Basics of CT

CT BASICS

1. BASICS OF COMPUTED TOMOGRAPHY PRESENTED BY : DR.B.BORTHAKUR PROFRSSOR AND HEAD, DEPT. OF ORTHOPAEDICS,SMCH

2. Computed Tomography (CT)  scanning, also known as, Computed Axial Tomography (CAT)  scanning, is a diagnostic imaging procedure that uses x-rays to build cross-sectional images of the body. CT is based on the fundamental principle that the density of the tissue passed by the x-ray beam can be measured

3. The patient lies on the table within the CT gantry, During each slice acquisition, an X-ray tube circling the patient produces an X-ray beam that passes through the patient and is absorbed by a ring of detectors surrounding the patient. The intensity of the X-ray beam that reaches the detector is dependent on the absorption characteristics of the tissues it passes through.

4. Since the beam is moving around the patient, each tissue will be exposed from multiple directions. The basic principle of CT lies in the reconstruction of the internal structure of an object from multiple projections of the object. Using a process called Fourier analysis, the computer ultimately forms the image.

5. Components of CT scanner:- Gantry assembly & patient table/couch Computer Operating console

6. Gantry assembly & patient table/couch

7. Console Room and Computer Operating console

8. Image reconstruction The computer receives a signal in analog form and converts it to a binary digit. The digital signal is stored and the image is reconstructed after the scan is over. In CT, a cross-sectional layer of the body is divided into many tiny blocks, called as voxels. Voxel is a 3D tissue element. Pixel is the 2D projection of a voxel on the computer screen Therefore, a voxel is a volume element and a pixel is a picture element. Each is displayed on a matrix.

9. CT number and Hounsfield units:- Each pixel is assigned a numerical value (CT number), which is the average of all the attenuation values contained within the corresponding voxel. This number is compared to the attenuation value of water and displayed on a scale of arbitrary units named Hounsfield units (HU) after Sir Godfrey Hounsfield.

10. Evolution of CT scanners First generation   detectors: one type of beam: pencil-like x-ray beam tube-detector movements: translate-rotate duration of scan (average): 25-30 mins single X-ray source and single X-ray detector cell to collect all the data for a single slice. Process is repeated once for each projection angle until 180 projections , across a 24 cm FOV

11. First Generation CT

12. Second generation  detectors: multiple (up to 30) type of beam: fan-shaped x-ray beam tube-detector movements: translate-rotate duration of scan (average): less than 90 sec By adding detectors angularly displaced , several projections could be obtained in a single translation

13. Third generation detectors: multiple, originally 288; newer ones use over 700 arranged in an arc type of beam: fan-shaped x-ray beam tube-detector movements: rotate-rotate duration of scan (average): approximately 5 sec Sampling considerations required scanning an additional arc of one fan angle beyond 180°, although most scanners rotate 360°for each scan.

14. Fourth generation detectors: multiple (more than 2000) arranged in an outer ring which is fixed type of beam: fan-shaped x-ray beam tube-detector movements: rotate-fixed  duration of scan (average): few seconds Design: stationary detector ring & rotating X-ray tube

15. Spiral/Helical CT Design: x-ray tube rotates as patient is moved smoothly into x-ray scan field Simultaneous source rotation, table translation and data acquisition Produces one continuous volume set of data for entire region Data for multiple slices from patient acquired at 1sec/slice

16. SSCT VS MSCT MSCT Detectors The primary difference between single-slice CT (SSCT) and MSCT hardware is in the design of the detector arrays. SSCT detector arrays are one dimensional, that is, they consist of a large number (typically 750 or more) of detector elements in a single row across the irradiated slice to intercept the x-ray fanbeam.

17. MDCT or MSCT MDCT is a commonly used acronym that stands for Multiple-row Detector Computed Tomography. The other commonly used term MSCT stands for Multi Slice Computed Tomography. It is the latest breakthrough in CT technology. It has transformed CT from trans-axial cross sectional technique into a true three dimensional imaging modality.

18. Advantages of MDCT Shorter scan time: Reduces movement artifacts. Useful in children, trauma patients & acutely ill patients. Improved scanning of parenchymal organs:- different phases of contrast enhancement can be elicited Minimization of partial volume averaging effects. Thinner sections: - excellent multiplaner reformations can be performed. Reduction of radiation exposure.

19. Disadvantages:- Radiation exposure. Relatively poor tissue contrast when compared to MRI is a problem, There can be contrast media-related complications, including allergic reactions and renal toxicity.

20. THANK YOU