Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

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Technique

Computed Tomography

Contrast-enhanced computed tomography (CT) can visualize the major aortic branches on early-phase and venous branches on late-phase images. Late images also evaluate the portal phase. Most radiologists use a nonionic contrast agent, with a typical intravenous (IV) injection consisting of 150mL at a rate of about 4 to 5mL/sec. A faster injection rate produces earlier and greater vascular enhancement than a slower rate. A test bolus injection or automatic bolus triggering optimizes contrast enhancement (more details on CT angiography are in Chapter 7).

Three-dimensional (3D) image reconstruction is helpful in evaluating overlapping arteries and veins; the aorta, main arterial branches, as well as vena cava and portal vein branches are usually identified. Preoperative reconstructed 3D CT angiography is useful to surgeons in planning complex intraabdominal procedures, especially if unusual vascular paths are present. Reconstruction techniques include surface rendering, maximum intensity projection (MIP) and volume rendering; the latter utilizes more of the available data and generally is more accurate. A multiple threshold display technique decreases artifacts and allows better small vessel depiction than a shaded surface display technique. Each technique has its advantages and disadvantages (Fig. 17.1).

Similar to virtual colonoscopy, virtual CT arterial endoscopy is also feasible. In theory, a stenosis, aneurysm, or stent could be evaluated from its endoluminal surface. In practice, such applications are still being developed.

Computed tomography arterial portography and CT hepatic arteriography are discussed in Chapter 7. These techniques are of value primarily in the study of the liver blood supply. Evaluation of CT arterial portography images is not always straightforward, most often due to flow artifacts caused by nonopacified blood.

Ultrasonography

Doppler ultrasonography (US) measures blood flow velocity in larger vessels. Superior mesenteric artery and renal artery blood flow velocities can be measured even in neonates.

The use of intravascular contrast agents enhances portal vein and collateral blood flow during Doppler US studies. One IV US contrast agent is Perflenapent emulsion (EchoGen, Sonus Pharmaceuticals, Bothell, WA), providing enhancement lasting 5 to 15 minutes. Continuous infusion, rather than bolus injection, prolongs enhancement and decreases saturation artifacts.

Transesophageal echocardiography has evolved into a valuable thoracic aortic study, especially in a setting of suspected aortic dissection, postoperative repair, and follow-up. Echocardiography also evaluates the extent

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advantages and limitations. A 2D time-of-flight technique permits 3D reconstruction images. The phase-contrast technique is useful in evaluating portal vein blood; it measures flow direction and velocity.

The relative merits of MRA versus computed tomography angiography (CTA) are still evolving. Mention of angiography in CT implies the use of an intravascular contrast agent. This is not necessarily true with MRA, because MRA without contrast enhancement is feasible. Flowing fluid can be made to produce a signal void and thus appear black (black blood technique) or appear bright. Limitations of MRA without contrast include the presence of slow flow and spin-dephasing signal voids induced by turbulence. In general, contrastenhanced MRA of abdominal vessels is superior to noncontrast time-of-flight or phase-contrast MRA.

A T1-weighted 3D MRA technique using gradient-echo [fast imaging with steady-state precision (FISP)] with ultrashort echo and repetition times obtained during the arterial phase after IV contrast overcomes a number of limitations of no-contrast techniques; images are obtained within 20 to 40 seconds during a breath-hold. At times a subtraction technique using precontrast images is helpful. Contrastenhanced MRA of the portal vein is hampered by slow flow, and in this situation a time-of- flight technique is more appropriate.

Both 2D and 3D gadolinium-enhanced MRA evaluate major vessels. Thus MRA performed with four 3D acquisitions (at 0, 30, 60, and 90 seconds after IV gadolinium) provides an arteriogram (without veins) in most patients during the second or third acquisitions and a venogram by subtracting the arterial phase from an arteriovenous phase (third or fourth acquisition) (1); all major vessels, including portal vein and superior mesenteric vein, can be visualized, but secondary branches are better shown by conventional angiography. Veins <5mm in diameter are often not well seen with MRA.

