Computed tomography

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(Redirected from CAT scan)

Computed tomography (CT), originally known as computed axial tomography (CAT) and body section roentgenography, is a medical imaging method employing tomography where digital processing is used to generate a three-dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word "tomography" is derived from the Greek tomos (slice) and graphia (describing).

Although most common in healthcare, CT is also used in other fields, e.g. nondestructive materials testing.

Contents

History

The CT system was invented in 1972 by Godfrey Newbold Hounsfield of EMI Central Research Laboratories (now Sensaura [1] owned by Creative Technology Ltd.) using X-rays. Allan McLeod Cormack of Tufts University independently invented the same process and they shared a Nobel Prize in medicine in 1979. The first scanner, known as the EMI Scanner, took several hours to acquire the raw data and several days to produce the images. The first EMI scanner was limited to making tomographic sections of the brain. It required the use of a water-containing device that enclosed the patient's head. The first CT system that could make images of any part of the body, and did not require the "water bottle" was the ACTA scanner designed by Robert S. Ledley, DDS at Georgetown University.

The first generation CT scanners used a pencil-thin beam of radiation directed at one or two detectors. The images were acquired by a "translate-rotate" method in which the x-ray source and the detector in a fixed relative position move across the patient followed by a rotation of the x-ray source/detector combination by one degree. Pairs of images were acquired in about 5 minutes. The first generation EMI scanner was limited to examining the brain.

The second generation of CT scanners increased the number of detectors and changed the shape of the radiation beam. The x-ray source changed from the pencil-thin beam to a fan shaped beam. The "translate-rotate" method was still used but there was a significant decrease in scanning time. Rotation was increased from one degree to thirty degrees.

The third generation of CT scanners made a dramatic change in the speed at which images could be obtained. In the third generation a fan shaped beam of x-rays was directed to an array of detectors that was fixed in position relative to the x-ray source. The slow "translate" portion of the scan was eliminated. Scan time per slice was reduced to 10 seconds initially.

The fourth generation of CT scanners achieved scan time similar to the third generation by employing a 360 degree ring of detctors that encircled the patient. The fan shaped x-ray beam rotated around the patient directed at detectors in a non-fixed relationship.

Improvements in CT scanner technology have developed with improvements in computer capabilities and detector technology and other improvements of movement of patients through the scanner.

Modern multi-detector, multi-row CT systems can complete a scan of the chest, for example, in less time than it takes for a single breath hold and display the computed images in near real time. Images that used to take hours to acquire and days to process are now accomplished in seconds. The number of cross sectional images that can be produced has increased from about a dozen to many hundreds.

In recent years, tomography has also been introduced on the micrometer level and is named Microtomography. But these machines are currently only fit for smaller objects or animals, and cannot yet be used on humans.

Principles

X-ray slice data is generated using an X-ray source that rotates around the object; X-ray sensors are positioned on the opposite side of the circle from the X-ray source. Many data scans are progressively taken as the object is gradually passed through the gantry. They are combined together by the mathematical procedure known as tomographic reconstruction.

Newer machines with faster computer systems and newer software strategies can process not only individual cross sections but continuously changing cross sections as the gantry, with the object to be imaged, is slowly and smoothly slid through the X-ray circle. These are called helical or spiral CT machines. Their computer systems integrate the data of the moving individual slices to generate three dimensional volumetric information, in turn viewable from multiple different perspectives on attached CT workstation monitors.

In conventional CT machines, an X-Ray tube is physically rotated behind a circular shroud (see the image above right); in the less used electron beam tomography (EBT) the tube is far larger, note the internal funnel shape in the photo, with a hollow cross-section and only the electron current is rotated.

The data stream representing the varying radiographic intensity sensed reaching the detectors on the opposite side of the circle during each sweep— 360 or just over 180 degrees in conventional machines, 220 degree in EBT —is then computer processed to calculate cross-sectional estimations of the radiographic density, expressed in Hounsfield units.

CT is used in medicine as a diagnostic tool and as a guide for interventional procedures. Sometimes contrast materials such as intravenous iodinated contrast is used. This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues.

