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).

Coil Choice

Signal-to-noise ratio (SNR) can be maximized with a surface phased array torso coil, positioned to cover the region from the suprarenal abdominal aorta to the iliac vessels. More than one coil may be used in series to maximize range in the z-axis.

The extra SNR, greater signal uniformity, and number of coil elements provided by phased array torso coils enable the use of parallel imaging techniques, higher spatial resolution, more rapid signal processing (if there are enough receiver channels), and lower doses of intravenous contrast. In obese or especially tall patients, a body coil can be employed, although this will result in lower SNR.

MR Pulse Sequences

Precontrast Techniques

Localizers: During a breath-hold, localizers are initially obtained in the sagittal, coronal, and axial planes. Performing localizers during breath-hold ensures that the intra-abdominal organs will be located in a similar place to the post-gadolinium sequences which are typically acquired using breath-holding. Localizers can be performed using a variety of sequences, but nongated two-dimensional steady-state free precession sequences (SSFP) provide good tissue contrast and bright-blood images of vessels. Alternatively, HASTE (half-Fourier acquisition single shot turbo spin echo) or SSFSE (single shot fast spin echo) sequences can be performed, which provide a single breath-hold overview of the abdominal organs, with rapidly flowing blood appearing black.

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.

Figure 102-1 Coronal (A) and axial (B) steady-state free precession sequence. A rapid breath-hold sequence without intravenous gadolinium, providing an overview of the aneurysm (arrows) location, relationship to major branch vessels, size, and internal 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 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.

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.

Clinical Manifestations of Disease

Most AAAs are asymptomatic and are discovered incidentally on routine physical examination or during other imaging studies. Patients with ruptured AAAs often present with an abrupt onset of back pain as well as abdominal pain and tenderness. Patients may present critically ill. Most will have a pulsatile abdominal mass. Rupture has a high mortality rate, with 25% mortality prior to arriving at the hospital and an overall 30-day survival rate of approximately 10%.¹²

The rate of aneurysm growth increases with increasing diameter in concordance with Laplace law. Aneurysms smaller than 4 cm grow at 2 to 4 mm/year, those measuring 4 to 5 cm grow at 2 to 5 mm per year, and aneurysms >5 cm grow at 3 to 7 mm per year.¹³ The risk of rupture increases with aneurysm size and rate of aneurysm expansion. In the U.K. Small Aneurysm Trial,¹⁴ aneurysms with a cross-sectional diameter of 5 to 5.9 cm in size had an annual risk of rupture of 6.5%. Elective AAA repair carries a 4% to 6% mortality. Recommendations¹⁰ suggest elective repair of aneurysms greater than 5.5 cm in males, and greater than 4.5 to 5 cm in women. Aneurysm length is not thought to be associated with rupture risk.¹⁵

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

* Also consider referral to a vascular surgeon.

Imaging Techniques and Findings

Radiography

The presence of calcification in the abdominal aortic wall, although commonly present, is not invariable. Moreover, a tortuous and calcified aorta may mimic an AAA. A lateral radiograph (Fig. 102-2) may depict aortic calcification more clearly. However, radiographs are unreliable for diameter measurements of the aortic wall and are not recommended for diagnosis or surveillance.¹⁷

Figure 102-2 Lateral radiograph of an abdominal aortic aneurysm. Fusiform dilation of the infrarenal aorta with calcification of the intima (arrows). The superior and inferior extent of the aneurysm sac are not visible.

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.

Figure 102-3 A and B, Abdominal aortic aneurysm on ultrasound. Transverse and sagittal ultrasound images of the aorta demonstrate a small aortic aneurysm, not appropriate for surgical repair. C, Abdominal aortic aneurysm containing mural thrombus. Measurements of the aneurysm are from outer wall to outer wall, not the caliber of the patent lumen.

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

Figure 102-4 Bedside ultrasound (A) and subsequent CT (B and C) on a hypotensive patient with a pulsative abdominal mass. On ultrasound, a 9 cm aneurysm (asterisk) was diagnosed, but retroperitoneal blood was not suspected. On subsequent CT (B), a small periaortic hematoma was identified (arrow), in addition to a “draped aorta sign” of a deficient posterior wall. More inferiorly (C), a small fissure has formed within the posterior aspect of intramural thrombus (arrow). Ultrasound is not accurate in excluding small retroperitoneal ruptures.

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.

