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Since its introduction in 1972, the use of this modality has grown rapidly. Because it is noninvasive and has high contrast resolution, it has replaced some radiographic procedures using contrast media. It also has a better spatial resolution than scintillation imaging (about 1 mm for CAT compared to 15 mm for a scintillation camera).
A CAT scan is divided into a square matrix of pixels (picture elements). The newer CAT scanners use a high resolution matrix with 256 × 256 or 512 × 512 pixels. 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 actual measurements made by the scanner are the x-ray attenuations along thousands of rays traversing the slice at all angles. The attenuation value for a ray is the sum of the values for all of the voxels it passes through. 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 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. 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.
Most of the isotopes used in PET scanning have a half-life of only 2 to 10 minutes. Therefore, they must be produced by an on-site cyclotron and attached chemically to the tracer and used within minutes. Because of the expense of the scanner and cyclotron, PET is used only in research centers. However, PET is important because it provides information that cannot be obtained by other means. By labeling the blood with 11C-carbon monoxide, which binds to hemoglobin, images can be obtained showing the regional perfusion of an organ in multiple planes. By using labeled metabolites, images can be obtained showing metabolic activity of an organ. 15O-oxygen and 11C-glucose have been used for brain imaging and 11C-palmitate for heart imaging. 81Rb, which is distributed like potassium, is also used for heart imaging. By using labeled neurotransmitters, hormones, and drugs the distribution of receptors for these substances in the brain and other organs can be mapped.
computed tomography (Medspeak-UK)
Centre for Information Technology (Medspeak-UK)
circadian time (circadian rhythm, see there)
clinical tutor (Medspeak-UK)
core training (Medspeak-UK)
cortical plate thickness
cover test (ophthalmology)
CTComputed tomography, see there; also
CT or CAT
Patient discussion about CT
Q. My brother is alcoholic.Doctor had told to have blood test, CT scan.What exactly he facing? My brother is alcoholic. He had made his family life like a hell. He beats his wife every day. He argues with mom on every simple issue and makes it complex. He dislikes his children and has no compassion for them. He had lost on his job. He fights with our neighbors and has made life hell for us to live here in this apartment. I don’t know now a days he going though a very bad problem he dislikes eating. Due to that he had lost on his weight and feels weak. His eyes are pale and body temperature is very high. Doctor had told to have blood test, CT scan. What exactly he facing...is this very serious? Please give some genuine information.
Q. I get bad headaches had ct scans and m.r.i. even sinus surgery, suffering 2yrs now, dizzness occurs too..