- Computed tomography (CT),
originally known as computed axial tomography (CAT or CT scan) and body section , is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensionalimage 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 graphein (to write). CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the X-ray beam. Although historically, the images generated were in the axial or transverse plane (orthogonal to the long axis of the body), modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.
Although most common in healthcare, CT is also used in other fields, for example nondestructive materials testing
The prototype CT scanner
The original 1971 prototype took 160 parallel readings through 180 angles, each 1° 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 production X-ray CT machine (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
CT's primary benefit is the ability to separate anatomical structures at different depths within the body, which on conventional radiographs are superimposed. A form of tomography can be performed by moving the X-ray source and detector during an exposure. Anatomy at the target level remains sharp, while structures at different levels are blurred. By varying the extent and path of motion, a variety of effects can be obtained, with variable depth of field and different degrees of blurring of 'out of plane' structures.
Although largely obsolete, conventional tomography is still used in specific situations such as dental imaging (orthopantomography) or in intravenous urography.
A Scout image is used in planning the exam and to establish where the target organs are located. The beginning and end of the scan are set by the target region and the location of the patient on the table. Once the Scout image is created it is used to determine the extent of the desired Axial/Helical scan. During the Scout scan the gantry is rotated to a fixed position and the table is translated as x-ray is delivered. The image appears similar to a radiograph.
In axial "step and shoot" acquisitions each slice/volume is taken and then the table is incremented to the next location. In multislice scanners each location is multiple slices and represents a volume of the patient anatomy. Tomographic reconstruction is used to generate Axial images.
A cine acquisition is used when the temporal nature is important. This is used in Perfusion applications to evaluate blood flow, blood volume and mean transit time. Cine is a time sequence of axial images. In a Cine acquisition the cradle is stationary and the gantry rotates continuously. Xray is delivered at a specified interval and duration.
Helical is a very fast way to examine the target anatomy. The volume is scanned very quickly because the table is in constant motion as the gantry rotates continuously. There is no interscan delay between slices as in a Axial acquisition.
A Digitally Reconstructed Radiograph (DRR) is used to help plan the treatment. The DRR is corrected for the fan beam effect. This is the prime difference from the Scout/Pilot image.
Since the introduction of the first clinical system by Hounsfield, several generations of scanners have been produced, with distinguishing tube-detector configuration and scanning motion (Canadian Association of Medical Radiation Technologists CT Imaging 1-Theory textbook).
Although numbered sequentially, the 3rd and 4th generation designs developed at approximately the same time. The concept of electron beam CT, which some authors have called 5th generation, followed later. Some authors have described up to 7 generations of CT design. However, it is only generations one to four that are widely, and consistently, recognized.
In the first and second generation designs, the X-ray beam was not wide enough to cover the entire width of the 'slice' of interest. A mechanical arrangement was required to move the X-ray source and detector horizontally across the field of view. After a sweep, the source/detector assembly would be rotated a few degrees, and another sweep performed. This process would be repeated until 360 degrees (or 180 degrees) had been covered. The complex motion placed a limit on the minimum scan time at approximately 20 seconds per image.
In the 3rd and 4th generation designs, the X-ray beam is able to cover the entire field of view of the scanner. This avoids the need for any horizontal motion; an entire 'line' can be captured in an instant. This allowed simplification of the motion to rotation of the X-ray source. Third and fourth generation designs differ in the arrangement of the detectors. In 3rd generation, the detector array is as wide as the beam, and must therefore rotate as the source rotates. In 4th generation, an entire ring of stationary detectors are used.
The third generation design suffers because it is highly sensitive to detector performance. Because of the fixed relationship of a detector to a specific part of the beam, any miscalibration or malfunction of an individual detector will appear as a ring in the final reconstructed image. As the detectors moved and were exposed to physical stress, loss of calibration and subsequent 'ring artifacts' were commonplace. The fourth generation, with its fixed detectors benefited not just from improved reliability of the detectors, but because the detectors could be automatically calibrated as the X-ray beam approached, and because the different reconstruction geometry meant that a malfunction would lead only to subtle loss of image contrast (fogging) rather than a visible ring.
Solving the issue of detector stability has led 3rd generation designs to the dominant position in contemporary designs. 4th generation designs suffered very high cost (due to the large number of detectors) and had very high susceptibility to 'streak artifacts' (due to compton scattering). All current CT scanners are of the 3rd generation design.
Electron beam tomography (EBCT) was introduced in the early 1980s, by medical physicist Andrew Castagnini, as a method of improving the temporal resolution of CT scanners. Because the X-ray source has to rotate by over 180 degrees in order to capture an image the technique is inherently unable to capture dynamic events or movements that are quicker than the rotation time.
Instead of rotating a conventional X-ray tube around the patient, the EBCT machine houses a huge vacuum tube in which an electron beam is electro-magnetically steered towards an array of tungsten X-ray anodes arranged circularly around the patient. Each anode is hit in turn by the electron beam and emits X-rays that are collimated and detected as in conventional CT. The lack of moving parts allows very quick scanning, with single slice acquisition in 50-100 ms, making the technique ideal for capturing images of the heart. EBCT has found particular use for assessment of coronary artery calcium, a means of predicting risk of coronary artery disease.
The very high cost of EBCT equipment, and its poor flexibility (EBCT scanners are essentially single-purpose cardiac scanners), has led to poor uptake; fewer than 150 of these scanners have been installed worldwide. EBCT's role in cardiac imaging is rapidly being supplanted by high-speed multi-detector CT, which can achieve near-equivalent temporal resolution with much faster z-axis coverage.
originally known as computed axial tomography (CAT or CT scan) and body section , is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensionalimage 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 graphein (to write). CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the X-ray beam. Although historically, the images generated were in the axial or transverse plane (orthogonal to the long axis of the body), modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.
