Abdominal Arteries Anatomic Variants

Anatomic variants are incidental findings in examinations of the abdomen. They assume importance when they cause symptoms or have a bearing on planning of surgery or interventional radiological procedures.

Anatomic variants of the arterial supply to the liver are found in up to 50% of patients [8]. They are typical incidental findings in arterial phase CT examinations of the abdomen but can also be picked up on MR. They become diagnostically relevant for planning partial liver resection or interventional radiological procedures. The following signs suggest a variant:

• Branches of the celiac trunk arising separately from the aorta.

• A vessel running posterior to the portal vein may be an accessory right hepatic artery, a replaced right hepatic artery, or a replaced proper hepatic artery. Accurate identification requires tracing the vessel into the liver and toward its origin.

• A vessel coursing between the caudate and left lobes of the liver superior to the porta hepatis may be an accessory or replaced left hepatic artery arising directly from the left gastric artery.

• An accessory right hepatic artery that arises from the gastroduodenal artery presents as a second vessel just below the right hepatic artery. Such vessels are easily missed and require thin-section imaging. They are best seen on thin-slab MIP (1-2 cm thickness) parallel to the course of the proper hepatic artery.

Superior mesenteric artery syndrome is caused by compression of the duodenum by the superior mesenteric artery (SMA) in thin individuals [9]. The nutcracker syndrome is due to compression of the left renal vein and may present as painless hematuria in slim patients.

Accessory renal arteries occur in up to 25% of individuals. Usually one vessel is dominant on either side. However, multiple small vessels may be present, and accessory arteries may originate even above the SMA (exceedingly rare) or the iliac vessels (some 1% of cases). Main renal arteries enter the kidney via the hilum, while polar arteries enter the renal parenchyma directly. Such variants are important for interventional radiological dilatation of renal artery stenosis and for the work-up of living renal donors [10]. The gonadal arteries usually arise from the aorta just below the level of the kidneys but may also rarely originate from the renal artery (more common on the right). Early branching of a normal sized artery or multiple or abnormal arteries or veins may make harvesting a kidney from a living renal donor difficult.

Renal Artery Stenosis

Renal artery stenosis (RAS) is an uncommon cause of arterial hypertension (approximately 5%). The commonest cause of RAS is atherosclerotic disease (65-70%). Angioplasty has been considered the treatment of choice for these hypertensive patients, but newer longer-term studies suggest that optimum medical treatment is equally effective [11]. There is also an ongoing discussion about the positive effects of angioplasty or stenting on the preservation of renal function in these patients. Thus, the final role of noninvasive techniques (CTA, MRA, ultrasound) for diagnosis of renal artery stenosis will depend on whether or not interventional therapy is considered.

The sensitivity and specificity of all imaging methods are well above 90% for the detection of significant stenoses if imaging is performed by experienced radiologists [12]. However, substantially lower numbers have been reported as well.

Indirect signs can already be appreciated on standard examinations. Differences in size (right kidney 2 cm smaller than left kidney, or left kidney 1.5 cm smaller than right kidney), cortical thickness (on corti-comedullary phase images), level of enhancement, and time to excretion between the two kidneys all suggest unilateral significant renal artery stenosis.

Effective interpretation of CTA or MRA examinations is based primarily on 3D views such as volume-rendered displays (VRT) or curved thin-slab MIP. In order not to miss eccentric plaques, anteroposterior (AP) views as well as caudocranial views should be used. Whenever a suspicious finding is encountered, it should be confirmed by cross-sectional images. This can either be done by interactive analysis of cross-sectional images, or by generating CPR through the vessel segment of interest.

Fibromuscular dysplasia is the next most common cause of renal artery stenosis (25%). It affects younger patients and is more prevalent in women (3:1). Angioplasty is usually a very effective treatment. Fibromuscular dyspla-sia may present as a unifocal stenosis in the middle and distal third of the main renal artery, multifocal stenosis with string-of-pearls appearance, or aneurysm formation. The right renal artery is most commonly affected, but bilateral disease is seen in up to two thirds of cases. It is characteristic that the proximal third of the main renal artery is spared (in over 95% of cases). Renal artery branches are rarely affected without concomitant disease in the extrarenal arteries. Such cases of isolated intrarenal fibromuscular dysplasia are extremely hard to detect with cross-sectional imaging techniques.

Celiac and Mesenteric Stenosis

Abdominal angina is a postprandial abdominal pain that occurs some 15-20 minutes after food ingestion. It is considered to be due to gastric steal, which diverts blood from the intestine. Patients have a food aversion, complain of weight loss, and can develop malabsorption or bowel strictures. This intermittent mesenteric ischemia is caused by severe stenosis of splanchnic arteries without adequate collateralization: combined severe stenoses of at least two of the three vessels supplying the bowel (celiac, superior, and inferior mesenteric artery) are present. More rarely, successive proximal and peripheral stenoses may be found in such patients. A lateral MIP or VRT (the width of the volume of interest [VOI] covers only the aorta) is best for demonstrating proximal stenoses of the celiac trunk and superior mesenteric artery. Coronal thin-slab MIP parallel but anterior to the aorta provide the best anatomic overview. Volume-rendered views are a good alternative to demonstrate more peripheral stenoses as well as collater-alization via the arc of Riolan [13]. Dilated collaterals display a typical pattern on axial sections.

