الفهرس 
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Abstract
MDCT is a rapidly evolving technique that significantly improves CT imaging for several indications. The advantages of MDCT include a reduced image acquisition time allows more images to be obtained within one breath hold, easier timing of scanning in synchronization with liver vascular phases, and better spatial resolution allowing high quality non axial reformations and 3D reconstructions. MDCT currently allows the acquisition of thin slices in daily routine diagnostics providing an improved detection rate of small liver lesions. Whereas large benign focal liver lesions exhibit typical patterns of morphology, attenuation and perfusion, which also may be assessed with SSCT, small lesions remain challenging even with MDCT, since the specific criteria for confident diagnosis become more ambiguous. Here, MR imaging provides more detailed information about tissue components and the availability of liver-specific contrast agents, adding further impact to this technique. For diagnosing a focal benign or malignant lesion the role of CT is to demonstrate the nature of the tumor and its relationship to the surrounding liver tissue and vessels as well as the extrahepatic structures. In this regard CT has to be highly reliable and specific in order to avoid further investigations and to give the best image to the referring physicians for possible intervention. In cases of extrahepatic disease, the role of CT is to rule out intrahepatic metastasis where maximum sensitivity and negative predictive value are requested. Taking into account the broad prevalence and incidence of focal liver lesions, clinical imaging and differential diagnosis should primarily focus on the most common entities and thereafter will have to consider the more rare entities. MDCT of the liver is challenging in terms of distinguishing benign lesions, which in most instances can be left alone, from malignant neoplasms requiring therapy. Whether liver CT derives any benefit from the evolution to 64-detector-row scanners from 16- row or not is still quationable. Indeed, even with 4-row scanners, thin-slice multiphase data sets with short breath-holding periods are possible. However, over beaming was a substantial problem in the early MDCT systems with up to 4 detector rows since relevant radiation exposure occurs at the margins of the beam fan, which can not be translated into image reconstruction but contributes to an increase of the patient’s effective dose. This drawback is almost negligible in the advanced scanners with at least 16 rows . However, until now, there has been no concrete evidence as to whether increasing the number of detector rows beyond 16 is of any further benefit for liver CT with respect to detection and characterization of focal tissue lesions. Liver CT perfusion imaging has opened up a new area for its clinical application. One single CT scan can provide both morphologic and functional information, so that clinicians can detect the disease before morphological changes and evaluate the effect of treatment. Multi-slice CT has a high time and spatial resolution for the measurement of perfusion. It is an effective non-invasive method for assessment of tumor vascularity and follow up after intervention. CT perfusion imaging is safe, reproducible, easy to operate. It can aid in detection of dysplastic nodules in cirrhotic liver before being frank HCC which may be critical to patient outcome, but it cannot be used to indicate the grade of HCC and differientiation between various types of hypervascular lesions . Radiation dose may be a limitation to the frequent use of perfusion CT .However as CT technology evolves, the radiation exposure is reduced because of decreased scanning time and improved detector sensitivity.