The orientation marker on the probe should be directed toward the patient’s head (cephalad: sagittal or coronal section; Fig. 12.2), but sometimes you may want to put the marker to the patient’s right to scan the body horizontally (transverse section; Fig. 12.3). On the screen, an orientation marker should be on the left upper corner of the screen.

Start with the depth set at 13–19 cm, and, occasionally, even more depth is required. Always start with sufficient depth so that important structures or pathology located deep in the abdomen are not missed.

Overgain is another common error made by the novice ultrasonographer. Try to undergain as much as possible.

Figure 12.4 illustrates control layout on a typical portable ultrasound machine.

If abdominal subcutaneous tissue is preventing an effective examination of the anterior abdomen, depth may be increased. If the machine function allows selection of different frequencies, choose a lower frequency for better penetration. Some structures, such as the inferior vena cava (IVC) or aorta, occasionally are better observed from a lateral approach.

Gas has a high attenuation coefficient, meaning abdominal gas will prevent ultrasound from penetrating deep to a gas-filled viscus to examine other intra-abdominal structures. Performing ultrasound from a frontal approach is difficult in patients with abdominal gas. In such a setting, important vascular structures, such as the IVC and aorta, may not be well examined from a frontal examination, and laterally placing the probe in the anterior to mid-axillary line can prove helpful.

Very few people in the ICU are not supine. The diaphragm may therefore be higher than expected in these patients. Therefore, identifying the diaphragm is of particular importance when you are planning procedures, such as thoracentesis or paracentesis. Scan higher in the thorax to definitively identify the diaphragm level prior to deciding on a spot for a procedure.

When examining the retroperitoneal organs, particularly the kidneys (especially the left one), a supine position makes the ultrasound more difficult. Place the transducer deep into the small space between the patient’s back and the bed. Sometimes help from other members of the critical care team in lifting the subcutaneous tissue or repositioning the patient is needed so that a space to scan can be created.

When imaging the abdomen, an ultrasonographer can take an organ-specific or a location-specific approach. A so-called organ-specific approach will focus on one specific organ at a time and then move, if desired, to the next organ. This is the typical approach taken in comprehensive abdominal ultrasounds performed in the radiology suite. Focused Assessment with Sonography for Trauma (FAST) examination [3] (see Chap. 14) is an example of a context-driven, location-specific ultrasound approach that was introduced to assess rapidly for the presence of free fluid as a sign of intraperitoneal injury following trauma. In the FAST exam, the examination is limited to a few key points of interest to allow a quick, stereotyped examination that can be carried out in the chaotic trauma bay. The concept of limiting the number of points of interest can be applied to the general critical care abdominal ultrasound as well.

A location-specific approach focuses on one specific point of examination, and all of the important organs are visualized from that location. This approach is efficient, and provides a logical and consistent framework to follow, so that with repeated examinations the ultrasonographer will gain confidence and experience in obtaining and interpreting a few critical views. There are inherent limitations in the comprehensiveness of this approach, and this must be kept in mind when assessing for specific pathologies, such as possible abdominal aortic aneurysm, in which imaging of a more complete extent of the organ is needed.

When using the phased array 1–5 MHz transducer at Point A, the orientation marker should be facing cephalad (Fig. 12.6) or directed toward the right shoulder, thus slightly rotated to the back (parallel to rib). Place the transducer at the mid-axillary line at the level of the 8th to 10th intercostal space. The probe may be rotated so that the acoustic shadow of the rib does not interfere with the image.

To scan the kidney, start the scan from the mid- to posterior axillary line, moving slowly forward to the anterior axillary line. Start scanning from the back so that the hand holding the transducer is almost touching the bed when the patient is in a supine position. If orientation is lost, look for the diaphragm, an important landmark. Placing the patient in the left lateral decubitus position, if possible, may be helpful. The transducer may be rotated slightly to minimize the effect of the acoustic artifact from the rib cage. Novice examiners may find it difficult to locate the kidney; this is often due to the fact that they start scanning too far anteriorly. Because the kidney is a retroperitoneal organ, scanning from back to front allows the sonographer to identify the kidney more easily (this is particularly the case with the left kidney). Angulation is also important: angle the transducer so that the beam is aimed posteriorly, almost scanning in a sagittal plane, i.e., not with the beam in a perpendicular position (coronal view), which is more suitable for assessment of IVC and aorta. Look for free fluid in the hepatorenal recess (Fig. 12.7) between the right kidney and the liver (Morison’s pouch). Be sure to image the kidney through its inferior pole to avoid inadvertently missing free fluid in the most inferior aspect of Morison’s pouch.

Goals when scanning the kidney include an assessment of kidney size, observation of the pelvis for evidence of hydronephrosis, and qualitative examination of renal cortex echogenicity. First, assess kidney size. Freezing an image and using calipers may be helpful. The size of the normal kidney usually ranges from 9 to 12 cm [1]. Reduction in kidney size typically indicates chronic/irreversible renal pathology, i.e., chronic kidney disease with atrophy [4]. Next, observe the renal pelvis to see if there is any sign of hydronephrosis (dilation of renal pelvis and collecting system) (Fig. 12.8). This will appear as hypoechoic regions that communicate and coalesce in the renal pelvis and drain into the ureter. More subtle evidence of early hydronephrosis may require comprehensive abdominal ultrasound for confirmation. Finally, assess the renal cortex. The normal renal cortex appears thick; a thin cortex suggests the presence of chronic kidney disease. In acute kidney injury, the cortex is usually hyperechoic. By contrast, in chronic kidney disease the renal cortex is usually thin and hypoechoic.