Computed Tomography: Hardware and Instrumentation

Greek words: Tomos (cut/slice/section) and Graphein (write/record). Literal definition: A form of tomography (imaging technique to provide cross sectional images of the body) in which a computer controls the motion of the X-ray source and detectors, processes the data, and produces the image. Computed Tomography is fundamentally a method of acquiring and reconstructing an image of a thin cross section of an object. CT is a technique of creating tomographic images from digitized data obtained by exposing the patient to x-rays from many different angles. In this presentation, we start from one of the greatest discovery in the history of mankind the CT scan and how we have evolved to reach the current level of modern CT imaging in the world of radiology.

In slide no. 60, regarding pitch typographical error is corrected as "Slower table movement creates a low pitch values that will give us good spatial resolution but at the expense of increased radiation dose."

1. Hardware and Instrumentation in COMPUTED TOMOGRAPHY Presented by: Dr. Anish Dhakal Resident, MD Radiodiagnosis KUSMS, Dhulikhel Hospital 27 th August, 2025

2. In a world before CT Entire body areas were inaccessible to radiography - brain, mediastinum, retroperitoneum Diagnostic procedures showing better detail in these areas were potentially harmful and or poorly tolerated by the patient viz. pneumoencephalography, diagnostic pneumomediastinum, diagnostic laparotomy

3. Why CT? Limitations of General Radiography : Superimposition of 3D structures over a 2D object Image is created directly on the image receptor and is low in contrast because of Compton scatter radiation

4. Historical Perspective Godfrey Hounsfield (an engineer with Electric and Musical Industries (EMI) Ltd. in England) is credited with the invention of CT. Early work on the mathematics used to reconstruct CT images: Allan Cormack, a physicist at Tufts University A first prototype of CT was installed at Atkinson Morley Hospital under the aegis of Dr. James Ambrose in 1971 and its clinical application started in 1972 The official announcement of invention of CT was made by G N Hounsfield at the annual congress of British Institute of Radiology in April 1972 The first body CT was installed by EMI at North Park Hospital in 1974 G N Hounsfield and Allan Cormack were awarded the 1979 Nobel Prize for medicine for the invention of CT

5. First CT Scanner (EMI Mark I) CT scanner in the early 1970s required 9 days to scan an object & produce a single-section image.

6. Histological perspective 1973-74: The first II Generation CT system total-body scanner has been realized in the U.S., the acquisition time for 1 tomogram was 18 sec. 1976-77: III and IV Generations CT with acquisition time for 1 tomogram lower than 5 sec. 1983 : First electron beam CT , very expensive. 1989 : First helical CT , very low acquisition time (less than 1 sec) and able to explore a large body volume 2000 : Multislice CT scanner, multiple arrays of detectors; continuous development.

7. Introduction: Computed Tomography Greek words : Tomos (cut/slice/section) and Graphein (write/record) Literal definition: A form of tomography (imaging technique to provide cross sectional images of the body) in which a computer controls the motion of the X-ray source and detectors, processes the data, and produces the image. Computed Tomography is fundamentally a method of acquiring and reconstructing an image of a thin cross section of an object CT is a technique of creating tomographic images from digitized data obtained by exposing the patient to x-rays from many different angles . Compared to radiographs, CT images are free of superimposing tissues and are capable of much higher contrast due to elimination of scatter.

8. BASIC PRINCIPLE OF CT The internal structures of an object can be reconstructed from multiple projection of the object Basically, a narrow beam of X ray scans across a patient in synchrony with an array of radiation detector on the opposite side of the patient. A sufficient no. of transmission measurements are taken at different orientation of X ray source & detectors, the distribution of attenuation coefficients within the layer are determined By assigning different gray levels to different attenuation coefficients, an image can be reconstructed with aid of computer that represent various structures with different attenuation properties.

9. How CT differs from conventional radiography? CT scan differs from conventional projection in two significant ways: It forms a cross-sectional image, eliminating superimposition. Sensitivity of CT to subtle differences in x-ray attenuation is at least a factor of 10 higher than normally achieved by radiography, due to the elimination of scatter . The CT scan makes many measurements of attenuation through a cross section. Then these data are used to reconstruct a digital image of the cross section, with each pixel in the image represent a measurement of the mean attenuation of a volume element – voxel.

