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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 - PowerPoint PPT Presentation
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Operational ModesOperational Modes
Chapter 2Chapter 2
Stewart C. BushongStewart C. Bushong
Major Early DevelopmentsMajor Early Developments
Major early computed tomography Major early computed tomography developments were given the misnomer developments were given the misnomer generation, as in genealogygeneration, as in genealogy
Progress was rapid so that fourth-Progress was rapid so that fourth-generation CT imagers appeared in 1978, generation CT imagers appeared in 1978, just 6 years after the first CT imagerjust 6 years after the first CT imager
Unlike Hounsfield’s early experiments, the Unlike Hounsfield’s early experiments, the patient does not move during CT, except patient does not move during CT, except for spiral CT, rather, the x-ray source and for spiral CT, rather, the x-ray source and the image receptor movethe image receptor move
First GenerationFirst Generation
Finely collimated x-ray beam (pencil Finely collimated x-ray beam (pencil beam) was used in first-generation beam) was used in first-generation CT imagersCT imagers
Fan-shaped x-ray beam (fan beam) is Fan-shaped x-ray beam (fan beam) is used in all current CT imagersused in all current CT imagers
Single radiation detectorSingle radiation detector Translate-rotate motionTranslate-rotate motion
First GenerationFirst Generation
180 translation with 1 degree 180 translation with 1 degree between translationsbetween translations
Single image projection per Single image projection per translationtranslation
Single image projection per Single image projection per translationtranslation
Five minute image timeFive minute image time Head imager only, not capable of Head imager only, not capable of
body imagingbody imaging
Second GenerationSecond Generation
Fan-shaped x-ray beamFan-shaped x-ray beam Multiple radiation detectors (detector Multiple radiation detectors (detector
array)array) Translate-rotate motionTranslate-rotate motion Usually 18 translations with 10 Usually 18 translations with 10
degree rotation between translationsdegree rotation between translations Multiple image projections per Multiple image projections per
translationtranslation
Second GenerationSecond Generation
Approximately, 30 s imaging timeApproximately, 30 s imaging time Head and body imagerHead and body imager
Third GenerationThird Generation
A fan beam x-ray source is used and it A fan beam x-ray source is used and it views the entire patient during imagingviews the entire patient during imaging
As many as several hundred radiation As many as several hundred radiation detectors are incorporated into the detectors are incorporated into the curvilinear detector arraycurvilinear detector array
The curvilinear detector array provides The curvilinear detector array provides constant distance between source and constant distance between source and each detector, resulting in good image each detector, resulting in good image reconstructionreconstruction
Third GenerationThird Generation
This development is based on 360 This development is based on 360 degree rotate-rotate motion. Both degree rotate-rotate motion. Both the x-ray source and the detector the x-ray source and the detector array rotate about the same axisarray rotate about the same axis
Hundreds of image projections are Hundreds of image projections are acquired during each rotation, acquired during each rotation, resulting in better contrast resolution resulting in better contrast resolution and spatial resolutionand spatial resolution
Third GenerationThird Generation
Imaging time is reduced to 1s or lessImaging time is reduced to 1s or less Various arc scans are possible in Various arc scans are possible in
order to improve motion blur-half order to improve motion blur-half scan, full scanscan, full scan
Ring artifacts are characteristic of Ring artifacts are characteristic of third generation imagersthird generation imagers
Fourth GenerationFourth Generation
Fourth generation was developed Fourth generation was developed principally to suppress ring artifactsprincipally to suppress ring artifacts
The x-ray source is collimated to a The x-ray source is collimated to a fan beam as in third generationfan beam as in third generation
The detector array can contain The detector array can contain several thousand individual detectorsseveral thousand individual detectors
Fourth GenerationFourth Generation
The mechanical motion is rotation of The mechanical motion is rotation of the x-ray source around a fixed the x-ray source around a fixed detector array (rotate-stationary)detector array (rotate-stationary)
There is a modest sacrifice in There is a modest sacrifice in geometry; however, the un-geometry; however, the un-attenuated leading edge and un attenuated leading edge and un attenuated trailing edge of the fan attenuated trailing edge of the fan beam allows for individual detector beam allows for individual detector calibration during each scancalibration during each scan
