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Operational Modes

Operational Modes. Chapter 2 Stewart C. Bushong. Major Early Developments. Major early computed tomography developments were given the misnomer generation, as in genealogy Progress was rapid so that fourth-generation CT imagers appeared in 1978, just 6 years after the first CT imager

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Operational Modes

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  1. Operational Modes Chapter 2 Stewart C. Bushong

  2. Major Early Developments • Major early computed tomography developments were given the misnomer generation, as in genealogy • Progress was rapid so that fourth-generation CT imagers appeared in 1978, just 6 years after the first CT imager • Unlike Hounsfield’s early experiments, the patient does not move during CT, except for spiral CT, rather, the x-ray source and the image receptor move

  3. First Generation • Finely collimated x-ray beam (pencil beam) was used in first-generation CT imagers • Fan-shaped x-ray beam (fan beam) is used in all current CT imagers • Single radiation detector • Translate-rotate motion

  4. First Generation • 180 translation with 1 degree between translations • Single image projection per translation • Single image projection per translation • Five minute image time • Head imager only, not capable of body imaging

  5. Second Generation • Fan-shaped x-ray beam • Multiple radiation detectors (detector array) • Translate-rotate motion • Usually 18 translations with 10 degree rotation between translations • Multiple image projections per translation

  6. Second Generation • Approximately, 30 s imaging time • Head and body imager

  7. Third Generation • A fan beam x-ray source is used and it views the entire patient during imaging • As many as several hundred radiation detectors are incorporated into the curvilinear detector array • The curvilinear detector array provides constant distance between source and each detector, resulting in good image reconstruction

  8. Third Generation • This development is based on 360 degree rotate-rotate motion. Both the x-ray source and the detector array rotate about the same axis • Hundreds of image projections are acquired during each rotation, resulting in better contrast resolution and spatial resolution

  9. Third Generation • Imaging time is reduced to 1s or less • Various arc scans are possible in order to improve motion blur-half scan, full scan • Ring artifacts are characteristic of third generation imagers

  10. Fourth Generation • Fourth generation was developed principally to suppress ring artifacts • The x-ray source is collimated to a fan beam as in third generation • The detector array can contain several thousand individual detectors

  11. Fourth Generation • The mechanical motion is rotation of the x-ray source around a fixed detector array (rotate-stationary) • There is a modest sacrifice in geometry; however, the un-attenuated leading edge and un attenuated trailing edge of the fan beam allows for individual detector calibration during each scan

  12. Fourth Generation • Patient dose may be somewhat higher with fourth-generation scanners because of interspace between detectors • When there is an interspace between detectors, some x-radiation falls on the interspace, resulting in a wasted dose • As the fan beam passes across each detector, an image projection is acquired

  13. Fourth Generation • Imaging time is 1s or less • Various arc scan are available – half scan, full scan, over scan

  14. Electron Beam CT (EBCT) • This CT imager was developed specifically for fast imaging • Images can be obtained in less than 100ms, about the time of a radiograph • The x-ray source is not an x-ray tube but rather a focused, steered, and microwave accelerated electron beam incident on a tungsten target

  15. EBCT • The target covers one-half of the imaging circle; the detector array covers the other half • The electron beam is steered along the curved tungsten target creating a moving source • There are four targets, or focal tracks, and four detector arrays, resulting in four contiguous images simultaneously

  16. EBCT • Electron beam CT is principally applied to cardiac imaging and frequently advertised as a heart scan • Electron beam CT has no moving parts • Electron beam CT uses a focused electron beam on a tungsten target ring as an x-ray source • Heat dissipation is no problem in EBCT

  17. EBCT • Electron beam CT can produce up to eight slices simultaneously • Electron beam CT scan times as short as 50ms are possible • Principal application for EBCT is cardiac imaging

  18. Spiral CT • Spiral CT was introduced to clinical practice in 1989 and is now the standard CT imager • If a third or fourth generation is CT imager is caused to continually rotate while the patient couch is moved through the imaging plane, spiral CT results

  19. Spiral CT • The development of slip rings was the technology breakthrough that made spiral CT possible • Spiral CT requires slip ring technology for data transfer from the rotating gantry • Spiral CT requires either an on-board high voltage supply so that coiled high voltage cables are unnecessary or slip rings for high voltage transfer

  20. Spiral CT • The principal advantage to spiral CT is the ability to image large volumes of anatomy in less time • Single breath-hold imaging of the entire torso is possible with spiral CT

  21. Comparison

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