Computed Tomography

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Computed Tomography
CT, CAT tomos slice, graphein to write

• Magdalena Bazalova

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1. What is a CT scanner?

• an X-ray device capable of cross-sectional
  imaging
• creates images of slices through the patient

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What is a CT scanner?

• doughnut shaped gantry with moving patient table

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Why CT?

• conventional radiography suffers from collapsing
  of 3D structures onto a 2D image
• although the resolution of CT is lower, it has
  extremely good low contrast resolution enabling
  the detection of very small changes in tissue
  type
• CT gives accurate diagnostic information about
  the distribution of structures inside the body

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CT scanning applications

• very wide ranging good for imaging bone and
  soft tissue
• diagnostic imaging
• radiotherapy planning
• 3D applications

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CT imaging applications
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CT imaging applications
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CT imaging applications
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Why CT for radiotherapy?

• Radiation therapy planning is done on the basis
  of patient CT images and is therefore patient
  specific
• the target and organs at risk are delineated in
  CT images (possibly with help of other imaging
  modalities PET)
• dose calculation algorithms use CT images for
  determination of dose delivered to the patient
  during treatment

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Why CT for radiotherapy?

• Tissue inhomogeneities can be taken into account
  in most treatment planning systems
• Dose to soft tissue is different than dose to
  cortical bone - mass density variations between
  tissue types are the most important factor
• Therefore, mass densities of tissues have to be
  known for an accurate dose calculation
• CT images do not represent mass densities of
  patient body directly but they can be converted
  into mass densities using a calibration curve

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2. CT scanner components
X-ray tube
X-ray beam
detector ring
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X-ray journey
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X-ray tube
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Beam shaping filter
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Detectors
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Detector arrangement
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Philips CT simulator
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Questions on CT apparatus

• How do we call the device that produces X-ray
  beam?
• (X-ray tube ?)
• What have the X-rays pass through on their way to
  the detector ring?
• (beryllium window, Al filters, bow-tie filter,
  patient, anti-scatter grid)

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3. CT image definition and formation
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What are we measuring in CT?

• the linear attenuation coefficient, µ, between
  the X-ray tube and
• the detector
• the linear attenuation
• coefficient is a measure
• of how rapidly are X-ray
• attenuated

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2D-projection data set - sinogram

• projections

• I, I0 - intensities

-d
x-ray source
-d
d
d
Projection angle
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Reconstruction algorithms

• Computer based
• simple back-projection
• filtered back-projection
• iterative techniques

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Simple back-projection

• reverse the process of measurement of projection
  data to reconstruct an image
• each projection is uniformly distributed across
  the reconstructed image

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Simple back-projection
1/r blurring
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Filtered back-projection

• simple back-projection produces blurred images
• projection data need to be filtered before
  reconstruction
• different filters can be applied for different
  diagnostic purposes
• smoother filters for viewing soft tissue
• sharp filters for high resolution images
• back-projection is the same as before

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Filtered back-projection
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Image reconstruction
Simple back-projection
FBP
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Patient image reconstruction
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Patient filtered back-projection
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CT number scale
HU represents the linear attenuation of a
material.
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CT number window
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CT number window
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CT for radiotherapy calibration, HU to mass
density conversion

• HU do not represent mass density, needed for dose
  calculation, directly. To obtain mass densities
  of each voxel
• A set of tissue equivalent
• materials with known
• mass densities is scanned
• and a calibration curve
• is created

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Calibration curve for treatment planning
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Questions on reconstruction

• How do we call picture and volume elements?
• (pixels and voxels)
• What do CT images represent?
• (linear attenuation coefficients of voxels)
• How do we call raw detector data?
• (a sinogram)
• Name two reconstruction techniques?
• (simple and filtered back-projection)

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4. CT technology
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Third generation CT scanners
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Fourth generation CT scanners
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Fifth generation CT scanners
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Helical CT scanning
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Advantages of helical mode
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Questions on CT technology

• How many CT generations exist?
• 5 (maybe more)
• Which one is the third one?
• rotate/rotate
• What are the advantages of helical scanning?
• arbitrary image position, faster scanning

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5. CT image quality
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Image noise
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Image noise
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Image contrast
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Image contrast
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Factors affecting image noise
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Reconstruction filters
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Factors affecting detector signal
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Radiation dose
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Questions on image quality

• Name three factors that influence image quality.
• kVp, mA, time, filteration of the beam, slice
  thickness, reconstruction filter, pitch
• Name three parameters that describe image quality
• spatial resolution, contrast, noise
• What is noise?
• variation in HU in a uniform image
• What is contrast
• ability to resolve details without blurring

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6. Artefacts in CT
If not recognized, CT artifacts can cause
misdiagnosis and incorrect outcomes of
radiotherapy treatment planning.
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Definition of CT artefacts
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Types of CT artefacts
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Origin of artefacts
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Beam hardening cupping
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Beam hardening correction
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Beam hardening correction
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Reduction of streaks and bands
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Partial volume artefacts
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Partial volume artefacts
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Minimizing partial volume artefacts
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Photon starvation
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Avoidance using mA modulation
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Metal artefacts
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Metal artefact reduction
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Patient motion artefacts

• Voluntary and involuntary motion cause artefacts
  in the reconstructed image

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Minimizing motion artefacts
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Motion artefact correction
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Detector sensitivity ring artefacts
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Ring artefacts
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CT artefacts summary
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Questions on CT artifacts

• Remedy for photon starvation?
• mA modulation, adaptive filtering
• What can cause metal artifacts in patients?
• dental implants, surgical clips, electrodes,
  prostheses
• In which CT generation can occur ring artifacts?
• in the 3rd where detector ring rotates with X-ray
  tube