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 150 mL at a rate of about 4 to 5 mL/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 reconstruc- tion is helpful in evaluating overlapping arter- ies 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 accu- rate. A multiple threshold display technique decreases artifacts and allows better small vessel depiction than a shaded surface display tech- nique. 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 prima- rily in the study of the liver blood supply. Eval- uation 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 mesen- teric artery and renal artery blood flow veloci- ties 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. Continu- ous infusion, rather than bolus injection, pro- longs enhancement and decreases saturation artifacts.
Transesophageal echocardiography has evolved into a valuable thoracic aortic study, especially in a setting of suspected aortic dis- section, postoperative repair, and follow-up.
Echocardiography also evaluates the extent
Abdominal Vasculature
975
of atherosclerosis, aortic ulcers, intramural hematomas, and blood flow.
Intravascular US holds promise in evaluating vessel wall morphology. It has been called a technology in search of an application and is gradually finding a role in endovascular inter- vention procedures.
Magnetic Resonance
Currently magnetic resonance angiography (MRA) is an alternate vascular imaging modal- ity in patients with a contraindication to iodi- nated contrast agents. Its future indications will undoubtedly increase considerably, mostly at the expense of invasive angiography.
Intraluminal blood has a varied MR appear- ance depending on the technique used; it ranges from a signal void on spin echo (SE) sequences to being hyperintense on gradient-recalled echo (GRE) sequences. Conventional T1- and T2- weighted SE techniques result in flowing blood appearing dark. An exception is with slow- flowing blood, which tends to mimic a thrombus and appears bright. Due to the T1-
shortening effect of IV contrast, contrast- enhanced MRA sequences result in flowing blood being bright while stationary objects are saturated and appear “dark.” A further refinement of this technique is subtraction of precontrast images from postcontrast ones, improving vessel contrast resolution.
Compared to digital subtraction angiography (DSA), the sensitivity and specificity percents of MRA in detecting major vessel stenoses or occlusions are in the high 90% range. Currently, the major constraints of abdominal vessel MRA include the pulsatile nature of blood flow, respiratory motion, and peristalsis. The use of heavily T2-weighted fast turbo spin echo sequences instead of conventional T2-weighted SE sequences improves image quality. Fast GRE sequences allow dynamic contrast-enhanced vascular studies using single breath-hold image acquisition and result in few motion artifacts.
Useful techniques include time of flight and
phase contrast. The former consists of a varia-tion in signal intensity with time due to proton motion within a magnetic field, while the latter monitors phase variations; each has certain
Figure 17.1. Computed tomography (CT) portal vein techniques.A: Maximum intensity projection. B: Shaded surface display. C:
Volume-rendered CT portal venography. Dilated left gastric vein (arrow) is seen in A.The maximum intensity projection technique provided best visualization of collateral vessels. (Source: Kang HK, Jeong YY, Choi JH, et al. Three-D Multi-detector Row CT Portal Venography in Evaluation of Portosystemic Collateral Vessels in Liver Cirrhosis. RadioGraphics 2002;22:1053–1061, with permis- sion from the Radiological Society of North America.)
A B
C
advantages and limitations. A 2D time-of-flight technique permits 3D reconstruction images.
The phase-contrast technique is useful in eval- uating portal vein blood; it measures flow direc- tion and velocity.
The relative merits of MRA versus com- puted 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, contrast- enhanced 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 rep- etition times obtained during the arterial phase after IV contrast overcomes a number of limi- tations 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. Contrast- enhanced 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 arte- riogram (without veins) in most patients during the second or third acquisitions and a venogram by subtracting the arterial phase from an arte- riovenous 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 con- ventional angiography. Veins <5 mm in diame- ter are often not well seen with MRA.
Using gadolinium-enhanced MRA, MIP post- processing 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 gadolinium- enhanced gradient echo MRI and a postpro- cessing 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 con- trast MRA provides additional information than is possible with each technique separately.
Both the aorta and renal arteries can be evalu- ated; 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 acquisi- tion 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 enhance- ment compared to manual triggering; venous enhancement is also less than with manual trig- gering. The advantages of such automatic trig- gering 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 dis- cussed 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 eval- uates 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 feasi-
ble. An electrocardiogram (ECG)-triggered
black-blood half-Fourier acquisition single- shot 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 intra- vascular tumors can be detected. Contrast- enhanced 3D MRA is necessary for more detailed study.
Contrast enhanced MR lymphography, performed after interstitial or iv injection is the- oretically feasible and undoubtedly will be per- formed 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) pertech- netate 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 stan- dard when evaluating other imaging modalities.
Although DSA is often supplanted in this role by CTA and MRA, conventional angiography con- tinues to serve as a framework for angioplasty, stent placement, and various embolization techniques.
Compared to conventional DSA, 3D images obtained with subtraction angiography per- formed by rotating the x-ray tube during con- trast injection (digital rotational subtraction
angiography) tend to be superior in mostpatients (5).
Percutaneous arterial puncture and catheter manipulation should be performed with caution in patients with Behçet’s disease. These
patients have a higher than normal prevalence of aneurysms and are at increased risk of (pseudo)aneurysm formation and throm- bophlebitis at a puncture site. If feasible, either CTA or MRA is a viable alternate study.
Diagnostic renal angiography and percuta- neous renal intervention procedures using carbon dioxide are options in patients with renal insufficiency.
Although gadolinium contrast has been occa- sionally 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 pharmacoki- netic 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 equiva- lent x-ray stopping power, is greater than with nonionic iodinated agents (7). This is in dis- tinction to the use of approved lower gadolin- ium 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 intrahep- atic portosystem shunt is an option in select patients. Indirect portography is an arterio- graphic procedure consisting of contrast injec- tion 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-
vides excellent opacification of the portal vein and its branches. Selective superior mesenteric vein catheterization permits dynamic portal venous blood flow studies. Direct splenoportog- raphy enjoyed its golden age in the early 1970s, being supplanted initially by other angiographic procedures and then by CT arterial portogra- phy. It is currently little used, although an occa- sional investigator rediscovers it as a “new procedure.” Computed tomographic portogra- phy 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 abnormal- ities 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 con- tinuation of a patent infrarenal segment, with hepatic veins draining directly into the right atrium. Such azygos inferior vena cava con- tinuation 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 continua- tion 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 drain- ing into the portal vein and hepatic veins drain- ing into a suprahepatic inferior vena cava (12).