Fourth GenerationFourth Generation
Patient dose may be somewhat higher Patient dose may be somewhat higher with fourth-generation scanners because with fourth-generation scanners because of interspace between detectorsof interspace between detectors
When there is an interspace between When there is an interspace between detectors, some x-radiation falls on the detectors, some x-radiation falls on the interspace, resulting in a wasted doseinterspace, resulting in a wasted dose
As the fan beam passes across each As the fan beam passes across each detector, an image projection is acquireddetector, an image projection is acquired
Fourth GenerationFourth Generation
Imaging time is 1s or lessImaging time is 1s or less Various arc scan are available – half Various arc scan are available – half
scan, full scan, over scanscan, full scan, over scan
Electron Beam CT (EBCT)Electron Beam CT (EBCT)
This CT imager was developed This CT imager was developed specifically for fast imagingspecifically for fast imaging
Images can be obtained in less than Images can be obtained in less than 100ms, about the time of a 100ms, about the time of a radiographradiograph
The x-ray source is not an x-ray tube The x-ray source is not an x-ray tube but rather a focused, steered, and but rather a focused, steered, and microwave accelerated electron microwave accelerated electron beam incident on a tungsten targetbeam incident on a tungsten target
EBCTEBCT
The target covers one-half of the imaging The target covers one-half of the imaging circle; the detector array covers the other circle; the detector array covers the other halfhalf
The electron beam is steered along the The electron beam is steered along the curved tungsten target creating a moving curved tungsten target creating a moving sourcesource
There are four targets, or focal tracks, and There are four targets, or focal tracks, and four detector arrays, resulting in four four detector arrays, resulting in four contiguous images simultaneouslycontiguous images simultaneously
EBCTEBCT
Electron beam CT is principally applied to Electron beam CT is principally applied to cardiac imaging and frequently advertised cardiac imaging and frequently advertised as a heart scanas a heart scan
Electron beam CT has no moving partsElectron beam CT has no moving parts Electron beam CT uses a focused electron Electron beam CT uses a focused electron
beam on a tungsten target ring as an x-ray beam on a tungsten target ring as an x-ray sourcesource
Heat dissipation is no problem in EBCTHeat dissipation is no problem in EBCT
EBCTEBCT
Electron beam CT can produce up to Electron beam CT can produce up to eight slices simultaneouslyeight slices simultaneously
Electron beam CT scan times as Electron beam CT scan times as short as 50ms are possibleshort as 50ms are possible
Principal application for EBCT is Principal application for EBCT is cardiac imagingcardiac imaging
Spiral CTSpiral CT
Spiral CT was introduced to clinical Spiral CT was introduced to clinical practice in 1989 and is now the practice in 1989 and is now the standard CT imagerstandard CT imager
If a third or fourth generation is CT If a third or fourth generation is CT imager is caused to continually imager is caused to continually rotate while the patient couch is rotate while the patient couch is moved through the imaging plane, moved through the imaging plane, spiral CT resultsspiral CT results
Spiral CTSpiral CT
The development of slip rings was the The development of slip rings was the technology breakthrough that made spiral technology breakthrough that made spiral CT possibleCT possible
Spiral CT requires slip ring technology for Spiral CT requires slip ring technology for data transfer from the rotating gantrydata transfer from the rotating gantry
Spiral CT requires either an on-board high Spiral CT requires either an on-board high voltage supply so that coiled high voltage voltage supply so that coiled high voltage cables are unnecessary or slip rings for cables are unnecessary or slip rings for high voltage transferhigh voltage transfer
Spiral CTSpiral CT
The principal advantage to spiral CT The principal advantage to spiral CT is the ability to image large volumes is the ability to image large volumes of anatomy in less timeof anatomy in less time
Single breath-hold imaging of the Single breath-hold imaging of the entire torso is possible with spiral CTentire torso is possible with spiral CT
ComparisonComparison
First GenFirst Gen Spiral CTSpiral CT
Scan TimeScan Time 300s300s Less than 1sLess than 1s
Data/imageData/image 60kb60kb 2 Mb2 Mb
Matrix SizeMatrix Size 80x8080x80 1024x10241024x1024
Energy/Energy/imageimage
2kJ2kJ 60kJ60kJ
Slice Slice ThicknessThickness
13mm13mm 1-10mm1-10mm
Spatial Res.Spatial Res. 3 lp/cm3 lp/cm 15 lp/cm15 lp/cm