CT SCAN BASICS
- 2. History Of CT Scan Sir Godfrey N. Hounsfield invented the CT scan in the year 1972 and called computerized axial transverse scanning. He presented a cross-sectional image of the head that revealed the internal structures of the brain in a manner previously only seen at surgery or autopsy and for the first time pathologic processes such as blood clots, tumors, and strokes could be easily seen noninvasively.
- 3. BASIC PRINCIPLE: The Internal Structures of An Object Can Be Reconstructed From Multiple Projections Of The Object. By assigning different levels to different attenuation coefficients, an image can be reconstructed with aid of computer that represent various structures with diff attenuation properties.
- 4. 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.
- 5. VARIOUS PARAMETERS OF CT SLICE MATRIX PIXEL VOXEL CT NUMBER WINDOWING WINDOW WIDTH WINDOW LEVEL PITCH
- 6. 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.
- 7. 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.
- 8. Pixel Each square in a matrix is called a pixel. Also known as picture element.
- 9. VOXEL
- 10. CT Number The CT Scanner calculate from the collected data from the linear coefficient in each pixel. After computer calculates a value for the linear attenuation coefficient in a pixel. the value is converted into new number called CT number
- 11. Windowing Windowing is a system where the CT no. range of interest is spread cover the full grey scale available on the display system
- 12. 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 center of the range of the no. displayed on the image. It corresponds to Brightness of image.
- 13. 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.
- 14. First Generation
- 16. Third Generation
- 18. Fifth Generation(EBCT) 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
- 19. Sixth Generation( Spiral CT)
- 20. Seventh generation Single x ray tube Cone beam Multiple of detectors array Fast 3D images Disadvantages – Expensive
- 21. COMMON CT ARTIFACTS: Artifacts Cause Correction Motion Artifact Patient movement (Resp/heart) Reduction in scan time Clear and concise instruction to the patient Proper patient immobilization If needed, administration of sedatives/antiperistaltic drugs Streak Artifact - Due to Metals or implants Using Gantry Angulation Metal/Streak reduction protocols Beam Hardening Due to Polyenergetic Spectrum Beam Hardening Correction Algorithm Partial Volume Averaging Presence of tissues with highly varying absorbtion properties in a voxel. Usage of Thinner CT slices Ring Artifact Due to Detector failure or miscalibration Regular quality assurance checks