Hypoplasia of the inferior vena cava is also rare;
these patients tend to have an extensive collat- eral 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 mesen- teric arteries is very rare (13). A middle mesen- teric 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 por-
tacaval 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.
Veins
Systemic Veins
A retroaortic left renal vein, located either at the same level as a normal renal vein or more caudally, occurs in about 5% of the population (Fig. 17.2). Circumaortic left renal veins consist of a true vascular ring and also occur roughly in about 5%, findings detected with contrast enhanced CT. Gadolinium enhanced 3D MRA detects a retroaortic and circumaortic left renal veins with roughly the same frequency.
An abdominal aortic aneurysm can compress a retroaortic left renal vein and result in renal vascular congestion and induce hematuria (14).
Although at times called a nutcracker phenom- enon, this term is best avoided to prevent con- fusion with other similarly named conditions.
Portal Venous System
Congenital portal venous absence is rare; most occur in females and tend to be associated with liver tumors and other congenital abnormalities (Fig. 17.3). With an absent portal vein, intestinal
Figure 17.2. Retroaortic left renal vein. Frontal (A) and oblique (B) 3D CT reconstructions reveal the left renal vein (arrows) poste- rior to a tortuous aorta. (Courtesy of Patrick Fultz, M.D., University of Rochester.)A B
Figure 17.3. Congenital absence of portal vein in an 11-year-old boy. A: Transverse magnetic resonance (MR) imaging shows the splenic vein (arrows) joining the superior mesenteric vein and emptying into the inferior vena cava. B: An MR angiogram identifies the superior mesenteric vein (arrow) draining into inferior vena cava. (Source: Kohda E, Saeki M, Nakano M, et al. Congenital absence of the portal vein in a boy. Pediatric Radiology 1999;29:235–237, with permission from Springer-Verlag.)
A B
and splenic venous blood bypasses the liver and drains directly into systemic veins. Some of these patients develop hepatofugal drainage of a large inferior mesenteric vein into systemic veins.
Congenital portal vein duplication or an accessory portal vein is also rare. Some of these mimic a liver hilar tumor. At times direct portography shows an accessory smaller vein located parallel to the main portal vein and draining into the right liver lobe; some of these accessory portal veins drain the coronary veins, thus modifying results obtained after a transjugular intrahepatic portosystemic shunt (TIPS) and preventing coronary vein embolization.
A portal vein located cranial to the gallblad- der bed and feeding the right anterior segment is associated with rightward deviation of the ligamentum teres. A portal vein located anterior to the duodenum (and thus prepancreatic in location) is associated with malrotation and polysplenia. Computed tomography and mag- netic resonance imaging (MRI) reveal a vascu- lar structure anterior to the head of the pancreas.
Aberrant gastric venous drainage is common and accounts for some of the unusual liver enhancement patterns detected.Veins of Retzius are intestinal veins draining directly into vena cava or its branches—usually the gonadal or renal veins—rather than into portal vein branches. Whether they represent a normal variant or should be considered congenital anomalies is conjecture. These veins can be identified if searched for, including with CT arterial portography.
Ultrasonography can detect portocaval anas- tomoses in infants; these vessels probably rep- resent continued ductus venosus patency.
Normally the superior mesenteric vein lies to the right and anterior to the superior mesen- teric artery. A reversed position of these two vessels suggests but is not pathognomonic of midgut malrotation.
An aberrant right gastric vein supplies segment 4 of the liver and is a cause of a pseudolesion during contrast-enhanced CT or CT portography. An aberrant left gastric vein is less common and is identified on postcontrast CT along the hepatogastric ligament. These veins provide a partial collateral pathway for the portal system.
Trauma
Abdominal aortic injury is considerably less common than to the thoracic aorta. Neverthe- less, abdominal aortic and inferior vena caval injuries are associated with a high morbidity and mortality. Unchecked bleeding, shock, and injury superior to the renal vessels all play a role in increased mortality.
On rare occasions intravascular gas is detected after trauma; portal venous gas, hepatic venous gas, and inferior vena cava gas in five patients cleared on follow-up studies and no cause was determined (15).
Aortic Injury
Thoracic aortic injuries are not discussed in this book, but mention should be made of delayed aortoesophageal fistulas developing after swal- lowing sharp objects or after gunshot wounds.
Abdominal aortic injury should be suspected with lumbar spine transverse process fractures.
Handlebar injuries to the duodenum are well known to trauma physicians. Less well known is aortic injury due to a similar mechanism.
Fatal delayed abdominal aortic ruptures have occurred after handlebar injury.
In the United States, contrast-enhanced CT is the current imaging modality used to evalu- ate suspected aortic trauma. A periaortic hematoma detected near the diaphragm level is an insensitive but specific sign of aortic injury (16).
Blunt abdominal aortic injury carries a high morbidity and mortality. Some traumatic inframesenteric abdominal aortic dissections can be successfully treated with implanted stents. In general, surgical repair is preferred for an unstable patient or those with threatened extremities, but angiographic endovascular stent placement appears to be an option in a stable patient with viable limbs.
Arterial Trauma
An arterial (pseudo)aneurysm is a known com- plication of trauma in both children and adults.
The hepatic artery is a common location for
these aneurysms, although any artery, even the
inferior epigastric artery, can be involved. These
aneurysms tend to remain silent until manifest-
ing by bleeding, often weeks or, rarely, even years later. Most such delayed bleeding is into the gastrointestinal tract, with an occasional one to respiratory or urinary tracts. At times bleed- ing is massive, stops spontaneously, but invari- ably recurs later.
