Physical principle of Computed Tomography (Scanning principle & Data acquisition).pptx

Principles of CT

1. Physical principle of Computed Tomography (Scanning principle & Data acquisition) Presented by : M.Naga Theja 3rd Semester,M.Sc.Radio-Imaging Technology Reg.No.: 210513003

2. Table of content What is Computed tomography ? History of CT Components of CT equipment Principle of CT Data acquisition in CT Questions Reference

3. What is Computed tomography ??? “Computed Tomography,” or CT, refers to a computerized x-ray imaging procedure in which a narrow beam of x-rays is aimed at a patient and quickly rotated around the body, producing signals that are processed by the machine’s computer to generate cross-sectional images, or “slices.” These slices are called tomographic images and can give a clinician more detailed information than conventional x-rays. Once a number of successive slices are collected by the machine’s computer, they can be digitally “stacked” together to form a three-dimensional (3D) image of the patient that allows for easier identification of basic structures as well as possible tumors or abnormalities.

4. History of Computed Tomography Since the introduction in 1970’s, Ct had undergone many continuous development improvement in performance, including increases in acquisition speed, amount of information in individual slices, and volume of coverage. A graph of these parameters versus time looks similar to Moore’s Law for computer price-performance, which observes that computer metrics (clock speed, cost of random access memory or magnetic storage, etc.) double every 18 months. In the case of CT technology, the doubling period is approximately 32 months, still an impressive rate. For example, scan time per slice has decreased from 300 seconds in 1972 to 0.005 seconds in 2005 Evolution of computed tomography scanner performance: plot of acquisition performance versus time, for computed tomography scanners. The slope implies a doubling of performance approximately every 2 years. (Data from Siemens Medical Systems, www.medical.siemens.com , “CT History and Technology.”).

5. Generations of CT

6. A fifth-generation of CT scanners, the EBCT(Electron Beam CT), uses a stationary X-ray tube and a stationary detector ring, thereby avoiding any mechanically moving parts. In these systems, an electron beam is emitted from a powerful electron gun and magnetically deflected to hit a semi-circular anode surrounding the patient. 5th Generation CT This is a helical or spiral CT scan or Volumetric Scanners.The x-ray source and detectors in 3rd generation is design to freely moves and attach to the rotating gantry,in this table with the patient moves smoothly through the scanner, this derives its name from the helical path traced out by the x ray beam. It is also uses a slip ring technology with a multiplanar reformation excels in 3D. 6th Generation CT

7. The differences in CT generations can also be clearly highlighted in table form for a quick means to understand the geometric differences.

8. Components of CT Equipment CT scanners are composed of many different connected parts, with many d ifferent components involved in the process of creating an image. Gantry Slip rings Generator Cooling system X-ray source (CT X-ray tube) Filtration Collimaton Detectors

9. Gantry The gantry is ring shaped part of the CT scanner having components necessary to produce and detect x rays . These components are mounted on a rotating scan frame. The range size of aperture is typically 70 to 90 cm, designed to be tilted either forward or backward according to examination protocols more or less 15 degrees to 30 degrees is usual. Include a laser light that is used to position the patient within the scanner and Control panels located on either side of the gantry opening allow the radiologic technologist to control the alignment lights, gantry tilt, and movement of the table.

10. Slip Rings Old model design CT scanner used recoiling system cables to rotate the gantry frame. This design limited the gantry rotation times. Newer systems use electromechanical devices called slip rings which use a brush like apparatus to provide continuous electrical power and electronic communication across a rotating surface permit the gantry frame to rotate continuously, eliminating the need to straighten twisted system cables. Slip rings allows the gantry frame to rotate continuously making helical scan modes possible.

11. Cooling System Cooling mechanisms are included in the gantry. They can take different forms, such as blowers, filters, or devices that perform oil to air heat exchange. Cooling mechanisms are important because many components can be affected by temperature fluctuations. Generator Highly stable 3-phase high frequency generator is currently used in CT scanners, designed to be small enough so that it can be located within the gantry. CT generator produce high kV generally 120 – 140 kV to increase the intensity of the beam and thereby reduce patient dose.

