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Chapter 3
Computed Tomography
Stewart C. Bushong
CT Gantry
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Every CT imager has three
distinguishing components – the
operating console, the computer, and
the gantry
The operating console performs two
major functions – imaging control
with pre-selected technique
conditions and image viewing and
manipulation (window/level)
CT Gantry
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There may be several operating consoles,
each dedicated to a separate function,
such as CT control or post-processing and
image analysis (3D, diffusion/perfusion
analysis, cardiac scoring, measurements,
region of interest)
The CT computer has no physically
distinguishing features (it typically looks
like any other computer)
CT Gantry
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The CT computer has high capacity and is
very fast due to the large number of
computations required on an extensive
data set – e.g. if there are 750 detectors
and 1500 projections are acquired in 360
degrees of rotation that would equal
1,125,000 samples (750 x 1500) for EACH
SLICE!!!! Each image at a 1024 x 1024
matrix requires approximately 2 MB of
memory
CT Gantry
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Some CT imagers have the computer
built into the operating console
Computers capable of
multiprocessing are used in CT
(multiprocessing means that each
processing unit works on a different
set of instructions to increase speed
or computing power)
CT Gantry
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Multiprocessing allows a computer to
perform several functions at the same
time, which reduces reconstruction time
and increases capacity
The gantry is special to CT. It houses the
x-ray source, the detector array, the
collimator assembly and a generator.
Sometimes the generator is attached to
the rotating framework along with the
tube and detectors. Other times the
generator is positioned on the floor of the
gantry and does not rotate
CT Gantry
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The patient aperture of a CT gantry has a
diameter of approximately 70 cm.
The CT gantry can be tilted in a cephalic
or caudal angle plus or minus 30 degrees.
The capability to tilt is especially useful for
extremity imaging and facial imaging.
E.g. by having a patient lie prone with
their head extended, coronal images of
the sinuses may be obtained
Coronal Sinus CT
The X-ray Source
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CT imaging places two demands on
an x-ray tube – high x-ray intensity
and rapid heat dissipation.
High x-ray intensity is accomplished
with a high mA generator and a
generous focal spot size, up to 2mm
Rapid heat dissipation is provided by
large diameter, thick anode disks
rotating at 10,000 rpm
Thick Anode
X-Ray Tube Components
The X-Ray Source
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X-ray tubes developed for CT have
very high heat capacity
Anode heat capacity of 6 MHU
(million heat units) are common.
That compares to less than 1 MHU
for general radiography.
The anode-cathode axis is
perpendicular to the patient axis to
avoid the heel effect
Anode Heel Effect
http://learntech.uwe.ac.uk/radiography/RScience/diag_xray_tube/anode_heel.htm
Anode Heel Effect
http://learntech.uwe.ac.uk/radiography/RScience/diag_xray_tube/anode_heel.htm
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Referring to slide # 13 “Close examination
of the x-rays emitted from the target
shows that because they are produced
below the surface they have to pass
through some tungsten before they can
escape from the tube.”
“ X-Ray A has to pass through a much
greater thickness of anode material before
escaping from the x-ray tube”
Anode Heel Effect
http://learntech.uwe.ac.uk/radiography/RScience/diag_xray_tube/anode_heel.htm
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“X-ray B only has to pass through a
small amount of tungsten”
“As the angle of the anode is
increased, the anode heel effect
increases”
The X-Ray Source
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Computed tomography x-ray tubes
have high speed (10,000) rpm rotors
X-ray tube failure is the principle
cause of CT imager malfunction
X-ray tube current of 200 to 800 mA
are common. Too low mA can result
in unacceptable image noise (caused
by a lack of sufficient x-rays striking
the detectors)
The X-Ray Source
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X-ray tube potential is usually 120 kVp to
140 kVp three phase of high frequency
Such high kVp is used for higher intensity
and penetrability, and therefore, less x-ray
tube loading and lower patient dose.
