4 CT instrumentation Radiation CT scan Lecture
- 1. COMPUTED TOMOGRAPHY COMPUTED TOMOGRAPHY INSTRUMENTATION 1 INSTRUMENTATION 1
- 2. Objectives : By the end of this lecture the students will be able to: To Describe and identify all CT system operations and components. including imaging system which contain: Gantry -Xray tube -Generator -Filter -Collimation
- 4. CT scanner instrumentation: CT scanner instrumentation: The The basic components of CT equipment are the:- basic components of CT equipment are the:- 1. 1.Imaging system is located in the scanner room. Imaging system is located in the scanner room. 2. 2.Computer system is located in computer room. Computer system is located in computer room. 3. 3.Image display, recording storage and Image display, recording storage and communication system is located in the communication system is located in the operator's room. operator's room.
- 5. Components of a CT Imaging system. I, Gantry; 2, patient couch; 3, integrated console; 4, optical disk system including cassette storage; 5, high-speed processor system; 6, x-ray high-voltage generator; 7, couch control unit; 8, system transformer I; 9, system transformer II; 10, patient observation system
- 6. SCANNER Gantry Patient Couch The Computer
- 7. 1 1 . . Imaging system Imaging system : : The purpose of imaging system is to produce x-rays, The purpose of imaging system is to produce x-rays, shape and filter x-ray beam to pass through only shape and filter x-ray beam to pass through only defined cross-section of the patient and convert the defined cross-section of the patient and convert the transmitted photons into digital information. transmitted photons into digital information. The major components of the imaging system is the The major components of the imaging system is the gantry which housed imaging component such as x-ray gantry which housed imaging component such as x-ray tube and generator, collimators, filter, detectors and tube and generator, collimators, filter, detectors and detector electronics. detector electronics.
- 8. ( 1 ) Gantry Gantry is a circular device that houses the Data Acquisition system (DAS) = Tube, Detectors, Filters, Collimators & Analog-to – Digital Converter (ADC)
- 9. ( ( 1 1 ) ) Gantry Gantry The gantry includes : The gantry includes : (a) The x-ray tube (a) The x-ray tube (b) Slips ring (b) Slips ring (c) The high-tension generator (c) The high-tension generator (d) Data acquisition system (d) Data acquisition system (e) Collimator , filter and Detectors (e) Collimator , filter and Detectors
- 12. . . There are Two important features of the gantry: There are Two important features of the gantry: 1- gantry aperture: gantry aperture is the opening in 1- gantry aperture: gantry aperture is the opening in which patient is positioned during scanning procedure. which patient is positioned during scanning procedure. Most scanners have a 70 to 90 cm aperture. Most scanners have a 70 to 90 cm aperture. 2- gantry tilting: CT gantry must be capable of tilting 2- gantry tilting: CT gantry must be capable of tilting to accommodate all patients and clinical examinations. to accommodate all patients and clinical examinations. the degree of tilt varies between ± 15 to ± 30 the degree of tilt varies between ± 15 to ± 30 degrees. the gantry also includes a set of laser beams degrees. the gantry also includes a set of laser beams to aid patient positioning to aid patient positioning
- 13. 70 CM
- 15. Gantry Can be tilted forward or backward up to 30 degrees to compensate for body part angulations.
- 16. The gantry Control panels located on either side of the gantry Control panels located on either side of the gantry opening allow the technologist to control the opening allow the technologist to control the alignment lights, gantry tilt, and table movement. alignment lights, gantry tilt, and table movement. In most scanners, these functions may also be In most scanners, these functions may also be controlled via the operator’s console. controlled via the operator’s console. A microphone is embedded in the gantry to allow A microphone is embedded in the gantry to allow communication between the patient and the communication between the patient and the technologist throughout the scan procedure. technologist throughout the scan procedure.
- 18. X-ray generator : CT scanners use three-phase power for the CT scanners use three-phase power for the efficient production of x-rays. efficient production of x-rays. In the past, the high-voltage generator was In the past, the high-voltage generator was bulky piece of equipment located in a corner bulky piece of equipment located in a corner of the x-ray room. of the x-ray room.
- 19. X-ray generator : CT scanners now use high frequency CT scanners now use high frequency generators, which are small, compact, and generators, which are small, compact, and more efficient than conventional generators. more efficient than conventional generators. These generators are located inside the These generators are located inside the gantry. gantry.