Figure 102-5 Infrarenal abdominal aortic aneurysm (arrow) demonstrated on maximum intensity projection (MIP) reconstruction of a gadolinium-enhanced MRA. The neck of the aneurysm and relationship to the renal arteries is clearly shown, as are the associated stenoses of the renal arteries and common iliac arteries.

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).

Figure 102-6 Noncontrast CT of the abdomen demonstrates a high-density crescent in the periphery of the aneurysmal wall suggesting impending rupture (arrow).

Figure 102-7 In patients unable to receive intravenous contrast, noncontrast CT is useful in evaluating the presence of an aneurysm and retroperitoneal hematoma. A large left heterogeneous retroperitoneal hematoma is present on noncontrast abdominal CT. Crescentic hyperattenuating thrombus is also present within the posterior wall (arrows).

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.

Figure 102-8 A, Early rupture of an abdominal aortic aneurysm. Contrast-enhanced axial CT of the abdomen shows a large abdominal aortic aneurysm with adjacent fat stranding (arrow) suggesting aortic leak or impending rupture. B, Contrast-enhanced coronal CT of the abdomen demonstrates a saccular aneurysm of the infrarenal aorta (black arrow) with adjacent fat stranding (white arrows) suggesting aortic leak or impending rupture.

Figure 102-9 A and B, Ruptured abdominal aortic aneurysm. Contrast-enhanced axial CT and VR images of the abdomen demonstrate a saccular infrarenal abdominal aortic aneurysm with frank rupture into the left retroperitoneum. Active contrast extravasation is also present (arrows). C, Contrast-enhanced coronal CT of the abdomen demonstrates a saccular infrarenal abdominal aortic aneurysm with a massive left retroperitoneal hemorrhage (thin arrows). Active contrast extravasation is again present (thick arrow). Sparing of the perinephric space excluded the kidney as the bleeding source.

Figure 102-10 Two ruptured abdominal aortic aneurysms. A, Intraperitoneal contrast extravasation (arrow) is uncommonly encountered. B, More commonly retroperitoneal extravasation is seen (arrow).

Figure 102-11 A, Abdominal aortic aneurysm with a fistula between the aorta and inferior vena cava. Axial images from arterial phase of a contrast-enhanced CTA demonstrates a large infrarenal abdominal aortic aneurysm; a crescentic structure just lateral to the aorta demonstrates enhancement during this arterial phase study (arrow). B, Sagittal reconstruction demonstrates a fistula connection between the abdominal aortic aneurysm and the inferior vena cava (arrow). C, Volume rendered AP image of the aorta demonstrates contrast filling the right iliac vein and the lower IVC (arrows).

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.

Differential Diagnosis

Nonruptured AAAs are often asymptomatic. Pain associated with a ruptured aneurysm is nonspecific. Other diagnoses which could manifest similarly include biliary disease, renal colic, diverticulitis, pancreatitis, cardiac ischemia, and mesenteric ischemia.

The imaging findings of AAA are pathognomonic in the correct clinical situation. The differential diagnosis of atherosclerotic AAA includes mycotic aneurysms and traumatic pseudoaneurysms.

Treatment Options

Definition

Inflammatory abdominal aortic aneurysms are defined by the presence of a thickened aneurysm wall, marked peri-aneurysmal and retroperitoneal fibrosis, and dense adhesions of adjacent abdominal organs.¹¹

Prevalence and Epidemiology

IAAAs constitute approximately 3% to 10% of abdominal aortic aneurysms. Male sex and smoking are both strong risk factors, with a male-to-female ratio ranging from 6 : 1 to 30 : 1. From 77% to 100% of patients smoke.¹¹ Other risk factors include northern European descent and autoimmune disease.

Etiology and Pathophysiology

Like AAA, the etiology of IAAAs is multifactorial. Current understanding suggests a common immune-mediated inflammatory pathogenesis shared with degenerative AAA, but in IAAA, the inflammation is more pronounced. White blood cells in the aortic wall cause degradation of the extracellular matrix by releasing proteolytic enzymes and cytokines. Based on this knowledge, steroid and other anti-inflammatory agents have been found by some to inhibit aneurysm growth. Some have postulated a role of infection in the aortic wall. Suggested agents include herpes simplex virus, cytomegalovirus, and Chlamydia organisms.

CLINICAL MANIFESTATIONS OF DISEASE

Unlike degenerative AAAs, in which back or flank pain is present in a minority of patients, a great majority of patients with IAAAs have back pain. In patients with a known aneurysm, the classic triad of back pain, weight loss, and elevated erythrocyte sedimentation rate (ESR) is highly suggestive of an inflammatory aneurysm.