Computerized tomography (CT) has also been known as Computerised axial tomography (CAT), Computerised transaxial tomography (CTAT), and digital axial tomography (DAT). Tomography is taken from the Greek word ‘tomos”, meaning section (wikipedia. org). Since the technology involved the computer, hence the name. If we consider very briefly about the history, Housefield demonstrated the CT in 1972, while the first whole body was done in 1975 (Moss, 1993). In 1979, Hounsefield, and Cormack received the Nobel Prize for their work on the CT machine (Fullreton, 1994).
The Spiral CT came up in 1979, and the multislice CT was introduced in 1991 (Siemens. com, Rydberg, 2003). Over the last 10-15 years, the rapid advance of CT machinery from a single slice to now multislice scanners has been mindblowing. A number of advances in hardware and software has made this possible. In particular the development of the slip rings to allow the gantry to rotate continuously in one direction (Rydberg, 2003). The development to x ray tubes to bear the heat produced so that large exposure times are possible, even upto 60 sec.
cheaper, more stable solid state conductors and multidetector arrays, improved computer power and hardware to handle and store the large amount of data produced, and the improvement of the software to allow rapid postprocessing(Claudia, 2000). In less than 4 years after the introduction of the first EMI machine in 1972 (Wikipedia. org), CT evolved through four generations of scanners. This rapid evolution was due primarily to improvements in the mechanical design for the scanners, which yielded shorter scan times and therefore better control of patient motion.
The principle advantage of CT imaging over other x ray imaging is improved contrast resolution, no superimposition of tissue, less scatter radiation, 3-dimesional imaging, and bone mineral assay (Claudia, 2000). The principle limitations are spatial resolution, radiation dose to the patient, and artifacts (Claudia, 2000). Basics about computerized tomography CT involves the transmission of x rays through the patient. Here the radiograph is obtained with a moving source image receptor assembly. The development of the CT required the emergence of the digital computer and special mathematics.
The basic approach is the translation of an object through a collimated x ray beam, and to detect an image projection. A computer generated matrix of a section of the object is produced by back projection reconstruction through a special sequence of computerized algorithms Moss, 1993). Basic Parts of the CT machine Every CT machine has three distinguishing components – the operating console, the computer and the gantry. The operating console performs two major functions- imaging control with preselected technique and image viewing and manipulation. The CT computer has no distinguishing features.
It may be inbuilt into the console. The gantry is special to the CT. it houses the x ray source, the detector assembly, and the high volatge generator (Siemens. com). The patient aperture of a CT gantry has a maximum diameter of approximately 70 cm. and the gantry can be tilted either way approximately 30 degrees (Fullerton, 1994). First generation CT scanner (Fullerton, 1994, Claudia, 2000, Moss, 1993) The original CT machine of the first generation was the EMI unit installed in 1971 at Atkinson Morley’s Hospital at Wimbledon (wikipedia. org). Later version of the scanner was marketed by EMI as the Mark 1 scanner.
In the scanner, the x-ray beam is collimated into two parallel pencil like beam. These beams are directed towards two NaI scintillation detectors adjacent to each other, on the opposite side of the patient. In this manner transmission data can be simultaneously for two adjacent tomographic images of the patients head. The x-ray tube, collimator, and detector are part of the common frame that scans across the patient so that the x-ray tube and detectors move in a synchrony on opposite sides of the patient. During this linear scanning motion, 240 measurements of x-ray transmission are obtained.
When the scanner has reached the end of the end of its translational motion, the frame is rotated through 1 degree and the process is repeated. The second scan adds 240 measurements of transmission during the scan. At the end of each translational scan, the rotation through 1 degree is repeated until the frame has rotated through 180 degrees. To correct the varying x-ray intensity during the scanning process, a separate detector is positioned, which monitors the intensity of the beam, and its signal is used to correct transmission readings for the fluctuating x-ray instensity (Claudia, 2000, Moss, 1993)
The x-ray transmission measurements are delivered to a computer, which generates a set of simultaneous equations equal in number to the number of transmission measurements. From these equations, a series of CT numbers is computed to be displayed as a gray scale image on the display unit. In the first scanner, the display unit provided an 80 x 80 matrix of 6400 viewing elements (called picture elements or pixels). In the first machines, the head of the patient was positioned in a rubber cap with a circular orifice.
