Computed Tomography

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Computed Tomography

• FRCR Examination
• Craig Moore
• Clinical Scientist
• Radiation Physics Service

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Brief History of CT Scanning
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First CT Scanner - 1972

• 80 x 80 resolution
• 4 min. per rotation
• 8 grey levels
• overnight reconstruction
• dual slice.

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Here and Now

• 512 x 512 or 1024 x 1024 resolution
• 0.4 sec. per rotation
• 4096 grey levels
• Reconstruction16 secs for 30 slices
• 64 slices.

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Basic Principles of CT ScanningWhy CT?

• Conventional radiography suffers from the
  collapsing of 3D structures onto a 2D image
• Although lower resolution than general
  radiography, 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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Collapse of 3D Data into 2D Plane

• Planar imaging
• 2D representation of 3D Distribution of Tissue
• No depth information
• Structures at different depths are superimposed
• Loss of contrast

Object Contrast 41
X rays
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X-ray Tube

• X-rays are produced by firing electrons at a
  metal target
• Beam filtered (0.5 mm Cu and/or 1.3 mm Al
  filters) to optimise spectrum
• Stops attenuation coefficients varying with depth
  via beam hardening

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• Compton scatter predominates over the kV range
  used
• CT is very demanding of X-ray tubes
• Up to 500 mA instantaneously and must provide
  X-rays for 30 secs
• Large heat capacity and fast cooling rates
• Up to 1.4 MHU/min
• Tube rotation causes mechanical stresses (up to
  13 G)

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CT Numbers

• Ideally the linear attenuation coefficient ?,
  calculated by the reconstruction algorithm would
  be absolutely correct
• Coefficients can be regarded as correct in a
  relative fashion only
• Relative accuracy of modern scanners lt 0.1
• A CT number is a relative value of the
  attenuation coefficient

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Window Width and Level

• The appearance of the image on the screen can be
  changed by altering the window width and level
• Window width refers to the range of CT numbers
  selected for display
• This range of CT numbers is centred at a
  particular level called the window level
• e.g. if imaging bone window level should be 1000
• Can spread a small range of CT numbers over a
  large range of grayscale values

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Window Level 593 Window Width 529 Good contrast
in lungs
Window Level 12 Window Width 400 Good soft
tissue contrast
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How do we get the picture?

• Back Projection
• Reverse the process of measurement of projection
  data to reconstruct image
• Each projection if smeared back across the
  reconstructed image

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Back Projection

• Back Project each planar image onto three
  dimensional image matrix

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Back Projection

• Back Project each planar image onto three
  dimensional image matrix

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Back Projection

• Back Project each planar image onto three
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Back Projection

• Back Project each planar image onto three
  dimensional image matrix

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Back Projection

• Back Project each planar image onto three
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Filtered Back Projection

• Back projection produces blurred transaxial
  images
• Projection data needs to be filtered before
  reconstruction
• Different filters can be applied for different
  diagnostic procedures
• Smoother filters for viewing soft tissue
• Sharp filters for high resolution imaging
• Back projection same as before

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Back Projection

• More views better reconstruction
• 1/r blurring, even with infinite number of views

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Filtered Back Projection

• Filter planar views prior to back projection
• Correction of 1/r blurring requires Ramp Filter
• Gives increasing weight to higher spatial
  frequencies
• Amplifies Noise

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Filtered Back Projection

• In Practice
• Use modifications of Ramp Filter
• Compromise between Noise and Spatial Resolution

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Problems with Filtered Back Projection

• Back projection is mathematically correct, but
  real life images require Filtered Back Projection
• Back Projection can introduce noise and streaking
  artefacts
• Not good with attenuation correction
• Filtered Back Projection can reduce noise and
  artefacts, but may degrade resolution

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Xenon v Solid State

• Xenon Single detector chamber sub-divided
  by electrodes
• Solid state Detector array made up of
  individual elements

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Number of detectors and projections

• Typically, for a 3rd generation scanner
• 650 900 detectors
• 1000 to 2000 projections per rotation

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Gantry Rotation

• Data transmission and power cables in old
  scanners meant start stop rotation
• Change direction at end of rotation to rewind
• Series of images acquired by rotating clockwise
  and anti-clockwise
• Scan times as fast as 1 s

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Helical and Multi-Slice Scanning
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Requirements for Helical Scanning

• Slip rings
• Continuous table feed through gantry

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Advantages of Helical Scanning

• Speed
• No need to pause between scans for table movement
• Pitches greater than 1 allowed
• Longer scan lengths within breath hold
• Reduced patient movement artefacts

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Disadvantages of Helical Scanning

• Broadening of Slice profile
• Effective slice thickness increases poorer z
  axis resolution
• Higher noise
• Helical artefacts not seen in axial scanning

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Multi Slice CT
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Each element is 1.25 mm wide
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Beam width is varied
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Physical acquisition
Computer reconstruction
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Single slice
Multi slice
Single slice
Multi slice
3 rotations
One rotation
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Dose and Multi-Slice Scanners

• Considerations similar to those of single slice
  scanners
• Dose utilisation on z axis usually poorer than
  with single slice scanners
• X ray beam width is generally broader than the
  total imaged width
• Geometric efficiency down to 50 for very small
  slice thicknesses (sub mm)

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Geometric Efficiency
Broader beam to negate the effect of penumbra
Extra beam/x geo eff
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Dose and Helical CT

• All helical scanning requires extra irradiation
  at the end of each run to obtain sufficient
  interpolation data to reconstruct the required
  volume
• On multi-slice scanners this extra length can be
  quite long

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Helical multi-slice pitch

• Pitch can be defined with respect to total beam
  collimation (x), or individual detector width (d)
• In practice pitchx is useful when looking at
  patient dose, pitchd for image quality
• Pitchx table travel/xray beam width
• Pitchd table travel/detector width

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Helical Multi-Slice in the UK

• Helical Multi-Slice scanning represent
  significant steps forward in CT
• Better scanning of previous scans
• Expansion of workload
• Nearly all scanners sold in UK are multi-slice
• Technology is still advancing
• 32/40/64/256 slice scanners now available
• More slices in the future?