Imaging Techniques

The advent of multidetector CT resulted in the concept of organ imaging in multiple planes, rather than single sequential slices. Thus, with multi-detector systems of more than 40 channels, one can image the entire abdomen in less than ten seconds, and the liver in five seconds or less, using the thinnest detector configuration of less than a millimeter, maximizing multiplanar reconstruction capabilities. When viewed electronically, reconstructed sections of 3-5 mm are preferred in order to realistically view these images. Timing of image acquisition in relation to contrast material administration depends on whether one needs to image during early arterial phase (for arterial anatomy), late arterial phase (for hypervascular tumor detection/characterization) or portal venous phase (for most routine imaging and hypovascu-lar tumor detection). Typically, 42-45 g of iodine should be administered, which can be accomplished with 120150 ml of contrast, depending on concentration. We administer contrast at 3 cc/sec when only portal phase imaging is to be done, and at 5 cc/sec for arterial phase imaging, with timing at 20 sec for early arterial phase (arterial mapping) and 30-35 sec for late arterial (tumor visualization). Timing of portal phase imaging varies depends on the rate of infusion and patient's fluid volume status and vascular circulation time (approximately 70 sec for 3 cc/sec, and 60 sec for 5 cc/sec). The choice of combinations of these phases of imaging depends on the individual indication [1, 2].

MR examination of liver includes sequences providing T1, T2, and with contrast enhancement. Specific pulse sequences depend on the make of the scanner, patient compliance and the clinical question being addressed [3]. In-and out-of-phase T1 imaging is recommended to allow for maximal tumor detection and characterization of fat. In general, dynamic imaging with extracellular gadolinium-based contrast agents is used for lesion characterization and detection of tumors in a setting of cirrhosis, and tissue-specific agents are also used to maximize detection of metastases in a non-cirrhotic liver [4, 5].

US can be performed with and without the use of contrast agents, which otherwise improve lesion detection and characterization, while scintigraphy may be used in selected cases for differentiation of benign and malignant tumors. However, new MR hepatocyte specific agents can perform this function as well, avoiding additional imaging tests.

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