10. CT scanner Generations Regardless of the CT scanner generation the latent image is acquired and archived in a similar manner. The exit radiation is detected & converted into a digital signal by the analog-to-digital converter (ADC). Data from many different entrance angles are processed in a computer to determine the transmission & attenuation characteristics of the tissues in the section under examination. Data are stored in a matrix of pixels . The digital pixel data are processed in a digital-to-analog converter (DAC) before being displayed.

11. What do CT generation mean? Generation: Order in which CT scanner design has been introduced, and each has a number associated with it. Classification based on arrangement of components and mechanical motion required to collect data. Higher generation number doesn’t necessarily indicate higher performance system.

12. First Generation Very first CT scans were performed on a first generation geometry on a CT benchtop. In the benchtop systems patients were not imaged but rather an object to be imaged is placed on a stage that can rotate (i.e. like a slow and well calibrated record player) Was a single ray system designed to examine only head. NaI scintillation detectors (one or two) with photomultiplier tubes X-ray beam: Finely collimated pencil thin slit field. X-ray beam turned on while scanning and off during rotation. Tube rotation 180 degrees, Translate-rotate system . X-ray tube and two detectors connected in a C-arm fashion to move synchronously from one side to other while scanning (translation). The x-ray tube then rotates 1 degree into the next position and scans again. This process was performed 180 times for each scan . Nearly 5 minutes to complete one slice. Use of two detectors split the finely collimated x-ray beam so that two contiguous slices could be imaged during each procedure.

13. Early detector system couldn’t accommodate large change in signal so patient head was recessed via a rubber membrane into a water filled box/water bath. Acts to bolus the x-rays so that the intensity outside the head is similar to the intensity inside head . Water bath allowed Hounsfield units (HU) to maximize accuracy of attenuation coefficient measurement (limitation of dynamic range, beam hardening correction)

14. First generation CT Advantage: With regard to scatter rejection , pencil beam geometry used in 1 st generation scanners were best. Disadvantage: Nearly 5 minutes was required to complete a single image. Contrast resolution of internal structures was unprecedented, images had poor spatial resolution

15. Second Generation Developed & installed by Ledley et al. at Georgetown University in February 1974 (first waterless full body scan) Use a single projection fan-shaped beam Introduce rest of the body part scanning , table movement through, gantry angulation, & a laser indicator. Translate-rotate system Narrow fan beam (3-10 degrees) and multiple detector elements (linear array of 30 detectors) With 10 degree rotation increment, only 18 translation required for 180 degrees image acquisition . Shortest scan time: 18 sec/slice. 15 times faster than with 1 st generation CT.

16. Limitations in second generation CT Though speed improved, it was still limited by mechanical complexity of translate – rotate geometry . Even small deviations (because of vibration or other misalignment) of scanner hardware position relative to reconstruction voxels would cause data to be back projected through wrong voxels creating severe artifacts . Due to fan beam: Increased radiation intensity towards the edge. Compensated with the use of bow-tie filter (limits the range of intensity reaching detector and hardens beam)

17. Third generation CT The principal limitation of second-generation CT imaging systems was examination time. Because of the complex mechanical motion of translation and rotation and the enormous mass involved in the gantry, most units were designed for imaging times of 20 seconds or longer. This limitation was overcome by third-generation CT imaging systems. Translation of source within each view was eliminated by having a fan-beam shaped x-ray beam acquiring all the data (for a slice) within each view by use of simple and pure rotational motion. Accomplished by widening the x-ray beam encompassing the entire patient width and using an array of detectors to intercept the beam. With these imaging systems, the source and the detector array are rotated about the patient. As rotate-only units, third generation imaging systems can now produce an image in less than 100 ms.

18. Third Generation Use a wider fan-shaped beam & a curved array of 250 to 750+ detectors. Rotate 360 degrees within the gantry. Rotate-rotate mechanism. Continues rotation of the detectors and x-ray tube in a circle around the patient and the x-ray beam slices through the body to produce image data. 1 rotation, 1 second, 1 slice Solve the differential magnification problem which is caused by the use of linear array of detectors, but problem of ring artifacts. The number of detectors and the width of the fan beam—between 30 and 60 degrees—are both substantially larger than for second-generation CT imaging systems.

