CT PHYSICS INTRODUCTION WITH BASICS.pptx

CT PHYSICS

1. CT PHYSICS SUKHESWER

2. INTRODUCTION: Process of generating a two-dimensional image of a slice or section through a 3-dimensional object (a tomogram). Technique that blurs out the shadows of superimposed structures to show more clearly the principal structure Stands for Computed Tomography scan

3. 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.

4. COMPUTER TOMOGRAPHY: 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 sagittal or coronal plane by reconstruction of the images by computer. CT has undergone several evolutions & now the days multi-detectors CT scanners have been evolved which have application in the clinical field.

5. 3 Step Processing Of CT Imaging:

6. PRINCIPLE OF COMPUTED TOMOGRAPHY: The internal structure of an object can be reconstructed from multiple projections of the object. Mathematical principles of CT were first developed in 1917 by Radon Proved that an image of an unknown object could be produced if one had an infinite number of projections through the object.

7. Basically, a narrow beam of X ray scans across a patient in synchrony with a radiation detector on the opposite side of the patient. The sufficient no. of transmission measurements are taken at different orientation of X ray source & detectors, the distribution of attenuation coefficients within the layer may be determined. By assigning different levels to different attenuation coefficients, an image can be reconstructed with aid of com. that represent various structures with different attenuation properties.

8. SLICE: 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.

9. 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.

10. PIXEL: VOXEL: Each square in a matrix is called a pixel. Also known as picture element. Each individual element or number in the image matrix represents a three dimensional volume element in object called VOXEL .

11. 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.

12. Windowing: 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 .

13. IMAGE QUALITY:

14. 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.

15. TECHNICAL TERMS:

16. Tomographic acquisition: Single transmission measurement through the patient made by a single detector at a given moment in time is called a ray. A series of rays that pass through the patient at the same orientation is called a projection or view. Two projection geometries have been used in CT imaging: Parallel beam geometry with all rays in a projection parallel to one another. Fan beam geometry , in which the rays at a given projection angle diverge.

17. ACQUISITION: Purpose of CT scanner hardware is to acquire a large number of transmission measurements through the patient at different positions. Single CT image may involve approximately 800 rays taken at 1,000 different projection angles. 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).

18. ALGORITHMS FOR IMAGE RECONSTRUCTION: An algorithm is a mathematical method for solving a problem. Thousand of equation must be solved to determine the linear Attenuation coefficient of all pixel in the image matrix. The three mathematical method of image reconstruction will be described are:- 1.Back projection 2.Iterative methods 3.Analytical methods

19. ANALYTICAL METHOD: Two popular method used in that method are:- 1. 2-D FOURIER ANALYSIS 2.FILTERED BACK PROJECTION

20. 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 c osine 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.

21. 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. The density of projected ray is adjusted to compensate - The star effect.

22. D isplay: 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

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

24. Generations Of CT Machines: There are 7 generations of CT scanner: First generation : Parallel beam; translate- rotate motion Second generation : Fan beam; translate- rotate motion Third generation : Multi detector ;Rotate- Rotate motion Fourth generation : Ring detector; Rotate-fixed motion Fifth generation : Electron beam scan;dynamic spatial r econstruction Sixth generation : Dual source ; multi detector Seventh generation : Flat panel digital area detector

25. SLIP RING TECHNOLOGY:

26. CT CLINICAL APPLICATION-CONVENTIONAL TO SPIRAL: It is a cross sectional image acquisition. It can scan head to toe (SPIRAL). TRENDS Conventional. Helical/Spiral CT scanner. Modern Volume scanner.

27. Conventional: Static scan. Table not moving continuously. Between the slices there may be gap. Poor resolution reformations. Increased scan Time. scanners outdated. Limited Study interpretation .

28. Helical / spiral CT scanners: Scan done with continuous table movement. Scan time significantly decreased than conventional. Improved image quality. Multiplanar reconstruction can be done (limited). Radiation dose reduced than conventional. Study interpretation. Better utilisation of contrast.

29. MODERN Volume CT scanner: Available higher area Z axis coverage of scan. Decreased scan time than earlier spiral single slice scanner. Better Image quality. Fine resolution of multi planner reconstruction. Scan can be done in single breath hold time. Scan can be completed in few seconds. Angiogram can be performed for all regions from head to toe. especially cardiac CT angio is one of the milestone achieved by Modern CT.

30. In a helical CT scanner, pitch is defined as: A) Table travel per gantry rotation divided by slice thickness B) Gantry rotation time divided by detector width C) Table travel per second divided by gantry speed D) Slice thickness divided by table travel per gantry rotation

31. The noise in a CT image is most significantly reduced by increasing: A) Pitch B) Reconstruction kernel sharpness C) mAs D) Field of view (FOV)

32. In a modern multi-detector CT (MDCT) scanner, what is the primary reason for using flying focal spot technology? A) Reduce patient dose B) Improve spatial resolution by increasing sampling C) Increase tube efficiency D) Reduce beam hardening artifacts

33. What is the primary disadvantage of using a very high pitch (e.g., > 1.5) in helical CT scans? A) Increased patient dose B) Loss of spatial resolution and potential for artifacts C) Increased scan time D) Increased radiation scatter to detectors

34. The term "z-axis resolution" in CT refers to resolution in which direction? A) Axial plane B) Coronal plane C) Sagittal plane D) Longitudinal (along the patient’s body)

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