PRESAGE polyurethane samples

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15 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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S

Optical CT scanning of
PRESAGE
TM

polyurethane samples
with a CCD
-
based readout system




S J Doran
1*
, N Krstajic

1
, J Adamovics
2

and P M Jenneson
1


1
Department of Physics, University of Surrey

2
Heuris 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 PRESAGE
TM

samples


Characterisation of scanner and future prospects


Scanner schematic

Hg

lamp

Cylindrical lens, pinhole
and filter


pseudo
point
-
source

Lens


parallel beam

Scanning tank with
matching medium

Exposed gel

Unexposed gel

Diffuser screen on which real
shadow image forms

CCD

detector

Standard 50mm

camera 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 PRESAGE
TM
, 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




PRESAGE
TM

samples supplied as cylinders of
radius ~7 cm


Two experiments undertaken


2.7 x 2.7 cm
2

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

PRESAGE
TM

Results on PRESAGE
TM

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
PRESAGE
TM
. 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.



m
PRESAGE
<
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 PRESAGE
TM

characteristics


Increase digitisation depth of camera (10


ㄶ⁢楴猩s
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 PRESAGE
TM

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 PRESAG
TM
.