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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 fourthgeneration 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

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
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
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
Second Generation
 Approximately,
30 s imaging time
 Head and body imager
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

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
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
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
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 unattenuated leading edge and un
attenuated trailing edge of the fan
beam allows for individual detector
calibration during each scan
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

Fourth Generation
 Imaging
time is 1s or less
 Various arc scan are available – half
scan, full scan, over scan
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
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

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 xray source
 Heat dissipation is no problem in EBCT

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
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
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

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
Comparison
First Gen
Spiral CT
Scan Time
300s
Less than 1s
Data/image
60kb
2 Mb
Matrix Size
80x80
1024x1024
Energy/imag
e
Slice
Thickness
Spatial Res.
2kJ
60kJ
13mm
1-10mm
3 lp/cm
15 lp/cm