Ct instrumentation and types of detector configuration

CT INSTRUMENTATION AND DETECTOR ARRAY CONFIGURATIONS

1. CT INSTRUMENTATION AND TYPES OF DETECTOR CONFIGURATION Presenter :Sujan Karki B.Sc. MIT 3 rd year National Academy of Medical Sciences(NAMS) Bir Hospital

2. Contents Introduction Generations of CT System components References

3. Introduction Computed tomography(CT) is a revolutionary tool of medicine, particularly in medical imaging. tomos = slice, graphein = to write Method for acquiring and reconstructing an image of a thin cross section of an object Based on measurement of x-ray attenuation through the section plane using many different projection. Image courtesy :Damien Hirst Autopsy with Sliced Human Brain 2004

4. Historical perspectives Invention of CT was made possible through the work of several individuals, most notably Godfrey Newbold Hounsfield and Allan MacLeod Cormack . In 1967, Hounsfield was investigating pattern recognition and reconstruction techniques by using the computer. He concluded that if x-rays are passed through an object from all direction and if are measured then information of internal structure could be obtained. The radiation used was from an americium 241 gamma source coupled with a crystal detector. Because of the low radiation output, the apparatus took about 9 days to scan the object. Because this procedure was too long, various modifications were made and the gamma radiation source was replaced by a powerful x-ray tube. ( Hounsfield, 1980). Fig: second prototype CT scanner

5. Limitation of conventional tomography and radiography The major shortcoming of radiography is that the superimposition of all structures on the radiograph (3D collapsed to 2D) and Low soft-tissue contrast The limitations of tomography include persistent image blurring that cannot be completely removed, degradation of image contrast because of the presence of scattered radiation created by the open geometry of the x-ray beam, and other problems resulting from film-screen combinations.

6. Imaging 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 - pneumoencephalography, diagnostic laparotomy Fig: ventriculography Fig: pneumoencephalography Fig: preparing patient and positioning during pneumoencephalogram.

7. Computed Tomography Developmental chronology 1924 - mathematical theory of tomographic image reconstructions (Johann Radon) 1930 - conventional tomography (A. Vallebona ) 1963 - theoretical basis of CT (A. McLeod Cormack) 1972 - first commercial CT (Sir Godfrey Hounsfield) 1979 - Nobel price (Cormack & Hounsfield) 1989 - single slice helical CT scanner 1992 – dual slice CT scanner 2004 – 64 slice CT scanner 2006 – dual source CT scanner 2007 – 320 slice CT scanner (per revolution) 2011 – Steller detector 2012 – 640 slice scanner 2015 – GE introduced 512 slice ct with 256 rows of detector 2018 – 360 degree rotation only in 0.24 sec (x-ray-tube)

8. Original EMI Scanner Designed specifically for brain imaging. Head is enclosed in a water bath between x-ray tube and pair of detectors. Patient remains stationary and gantry move through rotation and translation motion Linear motion was repeated 180 times and 1degree gantry rotation after each linear motion. The x-ray tube was on throughout linear motion and off during rotary motion. The transmitted radiation was measured 160 times in each linear movement. (180x160=28,800) Each pair of tomographic sections took 5 minutes, so scan time was 25 minutes. Fig: EMI Mark 1

9. EMI contd. CT image was reconstructed and displayed in 80x80 matrix in 2 different formats: a paper printout and visual image on cathode ray tube. The x-ray field was collimated to 3x3x13 mm. Data from the measurement were computer processed and presented in 3x3 mm of patient cross section. An oil cooled stationary x-ray tube was used with focal spot size 2.25x12mm and operated at 120kVp and 33mA. A large difference in attenuation between adjacent areas such as air surrounding the head makes computation complicated so water bath was used. Fig: pixel and voxel in EMI scanner

10. First generation : Rotate/Translate, Pencil Beam Original EMI unit was first generation scanner. Used a pinhole collimator NaI as detector element with PM tube X-ray tube and detector system translate linearly across the 24 cm FOV acquiring 160 parallel rays pencil beam and a single detector.  A five view study of head took about 25-30 minutes. fig: Image of first generation CT

11. Second generation : R otate/Translate, Narrow Fan Beam Narrow fan beam was used with an angle of 10-30 degree and detector 5-30 detectors . with 10 degree rotation increment, only 18 translation would be required for 180 degree image acquisition. Produced a tomographic section between 20-90 seconds. Using fan beam increased radiation intensity towards edge and was compensated by bow tie filter . Acquired more data (600 rays X 540 views=32400 data point) Limited only for head scan .

