

The time that it takes for a contrast bolus to pass from the site of intravenous injection until it reaches the abdominal aorta varies widely between individuals, from as little as 10 seconds to as long as 60 seconds. Consequently, varying techniques are available to synchronize MR and CT acquisition with peak arterial enhancement.

Contrast Media Dose

For abdominal aortic CTA, doses of up to 150 mL of nonionic contrast delivered at 3 to 6 mL per second have been routinely used in the past. However, with rapid acquisition techniques, doses as low as 50 mLs have been used, especially in smaller patients.5 For MRA, a typical gadolinium-chelate contrast agent dose is 0.2 mmol/kg injected at 2 mL/sec.

ABDOMINAL AORTIC ANEURYSM

Abdominal aortic aneurysm (AAA) is enlargement of the abdominal aorta above a diameter of 3 cm.7

Prevalence and Epidemiology

Abdominal aortic aneurysms are nearly five times more common in men than in women and almost twice as common in people of European descent than African Americans. The prevalence of aneurysm is also increased in those with a family history of AAA, and is strongly related to a history of smoking.8 Other risk factors include age, coronary artery disease or another manifestation of atherosclerosis, high cholesterol, and hypertension. In a large ultrasound screening study,9 an abdominal aortic aneurysm was detected in approximately 5% of males older than 65 years. Ruptured AAA occurs in 1% to 3% of men per year aged 65 years or more, and mortality is 70% to 95%. Left untreated, AAA leads to death in about one third of patients.8

Etiology and Pathophysiology

Although the exact etiology of AAA remains unclear, it appears that degradation of elastin, collagen, and other structural proteins in the aortic wall is a major factor.10 Atherosclerosis is considered to play an important role in the etiology, likely via chronic inflammation in the aortic wall. An imbalance of T-helper and T-suppressor lymphocytes leads to a proliferation of B-lymphocytes. Elastin-derived peptides (EDPs), which are breakdown products of medial elastin, are thought to be the initiating and propagating antigen in this process. Chronic inflammation leads to excess matrix metalloproteinases and degradation of medial elastin and collagen.11 However, the etiology is unquestionably multifactorial. Current research suggests that genetic, environmental, hemodynamic, and immunologic factors all contribute to the development of aneurysms.

Imaging Indications and Algorithm

The U.S. Preventive Services Task Force (USPSTF)16 recommends screening for AAA in men aged 65 to 75 years of age who have ever smoked. It can also be considered in patients with a strong family history of AAA. Ultrasound is the modality of choice for screening in most patients. Its advantages include the absence of intravenous contrast, absence of ionizing radiation, and relatively low cost.

For asymptomatic and smaller aneurysms, ultrasound surveillance is recommended (Table 102-1). For aneurysms >4.5 cm, CT or MRI offer the advantages of greater measurement accuracy, better depiction of the suprarenal aorta and branch vessels, and superior reproducibility versus ultrasonography.

TABLE 102-1 Rescreening Intervals for Asymptomatic AAAs

Diameter up to Re-image aorta in
<3.5 cm 3 years
<4.0 cm 2 years
<4.5 cm 1 year
<5 cm 6 months
5-5.5 cm 3-6 months*

* Also consider referral to a vascular surgeon.

Derived from Isselbacher EM. Thoracic and abdominal aortic aneurysms. Circulation 2005; 111(6):816-828.

Imaging Techniques and Findings

Ultrasonography

Examination of the abdominal aorta is an essential component of a complete abdominal ultrasound study, and should be examined in all patients presenting with acute abdominal pain. Abdominal aortic aneurysms should be detectable by sonography in up to 100% of patients.18

The aorta appears on ultrasound as a hypoechoic tubular structure with echogenic walls (Fig. 102-3A). The anterior and posterior walls of the aneurysm are usually better seen than are the lateral walls. Mural thrombus (see Fig. 102-3B) has low to medium echogenicity and is attached to the margins of the aortic wall. At times, mural thrombus will have a lamellated appearance. Thrombus that appears to “flutter” during the cardiac cycle may be at risk of embolization.

Ultrasound measurements of abdominal aortic aneurysms are accurate and repeatable. The measurements are taken from the outer-to-outer wall of the aorta in a plane perpendicular to the long axis of the aorta.

In suspected aneurysm rupture, a bedside ultrasonogram may be helpful for those patients who are too unstable for CT, or if CT is not readily available. Ultrasound scanning may assist in determining the aneurysm size and the presence of retroperitoneal or intraperitoneal fluid (Fig. 102-4), although the role of ultrasonography in identifying impending rupture is limited.19

CT and MRI

Compared to ultrasonography, CT and MRI (Fig. 102-5) provide superior depiction of the extent and shape of the aneurysm, involvement of the renal, mesenteric, and iliac arteries, and the suprarenal abdominal and thoracic aorta. On average, ultrasound underestimates aneurysm size by 3 to 9 mm compared to CT angiography.20 Due to their excellent contrast resolution and multiplanar capabilities, CT and MR angiography are now the mainstay of aneurysm characterization prior to endovascular or surgical repair. CT is the modality of choice for the evaluation of suspected rupture of the abdominal aorta19; usually it can be performed within minutes, and has clear benefits in showing alternative causes of acute abdominal pain. Contrast-enhanced CT provides information about the lumen size, location, extent, relationship to branch vessels, presence of active contrast extravasation, and complications secondary to aneurysm rupture.

Signs of Rupture, Impending Rupture, and Contained Rupture

Noncontrast CT is useful in demonstrating the presence of an abdominal aortic aneurysm, maximum aneurysm size, and the presence of retroperitoneal hemorrhage. A high attenuating crescent within the wall of aneurysm (Figs. 102-6 and 102-7) is a sign of impending or frank aneurysm rupture.21 A high attenuation crescent is denser than the psoas muscles (on enhanced CT) and the lumen (on nonenhanced CT scans).

The most common finding in aneurysm rupture is a retroperitoneal hematoma adjacent to the abdominal aortic aneurysm (see Figs. 102-4, 102-7, and 102-8). This blood usually tracks into the pararenal and perirenal spaces (Fig. 102-9). Active extravasation (Figs. 102-9 and 102-10) is frequently visualized on contrast-enhanced CT images. The “draped aorta sign,” where the abdominal aorta is closely applied to the spine with lateral “draping” of the aneurysm around the vertebral body (see Fig. 102-4), has been described as a finding of a deficient posterior wall of the aorta and a contained leak.22 Other sites of rupture include the bowel (most commonly the duodenum), and inferior vena cava (Fig. 102-11).23 Signs of AAA rupture, impending rupture, and contained rupture are (1) periaortic and retroperitoneal hemorrhage; (2) contrast extravasation; (3) high attenuating crescent sign; and (4) draped aorta sign.

FDG-PET

Recent evidence suggests that increased aortic wall metabolism, as measured by FDG-PET, may suggest increased rupture risk. Increased metabolism may represent increased activation of inflammatory cells in the aortic wall, which leads to increased degradation of elastin and collagen in the aneurysm wall.24 Pilot studies examining the role of FDG-PET-CT in asymptomatic and symptomatic AAAs,25 have demonstrated increased activity in those with symptomatic AAAs, and increased activity in focal areas of increased inflammation and collagen degradation.

INFLAMMATORY AORTIC ANEURYSM

A variant of aortic aneurysm, inflammatory abdominal aortic aneurysms (IAAAs) are characterized by peri-aneurysmal fibrosis and adhesions and are a surgical challenge. Surgical repair is associated with a higher morbidity and mortality than surgery for degenerative AAAs.

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