19. In third generation CT imaging systems the fan beam and the detector array view the entire patient at all times. As the x-ray tube and detectors rotate continuously, projection profiles are collected and a view is obtained for every fixed point of the tube and detector and image is reconstructed. The curvilinear detector array produces a constant source-to-detector path length, which is an advantage for good image reconstruction (allows for better x-ray beam collimation and reduces the effect of scatter radiation)

20. A finely collimated fan shaped x- ray beam covering an angle of 30- 50 degree exposes the patient’s body in many different angles (about 1000) in a circular fashion The beam is accurately aligned to an array of about 800- 900 small radiation detectors in the form of an arc which measures the transmitted intensity The measurements are digitized and fed to a computer, which is later reconstructed as tomographic images of the body part being exposed

21. Earlier 3rd generation CT: Use of wraparound cable for gantry rotation. Used Xenon detector arrange. Xenon detector : Inherently stable and well matched because factor affecting detector response were either uniform for the entire array or constant over chamber. Xenon detectors eventually replaced by solid state detectors. Early third generation CT scanners installed on late 1975 could scan in less than 5 sec , Modern variants of 3rd generation (MDCT): Uses slip ring technology : K/a Continuously rotating fan beam scanning. Scan time: 0.2-0.5 sec. Increases patient throughput Limits the production of artifacts caused by respiratory motion

22. Disadvantage of 3 rd generation CT: Requires extremely high detector stability and matching of the detector response. Any error or drift in the calibration of detectors relative to other detectors result is the ring artifact. Sample size and spacing are fixed by detector design Ring artifacts are never completely eliminated , rather they are minimized by high quality detectors design and frequent calibration . Residual ring artifacts removed by image processing algorithms. Despite these limitations , 3 rd generation CT was highly successful (Slip Ring Technology) and remains the basic geometry of most CT scanners manufactured today.

23. Fourth Generation Use a single projection fan shaped beam with 600 to 2,000+ detectors mounted to an array which forms a 360 degrees ring. Rotate only movement, detectors remains stationary The tube first scans in a clockwise direction & then counterclockwise; this motion continues until the exam is complete. 1 minute for multiple slices. Spatial resolution: >20 lp /cm.

24. Radiation detection is accomplished through a fixed circular array of detectors, which contains as many as 4000 individual detectors. The x-ray beam is fan shaped with characteristics similar to those of third-generation fan beams. The fixed detector array of fourth-generation CT imaging systems does not result in a constant beam path from the source to all detectors, but it does allow each detector to be calibrated and its signal normalized for each image , as was possible with second generation imaging systems. Fourth-generation imaging systems were developed because they were free of ring artifacts.

25. X-ray tube rotates outside the detector ring During rotation, the detector ring tilts so that the fan beam strikes an array of detectors located at the far side of the x-ray tube while the detectors closest to the x-ray tube move out of the path of the x-ray beam. Nutating: Tilting action of the detector ring during data collection.

26. Drawbacks: Size and geometric dose inefficiency Required large detector ring diameter. Reduced spatial resolution limited detector aperture to approx. 4 mm . More scatter radiation, more patient dose. Scatter absorbing septa used in 3 rd generation could not be used in 4 th generation because septa could necessarily be aimed at center of the ring which was the source of scatter. If such septa would used with stationary detectors they would block primary x-rays from reaching the detectors at oblique angles. Overcome by narrow collimation of x-ray tube and software scatter correction. Fourth Generation CT

27. Inside the geometry of 3 rd vs. 4 th generation CT 3 rd generation: Fan beam geometry has the x-ray tube as the apex of the fan; 4 th generation has the individual detector as the apex. 3 rd generation-Detectors near the edge of the detector array measure the reference x-ray beam 4 th generation-Reference beam is measured by the same detector used for transmission measurement

28. Flying focal spot (FFS) is used in Computed Tomography (CT) and other advanced X-ray imaging systems like Digital Breast Tomosynthesis (DBT) to improve image quality by increasing sampling density and reducing artifacts. By deflecting the X-ray focal spot, FFS technology enables the collection of more projection data with a stationary detector, leading to higher spatial resolution, thinner slices, and the suppression of artifacts.

29. Fifth Generation Is a dedicated cardiac unit designed around a rotating electron beam . Also called the Electron Beam CT scanner ( EBCT ). Produces high-resolution images of moving organs such as the (heart) without motion artifact . The x-ray tube has been replaced with an electron gun which uses deflection coils to direct a 30 degrees beam of electrons in an arc around four adjacent tungsten target rings (are stationary and span a 20-degree arc). Ten times faster than conventional CT scanners, fast enough to provide real-time dynamic sectional images of the beating heart.

