1. Chapter 2 Stewart C. Bushong
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
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 First Gen Spiral CT Scan Time 300s Less than 1s Data/image
60kb
2 Mb
Matrix Size
80x80
1024x1024
Energy/image
2kJ
60kJ
Slice Thickness
13mm
1-10mm
Spatial Res.
3 lp/cm
15 lp/cm