Computed tomography

Principle and generation of Computed Tomography

1. Computed Tomography :Basic Principle and its generations Presented by : Rituraj Mishra . B.Sc.MIT , 2013 Moderator: Mr. R anjit Kumar Jha

2. INTRODUCTION Computed Tomography is a well accepted imaging modality for evaluation of the entire body. Computed Tomography(CT) Scan Machines Uses X- rays, a powerful form of Electromagnetic Radiation. The images are obtained directly in the axial plane of varying tissue thickness with the help of a computer. Some pathology can be seen in saggital or coronal plane by reconstruction of the images by computer. CT has undergone several evolutions and nowadays multi- detectors CT scanners have been evolved which have better application in clinical field.

3. COMPARISION OF CT WITH CONVIENTIONAL RADIOGRAPHY Conventional radiography suffers from the collapsing of 3D structures onto a 2D image. CT gives accurate diagnostic information about the distribution of structures inside the body.

4. COMPARISION OF CT WITH CONVIENTIONAL RADIOGRAPHY. A conventional X-ray image is basically a shadow. Shadows give you an incomplete picture of an object's shape. This is the basic idea of computer aided tomography. In a CT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles

5. Comparison of CT with Conventional Radiography Radiographic procedure is qualitative and not quantitative

6. ADVANTAGE OF COMPUTED TOMOGRAPHY OVER CONVIENTIONAL RADIOGRAPHY. To overcome superimposition of structures. To improve contrast of the image. To measure small differences in tissue contrast.

7. TOMOGRAPHY Imaging of Layer/Slice. Principle Images of structures lying above and below the plane are blurred out due to motion unsharpness while the structures lying in plane of interest appear sharp in in the image.

8. Tomography

9. PRINCIPLE OF COMPUTED TOMOGRAPHY The internal structure of the object can be reconstructed from multiple projections of the object. Mathematically principle of CT was first developed in 1917 by Radon. Proved that image of unknown object could be produced if one had several number of projections throughout the object.

10. VARIOUS PARAMETERS OF CT SLICE MATRIX PIXEL VOXEL CT NUMBER WINDOWING WINDOW WIDTH WINDOW LEVEL PITCH

11. SLICE/CUT The cross section portion of body which is scanned for production of CT image is called Slice. The slice has width and therefore volume. The width is determined by width of the x rays beam.

12. Cross Sectional Slices Think like looking into the loaf of bread by cutting into the thin slices and then viewing the slice individually.

13. MATRIX The CT image is represented as the Matrix of the number. A two dimensional array of numbers arranged in rows and columns is called Matrix. Each number represent the value of the image at that location.

14. PIXEL Each square in a matrix is called a pixel. Also known as picture element.

15. VOXEL Each individual element or number in the image matrix represents a three dimensional volume element in object called VOXEL.

16. CT NUMBER The numbers in the image matrix is called CT NUMBER. Each pixel has a number which represents the x-ray attenuation in the corresponding voxel of the object.

17. HOUNSFIELD UNITS(HU) Related to different composition and nature of Tissue. The CT NUMBER is also known as Hounsfield units(HU). Represent the density of tissue. Different Tissue have different CT number Range in HU.

18. Air - 1000 Fat -100 Pure water CSF 15 White matter 45 Gray matter 40 Blood 20 Bone/calcification +1000 TISSUE AND CT NUMBER APPROXIMATE

19. WINDOWING is a system where the CT no. range of interest is spread cover the full grey scale available on the display system WINDOW WIDTH –Means total range of CT no. values selected for gray scale interpretation. It corresponds to contrast of the image. WINDOW LEVEL – represents the CT no. selected for the centre of the range of the no. displayed on the image. It corresponds to brightness of image .

20. Pitch The relationship between patient and tube motion is called Pitch. It is defined as table movement during each revolution of x-ray tube divided by collimation width. For example: For a 5mm section, if patient moves 10mm during the time it takes for the x-ray tube to rotate through 360˚, the pitch is 2. Increasing pitch reduces the scan time and patient dose.

