There are two major limitations to the sonographic examination of the abdomen: bowel gas and patient obesity. Air in the bowel causes scattering of the ultrasound beam resulting in an inability to appropriately image structures deep to the air. This is a particular challenge in the abdomen where bowel gas is often abundant. To decrease the amount of air in the bowel, the patient is instructed to fast for eight to ten hours prior to the ultrasound examination. Medications are to be taken with sips of water only. Patients are also asked to refrain from cigarette smoking and gum chewing as both result in significant amounts of swallowed air. Gas-reducing agents are occasionally used in some patients, with physician approval, but generally are not required in the fasted patient. To minimize the amount of abdominal gas and to optimize the chances of patient compliance, abdominal examinations are performed in the morning. Diabetic patients are permitted to have clear tea and dry toast, as necessary, to prevent hypoglycemia and their studies are prioritized to early morning. Examinations of postoperative patients may be particularly challenging due to diminished intestinal motility and gas- filled bowel.
Multiple patient positions will be used throughout the examination to optimize acoustic windows. The abdominal examination is most often initiated in the upper and central abdomen with the patient lying supine. The patient’s head should be slightly elevated on a low pillow. The examination table is tilted into the reverse Trendelenburg position (feet 15-20 degrees below heart level) to create better acoustic windows by allowing the viscera to descend into the abdomen. Lateral decubitus, coronal and coronal oblique positions may be used to optimize access to the abdominal organs and vessels. Keep in mind that you can quite often create better windows by moving organs out of the way by having the patient stretch their legs out straight to pull the hip downward or by placing their arm over their ear, rather than across their chest, when they are lying in the lateral decubitus position. On occasion, it may be helpful to have the patient sit up and raise their arms over their head to elevate the rib cage for access to the proximal mesenteric arteries.
It is difficult to predict from the patient’s body habitus whether imaging will be adequate. However, the more obese the patient, the greater the likelihood of poor imaging. The location of body fat (subcutaneous, intraperitoneal and/or retroperitoneal) and the particular sonographic characteristics of the fat are factors that affect the quality of the sonographic image.
Now let’s begin our sonographic journey through the abdomen by following the vascular roadmap!
