Branches of the Aorta
Earlier, I mentioned the superior mesenteric artery (SMA). From a longitudinal approach, it is seen rising anteriorly and then turning and coursing distally, maintaining a parallel course to the aorta. The SMA is the second branch of the abdominal aorta, and the celiac axis (CA) or trunk, is actually the first. It, too, rises anteriorly as a very short trunk which quickly divides into 3 branches, the common hepatic, splenic, and left gastric arteries. The first 2 branches are easily identified by ultrasound, whereas the left gastric is rarely seen.
The CA will often parallel the proximal SMA for a short distance and then gives rise to the common hepatic artery (CHA) coursing towards the liver and appearing in longitudinal fashion with the transducer in transverse approach. At the same level, the splenic artery (SA) courses toward the spleen, also in a longitudinal fashion. These 2 branches often create what is known as the “Seagull Sign”.
This sign can be very useful in identifying the visceral vessels.
Other Vessels
The left renal vein (LRV) is an important abdominal landmark due to its typical location and ease of identification. Because it has to course further to reach the IVC, it has a greater length than the right renal vein (RRV). The LRV is seen in a longitudinal fashion, anterior to the aorta and posterior to the SMA in a transverse approach as you scan just distal to the SMA.
The LRV empties into the IVC which will be appear in transverse fashion. The right and left renal arteries arise laterally from the aorta just distal to the origin of the SMA and at the level of the LRV. They both course posteriorly toward their respective kidneys. It is not unusual to see multiple renal arteries. The right renal artery (RRA) typically originates at the 10-11 o’clock position and the left renal artery (LRA) close to the 4 o’clock position. The renal arteries can be much harder to image than the LRV; however, identification of the LRV assures you that you are juxtarenal in location. Knowing the relative extension of an abdominal aortic aneurysm (AAA) is critical to surgical planning.
Gallbladder and Liver
Spectral Doppler
Depending on the end-organs’ blood supply demands, arteries supplying the viscera have characteristic spectral Doppler tracings. Comparison of hemodynamics of the carotid arteries to the hemodynamics of the visceral vessels can be applied when discussing high-vs low-resistance vascular beds. Because the brain always has a high demand for oxygenated blood, the waveforms obtained in the internal carotid arteries (ICA) display a low-resistance pattern with continuous forward flow throughout diastole. Waveforms obtained in the external carotid arteries (ECA), which have numerous branches supplying oxygenated blood to the face, scalp, etc., display a high-resistance pattern with little-to-no continuous forward flow during diastole. ECA waveforms also exhibit a sharp, rapid upstroke at the onset of systole.
Other Visceral Vasculature
Another vessel often seen when trying to scan the IVC is the main portal vein as it enters the portal triad of the liver, along with the CHA and the common bile ducts. As all of these structures are tubular conduits, it can be helpful to know the characteristic spectral Doppler tracings. The main portal vein supplies 70% of the liver’s blood supply; therefore, its larger size compared to the CHA is easy to recognize. The flow in the PV should always be towards the liver and appear continuous in nature. In contrast, no color or spectral Doppler waveforms are obtainable in the common bile duct. The flow in the hepatic veins is towards the IVC and has a pulsatile flow pattern which can be affected by the RA pressure, causing reversal of flow in some cases.
Patient Positioning Pearls
Taking advantage of your patient’s ability to change positions can make the seemingly impossible become possible. If you are struggling with imaging the IVC and proximal aorta with your patient supine, roll them onto their left side (left lateral decubitus – LLD) and use the liver as your window. Place the transducer in a coronal plane mid-way of the ribcage, have the patient breathe in deeply and hold, and find the large right lobe of the liver. Mid-way between the image of the right lobe of the liver (RLL) with the IVC well-displayed, and the image of the RLL with the right kidney displaying its pole-to-pole length, is the mid-clavicular line, along which the length of the liver is assessed.
Having the patient turn into a right lateral decubitus (RLD) position can also be used to image the abdominal aorta by scanning through the spleen, similar to the method described above for using the liver as a window. Oblique body angles may prove valuable especially when dealing with patients with large, pendulous abdomens, as turning the patient away or towards you will allow the lipomatous abdomen to move out of the way, thus decreasing the depth needed to reach the aorta, etc., and facilitates the use of less probe pressure.
One Final Thought
One thing I always teach my students about scanning in the abdomen is that no matter what size or shape the patient is, the key structures still have the same anatomical relationships. Learn to visualize the skeleton within the body and, through purposeful practice, learn to recognize the key landmarks mentioned in this blog. Once you have learned the spatial relationships between the organs, vessels, and bony structures, you will be able to navigate your way through the toughest terrain you encounter.
Summary
Being able to recognize ultrasound landmarks provides a reliable method to correctly identify visceral vessels, providing a more accurate assessment of each patient. By following the methods outlined above, you can rest assured you are doing your part in providing each patient the right care at the right time.
