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M. A. Bero, P.M. Jenneson,
W. B. Gilboy, P.M. Glover
and S.J. Doran
Department of Physics,
University of Surrey,
Guildford, GU2 5XH, UK
Faster optical tomography with parallel-beam “white” light for 3-D dosimetry
(a)
Introduction
Light source
Three-dimensional gel dosimetry is a novel method for radiation measurement motivated by the need to
verify experimentally the doses produced by stereotactic and conformal techniques in radiotherapy and by
other applications of radiation dosimetry such as radiobiology and food sterilisation. Until recently,
Magnetic Resonance Imaging (MRI) was the only technique to enable “readout” of a spatially-varying
radiation dose distribution in a non-destructive and non-invasive fashion, with both high resolution and
the ability to make time-course measurements [1]. However, this dosimetric method suffers from the
same limitations that affect MRI generally, namely low signal-to-noise ratio, an often lengthy imaging
time and the need for highly sophisticated and expensive equipment.
In the search for a readout technique that overcame the above drawbacks Gore et al. [2] developed a
novel technique in which radiation dose measurements can be achieved using an optical tomographic
densitometer. The sample used to detect the radiation and “fix” its pattern in space was a
poly(acrylamide) gel as previously introduced for MRI studies [3]. Here we report on a more powerful
approach to optical tomography, made possible by the use of a novel phantom material.
Diffuser
Irradiated
FXG gel
Filter
(b)
3 cm
Computerised data
aqusition and
reconstraction
software
Lens
Matching
solution
Unirradiated
FXG gel
CCD camera
Figure 2: (a) Schematic diagram of the optical tomography scanner. Our current scanner is designed for small-scale imaging, but we are currently
modifying the design to enable us to image head-sized phantoms to make the system truly competitive with MRI dosimetry. (b) The CCD camera
element at the heart of the scanner.
Results from the prototype scanner
Advantages of the new gel system
Figure 3(a) shows a selected 2-D slice from the 3-D image data-set obtained from an FXG gel phantom exposed to a
cone-beam of X-rays. The silver X-ray tube target was operated at 50kVp, 0.6mA and the exposure time was about
30 minutes. 128 projections, each consisting of a plane of 128128 pixels, were acquired in just 20 minutes, as the
sample rotated through 180. Projections were reconstructed onto a 1283 matrix using a Shepp-Logan filtered backprojection routine, leading to a 3-D image with isotropic resolution of 0.3 mm. This system is already competitive
with a corresponding MRI scan. However, there are many improvements that can be made to the prototype scanner.
For example, the source of the prominent ring artifacts is known, but the correction method (already implemented on
our X-ray microtomography system) requires a more sophisticated motor system, which we are currently designing.
The goal of this work was to construct an optical absorption
tomography scanner using a polychromatic light source and
to use it for 3D-radiation dosimetry employing a new colourchange radiation-sensitive gel system, Ferrous Xylenol
Gelatin (FXG). In addition to its good sensitivity, this gel is
transparent. This is a major advantage over the
poly(acrylamide) system, since the lack of scatter allows us
to operate in a parallel beam configuration, with consequent
increase in imaging speed.
Secondly, the absorption spectrum (shown below in
Fig. 1) has maxima centred at 430nm, and 530nm [4] and
the relative sensitivity at each of these locations is different.
This means that FXG may be used to perform multi-spectral
optical imaging, with the potential to obtain data over a
much wider range of doses than would be possible at a
single wavelength. This will be key to brachytherapy
dosimetry, where current gel dosimetry methods fail.
Figure 3: (a) Representative 2-D optical absorbance
map, extracted from the complete 128-slice 3-D data-set
for an irradiated cylindrical FXG phantom; (b)
absorbance profile across the sample that showing the
characteristic build-up and exponential decay regions.
The data are currently contaminated with a serious ring
artifact, but this can be corrected by improved
acquisition.
Figure 1: (a) FXG absorbance spectra, for
doses up to ~12Gy; (b) dose response relations
for four different wavelengths.
The optical tomography scanner
The optical CT scanner illustrated in Figure 2, uses an incoherent light source and an interference filter
that can be selected to cover the range of wavelengths at which the scanner is operated. The key novel
feature of our scanning system is to employ a parallel beam of light, obtained using a suitable lens. This
beam is directed onto the gel sample, which is surrounded by a matching solution of suitable refractive
index, and a shadow projection of the irradiated gel sample, consisting of a complete plane, is taken on a
ground glass screen. The light intensity is then recorded on a CCD-camera. The sample motion is
controlled by a stepper-motor and data acquisition and processing are performed using in-house software.
(a)
(b)
Conclusions
We have shown that it is possible to obtain 3D-dose distribution with a rapidity unmatched by either MRI or previous
optical methods. We use a novel parallel-beam optical tomography system that is convenient, easy to construct and
uses low cost and widely available components. “White” light offers the possibility of making multi-spectral images,
that have potential applications in extending the range of doses that may be measured.
Acknowledgements
References
The authors are indebted to Dr. E. J. Morton for his
support of the project and to BBSRC for the grant
under which much of the tomography control
software and reconstruction were written.
[1] M.J. Day, Phys. Med. Biol. 35, 1605, 1990
[2] J.C. Gore et al., Phys. Med. Biol. 41, 2695, 1996
[3] M.J. Maryanski et al., Magn. Res. Im. 11, 253, 1993
[4] M.A. Bero et al., Nucl. Inst. Meth. A422, 617,1999