CT Generation (Generation of CT)

Different Generation of CT By Upakar Paudel

1. GENERATIONS OF CT UPAKAR PAUDEL B.SC.MIT ROLL NO:6 UCMS BHAIRAHAWA

2. CONVENTIONAL RADIOGRAPHY Limitation of conventional radiography Inefficient x-ray absorption High scatter to primary x-ray ratios Superimposition Inconspicuity

3. CONVENTIONAL TOMOGRAPHY These issues were recognized and led investigators to consider improvements . One such innovation was conventional tomography .

4. COMPUTED TOMOGRPHY In the last 30 years X-ray Computed Tomography development produced a great change in the role of diagnostic imaging in medicine. The basics of CT is to take a series of conventional cross-sectional x-rays while the patient “is rotated” slightly around an axis between each exposure. A series of projection data is obtained, these data are used to reconstruct cross-sectional images

5. 1917: The austrian mathematician Radon develop a way to reconstruct the density distribution of an object if the line integrals (for every direction)are available. 1971: The first CT system has been created by Godfrey N. Hounsfield, he received the Nobel prize in 1979 (together with Cromack) 1957–63: The physician Cormack develops a lot of theoretical work on x-rays creating the basis of CT scanning (without knowing Radon work) 1972: The first clinical use of a CT system in London, the acquisition time of a couple of tomogram was 5 mins. COMPUTED TOMOGRPHY

6. 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 (scarce diffusion) 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. COMPUTED TOMOGRPHY

7. Computed tomography

8. INTRODUCTION Generation is the 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.

9. FIRST GENERATION CT It is functional marriage of diverse technology including computer hardware , control systems , x-ray detectors , sophisticated reconstruction algorithms , x-ray tube , generator system etc . X-ray beam was collimated down to a narrow (pencil width) beam . X-ray tube and scintillation detectors are rigidly linked and scan across subject sweeping a narrow x-ray beam through the slice.

10. Starting at a particular angle , the x-ray tube and detector system translated linearly across the FOV acquiring 160 parallel rays per view . After end of translation , tube and detector assembly rotated around the subject by 1 degree. This procedure was repeated until 180 projection. A total of 180 x 160 = 28,800 rays were measured. This combination of linear translation followed by incremental rotation is called translate – rotate motion . FIRST GENERATION CT

11. FIRST GENERATION CT

12. 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 . It acted to bolus the x-rays so that the intensity outside the head is similar to the intensity inside head . Though water bath cannot be used for body scanning , it was used because it allowed Hounsfield to maximize accuracy of attenuation coefficient measurement (limitation of dynamic range , beam hardening correction) FIRST GENERATION CT

13. ADVANTAGE - With regard to scatter rejection , pencil beam geometry used in 1 st generation scanners were best . DISADVANTAGE -The major drawback was nearly 5 minutes was required to complete a single image. -Contrast resolution of internal structures was unprecedented , images had poor spatial resolution .

14. SECOND GENERATION CT 1 ST waterless full body CT scanner was developed and installed by Ledlay et al at Gorgetown University in February 1974 . It consist of narrow fan beam (3-10 degrees) and multiple detectors(linear array of 30 detectors) With 10 degree rotation increment , only 18 translation would be required for 180 degrees image acquisition . Shortest scan time with 2 nd generation CT was 18 s per slice , 15 times faster than with 1 st generation CT.

15. SECOND GENERATION CT

16. DISADVANTAGE However further speed improvements were 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. One disadvantage of fan beam is the increased radiation intensity towards the edge . But it is compensated with the use of bow-tie filter (limits the range of intensity reaching detector and hardens beam) SECOND GENERATION CT

17. THIRD GENERATION CT The translation motion of 1 st and 2 nd generation CT scanner was fundamental impediment to fast scanning . 1 st and 2 nd generation had to be dynamically recalibrated at the end of each translation . Faster scans required the elimination of translational motion and the use of simple and pure rotational motion. This is accomplished by widening the x-ray beam encompassing the entire patient width and using and array of detectors to intercept the beam.

18. THIRD GENERATION CT The design , characterized by linked tube – detector system undergoing only rotational motion is called third generation of CT. Wide angle fan beam of 50-55 degrees was used and no. of detectors was increased to more than 800 detectors - limits spatial resolution – 5 to 10 lp/cm . The early third generation CT scanners installed on late 1975 could scan less than 5 sec , current designs can scan as quickly as one third of a sec for cardiac application .

