Spiral CT Scanning:Principles and Clinical Applications in Diagnostic Imaging of the Liver and Pancreas
The introduction of spiral CT or helical CT,also known as volumetric spiral CT,represents the most important step forward in the evolution of CT scanning technology in recent years. The major components of a typical spiral CT scanner consist of a rotating gantry,a high heat capacity X-ray tube,a mobile examination table,a sophis- ticated computer system and a powerful imaging processing workstation.By replacing traditional electric cables with slipring techniques,spiral CT enables the simultaneous acquisition of data and pa- tient translation over an area of the human body in a very brief scanning time.Compared to conventional CT,the advantages of spiral CT are obvious: 1.A volume of data will be generated in a set,in- stead of individual slices. 2.The scanning time will be dramatically short- ened,toa subsecond level(0.5-0.75see.).The total scanning will be completed during a brief single breath hold period(15-20 sec.) so that respira- tion related motion artifact and slice misregistration can be significantly avoided. 3.Data acquisition can be done at the peak en- hancement of the targeted organ,due to the quick- ness of the spiral CT scan speed.As a result,the detectability of lesions,especially those with early subtle lesions,will be greatly improved as the lesion is conspicuously highlighted. 4.Various imaging processes can be performed to the obtained a volumetric data-set,such as two- dimensional and three dimensional image recon- struction.One unique advantage of spiral CT image processing is that any axial image reconstruction can be obtained at arbitrarily determined section posi- tions and spacing.Therefore,partial volume aver- aging of small lesions can be effectively minimized 5.With the support of powerful computer software and color encoding techniques,virtual endoscopy of spiral CT can be performed for the delineation of the lumens and cavities of the human body,such as the airway,gastrointestinal tract,blood vessels,biliary tree and paranasal sinuses,etc. The liver receives double the blood supply(hepatic artery and portal vein),and hepatic lesions of dif- ferent pathology demonstrate different patterns of vascular drainage.These vascular features of the liver provide spiral CT with many important clinical application potentialities.The dual-phase,tri- phase or multi-phase spiral CT imaging of the liver (hepatic arterial phase,portal venous phase,equi- librium phase and delayed phase) not only reflect the vascular characteristics of hepatic lesions,but also greatly increase the lesions conspicuousness.There- fore,better detection and characterization of hepatic lesions can be achieved. Spiral CT technology also represents the state-of- the-art in the imaging evaluation of known or sus- pected pancreatic pathology.Dual phase scanning of the pancreas is helpful for improving lesion de- tectability and characterization accuracy.Three- dimensional depiction of peripancreatic vasculature offers valuable assistance in the preoperative staging and the possibility for resection assessment of pan- creatic malignancies. In summary,the advent of spiral CT has resulted in the improved efficiency of existing applications of CT and the creation of many new applications.It represents the future direction in the development of CT technology and it is believed it will play a more and more important role in the medical evaluation of human body pathology.