Not all echoes that originate at an acoustic interface return to the transducer without further reflection. If several strongly reflecting boundaries are encountered, the sound waves can be reflected back and forth before they eventually return as echo to the transducer. The resultant delay in registering these echoes leads to reverberation echoes (51). These reverberation echoes project as several parallel lines in the anterior aspect (near the transducer) of the urinary bladder (Figs. 10.1 and 10.2) or gallbladder (Fig. 34.3), since the processor calculates the site of the reflection solely from the registered time that has elapsed between emission and recording of the sound pulse by the transducer.
Section-thickness artifacts (51) (Fig. 10.2) are caused when the boundary between the wall of a cyst, gallbladder, or urinary bladder (77) and the containing fluid is not perpendicular to the interrogating sound beam. The echoes within the returning beam include echoes from liquid as well as from solid structures and are averaged by the processor. Consequently, the boundary between solid tissue and fluid is seen as a low echogenic and indistinct structure. Section-thickness artifacts can occasionally mimic sludge or layered material (concrements, blood clots) (52) in the urinary bladder (38) (Fig. 10.3).
Strongly reflecting interfaces can cause a scattered reflection of the echoes, spuriously displacing the acoustic interface laterally as a so-called arch artifact. For instance, the duodenal wall occasionally projects in the lumen of the neighboring gallbladder, or an air-containing bowel loop can be seen within the urinary bladder (Fig. 57.4). Finally, mirror artifacts are primarily produced by the diaphragm and visceral pleura, causing intrahepatic structures to be seen as a mirage on the pulmonary side of the diaphragm (Fig. 27.2b).
Did you already mark a cone coffee filter with the location of the structures visualized on sagittal sections, as described on page 4? If not, please do so and compare your drawings with the results on page 78. Only thereafter should you proceed.
The transducer should be perpendicularly placed on the epigastric region along the linea alba and the sound beam swept through the upper abdomen in a fan-like fashion (Fig. 11.1). For the time being, it should suffice to memorize the appearance of the normal anatomy. With the transducer inclined to the patient's right side (Fig. 11.2), aorta (15), celiac axis (32), and superior mesenteric artery (SMA) (17) are found paravertebral^ on the left and dorsal to the liver (9). Normally, all major vessels are hypoechoic (dark) or an-echoic (black).
The image displays the superiorly located diaphragm (13) on the left and the more inferiorly located pancreas (33) and confluens (12) of the portal vein (11) on the right. The hypo-echoic extensions of the diaphragm (the diaphragmatic crura) (13) and the gastroesophageal junction (34) are shown anterior to the aorta and immediately below the diaphragm. It is important to note where the left renal vein (25) crosses the aorta to reach the right-sided inferior vena cava. It travels through the narrow space between aorta and SMA, immediately caudal to the aortic origin of the" SMA. If not well demonstrated, the uninitiated examiner might mistake this vessel for a hypoechoic lymph node. Comparison with the transverse section at the same level clarifies this finding further (Fig. 18.3).
Now the transducer is inclined to the patient's left side (Fig. 11.3) for the visualization of the right paravertebral^ situated inferior vena cava (16), including its continuation into the right atrium. At the same level, the hepatic veins (10) can be distinguished from intrahepatic branches of the portal vein (11).
The presence of air prevents evaluation of the lungs (47). The diameter of the inferior vena cava should not exceed 2.0 cm or, in young athletes, 2.5 cm. The maximum diameter of 2.5 cm also applies to the aortic lumen at this level. The luminal diameter is always measured perpendicular to the vessel's longitudinal axis. The dAO = 1.8 cm and dVc = 2.3 cm in the cases illustrated here (Figs. 11.2, 11.3) are within the normal range.
After the upper retroperitoneum has been scanned, the transducer is moved inferiorly (arrow) along the aorta and inferior vena cava (Fig. 12.1 a). While the transducer is being moved, the vascular lumina should be visualized and evaluated and the perivascular spaces searched for space-occupying lesions. Preferably, the examination should be biplanar by adding transverse sections (see pp. 17 and 18). Enlarged lymph nodes are characteristically visualized as ovoid to lobulated space-occupying lesions with a hypoechoic pattern (see pp. 14 and 21). Distal to the aortic bifurcation, the branching iliac vessels are delineated and evaluated in two planes by sweeping the sound beam parallel (Fig. 12.1 b) and perpendicular (Fig. 12.1c) to the longitudinal vascular axis.
The confluence of the external (22) and internal (23) iliac veins is a frequent site for regional nodal enlargement (Fig. 12.2). The iliac artery (21) is anterior (i.e., superior on the image) to the vein. In unclear cases, the compression test can differentiate these structures, with the vein as a low pres sure system showing easy compressibility. On transverse section (Fig. 12.3), the iliac vessels can be easily distinguished from hypoechoic fluid-filled intestinal loops (46) by the peristalsis of the intestinal wall.
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