Using gadolinium-enhanced MRA, MIP postprocessing provides vascular phase images that can then be reformatted into 3D images viewed along a 360-degree axis. The use of gadolinium results in arterial phase images without the need for subtraction, but a subtraction technique is necessary for venous-phase imaging. The entire intraabdominal aorta can be visualized with 3D

gadolinium-enhanced MRA, and this technique appears useful in studying aortic aneurysms and dissections. Virtual MR intraarterial endoscopy is feasible using gadoliniumenhanced gradient echo MRI and a postprocessing algorithm to obtain aortic and renal artery images; the clinical usefulness of such a technique remains to be established. Combining gadolinium-enhanced MRA with 3D phase contrast MRA provides additional information than is possible with each technique separately. Both the aorta and renal arteries can be evaluated; the former results in images similar to a contrast arteriogram, while the latter is based on flow characteristics and is useful to identify vascular stenoses.

Similar to CT, either a test dose or an assumed time interval establish a delay between the start of contrast injection and initial image acquisition for optimal arterial enhancement. Another approach is to monitor a single voxel within the aorta and use an increase in signal within this voxel (corresponding to the arrival of injected gadolinium) to trigger MR angiography. Such automatic triggering improves arterial enhancement compared to manual triggering; venous enhancement is also less than with manual triggering. The advantages of such automatic triggering can be used two ways—either to obtain increased arterial enhancement or a similar enhancement to manual triggering but using a reduced contrast dose.

Magnetic resonance contrast agents are discussed in more detail in Chapter 7. Ultrasmall superparamagnetic iron oxide (SPIO) particles shorten both T1 and T2 relaxation times, have a blood half-life measured in hours, and thus are known as blood-pool MR contrast agents. Equi- librium-phase 3D MRA using SPIO readily evaluates abdominal and pelvic arteries, but venous overlap limits their use. They also depict the portal vein and its major branches. These small iron oxide particles pass through capillaries and are eventually cleared by lymph nodes.

Contrast enhanced 3D MR portography identifies the main portal vein and right and left intrahepatic portal veins with their main branches (2); MR portography also detects varices and portosystemic shunts. In a setting of liver tumors it can confuse vein occlusion with narrowing.

Magnetic resonance venography is also feasible. An electrocardiogram (ECG)-triggered

black-blood half-Fourier acquisition singleshot turbo spin echo (HASTE) sequence assesses the vena cava and major veins. Good quality pelvic and abdominal vein images are obtained by using a 2D fast low-angle shot (FLASH) time-of-flight technique without breath-hold but with arterial presaturation (3). Major venous obstructions, thrombi, and intravascular tumors can be detected. Contrastenhanced 3D MRA is necessary for more detailed study.

Contrast enhanced MR lymphography, performed after interstitial or iv injection is theoretically feasible and undoubtedly will be performed in the future once appropriate contrast agents are available.

Scintigraphy

An estimate of portal blood shunting can be obtained with perrectal portal scintigraphy. A solution of technetium–99m (Tc-99m) pertechnetate is instilled into the rectum, and serial liver and heart scintigrams are obtained and a portal shunt index calculated. In a longitudinal study of patients with liver disease ranging from chronic hepatitis to cirrhosis and varices, the shunt index initially increased gradually as the patient’s disease progressed to cirrhosis and then increased rapidly as varices developed (4). The clinical role of this test is not clear.

Angiography

Systemic Circulation

Catheter-based DSA is an often-used gold standard when evaluating other imaging modalities. Although DSA is often supplanted in this role by CTA and MRA, conventional angiography continues to serve as a framework for angioplasty, stent placement, and various embolization techniques.

Compared to conventional DSA, 3D images obtained with subtraction angiography performed by rotating the x-ray tube during contrast injection (digital rotational subtraction angiography) tend to be superior in most patients (5).

Percutaneous arterial puncture and catheter manipulation should be performed with caution in patients with Behçet’s disease. These

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patients have a higher than normal prevalence of aneurysms and are at increased risk of (pseudo)aneurysm formation and thrombophlebitis at a puncture site. If feasible, either CTA or MRA is a viable alternate study.

Diagnostic renal angiography and percutaneous renal intervention procedures using carbon dioxide are options in patients with renal insufficiency.

Although gadolinium contrast has been occasionally used for angiographic studies and CT imaging in patients with renal insufficiency or prior severe reaction to an iodinated agent (6), one should keep in mind that the pharmacokinetic properties of Gd-DTPA, with only one gadolinium ion, are similar to iodinated agents containing 3 to 6 iodine atoms. Toxicity of gadolinium agents, at doses achieving equivalent x-ray stopping power, is greater than with nonionic iodinated agents (7). This is in distinction to the use of approved lower gadolinium MR doses, which are insufficient for useful x-ray contrast, but which have negligible nephrotoxicity (8). The European Society of Urogenital Radiology position is that gadolinium-based contrast agents are more nephrotoxic than iodinated contrast agents in equivalent x-ray attenuation doses (9) and their use for angiography and CT is not recommended.

