Positron emission tomography (PET) a combination of computed tomography and scintillation scanning. CT numbers are sometimes expressed in Hounsfield units, named after Godfrey Hounsfield, the inventor of the CT scanner Hounsfield and Allan Cormack were co-winners of the Nobel Prize in physiology or medicine in 1979 for the development of computerized axial tomography. The attenuation values are converted to CAT numbers by subtracting the attenuation value of water and multiplying by an arbitrary coefficient to produce values ranging from −1000 for air to +1000 for compact bone with water as 0. A computer program called a reconstruction algorithm can solve the problem of assigning attenuation values for all the pixels that add up to the measured values along each ray. The attenuation value for a ray is the sum of the values for all of the voxels it passes through. The actual measurements made by the scanner are the x-ray attenuations along thousands of rays traversing the slice at all angles.
The region of the tissue slice corresponding to a pixel has a cross-sectional area of 1 × 1 mm to 2 × 2 mm because of the thickness of the slice, it has a finite height and is therefore referred to as a voxel (volume element). The newer CAT scanners use a high resolution matrix with 256 × 256 or 512 × 512 pixels. It also has a better spatial resolution than scintillation imaging (about 1 mm for CAT compared to 15 mm for a scintillation camera).Ī CAT scan is divided into a square matrix of pixels (picture elements). Because it is noninvasive and has high contrast resolution, it has replaced some radiographic procedures using contrast media. Since its introduction in 1972, the use of this modality has grown rapidly. Cross-sectional anatomy can be reconstructed in several planes without exposing the patient to additional radiation.
These images are spaced at intervals of 0.5 to 1 cm. Computed tomography (CT) ( computerized axial tomography (CAT)) a radiologic imaging modality that uses computer processing to generate an image ( CAT scan) of the tissue density in a “slice” as thin as 1 to 10 mm in thickness through the patient's body.
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