Less common are posttraumatic arteriove- nous fistulas; most are due to penetrating injury. Although CT is often useful in suggest- ing the diagnosis, arteriography is generally necessary to define the location and extent of these fistulas.
Many detected aneurysms are treated suc- cessfully by transcatheter embolization. An unresolved question is whether such emboliza- tion is indeed advantageous to the patient or whether conservative management achieves a similar end, especially with smaller, uncompli- cated aneurysms. In patients with arterial injuries managed nonoperatively, follow-up arteriography reveals that some injuries heal spontaneously, some improve with residual deformity and others worsen; the dilemma is that pretherapy imaging is not accurate enough in selecting those who are expected to improve and those who progress.
Venous Trauma
Trauma to the inferior vena cava consists of con- tusion, laceration, transection, and, rarely, dis- section. In particular, injury to the retrohepatic portion of the vena cava is associated with a poor prognosis.
Peritoneal lavage readily misses inferior vena cava injuries. Typically CT shows a hematoma surrounding the vena cava. Some hematomas are associated with an intraluminal thrombus.
At times CT in patients with traumatic inferior vena cava rupture identifies contrast extravasa- tion. Computed tomography detection of a col- lapsed inferior vena cava during a trauma study suggests hypovolemia.
Trauma is a rare cause of Budd-Chiari syn- drome. Presumably bleeding and extrinsic compression of the intrahepatic inferior vena cava by a hematoma obstruct the hepatic veins or intrahepatic portion of the inferior vena cava, findings detectable by CT. Ascites is common and should be differentiated from intraperi- toneal bile or blood, which suggest other etiolo- gies for intraperitoneal fluid. An occasional patient develops traumatic Budd-Chiari syn-
drome secondary to inferior vena cava throm- bosis. On the other hand, with hepatic vein rupture generally not enough time elapses for Budd-Chiari syndrome to develop.
Bleeding
Active hemorrhage in a trauma setting is recognized as contrast extravasation during contrast-enhanced CT. Rapid infusion of a relatively large contrast bolus aids in detecting a site of arterial extravasation, less so with venous bleeding. Multislice CT appears to be superior to more conventional CT in detecting more subtle bleeds. Even if active extravasation is not detected, a high-density region, having a Hounsfield density close to that of adjacent arteries and surrounded by lower density clot, should suggest recent extravasation. These are often subtle findings requiring expertise in interpretation.
Occasionally Tc-99m–red blood cell scintig- raphy locates a bleeding site; the radiotracer accumulates in one particular region, with no change in location during the study.
The appearance of a pseudoaneurysm is similar to that of arterial bleeding (Fig. 17.4).
Some authors consider bleeding to represent a
Figure 17.4. Traumatic pseudoaneurysm secondary to gunshot wound. Computed tomography identifies the aneurysm (arrows) between the aorta and inferior vena cava. (Courtesy of Patrick Fultz, M.D., University of Rochester.)
pseudoaneurysm if it is contained, but this distinction is rather subtle. In either case, a pseudoaneurysm should be isodense to its connecting vessel and, being contained, should be sharply marginated.
Aorta
Atherosclerosis/Stenosis
For unknown reasons, smokers and patients with chronic pancreatitis have a higher preva- lence of aortic calcifications than controls. Alag- ille syndrome includes arteriohepatic dysplasia;
occasionally aortic calcifications develop even in teenagers with this syndrome.
High-resolution T2-weighted MRI aids in detecting and classifying atherosclerotic plaques (17).
Abdominal aortic stenosis is occasionally dis- covered in children. A majority of these consist of congenital malformations and a minority of inflammatory aortitis; associated renal and vis- ceral artery involvement is common, together with arterial hypertension. Some stenoses in young adults are isolated, with the aortic bifur- cation being a common site; some of these stenoses calcify, even in young patients. Athero- sclerotic aortic stenoses are quite common in the elderly, although clinically they are often overshadowed by stenoses at the origin of great vessels.
MRA is useful in detecting and evaluating aortic stenoses.
Isolated stenoses are amenable to surgical correction. More common is diffuse involve- ment of the aorta, iliac arteries and distal vessels. Percutaneous transluminal balloon angioplasty and, if needed, intraluminal stent placement are the interventional modalities used to treat aortic stenosis. Although generally performed under angiographic guidance, intravascular US guidance is also feasible.
Angiography alone probably underestimates vessel diameter in almost two-thirds of patients;
incomplete stent deployment is also more readily identified by intravascular US than by angiography.
Aortic stent placement is feasible in patients with failure of percutaneous transluminal angioplasty or presence of ulcers, which increases risk of embolization with angioplasty.
In general, similar long-term restenosis rates are found for transluminal angioplasty and stent placement (18); a small aortic diameter is a pre- dictive factor for restenosis.
Follow-up after percutaneous transluminal angioplasty of patients with infrarenal athero- sclerotic aortic stenosis shows a clinical patency rate similar to open surgery.
Aneurysm
Atherosclerotic Aneurysm
Most abdominal aortic aneurysms are athero- sclerotic in origin. An occasional mycotic one is encountered.
Screening for an abdominal aortic aneurysm is not widely practiced even in hypertensive patients. An abdominal aortic aneurysm in these patients is associated with claudication, and these patients appear to benefit from screening US. An occasional aortic aneurysm is associated with a coagulopathy, which often clears after aneurysm repair.
From a potential therapeutic perspective, an abdominal aortic aneurysm’s size and location are of obvious importance. One classification is into infrarenal, juxtarenal, and pararenal aneurysms. Most common are infrarenal ones, fusiform in shape. Pararenal aneurysms extend distal to the superior mesenteric artery and involve the renal arteries.
The role of imaging is to establish that an aortic aneurysm is indeed present, provide information about its size and shape, detect complications, and outline the preoperative anatomy.
