Measuring optical density in 3-D using optical tomography –
application to radiotherapy treatment planning
Nikola Krstajić, Simon J. Doran
Department of Physics, University of Surrey, Guildford, GU2 7XH, United
Kingdom
email: n.krstajic@surrey.ac.uk
Abstract. Since the discovery of X rays radiotherapy has had the same aim: to deliver
a precisely measured dose of radiation to a defined tumour volume with minimal
damage to surrounding healthy tissue. Recent developments in radiotherapy such as
intensity modulated radiotherapy (IMRT) can generate complex shapes of dose
distributions. However, until recently it has not been possible to verify that the
delivered dose matches the planned dose. This is slowly becoming a real possibility
with three-dimensional radiation dosimeters which have been developed since the
early 1980s. Most chemical formulations involve a radiosensitive species immobilised
in space by gelling agent. Magnetic Resonance Imaging (MRI) and optical computed
tomography have been the most successful imaging modalities so far applied to gel
scanning. Optical techniques rely on gels changing optical density once irradiated.
Parallel beam optical computed tomography has been developed at the University of
Surrey since the late 1990s. The apparatus involves light emitting diode light source
collimated to a wide (12cm) parallel beam. The beam is attenuated or scattered
(depending on the chemical formulation) as it passes through the gel. Focusing optics
project the beam onto a CCD chip. We describe the apparatus and report the recent
results including calibration, measurement noise and discuss issues regarding
reconstructing optical density in 3-D. The apparatus uses the simplest form of optical
tomography directly equivalent to X-ray computed tomography and is a promising
readout method for 3-D radiation dosimetry.
1. Introduction
3-D radiation dosimetry (another term used is gel dosimetry) is relatively new field with the
clear aim to help radiotherapy treatment planning especially in new techniques such as
intensity modulated radiotherapy (IMRT). These techniques are capable of generating
complex shapes of radiation dose distributions. For example, in case of head and neck
cancer one would want a torus shaped dose distribution that spares the spinal cord, but
attacks the tumour surrounding it. In order to verify and plan sophisticated radiation dose
delivery one would ideally want a material that: 1) responds to radiation as the target human
tissue (i.e. it is tissue equivalent), 2) integrates dose across its volume and 3) allows for
quick, precise and accurate readout of 3-D dose distribution. These are broadly the
characteristics of 3-D dosimeters which have been developed during the last 20 years ([1-3]).
Although a number of different imaging modalities (magnetic resonance imaging (MRI),
optical-CT, X-ray CT and ultrasound) have been suggested for the readout of information
from 3-D dosimeters, to date only MRI and laser based optical-CT have been characterized
in detail. Reviews of all techniques can be found in the proceedings from recent DOSGEL
conferences [4-7]. This abstract describes the initial steps we have taken in establishing
CCD based optical-CT as a viable alternative for 3-D radiation dosimetry. The main issue to
bear in mind is that 3-D dosimeters are designed so that their optical density (OD) changes
linearly with absorbed dose. Therefore, mapping spatial distribution of OD is equivalent to
measuring the dose distribution.
First, we compare OD measurements from a high quality test target and variable neutral
density filter (VNDF). A modulation transfer function (MTF) of individual projections is derived
for three positions of the sinusoidal test target within the scanning tank. Our CCD is then
characterized in terms of its signal-to-noise ratio (SNR). Finally, a sample reconstruction of a
scan of a PRESAGETM 1 dosimeter is given, demonstrating the capabilities of the apparatus.
The work presented follows from our previous efforts [8, 9] and parallels similar analysis
performed on laser based optical-CT systems [10-12].
2. Methods
A schematic of the apparatus is shown in figure 1(a). The only difference from the description
in [8] is the low-noise, air-cooled CCD (Hamamatsu, Orca 1024 BTII, Japan, part C4742-9826KAG). A high quality test target (Edmund Optics, Barrington, NJ, US, part NT54-803) was
used for OD and MTF measurement, and a transmission image of the target is shown in
figure 1(b). All measurements were made in air (as opposed to the matching liquid) and thus
represent a best-case scenario. Prior to saving projections of the test target, a clear light field
was obtained. Thus the “true” OD (provided by the target supplier) can be compared with the
OD obtained from the light field and the test target projection. The line profile across
sinusoidal pattern on the test target projection allows for the MTF to be determined. The
square OD steps on the test target are used to investigate projection image quality by
obtaining the standard deviation of pixels in the projection. To demonstrate the potential of
optical-CT, a PRESAGETM [13] sample was irradiated by 2cm x 2cm, 6MV external photon
beam, with SSD 100cm.
3. Results
Figure 2(a) demonstrates that with high quality test target and a low noise CCD, linear
measurements of the OD are possible directly without the need for calibration curve as in [9],
providing the dark noise is taken into account. The VNDF measurement deviates from high
quality test target due to VNDF manufacturing tolerances (+/-10%). Figure 2(b) shows the
MTF in focus and 100mm away from focus. This suggests that even for large dosimeters,
excellent spatial resolution will be possible., The reason for the deviation of the in-focus
curve from its theoretical optimum is still under investigation. Figure 2(c) shows how the
projection standard deviation (SD) varies with the transmission values. The low values given
by triangles represent the low limit obtained by subtracting two successive test target
projections. The upper values represented by squares are SD measured across 35x35 pixels
region across the OD steps on the test target. Figure 2(d) shows SNR in OD values for the
same projection as in 2(c). The best possible SNR is derived by assuming presence of
photon and readout noise only. It is not achieved for reasons of optical clarity. Figure 3
shows a slice and profile of the PRESAGETM.
1
PRESAGETM is a registered trademark of Heuris Pharma, NJ, Skillman, USA.
a
b
Figure 1 – (a) shows the outline of the apparatus and (b) the test target used, pixel size 0.2mm.
Max possible MTF
In focus MTF
+100 mm defocus
-100mm defocus
high quality
test target
VNDF
a
b
SD measured
theoretical minimum SD
theoretical maximum SNR
SNR measured
c
d
Figure 2 – (a) demonstrates the capability of the CCD to measure OD directly, (b) gives MTF for
three positions of the test target. (c) and (d) contrast the best possible performance in terms of
SD of projections and SNR in OD with the measured values. SNR is simple ratio, i.e. not in
decibels.
a
b
Figure 3 – (a) shows a reconstruction slice of an irradiated PRESAGE TM sample and (b) gives a
line profile across the reconstruction over the line indicated in (a). Slice thickness is 1.1mm
and the pixel size is 0.38mm x 0.38mm.
4. Discussion
The detailed projection analysis is justified since the noise performance of the reconstruction
depends solely on noise performance of the individual projections and the number of views
[14]. While the CCD technology is maturing [15], there are big developments in CMOS
imaging chips with higher dynamic range (via so called “active pixels”), faster scan times and
cheaper price [16].
5. Conclusion
High quality and low noise CCDs bring superior performance in OD measurements. They
allow measurements with high dynamic range in OD, excellent linearity and low readout
noise. The main problem in the context of optical-CT is that the low noise performance is not
reached because of optical clarity.
6. Acknowledgments
The research was supported by UK EPSRC studentship. NK thanks the Institute of Physics,
Optical Physics Group, for generous financial support in attending the Photon06 conference.
We wish to thank Dr John Adamovics, Heuris Pharma, Skillman, NJ, USA, for providing the
PRESAGETM sample. We wish to thank as well Dr David Bonnett, Medical Physics
Department, Maidstone Hospital, Kent Oncology Centre, Maidstone, UK for irradiating the
PRESAGETM.
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