Aortic ultrasound

Blood flow visualization using ultrasound Doppler (US) has expanded the capabilities of the ultrasound method in examining abdominal organs. Ultrasound Doppler is performed according to certain clinical indications that require a specific examination protocol and quantitative assessment of blood flow, for example, during monitoring after interventional procedures for the imposition of a transjugular intrahepatic portosystemic shunt. Also, the color mode can be used during an ultrasound examination to identify the vascular nature of undefined hypoechoic or anechoic formations.

When performing an ultrasound examination of the abdominal cavity, the ultrasound specialist faces a large number of clinical problems and the need to visualize all vascular pools. Precise selection of settings is necessary to optimize the image. Traditional image planes can be modified to examine altered vessels at a convenient Doppler angle.

This chapter presents the normal ultrasound appearance of the abdominal vascular beds and the pathological changes detected by ultrasound. Parenchymal diseases are limited to neoplasms because of their high clinical significance. The aim is not to fully demonstrate the capabilities of color duplex sonography of the abdomen, but to give an idea of its key aspects and thus help diagnosticians take the first step in this complex field.

Ultrasound anatomy of the aorta and its branches

The abdominal aorta is located paravertebrally to the left of the diaphragmatic aperture to the level of the L4 vertebra, where it divides into the common iliac arteries. Its diameter varies from 25 mm or less at the subdiaphragmatic level to 20 mm or less at the bifurcation level.

The first unpaired branch of the abdominal aorta, the celiac trunk, originates to the left of the midline. It deviates slightly to the right before the common hepatic artery, a vessel of approximately the same caliber as it, the splenic artery, and the small-caliber left gastric artery originate. The common hepatic artery runs in the hepatoduodenal ligament to the liver, passing anterior to the portal vein. The splenic artery, accompanied by the vein of the same name, runs along the posterior edge of the pancreas to the hilum of the spleen.

The superior mesenteric artery usually arises from the abdominal aorta 1 cm distal to the celiac trunk. Its main trunk runs parallel to the aorta and can be followed by ultrasound over a long distance when the mesenteric vascular arches are no longer visible.

The inferior mesenteric artery arises about 4 cm before the bifurcation and runs for some time to the left of the aorta before dividing into branches. The Buhler anastomosis connects the celiac trunk and the superior mesenteric artery via the pancreaticoduodenal arteries. The anastomosis between the superior and inferior mesenteric arteries (Riolan anastomosis) is via the middle and left colic arteries.

Survey methodology

The patient is examined in the supine position using an intermediate frequency convex probe (usually 3.5 MHz). A cushion under the knee joints allows the patient to feel comfortable and improves the scanning conditions, since the abdominal wall is relaxed. The abdominal aorta is completely examined first in longitudinal and transverse B-mode, after which the color mode is applied.

Normal picture

The blood flow pattern in the aorta is variable. Above the renal level, the postrenal peak gives way to a constant forward flow in diastole. Scanning below the renal level normally reveals early diastolic reverse flow, as in peripheral arteries. This should not be considered abnormal flow or "blurring."

The blood flow velocity in the abdominal aorta is approximately 50 cm/m lower than in the peripheral arteries, which is associated with the large caliber of the aorta. Velocities and the component of reverse blood flow are variable.

Color mode scanning of the aorta below the level of the kidneys is often unsuccessful in the examination of the upper abdomen, since the angle between the sound track and the direction of blood flow is unacceptable (90°) when using a convex probe, and changing the angle has little effect on the situation. The position of the probe in the caudal direction gives a better Doppler angle, but the gas-filled transverse colon often falls into the scanning area at the level of the mid-abdomen, superimposing it on the image.

The most common disease of the aorta is atherosclerosis. Ultrasound allows us to determine the dynamics of combined changes, such as stenosis, occlusion and aneurysms.

Aortic dilation criteria

1. Blood flow is laminar or turbulent
2. The maximum diameter of the aorta is less than 2.5 cm. An indication for surgical intervention is a diameter of more than 5 cm, progression of more than 0.5 cm per year.
3. Width and location of perfused, thrombosed or false lumen: eccentric location
4. Abdominal visceral arterial disease, hepatic or iliac artery disease? (surgical strategy and choice of implant)
5. Peripheral aneurysmosis?
6. Spectra in the true and false lumen? (threat of ischemia, indications for surgical intervention)

Aneurysms

Abdominal aortic aneurysms are usually clinically asymptomatic. Their enlargement and formation of peripheral emboli lead to nonspecific symptoms such as back and abdominal pain.

Classification

Isolated aneurysms are relatively common and are usually located below the level of the kidneys. The iliac arteries may also be involved. The location of the less common thoracoabdominal aneurysm is determined by the four-stage Crawford classification. Type I (not shown) involves the aorta above the level of the kidneys. Stages II–IV define the level of thoracic involvement by an aneurysm located below the kidneys.

Abdominal aortic aneurysm and marginal thrombosis are clearly defined by ultrasound. The extent of thoracic aortic lesions and spatial relationships necessary for surgical planning are assessed by Doppler spectra and CT.

In a dissecting aneurysm, blood enters between the intima and media through a gap in the vessel wall. The intimal flap separates the true and false lumens and oscillates with the movement of blood. The extent of the aneurysm can be assessed using CT or ultrasound, using the Stanford or DeBakey classification. Ultrasound can provide additional information about the state of the arteries of the internal organs and pelvis, and is also used for dynamic observation at short intervals.

Leriche syndrome

Leriche syndrome is an occlusion of the abdominal aorta at the bifurcation. Flow at the level of the superior mesenteric artery can still be visualized on longitudinal and transverse images. Distally, there is no flow signal on transverse scans at the level of the mesenteric vault and caudal to the bifurcation. Note that focal color voids may be due to poor scanning angle or anterior obscuring plaques. Poor settings may lead to false-positive results.