19. THIRD GENERATION CT

20. DISADVANTAGE It requires extremely high detector stability and matching of the detector response. Any error or drift in the calibration of detectors relative to other detectors is back projected along these ray path and reinforced along a ring where they cross . The result is the ring artifact. Another disadvantage is sampling – sample size and spacing are fixed by detector design . Samples cannot be closure together than distance between rays associated with detector at the level of center of rotation. THIRD GENERATION CT

21. A solution was provided by a Xenon detector arrange. Xenon arrays were inherently stable and well matched because factor affecting detector response were either uniform for the entire array or constant over chamber. Xenon were eventually replaced by solid state detectors. THIRD GENERATION CT

22. Ring artifacts are never completely eliminated , rather they are minimized by high quality detectors design and frequent calibration . Residual ring artifacts are then removed by image processing algorithms. Despite these limitations , 3 rd generation CT was highly successful and remains the basic geometry of must CT scanners manufactured today . THIRD GENERATION CT

23. FOURTH GENERATION CT By 1976 , a design was incorporated with a large stationary 360 degrees ring of detectors with the x-ray tube alone rotating round the patient. This approach of wide angle fan beam and rotate/stationary motion with sub second imaging time is referred to as fourth generation of CT. It uses about 4800 individual detectors and spatial resolution of more than 20 lp/cm.

24. FOURTH GENERATION CT

25. DRAWBACKS Size and geometric dose inefficiency Because tube rotated inside the detector ring large ring diameter was needed . On the other hand acceptable spatial resolution limited detector aperture to approx. 4mm . Scatter The 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. FOURTH GENERATION CT

26. 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-the reference beam is measured by the same detector used for transmission measurement 3 rd Vs 4 th GENERATION

27. FIFTH GENERATION CT Cardiac imaging required ultra fast scan times(<50ms) 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 10-20ms. The idea behind the ultrafast scanner is a large bell shaped x-ray tube.

28. It doesn’t use conventional x-ray tube , instead a large arc of tungsten encircles the patient and lies directly opposite to the detector ring. X-rays are produced from a focal track as a high energy electron beam strikes the tungsten. There are no moving parts in the gantry. 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) FIFTH GENERATION CT

29. EBCT is also restricted to single slice acquisition for ECG- triggered scan examination times may be still beyond a single breathe hold . Typical scan times are 30-40s for a 12 cm volume. 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 . FIFTH GENERATION CT

30. SIXTH GENERATION CT Though 3 rd and 4 th generation CT scanners eliminated the translational motion , the gantry had to be stopped after each slice was acquired. 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. The result was poor temporal resolution and long procedure time.

31. The development of helical or spiral CT was truly revolutionary advancement in CT scanning that finally allowed true 3D image acquisition within a single breath hold technique. As the table is smoothly moved through the rotating gantry , the resulting trajectory of the tube and detector relative to patient traces out a helical or spiral path. Three technological developments were required; -slip ring technology -high power x-ray tubes -interpolation algorithms SIXTH GENERATION CT

32. The patient is continuously translated while multiple rotations of scan data are acquired. 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 image position. SIXTH GENERATION CT

33. Slip ring Eliminating interscan delays required continuous rotation , a capability made possible by low voltage slip ring. A slip ring passes electrical power to the rotating components without fixed connections. It allows the 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. SIXTH GENERATION CT

34. SIXTH GENERATION CT

35. High power x-ray tubes stationary tubes were used in 1 st and 2 nd generation CT scanner – long scan time – allowed heat dissipation. Shorter scan time required high power of x-ray tubes and use of oil cooled rotating anodes for efficient thermal dissipation. Largest heat capacities are achieved with thick graphite backing of target disks , anode diameters of 200mm or more , metal housing with ceramic insulator. The working life of tubes ranges from 10,000 – 40,000 hours SIXTH GENERATION CT

36. Pitch It is defined as the table movement per rotation divided by beam width. If beam width is 10mm , table moves 10mm during one tube rotation then pitch is 1 – x-ray beam associated with consecutive helical loops are contiguous. If beam width is 10mm and table moves 15mm per tube rotation then pitch is 1.5 – gap exists between x-ray beam edge of consecutive loop. If beam width is 10mm and table moves 7.5mm then pitch is 0.75 – beams and consecutive loops overlap by 2.5mm(doubly irradiating the underlying tissues) SIXTH GENERATION CT

37. Interpolation Helical CT scanning produces a data set in which the x-ray source has travelled in helical trajectory around the patient. Present day CT reconstruction algorithms assume that x-ray source has negotiated a circular not a helical path around the patient. With helical scanning ,CT images can be reconstructed at any position along the length of scan. It allows the production of additional overlapping images with no additional dose to the patient. SIXTH GENERATION CT

38. Advantages Fast scan times and large volume of data collected. Minimizes motion artifacts. Less mis-registration between consecutive slices. Reduced patient dose. Improved spatial resolution. Enhanced multiplaner or 3D renderings. Improved temporal resolution. SIXTH GENERATION CT

39. SEVENTH GENERATION CT MS/MD CT It is introduced in 1998. Allows acquisition of multiple slice in single row. A body section can be scanned faster with a multiple row of detectors system with multiple fan beams scanning simultaneously. Crucial for covering a large body section with thin beams for producing thin , high-detail slice images or 3-D images.