30. Why 5 th generation CT was developed? Cardiac imaging required ultra fast scan times (<50 ms ) which was a hurdle with previous existed generation . A novel CT scanner was developed specifically for cardiac imaging which was capable of performing complete scans in a little as 10-20 ms. The idea behind the ultrafast scanner is a large bell shaped x-ray tube. Do not used conventional x-ray tube , instead a large arc of tungsten encircles the patient and lies directly opposite to the detector ring. No moving parts in the gantry.

31. Electron beam is produced in cone like structures behind the gantry and is electronically steered around the patient so that it strikes the annular target. Wherever it strikes – produces x-rays. The concept is known as EBCT (Electron Beam CT) Uses an electric gun that deflects & focuses the fast moving electron beam along a 210 arc of a large tungsten target ring in the gantry. When the focused electron beam scans this large target area, X- rays are produced & collimated into a 2 cm wide beam by a set of circular collimators. X-ray beam traverses the patient & strikes the detector ring. Two detector rings permits the simultaneous acquisition of 2 image sections. Image of the whole heart can be acquired in ~0.2 s.

32. Although still available, EBCT was limited to cardiac screening mostly because of image quality for general screening was lower than that of conventional CT (because of low mAs values) and higher equipment costs. With progress being made cardiac scanning by multi slice CT, the future of EBCT is uncertain.

33. Sixth Generation Generations one through four utilized electric cables to move the components and to make X-ray exposure. Designed to use slip-ring technology to replace them. Allows continuous rotation of the x-ray tube & detectors around the patient allowing for a continuous set of attenuation data to be obtained in a helical/spiral manner (hence, helical/spiral scanner). As the x-ray tube circles around the patient, the patient table is continuously move through the gantry aperture. When the examination begins, the x-ray tube rotates continuously. While the x-ray tube is rotating, the couch moves the patient through the plane of the rotating x-ray beam. The x-ray tube is energized continuously, data are collected continuously, and an image then can be reconstructed at any desired z-axis position along the patient

34. 3 rd and 4 th generation CT scanners eliminated the translate-rotate motion , the gantry had to be stopped after each slice was acquired. Before Helical CT era: Cables are spooled onto a drum, released during rotation and respooled during reversal. Scanning, braking and reversal required at least 8-10 sec of which only 1-2 sec were spent for data acquisition. Results in poor temporal resolution and long procedure time. Three technological developments: -Slip ring technology -High power x-ray tubes -Interpolation algorithms

35. After these improvements Allowed true 3D image acquisition within a single breath hold technique. Patient is continuously translated while multiple rotations of gantry (X ray tube and detector) The path of x-ray tube and detector relative to the patient is a helix. An interpolation of the acquired measurement data has to be performed in the z-direction to estimate a complete CT data set at the desired position.

36. Interpolation Algorithms Reconstruction of an image at any z-axis position is possible because of a mathematical process called interpolation . If we estimate a value between known values, that is interpolation ; if one wishes to estimate a value beyond the range of known values, that is extrapolation . During helical CT, image data are received continuously and when reconstructed, the plane of the image does not contain enough data for reconstruction. Data in that plane must be estimated by interpolation. Data interpolation is performed by a special computer program called an interpolation algorithm . The first interpolation algorithms used 360-degree linear interpolation.

37. The plane of the reconstructed image was interpolated from data acquired one revolution apart. When these images are formatted into sagittal and coronal views, blurring can occur. The solution to the blurring problem is interpolation of values separated by 180 degrees—half a revolution of the x-ray tube.’ This results in improved z-axis resolution and greatly improved reformatted sagittal and coronal views. Allows production of additional overlapping images with no additional dose to the patient.

38. KNOWN DATA KNOWN DATA INTERPOLATED DATA

39. Spiral scanning differs from conventional CT scanning in that the table is not stopped at the center of each slice location while the data are collected. Advantage: CT examinations time <1 minute , single breath-hold examination, lower amount of contrast media, decrease in motion artifacts. Acquisition time about 30 seconds , the acquisition time. and the examination time are the same. A conventional axial CT examination requires several minutes to complete, which is much longer than a spiral CT examination.