Aneurysms vary in size considerably.
Measurement of aneurysm dimensions before endovascular therapy is of obvious importance, yet DSA measurements of an aneurysm’s diam- eter and length are inaccurate by up to 15% (19);
an indwelling catheter is the only available reference standard. Computed tomography, US, or MRA provides more reliable aneurysm dimensions.
Calcifications develop in long-standing aneurysms, but aortic calcifications do not imply that an aneurysm is present. An occa- sional aneurysm is suggested from a conven- tional abdominal radiograph, but this study is rarely employed when suspecting an aneurysm.
In particular, measurement of a suspected
aneurysm’s diameter is notoriously inaccurate on conventional radiographs, but if such a measurement is necessary, a lateral projection rather than a frontal one should be chosen.
Computed tomography angiography is the current preoperative imaging study of choice.
Numerous studies confirm accuracy of CT in measuring an aneurysm’s diameter. Multidetec- tor CT in patients evaluated for aortoiliac aneurysms, performed with a 25-second delay after the start of IV contrast injection resulted in aortic enhancement of >200 Hounsfield units (HU) and no superimposed venous filling (20). A 3D shaded surface display is helpful in viewing aneurysms, with image rotation providing multidirectional images. In differ- entiating among suprarenal, juxtarenal, and infrarenal aneurysms, the use of narrrow colli- mation and overlapping axial reconstructions aids in correctly classifying most aneurysms and identifies main and accessory renal arteries.
Ultrasonography should identify an aortic aneurysm, outline its shape, and measure its width and rough length in most patients and is often the first screening examination obtained in a patient with a pulsatile abdominal mass.
Involvement of other vessels is difficult to evaluate. Obesity and bowel gas limit the information obtained. Doppler US does provide additional information but is generally sup- erfluous if subsequent angiography (CT or other) is obtained.
Magnetic resonance imaging and MRA are evolving into primary imaging modalities for preoperative aneurysm study, at times pro- viding information superior to CT. Potentially 3D MRA can provide definitive pretherapy studies and replace both CT and DSA. Some studies achieve almost perfect agreement between conventional angiography and MRA interpretations of aortic disease. Conventional angiography is often considered the gold stan- dard in aneurysm evaluation primarily because many surgeons are comfortable with its results and rely on its information. Nevertheless, the use of angiography is declining for this indica- tion and is gradually being relegated to those situations where CT and MR are inconclusive.
Magnetic resonance imaging can identify an aneurysm’s size and shape. Transverse images tend to provide more information about the distal aorta and iliac vessels compared to coronal images, but sagittal images are helpful
in identifying major vessel origins. Gadolinium- enhanced MRA achieves high sensitivity and specificity in determining whether the renal arteries and iliac arteries are involved by an aneurysm; also, gadolinium aids in differen- tiating between slow blood flow and a mural thrombus. A 3D gadolinium-enhanced MRA technique identifies an aneurysm and aids in establishing its relationship to renal and other major arteries. Stenosis of major vessels is also detected and atherosclerotic plaques and thrombi are evaluated.
One inherent limitation of MRI is its inabil- ity to visualize calcifications. Also, multiple sequences are generally necessary. Thus although arterial-phase MRA visualizes a patent aortic lumen, it does not outline the aortic wall or identify thrombi; for the latter axial imaging is necessary.
Inflammatory Aneurysm
An inflammatory abdominal aortic aneurysm is characterized by marked thickening and inflammation in the aneurysm wall. The etiol- ogy is unknown, although an immune response appears to be involved in some patients.
Many inflammatory aneurysms are associ- ated with extensive surrounding extraperitoneal fibrosis. At times the ureters become encased and obstruct. The duodenum or inferior vena cava can also be entrapped. The inflammation often subsides after aneurysm repair and ureteric obstruction is relieved. Computed tomography performed several years after inflammatory aortic aneurysm repair reveals no or little persisting inflammatory tissue in most patients.
A MR study of an inflammatory aortic aneurysm reveals a complex, concentric, layered outline; homogeneous enhancement postcon- trast distinguishes this condition from the more common atheromatous intima.
Dissecting Aneurysm
Most dissecting aortic aneurysms are thoracic
in origin and dissect into the abdomen. These
aneurysms are generally subdivided into those
involving the ascending aorta and those origi-
nating distal to the great vessels. A dissection is
diagnosed by detecting both true and false
lumens and identifying an intimal flap. Most
false aneurysms are saccular in outline and communicate with the aortic lumen. Some false lumens spiral around the aortic circumference;
in these, dissection the inner lumen is invariably the true lumen (21).
Aortography was the traditional gold stan- dard in detecting a dissection, identifying an intimal flap, and establishing an entry site and possible exit site. Postcontrast CT and lately MRI are evolving as noninvasive alternatives to aortography in suggesting the extent and length of a dissection (Fig. 17.5). A helical CT study
evaluating various protocols concluded that an optimal CT study consists of two separate but adjacent scans—3-mm collimation for the aortic arch and 5-mm collimation for remain- ing aorta (22); such a study achieved almost a 100% sensitivity and specificity for detecting a dissection and identifying entry and exit sites.
Some acute dissecting aneurysms have an eccentric hyperdense aortic wall on precontrast CT. A hyperdense wall is also found in some intramural hematomas and nondissecting
Figure 17.5. Dissecting abdominal aortic aneurysm. Transverse (A), coronal (B), and 3D (C) CT reconstructions each provide slightly different information about the shape and scope of this aneurysm. (Courtesy of Patrick Fultz, M.D., University of Rochester.)
A
B C
aneurysms, but with the latter the hyperdensity is circumferential rather than eccentric.
Contrast-enhanced CT in a minority of patients with a dissecting aneurysm reveals linear hypodense structures in the false lumen;
these fibroelastic bands, described as aortic
cobwebs, extend from the intimal flap and rep-resent portions of aortic wall incompletely sheared during dissection. These cobwebs identify the false lumen and distinguish it from the true lumen.