12. Third generation :  Rotate/Rotate, Wide Fan Beam Translational motion was oblirated . The path traced by the tube describe circle rather than the semicircle characteristic of 1 st and 2 nd generation CT scanner.  The main goal was to cut the acquisition time to less than 20 seconds . W ide aperture fan beam angle ranging between 40 to 60 degrees.  This new array consisted of 400 to 1000 detector elements. M ajor disadvantages: ring artefact(occasional ) Software corrected image reconstruction algorithms now remove such artefacts.

13. Fourth generation : Rotate/Stationary D eveloped specifically to alleviate the ring artifacts produced by the third generation.  R emoving the detectors from the rotating gantry and putting them in a stationary ring around the patient. This stationary 360 degree ring of detectors required an increased number of detector elements (~5000 total). x-ray tube can rotate either outside or inside the detector ring . If the x-ray tube rotates outside of the detector ring, it is crucial that the detector ring is tilted so that the x-rays only interact with the detectors once they pass through the patient .

14. Fifth generation :  Stationary/Stationary Developed for cardiac imaging, the EBCT reduces scan time to as little as 50 ms. does not use an x-ray tube; instead it uses a beam of electrons generated outside the gantry . Inside the gantry there are 180-degree rows of fixed detectors on one side and 180 degrees of tungsten arcs opposite .  The electron beam is rapidly moved to bombard the tungsten arcs, producing an x-ray beam . The x-rays then pass through the patient and transmission information is collected by the detectors. Capable of producing 17slice/second.

15. Sixth Generation: Helical CT In previous generation detectors, the gantry had to be stopped after ever slice, so that data acquisition could not be a continuous process. This problem was solved in the 1990's when slip ring technology was introduced to the field of medical imaging. Three technological developments were required: slip ring gantry designs, very high power x-ray tubes, and interpolation algorithms. Interpolation is essentially a weighted average of the data from either side of the reconstruction plane, with slightly different weighting factors used for each projection angle. Z-AXIS

16. Seventh Generation: Multiple Detector Array The most recent generation of CT scanner consists of a multiple detector array and a cone shaped x-ray beam. Unlike the pencil beam and fan beam, the cone beam does not pass through a narrow collimator. Therefore, the intensity of the initial x-ray beam is not as strongly reduced and hence can interact more efficiently and effectively with the detector array  . In order to use a cone beam x-ray geometry, the linear detector array found in previous generations of CT scanners had to be modified to multiple detector array. With multiple detector array scanner, slice thickness is determined by the detector size, not by the collimator. Rows or detector along z-axis are increased.

17. Flat panel detector CT ….which generation????? Flat-panel digital detectors similar to the ones used in digital radiography are now being considered for use in CT The detector consists of a cesium iodide ( CsI ) scintillator coupled to an amorphous, silicon thin-film transistor array. These flat-panel detectors produce excellent spatial resolution but lack good contrast resolution. Its fluoroscopic and angiographic capabilities are useful for intraoperative, vascular, interventional and intraoperative applications makes it a system of choice. flat-panel detectors are also being investigated for use in CT of the breast. Fig: Photograph of a prototypic flatpanel volume CT scanner Fig: cadaveric temporal bone and 3D model of auditory apparatus

18. System Components The three major components of CT scanner are: 1)Gantry 2)Computer and operators console 3)Patient Table

19. Gantry The gantry is a circular device that houses the x-ray tube, DAS, slip rings and detector array . Helical CT unit have continuous slip ring and high voltage generator in the gantry. The gantry can be tilted forward and backward to 30 degree aperture measures about 28 inches(71.1cm ) wide to accommodate variety of patient sizes.