40. Advantages of sixth generation CT: Fast scan times and large volume of data collected. Minimizes motion artifacts. Less misregistration between consecutive slices. Reduced patient dose. Improved spatial resolution. Enhanced multiplaner or 3D renderings. Improved temporal resolution.

41. Seventh generation CT (MS/MD CT) Multisection / Multislice /Multidetector CT (MSCT/MDCT). Introduced in 1998. Multiple rows of detectors- allows beam utilization. Allows acquisition of multiple slice in single rotation . Faster scanning with a multiple row of detectors system with multiple fan beams scanning simultaneously. Large volume imaging possible with thin beams for producing thin , high-detail slice images or 3-D images. Minimum slice thickness- Minimum detector width not by post patient collimator.

42. 8, 16, 64, 128, 256, 320 and up to 640 slice CT machine are available. Able to expose multiple detectors simultaneously due to detector technology which permits an array of thousands of parallel bands of detectors to operate at the same time. Coupled with helical scanning reduces the total exam time for an entire chest or abdomen to 15 to 20 seconds. Mathematically: For e.g. A 128 slice CT refers to the number of detector rows (slices) the scanner can acquire. Simultaneously per gantry rotation. If each slice is 0.625 mm, coverage per rotation= 128x0.625 mm= 8 cm (wide Z-axis coverage)

43. Pitch : Table movement per rotation divided by beam width. If beam width is 10mm , table moves 10 mm during one tube rotation then pitch is 1 – x-ray beam associated with consecutive helical loops are contiguous. If beam width is 10 mm and table moves 15 mm per tube rotation then pitch is 1.5 – gap exists between x-ray beam edge of consecutive loop. If beam width is 10 mm and table moves 7.5 mm then pitch is 0.75 – beams and consecutive loops overlap by 2.5mm (doubling exposure to the underlying tissues)

44. Increasing pitch to above 1 : 1 increases the volume of tissue that can be imaged at a given time. This is one advantage of multislice helical CT: the ability to image a larger volume of tissue in a single breath-hold. In practice, the pitch for multislice helical CT is usually 1.0. Because multiple slices are obtained and z-axis location and reconstruction width can be selected after imaging, overlapping images are unnecessary. An exception is CT angiography (CTA), which requires a pitch of less than 1.0 : 1. Because of multislice capability, more slices are acquired per unit time. This results in a much larger volume of imaged tissue. Higher pitch has faster coverage but reduced image quality due to gaps in data.

45. Faster table movement creates a low pitch values that will give us good spatial radiation but at the expense of increased radiation dose.

46. Components of a CT system 1. A gantry 2. A patient support table/couch 3. A computer system 4. An operator’s console with display 5. Accessories

47. 1. The Gantry Is a doughnut-shaped structure with X-ray circuit, the x-ray tube, the radiation detectors, the high-voltage generator, rotational components including slip ring systems, gantry angulation motors, patient positioning laser lights and mechanical supports. The gantry frame maintains the proper alignment of the x-ray tube and detectors. Gantry has a 50 to 80 cm aperture for the patient to pass through during the scan.

48. Inside the gantry cover is a large ring that holds the detectors and the track for the x-ray tube while it rotates around the patient. Can be angled up to 30 degrees toward or away from the patient table to permit positioning the patient for coronal images and to align the slice plane to certain anatomy such as the base of the skull or lumbar spine curvature. Three intense white or low-power red laser lights: used to accurately line the patient up for sagittal, coronal, and transverse plane centering in the aperture.

49. X-ray generator Earlier : Low frequency (60 Hz), located outside the gantry in the CT room & is connected to the rotating x-ray tube by thick, flexible high-voltage cables. The cables prevent the tube from rotating >360 degrees without rewinding so axial CT examination collects data one slice at a time. Recent : High-frequency (3000 Hz) circuit, small enough to be mounted with the x-ray tube on the rotating frame inside the gantry. Both the tube and the circuit rotate together around the patient. The input voltage is connected to the circuit through slip rings allowing the circuit and the x-ray tube to continuously rotate.

50. X-Ray Filtration Absorb soft, low energy X-rays. Make uniformity of X-ray spectrum . Reduce scatter, reduce patient dose Improve image quality. 3 mm Al equivalent thickness Flat: Copper or Aluminium . Comb shaped/Bow-tie: Teflon (a material with a low atomic number & high density).