12. X ray source - CT x ray tube A standard rotating anode tube, Tungsten is often used for the anode target material because it produces a higher intensity xray beam. CT scan tubes contain more than one size of focal spot; 0.5 and 1.0 mm are the common size of focal spot. Just like as in standard xray tubes, because of reduced penumbra small focal spot in Computed tomography tubes produce sharper images like better spatial resolution. A CT scan tube must be designed to handle such stress as they expose for a long time. Collimators Collimation restrict the xray beam to a specific area, as a result it helps reduce scatter radiation. In MDCT systems, slice thickness is also influenced by the detector element configuration. Scanner vary in the choices of slice thickness available. Choices range from 0.5 to 10 mm. Some CT scan systems also use predictor collimation. This is located below the patient and above the detector array( postpatient collimation). . The primary functions of predetector collimations are to ensure the beam is the proper width as it enters the detector and to prevent scatter radiation from reaching the detector.

13. Filtration Used to shape the xray beam & reduce the radiation dose to the patient and help to minimize image artifact. Different filters are used when scanning the body then when scanning the head. Human body anatomy having distinctive quantities has a round cross section that is thicker in the middle than in the outer area. Hence, body scanning filters are used to reduce the beam intensity at the periphery of the beam, corresponding to the thinner areas of a patient’s anatomy. Because of their shape they are often referred to as bow tie areas.

14. Detectors The detectors is a component of CT scan machine which collect information regarding the degree to which each anatomic structure attenuated the beam. Xenon gas detectors Pressurized xenon gas fills hollow chamber to produce detectors that absorb nearly 60% to 87% of the photons that reach them. Xenon gas is used as its stable under pressure, significantly less expensive compared with the solid – state variety. a photon enters the channel, it ionizes the xenon gas & accelerated and amplified by the electric field. The collection charge produces an electric current. This current is then processed as raw data. 2. Solid state crystal detectors Also called scintillation detectors because they use a crystal that fluoresces when struck by a xray photon. A photodiode is attached to the crystal and transforms the light energy into electrical (analog) energy. The individual detector elements are affixed to a circuit board. These solids have high atomic numbers and high density in comparison to gases, solid state detectors have higher absorption coefficients nearly 100% of the photons that reach them.

15. Xenon gas detectors Solid state crystal detectors ( cadmium tungstate, bismuth germinate, cesium iodide, and ceramic rare earth compounds such as gadolinium or yttrium)

16. Principle of CT CT images are the explanation of relative attenuation of x ray passing through the body of different densities. A tissue’s CT attenuating ability is related to its density and represents the likelihood that an x ray photon will pass through the tissue to be recorded by the detectors rather than interacting with tissue’s atoms (absorption of the x rays into the tissue) which prevents the photon from reaching the detector at all. A particular tissue’s x-ray attenuating ability is expressed by its attenuation coefficient , μ . The higher the μ value, the lower the number of photons that reach the detector,The higher the μ value, the lower the number of photons that reach the detector. The attenuation coefficient of a tissue is not constant and may be altered by the tissue thickness and the energy of the x ray photon

17. Cotd., The μ value is directly related to the tissue’s density, that is, the higher the tissue density, the higher its μ value. The CT detector is a photon flux counter where a scintillation detector measures, records, and converts to light the incident x ray photons exiting the patient. This light is then transformed to an electrical signal by the photodiode and the electrical signal is then converted to a digital signal by the computer. The photon flux measured by the detector is represented by the term I where I is a function of the photon flux emitted by the x ray tube ( I ) and the attenuation coefficient (μ) of the tissue. I = I   × e −μ . As μ increases, the fraction e−μ decreases such that as μ increases, the fraction of the incident photons leaving the x ray tube that are detected by the detector decreases.