21. STEPS OF CT IMAGE FORMATION

22. Phase of CT scanning 1.Scanning the patient or data Acquisition a)X-ray Generator b)X-ray Tube c)X-ray Filtration System d)Detector System 2.Reconstruction a)Simple back projection b)Iterative method c)Analytical method 3.Display

23. DATA ACQUISTION The scanning process begins with data acquisition Data Acquisition refers to a method by which the patient is systematically scanned by the X ray tube and detectors to collect enough information for image reconstruction

24. Major components of Data Acquisition System(DAS) a )X-ray Generators Generators are located on rotating scan frames within the CT gantry to accommodate slip Ring. Power: 50 to 80kw Frequency: 5 to 50kHz KVp : 80-120 mA:80-500

25. b) X-ray Tube Rotating anode x-ray tube with unique cooling. Small focal spot size (0.6mm) to improve spatial resolution. Anode heating capacity:1MHU to 7MHU Cooling rate:1MHU per minute. c)X-ray Beam Filtration System CT employs monochromatic beam but radiation from CT X-ray tube is polychromatic . so, X-ray beam is shaped by compensation filter. a)Pre patient Collimators : Reduces the patient dose. b)Post patient Collimators : Reduces the scattered radiation detectors .

26. Overall Functions of Collimators. To decrease scatter radiation To reduce patient dose To improve image quality Collimator width determines the slice thickness

27. d)Detectors The detectors gather information by measuring the x-ray transmission through the patient. Two types: Scintillation crystal detector (Cadmium tungstate + Si Photodiode) Can be used in third and fourth generation scanners Xenon gas ionisation chamber Can be used in third generation scanners only

28. 2)Reconstruction Reproduction of an image from raw data is called Reconstruction. A)Simple back projection The image is created by reflecting the attenuation profiles back in same direction they were obtained.

29. B)Iterative method It start with assumption that all point in matrix have same value and it was compared with measured value and make correction until Values come with in acceptable range. It contain three correction factor 1. SIMULTANEOUS RECONSTRUCTION 2. RAY BY RAY CORRECTION 3. POINT BY POINT CORRECTION

30. C)Analytical Method Today commonly used . Two popular method used in that method are:- 1. 2-D FOURIER ANALYSIS 2.FILTERED BACK PROJECTION

31. 2-D FOURIER ANALYSIS In it any function of time or space can be represented by the sum of various frequencies and amplitude of sine and cosine waves. For example the actual projected image of original object is more rounded than those shown which would be slowly simplify and corrected by Fourier transformation .

32. 2-D FOURIER ANALYSIS

33. FILTERED BACK PROJECTION Same as back projection except that the image is filtered, or Modified to exactly counterbalance the effect of sudden density Changes , which cause blurring(star like pattern) in simple back projection.

34. 3)Display The reconstructed image is displayed on the monitor. It is a digital image. It consists of 2D representation of 3D object in the form of pixels. CT pixel size is determined by dividing the FOV by matrix Size which is generally 512*512. PIXEL SIZE= FOV (mm)/ MATRIX SIZE

35. Generations of CT Scan First Generation Narrow pencil beam Single detector Detector used is made up of NaI . Translate –Rotate movements of Tube- detector combination Scan time-5mins. Designed only for evaluation of brain.

36. First generation CT Scanner Head kept enclosed in a water bath Paired detectors A reference detector

37. Second Generation CT Scanner

38. Second Generation Narrow fan beam Linear detector array(5 to30) Translate-Rotate movements of Tube-Detector combination Fewer linear movements are needed as there are more detectors to gather the data. Between linear movements, the gantry rotated 30 o Scan time~30secs(advantage over first generation)

39. Third Generation Rotate(tube)Rotate(detectors) Motion. Pulsed wide fan beam. Arc of detectors(600-900) Detectors are perfectly aligned with the X-Ray tube Both Xenon and scintillation crystal detectors can be used Scan time< 5secs Disadvantage: Ring Artifacts due to electronic drift between many detectors.

40. Fourth Generation

41. Fourth Generation Complete circular array of about 1200 to 4800 stationary detectors Single x-ray tube rotates with in the circular array of detectors Wide fan beam to cover the entire patient Scan time of newer scanners is about ½ s or, <2s. Designed to address ring artifacts by keeping detector assembly stationary. Disadvantage: High cost.

42. Fifth 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

43. Electron gun Large Arcs of tungsten targets Detector ring 17 slices per second

44. CT SCAN IN OUR RADIOLOGY DEPARTMENT.(16 SLICES)

45. Patient Positioning and Data Acquisition

46. TECHNICAL SPECIFICATION (Hitachi ECLOS 16) CT scanner mode: Multislice Slices per rotation: 16 Other rotation speed options: 1.0, 1.5, 2.0, 3.0 seconds Minimum rotation speed: 0.8 seconds Gantry diameter: 70 cm Maximum beam width (cm): 2 cm Table weight limit: 495 lbs Table movement range vertically/longitudinally: 42 to 100 cm vertical, 186 cm horizontal X-ray generator kV range: 100, 120, 130 kVp Maximum scan range: 175 cm X-ray tube heat capacity: 5.0 MHU Power requirements: 3-phase, 200 Amp max.