51. X-ray Tube CT X-ray tube: Produces a continuous beam, high-heat capacity tube, capable of operating up to 400 mA & 120 to 150 kVp for several seconds. Heat storage capacity (3.5-7.5 MHU), designed to absorb high heat levels generated from the high speed rotation of the anode & the bombardment of electrons upon the anode surface. Exceed to a maximum heat value it will not operate until it cools down to an acceptable level. Anode: Larger diameter with graphite backing, which allows the anode to absorb & dissipate large amounts of heat. Focal spot size smaller ( 0.6 mm ). Bigger filament size, increased effective focal spot. Anode target angle 7-10 degrees to diminish heel effect .

52. X-Ray Tube ( Metal Ceramic type) Glass envelope has been replaced by metal casing and ceramic is used as insulation of high voltage cable e.g.-super rotalix ceramic x-ray tube. Metal envelope is grounded thus positive with respect to electrons. Anode rotates on an axle with bearing at each end providing greater stability and reduced stress on shaft permits massive anode approx-2 kg. Ceramic insulator (Al oxide) are used to insulate high voltage parts of x-ray tube from metal envelope allowing more compact tube design. Metal used is an alloy of chromium and iron. When metal enclosure is grounded to increase tube life. Higher tube loading, reduce off focus radiation, allow high tube current.

53. X-Ray Tube ( Metal Ceramic type) Fig. Schematic diagram of a Super Rotalix ceramic x-ray tube: 1. Metal casing, 2. Anode, 3/6. Ball bearings, 4/8. Ceramic insulators, 5. Cathode, 7. Stator windings, 6. Anode shaft, 10. Beryllium window

54. X-Ray Tube (Metal Ceramic type: Maximum rotalix ) Introduced Maximum rotalix ceramic (MRC) tube by Philips-1989AD. Features & advantage of MRC tube: Improvement in rotating anode x-ray tube. Noiselessly continuous rotating anode that could be switched on in the morning and switched off in the evening. Liquid metal alloy as lubricant. No waiting time during & between examination.

55. X-Ray Tube (Metal Ceramic type: Maximum rotalix ) MRC x-ray tube (Philips) with metal spiral groove Possible to achieve dose saving filter technique in angiography. Higher output & longer tube life. Cont. rotation overcome the time to speed barrier. 200 mm graphite backed anode. Anode heat storage capacity -8 MHU Directly cooled Anode. Tube voltage 90 to 140 kV. Tube current 20 to 500 mA . Anode angle – 7 o . Uses: Cardiovascular imaging, Multi-slice CT.

56. Metal Ceramic X-Ray Tube: Aquilion Toshiba High capacity multi-slice CT tube Heat storage capacity-7.5 MHU Cooling rate- 1.7 MHU/min Anode grounded Focal spot 1.4 mm×1.6mm Air cooled. Aquilion x-ray tube

57. Straton X-Ray Tube Introduction of rotating envelope tube (RET) Idea was first documented in 1917. This design is very close to the realized Straton tube (Siemens) having used magnetic & deflecting coils to deflect (deflect focal spot 4,640 times) the electron beam for striking two areas on the anode to get better z-axis resolution. Anode being direct contact with the cooling oil enables an extremely high cooling rate (up to 7 MHU /min). Made MDCT possible with improved workflow, increased resolution. Focused and deflected beam of thermal electron Whole tube and anode assembly rotates. Bearings out side. Oil cooled, Zero heat storage capacity. Cooling rate- 4.7 MHU/min, Cooled down within - 20 sec. Enables gantry speed of 0.37 seconds per rotation. Tube current-500 mA.

58. In the early 1990s, the design of third- and fourth-generation scanners evolved with the incorporation of a new technology called slip ring . A slip ring is a circular contact with sliding brushes which supplies electrical power to the CT system and allows the gantry to rotate continuously, uninterrupted by wires. E lectrical conductive rings and brushes transmit electrical energy across a rotating interface. The use of slip-ring technology eliminated the inertial limitations at the end of each slice acquisition, and the rotating gantry was free to rotate continuously throughout the entire patient examination. Slip ring Technology

59. Early CT- Used cables for power supply and data transfer. Leads to interscan delay. No single breathhold examination possible.

