Basic principle of CT and generation of CT.pptx

presentation of basic principle of CT and generation of CT

1. Basic principle of CT and Generation of CT Presented by :-Sarita gaire Roll no :- 160 BSc. MIT 2 nd year Maharajgunj medical campus,iom

2. Contents Introduction History Principle of conventional tomography Principle of CT Generations of CT Advancements in CT Summary References

3. INTRODUCTION The main limitation of the conventional radiography is the overlapping of different tissue. To overcome with this problem computed tomography is introduced which creates cross sectional images from 3-D body structure. It utilizes mathematical algorithm called reconstruction to complete the task. The main goal of CT is to reconstruct the internal structures of the body as 2-D cross sectional images.

4. The goal is accomplished by its superior ability to overcome superimposition of structures and demonstrates slight differences in tissue contrast. A CT image is the result of “ BREAKING APART” a 3- D structure and mathematically putting it back together again and displaying it as a 2-D image in a computer monitor .

5. Comparison of CT with conventional radiography CT Highly tissue sensitive Provides more anatomical detail and differentiation Scan data can be manipulated into different views without additional imaging (i.e. axial,sagittal,coronal and 3D reconstruction) Increased radiation dose, examination time and examination cost than x-ray. X ray Less tissue sensitive than CT Provides less anatomic detail and differentiation 2D image ,with superimposing anatomical information Requires separate exposures for each unique views (i.e. AP, lateral, and oblique views) Less radiation dose, examination time and examination cost than CT.

6. History 1917 - Mathematical theory of tomographic image reconstructions (Johann Radon) 1931- Conventional tomography (Alessandro Vallebona) 1963 - Theoretical basis of CT (Allan McLeod Cormack) 1971 - First commercial CT (Sir Godfrey Hounsfield) 1974 -Fourth generation CT 1979 - Nobel prize (Cormack & Hounsfield) 1998 - Multiple scanner were introduced 2001 - 16-row spiral CT 2007 - 320-row spiral CT 2008 - Dual Energy CT 2010 - 640 slices MDCT 2014 - Use of stellar detector

7. Principle of conventional tomography Tomographic units synchronize the movement of x ray tube and the image receptor in opposite direction around stationary fulcrum(pivot point) The fulcrum area is sharp. The farther the anatomical structure from the object plane, the more blurred its image is.

8. Components of CT

9. Principle of CT The internal structure of an object can be reconstructed from multiple projection of the object. Mathematical principles of CT were first developed in 1917 by radon. Image of an object can be produced if one had an infinite projection through the object. Basically a narrow beam of X-ray scans across a patient in synchrony with a radiation detector on the opposite side of the patient.

10. Principle of CT The sufficient no. of transmission are taken at different orientation of X-ray source & detectors, the distribution of attenuation coefficients with in the layer may be determined. By assigning different levels of different attenuation coefficients, an image can be reconstructed with aid of computer that represent various structure with different attenuation properties.

11. Steps of CT image formation A computed tomography (CT) image is a display of the anatomy of a thin slice of the body developed from multiple x-ray absorption measurements made around the body's periphery. All CT systems use a three step process: 1. Scan or Data Acquisition 2. Image Reconstruction 3. Image Display

12. Scan or Data Acquisition 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 collect the enough information by the detector for image reconstruction. The total X-ray transmission measured by each detector is the result of ray sum. The collection of X-ray sum for all the detector at a given tube position is called a projection. Projection data sets are acquired at different angle around the patient.

13. Two projections geometries have been used in CT imaging Parallel beam geometry with all rays in projection parallel to one another . Fan beam geometry in which the rays at a given projection diverge with different angles. Cone beam geometry

14. Scan mode Step and shoot scanning Helical(spiral) scanning Multi detector row scanning

15. Step and shoot scanning The x ray tube rotated around the patient to acquire data for a single slice. The motion of the x ray tube was halted while the patient was advanced on the CT table to the location appropriate to collect data for next slice. Steps 1 and 2 were repeated until desired area were covered. This method is also called axial, conventional or serial scanning .