Ultrasonography identifies most true and false lumens, separated by an intimal flap. Some false-positive Doppler imaging results are prob- ably due to a heavily calcified aorta acting as a strong acoustic reflector, resulting in mirror image artifacts. At times a crescent-shaped hypoechoic zone between a thrombus and aortic wall suggests dissection, but Doppler US does not detect any flow in this region, a condi- tion called pseudodissection.
Intravascular US also differentiates the true from the false lumen. It identifies an acute angle between the dissecting flap and false lumen outer wall, and differentiates the three-layered true lumen wall from single-layered false lumen wall. Intravascular US is also useful to differen- tiate causes of branch vessel compromise—
whether a dissection intersects a branch vessel origin or whether a vessel origin was spared but a dissection flap simply compresses the true lumen and covers its origin.
Magnetic resonance detects an intimal flap and identifies both true and false lumens.
Flowing blood results in a signal void in both lumens with a SE technique, a finding modified by slow flow or a thrombus. As an aneurysm diameter increases, the false-to-true lumen cross-sectional area ratio also increases, but peak average velocity in the true lumen decreases, findings obtained from MR phase- contrast images. T1-weighted SGE images reveal slow flow in either a true or false lumen as high signal intensity, although, in general, postcon- trast 2D or 3D techniques better identify both lumens and an intimal flap, and differentiate slow flow from a thrombus. Contrast-enhanced MR establishes flow patterns in a dissection, including flow in major vessels, and is useful if iodine-based contrast agents are contraindi- cated. Yet exceptions do occur; in an occasional patient aortic wall thickening is the only sign of dissection on T1-weighted MRI.
The diagnosis is difficult with a thrombosed false lumen; the appearance is similar to that of an eccentric mural thrombus. An intimal flap is not detected with a thrombosed false lumen.
Inward displacement of intimal calcifications suggests dissection, but this appearance is mimicked by calcifications within a thrombus and an intramural hematoma. A thrombosed false lumen does not enhance with contrast during an early phase; for reasons unknown, occasionally a false lumen enhances on late- phase images.
Intramural Ulcer/Hematoma
A penetrating atherosclerotic ulcer-like defect is detected in some patients with extensive ather- osclerosis, an entity probably distinct from a dissecting aneurysm. The spectrum of findings ranges from an asymptomatic ulcer penetrating through lamina elastica and forming an intra- mural hematoma, to intramural dissection, aneurysm formation, and even aortic rupture.
The evidence suggests that a hematoma, caused by bleeding from vasa vasorum, is the inciting event in this condition, weakening the aortic wall sufficiently to lead to a dissection.
Follow-up CT of some of these ulcer- intramural hematomas reveals the hematoma regressing in size with time (23); some of these patients, however, have ulcer-like cavities that tend to evolve into saccular aneurysms. In others, an intramural hematoma without a patent false lumen is associated with intramural dissection; nonenhanced CT of these focal hematomas often shows a hyperdense rim. A typical appearance is a focal outpouching in a region involved by atherosclerosis and sur- rounded by a thickened, hyperdense aortic wall that does not enhance with contrast. Aortic intramural hematomas probably need to be followed with imaging.
Transesophageal US identifies an intramural hematoma as a homogeneous aortic wall thick- ening with intraluminal displacement of intimal calcification.
Hematomas appear hyperintense on T1- and T2-weighted MR images.
Mycotic Aneurysm
An abdominal aneurysm in a patient with fever,
abdominal or back pain, and leukocytosis sug-
gests a mycotic etiology, a life-threatening con- dition. Aneurysm wall tissue and blood often yield positive blood cultures for the involved organism. Complicating matters in the elderly, some atherosclerotic aneurysms become infected. These aneurysms tend to change size faster than atherosclerotic aneurysms.
Less common organisms encountered include
Salmonella enteritides and other Salmonellaspecies, Yersinia enterocolitica, and even a hydatid origin. Syphilitic aneurysms are mostly of historical interest. Rare reports describe
Mycobacterium bovis infected aortic aneurysmsafter intravesical bacillus Calmette-Guérin therapy for bladder cancer, even to the point of aneurysm rupture. Some aneurysms develop from an adjacent spinal abscess.
Most of these aneurysms are saccular.
Early-stage CT findings consist of a hazy aortic wall outline, evolving into a periaortic infiltrate and paraaortic fluid. The false lumen is often thrombosed. An increase in surrounding fat density can suggest the diagnosis, although similar findings are seen with other causes of extraperitoneal fibrosis or hemorrhage. Aortic wall gas is only an occasional finding.
Complications Rupture
The sudden onset of severe pain is the hallmark of a ruptured abdominal aortic aneurysm. The risk of rupture is directly related to aneurysm size. Rupture continues to be associated with high morbidity and mortality. It is fatal without surgical repair and is considered a surgical emergency. Rupture is unpredictable and can occur during a radiologic examination. Patients with a leaking aneurysm are often hypotensive and hemodynamically unstable. A ruptured aortic aneurysm occasionally occludes the infe- rior vena cava. A chronic ruptured but con- tained abdominal aortic aneurysm can result in extensive anterior lumbar spine erosions, findings readily detected with imaging.
A hemodynamically stable patient suspected of harboring a ruptured abdominal aortic aneurysm usually undergoes a CT study. The aneurysm is recognized and the extraperitoneal hemorrhage identified as isodense or hyper- dense regions. Less often detected is an actual aortic wall defect. Extravasation of contrast signifies active bleeding.