60. There are usually 3 slip rings on a gantry: One provides high voltage power to the x-ray tube or low voltage power to the high tension generator Second provides low voltage power to control systems on the rotating gantry The third transfers digital data from the rotating detector array

61. Slip ring p rovides electrical power to the rotating components without fixed connections. Complete elimination of interscan delays except for the time required to move the table to next slice position. For eg : If scanning and moving the table each take 1s , only 50% of the time is spent acquiring the data . Furthermore, rapid table movement may introduce tissue jiggle artifact.

62. F ig. Slip rings used to bring power to x-ray tube on rotating gantry of CT machine. (a) The shiny metal strips carry electric signals that are swept off by special brushes. (b) The brushes are not in the form of bristles but rather of metal blocks (in this case a silver alloy). The five pairs of larger brushes provide the voltage required by the x-ray tube, and the three pairs of smaller ones transfer signals from the gantry controller.

63. Designs: DISC CYLINDER

64. BRUSH DESIGN: 2 Common Design Wire brush: Uses conductive wire as sliding contact Composite brush: Uses a block of some conductive material ( e.g. silver alloy graphite ) as sliding contact

65. Based on the power supply: 1) Low voltage slip ring AC power and x-ray control signal transmitted to slip rings by means of low voltage brushes that glide in contact groove on the stationary slip ring. Slip rings then provide power to the high voltage transformer which subsequently transmit to x-ray tube and gantry. X-RAY

66. 2) High voltage slip ring AC delivers power to the high voltage generator which subsequently supply high voltage to the slip ring. High voltage from slip ring transmitted to x-ray tube. In this design, high voltage generator does not rotate with the x-ray tube X-RAY Tube

67. Recent advance in the design of slip rings: Contactless slip ring. Utilize IPT (Inductive power transfer) to transfer power. Electromagnetic field created by coils placed in the stationery transmitter and rotating reciever.

68. Advantages of slip ring technology Contactless slip ring removes the inherent need for friction or contact to generate an electrical current. Possible to transfer electrical energy across a rotating interface without the use of electrical contacts. Reduces the maintenance costs. Removal of cable wrap around process Elimination of the start–stop process characteristic of conventional CT scanners Provides capacity for continuous data acquisition protocol like dynamic studies and CTA. Faster scan time and minimal interscan delay

69. Collimation Collimation is required in CT scanning for exactly the same reasons as in radiography. Reduces patient dose by restricting the amount of tissue which is irradiated. Type of collimators: Pre-patient collimator (Tube or Source collimators): Is mounted on the tube housing and limits the area of the patient that is exposed to the primary beam. This collimator determines the slice thickness and patient dose. Post-patient collimator (Pre-detector or Anti-scatter collimators or Anti-scatter septa or grid ): Is comprised of thin plates formed from a suitable X-ray absorbing material like lead or tungsten. Located directly below the patient & above the detectors restricts the x-ray field viewed by the detector array.

70. Lead or Tungsten Plates are focused at the x-ray focal spot and generally located between columns of detectors (z-direction) but not between rows of detectors. This collimator is referred to as a " 1D " anti-scatter collimator. In multi-slice scanners : shielding between both columns & rows of detectors; both directions are focusing to the X-ray source . This type is called a " 2D " anti-scatter collimator. 1-D Anti-scatter Collimator 2-D Anti-scatter Collimator

71. Radiation Detectors Detectors measure the intensity of radiation transmitted through the patient. Characteristics: Cost as minimum as possible. High absorption efficiency. High conversion efficiency. Responsiveness- Response time refers to length of time it takes for a detector to review & discard signals. Dynamic range- Refers to the detector ability to receive wide range of X ray intensities with proportional output. If exceed dynamic range streak artifacts may occur. Types of detectors: Gas ionization detectors Scintillation crystal detectors

72. Gas-filled Ionization Detectors Use xenon gas to produce an electrical signal when x-ray photons pass through the chamber. Thin tungsten plates spaced 1.5 mm apart make up the electrodes in the ionization chamber. Space between the electrodes acts as radiation detectors. Each electrode produces an electric field across the chamber . X-ray photons produce ionization in the gas. The electric field pulls the negative ions to the positive electrode & the positive ions to the negative electrode producing electric current.

73. Gas-filled Ionization Detectors The current produced is proportional to the ionization in the chamber and the energy of the radiation passing through the chamber. The detected energy makes up the digital signal that is sent to the computer. Detector efficiency approximately 50 - 60%.