16. Helical (spiral) scanning Many technical development of the 1990s allowed for the development of a continuous acquisition scanning mode called spiral or helical scanning. It eliminates the cables and there by enabled continuous rotation of the gantry which allowed for uninterrupted data acquisition that traces a helical path around the patient.

17. Multi detector row CT scanning The first helical scanner emitted X-ray that were detected by a single row of detectors, yielding one slice per gantry rotation. This technology was further expanded on in 1992 when scanners were introduced that contained two rows of detectors, capturing data for two slice per gantry rotation. Further improvement equipped scanners with multiple rows of detectors, allowing data for many slices to be acquired with each gantry rotation.

18. Digital image components Pixel: It is 2-D(i.e.X&Y) picture element that makes of matrix. Pixel size =FOV/Matrix size Voxel: It is 3-D(i.e. X,Y&Z) volume element. Voxel=Pixel X slice thickness(Z)

19. Image reconstruction When source-detector makes one sweep across the patient, the internal structures of the body attenuate the X-ray beam according to their mass density and effective Z. The intensity of radiation detected varies according to this attenuation pattern and an intensity profile or projection is obtained. These projections are not displayed visually, stored in digital form in the computer. The computer processes the projections that involve super imposition of each projection to reconstruct an image of the anatomic structures within that slice.

20. Image reconstruction The individual value of the matrix elements( ) are obtained by solving the simultaneous equations. The matrix of values that are obtained represents cross sectional anatomy. Dedicated array processor is used to do calculation and instantaneous image display. The image reconstruction algorithm are Iterative technique Back projection Filtered back projection

21. Beam attenuation The degree to which an X-ray beam is reduced by an object. The amount of the Xray beam that is scattered or absorbed per unit thickness of the absorber is expressed by the linear attenuation coefficient. Attenuation is followed by Lambert’s law of absorption. Difference in linear attenuation coefficient among tissue are responsible for X-ray image contrast. The number of photons interaction depends on the thickness, density, and atomic number of the object.

22. CT number and Hounsfield unit The attenuation coefficient is useful for computation, but not for display of images. Hence, after reconstruction, the attenuation values of each pixel is normalized to that of water CT no=k(  t -  w )/  w When k = 1000, CT numbers are called Hounsfield Units (HU). CT numbers drive their contrast from physical properties of the tissue that influence Compton scatter; e.g. density, electron density, relative abundance of hydrogen.

23. CT number and Hounsfield unit CT numbers are quantitative, useful for clinical diagnosis such as pulmonary nodule, levels of calcification, bone density, fracture risk, and tumor volume or lesion diameter. The attenuation coefficient of body organs vary with kV and filtration. Conversion of attenuation coefficient into CT number makes the image, independent of machine parameters and makes the image more dependent on patient anatomy.

24. Volume averaging Refers to the process in CT by which different tissue attenuation values are averaged to produce one less accurate pixel reading. Thicker CT slices increase the likelihood of missing very small objects. For example if 10-mm slices are created, and the area of pathologic tissue measures just 2 mm, normal tissue represents 8 mm and is averaged in with the pathologic tissue, potentially making the pathologic tissue less apparent on the image. This process is referred to as volume averaging, or partial volume effect.

25. Volume averaging The X and Y dimensions of the pixel also affect the likelihood of volume averaging. For example, imagine a pixel that contains equal parts calcium (measuring 600 HU) and lung tissue (measuring −600 HU). The resulting density of the specific pixel is the average of the two tissues, or 0 HU. In this case, the image pixel does not accurately reflect either the calcium or the lung tissue. Using a small pixel size reduces the likelihood of volume averaging by limiting the amount of data to be averaged.

26. Image display Image display includes all of the system components necessary to convert the digital data created from the reconstruction process to electrical signals needed by the CT display monitor. The reconstructed image is displayed either on a cathode ray tube or some form of flat panel such as a TFT LCD , by allotting shades of gray to each CT number. There are 256 shades of gray in the system and each CT number is allotted one shade of gray .

27. Image display The range of CT numbers above and below the window level (center number) is called window width, which determines the contrast. A narrow window width provides higher contrast than wide window width. The window level and window width can be set to any desired value of CT number. The window width and window level settings only affect the displayed image, not the reconstructed image data.