The CT crescent sign, consisting of a hyper- dense curvilinear structure within a mural thrombus or aneurysm wall, is found in larger aortic aneurysms and often signifies an aneurysm at risk of rupture or one already rup- tured. This sign appears to be caused by hem- orrhage within a thrombus or in the aneurysmal wall. It is more common in ruptured aneurysms, and if detected in an unruptured aneurysm, suggests impending rupture. A periluminal halo, on the other hand, consisting of a hypo- dense internal structure within a thrombus around the lumen, is found in roughly 10% of both nonruptured and ruptured aneurysms and does not have a connotation similar to that of the crescent sign.
In a Dutch study of consecutive patients admitted with a ruptured abdominal aortic aneurysm, an US diagnosis was consistent with rupture in only 51% (24).
A Tc-99m–red blood cell study will occasion- ally suggest rupture of an aortic aneurysm.
Fistula
Vascular fistulas in general are discussed later (see Vascular Fistulas).
A rare ruptured atherosclerotic aortic aneurysm results in an aortocaval fistula.
Surprisingly, some of these patients are rela- tively asymptomatic. These fistulas can be detected by CT. Early caval contrast enhance- ment is diagnostic. Multislice CT with over- lapping slices improves anatomic depiction of these fistulas.
An aortoduodenal fistula and resultant massive gastrointestinal hemorrhage is a recog- nized complication after aortic aneurysm repair but is quite rare in the absence of prior surgery.
An occasional small aneurysm communicates with a cavity and adjacent duodenum (25); the diagnosis can be suggested by CT.
Mural Thrombus
Mural thrombi are common in an aortic
aneurysm. These thrombi tend to be soft and
loosely attached to the aortic wall; as a result,
they are a source of distal emboli. These thrombi
do not strengthen the wall of an aneurysm. In
time, calcifications develop within a thrombus.
Mural thrombi can be evaluated and charac- terized by MRI. Thrombi range from hyperin- tense signal on T1- and T2-weighted images for an unorganized thrombus, an inhomogeneous signal intensity for a partially organized throm- bus, and hypointense signal on both T1- and T2- weighted images for an organized thrombus.
A mural thrombus can be suspected with radionuclide aortic angiography. A photon- deficient region along the aneurysm wall corre- sponds to a mural thrombus.
Therapy Surgical
Indications for surgical repair of an abdominal aortic aneurysm vary somewhat among sur- geons. Because of the risk of rupture in aneurysms >5 cm in diameter, one recommen- dation is to consider elective surgery in fit patients with an aneurysm >4.5 cm in diameter.
Other indications include significant occlusive disease requiring repair, pain, or a suspected mycotic etiology.
An open, elective aortic abdominal aneurysm repair carries a mortality of about 2% to 4%, which is in contrasts to a mortality of about 80%
for emergent repair of a ruptured aneurysm.
Endovascular
The introduction of endoprostheses has markedly changed aortic aneurysm surgery.
Instead of an abdominal incision, vessel clamp- ing, and associated major blood loss, the pro- cedure is performed via a femoral artery approach. Instead of direct aneurysm inspec- tion, oversawing of feeding vessels and surgical repair, both initial planning and repair are per- formed under imaging control. Preoperative CT, often with 3D reconstruction, is used for aneurysm evaluation and to select an appropri- ate prosthesis, although 3D MRA is assuming an increasing role in preoperative management of these patients. At times arteriography is helpful.
C-arm fluoroscopic guidance is used for pros- thesis insertion to bridge the aneurysm. If needed, a bifurcation graft or an aorto-uni-iliac graft is inserted. Such percutaneously inserted and deployed grafts have had a variable success rate. Long-term success rates are not available and the borderline between surgical correction
and percutaneous stent therapy is poorly defined. Patients with a long life expectancy and low surgical risk continue to undergo surgical repair. Also, infected aneurysms remain in the surgeon’s domain.
Endovascular aneurysm repair performed using stent-grafts in patients considered too high risk for conventional repair resulted in complete aneurysm exclusion (based on CT cri- teria) in 88% (26). Deployment and complete aneurysm exclusion with covered stents is more successful with tube grafts than with bifurcation grafts. Surgical correction is necessary for migrating grafts. Nevertheless, a number of authors have commented that their morbidity rates compare favorably with those of open surgery in these high-risk patients.
A limitation of a transfemoral approach is that the iliac arteries must be accessible and relatively free of major atheromatous disease.
A major reason for failure of stent-graft inser- tion is excessive iliac artery tortuosity or arte- riosclerosis, with rates of failure dependent on patient selection factors and operator experience.
Covered stent-grafts have been successfully inserted in patients with aortic aneurysms close to renal artery orifices, with the proximal uncovered stent portion placed across one or both renal artery orifices. Stent-grafts have been deployed across saccular aneurysms.
Complications after initial stent-graft inser- tion are common, and secondary intervention is often necessary. The significance of leakage outside the graft is not clear.
Postoperative Findings
Postoperative imaging appearances differ after various surgical repairs and after percutaneous endoprosthesis insertion. Some of the compli- cations encountered also differ. Knowledge of specific therapeutic technique used is necessary for postoperative imaging evaluation.
Delayed aneurysm rupture after endovascu- lar repair is rare.
Normal Imaging
Serial postoperative contrast-enhanced CT
or MR identifies most major complications
after surgical repair, including bleeding, false
aneurysm formation, vessel occlusion, and most
fistulas (Fig. 17.6). With an end-to-end anasto- mosis, the native aneurysm wall is wrapped around a graft. Postoperative fluid is common between a graft and native aorta and is detected with CT or MR. Perigraft fluid in some patients persists for several months. This fluid is gradu- ally reabsorbed; increasing amounts of fluid with time suggest infection.
Postoperative surveillance includes measure- ment of residual aneurysm size. A measure of maximum residual aneurysm diameter using CT angiography is common, although aneurysm volume is probably more accurate.
Magnetic resonance imaging is limited in the immediate postoperative period if metal components are present. Covered nickel tita- nium stent-grafts used for abdominal aortic aneurysm repair have been safely imaged by 1.5-T MRI, with no ferromagnetism or heating detected (27). In an in vitro contrast-MR study of nitinol, tantalum, stainless steel, and cobalt alloy stents, only the latter stent resulted in a signal void inside the lumen (28); MR of some of the stents produces an artificial diameter narrowing.
