Abdominal Aorta

Introduction

Patients with aortic pathology often present with nonspecific complaints or presentations such as back pain, abdominal pain, myocardial infarction (MI), or even stroke, making the diagnosis difficult. The adage of “abdominal pain plus one other organ system” should prompt the physician to consider aortic pathology as the unifying diagnosis. The level and acuity of pain often is secondary to the etiology of the pain. For example, chronic pain may be secondary to an expanding aortic aneurysm, whereas more acute pain may be secondary to aortic dissection or traumatic aortic rupture. Abnormalities of the liver, spleen, and colon should also be considered in the differential listed in Box 20.1 .

Normal Anatomy

The aorta originates from the left ventricle of the heart and ascends, giving off branches to supply blood to the upper body and brain. The ascending aorta courses left and caudally, forming the aortic arch and the descending aorta. The aorta tapers as it descends from the thoracic into the abdominal cavity, ultimately bifurcating into the common iliac arteries ( Fig. 20.1 ).

The first and most proximal branch of the abdominal aorta is the celiac axis. It divides into three main branches: the splenic, common hepatic, and left gastric arteries. The left branch becomes the splenic artery, and the right branch the common hepatic artery. There is a smaller division of the celiac axis, the left gastric artery, which is often not well visualized on ultrasound.

The second large branch of the abdominal aorta, approximately 1 to 2 cm caudal but sometimes abutting the celiac axis, is the superior mesenteric artery (SMA), which lies anterior to the aorta. The splenic vein passes over the SMA, emptying into the portal vein just right of midline. The left renal vein passes between the SMA and the aorta before emptying into the inferior vena cava (IVC).

Sonographic Findings

Imaging of the aorta is normally performed using a curved array transducer, typically 3 MHz frequencies, depending on body habitus. The probe indicator should face the patient’s head for coronal and sagittal views or the anatomic right side for axial views. It is important to image the entirety of the abdominal aorta in both long and short axis, starting superiorly in the epigastric region and scanning through the proximal common iliac vessels. As the aorta descends, it follows the lordosis of the spine, allowing for more superficial ultrasound imaging.

It is best to start the examination by identifying posterior landmarks such as the vertebral body, a rounded, hyperechoic structure with posterior shadowing. Depth is typically set in the 20- to 30-cm range to identify these structures and set the far field, depending on abdominal girth. Once the vertebral body is identified, the depth can be adjusted to visualize the structures of interest, keeping the vertebral body visible ( Fig. 20.2 , Video 20.1 ).

The aorta will rest anterior and to the anatomic left of the vertebral body. On the right, the IVC runs alongside the aorta; thus the two structures must be sonographically distinguished from one another. The IVC is usually respirophasic, thin walled, and collapsible, depending on patient’s hydration status and phase of respiration. The aorta is thicker walled, non-collapsing, and pulsatile. When pulse wave Doppler is placed on an aorta with normal flow, it has a triphasic appearance ( Fig. 20.3A ) compared with the IVC, which has a monophasic waveform of differing morphology, depending on the phase of respiration (see Fig. 20.3B ).

The major branches of the aorta often have characteristic sonographic appearances. The axial view of the celiac axis originating from the aorta is commonly known as a seagull sign ( Fig. 20.4 , Video 20.2 ). The axial view of the SMA takeoff surrounded with hyperechoic fatty tissue is known as the Mantel clock sign . The pancreas and the liver are tissues anterior to the splenic vein that can be seen at the level of the SMA ( Fig. 20.5 , Video 20.3 ).

Several centimeters inferior to the takeoff of the SMA, the origin of the right and left renal arteries are seen ( Fig. 20.6 , Video 20.4 ). The abdominal aorta ultimately bifurcates into the right and left common iliac arteries ( Fig. 20.7 , Video 20.5 ).