CT Physics
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Computed tomography (CT) utilizes X-rays and computer processing to produce cross-sectional images of the body. In CT, X-rays pass through the body and are measured by a detector array, with the data used to reconstruct tomographic slices. The key components of a CT scanner include an X-ray tube, detector array, data acquisition system, computer system, and display system. CT has advantages over plain films by eliminating superimposition of structures and increasing contrast, allowing clinicians to better distinguish between tissues.
CT Physics
- 2. -Dr.Bhanupriya Singh. JR-1. PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY
- 3. History Internal structure of an object can be reconstructed from multiple projections to object. “Tomos:Greek: to cut section” & “Graphein:to write” J.Radon (1917) “Two dimension and three dimension object can be reconstructed from the infinite set of projection data”. 1972 G.N.Housefeild(EMI) announced ‘COMPUTERIZED AXIAL TRANSVERSE SCANNING’
- 4. CT MAIN SYSTEMS IMAGING SYSTEM COMPUTER SYSTEM DISPLAY, RECORDING, STORAGE SYSTEM DATA ACQUISITION SYSTEM
- 5. Gantry and Table Operation console
- 6. 70 CM
- 7. Y
- 8. Z
- 9. ISOCENTER
- 10. IMAGING SYSTEM PRODUCTION OF X-RAYS SHAPING OF X-RAY BEAM ENERGY FILTERING X-RAY BEAM
- 11. Gantry COMPONENTS circular device-that houses Data Acquisition system (DAS) X Ray Tube Detectors Filters Collimators Analog-to –Digital Converter (ADC) Voltage Generators
- 12. THERMIONIC EMISSION CATHODE HEATED UP TO AT LEAST 2,200 DEG. CELSIUS TO LIBERATE ELECTRONS FOR TRANSIT TO ANODE
- 13. FOCAL SPOT- CT UTILIZES DIFFERENT FOCAL SPOTS THE FILAMENT SIZE – LENGTH – FOCALFOCAL SPOTSPOT SMALLER FOCAL SPOT - Low mA SMALLER FOCAL SPOT – sharper image
- 14. mA – tube current The number of electrons flowing from cathode to anode
- 20. CT NUMBER:Hounsfield units A relative comparison of x-ray attenuation of a voxel of tissue to an equal volume of water.
- 21. ATTENUATION
- 25. CT # vs BRIGHTNESS LEVEL + 1000 -1000
- 27. PIXEL SIZE PIXEL SIZE= FOV (mm)/ MATRIX SIZE
- 28. EXAMPLE: FOV= 40 CM= 40 X 10 MM=400 mm MATRIX= 512 X 512 = 5122 400/512 = 0.78 mm 0.8 mm
- 30. VOXEL SIZE DEPENDS -FOV -MATRIX SIZE -SLICE THICKNESS Each VOXEL- a number. Extent of Xray attenuation passing.
- 31. CT ELIMINATES SUPERIMPOSITION INCREASES CONTRAST OF LOW SUBJECT CONTRAST TISSUES HELPS IN DISTINGUISHING BETWEEN HOMOGENOUS OBJECTS OF NON- UNIFORM THICKNESS. ABILITY TO ADJUST & MANIPULATE IMAGE AFTER SCANNING
- 32. Computed tomography X-rays irradiated on body, some rays are absorbed and some pass through the body to produce an image. In plain X-ray imaging, the film directly absorbs penetrated X- rays. In CAT scanning, an electronic device called a "detector array" absorbs the penetrated X-rays, measures the X-ray amount, and transmits the data to a computer system. A sophisticated computer system, in turn, calculates and analyzes data from each detector in each level, and finally reconstructs multiple, two- dimensional, cross-sectional images.
- 33. Gantry tilt: depend on examination. Lateral scout view. Scout view is used for planning
- 34. Plain film imaging reduces the 3D patient anatomy to a 2D projection image Density at a given point on an image represents the x-ray attenuation properties within the patient along a line between the x- ray focal spot and the point on the detector corresponding to the point on the image
- 35. Tomographic image-picture of a slab of patient’s anatomy 2D CT image corresponds to 3D section of pt CT slice thickness is very thin (1 to 10 mm) and is approximately uniform The 2D array of pixels in the CT image corresponds to an equal number of 3D voxels (volume elements) in the patient Each pixel on the CT image displays the average x-ray attenuation properties of the tissue in the corrsponding voxel
- 36. CT SYSTEM GANTRY DETECTORS S/H ADC ARRAY PROCESSOR HOST COMPUTER STORAGE CONSOLE SCAN CONTROLLER DAC GANTRY CONTROL HIGH VOLTAGE GENERATOR X-RAY TUBE
- 37. Ray, Ray sum, View & Attenuation ProfileRay, Ray sum, View & Attenuation Profile RayRay – Imaginary line– Imaginary line between Tube &between Tube & DetectorDetector Ray SumRay Sum – Attenuation– Attenuation along a Rayalong a Ray ViewView – The set of ray– The set of ray sums in one directionsums in one direction The attenuation for eachThe attenuation for each ray sum when plottedray sum when plotted as function of itsas function of its position is called anposition is called an attenuation profileattenuation profile View Attenuation profile Ray Ray sums
- 38. DATA ACQUISITIONDATA ACQUISITION X-ray tube Collimators Detector • SET OF ELECTRONICS BETWEEN DETECTORS AND HOST COMPUTER. • AMPLIFIER • ADC • DAC • GENERATOR • S/H. SET OF ELECTRONICS BETWEEN DETECTORS AND HOST COMPUTER
- 39. AMPLIFIER SIGNAL FROM DETECTORS GOES TO AMPLIFIERS FOR SIGNAL MAGNIFICATION AND THEN IS SENT TO SAMPLE/HOLD UNIT
- 40. ADC CONVERTS ANALOG SIGNAL OUTPUT FROM THE SCANNING EQUIPMENT TO A DIGITAL SIGNAL SO IT CAN BE PROCESSED BY A COMPUTER.
