CT的基本知识.ppt

CT number of different tissues CT number (HU) of the substance of interest = 1000 (u- uw)/(uw - ua) Here, uw, ua, and u are the linear attenuation coefficients of water, air and a substance of interest. 2000 or more HU are resolved by CT, only 16 gray-scales can be resolved by human eyes，so window technique of image must be used Window width, the range of CT number used in image Window level: the center of used CT numbers in image Proper window width and window level are essential for CT imaging ★ WW=100,WL=50 2.3 Classification of CT According to the scanning pattern Conventional CT Spiral CT Single slice Multi-slice spiral CT 2.4 Examination methods of CT 1.Plain scan: without intravenous injection of contrast agent 2.Contrast-enhanced scan With intravenous injection of contrast agent Increase CT number of tissues of interest Conventional contrast-enhanced scan Dynamic and muti-phase contrast-enhanced scan 2.5 CT Image post-processing Multiplanar reformation (MPR) Maximum of minimum intensity projection (MIP) Surface shaded display (SSD) Volume rendering (VR) Virtual endoscopy (VE) CT angiography (CTA) In this unit system, water is assigned a value of zero and air is assigned a value of -1000. peripheral nodular enhancement and progressive centripetal fill-in Plain scan Arterial phase Venous phase Multiphase scan Acquiring a series of CT images of the same slice locations in (rapid) succession such that time dependent changes of contrast enhancement can be studied. HCC Arterial phase Venous phase Late venous phase Plain scan Focal Nodular Hyperplasia Multiphase scan Nodular enhancement appears in the rim of the lesion(A)，showing similar density with blood vessel. With the time passing by, enhancement spreads to the center and fill the whole lesion(C) A B C Hemangioma Delay phase Arterial phase Venous phase Multiphase scan: CT Myelography involves opacification of the subarachnoid space by injection of contrast media and followed by CT sc