Imaging the liver
The major advances which have taken place over
recent years in surgical approaches to the liver and the enormous improvement in
the safety of liver surgery are due to the careful individualised planning of
surgery which is possible owing to the information obtained by preoperative
imaging. The ideal choice of imaging modality is determined by the likely liver
pathology and the locally available equipment and radiological expertise.
Ultrasound
This is the first-line test owing to its
safety and availability. It is entirely operator dependent. It is useful for
determining bile duct dilatation, the presence of gallstones (Fig.
52.4) and the
presence of liver turnouts. Doppler ultrasound allows flow in the hepatic
artery, portal vein and hepatic veins to be assessed. In some countries it
is used as a screening test for the development of primary liver turnouts in
a high-risk population. Ultrasound is useful in guiding the percutaneous biopsy
of a liver lesion.
Computerised
tomography (CT)
The current ‘gold standard’ for liver
imaging is triple-phase spiral CT. This provides fine detail of liver lesions
down to less than 1 cm in diameter and gives information on their nature (Fig. 52.5).
Oral contrast enhancement allows visualisation of the stomach and duodenum
in relation to the liver hilum. The early arterial phase of the intravenous
contrast vascular enhancement is particularly useful for detecting small liver
tumours owing to their preferential arterial blood supply. The venous phase
maps the branches of the portal vein within the liver and the drainage via the
hepatic veins. Inflammatory liver lesions often exhibit rim enhancement with
intravenous contrast, whereas the common haemangioma characteristically shows
late venous enhancement. The density of any liver lesion can be measured, which
can be useful in establishing the presence of a cystic lesion.
Magnetic
resonance imaging (MRI)
MRI (Fig. 52.6) would appear to be as
effective an imaging modality as CT in the majority of patients with liver
disease. It does, however, offer several advantages. First, many patients are
precluded from iodine-containing intravenous contrast agents because of a
history of allergy. These patients should be offered MRI rather than contrast
CT. Second, magnetic resonance
cholangiopancreatography (MRCP) can produce excellent quality imaging of the
biliary tract noninvasively. The image quality is currently below that available
from endoscopic retrograde cholangiopancreaography (ERCP) or percutaneous
transhepatic cholangiography (PTC) but this is rapidly improving. Currently,
it should be considered for diagnostic questions where ERCP has failed or is
impossible due to previous surgery. Magnetic resonance angiography (MRA) similarly provides high-quality
images of the hepatic artery and portal vein without the need for arterial
cannulation. It is used as an alternative to selective hepatic angiography for
diagnosis. It is particularly useful in patients with chronic liver disease and
a coagulopathy in whom the patency of the portal vein and its branches is in
question.
Endoscopic
retrograde
cholangiopancreatography
ERCP (Fig. 52.7) is required in patients with
an obstructive pattern of liver function tests or in whom imaging has suggested
an abnormality of the biliary tract. A preoperative check of coagulation is
essential, along with prophylactic antibiotics and an explanation of the main
complications which include pancreatitis, cholangitis and bleeding or
perforation of the duodenum related to sphincterotomy. Endoluminal ultrasound of the biliary tract is possible using a
‘baby’ scope, and may provide additional information on the extent of hilar
tumours. Therapeutic interventions are also possible at the time of ERCP and
include stone retrieval, balloon dilatation of strictures, endoprosthesis
insertion and brush cytology of turnouts to provide a tissue diagnosis.
Percutaneous
transhepatic cholangiography
PTC is indicated where endoscopic
cholangiography has failed or is impossible, as in patients with previous Polya gastrectomy. It is often required in patients with hilar bile duct
tumours where
endoscopic cholangiography fails to visualise the intrahepatic bile ducts (Fig.
52.7).
Angiography
Selective visceral angiography (Fig.
52.1) may
be required both for diagnostic purposes and for therapeutic intervention.
Prior to liver resection it may be
used to visualise the anatomy of the hepatic artery to the right and left sides
of the liver and to confirm patency of the portal vein. It can also
Nuclear
medicine scanning
Radioisotope scanning can provide diagnostic
information which cannot be obtained by other imaging modalities. Iodoida is a
technetium-99m (99mTc)labelled radionuclide which is administered
intravenously, removed from the circulation by the liver, processed by
hepatocytes and excreted in the bile. Imaging under a gamma camera allows
Laparoscopy and
laparoscopic ultrasound
Laparoscopy is useful for the staging of
hepatopancreatobiliary cancers. Lesions which have failed to be detected by
conventional imaging are mainly peritoneal metastases and superficial liver
tumours. Approximately 30 per cent of patients may have additional lesions
detected by laparoscopy which have not been shown on good-quality planar imaging
by CT or MRI. Laparoscopic ultrasound may increase this figure and provides
additional information with liver tumours on their proximity to the major
vessels and bile duct branches.
Fluorodeoxyglucose—position
emission tomography
(FDG—PET)
This new imaging modality depends on the avid
uptake of glucose by cancerous tissue in comparison to benign or inflammatory
tissue. At present it is mainly used
to determine the nature of a mass lesion demonstrated on another form of
imaging. Deoxyglucose is labelled with the positron emitter fluorine-18 (15FDG)
and this is administered to the patient prior to imaging by positron emission
tomography (PET). A three-dimensional image of the whole body is obtained,
highlighting areas of increased glucose metabolism (Fig. 52.8).