Computed tomography

Image:CT Scanner.jpg Computed tomography (CT), originally known as computed axial tomography (CAT) and body section roentgenography, is a medical imagingmethod employing tomographywhere digital geometry processingis used to generate a three-dimensionalimageof the internals of an object from a large series of two-dimensional X-rayimages taken around a single axis of rotation. The word "tomography" is derived from the Greektomos (slice) and graphia (describing). CT produces a series of axial images which can be manipulated, through a process known as windowing, in order to recreate the image in a different plane.

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

History

The CT system was invented in 1972by Godfrey Newbold Hounsfieldof EMICentral Research Laboratories in Hayes, England (now Sensaura [1]owned by Creative Technology Ltd.) using X-rays. Allan McLeod Cormackof Tufts Universityindependently invented the same process and they shared a Nobel Prizein medicinein 1979. One of Hounsfield's early prototype scanners took 9 days to scan the object and acquire the raw data, because of the low density gamma-ray source that was used. The images from these scans took 2.5 hours to be produced on a large computer.

The first commercial CT machine using X-rays (called the EMI-Scanner) was limited to making tomographic sections of the brain, but aquired the image data in about 4 minutes (scanning 2 adjacent slices) and the computation time (using a Data General Novaminicomputer) was about 7 minutes per picture. This scanner required the use of a water-filled Perspextank with a pre-shaped rubber "head-cap" at the front, which enclosed the patient's head. The water-tank was used to reduce the dynamic range of the radiation reaching the detectors (between scanning outside the head compared with scanning through the bone of the skull). The images were relatively low resolution, being composed of a matrix of only 80 x 80 pixels. The first EMI-Scanner was installed in Atkinson Morley's Hospital in Wimbledon, England, and the first patient brain-scan was made with it in 1972. In the US, the machine sold for about $390,000, with the first installations being at the Mayo Clinicand Massachusetts General Hospitalin 1973.

The first CT system that could make images of any part of the body, and did not require the "water tank" was the ACTA scanner designed by Robert S. Ledley, DDS at Georgetown University.

The first generation CT scanners used a pencil-thin beam of radiationdirected 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 (gantry) by one degree. In the EMI-Scanner, a pair of images was acquired in about 4 minutes with the gantry rotating a total of 180 degrees. Three detectors were used (one of these being an X-ray source reference), each detector comprising a sodium iodide scintillatorand a photomultipliertube.

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 photomultiplier tubesthat were 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 photomultiplier tubes that encircled the patient. The fan shaped x-ray beam rotated around the patient directed at detectors in a non-fixed relationship. Eventually, the photomultiplier tubeswere replaced with rectangular, solid-state photodiodes, coated with a rare earththat glows when irradiated. With photomultiplier tubes, 600 tubes, 1/2" in diameter, could fit in the detector ring. Three photodiode units could replace one photomultiplier tube. This change resulted in increasing both the acquisiton speed, and image resolution. The method of scanning was still slow, because the X-ray tube and control components interfaced by cable, limiting the scan frame rotation.

With the cable interface method, the scan frame was limited to a 380° rotation. after it completed its scanning rotation, the scan frame would 'reset,' or return to its home position. Later changes allowed the scan frame to acquire images in both directions, which allowed slightly faster performance.

Another limiting factor in image acquisition was the X-ray tube. Essentially a vacuum tubediode, it's mounted in a lead-lined, dielectricoil filled, metal housing. The oil is circulated through an external cooler, to remove heat from the X-ray tube. Heat is generated by the production of X-rays, and a negligible amount comes from the X-ray tube motor used to spin the anode. This can reduce the life and operating time of the X-ray tube, or cause catastrophic damage to the tube housing. The heat is measured in "Heat Units" or "HU." The X-ray tube is rated in HU's, maximum voltage (arc over), and maximum current (filament). Heat Units are calculated by multiplying the selected voltage (KV), current (mA), and exposure time.

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. Slip-ring technology replaced the spooled cable technology of older CT scanners, allowing the X-ray tube to continuously spin within the detector ring. This, along with faster computers and increased memory capacity, provided substantially faster acquisition of images.

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 (3D-CT scan), in turn viewable from multiple different perspectives on attached CT workstation monitors.

