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96<br />
ABDOMINAL ULTRASOUND<br />
Table 4.3<br />
jaundice<br />
Condition<br />
Haemolysis<br />
Haematoma<br />
Gilbert’s disease<br />
Viral hepatitis,<br />
cirrhosis of all types,<br />
alcoholic or<br />
drug-induced liver<br />
disease<br />
Abscess,<br />
intrahepatic<br />
malignancy<br />
Causes of non-obstructive (‘medical’)<br />
Aetiology<br />
In which red cells are<br />
destroyed, releasing the haemoglobin<br />
(from which bilirubin is<br />
derived) into the surrounding<br />
tissue<br />
Haemolytic process<br />
A defect in the hepatic uptake<br />
of bilirubin<br />
Destruction of the liver cells by<br />
these diseases prevents the<br />
mechanism of hepatic uptake<br />
and excretion of bilirubin. Both<br />
conjugated and unconjugated<br />
bilirubin are present<br />
Multiple and/or large lesions<br />
prevent the take-up and<br />
excretion of bilirubin by the liver<br />
cells<br />
reflective. As the level of fat deposition increases, the<br />
level of echogenicity may reach that of the highly<br />
reflective portal tract walls. This has the effect of<br />
reducing the prominence of the portal tracts (Fig.<br />
4.19) and making the liver appear smooth and<br />
homogeneous, with closely packed, fine echoes.<br />
The contrast between the liver and parenchyma<br />
of the right kidney is therefore increased (a particularly<br />
useful sign confirming that the correct gain<br />
settings have been used). Hepatomegaly is also a<br />
feature, though not invariably.<br />
Finally, the attenuation of fat is greater than<br />
that of normal liver tissue; this has the effect of<br />
reduced penetration in the far field, rather as if the<br />
time gain compensation (TGC) paddles or slope<br />
control had been incorrectly set. In severe cases of<br />
infiltration, most of the sound is reflected back to<br />
the transducer in the first few centimetres, creating<br />
a highly reflective near-field band through<br />
which the sound is unable to penetrate.<br />
Fatty infiltration itself is not usually a significant<br />
finding; however it often occurs in conjunction<br />
with other significant diffuse processes such<br />
as cirrhosis. Its increased attenuation reduces the<br />
ability of ultrasound to exclude other disease or<br />
A<br />
B<br />
Figure 4.19 (A) Fatty infiltration increases the hepato-renal contrast. The portal tracts are reduced in prominence,<br />
giving a more homogeneous appearance. (B) Attenuation of the beam by fat prevents demonstration of far-field<br />
structures.
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