CT Generations 1.pptx

CT scan generations, principle

1. CT Generations By- Vaishnavi Kalbandhe

2. Topics: CT- Introduction History of CT Basic principle Generations

3. CT- Introduction CT- Computed Tomography Computed tomography is a medical Imaging technique used to obtain detailed internal images of the body. It includes internal organs, vessels and bone. It is used to diagnose conditions like damage to bones, injuries to internal organs, problems with blood flow, strokes and cancer. Another name: Computerized Axial Tomography (CAT). CT generates image in transaxial section, i.e. perpendicular to the axis of rotation. Some pathologies can also be seen in the sagittal or the coronal plane by reconstruction of the images by the computer.

4. HISTORY The first commercially available CT scanner was created by British engineer Godfrey N. Hounsfield of EMI laboratories in 1972. He co-invented this technology with physicist Dr. Allan Cormack. both were awarded with noble prize in 1979. The first commercial machine was designed to study head and later it was modified to scan any part of the body.

5. Basic Principle of CT Scan Basic principle of CT is that “The internal structure of an object can be reconstructed from multiple projection of the object”. CT refers to the computerized X-ray imaging procedure in which a narrow beam of X-rays is aimed at a patient and quickly rotated around the body producing signals that are processed by the machine’s computer to generate cross-sectional images or slices. Those slices are togetherly stacked to form a three dimensional (3-D) images of the patient.

6. Fig: Projections

7. Fig: slice

8. Generations of CT Classification of computed tomography (CT) based upon: arrangement of components and mechanical motion required to collect data. Ct scanners were first introduced with a single detector for brain study under the leadership of sir Godfrey Hounsfield. Thereafter it has undergone multiple improvements with an increase on the number of detectors and decrease In the scan time.

9. First generation Design- single X-ray source and single X-ray detector to collect all the data for a single slice. It was Rotate / Translate geometry. Beam: Pencil beam with parallel ray geometry. It took about 4-5 minutes per scan. NaI used as detector. It was designed specifically for the evaluation of brain. In this head was enclosed in a water bath.

10. Advantage : Pencil beam geometry allowed very efficient scattered reduction. Disadvantages : NaI is hygroscopic (absorb moisture from air). High scanning time. Poor spatial resolution. Complex mechanical motion of rotate- translate system.

11. First generation CT scanner image

12. Second generation It also has rotate- translate system scanner. Beam: narrow beam geometry. 30 detectors were used made up of NaI . Bowtie filter was firstly used in this generation. (Bowtie filter- pre patient attenuator, used to shape an x-ray beam) Scan time was reduced (10-20 sec)

13. Advantages: Shorter scan time. Speed increased of scanning due to increased number of detectors. Larger rotational increments(30 degrees). Disadvantages: Hygroscopic nature of NaI . Complex mechanical motion of translate / rotate. Scattered radiation increased due to narrow fan beam.

14. Third Generation Rotate/ rotate system. Wide fan beam geometry. 250-750 detectors were used in this generation. Detectors were aligned as an arc, which results in constant source to detector path length. Both xenon and scintillators detectors were used. Ring artifact is common in this generation. Scan time reduced to 1 sec.

15. Advantage: Elimination of complexity of translate- rotate motion. Shorter scan time than previous generation. Less patient dose. Alignment of detector assembly allows for better X-ray collimation. Disadvantage: More scattered radiation due to wide angled fan beam. Expensive than previous generation. Complex electronic circuitry. Ring artifact.

16. Ring artifact : It is never possible to have large number of detectors in a perfect balance with each other during continuous rotation of source detector assembly. If one of the detector is out of calibration, the detectors will give a consistently erroneous readings at each angular position resulting in a circular artifact called as ring artifact. So, ring artifact occurs due to miscalibration or failure of one or more detector elements.

17. Fourth generation Rotate/ stationary system. Beam: wide fan beam. Huge number of detectors (upto 4,800) were used. This generation was designed to overcome ring artifact. In this generation scanner, detectors are aligned in a ring and this assembly does not move. The detector ring completely surrounds the patient. The X-ray tube rotates in a circle inside the detector ring.

18. Advantage: Elimination of ring artifact. No complex electronic circuitry. Disadvantage: More scattered radiation due to wide angle of fan beam. More expensive than previous generations.

