Final answer:
The late arterial phase is best for detecting tumor nodules in a triphasic CT scan of the liver, as it highlights the vascularity of liver lesions. Subsequent venous and late venous phases provide further details on the liver's parenchyma and vascular structure.
Step-by-step explanation:
The phase that will help detect tumor nodules during a triphasic CT examination of the liver is the late arterial phase. This phase allows the radiologist to assess the vascularity of liver lesions. Typically, liver tumors receive their blood supply from the hepatic artery, and thus, they enhance during the arterial phase. Contrast enhancement during this phase can highlight hyper vascular lesions such as hepatocellular carcinoma, which may not be as apparent on other phases.
The other phases, which include the venous phase and the late venous phase, provide additional information. The venous phase aids in visualizing the overall liver parenchyma and detecting hypo vascular lesions, while the late venous phase is helpful in assessing lesion washout characteristics and the venous structures.