Pixels in an image obtained by CT scanning are displayed in terms of relative radiodensity. The pixel itself is displayed according to the mean attenuation of the tissue that it corresponds to on a scale from −1024 to +3071 on the Hounsfield scale. Water has an attenuation of 0 Hounsfield units (HU) while air is −1000 HU, bone is typically +400 HU or greater and metallic implants are usually +1000 HU.

Improvements in CT technology have meant that the overall radiation dose has decreased, scan times have decreased and the ability to reconstruct images (for example, to look at the same location from a different angle) has increased over time. Still, the radiation dose from CT scans is several times higher than conventional X-ray scans.

As of 2005, the cost of an average CT scanner is US$1.3 million.

Diagnostic use

Since its introduction in the 1970s, CT has become an important tool in medical imaging to supplement X-rays and medical ultrasonography. Although it is still quite expensive, it is the gold standard in the diagnosis of a large number of different disease entities.

Cranial CT

Diagnosis of cerebrovascular accidents and intracranial hemorrhage is the most frequent reason for a "head CT" or "CT brain". Scanning is done without intravenous contrast agents (contrast may resemble a bleed). CT generally does not exclude infarct in the acute stage, but is useful to exclude a bleed (so anticoagulant medication can be commenced safely).

For detection of tumors, CT scanning with IV contrast is occasionally used but is less sensitive than magnetic resonance imaging (MRI).

CT can also be used to detect increases in intracranial pressure, e.g. before lumbar puncture or to evaluate the functioning of a ventriculoperitoneal shunt.

CT is also useful in the setting of trauma for evaluating facial and skull fractures.

In the head/neck/mouth area, CT scanning is used for surgical planning for craniofacial and dentofacial deformities, evaluation of cysts and some tumors of the jaws/sinuses/nasal cavity/orbits, and for planning of dental implant reconstruction.

Chest CT

CT is excellent for detecting both acute and chronic changes in the lung parenchyma. For detection of airspace disease (such as pneumonia) or cancer, ordinary non-contrast scans are adequate.

For evaluation of chronic interstitial processes (emphysema, fibrosis, and so forth), thin sections with high spatial frequency reconstructions are used. For evaluation of the mediastinum and hilar regions for lymphadenopathy, IV contrast is administered.

CT angiography of the chest (CTPA) is also becoming the primary method for detecting pulmonary embolism (PE) and aortic dissection, and requires accurately timed rapid injections of contrast and high-speed helical scanners. CT is the standard method of evaluating abnormalities seen on chest X-ray and of following findings of uncertain acute significance.

Cardiac CT

With the advent of subsecond rotation combined with multi-slice CT (up to 64 slices), high resolution and high speed can be obtained at the same time, allowing excellent imaging of the coronary arteries. Images with a high temporal resolution are formed by updating a proportion of the data set used for image reconstruction as it is scanned. In this way individual frames in a cardiac CT investigation are sinificantly shorter than the shortest tube rotation time. It is uncertain whether this modality will replace the invasive coronary catheterization.

Abdominal and pelvic CT

Many abdominal disease processes require CT for proper diagnosis. The most common uses include diagnosis of renal/urinary stones, appendicitis, pancreatitis, diverticulitis, abdominal aortic aneurysm, and bowel obstruction. CT is also the first line for detecting solid organ injury after trauma. Oral and/or rectal contrast is usually administered (more often iodinated contrast than barium due to the tendency of barium to cause imaging artifacts that limit evaluation of abdominal structures).

CT has limited application in the evaluation of the pelvis. For the female pelvis in particular, ultrasound is the imaging modality of choice. Nevertheless, it may be part of abdominal scanning (e.g. for tumors), and has uses in assessing fractures.

CT is also used in osteoporosis research along side DXA scanning. Both CT and DXA can be used to asses bone mineral density (BMD) which is used to indicate bone strength, however CT results do not correlate exactly with DXA (the gold standard of BMD measurment), is far more expensive, and subjects patients to much higher levels of ionizing radiation, so it is used infrequently.

Extremities

CT is often used to image complex fractures, especially ones around joints, because of the ability to reconstruct the area of interest in multiple planes.

See also

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