19. Fifth generation [EBCT] Stationary / stationary system. As it uses electron beam , it is also called as electron beam computed tomography. EBCT is also called as ultrafast CT. It is used for cardiac imaging and calcium scoring and to visualize coronary arteries. Large arc of tungsten (210 degrees) encircle the patient and lie directly opposite to the detector ring. It consists of waveguide to accelerate an electron beam onto a tungsten target through a bending magnet. Actually there are four tungsten target ,so that four tissue slices are imaged at same time. The images are obtained in less than 50 microseconds.

20. Advantage: It can produce fast frame rate CT of beating artifact with minimum motion artifact. Increased speed. Shorter scanning time. Disadvantage: Complex circuitry. Bulky. High equipment cost. Image quality poor for general scanning (because of low mA).

21. Sixth generation Third/fourth generation CT + slip ring technology + helical motion. It is also called helical or spiral CT. Spiral CT is a technology in which a source and detector travel along a helical path relative to the object. In this generation, the x-ray tube rotates continuously and the couch move the patient through the plane of rotating beam.

22. Advantages: No motion artifact. High quality reconstruction. Reduced scan time. Smooth movement of gantry and other components. Disadvantages: More processing time. Expensive. Tube heating increased. Reduced resolution.

23. Slip ring technology Conventional CT scanning is performed with a pause between each gantry rotation. During pause, the patient couch is moved and gantry may be rebound to a starting position. In helical CT, slip ring technology is used that allows the gantry to rotate continuously without interruption. Slip rings are electrical conducting brushes and component of gantry transferring the data or electrical energy from stationary part of gantry to rotating part of gantry for continuous rotation if gantry. They are made up of conducting material, e.g. silver, graphite alloys. There are two slip rings designs in spiral CT scanner – Disk and Cylinder.

24. There are usually three slip rings on a gantry- First slip ring - provides high voltage power to x-ray tube and generator. Second slip ring - provides low voltage power to control system on rotating gantry. Third slip ring – transfers digital data from rotating detector array.

25. Seventh generation Third Gen+ slip ring technology+ helical/ spiral motion+ multislice detector array along z-axis = seventh generation. Multislice Computed tomography. [MSCT] Multidetector row computed tomography. [MDCT] This generation CT uses third generation CT with helical scanning and low voltage slip rings. Basic principle : “The reconstruction of an internal structure of an object from multiple projections of an object by utilizing row of detectors along the z-axis.” In MSCT, multiple rows of detectors are used instead of using only one row of detectors as previous generations. Multiple rows of detectors allows for registration of more than one channel one gantry rotation. Here, the same data set can be used to reconstruct two or more image data sets of varied thickness.

26. Other main features of MSCT: Cone shaped beam is used. Slip ring technology used as sixth generation. Collimator spacing is wider. Slice thickness is controlled by detector size, not by the collimator. Major clinical applications of MSCT: Angiographic studies including cardiac. Virtual endoscopies. Coronary calcium studies. Perfusion studies. Routine head and abdomen.

27. Advantages: Reduced duration of scanning. Reduced artifacts. High resolution images. Isotropic 3-D voxels. Ultrathin slices. Disadvantages: High radiation dose. Data overload. Higher cost.

28. Generations Year Why developed Anatomy scanned Source detector movement Time to acquire 1 image No. of detectors First Gen 1971 To show CT works (pencil beam) Head only Translate/ Rotate ~ 5 min Only one Second Gen 1974 Image faster (Narrow fan beam) Head only Translate/ Rotate 20 sec- 2 min 30 Third Gen 1975 Image faster (wide fan beam) All anatomy Rotate/ Rotate 1 sec 250- 750 Fourth Gen 1976 Makes image without rings All anatomy Rotate/ Stationary 1sec Upto 4,800 Fifth Gen [EBCT] 1980 s Fast cardiac CT (electron beam) Cardiac only Stationary/ Stationary 50 ms (17 slices/ sec) Stationary ring of detectors Sixth Gen (Helical or Spiral CT) 1989 No motion artifact All anatomy Spiral CT Very fast MDA Seventh Gen [MDCT] [MSCT] 1998 High resolution and ultrathin slices All anatomy Spiral CT Very fast MDA

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