47. SPIRAL/HELICAL CT Spiral/Helical scanning uses third generation or fourth generation slip ring design. Spiral computed tomography  (or helical computed tomography) is a computed tomography(CT) technology in which the source and detector travel along a helical path relative to the object. Typical implementations involve moving the patient couch through the bore of the scanner whilst the gantry rotates. Spiral CT can achieve improved image resolution for a given radiation dose, compared to individual slice acquisition. Most modern hospitals currently use spiral CT scanners.

48. SLIP RING TECHNOLOGY In conventional CT scanning there was a paused between each gantry rotation. But in Helical CT, Slip Ring technology is used which allows continuous rotation of gantry without interruption. Slip Rings are electrical conducting brushes and component of gantry transferring the data or, electrical energy to and from the stationary part of gantry to rotating part of gantry for continuous rotation of gantry.

49. Contd …. There are usually three slip rings made up of conduction materials(i.e. Sliver and Graphite.) First Slip Ring provides high voltage power to X-ray tube. Second provide low voltage to control system on rotating gantry and Third Slip Ring transfers digital data from rotating detectors arrays.

50. MULTISLICE/MULTIDETECTOR CT Multidetector computed tomography(MDCT) is a form of computed tomography (CT) technology for diagnostic imaging. In MDCT, a two-dimensional array of detector elements replaces the linear array of detector elements used in typical conventional and helical CT scanners. The two-dimensional detector array permits CT scanners to acquire multiple slices or sections simultaneously and greatly increase the speed of CT image acquisition. Image reconstruction in MDCT is more complicated than that in single section CT.

51. DUAL SOURCE CT Dual Source CT  (DSCT) is equipped with two X-ray tubes. Two corresponding detectors are oriented in the gantry with an angular offset of 90 degrees. ADVANTAGES High temporal resolution(in response to time domain) for cardiac imaging without β blockers which means heart rate is independent upon temporal resolution. Less radiation dose even for obese patient. Faster acquisition time with shortest breathe hold.

52. DUAL ENERGY CT Standard computed tomography (CT) scanners use normal X-rays to make cross-sectional ‘slice-like’ pictures or images of the body. A dual energy CT scanner is fairly new technology that uses both the normal X-ray and also a second less powerful X-ray to make the images. Two pictures are taken of the same slice at different energies.(i.e. 80/140KV ,100/140KV Or,70/150KV) This gives dual energy CT additional advantages over standard CT for a wide range of tests and procedures. Most commonly used for CT Angiography.

53. PORTABLE CT SCAN

54. PORTABLE CT SCAN As the world’s first portable, full-body, 32-slice CT (computed tomography) scanner, BodyTom is a multi-departmental imaging solution capable of transforming any room in the hospital into an advanced imaging suite. The system boasts an impressive 85cm gantry and 60cm field of view, the largest field of view available in a portable CT scanner. The battery-powered BodyTom with an innovative internal drive system can easily be transported from room to room and is compatible with PACS, planning systems, surgical and robotic navigation systems. Uniquely designed to accommodate patients of all sizes, BodyTom provides point-of-care CT imaging wherever high-quality CT images are needed, including the operating room, intensive care unit, radiation oncology suites, and the emergency department. 

55. RADIATION DOSE IN CT Volume Computed Tomography Dose Index ( CTDI vol ) is a standardized parameter to measure Scanner Radiation Output CTDIvol provides information about the amount of radiation used to perform the study. CTDI vol is NOT patient dose CTDI vol is reported in units of mGy for either a 16-cm (for head exams) or 32-cm (for body exams) diameter. AAPM (American Association of Physicts in Medicine) introduces a parameter known as the Size Specific Dose Estimate (SSDE) to allow estimation of patient dose based on CTDI vol and patient size. For the same CTDI vol , a smaller patient will tend to have a higher patient dose than a larger patient.

56. EFFECTIVE DOSE ( mSv ) CT HEAD------------ 2 CHEST ---------- 8 ABDOMEN -- 10 PELVIS ------- 10 GENERAL RADIOGRAPHY SKULL -------- 0.07 CHEST PA ---- 0.02 ABDOMEN --- 1 PELVIS -------- 0.7

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