Isolated stenosis of the celiac artery is a common incidental finding that is usually caused by a transverse ligament spanning the left and right crus of the diaphragm, just below the level of the thoracoabdominal junction. This stenosis is typically exacerbated by inspiration and causes no symptoms. Association with an increased rate of pancreatitis or complications of the liver transplantation has been discussed. Stenoses due to diaphragmatic trapping can be diagnosed in a celiac trunk running straight downward from its aortic origin with an elliptical distortion of the normally round cross-section of the artery. The trunk displays a normal diameter in the AP projection but shows marked narrowing on lateral projections.

Stenosis of the superior or inferior mesenteric artery are also relatively frequent incidental findings in patients with severe arteriosclerosis. They are asymptomatic unless adequate collateral supply can no longer be maintained between the superior and inferior mesenteric arteries through the Riolan anastomosis or between the superior mesenteric artery and celiac artery through gastro-duodenal vessels.

Ultrasound is well suited for the detection of proximal stenosis, but CTA and MRA are superior when it comes to display of collateral pathways. CTA and MRA are effective imaging procedures prior to surgical revascular-ization of patients with chronic mesenteric ischemia. For CTA, however, is important to use a negative contrast agent such as water for bowel distension.

Acute Mesenteric Ischemia

Acute mesenteric ischemia is responsible for 8% of cases of acute abdomen. It is a life-threatening event with a mortality that varies between 50 and 90%, depending on comorbidity and age of patients. Prognosis depends on the time of diagnosis, the extent of ischemia (mucosal versus transmural), secondary damage due to infection and pre-existent illnesses. Due to the unspecific symptoms in multimorbid patients, diagnosis is frequently delayed. Acute mesenteric ischemia can be caused by:

• Nonocclusive mesenteric insufficiency (NOMI, 2030%) or

Acute mesenteric ischemia remains a diagnostic challenge because of a large list of potential etiologies. Mesenteric angiography used to be the first-line technique for demonstrating suspected acute mesenteric ischemia, but the increased spatial resolution and its abili ty to detect alternative diagnoses has made multislice CTA the current technique of choice [14, 15].

The most frequent direct imaging sign is a perfusion defect, most frequently approximately 3-10 cm distal from the origin of the SMA (due to acute embolism). Next frequent is an acute thrombotic occlusion, usually on the basis of an atherosclerotic plaque. Proximal occlusions can be more easily compensated via collateral perfusion than distal occlusions that are closer to the bowel wall. Imaging shows the site of obstruction and any residual perfusion distal to the obstructing lesion. One should search for signs that might indicate the etiology, such as embolic disease, thrombosis of a stenosed vessel segment in atherosclerosis, aortic dissection, vas-culitis, direct trauma, intravascular coagulation, hypoper-fusion due to shock, hypovolemia, or endotoxins.

Associated findings include thickened bowel wall due to edema, reduced bowel opacification as compared to other segments, and gas within the bowel wall (pneumato-sis) or in the mesenteric or portal veins indicating infarction. Ascites may be present in the region of the affected bowel. Focal or diffuse bowel dilation is a nonspecific sign.

NOMI can be the consequence of hypovolemic shock, protracted bradycardia, sepsis, heart failure or digitalis intoxication. In these cases, the mesenteric vessels are exceedingly thin without apparent occlusion or focal stenosis, although secondary signs of ischemia are present. Distension ischemia is a form of NOMI: it occurs in a prestenotically dilated colon if the intraluminal pressure exceeds 50 mm Hg, thus causing a nonocclusive critical reduction of colonic microcirculation.

Acute mesenteric thrombosis can be caused by portal hypertension, acute inflammatory processes (e.g., diver-ticulitis, appendicitis), tumor compression, trauma, surgery (especially in young individuals), or by a parane-oplastic syndrome or as the consequence of coagu-lopathies. Strangulation of the mesenteric vessels by volvulus and various other conditions also has to be considered. The superior mesenteric vein is most commonly affected, hampering the venous drainage of the small bowel and right-sided colon. Massive bowel edema and hyperemia are the most frequent secondary signs. Local hemorrhage into the bowel wall is possible. Prognosis is generally better than that of arterial ischemia.

Arterial Bleeding

Arterial injury leads to substantial bleeding at the site of injury, with large hematoma formation that can extend into accompanying structures. Complete transection or the presence of an intimal flap may lead to occlusion of the affected vessel. Long-term sequelae are pseudoaneurysm formation or arteriovenous shunts in cases with simultaneous trauma to arteries and veins. The latter is particularly frequent after cardio-angiographic procedures at the site of arterial and venous access in the groin.

Cross-sectional imaging allows for demonstration of the hematoma, accompanying injuries to the skeletal system or to internal organs, as well as the site of active bleeding. In patients with a continuing blood loss, however, angiography with subsequent embolization of the affected artery will be a better choice. CTA is the technique of choice because it is both sensitive and fast. CTA may be helpful in the detection of occult gastrointestinal hemorrhage as well [16], but MRA with blood-pool agents is a good alternative in patients with occult bleeding.

As opposed to venous bleeding, the hematoma in arterial bleeding is usually extensive and often has a convex outer contour. The site of active bleeding should be searched for in the region of the hematoma. A small spot of brightly enhanced blood outside the vessel lumen is indicative of the site of bleeding. The size of the paravasa-tion indicates the amount of active bleeding. This spot will enlarge if a second scan is added in a venous phase. Such venous phase images are especially helpful to rule out injuries to abdominal organs in patients with blunt abdominal trauma.

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