Gelatin sponge is often used as an embolic material. Being inherently radiolucent, contrast agents are often mixed with it if visualization is desired. Some investigators find Lipiodol more useful than a water-soluble contrast material.

Portal System

Direct portography is performed either by injecting an intrahepatic portal vein branch or via splenic pulp injection (splenoportography). A transjugular approach through an intrahepatic portosystem shunt is an option in select patients. Indirect portography is an arteriographic procedure consisting of contrast injection into either the superior mesenteric or splenic arteries.

Direct transhepatic portography is useful to evaluate and treat some portosystemic collateral pathways in patients with portal hypertension; although this technique has been supplanted by newer procedures, in selected patients it pro-

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vides excellent opacification of the portal vein and its branches. Selective superior mesenteric vein catheterization permits dynamic portal venous blood flow studies. Direct splenoportography enjoyed its golden age in the early 1970s, being supplanted initially by other angiographic procedures and then by CT arterial portography. It is currently little used, although an occasional investigator rediscovers it as a “new procedure.” Computed tomographic portography performed by direct intrasplenic contrast injection using a needle-catheter assembly is a simple procedure having a role in an occasional patient when other access to the portal vein is impractical. Direct splenoportography using carbon dioxide through a small needle fills a useful but rather select niche (10).

Hepatic venography is performed by selective hepatic vein catheterization. If needed,a balloon occludes hepatic vein outflow, thus permitting contrast opacification of hepatic parenchyma, an aid in detecting intrahepatic venous collaterals.

Wedge hepatic venography using carbon dioxide is little used but at times is an adjunct to study the portal venous system. Carbon dioxide hepatic venography can visualize the portal vein in most patients and splenic and superior mesenteric veins in over half of patients.

Congenital Abnormalities

Aorta

Isolated congenital abdominal aortic abnormalities are uncommon; most are associated with other anomalies and are discussed in Chapter 14 under the heading Heterotaxy Syndrome.

Congenital abdominal aortic coarctation is uncommon. Symptoms in these patients range from hypertension, to intermittent claudication, and abdominal pain. A patient with Alagille’s syndrome and abdominal aortic coarctation also had an aberrant splanchnic blood supply (11).

Vena Cava

Most inferior vena caval abnormalities occur inferior to the renal veins. The absence of the infrarenal portion is rare; more often, mention

of an absent vena cava describes azygous continuation of a patent infrarenal segment, with hepatic veins draining directly into the right atrium. Such azygos inferior vena cava continuation is associated with cardiac and situs anomalies. In these patients the renal artery is located ventral to the azygos vein, a finding detectable by abdominal US. Azygous continuation of the vena cava is common in patients with polysplenia. Occasionally, indirect azygous continuation is associated with an inferior vena caval aneurysm. Hemiazygous continuation is rare.

Agenesis of other inferior vena cava segments is rare. One patient with agenesis of the hepatic segment had the infrahepatic vena cava draining into the portal vein and hepatic veins draining into a suprahepatic inferior vena cava (12). Hypoplasia of the inferior vena cava is also rare; these patients tend to have an extensive collateral circulation.

In inferior vena caval transposition a single vein ascends on the left, crosses the midline at the renal vein level, and continues its ascent to the right atrium. A duplicated left vena cava is rare; when present, the two vena cavae join at the renal vein level. A left vena cava can be differentiated from a dilated gonadal vein by tracing its cranial and caudal extensions. As expected, thrombophlebitis of a left vena cava is difficult to detect; CT and MR tend to suggest adenopathy, and aspiration biopsy or surgical exploration are often performed.

Arteries

An independent hepatic and splenic artery origin from the abdominal aorta occurs in approximately 1% of the population, an anomaly detected with contrast-enhanced CT and angiography. A common origin of the celiac, superior mesenteric, and inferior mesenteric arteries is very rare (13). A middle mesenteric artery, originating from the aorta, also a rare anomaly, usually supplies the right and transverse portions of the colon.

A hepatic artery identified by US in the portacaval space is often believed to have a superior mesenteric artery origin, yet one originating from the celiac artery can also be detected in this space.