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Optical CT scanning of PRESAGETM polyurethane samples with a CCD-based readout system

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Collimation arrangement. Scanning tank. Irradiation and imaging of ... Introduction of a properly designed collimation system greatly improves performance. ... – PowerPoint PPT presentation

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Title: Optical CT scanning of PRESAGETM polyurethane samples with a CCD-based readout system


1
Optical CT scanning of PRESAGETM polyurethane
samples with a CCD-based readout system S J
Doran1, N Krstajic 1, J Adamovics2 and P M
Jenneson1 1Department of Physics, University of
Surrey 2Heuris Pharma, Skillman, NJ
2
Structure of talk
  • Basic design of optical CT scanner
  • New design features of scanner (including works
    in progress)
  • Light source
  • Collimation arrangement
  • Scanning tank
  • Irradiation and imaging of PRESAGETM samples
  • Characterisation of scanner and future prospects

3
Scanner schematic
4
Disadvantages of existing design
  • Mercury lamp is too weak
  • Light intensity is split between three colours.
  • Fricke gels are moderately absorbing even before
    irradiation.
  • Light is at wrong wavelength for scanning
    PRESAGETM, which was optimised for a HeNe laser
    (632 nm).
  • Diffuser screen wastes light and introduces
    noise
  • Introduction of a properly designed collimation
    system greatly improves performance.
  • Noise from diffuser screen was coherent between
    projections and gave rise to ring artifact.
  • Perspex is not a suitable material for the end
    walls
  • New design will use higher quality optical glass.

5
Solutions (1) Light source
  • Light source changed to ultra-bright LED
  • Considerable investigation of different options
    (expanded laser beam, laser fibre optic, more
    powerful discharge lamp)
  • New LED products have appeared within the last
    year.
  • Range of wavelengths available one can pick one
    to suit any of the current families of gel.

6
Solutions (2) Collimation
  • New lens introduced at the exit of the tank
  • Light still passes through the sample as a
    parallel beam.
  • Second lens focuses light down to an appropriate
    acceptance angle for the camera lens.
  • Standard 50 mm camera lens focuses light onto CCD
    chip.

7
Sample irradiation
  • PRESAGETM samples supplied as cylinders of radius
    7 cm
  • Two experiments undertaken
  • 2.7 x 2.7 cm2 square field
  • grid irradiation using purpose built lead
    collimator

X-ray beam 30 Gy at lead surface
2 mm diameter holes
PRESAGETM
Cross-section
Top view
8
Results on PRESAGETM samples
  • Results of this first test show early promise,
    but highlight a number of problems
  • resolution is excellent (gt 7 pixels across 2 mm
    spot)
  • contrast / artifact ratio is poor
  • difficult to achieve good contrast in original
    projections for these very small features

9
Discussion (1) Artefacts
  • Artefacts are currently the limiting factor in
    both resolution and dose sensitivity.
  • Slice thickness was very large (2 cm) in images
    presented.
  • Ring artefact These initial experiments were
    performed on the original system with diffuser
    screen.
  • Solution With the modified system,
    significantly better results will be obtained.
  • Refraction artefacts These have two causes, poor
    mixing of the matching liquid and (possibly)
    internal inhomogeneities of the PRESAGETM. CCD
    optical-CT appears more sensitive to these than
    laser optical-CT.
  • Solution Commercial matching liquids under
    investigation
  • Absolute attenuation coefficients Current camera
    is not yet calibrated over full dynamic range.

10
Discussion (2) Sensitivity
  • CCD tomography scanner originally designed for
    use with Fricke gels.
  • m PRESAGE lt m Fricke for a given dose.
  • At present, light needs to pass through a
    relatively large path-length of gel to obtain a
    measureable effect
  • Solutions (both can be attempted in parallel)
  • Modify PRESAGETM characteristics
  • Increase digitisation depth of camera (10 ? 16
    bits) once the artifact problem has been solved.

11
Characterisation of scanner (1)
  • In order to investigate the resolution and 3-D
    capabilities of the scanner, a phantom was made
    from wires (c.f. needle phantom of Oldham et al.)

Raw projection
Sinogram
Single slice
3-D reconstruction
12
Characterisation of scanner (2)
  • A series of experiments investigated the effect
    of the diffuser screen and the system resolution.

Reconstructed slice from experiment with diffuser
screen
Corresponding surface plot
13
Characterisation of scanner (3)
  • Replacing the diffuser screen dramatically
    reduced the ring artefact, as expected.

Residual artifacts are due to the problem of an
infinite absorber
Reconstructed slice from experiment without
diffuser screen
Corresponding surface plot
14
Characterisation of scanner (4)
  • A profile through the thinnest wire demonstrates
    the extent of the ring artifact problem.

Original arrangement with diffuser
New arrangement with no diffuser
15
Characterisation of scanner (4)
  • From a knowledge of the true profile, we can
    estimate the MTF of the system by constrained
    deconvolution.

16
Conclusions
  • We have performed the first CCD-based optical-CT
    scans of the new PRESAGETM dosimeter.
  • We have identified a number of issues that need
    addressing before this method will yield
    high-quality results.
  • However, these problems appear to be soluble
    using available technology.
  • The excellent resolution and speed of the CCD
    optical-CT system should be compatible with the
    excellent dosimetric properties of PRESAGTM.
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