Current transducers have a range of frequencies. A 3.0–3.5 MHz range is a good choice for most patients when evaluating the liver. A 5 MHz frequency may be better for very thin patients, while a 2.5 MHz frequency is helpful for obese patients or those with steatosis. Again, it is important to develop a systematic approach to scanning; it should be done every time. To begin liver imaging, place the probe transversely in the subxiphoid position and identify the hepatic veins as they join the vena cava. This is an easily recognized image that helps to orient the examiner (Fig. 4.8). If the patient has a steep, angulated costal margin, ask the patient to take a half to whole breath and hold it. This pushes the liver toward the costal margin, making the superior liver easier to see. Once this view is found, a systematic approach (Table 4.4) using a combination of transducer movements allows mapping of the segmental hepatic anatomy. For instance, a majority of the liver can be seen by rocking and tilting the probe while in the subxiphoid window (Fig. 4.9). Next, the probe is slid toward the left and then the right, allowing views of the remaining left and right livers, respectively. Upon completing the transverse views, the probe is reoriented in a sagittal plane and the process repeated. Sometimes the probe must be angulated sharply toward the head to scan beneath the costal margin. If this does not permit adequate viewing, an intercostal window allows access to structures hidden beneath the ribs. A smaller footprint probe is useful in this instance. Both an anterior and lateral approach through the costal margin may be necessary to image the structures of interest.

The transducer and techniques are similar to those described for the liver. Ideally, the patient should fast for 6 h prior to the study to allow maximal gallbladder distension. With the patient supine, position the probe subcostal in the midaxillary line while oriented in the sagittal plane. The initial step is to find the gallbladder. Slight sliding and tilting in this position allows a long axis view of the gallbladder (Fig. 4.10). Rotating the probe in the same position allows transverse and oblique views of the gallbladder. From here, several standard probe positions are helpful for complete biliary scanning (Fig. 4.11) [1]. Intercostal windows typically are necessary for complete biliary scanning. Rolling the patient into the left lateral decubitus position is important to distinguish whether gallbladder masses are stones (move with repositioning) or polyps (stationary with repositioning) and sometimes to get an adequate view of the extrahepatic bile duct.

The extrahepatic bile duct is recognized by its position anterior to the portal vein. Place the probe in a longitudinal plane approximately perpendicular to the right costal margin between the midaxillary line and the epigastrium (Fig. 4.11, position b). Identify the portal vein at the hilar plate and follow it caudally to identify the bile duct in a longitudinal view anterior to the portal vein (Fig. 4.12). Sliding the probe toward the midline while in a longitudinal or oblique position allows viewing of the distal duct (Fig. 4.11, positions e and f). If the duct is not visible in this position, the best view may be obtained by placing the probe in a longitudinal, subcostal, and midaxillary position while the patient is in the left lateral decubitus position. Views of the retroduodenal duct are difficult to obtain as they often are obscured by duodenal or bowel gas. The intrapancreatic duct is best seen in transverse section through the head of the pancreas; however, this can be difficult to obtain due to bowel gas. Unless dilated, small intrahepatic ducts can be difficult to see. Intercostal windows facilitate imaging the right intrahepatic ducts, while the left ducts are viewed from a left subcostal window.

TAUS of the pancreas can be difficult due to bowel gas between the abdominal wall and the pancreas. The patient is examined in the supine position after fasting to minimize bowel gas. If the view still is impeded by gas, several other techniques can be used to improve the view. Deep inspiration and breath holding may push the liver below the costal margin where it can serve as an acoustic window to the pancreas. Compression scanning with deep pressure on the probe can push gas-filled structures aside. Placing the patient in a semi-upright position after drinking 500 ml of water may allow a viewing window through the fluid-filled stomach. A right lateral decubitus position may improve the image. Finally, the tail of the pancreas may be best seen from a left lateral flank view through the spleen.

The pancreas is difficult to find on TAUS. It is most easily identified by its relationship to surrounding landmarks. The transducer is positioned transversely in the subxiphoid, midline position. The pancreas is identified by first finding the vertebral body, aorta, vena cava, and the splenic vein junction with the superior mesenteric vein. The pancreas can be inferred by its relationship to these structures. Figure 4.13 shows the prototypical image used to identify the pancreas. Once the neck and body are seen, the remaining portions of the gland are examined in the transverse plane and then the longitudinal plane.

A flat, side-viewing, linear array transducer is favored for liver scanning. The long footprint allows efficient imaging of the whole organ and gives a rectangular image of the underlying structures (Fig. 4.14). Images in this configuration make interventions, such as needle biopsy, relatively easier compared to a curvilinear array. The probe’s low profile allows easy access in limited working spaces between the liver, abdominal wall, and diaphragm. While current probes are multifrequency, scanning in the 7.5 MHz range has the most utility for liver IOUS. This frequency allows adequate penetration to view the whole organ while showing the very fine detail of intrahepatic structures. A lower frequency, 5 MHz, may be necessary for a larger, steatotic, or cirrhotic liver since this frequency results in deeper sound penetration to examine the depths of the parenchyma in these organs.

Similar to TAUS, a critical component for successful IOUS is developing a systematic scanning approach for the liver. Fastidious adherence to this system insures an adequate and thorough evaluation every time an examination is performed. The first step of this approach is identifying the segmental anatomy. Once each segment is mapped, an evaluation for known and occult lesions is undertaken, noting their location and relationship to the segmental anatomy and intrahepatic structures.

Hepatic IOUS begins with contact scanning (Fig. 4.15, position A). Moistening the liver’s surface with saline is sufficient to create coupling. The probe is placed in direct contact with the liver; knowing its exact position on the organ is an advantage of IOUS, making image interpretation easier. Contact scanning is useful for imaging most of the liver; however, it is of limited utility when a mass or structure is within 5–10 mm of the probe (Fig. 4.16). Thus, superficial areas directly beneath the probe represent a “blind spot” during contact scanning. Similarly, irregular surfaces (e.g., cirrhosis) make scanning difficult, leading to poor image quality. It is these circumstances when a probe standoff technique for scanning is of use (Fig. 4.15, positions B and C). Saline is an effective interface to establish an acoustic window between the probe and the surface of the liver for a probe standoff technique. With the transducer separated from the liver surface, superficial structures or the irregular surface are seen with better clarity and resolution than during contact scanning. Probe standoff scanning can be done in several ways as illustrated in Fig. 4.15. The difficulty with image resolution of superficial lesions by IOUS emphasizes the importance of combining inspection and palpation when examining abdominal organs, since these techniques are complementary to IOUS. Each of these examination methods should be used to insure complete evaluation of the liver.