Operational Modes

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


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


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


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


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


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

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Operational Modes