A CT scan 24 to 48 hours after graft place- ment should reveal any partial or complete thrombosis of the aneurysmal sac; initially patent channels tend to close subsequently, but can recur. A mottled appearance within the aneurysmal sac is not uncommon. Maximum
intensity projections rendered from helical CT a week or so after aortic stent graft placement is useful to evaluate stent deformity or stent angulation; MIP also aids in detecting renal artery occlusion, leaks and thrombi. Computed tomography criteria of successful endovascular repair consist of a decrease or unchanged size aneurysmal sac without the presence of perigraft channels; the latter are often associ- ated with subsequent aneurysm enlargement.
Eventual fibrosis surrounding the surgical site appears hypointense with both T1- and T2-weighted images.
Some evidence suggests that MRA is as sen- sitive as CTA in detecting endoleaks (29); it also avoids iodinated contrast agents Eventual fibrosis surrounding the surgical site appears hypointense both with T1- and T2-weighted images.
No firm guidelines are established for long- term follow-up of asymptomatic patients after endovascular repair. Pre- and postcontrast CT scans annually appear reasonable. Any symp- toms or known complication require more frequent follow-up.
Bleeding/Leak
Precontrast CT is useful in detecting an acute postoperative hematoma; a recent bleed is slightly hyperdense to the surrounding struc-
Figure 17.6. Abdominal aortic aneurysm repair. A 3D reconstruction (A) and maximum intensity projection (MIP) reconstruction (B) identify relationship of aorta to adjacent structures. (Courtesy of David Waldman, M.D., University of Rochester.)A B
tures. Postcontrast CT readily detects graft occlusion. A new false aneurysm enhances post- contrast, unless it is thrombosed.
Leakage adjacent to a prosthesis is considered a perigraft leak, while leakage along an aneurysm border is a retrograde leak. Leaks are uncommon in the thrombosed portion of an aneurysm; rather, continued patency of feeding vessels, such as the lumbar and inferior mesen- teric arteries, the median sacral artery, and occasionally even the urethral and testicular arteries plays a role in the pathogenesis of leaks after stent-graft repair (30). Inferior mesenteric artery leaks are ventral to the prosthesis and lumbar or median sacral artery leaks are dorso- lateral in location. For most leaks CT can thus establish its origin. Perigraft leak rates vary con- siderably and depend, in part, on the extent of the initial aneurysm. As the number of patent feeding vessels increases, the leak rate also increases, reaching 60% when more than six lumbar arteries are patent (31). A rare contained leak developing after aneurysectomy is suf- ficiently extensive to result in inferior vena cava compression.
Computed tomography angiography appears reliable in detecting perigraft leakage. The sen- sitivities and specificities for detecting leakage were 63% and 77% for conventional angiogra- phy and 92% and 90% for CTA, respectively (32). An occasional side-branch endoleak is not detected by CT but is identified by duplex US.
Preliminary evidence suggests that contrast enhanced US also detects endoleaks after endovascular aneurysm repair, even detecting some missed by CTA (33).
Most leaks can be treated successfully with additional stent-grafts. Some leaks are amenable to coil embolization; embolization results in aneurysmal sac thrombosis. Others can be sealed off with balloon dilation at the end of stent insertion. Still others disappear sponta- neously. At times rather creative embolization is helpful. Thus endoleaks due to retrograde flow in the inferior mesenteric artery have been treated by selective superior mesenteric artery catheterization and embolization through the middle colic artery (34).
Infection/Fistula
Fever, leukocytosis, and anemia suggest graft infection as a complication of an aortic
aneurysm or aortic bypass surgery. While graft infection is not common, it is associated with a high morbidity and mortality. An extraperi- toneal lymphocele can develop after aortic reconstruction. It can become infected.
Some of the initial work with CT detection of graft infection suggested a sensitivity and specificity approaching 100%, but later studies tempered such enthusiasm, and the current belief is that CT detects mostly advanced infections.
Gas adjacent to a graft is identified by CT shortly after surgery in many patients even without infection. Gas developing several weeks or longer after surgery, on the other hand, implies infection or an aortoenteric fistula. Thus in a patient with anemia or gastrointestinal bleeding of unknown cause (sentinel bleed) and a remote history of aortic aneurysm repair, the presence of soft tissue gas, no matter how little, is presumptive evidence of graft infection and often serves as a harbinger for a future major bleed. A CT finding of periprosthetic wrap thickening, an inhomogeneous appear- ance and thickening of adjacent bowel wall and valvulae conniventes are common with an infection but are nonspecific findings. Perigraft fluid can persist for several months after surgery, although fluid collections for longer periods of time suggest an infection. If neces- sary, image-guided fluid can be obtained for culture. Some infectious organisms are notoriously difficult to culture, and prolonged incubation is necessary.
Some infections progress to an aortoenteric fistula, with the third and fourth parts of the duodenum most often involved. Direct signs of an aortoenteric fistula are not common; these include either IV contrast extravasating into bowel lumen or orally administered contrast leaking into soft tissues surrounding a graft.
One of the limitations of MR is an inability to detect small amounts of gas in soft tissues.
However, MR differentiates persistent inflam- mation and fluid in an abscess from a hematoma. Fluid appears hypo- to isointense on T1- and hyperintense on T2-weighted images, while a persistent hematoma is hyperintense with both sequences, although these findings vary, depending on age.
Indium-111–white blood cell (WBC) scintig-
raphy is useful as a primary test for suspected
infections or as an adjunct to ambiguous CT
results. Gallium scanning, or more recently, Tc-99m–hexamethylpropyleneamine oxime (HMPAO)–labeled leukocytes appear to have a role in suggesting an infection but have had limited application. An occasional test is false positive; thus an occasional patient with a non- infected pseudoaneurysm will have uptake of Tc-99m-HMPAO–labeled leukocytes.