Dual focus tubes are common, usually
having .5 and 1.0 mm focal spots, with
the smaller focal spot used for better
spatial resolution
Dual Focus Cathode
http://learntech.uwe.ac.uk/radiography/RScience/diag_xray_tube/components_cathode.htm
The X-Ray Source
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The improved spatial resolution does not
result from projection geometry as in
radiography, rather from better x-ray
beam – radiation detector collimation
Still, the principal effect os spatial
resolution is matrix size and field of view
(FOV)
For third generation CT imagers, the x-ray
source is pulsed. Each pulse creates an
image projection from each detector
The X-Ray Source
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When pulsed, up to 100 mA is used
with pulse widths of 1 to 5 ms at
pulse repetition rates of 60 Hz
For fourth generation imagers the xray tube is energized continuously
Each pass of a fourth generation fan
beam over a detector produces an
image projection
The X-Ray Source
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Computed tomography x-ray beam
are filtered to harden the beam and
make it more unifrom at the detector
array
Filtration produces a higher energy,
more homogeneous x-ray beam and
reduces the beam hardening artifact
A shaped x-ray beam filter is used in
CT to produce a more uniform
intensity at the detector array
The X-Ray Source
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A “bow tie” filter is often used to
even radiation intensity at the
detector array
High Voltage Generator
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High kVp is used to minimize
photoelectric absorption and,
therefore, patient dose
High kVp is used to reduce bone
attenuation relative to soft tissue
allowing a wider dynamic range of
the image
High kVp is used to increase
radiation intensity at the detector
array
High Voltage Generator
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High kVp is used to reduce x-ray tube
loading, and thereby , extend tube life
Three phase or high frequency voltage
generation is used for CT imagers
Three phase voltage is usually generated
by a stand alone module near the gantry.
Cables that will only wind 360 degree
must be used, causing a reversal of gantry
position
High Voltage Generator
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High frequency generators are small
enough that they can be mounted on
the rotating gantry
Heat units and joules are equivalent
measure of energy
Slip rings make possible continuous
rotation of the x-ray source leading
to spiral CT
High Voltage Generator
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Slip rings incorporate circular electrical
conductors, one type of which rotates and
passes power to the high-voltage
generator; the other passes signals from
the data acquisition system to the
computer: further explanation can be
found at
http://www.amershamhealth.com/medcyc
lopaedia/medical/Volume%20I/SLIP%20RI
NG%20TECHNOLOGY.ASP
High Voltage Generator
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Essentially all CT imager now use high
frequency generators
• Three phase power was used until the mid
1980’s
The high frequency generator can be
positioned on the rotating gantry with the xray source
The high frequency generator can be
positioned on the fixed part of the gantry and
connected to the x-ray source through slip
rings
High Voltage Generator
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The DAS is located between the
detector array and the computer
The DAS
• Amplifies the detector signal
• Converts the analog signal to digital
• Transmits the digital signal to the
computer
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High frequency generator voltage
generation eliminated the need for
massive high-voltage transformers
Detector Array
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The evolution of the CT radiation detector
has progressed with continuous
improvements
Detector efficiency is important because it
determines maximum tube loading and
controls patient dose
Three important features of the detector
array are efficiency, number of detectors,
and detector concentration
Detector Array
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Early CT imager used a scintillation
crystal photomultiplier tube as a
single element detector
A grouping of detectors is called a
detector array
There are two types of detector
arrays- gas filled and solid state
Detector Array
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Gas filled detectors – high pressure
xenon – have very fast response and
no afterglow but only about 50%
detection efficiency
Gas filled detectors can be packed
more tightly than solid state
detectors with less interspace septa
Most solid state detectors today use
a scintillator, cadmium tungstate,
optically coupled to a photodiode
Detector Array
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Solid state detectors have nearly 100%
detection efficiency but cannot be tightly
packed
The detector array consists of many