- 41. SAMPLE/HOLD UNIT (S/H) LOCATED BETWEEN AMPLIFIERAMPLIFIER AND ADCADC PERFORMS SAMPLING AND ASSIGNS SHADES OF GRAY TO THE PIXELS IN THE DIGITAL MATRIX CORRESPONDING TO THE STRUCTURES
- 42. DAS GANTRY DETECTORS S/H ADC ARRAY PROCESSOR HOST COMPUTER STORAGE CONSOLE SCAN CONTROLLER DAC GANTRY CONTROL HIGH VOLTAGE GENERATOR X-RAY TUBE
- 44. DATA FLOW IN CT REFERENCE DETECTOR REFERENCE DETECTOR ADC PREPROCESSOR COMPUTER RAW DATA CONVOLVED DATA BACK PROJECTORRECONSTRUCTED DATA PROCESSORS DISK TAPE DAC CRT DISPLAY
- 46. COLLIMATION IN CT ADC PRE-PATIENT COLLIMATION (Tube/Source) POST-PATIENT COLLIMATION (Detector) • Width of beam Leaving tube • Shapes beam • Defines Slice Thickness • Width of ATTENUATED beam- after passing pt- before reaching detector. • Fine tuning of beam profile • Removes scatter Radiation.
- 47. BASIC DATA AQUSITION SCHEME IN CT ADC FILTRATION
- 49. FILTRATION MATERIAL ALUMINIUM ( SPECIAL FILTER IN CT) BOWTIE TO MAKE THE BEAM HARDER AND MORE MONOENERGETIC
- 50. Filter Patient DEFINES SLICE THICKNESS REDUCES SCATTER RECHING THE Detector Detector
- 51. DETECTOR TYPES: SCINTILLATION Photomultiplier TUBE S. CRYSTAL S. CRYSTAL Silicon PHOTODIODE X ray energy-> Light-> Electrical energy.
- 52. SCINTILLATION CRYSTALS USED WITH PM TUBES: SODIUM IODIDE –AFTERGLOWAFTERGLOW +Hygroscopic+ LOW DYNAMIC RANGE+Hygroscopic+ LOW DYNAMIC RANGE ( USED IN THE PAST)( USED IN THE PAST) CALCIUM FLUORIDE Cesium Iodide(CsI) BISMUTH GERMANATE Short decay time-Afterglow not a problem.
- 53. S. CRYSTAL USED WITH PHOTODIODE CALCIUM TUNGSTATE RARE EARTH OXIDES – CERAMIC (requires less dose)-faster
- 54. DETECTOR TYPE: GAS IONIZATION XENON GAS 30 ATM Xray energy -> Electrical Energy
- 55. COMPUTER SYSTEM RECONSTRUCTION AND POSTPROCESSING CONTROL OF ALL SCANNER COMPONENTS CONTROL OF DATA ACQUSITION, PROCESSING, DISPLAY. DATA FLOW DIRECTION
- 56. Tomographic reconstruction t)I/Iln( II t0 t 0t µ µ = = − e • Each ray acquired in CT is a transmission measurement through the patient along a line • The unattenuated intensity of the x-ray beam is also measured during the scan by a reference detector • Before the acquisition of the next slice, the table that the patient lies on is moved slightly in the cranial-caudal direction (the “z-axis” of the scanner) •
- 57. Reconstruction There are numerous reconstruction algorithms Filtered backprojection reconstruction is most widely used in clinical CT scanners Builds up the CT image by essentially reversing the acquistion steps The µ value for each ray is smeared along this same path in the image of the patient As data from a large number of rays are backprojected onto the image matrix, areas of high attenutation tend to reinforce one another, as do areas of low attenuation, building up the image
- 59. Views & attenuation profiles for a slice 1 2 3 4
- 60. Image reconstructionImage reconstruction The image is createdThe image is created by reflecting theby reflecting the attenuation profilesattenuation profiles back in the sameback in the same direction they weredirection they were obtainedobtained This process is calledThis process is called BACK PROJECTIONBACK PROJECTION BACK PROJECTION
- 61. IN CT DIGITAL RECONSTRUCTED IMAGE IS CONVERTED INTO A GRAY SCALE IMAGE.