18. Cotd., The tube voltage (photon energy) is the energy possessed by each individual x ray leaving the x ray generator and its unit is the kiloelectron volt (KeV). The tube current or photon (x ray) flux is the frequency with which x rays leave the x ray tube and its unit is the milliampere (mA). Increasing the KeV will have the following effects. The tissue penetration will increase, the μ value will decrease the number of photons detected by the detector will increase, the image noise will decrease, the radiation dose will increase and the visualized tissue contrast will decrease. Therefore, increasing KeV will decrease the CT number of the iodinated contrast and result in less opacified coronary arteries. The opposite is also true. Decreasing the KeV will result in higher CT numbers and brighter contrast within the coronary arteries. Increasing mA serves to decrease image noise, to increase the radiation dose and to increase the visualized tissue contrast.

19. Cotd., The attenuation coefficient of a particular tissue is not measured directly but rather is calculated using the count data (I and I0) by manipulating the equation I = I0 × e−μ. The Hounsfield unit behind the image is known as the CT number. It is computed from the calculated attenuation coefficient using the equation CT number = [(μ tissue   − μ water )/μ water ] × 1,000 The CT number is inversely related to μ such that the greater the μ value, the higher the CT number. he CT number is directly related to the brightness of the image where higher CT numbers appear brighter on the screen. The CT number or Hounsfield unit (H.U) is not an absolute number. It is a representation of the density of the particular tissue relative to the density of water

20. Data Acquisition Generation of X rays : A rotatory anode x ray tube is the source of x ray production in CT. The x-rays from the target are spread over a wide solid angle. To minimize radiation dose and generation of background scatter, the x-ray beam is collimated by an aperture into a thin fan beam. For CT scanners, the beam is typically a few millimeters thick in the patient, angleing a fan of about 45 degrees. Additionally, because human anatomy typically has a round cross-section that is thicker in the middle than in the periphery, more x-ray flux reaches detectors in the center than on the edges. This means that patients receive more dose than is necessary on the periphery of their anatomy. To compensate for this effect, a bowtie-shape filter is placed in the beam, which is tapered such that its center is thinner than its edges, to equalize the flux reaching the detectors and minimize patient dose.

21. Cotd., The inefficiency in conversion of electron current into x-rays has been a significant practical limitation in the operation of x-ray imaging equipment . The tube is quickly heated to high temperatures, which must be limited to avoid damage. Anode targets have been designed to rotate on bearings, spreading out the area that is heated by the beam. Heat sinks are used to remove heat from the system by convection or water-assisted cooling. In typical clinical operation, an x-ray tube delivers on the order of 2 × 10 power of 11 x-rays per second to the patient, providing a high signal-to-noise ratio for measurements.

22. Cotd., Detection of X-Rays: Detection of x-rays is accomplished by the use of special materials that convert the high energies (tens of keV) of the x-ray quantum into lower energy forms, such as optical photons or electron-hole pairs, which have energies of a few electron volts. The detector materials, such as phosphors, scintillating ceramics, or pressurized xenon gas, ultimately produce an electrical current or voltage. Electronic amplifiers condition this signal, and an analog-to-digital converter converts it into a digital number. The range of signals produced in tomography is large, varying from a scan of air (no attenuation, or 100% transmission) to that of a large patient with metal implants (possible attenuation of 0.0006%), a factor of almost 10 to power of 5. Furthermore, even at the lowest signal levels, the analog-to-digital converter must be able to detect modulations of a few percent. Thus the overall range approaches a factor of one million, specifying the equivalent of a 20-bit analog-to-digital converter.

23. Cotd., Gantry electromechanics : To obtain required measurements at different angles, all the electrical components must be rotated around the patient. In modern scanners, this puts tremendous requirements on mechanical precision and stability. The gantry can weigh 400 to 1,000 kg, span a diameter of 1.5 m, and rotate 3 revolutions per second. While rotating, it may not wobble more that 0.05 mm. Originally, the gantry was connected by cables to the outside environment and had to change rotation direction at the end of each revolution. A major breakthrough in scanning operation occurred with the invention of slip-ring technology, which used brush contacts to provide continuous electrical power and electronic communication, allowing continuous rotation.