74. Gas-filled Ionization Detectors D iagram shows how an array of Xenon detector cells convert X-ray energy to the digital data that is used by the image reconstruction computer.

75. Scintillation/Solid state Detectors Comprised by sodium iodide crystals that give off a flash of light when an x-ray photon is absorbed. Light is produced in proportion to the intensity of the photon. Use a Photomultiplier (PM) tube to convert x-ray photons into an electrical signal which the computer uses to form the visible image. Light from the scintillation crystal is detected by a PM tube. The light photon strikes the cathode of the PM tube where it is converted into electrons. Electrons are then amplified by a chain of dynodes as they pass through the tube. The dynodes have progressively higher voltage which causes an increase in the number of electrons as they move toward the anode.

76. Once the electrons bombard the anode , they are converted into an amplified electrical signal which is then processed by the computer. This system were used in early generations of scanners but not to the modern scanners because of the phosphorescent afterglow properties of them. Sodium iodide was replaced by Bismuth germanate & Cesium iodide. The current crystal of choice is cadmium tungstate which has more than 90% efficiency with minimal afterglow. Photodiode detects the light photons emitted, and converts them into an electrical signal which is used by the computer to form the digital image.

77. Scintiliation Detector D iagram shows how an array of four Scintilating Crystal detector cells convert X-ray energy to the digital data that is used by the image reconstruction computer .

78. 2. Patient Support Table (Couch) Either curved or flat. Capable of moving up & down for ease in transferring patients onto and off of the table , and in positioning the patient correctly in the aperture. Table indexing must be accurate and reproducible within 1 millimeter (mm). Constructed of low atomic number carbon graphite fiber to reduce attenuation of the x-ray beam and to support patients weighing as much as 250-350 kg, if excess may alter indexing . Tabletop must be able to support the entire weight of the patient when the table is moved into the gantry aperture.

79. In conventional CT scanning, the tube rotates around the patient to collect data for one slice , the table indexes into the gantry at a preset distance, and the tube rotates again to collect data for the next slice. In helical scanning, table moves steadily through the gantry while the tube continuously rotates around the table and the patient. The entire helical CT examination can be completed in <1 minute.

80. 3. Computer System Unique component of the CT system. Sufficient speed & memory to solve several thousands calculations simultaneously. Is designed to control data acquisition, processing, display, retrieve and storage. Calculates the attenuation of the individual voxels using algorithm. Calculations of the CT numbers must be very fast to produce images for immediate viewing.

81. 4. Operator’s Console Permits control of all scan parameters including selecting proper technical factors, movement of the gantry and patient table, and computer commands that allow reconstruction and transfer of image data for storage in a data file. Operates a menu of directory operations. Is preprogrammed with the kVp and mA values for individual anatomic parts. The technologist uses a keyboard and mouse to indicate the desired operation for the anatomy to be scanned. Each scan must have patient information, such as identification and medical history entered prior to beginning the scan. Hard copy images are printed on film using a multiformat camera or printer.

82. Computed tomography imaging systems can be equipped with two or three consoles. One console is used by the CT technologist to operate the imaging system. Another console may be available for a technologist to postprocess images to annotate patient data on the image (e.g., hospital identification, name, patient number, age, gender) and to provide identification for each image (e.g., number, technique, couch position).

83. Display console Is often part of the main control and is a separate CRT or flat panel display with controls. May be a separate workstation that allows radiologists to display and manipulate images, and to electronically dictate the results, permit a wide range of features to enhance the digital image. Multiplanar Reconstructions (MPRs), Reverse Display, Magnification, Suppression, Selection of Region of Interest (ROI), Annotation, Maximum Intensity Projections (MIP), 3D Imaging etc .

84. 5. ACCESSORIES Head rests/supports, Knee support. Velcro straps for immobilization. Automatic contrast injector. ECG gating machine for Cardiac gated CT. Phantoms for BMD scanning. Stereo-tactic localizer for brain lesion. localization for stereo-tactic radio-surgery planning. Trolley with 2 compartments: sterilized and unsterilized items. Lead aprons, emergency drugs and management system etc.

85. References Christensen's Physics of Diagnostic Radiology, 4th edition Radiologic Science For Technologists Physics Biology And Protection by Stewart C. Bushong 12th edition

86. Thank you