Basics of Computed Tomography

Computed Tomography also known as computed axial tomography, or CAT scan, medical technology that uses X rays and computers to produce three-dimensional images of the human body. Unlike traditional X rays, which highlight dense body parts, such as bones, CT provides detailed views of the body's soft tissues, including blood vessels, muscle tissue, and organs, such as the brain. While conventional X rays provide flat two-dimensional images, CT images depict a cross-section of the body.

The CT scanner contains an X-ray source, which emits beams of X rays; an X-ray detector, which monitors the number of X rays that strike various parts of its surface; and a computer. The source and detector face each other on the inside of the scanner ring and are mounted so that they rotate around the rim of the scanner. Beams from the X-ray source pass through the patient and are recorded on the other side by the detector. As the source and detector rotate in a 360 circle around the patient, X-ray emissions are recorded from many angles. The resulting data are sent to the computer, which interprets the information and translates it into images that appear as cross-sections on a television monitor. By moving the patient within the scanner, a series of parallel images, called slices can be obtained. Doctors analyze a series of slices to understand the three-dimensional structure of the body.

To enhance an image, patients may be given an injection of a substance that will increase the contrast between different tissues. The patient may also be asked to drink a liquid that makes intestines more
clearly visible in the CT scan. Contrast agents are commonly used for scans of the chest, abdomen, and pelvis. The speed at which CT slices are obtained has increased dramatically since this technology was developed during the late 1960s and early 1970s by British engineer Sir Godfrey Newbold Hounsfield and American physicist Allan MacLeod Cormack. The first CT scanner required 4.5 minutes of scanning and 1.5 minutes of computer reconstruction to generate a single slice. The latest generation of CT scanners create a slice in fraction of a second.

CT was first used to view the tissues of the brain, and this is still its most frequent application. CT of the brain can show if an accident victim has sustained a brain injury, or if bleeding is occurring in the brain of a stroke victim. CT is also commonly used to diagnose disorders involving the chest, abdomen, spine, and pelvis. CT may be used to determine the location and size of a cancerous tumor and whether the cancer has spread to other parts of the body. Newer applications for CT include CT angiography, which provides detailed images of the interior of small blood vessels.

Some experts once expected that CT would be replaced by the newer technology of magnetic resonance imaging (MRI), which uses magnetically charged protons within the body to create diagnostic images. Nevertheless, CT continues to have several advantages over MRI, including lower cost, wider availability, shorter examination time, superior imaging of the chest and abdomen, and the ability to create images without the strong magnetic field required by MRI. At present, CT and MRI are complementary, rather than competitive technologies.

SPIRAL C.T. (SCT)

Spiral C.T. (SCT) with slip ring technology had a dramatic impact on the approach to C.T. scanning since its introduction, especially at 'speed' of scanning. Spiral C.T. allows 3-D imaging that is useful for vascular imaging imaging of hollow viscera especially the virtual endoscopy has become a reality with SCT. The new hardware and software packages and computer guided Automatic Scanning Technique (C.A.S.T) allows monitoring real time contrast enhancement following administration of I.V. contrast in target organs or vessels (not bolus contrast medium advance), which in turn reduces the amount of C.M. to be used.

Spiral C.T. had a dramatic impact of change of protocols in the form of change in parameters, length of spiral exposure, inter relationship between pitch and collimators, 3-D image displays and above all, proper understanding of I.V. contrast dynamics when compared to C.C.T.. In C.C.T., scans are generally performed with 2 seconds scan time and an interscan delay (ISD) of 6-7 seconds. But in SCT not only the scan but also the ISD for a long length of a segment of the body has been eliminated by volumetric scanning. SCT is expected to replace the CCT eventually by volumetric scanning due to its wide spread clinical applications and acceptance.Such ultrafast scanning has virtually eliminated artifacts occurring as a result of motion. Spatial and contrast resolutions are superior with SCT than CCT.

There are certain limitations of low end SCT. These are limitations of mAs (especially in large patients) in whom scan time have to be increased to 1.5 times. Otherwise with lower mAs, the images are excessively grainy, only higher MHU tubes and refinement of detector technology will decrease these limitations.

IMPORTANCE OF SPIRAL C.T. IN COmputerised Tomography Angiography

The area in which spiral scanning has made the greatest impact is in vascular imaging. CT Angiography is a minimally invasive medical test that helps physicians diagnose and treat medical conditions. CT angiography has been shown to be effective in a number of vascular systems including the renal arteries, the aorta and its main branches, the cerebral circulation, the portal venous system, and the pulmonary arteries. Maximum Intensity Projection (MIP) and surface rendering were the 3-D techniques which had been primarily applied to CT angiographic applications. Both of these techniques have important shortcomings which ultimately limit the clinical usefulness of the resulting images. But volume rendering is becoming the preferred 3-D rendering technique for CT angiography.

In CT angiography (CTA), computed tomography using a contrast material produces the detailed pictures. CT imaging uses special x-ray equipment to produce multiple images and a computer to join them together in cross-sectional views. A firm understanding of fundamental principles underlying CT angiography including spiral CT acquisition, image processing, and image display are required in order to get consistently excellent results over a wide range of clinical applications. One of the most compelling advantages of CT angiography is the ability to provide all of the information which previously required two or more radiological studies which may, in the case of conventional angiography, be much more expensive than CT. Image processing techniques such as volume rendering enhance this ability, allowing the radiologist and clinician to interactively explore different aspects of the dataset to address many specific questions which impact patient management.

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