Primary percutaneous drainage appears rea- sonable in patients with aortic graft infection and a fluid collection, although some of these patients later require removal of their infected prosthetic grafts.
Other Findings
Postoperative pseudoaneurysms develop both with and without an underlying infection.
Iliac artery tortuosity predisposes to iliac artery injury during percutaneous endopros- thesis insertion. An extreme complication of a pseudoaneurysm is blowout of an aortic stump.
In a setting of a pseudoaneurysm, aortogra- phy provides a roadmap for future repair, while CT is superior in evaluating surrounding infection.
Ureteric stenosis, periureteritis, and ureteric compression by a false aneurysm are some of the complications of aortic surgery (35); some of these complications manifest only years later.
Endovascular repair can also result in a periaor- titis and ureteral obstruction (36). At times a portion of a self-expanding stent covers a renal artery; evidence of renovascular compromise, however, is not common.
In a patient with postoperative hematuria the surgical graft coursed through the bladder (37);
presumably an intravesical tunnel was created during the original graft insertion.
Aortitis
Nonspecific aortoarteritis, or Takayasu’s arteri- tis, is a panarteritis of unknown etiology. Patient age at onset ranges from pediatrics to old age.
In a collection of 31 patients with Takayasu’s arteritis, 45% had aortic aneurysms (38); of note is that aortic wall thickening was detected on CT in several of these aneurysms, aneurysms increased rapidly in size, and ruptured during follow-up.
Patients with Takayasu’s arteritis develop vis- ceral artery stenoses. Although angioplasty and
stent insertion usually have an immediate benefit, these patients suffer from a high rate of restenosis.
Thrombosis Leriche’s Syndrome
Acute abdominal aortic thrombosis is not common. Rather than being acute, some of these thrombotic occlusions present with renal failure or congestive heart failure, and the diagnosis is suspected from renal scintigraphy.
A rare cause of aortic occlusion was intra- aortic growth of hydatid cysts (39); recurrent hydatid cysts developed after previous surgery for a paraspinal hydatid cyst.
Leriche originally described obstruction at the aortic bifurcation, but his name is now asso- ciated with symptoms due to infrarenal aortic obstruction. Varying degrees of claudication and impotence develop depending on the extent of atherosclerosis and collateral flow. Dimin- ished femoral artery pulses are common.
Three-dimensional contrast-enhanced MRA using MIP and a rotated display in patients with Leriche’s syndrome located the level of aortic occlusion as juxtarenal, infrarenal but cranial to inferior mesenteric artery, or caudal to the infe- rior mesenteric artery (40). Collateral pathways and concomitant renal artery stenoses can often be detected. Although in theory IV DSA pro- vides similar information, increased contrast conspicuity and a 3D rotational display make MRA superior, visualizing even small collater- als. Whether MRA image quality is superior to that of intraarterial DSA is debatable, but the lack of catheter manipulation and arterial injec- tion makes MRA a simpler study.
Inferior Vena Cava
Obstruction
Thrombosis
Most inferior vena caval thrombi originate in an adjacent vein and spread centrally. Thus a lower extremity venous thrombus can extend superi- orly, or a renal malignancy, especially orig- inating in the right kidney, not uncommonly invades and obstructs the inferior vena cava.
Caval thrombosis is a complication of Crohn’s
disease, systemic lupus erythematosus, and
Behçet’s syndrome. In these settings caval obstruction should be suspected if hepato- splenomegaly, ascites, and lower extremity dependent edema develop.
Imaging detects a thrombus as an intralumi- nal filling defect or simply as lack of caval filling with contrast (Fig. 17.7). A primary thrombus and a neoplastic thrombus have a similar imaging appearance and can be differentiated only if thrombus neovascularity or other evi- dence of a neoplasm is detected. Noncontrast CT reveals an acute thrombus to be isodense or slightly hyperdense to blood. With age, a throm- bus gradually becomes hypodense. The involved vessel diameter tends to be expanded focally, regardless of etiology. Gas within a thrombus is rare and suggests infection. Calcifications develop in some chronic thrombi.
With incomplete obstruction, postcontrast CT identifies most thrombi as a hypodense tumor surrounded by contrast-opacified blood.
A bland thrombus does not enhance postcon- trast, while a tumor thrombus does. Complicat- ing the issue is the occasional bland thrombus attached to a tumor thrombus. Still, caution is needed to differentiate a thrombus from incom- plete mixing of opacified and nonopacified blood and a resultant transient artifact.
Collateral vessels are common with obstruc- tion of the inferior vena cava. Lower extremity radionuclide venography reveals collateral flow;
an occasional patient has diffuse hepatic uptake of the radionuclide. With superior vena caval obstruction, the azygos vein, hemiazygos vein, internal mammary veins, vertebral venous plexus, and lateral thoracic and some superficial thoracoabdominal veins enlarge and become collateral vessels. Because of these collaterals, after contrast injection into an upper extremity CT can reveal enhancement of a liver segment or the inferior vena cava. Early and dense con- trast enhancement of liver segment IV occurs due to segmental liver perfusion from epigastric and paraumbilical veins; such a pseudolesion in segment IV is a potential pitfall seen occasion- ally during both arterial portography and helical CT.
Magnetic resonance readily detects a caval thrombus. With SE sequences the vena cava contains a tumor rather than a signal void as seen with flowing blood. Slow-flowing blood, however, also results in loss of the caval signal void. With GRE sequences flowing blood appears hyperintense, and this technique appears more reliable in detecting a thrombus than a SE technique. In some patients incom-
Figure 17.7. Idiopathic inferior vena caval obstruction. A: A lateral view from a barium enema and cystogram reveals widening of the presacral soft tissues (arrows), a common finding with caval obstruction. B: Venogram identifies extensive collaterals veins and confirms lack of vena cava filling.
A
B
