CT scan (aka computer axial tomography, CAT scan, computer-aided tomography) uses multiple x-rays taken from different angles, to generate a tomographic image (i.e. virtual cross-sectional slice), or even 3D image using a computer, permitting the ability to see inside the body w/o cutting it open.

• Digital gemoetry processing is used to generate a 3D image of the inside of the object from a large series of 2D radiographic images taken around a single axis of rotation
• CT produces a volume of data that can be manipulated to demonstrate various bodily structures based on their ability to block the x-ray beam
• Although istorically images were generated in either the axial or transverse plane, perpendicular to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as a volumetric 3D representation of structures

• [X-ray] CT, the most ocmmon form of CT
• PET (positron emission tomography), which is a functional imaging technique that produces a 3D image of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule. 3D images of tracer concentration within the body are then constructed by computer analysis. In modern PET-CT scanners, 3D imaging is often accomplished w/ the aid of a CT scan performed on the Pt during the same session, in the same machine. If the biologically active molecule chosen for PET is FDG (flurodeoxyglucose, i.e. an analog of glucose), the concentrations of tracer imaged will indicate tissue metabolic activity as it coresponds to the regional glucose uptake. Use of this tracer to explore the possiblity of cancer metastasis is the most common type of PET scan in standard medical care (90% of current scans). Sometimes, other radioactive tracers are used in PET to image the tissue concentration of other types of molecules of intrest. One of the disadvantages of PET scanners is their operating cost
• SPECT (single-photon emission CT)

• Hypodense (non-dense) structures are dark
• Hyperdense (dense) structures are bright

• Dx
• Tx
• Its advantage to x-ray, is its depiction as slices, and thus ability to visualize internal structures
• When MRI is unavailable
• Speed is necessary, as in emergency settings when hemorrhage, stroke, or traumatic brain injury are suspected. However, even in emergency, when head injury is minor as determined by clinical exam, CT head should be avoided
• Used to supplement x-rays and ultrasonography
• Screening for disease, including:
  • CT colonography, for Pt's w/ a high risk of colon cancer
  • Full motion heart scans for people with high risk of heart disease
• CT head:
  • Used to detect:
    • Infarctions, are dark structures
    • Edema, are dark structures
    • Tumors, are indicated by swelling and anatomical distorsion they cause, or by surrounding edema
    • Calcifications, are bright areas
    • Hemorrhage, are bright areas
    • Bone trauma, are indicated by disjunction in bone windows
  • Also used in CT-guided stereotactic surgery and radiosurgery, for Tx of intracranial tumors, AV malformations, and other surgically treatable conditions using the N-localizer
• CT lung:
  • Used to detect both acute/chronic changes in the lung parenchyma (i.e. internals of the lung), particularly useful because conventional x-rays do NOT show such defects
  • For evaluation of chronic interstitial processes (emhysema, fibrosis, etc), thin sections w/ high spatial frequency reconstructions are used, often scans are performed both in inspiration and expiration, known as high resolution CT, producing a sampling of the lung and not continuous images
  • An incidentally found nodule in the absence of Sx (an incidentaloma), may raise concerns it might represent a tumor, either benign or malignant. This should NOT result in an intensive schedule of CT's to surveil the nodules, beyond the recommended guidelines, i.e. in Pt's without a prior Hx of cancer, and whose solid nodules haven't grown over a 2-year period, are unlikely to have malignant cancer
  • CTPA (CT pulmonary angiogram), used to Dx PE. It employs CT and an iodine based contrast anget to obtain an image of the pulmonary arteries
• Cardiac CT, which with the advent of subsecond rotation combined w/ multi-slice CT, high resolution and high speed, allowing excellent imaging of the coronary arteries in cardiac CT angiography
• Abdominal CT and pelvic CT, which is sensitive to Dx abdominal diseases. It is frequently used to determine stage of cancer and to follow progress. It can Ix acute abdominal pain too
• Extremities, often used to image complex fractures, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes. Fractures, ligamentous injuries, and dislocations can easily be recognized with 0.2mm resolution

• Its disadvantage is its high level of radiation, so use only when necessary
• Kidney problems may occasionally occur following IV contrast agents

• The technology is also used in non-medical fields, including nondestructive materials testing, and archaeology
• Use of CT has increased dramatically over the last 2 decades, with 72m scans performed in the USA annually
• 0.4% of current cancers in the USA are due to CT's performed in the past, with rates as high as 2% in current anunal rates of CT usage
• Ambulances may have small bore multi-sliced CT scanners, to respond to cases involving stroke or head trauma

• Radiology (category, although it usually refers to conventional non-CT x-rays)
• X-ray
• MRI (non-radiological)
• CT of the abdomen and pelvis