Then the entire path for x ray transmission was filled by either water or the patients head, and there was no air in the path of the x rays. Thus the distance traveled by the rays was small, and the limited disturbance of signals made the calculations simple and the pictures produced were very accurate. In the original machine, data collection required 4? minutes and 5 minutes were needed to make the image of the scan for a single image. A minimum of 35 minutes was required to format 6 images. The radiation exposure was of the order of 2 R since the number of images was few.
The scan was reasonable only for the head, since, though the picture quality was very good, the time required made it difficult to keep patient still (Fullerton, 1994). Thus motion artefacts were very common with the first generation, thus scans of other areas such as the chest were very inaccurate. A summary of the first generation machine is 1. finely collimated x ray beam 2. single radiation detector 3. translate – rotate motion 4. 180 translations with 1 degree rotation between translations 5. single image projection per image 6. 5 minute imaging time 7.
head imager, not capable of body imaging. The efforts to improve the machine were in reducing the times to scan, this led to the development of the second generation CT scanners. Second Generation Scanners (Fullerton, 1994, Claudia, 2000, Moss, 1993) The problem with the first generation scanners was that the detectors in the scanners were placed at two adjacent tomographic sections. This was good for 2 sections but did not speed up the process of a single scan. Therefore in the second generation scanners the detectors were placed side by side in the same plane rather than adjacent planes.
In the first machines of this generation, three detectors were placed side by side which received three pencil like rays in a fan beam shape. Thus the x ray tube and the detector frame could be rotated through 3 degrees than the earlier 1 degree, at the end of each translation movement. With reduction in the data acquisition time by a factor of 2. other manufacturers further improved on this by as many as 52 detectors in the same plane, bettered by others, with introduction of two sets of multiple detectors to yield simultaneous measurements of transmission data in two adjacent scan planes (Claudia, 2000).
Now a practical problem with the increasing number of detectors was that with a pencil beam, all the detectors will not get equal signal intensity, thus the picture would not be uniformly accurate and clear. Thus a modification was made with a fan type of beam transmission. Further the detectors were further programmed to collimation such that they would receive a specific beam of light of equal intensity along a straight line. This fan beam irradiated not only a greater amount of tissue, also the scatter produced at the detectors would affect the image quality, so the detectors were shielded from this scattered radiation.
Each detector was further programmed such that each detector was equally sensitive to the radiation (Fullerton, 1994). With this form of symmetry, the fan shaped x ray radiation and multiple detectors, which move in the translation- rotation geometry, the data collection times for a single tomographic section to 20 seconds, which was further reduced to 5. 3 seconds. Even this time, is too much for examination of chest and abdomen, specially in a situation of trauma and critically ill patients, thus this led to the development of the third generation of scanners.
A summary of the second generation scanners is 1 fan shaped x ray beam 2 multiple radiation detectors – a detector array 3 translate-rotate movement 4 usually 18 translations with 10 degrees rotation between translations 5 multiple image projection per translation 6 approximately 30 second imaging time 7 head and body imager Third Generation Scanners (Fullerton, 1994, Claudia, 2000, Moss, 1993) The second generation scanners were limited by the need to move the x-ray tube and detectors through two degrees of movement, in translation and rotational planes.
In the third generation machines, the movement was simplified by limiting the movement in the rotation such that x ray tube and detectors rotate around the patient, with no translation al movement, and this resulted in further reduction of scanning time. GE electronics came up with the first third generation scanners in 1975 (Moss, 1993). This machine reduced scan time for a single section to 4. 8 seconds. Other comapanies came up with machines that reduced times to 2. 5 seconds (Cluadia, 2000, Siemens. com) These scanners employ over 500 detectors in an arc between 21 to 45 degrees on the side of the patient opposite to the x ray tube.
The detectors are placed slightly misaligned to the centre of rotation of the detectors and x ray tubes, making the number of independent measurements of x ray transmission to be increased by a factor of 2 or more. In the third generation scanner, most of the detectors of the scanner are in shadow of the patient during an examination. There is a chance of image artefacts caused by detector imbalance. Through the use of stable detectors and electronics, this has not proven to be a major problem in scanning with these machines (Claudia, 2000).
