Abdominal Sonology in the ICU
While ultrasound has always been an important imaging modality in the management of abdominal pathology, its use in the ICU is focussed on a few specific disease entities. A comprehensive, detailed ultrasound examination is often not urgent and can be requested from the radiology service. Critical care ultrasound should serve to answer specific questions which would have an immediate impact on the care of the patient. It has been shown that bedside ultrasound studies in the ICU helps to avoid more elaborate imaging studies.
The following organs / areas are usually imaged as part of an ICU abdomen scan.
- Peritoneal cavity
- Spleen and pancreas
All these scans are done using the standard abdominal convex probe or in some machines, the echo probe with abdominal settings, both of which have frequencies between 2 and 5 MHz.
The 2 lobes of the liver should be evaluated separately in 2 planes.
The right lobe is imaged by first placing the probe just below the costal margin on the mid-clavicular line with the marker dot pointing towards the head end of the patient. The probe is angled slightly to point the beam cephalad. The depth of the image should be adjusted to ensure visualization of the entire lobe. This is a longitudinal view of the left lobe. The liver will be seen to be bounded by the diaphragm postero-superiorly (i.e in the lower part of the image) separating it from lung. The probe must be rotated clockwise 90° to obtain a transverse scan and complete the imaging of the lobe in both planes.
The left lobe is imaged by first placing the probe on the midline of the epigastrium midway between the xiphoid and the umbilicus, with the marker dot pointing towards the head end of the patient. The probe is angled slightly to point the beam cephalad. The depth of
the image should be adjusted to ensure visualization of the entire lobe. The liver will be seen to be bounded by the diaphragm postero-superiorly (i.e in the lower part of the image) separating it from the heart. As with the right lobe, the probe must be rotated clockwise 90° to complete the imaging in both planes.
The normal dimension of the right lobe from lower border to the postero-superior aspect (a vertical caliper measurement on either right lobe view) is 13 to 15 cms. It is labelled as hepatomegaly when this dimension is more than 17 cms. The size of the left lobe is more varied. Clinically, it is said to be enlarged if the lower border extends lower than the midpoint between the xiphoid notch and the umbilicus. Cirrhosis and fulminant hepatitis are 2 conditions, where the liver is shrunken (ultrasound liver span of less than 13 cms). The right and the left lobes enlarge equally in most diffuse diseases except early cirrhosis, where the left lobe may be hypertrophied while the right lobe may be shrunken. Focal pathologies like abscesses and tumors cause enlargement of the lobe that is involved.
The liver should be homogenous and moderately echogenic throughout. Usually, the echogenecity should be the same as the adjacent right kidney. However, in critically ill patients, renal dysfunction is common and renal echogenicity may not be normal. Hence it cannot always be used as a standard to compare hepatic echogenicity against. Decreased hepatic echogenicity is seen in acute hepatitis, while fatty liver and alcohol related changes cause the parenchyma to have a hyperechoic appearance. Coarse echotexture can be seen in cirrhosis.
The surface of the normal liver is smooth and the capsule is seen as a thin echogenic line. Biconvex anechoic, hypoechoic or hyperechoic structures just below the capsule may indicate subcapsular blood or pus. Irregularity of the surface with blunting of the edge is usually seen in cirrhosis. isolated humping of the surface may be due to an intrahepatic tumor or a regenerating nodule.
A hyperechoic rounded or triangular structure may be seen appearing to divide the right and left lobes in a transverse scan of the liver. This is the Falciform ligament. It can also be seen sometimes on a longitudinal scan as a highly echogenic sickle shaped structure. The main lobar fissure and the ligamentum teres are other echogenic collagenous structures that can be visualized.
Numerous hypoechoeic structures are seen within the liver parenchyma. These include biliary radicals and vessels - Hepatric veins, branches of portal vein and hepatic artery. Some may appear round, being seen in cross section, while others may be visualized in longitudinal section. Biliary intrahepatic ducts are typically much smaller than the portal venous radicals that accompany them. Color flow imaging can be used to differentiate vessels from biliary radicals.
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