s optical ct scanning of presage tm polyurethane samples with a ccd-based readout system s j doran...
TRANSCRIPT
SOptical 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 Surrey2Heuris Pharma, Skillman, NJ
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
Scanner schematic
Hglamp
Cylindrical lens, pinhole and filter pseudo
point-source
Lens parallel beamScanning tank with matching medium
Exposed gel
Unexposed gel Diffuser screen on which real shadow image forms
CCDdetector
Standard 50mmcamera lens
PC with frame-grabber card
Turntable controlled by acquisition computer via stepper motors
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.
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.
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.
*LED light source
Scanning tank
CCD chip
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
Top view Cross-section
X-ray beam~30 Gy at lead surface
2 mm diameter holes
PRESAGETM
Results on PRESAGETM samples
• Results of this first test show early promise, but highlight a number of problems
resolution is excellent (> 7 pixels across 2 mm spot)
contrast / artifact ratio is poor
difficult to achieve good contrast in original projections for these very small features
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.
Discussion (2): Sensitivity
• CCD tomography scanner originally designed for use with Fricke gels.
• PRESAGE < 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.
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
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
Characterisation of scanner (3)
• Replacing the diffuser screen dramatically reduced the ring artefact, as expected.
Reconstructed slice from experiment without diffuser screen
Corresponding surface plot
Residual artifacts are due to the problem of an “infinite absorber”
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
Characterisation of scanner (4)
• From a knowledge of the true profile, we can estimate the MTF of the system by constrained deconvolution.
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.