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.