40. An approach to overcoming x-ray tube output limitation is it make better use of x-rays that are produced by x-ray tube. When multiple detector is used , the collimation spacing is wider therefore more of x-rays that are produced by x-ray tubes are used in producing image data. With conventional single detector array scanners , opening up the collimator increases slice thickness which is good for utilization of x-ray but reduces spatial resolution in the slice thickness dimension. With introduction of multiple detector arrays , the slice thickness is determined by the detector size and not by the collimator. SEVENTH GENERATION CT

41. Pitch With the introduction of multiple-row detector CT scanner the definition of pitch has changed. Pitch is equal to the table rotation per gantry rotation divided by width of detector. SEVENTH GENERATION CT

42. SEVENTH GENERATION CT

43. Multiple array designs

44. 16x0.75mm 4 x 1.5mm 4 x 1.5mm 24 mm 16x0.75mm 16x1.5mm 8x3mm 4x3mm 2 x 3mm 2 x 3mm 24 mm 8x1.5mm 4 x 1.5mm 4 x 1.5mm 24 mm

45. ADVANTAGES It is based on 3 rd generation geometry. Has better Z-axis resolution. Provides larger coverage. Faster scan times. Less motion artifacts. 8 , 16 , 64 , 256 slice CT machine are available. SEVENTH GENERATION CT

46. Advancement. Dual energy CT. Flat panel detector CT.

47. Advancement. DUAL SOURCE CT Dual source increase the temporal resolution by reducing the rotation angle required to acquire a complete image. It permits cardiac imaging without the use of heart rate lowering medication and imaging heart in systole. The use of dual x-ray source makes possible the use of dual energy imaging which allows an estimate of the average atomic number in a voxel , as well as the total attenuation. Principle – materials show different attenuation at different mean energies.

48. Graph of mass-attenuation coefficients for iodine (blue), calcium (green), and water (red) on CT images obtained at two different energies (vertical dashed lines) shows that these materials can be characterized by comparing their attenuation at the lower energy with that at the higher energy. When dual-energy images reconstructed for 50 and 80 keV are compared, iodine demonstrates a greater decrease in attenuation than calcium does at the higher energy, whereas the attenuation of water remains more or less constant

49. Types of Dual Energy CT Scanners Three types of dual-energy CT scanners are available that differ in the technique used to acquire high- and low-energy CT datasets: a dual-source dual-energy scanner a single-source dual-energy scanner with fast kilovoltage switching (ie, rapid alternation between high and low kilovoltage settings) a single-source dual-energy scanner with dual detector layers.

50. DUAL SOURCE CT:

51. NEW GENERATION CARDIAC CT New generation CT scanners are recommended as the first line imaging of coronary artery diseases in whom imaging is difficult with earlier generation CT scanners. These enhancement include better temporal resolution , better spatial resolution and shorter acquisition times. These are; Acquilion One (Toshiba) Brilliance iCT (Philips) Discovery CT750 (GE Health Care) Samatom Defination Flash (Siemens)

52. AQUILION ONE CT In 2007 Aquilion One CT systems were tested at Fujita Health University and the National Cancer Center in Japan AND Johns Hopkins University in U.S. Aquilion CT can see not only a 3D depiction of an organ but also shows how the organ and blood flow are dynamically functioning or moving within and around the organ. It can perform functional studies – reduces the amount of contrast media and radiation dose. It is fast enough to image entire heart in less than heart beat – 0.35 sec. Detector rows – 128x0.626 mm. Z-axis coverage – 80mm.

53. Brilliance iCT The Brilliance iCT scanner has 128x0.625 mm detector rows. It provides total z-axis coverage of 80mm. Each detector row is double sampled to increase spatial resolution. It is claimed that it can capture an image of the entire heart in two heart beats.

54. Discovery CT750 HD It is 64x0.625 mm detector dual – energy CT scanner. Contains single x-ray source that switches between two energy levels allowing two data sets – high and low energy. It uses Gemstone detector that contributes to high image quality and “snap shot” pulse – allows a complete picture of the heart to be captured in 3-4 sec. Snapshots are taken at precise table position and timed to correspond to a specific phase of cardiac cycle.

55. Samatom Defination Flash It is dual source CT scanner of detector 64x0.6mm. It provides high resolution image at a fast scanning speed with low-radiation dose. Maximum scan speed of 458mm/s. Two x-ray tubes and detector arrays are mounted at 95 degrees to each other.

56. commonly known as Industrial CT Scanner The term micro is used to indicate that the pixel sizes of the cross-sections are in the micrometer range. The machine is much smaller in design compared to the human version and is used to model smaller objects. There are two types of scanner setups- one setup, the X-ray source and detector are typically stationary during the scan while the sample/animal rotates. second setup, much more like a clinical CT scanner, is gantry based where the animal/specimen is stationary in space while the X-ray tube and detector rotate around. Micro-CT

57. MOBILE CT

58. SUMMARY

59. SUMMARY

60. REFERENCES Radiologic sciences for technologists – Stewart Carlyle Bushong The essential physics of medical imaging – Jerrold T. Bushberg Computed tomography for Technologist ,Loise E. Romans Various websites.

61. QUESTIONS Features of 1 st generation CT? Features of 2 nd generation CT? Features of 3 rd generation CT? Features of 4 th generation CT? Features of EBCT? Advantages of helical CT over conventional axial scanning? Advantages of 7 th generation CT? Principle of dual source energy CT?

62. THANK YOU !!!