- 20. X-ray generator : Current CT generators have maximum power ratings Current CT generators have maximum power ratings of about 30-60 kilowatts (kW) that allow kVp of about 30-60 kilowatts (kW) that allow kVp settings in the range of 80 to 140 kVp and tube settings in the range of 80 to 140 kVp and tube currents in the range of 100 to 500 mA. currents in the range of 100 to 500 mA. a higher kV setting will help to reduce the heat a higher kV setting will help to reduce the heat load on the x-ray tube by allowing a lower mA load on the x-ray tube by allowing a lower mA setting. setting. Reducing the heat load on the x-ray tube will Reducing the heat load on the x-ray tube will extend the life of the tube. extend the life of the tube.
- 21. CT Tubes X-ray tube for CT is similar in design to the conventional radiography tube, but is specially designed to handle and dissipate excessive heat units – much higher heat loading Ceramic target backing Decreases tube weight weight
- 22. X-Ray Tube Conventional Radiography Tube Siemens Straton Tube (Grounded Anode, Balanced Shaft, Radiant Cooling and Active Cooling)
- 23. X-ray Tube X-ray Tube The x-ray tube is responsible of x-ray The x-ray tube is responsible of x-ray production. First and second-generation production. First and second-generation scanners used fixed anode, oil-cooled x-ray scanners used fixed anode, oil-cooled x-ray tubes, but rotating anode x-ray tubes have tubes, but rotating anode x-ray tubes have become common in CT because of the demand become common in CT because of the demand for increase output. for increase output.
- 24. Contd … The anode disk is usually made of a rhenium, The anode disk is usually made of a rhenium, tungsten, and molybdenum (RTM) alloy and tungsten, and molybdenum (RTM) alloy and other materials with a small target angle other materials with a small target angle (usually 12 degrees). (usually 12 degrees).
- 25. TUNGSTEN TARGET TARGET MADE OF TUNGSTEN AND RHENIUM
- 26. Contd … The introduction of spiral CT with continuous rotation The introduction of spiral CT with continuous rotation scanners has placed new demands on x-ray tubes. Because scanners has placed new demands on x-ray tubes. Because the tube rotates continually for a longer period compared with the tube rotates continually for a longer period compared with conventional scanners, the tube must be able to sustain higher conventional scanners, the tube must be able to sustain higher power levels. power levels. Several technical advances in component design have been Several technical advances in component design have been made to achieve these power levels and deal with the made to achieve these power levels and deal with the problems of heat generation, heat storage, and heat problems of heat generation, heat storage, and heat dissipation. dissipation.
- 27. Special features of x-ray tube Special features of x-ray tube 1. 1. Tubes with metal envelopes, which are now common Tubes with metal envelopes, which are now common 2. The anode heating capacity must be high 2. The anode heating capacity must be high 3. High speed rotators are used for best heat 3. High speed rotators are used for best heat dissipation. dissipation. 4. Small focal spot size. 4. Small focal spot size.
- 28. Contd …
- 29. Contd … 5. 5. The cathode assembly consists of one or The cathode assembly consists of one or more tungsten filaments positioned in a more tungsten filaments positioned in a focussing cup. focussing cup. 6. The getter is usually made of barium to 6. The getter is usually made of barium to ensure a vacuum by the absorption of air ensure a vacuum by the absorption of air molecules released from the target during molecules released from the target during operation. operation.
- 30. Contd … 7. 7. The anode assembly consists of the disk, The anode assembly consists of the disk, rotor stud. The rotation speed of 3600 rpm to rotor stud. The rotation speed of 3600 rpm to 10,000 rpm (high-speed rotation). 10,000 rpm (high-speed rotation). 8. Large anode disks e.g. 2oo mm, (conventional 8. Large anode disks e.g. 2oo mm, (conventional tube disk 120 – 160 mm). tube disk 120 – 160 mm). 9. The anode disk is thicker than conventional all 9. The anode disk is thicker than conventional all metal disk. metal disk.