individual detector fashioned as a module
that are positioned on a receptor board for
easy exchange and service
A gas filled detector array uses small ion
chamber filled with high-pressure xenon
or other gas
Detector Array
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Each ion chamber is about 1 mm wide
with essentially no interspace
The geometric efficiency – the percent
area of the detector array that is detector,
not interspace – is more than 90%
The intrinsic detection efficiency for high
pressure xenon is approximately 50%
Total detector efficiency = geometric
efficiency x intrinsic efficiency
Detector Array
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Solid state detectors are made of a
scintillation crystal, which when
irradiated emits light that is
converted to an analog signal by a
photodiode
Solid state detectors have
approximately 90% intrinsic
detection efficiency. Essentially, all
incident x-rays are detected
Detector Array
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Total detection efficiency depends on
the number of detectors and how
tightly they are packed
When there is interspace between
detectors, detection efficiency is
reduced and patient dose increased
Eighty percent total detection
efficiency is common for solid state
detector arrays
Detector Array
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Solid state detectors are
automatically recalibrated between
scans
Solid state detectors are more
expensive than gas-filled detectors
and their increased efficiency can
result in less x-ray tube loading,
reduced image noise and reduced
patient dose
Detector Array
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The DAS is positioned just after the
detector array to amplify each signal,
convert each signal to digital form,
and properly sequence each signal to
the computer
Multiple detector array allow the
collection of two or more image data
sets simultaneously
Multiple detector arrays can reduce
the heat loading of the x-ray tube
Detector Array
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Multiple detector arrays allow
simultaneous imaging of two or more
slices
Collimator Assembly
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There are two collimator in CT – prepatient and post-patient
The pre-patient collimator is
positioned near the x-ray source
The pre-patient collimator controls
the patient dose and determines the
dose profile
As the pre-patient collimator is
narrowed, patient dose increases and
the dose profile becomes rounded
Collimator Assembly
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pre-patient collimation controls slice
thickness
The dose profile is a plot of dose
across the slice thickness
The dose profile should be square
but is rounded because of scatter
radiation
The post-patient collimator controls
the slice thickness (sensitivity
profile)
Collimator Assembly
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When the post-patient collimators
are narrowed, slice thickness is
reduced
Sensitivity profile is a plot of detector
response versus distance (mm)
The ideal sensitivity profile is square;
in practice, it is rounded because of
scatter radiation.
Collimator Assembly
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pre-patient and post-patient collimators
are controlled together to match dose
profile and sensitivity profile
If dose profile exceeds sensitivity profile,
the patient dose is excessive
If sensitivity profile exceeds dose profile,
image quality is compromised
Nominal slice thickness is controllable
between 1 and 10mm (sub-millimeter
scanning is available on newer multi-slice
system)
Collimator Assembly
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As the slice thickness is changed so is the
voxel size
Collimator Assembly
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Thinner slices are required for rapidly
changing anatomy, for example, the inner
ear
Thinner slices result in improved spatial
resolution
Thinner slices result in higher patient dose
because of increase overlap of slices
When imaging with thin slices they are
usually contiguous so that no tissue is
missed
Collimator Assembly
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High voltage slip rings are oil insulated
and transfer power from an external high
voltage generator to the gantry
Low voltage slip rings are air insulated and
transfer data from gantry to computer
When a spiral (helical) CT is based on low
voltage slip rings, the high voltage
generator is high frequency type and
mounted on the rotating gantry
Collimator Assembly
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Please refer to page 31 of your
textbook for a nice example of how
all the CT Gantry parts fit together.
Sources
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Computed Tomography: physical
principles… – Seeram
Helical Scanning – Blanck
Introduction to Computed Tomography –
Romans
Computed Tomography – Bushong
http://www.impactscan.org/slides/xrayct/i
ndex.htm
http://learntech.uwe.ac.uk/radiography/R
Science/diag_xray_tube/d_xray_contents.
htm