GI Radiology > Liver > Imaging Modalities > CT

 Modalities of Liver Imaging

CT

  • Uses X-ray to acquire data that can be displayed in axial, coronal, and sagittal planes.
     
  • I.V. iodinated contrast is commonly used in liver imaging to demonstrate any abnormal enhancement of a hepatic lesion and to show vascular structures.
     
  • An I.V. bolus of 100 to 150 ml of iodinated contrast is often used. The contrast agent is injected into veins, travels to the heart, aorta, celiac trunk, hepatic arteries, liver parenchyma and mixes with blood in portal veins drained into venules and then hepatic veins and then out to the IVC.
     
  • Thanks to this temporal and spatial disposition of the injected contrast agent, the hepatic enhancement can be further divided into 3 phases: arterial phase (when the contrast just fills up the aorta and the main hepatic arterial structures), portal venous phase (when the contrast disperses into the liver parenchyma and mixes with portal blood), equilibrium phase (when the contrast further scatters in the parenchyma and drains out the hepatic veins and also be seen in the renal collecting system).


  • When searching for hypervascular lesions, such as hepatoma or metastastic disease, a three-phase technique often should be used: non-contrast phase, arterial phase, and portal venous phase.
     
  • The appropriate delay times for scanning in the arterial phase and portal venous phase for a 2-3 ml/sec injection are 25 seconds and 70 seconds, respectively.
     
  • The rationale behind this technique is that primary and secondary malignancies of the liver typically have hepatic arterial supply, thus will enhance during the arterial phase, whereas benign entities and normal liver parenchyma have primarily portal venous supply, therefore, will enhance during portal-venous phase of I.V. contrast.

  • The liver parenchyma is normally homogeneous. Typically, the liver's attenuation is 54-60 Hounsfield units (HU), usually 8-10 HU greater than the spleen.  Thus, the liver will appear hyperdense to (brighter than) the spleen. If the density of the liver on CT is less (darker) than that of the spleen, fatty change is most likely cause. 
     
  • The vascular structures can be identified by their location on the unenhanced images and confirmed by enhancement with intravenous contrast. The intrahepatic biliary tree is not normally visualized although the main right and left hepatic ducts are increasingly demonstrated on modern systems and the common hepatic duct and common bile duct are normally seen.
 

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