Liver Ultrasound Imaging

B-mode images are two-dimensional, real-time ultrasound images that depict all the tissue traversed by the scan.

Real-time mode shows motion by displaying changing images in real time as the transducer is moved.

M-mode displays motion as a wavy line and is commonly used for cardiac ultrasound.

Acoustic enhancement is the increased echogenicity (echo brightness) of tissues that lie behind structures that cause minimal ultrasound wave attenuation.

Acoustic shadowing is the decreased echogenicity of tissues that lie behind structures that cause significant ultrasound wave attenuation.

An acoustic window is a tissue or structure that offers little obstruction to ultrasound waves and serves as a route to image deeper structures.

A cyst is a fluid-filled structure with thin walls and anechoic (echo-free) content, which may have strong back wall reflections and enhancement of echoes behind the cyst.

Debris are solid masses within a fluid-filled mass, which are echogenic and may be mobile, changing with the patient's position or movement.

Hyperechoic tissues create brighter echoes than adjacent tissues, such as bone, perirenal fat, and gallbladder wall.

Hypoechoic tissues create dimmer echoes than adjacent tissues, such as lymph nodes, some tumors, and fluid.

Internal echoes are ultrasound reflections from tissues of different density within an organ, such as gallstones or debris.

Ultrasound is a valuable tool for assessing liver size, texture, and functional status, as well as for identifying possible liver lesions and checking the patency of hepatic and portal veins.

Ultrasound indications include enlarged liver, suspected liver abscess, jaundice, abdominal trauma, ascites, suspected metastases, suspected liver mass, right upper abdominal pain, and screening for endemic echinococcosis.

In a normal liver, the parenchyma appears homogeneous, with portal vein and branches as linear tubular structures with reflective walls. The hepatic veins are non-reflective, and it should be possible to follow them to their confluence with the inferior vena cava.

The hepatic veins can be made to dilate with the Valsalva maneuver, and the inferior vena cava and aorta may be identified.

The liver is composed of right and left lobes, as well as the caudate lobe, which is limited posteriorly by the inferior vena cava. The gallbladder, a pear-shaped, echo-free structure, and the right kidney must also be identified.

The echogenicity of the normal liver parenchyma lies midway between that of the pancreas (more echogenic) and the spleen (less echogenic).

Simple hepatic cysts are common benign liver lesions, occurring in 5%-7% of the population. They are typically discovered incidentally and are almost always asymptomatic.

Simple hepatic cysts may be isolated or multiple, and vary from a few millimeters to several centimeters in diameter. They do not communicate with the biliary tree and have no malignant potential.

The origin of true hepatic cysts is believed to be hamartomatous tissue. They are composed of serous fluid and lined by cuboidal epithelium identical to that of bile ducts.

Sonographically, a well-defined solitary cyst appears as a rounded, echo-free mass with acoustic enhancement. It is usually less than 3 cm in diameter. Small hydatid cysts cannot always be differentiated sonographically.

Large or enlarged liver with homogeneous pattern:

• Congestive cardiac failure: hepatic veins dilated, inferior vena cava does not vary on respiration, and pleural effusion above the diaphragm.
• Acute hepatitis: liver enlargement and tenderness with no characteristic sonographic changes.
• Tropical hepatomegaly: liver enlargement and splenomegaly.
• Schistosomiasis: either sonographically normal or enlarged liver with thickening of the portal vein and main branches.

Large or enlarged liver with non-homogeneous pattern:

• Without discrete masses: increased echogenicity in the liver parenchyma, loss of highly reflective edges of the peripheral portal veins, and cirrhosis, chronic hepatitis, or a fatty liver.
• With multiple echogenic masses: various sizes, shapes, and echo textures, and may be consistent with macronodular cirrhosis, multiple abscesses, multiple metastases, or multinodular hepatocarcinoma.

Small or shrunken liver: micronodular cirrhosis, associated with portal hypertension, splenomegaly, ascites, and dilated splenic veins, and may be difficult to diagnose in the early stages.

Sonographic features of multiple echogenic masses in the liver: various sizes, shapes, and echo textures, and may be consistent with macronodular cirrhosis, multiple abscesses, multiple metastases, or multinodular hepatocarcinoma. It is not possible to distinguish between lymphoma and metastases by ultrasound.

Haemorrhagic cysts: irregular in outline, and may exhibit acoustic enhancement. However, once the blood has clotted, the haematomas may be hyperechogenic. It is important to obtain a clinical history of trauma or anticoagulant medication.