Anatomy
- The conspicuity of a liver lesion depends on the attenuation difference between the lesion and the normal liver.
- On a non enhanced CT-scan (NECT) liver tumors usually are not visible, because the inherent contrast between tumor tissue and the surrounding liver parenchyma is too low.
- Only a minority of tumors contain calcifications, cystic components, fat or hemorrage and will be detected on a NECT.
- So i.v. contrast is needed to increase the conspicuity of lesions.
- When we give i.v. contrast, it is important to understand, that there is a dual blood supply to the liver.
- Normal parenchyma is supplied for 80% by the portal vein and only for 20% by the hepatic artery, so it will enhance in the portal venous phase.
- All liver tumors however get 100% of their blood supply from the hepatic artery, so when they enhance it will be in the arterial phase.
- This difference in bloodsupply results in different enhancement patterns between liver tumors and normal liver parenchyma in the various phases of contrast enhancement (figure).
CT
Phases
Arterial
- In the arterial phase hypervascular tumors will enhance via the hepatic artery, when normal liver parenchyma does not yet enhances, because contrast is not yet in the portal venous system.
- These hypervascular tumors will be visible as hyperdense lesions in a relatively hypodense liver.
- However when the surrounding liver parenchyma starts to enhance in the portal venous phase, these hypervascular lesion may become obscured.
Portal venous phase
- In the portal venous phase hypovascular tumors are detected, when the normal liver parenchyma enhances maximally.
- These hypovascular tumors will be visible as hypodense lesions in a relatively hyperdense liver.
Equolbrium
- In the equilibrium phase at about 10 minutes after contrast injection, tumors become visible, that either loose their contrast slower than normal liver, or wash out their contrast faster than normal liver parenchyma.
- These lesions will become either relatively hyperdense or hypodense to the normal liver.
Hypervascular liver lesion
Non-neoplastic
Focal nodular hyperplasia (FNH) Haemangioma vascular shunts / fistulas focal hepatic hot spot sign hepatic artery aneurysm failing Fontan circulation 3 hepatic peliosis background liver disease (cirrhosis) 6
- regenerative nodules
- dysplastic nodules
Neoplastic
hepatocellular carcinoma (HCC) hepatic adenoma
Metastases Although the majority of liver metastases are hypodense and enhance less than the surrounding liver, metastases from certain primaries demonstrate an increase in the number of vessels, resulting in a hyperechoic ultrasound appearance, and arterial phase hyperenhancement on CT or MRI which washes out on delayed scan (cf. haemangioma which does not show washout). The primaries typically include:
- renal cell carcinoma (RCC)
- thyroid carcinoma
- neuroendocrine tumours
- carcinoid
- islet cell tumours
- phaeochromocytoma
- leiomyosarcoma
- choriocarcinoma
- melanoma
- breast cancer
- colonic carcinoma 5
- ovarian cystadenocarcinoma 5