20. High voltage/frequency Generator Xray tube require a high voltage generator to achieve the necessary power required of an x-ray tube AC power will supply x-ray tube with sinusoidal currents, resulting in peak and trough A single phase high voltage generator converts this AC into a half or full wave rectified supply with a measure in the thousands of volts The half wave rectification results in a peak voltage that will dip to zero reoccurring: this will consequently have an effect on this behavior of radiation produced and hence the name kilovoltage peak ( kVp ) was born The advancement of highvoltage generator from single phase to three phase to constant potential generators have overcome this voltage ripple creating a continuous, uninterrupted voltage. Modern x-ray units, which largely utilize constant high voltage generators have voltage ripple of lee than 1% and consequently employ the term kV rather than kVp .

21. High frequency generator Ratings can range from 20 to 100 kilowatts (kW; Kalender , 2005). More recently CT manufacturers have generators capable of 120 kW. An output capacity of, say, 60 kW will provide a range of kilovolt and milliampere settings, where 80 and 120 to 140 kV and 20 to 500 milliamperes (mA) with 1-mA increments are typical.

22. Filters Copper  or  aluminum filters  are used to filter the x-ray beam. The typical filtration on a CT x-ray tube is ~ 6 mm Al. A  bow tie filter  is used to  minimize  the  dynamic range  of exposures at the  detector. Bow tie filters attenuate little in the center, but attenuation increases with increasing distance from the central ray. Bow tie filters  are made of a low Z material such as  Teflon  to minimize  beam hardening  differences. Bow tie filters also  reduce scatter  and  patient dose. Fig: Two types of beam-shaping filters for use in CT Via: Radiology key

23. Collimators Collimation usually refers to the act of constraining the x-ray beam. In the early step and shoot scanner the beam collimation =slice thickness. Pre patient collimation - determines dose profile and patient dose. Pre detector collimation -restrict the X-ray beam viewed by detector array, this reduces the scatter on detector array. when properly coupled with pre patient collimator define the slice thickness. -It reduces scatter radiation there by improving contrast. Detector collimation Beam collimation Fig: 3D collimator

24. Over ranging and Over beaming in MDCT Over-ranging Over-beaming Over beaming “relates to x-ray beams being slightly wider than the detector which means that patients are exposed over a small area without the signal being detected” Over ranging “refers to exposure of the patient outside the imaged range which occurs for spiral CT with multi-row detectors at the start and the end of the scan ( Kalender , 2014 )

25. Adaptive Section Collimation The problems of over ranging and overbearing can be solved using a technique called adaptive section collimation. Parts of the x-ray beam exposing tissue outside of the volume to be imaged are blocked in the z-direction by dynamically adjusted collimators at the beginning and at the end of the CT scan (Deak et  al., 2009).

26. Data Acquisition System 1)The DAS system: ‘reads’ the measurements from the detector array, converts these analogue signals into digital format, and transmits the digital signal to the computer systems for reconstruction into the presented images. FIG: Data acquisition system (DAS) channels are used to specify slice width and thickness in multi-detector CT by electrically coupling detectors.

27. Slip ring Technology Slip rings are electromechanical conducting brushes that transmits power to gantry components for its continuous rotation. Due to the slip ring technology, helical scanning is possible. One surface is a smooth ring and other a ring with brushes that sweep the smooth ring. In a slip ring gantry system, power and electrical signals are transmitted through stationary rings within the gantry. Thus eliminating the need for electrical cables. Recently GE have introduced the contactless whisper slip ring technology.(Optical fibers rely on optical radiation to transfer the data and operate typically at infrared wavelength between 850 and 1550 nm allowing EMI free transmission at very high rate of several dozens of Gbps)

28. Slip ring contd. Two slip-ring designs are the disk (Fig A) or pancake type (Fig B) In the disk design, the conductive rings form concentric circles in the plane of rotation The cylindrical design includes conductive rings positioned along the axis of rotation to form a cylinder The brushes that transmit electrical power to the CT components glide in contact grooves on the stationary slip ring. Fig: A Fig: B

29. Slip ring contd. Two common brush designs are the wire brush and the composite brush Two brushes per ring are often used to increase either communication reliability or current carrying capacity. The composite brush uses a block of some conductive material (e.g., a silver graphite alloy) as a sliding contact. This design makes it possible to transfer electrical energy across a rotating interface without the use of electrical contacts.

