Computed Tomography

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Chapter 3

• Computed Tomography
• Stewart C. Bushong

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CT Gantry

• 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)

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CT Gantry

• 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)

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CT Gantry

• 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

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CT Gantry

• 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)

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CT Gantry

• 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

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CT Gantry

• 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

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Coronal Sinus CT
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The X-ray Source

• 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

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Thick Anode
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X-Ray Tube Components
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The X-Ray Source

• 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

13
Anode Heel Effect http//learntech.uwe.ac.uk/radio
graphy/RScience/diag_xray_tube/anode_heel.htm
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Anode Heel Effect http//learntech.uwe.ac.uk/radio
graphy/RScience/diag_xray_tube/anode_heel.htm

• 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

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Anode Heel Effect http//learntech.uwe.ac.uk/radio
graphy/RScience/diag_xray_tube/anode_heel.htm

• 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

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The X-Ray Source

• 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)

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The X-Ray Source

• 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

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Dual Focus Cathode http//learntech.uwe.ac.uk/radi
ography/RScience/diag_xray_tube/components_cathode
.htm
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The X-Ray Source

• 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

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The X-Ray Source

• 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 x-ray tube is
  energized continuously
• Each pass of a fourth generation fan beam over a
  detector produces an image projection

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The X-Ray Source

• 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

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The X-Ray Source

• A bow tie filter is often used to even
  radiation intensity at the detector array

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High Voltage Generator

• 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

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High Voltage Generator

• 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

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High Voltage Generator

• 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

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High Voltage Generator

• 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/medcyclopae
  dia/medical/Volume20I/SLIP20RING20TECHNOLOGY.AS
  P

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High Voltage Generator

• Essentially all CT imager now use high frequency
  generators
• Three phase power was used until the mid 1980s
• The high frequency generator can be positioned on
  the rotating gantry with the x-ray 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

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High Voltage Generator

• 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
• High frequency generator voltage generation
  eliminated the need for massive high-voltage
  transformers

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Detector Array

• 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

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Detector Array

• 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

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Detector Array

• 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

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Detector Array

• 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

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Detector Array

• 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

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Detector Array

• 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

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Detector Array

• 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

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Detector Array

• 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

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Detector Array

• 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

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Detector Array

• Multiple detector arrays allow simultaneous
  imaging of two or more slices

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Collimator Assembly

• There are two collimator in CT pre-patient 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

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Collimator Assembly

• 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)

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Collimator Assembly

• 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.

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Collimator Assembly

• 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)

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Collimator Assembly

• As the slice thickness is changed so is the voxel
  size

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Collimator Assembly

• 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

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Collimator Assembly

• 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

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Collimator Assembly

• Please refer to page 31 of your textbook for a
  nice example of how all the CT Gantry parts fit
  together.

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Sources

• Computed Tomography physical principles
  Seeram
• Helical Scanning Blanck
• Introduction to Computed Tomography Romans
• Computed Tomography Bushong
• http//www.impactscan.org/slides/xrayct/index.htm
• http//learntech.uwe.ac.uk/radiography/RScience/di
  ag_xray_tube/d_xray_contents.htm