24. Cotd., H elical/Spiral scanning : One of the primary goals of CT manufacturers has been to provide faster scan times and larger scan coverage. With the advent of slip-ring technology and continuous gantry rotation, the main limitation to scanning speed was the stepping of the patient bed to position sequential slices. In the late 1980s continuous motion of the patient table was introduced, which allowed faster scan times but required different data handling for image reconstruction. In helical scans the gantry is at continuously different table positions throughout each rotation. A good mathematical value for each gantry position is to interpolate a reconstruction plane from corresponding neighboring gantry positions. This approach provided adequate image quality, and in fact had the added benefit that slices could be reconstructed retrospectively for arbitrary table positions. Furthermore, analysis revealed that on average the spatial resolution was better with helical scans rather than sequential scans. A drawback was that the interpolation process could create stair-step artifacts on the boundaries of extended high-contrast objects.

25. Cotd., Detector configuration : One problem quickly encountered with single slice (single detector row) helical scanning (SSCT) was excess stress on the x ray tube. That is, the x ray tube would heat to extreme temperatures as very high energy was deposited onto the anode. This problem limited the ability to perform thin slice imaging necessary for acceptable coronary imaging. Alternatively, if thin slice imaging were performed, tube current was limited to 100 mAs, which in many instances was not satisfactory and produced noisy images. Thus, multislice CT scanners (MSCT) were created. MSCT is also known as multidetector CT (MDCT) and multirow CT.

26. Cotd., The primary difference between MSCT and SSCT is the detector arrangement. SSCT uses a one dimensional detector arrangement where many individual detector elements are arranged in a single row across the irradiated slice that receives the x ray signals. In MSCT, each detector in a single row is long enough in the slice thickness direction (z axis) to intercept the entire x ray beam width including the penumbra. Each individual detector in each row is then divided into multiple detector elements forming a two-dimensional array. In MDCT, there are multiple rows of detectors. By increasing the number of detector rows, the z axis coverage slab thickness increases thereby decreasing the number of gantry rotations necessary to image the selected field of view (scan length), theoretically reducing the strain on the x ray tube

27. Cotd., For example, 1.25 mm long detectors and the scanner had 16 rows of detectors, the z axis coverage (slab thickness) per gantry rotation would total 20 mm. However, a typical scan would acquire over 1,000 views per gantry rotation and collect data along over 800 detectors in each row, thus generating huge amounts of data. Due to early limitations in acquiring, handling and processing such large quantities of data, MSCT was limited to four detector rows only. Subsequent MSCT scanners possessed increasing numbers of detector rows starting at 16 rows and moving to 64, 156 and 320 rows.

28. Cotd., Depending on the manufacturers multi detector row systems are of many types : Parallel rows of equal sizes are referred as uniform or fixed array. Parallel rows with variable width detectors are adaptive array. Thinner rows centrally, wider rows peripherally are hybrid array. Slice width Fixer/Uniform array Adaptive array Hybrid array Slice thickness of MDCT, is not determined by physical collimation of x ray as in case with SDCT, but also impact by the width of detector in the slice thickness dimension.

29. Questions…? Use of CT number ? What is attenuation coefficient ? Detectors with high efficiency ? Wobbling of gantry should not be more than ? Advantage of helical scanning ?

30. References Computed tomography (CT) . (n.d.). Nih.gov. Retrieved August 13, 2022, from https://www.nibib.nih.gov/science-education/science-topics/computed-tomography-ct Themes, U. F. O. (2016, July 15). Basic Principles of Computed Tomography Physics and technical considerations . Radiology Key. https://radiologykey.com/basic-principles-of-computed-tomography-physics-and-technical-considerations/ CT Scan Components . (n.d.). Radtechonduty.com. Retrieved August 15, 2022, from http://www.radtechonduty.com/2017/03/ct-scan-components.html