30. Types of slip ring based on power supply low-voltage slip-ring system :AC power and x-ray controlling signal are passed to slip ring by low voltage brushes then slip ring provides power to high voltage transformer then high voltage is provided to x-ray tube. In this case, the x-ray generator, x-ray tube, and other controls are positioned on the orbital scan frame . High voltage slip ring system : AC delivers power to the high-voltage generator ,which subsequently supply to x-ray tube. In this case, the high-voltage generator does not rotate with the x-ray tube.

31. CT x-ray tube First- and second-generation scanners used fixed anode, oil-cooled x-ray tubes. Rotating anode x-ray tubes have become common in CT because of the demand for increased output. The disk is usually made of (RTM) alloy and other materials with a small target angle (12 degrees) and a rotation speed of 3600 to 10,000 rpm The introduction of spiral/helical CT with continuous rotation scanners has placed new demands on x-ray tubes. Because the tube rotates continually for a longer period compared with conventional scanners, the tube must be able to sustain higher power levels.

32. Technical advancement's Electrical arcing results from tungsten deposits on the glass caused by vaporization was solved by replacing metal envelops. Metal envelope tubes have larger anode disks 200-mm diameter compared with the 120- to 160-mm diameter typical of conventional tubes. Heat-storage capacity is also increased with an improvement in heat dissipation rates The cathode assembly consists of one or more tungsten filaments positioned in a focusing cup . Working life of recent tube are 10000 to 40000 hours compared to 1000hours of conventional tube. Heat storage capacity of modern tube is 5-8MHUor more compared to 1-3MHU of early 3 rd generation scanner tube. Upgrade

33. Disc design of x-ray tube in modern CT The brazed graphite anode disk consists of a tungsten-rhenium focal track brazed to a graphite base body. Graphite increases the heat storage capacity because of its high thermal capacity, which is about 10 times that of tungsten. CVD graphite disk consists of a graphite base body with a tungsten-rhenium layer deposited on the focal track by a chemical vapor process. (Fox, 1995). Fig: The anode assembly of a modern x-ray tube used in CT

34. STRATON X-RAY TUBE Siemens introduced a new design of x-ray tube in which entire tube body rotates , rather than just the anode, as is the case with conventional designs. This change allows all the bearings to be located outside the evacuated tube & enables the anode to be cooled more efficiently. Anode is in direct contact with the oil. The Straton has a low inherent heat capacity of 0.8 MHU , but an extremely fast cooling rate of 5 MHU / min. The heat capacity & cooling rate combine to produce a tube which Siemens claim is ‘ 0 MHU ’ The beam is deflected to strike the anode at two precisely located focal spots that vary in size. the sizes can be 0.6 mm × 0.17 mm, 0.8 mm × 1.1 mm, and 0.7 mm × 0.7 mm. The electron beam alternates at about 4640 times per second to create two separate x-ray beams that pass through the patient and fall on the detectors.

35. Comparison of Straton tube with conventional tube

36. MAXIMUS ROTALIX CERAMIC (MRC) TUBE Based on the technology of spiral groove bearing using liquid metal alloy as lubricant. The excellent heat dissipation of the bearings via the liquid metal lubricant gave the tube higher cooling capacity. Noiseless rotating anode and had a very long lifetime. Replaced ball bearings which has to run in vacuum , conventional oil based lubricants could not be used.

37. LIMAX (Liquid Metal Anode X-ray) by Philips Liquid metal jet of SnPb , GaInSn , turbulently streaming through a tube close to the cathode, is heated at the focal spot & subjected to fast electrons. . While the heated material is transported through the tubing, cold metal enters the focal spot area & cooled effectively by circulation through a heat exchanger. Liquid metal separated from the vacuum by a diamond, tungsten or molybdenum The requirements on mechanical stability (especially at elevated temperatures) and transparency to electrons (in the energy range of 50 – 150 keV) are much more demanding.