A summary of this scanner is 1 a fan beam views the entire patient during imaging 2 several hundred radiation detectors are incorporated into the curvilinear detector array 3 this curvilinear design, provides a constant distance between source and each detector, resulting in good image reconstruction 4 this development is based on a 360 degree rotate-rotate motion. Both the x ray and the detector rotate about the same axis 5 hundreds of image projections are produced during each rotation, resulting in better contrast resolution and spatial resolution.
6 imaging time is reduced to 1 sec or less 7 ring artifacts are common Fourth Generation Scanners (Fullerton, 1994, Claudia, 2000, Moss, 1993) American Science and Engineering Company, announced the fourth generation scanner in 1976. In this design, more than 600 scintillation detectors are positioned side by side to form a ring that completely surrounds the patient. In this design the detectors remain stationary, while the x ray tube moves inside the ring and around the patients body. The single image collection time is reduced to a paltry 2 seconds.
Another advantage is that during each cycle of the x ray tube, each detector receives the beam twice, increasing the intensity and resolution, while reducing artefacts. The major defect with the fourth generation scanner is the high cost due to the very large number of detectors ( > 2000 in some modern machines). There is also the chance of wasted radiation to the spaces in between the detectors. Thus the patient may end up taking a higher dose of radiation. If these advantages and disadvantages are compared, there may not be much significant difference versus the third generation scanner. Summary of the fourth generation
1 deals with the ring artifacts 2 x ray source is collimated to a fan beam 3 several thousand individual detectors are present 4 the mechanical motion is rotation of the x-ray source around a fixed detector array 5 patient dose is slightly higher 6 wasted dosage due to some radiation falling on the interspace 7 imaging time is 1 sec or less Fifth generation of scanners(Fullerton, 1994, Claudia, 2000, Moss, 1993) With the great reduction achieved in the previous machines primarily because of the movement restriction in the single plane, and the large increase in the number of detectors, a limit seemed to have reached.
To further reduce the scan times, either a new design for x ray tubes ( with increased speed of firing), or the introduction of multiple x ray tubes that fired successively have to be employed. The fifth generation is heralded by the dynamic spatial reconstructor; here 28 x ray tubes are positioned around a semicircular gantry. The tubes are in line with 28 light amplifiers and TV cameras, which are placed behind a curved fluorescent screen that occupies the remaining semicircle. The gantry of x ray tubes and imaging systems rotates at around the patient at 15 revolutions per minute.
This rotation will yield a set of 28 data- collection views every 1/10 second, and upto 240 equally spaced data collection views over 360 degrees in less than 2 seconds. Here the image may be produced in times as short as 16 milliseconds (Claudia, 2000) Electron beam CT (EBCT) Images are obtained in less thatn 100ms. .the source is not an x ray tube but rather a focused, steered and microwave accelerated electron beam on a tungsten target. The target covers one half of the imaging circle, the detector array covers the other half. There are four targets, or focal tracks, and four detector arrays, resulting in four contiguous images.
Electron beam CT is principally applied to cardiac imaging in the form of coronary angiogram. Electron beam CT uses a focussed electron beam on a tungsten target ring as an x ray source and it can take upto 8 slices simultaneously, with scan times as short as 50 ms (wikipedia. org/ electron beam tomography). Spiral CT (Rydberg, 2003, wikipedia. org) This was introduced in 1989 and is now the standard CT imager. Basically if the third or the fourth generation scanner is caused to continuously rotate while the patient couch is moved through the imaging plane, the spiral CT results.
The development of the slip rings has made this possible. Slip rings Technology is required to for data transfer from the rotating gantry. In a spiral CT coiled wires are not present, which necessitate returning of the gantry to the original position, wasting time, here a high voltage supply is given on board or the use of slip rings for high voltage energy transfer. Single breath holding imaging of the entire torso is possible with the spiral CT Slip rings Before the slip ring technology was developed, the high tension wires that supplied the power to the gantry, was made via cables.
This caused a twist in the cables when the gantry took a turn. Thus it was necessary for the gantry to return to its base position to unwind the cable. This led to longer scanning times and patient discomfort. Slip rings are large circumferentially conducting rings that allow power to be conducted to the tube on the CT gantry via electrical brushes rather than cables. The gantry is thus able to continuously rotate in one direction. The data may be transferred back to the console by the same slip rings. This allowed faster acquisition times, faster processing, better resolution (imaginis. com)
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