- 62. Window width: is the range of CT numbers that are used to map signals into shades of gray Window level: determines the midpoint of the range of gray levels to be displayed Darker or Lighter
- 63. 1st generation: rotate/translate, pencil beam Only 2 x-ray detectors used (two different slices) Parallel ray geometry Translated linearly to acquire 160 rays across a 24 cm FOV Rotated slightly between translations to acquire 180 projections at 1-degree intervals About 4.5 minutes/scan with 1.5 minutes to reconstruct slice
- 64. • LONG HIGH VOLTAGE CABELS WERE USED- • 1 Clockwise rotation-STOP-Anticlockwise- Unwind High Tension cabels • Tube-detector array moves continousely around pt-covers Volume
- 65. Slip Ring: : connects generator with tube Circular Electrical Conductive Ring:Overcame Voltage Cable Problem
- 66. 1st generation (cont.) Large change in signal due to increased x- ray flux outside of head – Solved by pressing patient’s head into a flexible membrane surrounded by a water bath NaI detector signal decayed slowly, affecting measurements made temporally too close together Pencil beam geometry allowed very efficient scatter reduction, best of all scanner generations
- 67. 2nd generation: rotate/translate, narrow fan beam Incorporated linear array of 30 detectors Rotated in an arc upto 300 More data acquired to improve image quality Shortest scan time was 18 seconds/slice Practicle to scan large distances Narrow fan beam allows more scattered radiation to be detected
- 69. 3rd generation: rotate/rotate, wide fan beam Detector & X-ray source rotate in unison Number of detectors increased substantially (more than 800 detectors-incr 30-40degree) Angle of fan beam increased to cover entire patient – Eliminated need for translational motion Mechanically joined x-ray tube and detector array rotate together Newer systems have scan times of ½ second
- 70. 4th generation: rotate/stationary Designed to overcome the problem of ring artifacts Stationary ring of about 4,800 detectors around pt. Wide FAN BEAM Geometry X-ray tube moves in circular motion within detector array.
- 72. 3rd vs. 4th generation 3rd generation fan beam geometry has the x- ray tube as the apex of the fan; 4th generation has the individual detector as the apex t)I/Iln(:gen4 t)I/Iln(:gen3 t0 th t201 rd µ µ = = gg gg
- 74. 5th generation: stationary/stationary Developed specifically for cardiac tomographic imaging No conventional x-ray tube; large arc of tungsten encircles patient and lies directly opposite to the detector ring Electron beam steered around the patient to strike the annular tungsten target Capable of 50-msec scan times; can produce fast-frame-rate CT movies of the beating heart
- 75. Electron beam CT scanners:uses e-gun,linear accelerators Electron beam is focused & deflected along Tungsten Target in FAN BEAM GEOMETORY
- 76. • Electron beam targeted on TUNGSTEN Target ring • Opposite to stationary Semi-circular detector array.
- 78. 6th generation: helical Helical CT scanners acquire data while the table is moving By avoiding the time required to translate the patient table, the total scan time required to image the patient can be much shorter Allows the use of less contrast agent In some instances the entire scan be done within a single breath-hold of the patient
- 81. Breathing Artifact. Motion Artifact
- 82. Streak Artifact
- 83. Motion Artifact Metal Implant
- 84. Ring artifacts The rotate/rotate geometry of 3rd generation scanners leads to a situation in which each detector is responsible for the data corresponding to a ring in the image Drift in the signal levels of the detectors over time affects the µt values that are backprojected to produce the CT image, causing ring artifacts
- 86. Ring Artifact
- 87. Ring Artifact Severe Hardening of Xray by Bone
- 88. Drawbacks in Back projectionDrawbacks in Back projection (Artifacts - Star shape & streaks)(Artifacts - Star shape & streaks) The resultant imageThe resultant image closely resembles theclosely resembles the original objectoriginal object But it shows starBut it shows star shaped patternsshaped patterns around objects andaround objects and streaksstreaks These are calledThese are called ‘Star’ and ‘streak’‘Star’ and ‘streak’ artifactsartifacts
- 89. Formation of Star artifact andFormation of Star artifact and streaksstreaks Consider aConsider a scan of ascan of a single highsingle high densitydensity objectobject suspended insuspended in airair
- 90. The back projections take the form of a stripeThe back projections take the form of a stripe through the center of the objectthrough the center of the object
- 91. Back projections are crated for each profileBack projections are crated for each profile
- 92. Addition of the attenuation profiles create anAddition of the attenuation profiles create an image with star and streak artifactsimage with star and streak artifacts Final back projection