28. Generation of CT “Generation” means the order in which CT scanner design has been introduced, and it has number associated with it. Generation of CT is determined by the configuration of X-ray tube and detector. Higher generation number doesn’t necessarily have higher system performance. Main objective of different generation is Scanning time reduction Simplification of mechanical motion

29. First generation Is a rotate/translate motion, pencil beam system. Pencil beam provides excellent X-ray scatter rejection. It had one or two X-ray detectors and used parallel ray geometry with NaI detector . It is translated linearly to acquire 160 rays across a 24 cm FOV and rotated between translations to acquire 180 projections at 1° interval. The combination of linear translation followed by incremental rotation is known as translate rotate motion.

30. First generation Early detector system couldn't accommodate large change in signal so patient head was recessed through 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) Is only used for the imaging of the brain. Also known as EMI scanner.

31. Drawbacks of first generation Images have poor spatial resolution. Scan time were very long i.e. up to 5min/slice

32. Second generation 1st waterless full body CT scanner. Also of translate and rotate type. Use fan shaped x-ray beam. These imaging system consisted of 5 to 30 detectors in the detector assembly ,therefore shorter imaging times were possible i.e.30sec Because of the multiple detector array a single translation resulted in the same no. of data points as several translation with a first generation CT. Consequently translations were separated by rotation increments of 5 degree or more .

33. Drawbacks of second generation The principal limitation of second-generation CT imaging systems was examination time. Because of the complex mechanical motion of translation and rotation and the enormous mass involved in the gantry, most units were designed for imaging times of 20 seconds or longer. One disadvantage of the fan beam is the increased radiation intensity towards the edge. It is reduced by use of bow-tie filter.

34. Third generation Is rotate/rotate type with wide fan beam geometry. The number of detectors(curvilinear) has increased substantially (> 800 detectors) and the angle of fan beam is increased to cover entire patient i.e between 30 and 60 degree. Can produce an image in less than 100ms. Reference detector is typically located at either end of the detector array to measure the unattenuated x-ray beam.

35. Third generation Tube is directly focused on the detector array & fixed relationship between x-ray source and the detector allows the beam to be highly collimated. The third generation design is the most widely used configuration in the industry today. All new multidetector CT systems sold in the United States use the third-generation design.

36. Third generation

37. Drawbacks of third generation There is chance of frequent occurrence of ring artifact since, each detector views ring anatomy, even a small misalignment of a single detector will result in visible ring artifact. These ring artif­acts were troublesome with early third generation CT imaging systems. So­ftware-corrected image reconstruction algorithms now remove such artifact.

38. Fourth generation Rotate and stationary type , fan beam geometry. It uses detector array of about 4800 detector which are fixed in 360° circle within gantry. The number of detector used in any one time is controlled by the width of the x-ray beam. Fourth generation imaging systems were developed because they were free of ring artifacts. Sub second imaging time.

39. Fourth generation Because the emerging beam does not strike the detectors at exactly the same time, motion artifacts are more of a problem Fourth generation systems often use overscans to address this problem. An over scan is a tube arc greater than 360°. The use of an over scan technique will increase the radiation dose to the patient.

40. Fourth generation In addition, because the tube is closer to the patient, the same milliampere-seconds ( mAs ) and kilovolt-peak ( kVp ) setting will produce a higher dose when a fourth- generation system is used (as compared with the same settings used in a third-generation system). However, because the x-ray source is closer to the patient, techniques necessary to produce an adequate image are generally somewhat lower than that used on a third generation system .

41. Fourth generation

42. Fifth generation Cardiac imaging required ultra fast scan times (<50ms), which was a hurdle with previously existed generation. A noble CT scanner was developed for cardiac imaging which was capable of performing complete scans as little as 10-20ms. Stationary/stationary system, developed specifically for cardiac tomography imaging. It doesn’t use conventional x-ray tube, instead a large arc of tungsten (210°) encircle patient and lies directly opposite to the detector ring. It uses an electron gun that deflects and focuses a fast moving electron beam along tungsten target ring in gantry.

