Computed tomography

Computed tomography (CT), originally known as computed axial tomography (CAT) and body section roentgenography, is a medical imaging method employing tomography where digital geometry 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). 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 by Godfrey Newbold Hounsfield in Hayes, England at EMI Central Research Laboratories (now Sensaura owned by Creative Technology Ltd.) using X-rays. Hounsfield conceived the idea in 1967, and it was publicly announced in 1972. Allan McLeod Cormack of Tufts University independently invented the same process and they shared a Nobel Prize in medicine in 1979. The original 1971 prototype took 160 parallel readings through 180 angles, each 1&deg; apart, with each scan taking a little over five minutes. The images from these scans took 2.5 hours to be processed by algebraic reconstruction techniques 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 acquired the image data in about 4 minutes (scanning two adjacent slices) and the computation time (using a Data General Nova minicomputer) was about 7 minutes per picture. This scanner required the use of a water-filled Perspex tank 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 Clinic and Massachusetts General Hospital in 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.

CT technology generations

 * First generation: These 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 (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 scintillator and a photomultiplier tube.


 * Second generation: This design 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.


 * Third generation: 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 is directed to an array of detctors that are fixed in position relative to the x-ray source. This eliminated the time consuming translation stage allowing scan time to be reduced, initially, to 10 seconds per slice. This advance dramatically improved the practicality of CT. Scan times became short enough to image the lungs or the abdomen; previous generations had been limited to the head, or to limbs.


 * Fourth generation: This design was introduced, roughly simultaneously with 3rd generation, and gave approximately equal performance. Instead of a row of detectors which moved with the X-ray source, 4th generation scanners used a stationary 360 degree ring of detectors. The fan shaped x-ray beam rotated around the patient directed at detectors in a non-fixed relationship.

Bulky, expensive and fragile photomultiplier tubes gradually gave way to improved detectors. A xenon gas ionization chamber detector array was developed for third generation scanners, which provided greater resolution and sensitivity. Eventually, both of these technologies were replaced with solid-state detectors: rectangular, solid-state photodiodes, coated with a fluorescent rare earth phosphor. Solid state detectors were smaller, more sensitive and more stable, and were suitable for 3rd and 4th generation designs.

On an early 4th generation scanner, 600 photomultiplier tubes, 1/2 in (12 mm) 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.

Initially, 4th generation scanners carried a significant advantage - the detectors could be automatically calibrated on every scan. The fixed geometry of 3rd generation scanners was especially sensitive to detector mis-calibration (causing ring artifacts). Additionally, because the detectors were subject to movement and vibration, their calibration could drift significantly.

All modern medical scanners are of 3rd generation design. Modern solid-state detectors are sufficiently stable that calibration for each image is no longer required. The 4th generation scanners' inefficient use of detectors made them considerably more expensive than 3rd generation scanners. Further, they were more sensitive to artifacts because the non-fixed relationship to the x-ray source made it impossible to reject scattered radiation.

Further advances
Another limiting factor in image acquisition was the X-ray tube. The need for long, high intensity exposures and very stable output, placed enormous demands on both the tube and generator (power supply). Very high performance rotating anode tubes were developed to keep up with demand for faster imaging, as were the regulated 150 kV switched mode power supplies to drive them. Modern systems have power ratings up to 100 kW.

Slip-ring technology replaced the spooled cable technology of older CT scanners, allowing the X-ray tube and detectors to spin continuously. When combined with the ability to move the patient continuously through the scanner this refinement is called Helical CT or, more commonly, Spiral CT.

Modern multi-detector-row CT systems further accelerated scans. It is possible to complete a scan of the chest in a few seconds. An examination that required 10 seperate breath-holds of 10 seconds each, can now be completed in a single 10 second breath-hold. Multi-detector CT can also provide isotropic resolution, permitting cross-sectional images to be reconstructed in arbitrary planes; an ability similar to MRI.

Improved computer technology and reconstruction algorithms have permitted faster and more accurate reconstruction. On early scanners reconstruction could take several minutes per image, a modern scanner can reconstruct a 1000 image study in under 30 seconds. Refinements to the algorithms have reduced artifacts and improved fidelity.

Dual source CT uses 2 x-ray sources and 2 detector arrays offset at 90 degrees. This reduces the time to acquire each image to about 0.1 seconds, making it possible to obtain high quality images of the heart without the need for heart rate lowering drugs such as beta blockers. A dual-source multi-detector row scanner can complete an entire cardiac study within a single 10 second breath hold.

Microtomography
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.

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&mdash; 360 or just over 180 degrees in conventional machines, 220 degree in EBT &mdash;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.

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 radiation and 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 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 with or without intravenous contrast agents. CT generally does not exclude infarct in the acute stage of a stroke, 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, diagnosis of the causes of chronic sinusitus, 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 significantly shorter than the shortest tube rotation time. It is uncertain whether this modality will replace the invasive coronary catheterization.

Cardiac MSCT carries very real risks since it exposes the subject to the equivalent of 500 chest X Rays in terms of radiation. The relationship of radiation exposure to increased risk in breast cancer has yet to be definitively explored.

Also a lot of MSCT technicians are trained cardiologist as opposed to radiologists. The positive (93-95%) and negative (97-98%) predictive values of the scan are calculated on the basis of a knowledgable staff which may not always be the case.

Much of the software is based on data findings from caucasian study groups and as such the assumptions made may also not be totally true for all other populations.

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 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 studies and 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.

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 different thresholds are coloured 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.) Based on this difference in the radiodensities the bones can be reconstructed in 3D as shown on the next image.

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.

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 increasing 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. 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.