Navigation Links
Computed axial tomography


CT apparatus in a hospital

Computed axial tomography (CAT), computer-assisted tomography, computed tomography, CT, or body section roentgenography is the process of using digital processing 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 Sensura [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 (See also history of brain imaging). The first scanner took several hours to acquire the raw data and several days to produce the images. Modern multi-detector CT systems can complete a scan of the chest in less time than it takes for a single breath (useful if the patient cannot hold his/her breath) and display the computed images in a few seconds.

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.

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 degree 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. See diagnostic uses of a CT scan for more detail.

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.

Presently, the cost of an average CT scanner was $1.3 million U.S.

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.

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.

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.

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


'"/>


(Date:1/22/2015)... YORK and POINT ROBERTS, Washington ... news source covering leading sectors including technology and tech stocks, ... Privacy marketplace featuring master pickpocket and security consultant Apollo ... is stolen and talks about the Wocket™ biometric smart wallet, ...
(Date:1/22/2015)... MUNICH , January 22, 2015 ... its tenth year  The European Patent Office to present ... Two British nominations to be featured: Christofer Toumazou ... week featuring former finalists and winners of the Award   ...
(Date:1/22/2015)...   EyeLock, Inc. , a market leader of iris-based identity ... to the new role of Senior Vice President of ... of mobile platforms and wearable solutions for EyeLock,s technologies. Gerber ... the semiconductor industry to his role at EyeLock, with expertise ...
Breaking Biology News(10 mins):Apollo Robbins Talks about the Illusion of Personal Safety and How Easily Your Wallet is Stolen; If a Wocket Wallet is Stolen the Thief Can't do Anything With it! 2Apollo Robbins Talks about the Illusion of Personal Safety and How Easily Your Wallet is Stolen; If a Wocket Wallet is Stolen the Thief Can't do Anything With it! 3Ten Years of the European Inventor Award: A Retrospective Look at the Inventors and Ideas That Have Changed Our Lives 2Ten Years of the European Inventor Award: A Retrospective Look at the Inventors and Ideas That Have Changed Our Lives 3EyeLock Names Steve Gerber as Senior Vice President of Mobile and Wearables 2
... Technologies, M2000, a pocket-sized rugged handheld that scans ... Embedded Handheld 6.5 operating system and a new ... capabilities. (Logo:   http://photos.prnewswire.com/prnh/20090408/LA96741LOGO ) ... specifically for enterprise handheld computers, which provide real-time ...
... Scientists at The Scripps Research Institute have devised ... used widely in the synthesis of drugs and other ... CF3 molecule to the original compound, often making it ... body longer. With the new technique, chemists can perform ...
... adapt to environmental challenges are likely influenced by variations ... the plant,s growth and the external environment, reports a ... The findings were revealed by a genomewide association ... common research plant. The researchers, led by UC Davis ...
Cached Biology News:DAP Technologies Adds Windows Embedded Handheld 6.5 and Biometrics Module to its M2000 Rugged Handheld 2Scientists find easier, cheaper way to make a sought-after chemical modification to drugs 2Genomewide mapping reveals developmental and environmental impacts 2
Other biology definitionOther Tags