Also found in: Dictionary, Thesaurus, Legal, Financial, Acronyms, Encyclopedia, Wikipedia.


any method that produces images of single tissue planes. In conventional radiology, tomographic images (body section radiographs) are produced by motion of the x-ray tube and film or by motion of the patient that blurs the image except in a single plane. In reconstruction tomography (CT and PET) the image is produced by a computer program.
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. These images are spaced at intervals of 0.5 to 1 cm. Cross-sectional anatomy can be reconstructed in several planes without exposing the patient to additional radiation.

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.
Computed tomography. Relative position of the x-ray tube, patient, and detectors in a fourth generation CT unit.
electron beam computed tomography (EBCT) ultrafast computed tomography done with a scanner in which the patient is surrounded by a large circular anode that emits x-rays as the electron beam is guided around it.
extended narrow tomography tomography involving an increase in amplitude and increase in exposure angle resulting in greater thinness of the cut for examination.
linear tomography tomography in which the tube and film move in the same direction.
narrow angle tomography a type of tomography that results in thicker sections for examination.
pluridirectional tomography tomography in which there is a great deal of movement in a variety of directions.
positron emission tomography (PET) a combination of computed tomography and scintillation scanning. Natural biochemical substances or drugs tagged with a positron-emitting radioisotope are administered to the subject by injection; the tagged substance (tracer) then becomes localized in specific tissues like its natural analogue. When the isotope decays, it emits a positron, which then annihilates with an electron of a nearby atom, producing two 511 keV gamma rays traveling in opposite directions 180 degrees apart. When the gamma rays trigger a ring of detectors around the subject, the line between the detectors on which the decay occurred is stored in the computer. A computer program (reconstruction algorithm), like those used in computed tomography, produces an image of the distribution of the tracer in the plane of the detector ring.

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.
single-photon emission computed tomography (SPECT) a type of tomography in which gamma photon–emitting radionuclides are administered to patients and then detected by one or more gamma cameras rotated around the patient. From the series of two-dimensional images produced, a three-dimensional image can be created by computer reconstruction. The technique improves resolution of, and decreases interference by, overlapping organs. It is used particularly for assessment of cardiac disease, stroke, and liver disease; for staging of cancer; and to diagnose physical abnormalities through evaluation of function.
ultrasonic tomography the ultrasonographic visualization of a cross-section of a predetermined plane of the body; see B-mode ultrasonography.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


Abbreviation for computed tomography.
Farlex Partner Medical Dictionary © Farlex 2012


1. Central Time
2. also Ct. Connecticut
3. computed tomography
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


Abbreviation for:
computed tomography (Medspeak-UK)
carbon tetrachloride
cardiac type
carotid tracing
carpal tunnel
cellular telephone
cellular therapy
Centre for Information Technology (Medspeak-UK)
cerebral thrombosis
cerebral tumour
chemical test
chest tube
child trends
cholera toxin
chorda tympani
circadian time (circadian rhythm, see there)
circulation time
clinical tutor (Medspeak-UK)
clotting time
coated tablet
cognitive therapy
collecting tubule
compressed tablet
confirmatory test
constitutive transcript
conventional therapy
connective tissue
continue treatment
contraceptive technique
Coombs’ test
core training (Medspeak-UK)
corneal transplant
coronary thrombosis
corrected transposition
corrective therapist
cortical plate thickness
cover test (ophthalmology)
crista terminalis
cystine-tellurite (medium)
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.


Computed tomography, see there; also
1. Carpal tunnel.
2. Chemotherapy.
3. Chest tube.
4. Cognitive therapy, see there.
5. Connective tissue.
6. Continue treatment.
7. Crossmatch:transfusion.
8. Cytotechnologist.
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


Abbreviation for computed tomography.


Abbreviation for concentration-time product.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


Computed tomography, a radiologic imaging that uses computer processing to generate an image of tissue density in slices through the patient's body.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


Abbreviation for computed tomography.
Medical Dictionary for the Dental Professions © Farlex 2012

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.

A. YES HIS PROBLEM IS VERY SERIOUS,but right now,HE is the ONLY one that can help himself....YOU AND YOUR FAMILY SHOULD WORRY ABOUT YOURSELVES at thi point.SUGGEST;;;for your family not to give him money---suggest..if he is living at home with mother/you..put him out.THAT IS THE ONLY THING THAT WILL STOP THE DRAMA,and ABUSE....HIS WIFE SHOULD GO TO COURT AND GET A ORDER FOR HIM NOT TO COME AROUND HER AND THE KIDS,NEXT TIME HE STARTS SHIT...CALL THR POLICE....THIS IS HIS PROBLEM NOT YOURS...you cant save your brother..he is not going to listen to you with alcohol and drugs in his system...you can suggest going into detox and then into rehab and a drug program,alcoholism and drug use can destroy your family..if you and the family dont get help----you can find people who will talk to you about your problem at ALCOHOLICS ANONYMOUS/NORCOTICS ANONYMOUS..800#..help line...GO TO (INTHEROOMS.COM).....peace mrfoot56

Q. I get bad headaches had ct scans and m.r.i. even sinus surgery, suffering 2yrs now, dizzness occurs too..

A. If all prior medical investigations turned out normal, and sinus surgery didn't help relieve your symptoms, I would suggest the reason for your headaches is probably migraine attacks, that can cause severe headaches, and no CT scan or MRI can diagnose them. The diagnosis is made clinically, by your doctor. Migraine headaches can be eased by proper medications, before and during an attack. You should consult a neurologist.

More discussions about CT
This content is provided by iMedix and is subject to iMedix Terms. The Questions and Answers are not endorsed or recommended and are made available by patients, not doctors.