Image:E-Speed EBT.jpgIn conventional CT machines, an X-Ray tubeis 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 Hounsfieldunits.

CT is used in medicine as a diagnostic tool and as a guide for interventional procedures. Sometimes contrast materials such as intravenousiodinatedcontrast 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.

Due to improvements in CT technology the overall radiation dose and 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. X-rays are a form of ionizing radiationand as such can be dangerous.

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 imagingto supplement X-raysand medical ultrasonography. Although it is still quite expensive, it is the gold standardin 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 with or without intravenous contrast agents. CT generally does not exclude infarctin the acute stage of a stroke, but is useful to exclude a bleed (so anticoagulantmedication 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 punctureor 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

Image:Chest CT scan with lung metastatis 2.jpg

CT is excellent for detecting both acute and chronic changes in the lungparenchyma. 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 mediastinumand 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-rayand 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 significantly shorter than the shortest tube rotation time. It is uncertain whether this modality will replace the invasive coronary catheterization.

Dual Source CT scanners, introduced in 2005, allow higher temporal resolution when acquiring images of the heart, allowing a greater number of patients to be scanned.

Abdominal and pelvic CT

Many abdominaldisease 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 contrastthan bariumdue 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, ultrasoundis 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 osteoporosisstudies and research along side DXAscanning. 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.

Three dimensional (3D) reconstruction

The principle

Mathematically the result of a CT scan is a 3 dimensional matrix of numbers representing the radiodensity of the different parts of the body examined. Let us call this matrix the volume. Now consider a certain level of radiodensity and cast an imaginary ray through the volume. There are two possibilities: (a) our ray goes through the volume without hitting a point of the given or greater radiodensity, (b) there is a point at which the ray first hits a value equal or greater than the treshold given. Mark this point. Then move the ray around (say, parallel to itself) and mark all these ?first hit? points. For instance, if one selects a value characteristic to the bone then one may expect that the set of the "first hit" points will depict the surface of the bone within the volume. Usually the surfaces belonging to differend tresholds are colored artificially so that they look like the original tissue.

An example

Some slices of a cranial CT scan are shown below. The bones are whiter than the surrounding area. (Whiter means higher radiodensity.) Image:Cranialslices.JPG Based on this difference in the radiodensities the bones can be reconstructed in 3D as shown on the next image. Image:Bonereconstruction.jpg

Segmentation

The difficulty with this technique is that structures of high radiodensity can hide other structures of equal or lower radiodensity. For instance the cranium hides the blood vessels of the brain even if their radiodensity is increased by some contrast agent. The solution is the so called segmentation, a manual or automatic procedure cutting the outer layers of higher density out.

The cranial slices above show blood vessels too appearing similar to the bone in white (due to an intravenous contrast agent; see the arrow). However, these blood vessels cannot be seen on the present 3D reconstruction because the cranium hides them. After some chopping around and coloring the blood vessels appear nicely as shown below. Image:Venesreconstruction.JPG

CT imaging as graphic art

Interesting graphical effects can be achieved by the 3D imaging technique described above. The attached image (Michelangelo?s dream) was created by using increasing radiodensity values for treshold. Starting with a small treshold the whole surface of the volume got marked. Then by icreasing this value first the textile then the skin loomed up. Had we gone further the bones, too, would have shown up, then everything would have disappeared. Four images were chosen and then artificially colored. Image:Michelangelo zgyorfi.jpg Women of the early twentieth century were afraid of the X-ray technique for it could reveil their naked body. This fear is gone despite the fact that with a CT one can be really undressed easily.

See also

• Cardiology diagnostic tests and procedures
• Computed Tomography Laser Mammography(CTLM)
• Fluoroscopy
• Medical ultrasonography
• Magnetic resonance imaging(MRI)
• Neuroimaging
• Positron emission tomography(PET)
• Single photon emission computed tomography(SPECT)

External links

• Computed Tomography from Siemens Medical
• Computed Tomography Applications in Medical, Industrial and Security applications From BIR
• RadiologyInfo- The radiology information resource for patients: Computed Tomographyde:Computertomographie

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It uses material from the http://en.wikipedia.org/wiki/Computed+tomography Wikipedia article Computed tomography.