- 31. Contd … 10. In spiral CT, 10. In spiral CT, The anode disk is brazed The anode disk is brazed graphite anode disk consists of a tungsten- graphite anode disk consists of a tungsten- rhenium. rhenium. Graphite increases the heat Graphite increases the heat storage capacity storage capacity
- 32. Contd … 11- 11- The working life of the tubes can range The working life of the tubes can range from about 10,000 to 40,000 hours, from about 10,000 to 40,000 hours, compared with 1000 hours, which is typical compared with 1000 hours, which is typical of conventional tubes .. of conventional tubes ..
- 33. Contd … Despite all these efforts, the problem of heat Despite all these efforts, the problem of heat transfer and slow cooling rates still persist with MSCT transfer and slow cooling rates still persist with MSCT scanners, especially for multiple longer scan times and scanners, especially for multiple longer scan times and cardiac CT imaging. cardiac CT imaging. The Straton x-ray tube The Straton x-ray tube has been introduced for use has been introduced for use with MSCT scanners to overcome these problems. with MSCT scanners to overcome these problems.
- 34. Contd …
- 35. kVp INTENSITY - ENERGY – KVP : Potential difference between cathode and anode (Volts) KVp in CT 80-140 TOO LOW – NOISE (NOT ENOUGH PENETRATION OF THE PATIENT )
- 36. TOO LOW OF kVp : NOISE !!!! NOISE !!!!
- 37. mA – tube current The number of electrons flowing from cathode to anode S –time of exposure mAs tube current for certain length of time CURRENT INTENSITY ENERGY – NO CHANGE
- 38. Why changing mA or time Avoiding motion – mA time Pediatric technique modification Reducing noise - mAs NOISE MOTION
- 39. An X-ray tubes heat capacity is expressed in heat units Modern CT systems utilize X-ray tubes that have a heat capacity of approximately 3.5 to 5 million heat unit (MHU). A CT X-ray tube must possess a high heat dissipation rate. Many CT X-ray tubes utilize a combination of oil and air-cooling systems to eliminate heat and maintain continuous operational capabilities
- 40. HEAT UNITS CALCULATION HU= kVp X mA x time MOST CT TUBES HEAT CAPACITY 3-5 MILLION HU
- 41. Filtration Filtration : : Radiation from x-ray tubes consists of long and short wave-lengths. In CT, FILTRATION SERVES A DUAL PURPOSE: 1- remove long wavelength x-rays because they do not play a role in CT image formation, but instead contribute to patient dose.
- 42. Contd Contd … … 2- also filtration shapes the energy distribution across the radiation beam to produce uniform beam hardening when the x-rays pass through the filter and the object. Filtration make the beam harder and more monoenergetic
- 43. Contd Contd … … Recall that the total filtration is equal to the sum of the inherent filtration and the added filtration. In CT the inherent filtration has a thickness of about 3 mm Al-equivalent.
- 44. Contd Contd … … The added filtration, consists of filters that are flat or shaped filters: ‘‘bowtie’’ filter, made of “Teflon”, that has low atomic number and high density , so as not to have a significant impact on beam hardening. The thickness of which can range from 0.1 to 0.4 mm
- 45. Contd Contd … … The filter is positioned between the x-ray tube and patient, and they shape the beam to produce more uniformity at the detectors.
- 47. Detectors X-Ray Tube X-Ray Beam Patient Cross-Section Bow-Tie or Equalizing Filter
- 48. Collimation Pre-patient collimator Post patient or pre-detector collimator . Require for the same reasons as that is required in conventional radiography : 1 - Reduces patient dose . 2 - Enhance the image contrast . 3- Reducing scatter radiation.
- 49. Set of collimators The tube or source collimators are located in the X-ray tube and determine the section thickness that will be utilized for a particular CT scanning procedure . A first set of collimators located directly below the tube collimators maintain the width of the beam as it travels toward the patient
- 50. . A final set of collimators called post-patient or pre-detector collimators are located below the patient and above the detector. the primary responsibilities of this set of collimators are to insure proper beam width at the detector and reduce the number of scattered photons that may enter a detector.
- 52. Pre-patient collimation • Depends on the focal spot size • Mounted on the tube housing • Creates more parallel beam • Reduces patient dose Pre-detector collimation • helps define slice (beam) thickness • Reduces the scatter radiation reaching detector
- 53. Summary: Collimation In CT : In CT, collimation is equally important because it affect patient dose and image quality. Collimation is pre-patient collimators and post patient or pre- detector collimators. The collimator section at the distal end of the collimator assembly also helps in defining the thickness of the slice to be imaged.