WHO criteria for diagnosing Hepatocellular carcinoma includes:
1. Alpha-foetoprotein (AFP)
2. Triphasic CT scan is the gold standard investigation
Four Phases of CT scan:
The four phases are pre contrast, arterial phase, portal venous phase, and delayed phase. Multiphase liver CT is used to detect and characterise liver lesions as different types of tumours enhance differently during each phase depending on whether they are hypervascular or hypovascular lesions.
Precontrast liver scans are used to detect calcifications, visualise haemorrhage from trauma, and demonstrate hypervascular lesions which appear hypodense compared to the surrounding liver parenchyma.
Arterial phase of scanning is performed approximately 30 seconds after the contrast injection is initiated and is most accurately detemined by using bolus tracking software (eg SmartPrep) to monitor the level of contrast enhancement in the aorta and automatically triggering the scan when it reaches a pre determined level of enhancement (eg 120HU). Hypervascular lesions enhance during the arterial phase and apper hyperdense. Arterial phase images are also used for pre operative evaluation of the arterial vasculature through the use of MIPs and 3D reconstructions.
Portal venous phase is performed 70-90 seconds post contrast and hypovascular lesions appear hypodense and hypervascular lesions appear isodense (same density as surrounding liver).
Delayed phase is performed 5-10 minutes post contrast and is used to further characterise lesions. Haemangiomas are slow to enhance and some HCC can appear hypodense due to rapid washout and CCC can appear hyperdense due to delayed washout.
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