43. Fifth generation Since the detector is also in the form of ring, it permits simultaneous acquisition of multiple image sections. 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)

44. Fifth generation EBCT is also restricted to single slice acquisition for ECG-triggered scan examination times may be still beyond a single breathe hold. 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 cost.

45. Drawback of fifth generation/EBCT Due to low spatial resolution it can not be used in general imaging. It is highly expensive.

46. Sixth generation Though 3 rd and 4 th generation CT scanners eliminated the translation motion , the gantry had to be stopped after each slice was acquired. Cables are spooled onto a drum , released during rotation and re spooled 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.

47. Slip ring technology Slip rings are electromechanical devices that conduct electricity and electrical signals through rings and brushes from a rotating surface onto a fixed surface. One surface is a smooth ring and the other a ring with brushes that sweep the smooth ring. Helical CT is made possible by the use of slip-ring technology, which allows the gantry to rotate continuously without interruption. Early CT imaging was performed with a pause between gantry rotations because high voltage and data cables passed from the gantry.

48. Slip ring technology During the pause, the patient couch was moved and the gantry was rewound to a starting position 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 and making continuous rotation possible. Brushes(silver graphite alloy) that transmit power to the gantry components glide in contact grooves on the stationary slip ring.

49. 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.

50. interpolation Reconstruction of an image at any z-axis position is possible because of a mathematical process called interpolation. During helical CT, image data are received continuously but when an image is reconstructed the plane of the image does not contain enough data for reconstruction. Data in that plane must be estimated by interpolation. Data interpolation is performed by a special computer program called an interpolation algorithm The first interpolation algorithms used 360-degree linear interpolation. The plane of the reconstructed image was interpolated from data acquired one revolution apart.

51. interpolation When these images are formatted into sagittal and coronal views, prominent blurring can occur compared with conventional CT reformatted views. The solution to the blurring problem is interpolation of values separated by 180 degrees—half a revolution of the x-ray tube. This results in improved z-axis resolution and greatly improved reformatted sagittal and coronal views.

52. Pitch Parameter to describe CT table movement. Defined as the ratio of table increment per 360 degree gantry rotation over collimator width. Pitch = table travel/collimator width Pitch is expressed as a ratio, such as 0.5:1,1:1,1.5:1or 2:1

53. Advantages of sixth generation Fast scan times and large volume of data collected. Minimizes motion artifacts. Less miss-registration between consecutive slices. Reduced patient dose. Improved spatial resolution. Enhanced multi-planer or 3D renderings. Improved temporal resolution

54. Seventh generation It is also known as Multi slice (MS) or Multi detector (MD) CT. It allows acquisition of multiple slice of image in single rotation. It has got faster scan time leading to less motion artifact. 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.

55. Seventh generation 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. Pitch is equal to the table movement per gantry rotation divided by width of detector.

56. Advantages of seventh generation 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

57. Advancements in CT Dual energy CT Flat panel detector CT(FPCT ).

58. Dual energy CT The principle of dual energy CT imaging is based on the differential absorption of energy at variable kvp setting.

59. Dual energy CT Types DS-DECT SS-DECT with fast kv switching SS-DECT with dual detector layers

60. Flat panel detector CT Also called flat panel volume tomography scanners that utilize a large area detector rather than a fixed array of detectors. has significantly wider z-axis area coverage that cover large areas in one rotation. can demonstrate real time fluoroscopy with a high spatial resolution. has high spatial resolution than MDCT making advantageous in the clinical setting of follow-up investigation of cardiovascular stents and clipped aneurysms. However, FPCT has lower contrast resolution than MDCT.

61. Summary In tomography only the image in the fulcrum is clear and other get blurred. The data obtained from the scanning body is known as raw data and the CT computer reconstruct raw data, image is created and called image data. Generation of CT is determined by the configuration of X-ray tube and detector. Higher generation number doesn’t necessarily have higher system performance.

62. Reference CT for Technologist Radiologic sciences for technologists – Stewart Carlyle Bushong The essential physics of medical imaging – Jerrold T. Bushberg