38. CT Detectors When the x-rays passes through the patient the x-rays that were not attenuated by the patient passes and reaches the film in screen-film radiography . A similar process occurs in CT but the film is replaced by the detectors. In CT there are two main types of detectors 1) Xenon gas detectors 2)Scintillation detectors

39. Detector Characteristics Detectors exhibit several characteristics essential for CT image production affecting good image quality. Some of them are as mentioned below: Efficiency :refers to the ability to capture, absorb, and convert x-ray photons to electrical signals. CT detectors must possess high capture efficiency, absorption efficiency, and conversion efficiency. Dynamic range: The dynamic  range describes the  range  of x-ray intensities a detector can differentiate. A high dynamic  range  provides the discrimination between small differences in x-ray  attenuation., the dynamic range is 1 million to 1) Afterglow :refers to the persistence of the image even after the radiation has been turned off. CT detectors should have low afterglow values, such as less than 0.01 percentage, 100 milliseconds after the radiation has been terminated Stability :refers to the steadiness of the detector response. If the system is not stable, frequent calibrations are required to render the signals useful.

40. Xenon Detector ( Convert x-ray energy directly into electrical signal ) Xenon detectors consist of two metal electrodes (an anode and a cathode) surrounding high-pressure(25-30atm pressure) xenon gas.  X-ray beam pass through the patient, it interacts with the xenon atoms ionizing the gas. I onization of xenon causes a build up of electrons within the compartment .  The electrons move toward the positively charged anode and continue to build up so that a charge accumulates on the anode. C harge is then amplified into an electronic signal . Xenon gas is used because of its ability to remain stable under pressure ,it has QDE(60-87%) It was used in third generation CT Xenon gas is the element of choice because of it's ability to remain stable under extreme amounts of pressure and high atomic no.

41. Scintillation Detectors ( Convert x-ray energy into light) The more modern version of the CT detector is a luminescent detector. The solid-state detectors consist of a scintillator and a photodetector that converts x-rays into light and then photodiode converts light into electrical signal. This type of photodetector is referred to as a photovoltaic detector array (PDA) and it is based on front illumination. Current CT scanners now make use of back-illuminated PDA .

42. Major types of detector

43. Design innovations in CT detectors FIG: Various types of latest detector Coverage of 64,128 and 320 rows of detector along z axis Image: Robert pelberg 2015

44. Detectors array configuration Matrix Type(uniform) Adaptive Type(non-uniform) Hybrid Type(mixed) Fig:4 slice MDCT Scanner

45. UFC Detector Ultrafast Ceramic is a hard yellow substances that resembles plastic and weighs about as much as gold It includes rare earth elements gadolinium , yttrium and sulfur. The material is formed through the process involves mixing, chemical reduction , sintering and pressing.

46. UFC COntd Ultrafast Ceramic scintillator material was developed with high X-ray absorption efficiency, a fast decay behavior and low afterglow for the CT system with the highest rotation speed and the shortest integration time .

47. Stellar Detector Third generation CT detector developed by siemens. Major components of the Stellar Detector, which include the UFC scintillator, the back-illuminated photodiode, and the metal oxide semiconductor (CMOS) wafer that includes the ADC, and a ceramics substrate. The principle of operation is as follows: the light emitted from the UFC scintillator reaches the backside-illuminated photodiode, a digital signal is then produced on the other side of the wafer. Stellar detectors can measure smaller signals over a wide dynamic range which reduces the noise in CT images and enhances CT image quality. The electrical components like ADC are integrated together at the back of scintillation layer so the electronic noise can be reduced compared to the second generation detector as shown in fig 2 Fig: 2 Fig : 1

48. Stellar detector contd.. Fig: Major components of Stellar Detector Fig: photograph of the detector array Fig: hip phantom with a resolution insert that shows the improvement in image quality using the Stellar Detector and a conventional CT detector

49. Gem Stone detector Transparent polycrystalline scintillator It has higher sensitivity to radiation and allow faster sampling rate. It is used in single source ultrafast dual energy switching , promising almost simultaneous spatial and temporal registration and material decomposition. It has primary decay time only 30nsec i.e. 100 fold faster than conventional scintillator. The most advantage of gemstone detector is the improved spatial resolution.( high definition imaging up to 230mm resolution .) Primary speed is only 0.03 µs, 100 times faster than GOS and is the fastest scintillator in CT industry, at 40ms, the afterglow is 0.001%, only 25% of GOS

50. Patient couch The table is automated device linked to the computer and gantry and move in increment according to scan programme . Indexing must be accurate and reliable, especially when thin slices(1 or 2 mm ) are taken through the area of interest. Ct table are made of up low density carbon composite which supports the patient without causing artefact All ct table have maximum patient weight limit this limit varies by manufacturing from 136-272kg

51. 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 & 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.