Although most common in healthcare, CT is also used in other fields, for example nondestructive materials testing
The prototype CT scanner
The original 1971 prototype took 160 parallel readings through 180 angles, each 1° 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 production X-ray CT machine (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
CT's primary benefit is the ability to separate anatomical structures at different depths within the body, which on conventional radiographs are superimposed. A form of tomography can be performed by moving the X-ray source and detector during an exposure. Anatomy at the target level remains sharp, while structures at different levels are blurred. By varying the extent and path of motion, a variety of effects can be obtained, with variable depth of field and different degrees of blurring of 'out of plane' structures.
Although largely obsolete, conventional tomography is still used in specific situations such as dental imaging (orthopantomography) or in intravenous urography.
A Scout image is used in planning the exam and to establish where the target organs are located. The beginning and end of the scan are set by the target region and the location of the patient on the table. Once the Scout image is created it is used to determine the extent of the desired Axial/Helical scan. During the Scout scan the gantry is rotated to a fixed position and the table is translated as x-ray is delivered. The image appears similar to a radiograph.
In axial "step and shoot" acquisitions each slice/volume is taken and then the table is incremented to the next location. In multislice scanners each location is multiple slices and represents a volume of the patient anatomy. Tomographic reconstruction is used to generate Axial images.
A cine acquisition is used when the temporal nature is important. This is used in Perfusion applications to evaluate blood flow, blood volume and mean transit time. Cine is a time sequence of axial images. In a Cine acquisition the cradle is stationary and the gantry rotates continuously. Xray is delivered at a specified interval and duration.
Helical is a very fast way to examine the target anatomy. The volume is scanned very quickly because the table is in constant motion as the gantry rotates continuously. There is no interscan delay between slices as in a Axial acquisition.
A Digitally Reconstructed Radiograph (DRR) is used to help plan the treatment. The DRR is corrected for the fan beam effect. This is the prime difference from the Scout/Pilot image.
Since the introduction of the first clinical system by Hounsfield, several generations of scanners have been produced, with distinguishing tube-detector configuration and scanning motion (Canadian Association of Medical Radiation Technologists CT Imaging 1-Theory textbook).
Although numbered sequentially, the 3rd and 4th generation designs developed at approximately the same time. The concept of electron beam CT, which some authors have called 5th generation, followed later. Some authors have described up to 7 generations of CT design. However, it is only generations one to four that are widely, and consistently, recognized.
In the first and second generation designs, the X-ray beam was not wide enough to cover the entire width of the 'slice' of interest. A mechanical arrangement was required to move the X-ray source and detector horizontally across the field of view. After a sweep, the source/detector assembly would be rotated a few degrees, and another sweep performed. This process would be repeated until 360 degrees (or 180 degrees) had been covered. The complex motion placed a limit on the minimum scan time at approximately 20 seconds per image.
In the 3rd and 4th generation designs, the X-ray beam is able to cover the entire field of view of the scanner. This avoids the need for any horizontal motion; an entire 'line' can be captured in an instant. This allowed simplification of the motion to rotation of the X-ray source. Third and fourth generation designs differ in the arrangement of the detectors. In 3rd generation, the detector array is as wide as the beam, and must therefore rotate as the source rotates. In 4th generation, an entire ring of stationary detectors are used.
The third generation design suffers because it is highly sensitive to detector performance. Because of the fixed relationship of a detector to a specific part of the beam, any miscalibration or malfunction of an individual detector will appear as a ring in the final reconstructed image. As the detectors moved and were exposed to physical stress, loss of calibration and subsequent 'ring artifacts' were commonplace. The fourth generation, with its fixed detectors benefited not just from improved reliability of the detectors, but because the detectors could be automatically calibrated as the X-ray beam approached, and because the different reconstruction geometry meant that a malfunction would lead only to subtle loss of image contrast (fogging) rather than a visible ring.
Solving the issue of detector stability has led 3rd generation designs to the dominant position in contemporary designs. 4th generation designs suffered very high cost (due to the large number of detectors) and had very high susceptibility to 'streak artifacts' (due to compton scattering). All current CT scanners are of the 3rd generation design.
Electron beam tomography (EBCT) was introduced in the early 1980s, by medical physicist Andrew Castagnini, as a method of improving the temporal resolution of CT scanners. Because the X-ray source has to rotate by over 180 degrees in order to capture an image the technique is inherently unable to capture dynamic events or movements that are quicker than the rotation time.
Instead of rotating a conventional X-ray tube around the patient, the EBCT machine houses a huge vacuum tube in which an electron beam is electro-magnetically steered towards an array of tungsten X-ray anodes arranged circularly around the patient. Each anode is hit in turn by the electron beam and emits X-rays that are collimated and detected as in conventional CT. The lack of moving parts allows very quick scanning, with single slice acquisition in 50-100 ms, making the technique ideal for capturing images of the heart. EBCT has found particular use for assessment of coronary artery calcium, a means of predicting risk of coronary artery disease.
The very high cost of EBCT equipment, and its poor flexibility (EBCT scanners are essentially single-purpose cardiac scanners), has led to poor uptake; fewer than 150 of these scanners have been installed worldwide. EBCT's role in cardiac imaging is rapidly being supplanted by high-speed multi-detector CT, which can achieve near-equivalent temporal resolution with much faster z-axis coverage.
