The liverModalities
With the introduction of cross-sectional imaging methods such as US, CT, and MRI, direct imaging of the liver parenchyma became possible where previously only angiography and radionuclide imaging had been available.
Ultrasonography
Because of the location, size and structure of the liver, US is very well suited for imaging its parenchyma and is therefore usually the first method employed. It is widely available, easily performed and has no contraindications. An excessive amount of bowel gas may degrade the study.
Ultrasound gives information on the size and structure of the liver and demonstrates both localized lesions (e.g. hepatic tumours, cysts and abscesses) and diffuse disease. Intrahepatic structures, such as portal vessels and biliary ducts can be identified. The vascular systems in the liver may be studied with Doppler US, which can give important differential diagnostic information especially when a colour-system is used (Fig. 1).
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Figure 1.
US of the liver. The normal echo pattern of the liver is demonstrated. The echo-free, tubular structures within the liver are hepatic veins (arrows). |
Computed tomography
A CT study of the liver entails imaging of the entire organ from its superior border at the dome of the diaphragm to its caudal tip. Contiguous 10 mm thick slices are obtained, usually before and after the intravenous injection of contrast medium (Fig. 2). Because of its iron content, the density of the liver is slightly higher than that of other intra-abdominal organs, usually of the order of 65 ± 5 HU. Most pathological lesions have a density less than that of normal parenchyma. This difference is accentuated following a contrast medium injection, but because of pharmacodynamic considerations care has to be taken to perform the enhanced study during a narrow time window of 30-60 seconds following the bolus injection of contrast medium. Because of the size of the liver, the contrast injection may have to be repeated in order to study the entire organ but this requirement has diminished considerably since the advent of fast CT scanning and spiral CT. Sequential scans at a single, predetermined level after a bolus of contrast medium are useful for determining contrast enhancement dynamics, which are of decisive importance in the diagnosis of, for example, a haemangioma.
a
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Figure 2.
CT of the liver. (a) Without i. v. contrast enhancement the texture of the liver is even. The blood vessels are seen faintly as low-attenuating structures against the liver parenchyma. The attenuation of the liver (l) is equal to that of the spleen (s). (b) After i.v. contrast enhancement the attenuation of the liver increases, as does that of the spleen. The hepatic veins are now clearly visible as highly attenuating (white) structures and are well discerned against the liver parenchyma. On this scan contrast medium is also seen in the aorta (a).
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b
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The size of the liver and information on both
focal and
diffuse parenchymal disease are all clearly evaluated by
CT. Newer
CT technology has also made it possible to visualize blood vessels (
CT -angiography,
CT -portography) and to perform 3-dimensional reconstructions which are important in studying anatomically complex areas, such as the liver hilum.
Magnetic resonance imaging
MRI is, in many respects, equal to CT in imaging of the liver. It has, however, certain advantages, that probably make it the best available method for studying disease in this organ. The free choice of imaging planes permits better anatomical orientation (Fig. 3) and the utilization of multiple imaging sequences facilitates the identification of smaller lesions, especially those associated with oedema. MRI gives new information on parenchymal and metabolic disease, 3-dimensional imaging enables visualization of, for instance, the biliary tree and the liver hilum,
 | Figure 3.
MRI of the liver. In these T1-weighted images the anatomy of the liver is well displayed in the transverse, sagittal and coronal projections. The portal and hepatic veins are seen as dark tubular structures. |
and MR -angiography delineates the blood vessels. Various types of magnetic and paramagnetic contrast media that increase the signal intensity of either the
lesion or the
parenchyma, add significantly to the precision of MR studies. Drawbacks of
MRI include its
sensitivity to movement artefacts, and the rather long duration of study, though with new technology and rapid sequences examination times can be considerably reduced. MR spectroscopy may become clinically useful in the study of, for example, metabolic liver disease.
Arteriography us ed to be the most precise method for evaluating liver disease, but its diagnostic use is now limited to the investigation of certain special problems such as the pre-operative mapping of liver vessels or the detailed evaluation of certain liver tumours.
Therapeutic angiography is particularly important in the liver as the organ has a dual blood supply (making embolization a relatively safe procedure) and interventional procedures are associated with a far lower morbidity than surgery in a variety of circumstances such as acute arterial bleeding and porto-systemic shunting.
Hepatic venography is valuable in the evaluation of the Budd-Chiari syndrome.
Percutaneous transhepatic portography is used for studying the portal circulation or for venous sampling of the pancreas. The use of this approach for the embolization of varices is now virtually obsolete except in the most special circumstances.
Radionuclide imaging used to be an important method for studying the liver, particularly focal lesions in the organ, but it has diminished in importance mainly because of its poor spatial resolution and non-specificity in comparison with other methods. Nevertheless, several specialised agents may be useful for imaging specific pathology, such as radiolabelled leucocytes for intrahepatic abscess, In-III octreotide for GFP tumours metastasing to the liver, and 1-123 SAP (serum amyloid P component) for hepatic amyloidosis. Hepatic haemangioma is a lesion which is characteristically associated with low blood flow but high blood volume and may be diagnosed by dynamic Tc-99m labelled red cell imaging.
Biliary imaging
Proper evaluation of the liver frequently requires imaging of the biliary tract. This is considered separately in the succeeding section.
David J. Allison and Carl-Gustaf Standertskjold-Nordenstam