52. Operator’s Console Permits control of all scan parameter including selecting proper technical factor, movement of the gantry and patient table Commands computer to reconstruct and transfer of image data for storage in data file Pre programmed with the kV an mA values for individual anatomic parts

53. Display console In the display and manipulation of grayscale images for diagnosis, it is important to optimize image fidelity (i.e., the faithfulness with which the device can display the image) This is influenced by physical characteristics such as luminance, resolution, noise, and dynamic range. Resolution, however, is an important physical parameter of the grayscale display monitor and is related to the size of the pixel matrix, or matrix size. The display matrix can range from 64 × 64 to 1024 × 1024, but high-performance monitors can display an image with a 2048 × 2048 matrix ( Dwyer et al., 1992).

54. Other Accessories Head rest/support Table straps for immobilization Automatic contrast injector ECG machine Phantoms for quality, performance and dose test Emergency trolly Radiation protective materials

55. Bir CT Specifications ( Philips Ingenuity -128 slice CT Xray tube Anode heat storage:8MHU, Anode cooling rate: 1608KHU/min dual focus (1x1mm)and (0.5x0.1mm),Generator rating 80 KW, KV Range: 80-140,MA range: 20-665 gantry rotation:0.4 sec for 360 degree rotation with 70 cm gantry aperture Detector scintillation detector ,gadolinium oxysulphide no of detector rows 64 (128 slice per rotation by z-flying focal spot technique) Reconstruction iterative reconstruction (idose4) standard reconstruction matrix:512x512 standard reconstruction speed:25images/sec temporal resolution : 0.053sec operating monitor:2 lcd monitors with screen resolution of 1280x1024

56. Bibliography Computed Tomography (CT or CAT Scan). RadiologyInfo.org Website : http://radiologyinfo.org/en/sitemap/modal-alias.cfm?modal=CT . Published 2013. Updated 2013. Accessed November 20, 2013. Bharath AA. Introductory Medical Imaging (Synthesis Lectures on Biomedical Engineering). Morgan and Claypool Publishers; 2008. Ulzheimer S, Flohr T. Current Technology and Future Direction. In: Multislice CT. Medical Radiology. Germany, Springer Berlin Heidelberg. 2009; 3-23. Dr. EUCLID SEERAM, computed tomography ,Physical Principles, Clinical Applications, and Quality Control Multidetector CT in children: current concepts and dose reduction strategies Rutger A. J. Nievelstein ,  Ingrid M. van Dam  &  Aart J. van der Molen Robert Pelberg Cardiac CT Angiography Manual Second Edition Flat-Panel Volume CT: Fundamental Principles, Technology, and Applications Rajiv Gupta, MD, PhD • Arnold C. Cheung, MD A Comparison of Front- and Backside-Illuminated High-Saturation Power Partially Depleted Absorber Photodetecters Xiaowei Li, Ning Li, Stéphane Demiguel , Joe C. Campbell, Fellow, IEEE, David Tulchinsky , and Keith J. Williams, Liquid metal anode x-ray tube Bernd Davida , Hans Barschdorfa , Volker Doormanna , Rainer Eckarta , Geoffrey Hardinga , JensPeter Schlomkaa , Axel Thrana ADVANCES IN CT IMAGING (NJ PELC, SECTION EDITOR) CT Systems Thomas Flohr second edition. Instrumentation and Principles of CT Douglas P. Boyd Computed Tomography: Ian A. Cunningham Victoria Hospital, the John P. Robarts Research Institute, and the University of Western Ontario Philip F. Judy Brigham and Women’s Hospital and Harvard Medical School Radiologic Science for Technologist: physics, biology and protection tenth edition, Stewart Carlyle Bushong

57. Questions What is the advantage of slip ring technology? What were the technological developments of helical CT? What do you understand by overranging and overbeaming ? How are scintillation detectors different from gas ionization detectors? What are the type of detector array configuration ? Which type is commonly used in modern CT scanners? How is modern CT tube different from Conventional tube ? What is the difference between Straton ,maximus rotalix and liquid metal xray tube ?