Optical-CT scanning of polymer gels

Abstract

The application of optical-CT scanning to achieve accurate high-resolution 3D dosimetry is a subject of current interest. The purpose of this paper is to provide a brief overview of past research and achievements in optical-CT polymer gel dosimetry, and to review current issues and challenges. The origins of optical-CT imaging of light-scattering polymer gels are reviewed. Techniques to characterize and optimize optical-CT performance are presented. Particular attention is given to studies of artifacts in optical-CT imaging, an important area that has not been well studied to date. The technique of optical-CT simulation by Monte-Carlo modeling is introduced as a tool to explore such artifacts. New simulation studies are presented and compared with experimental data.

Figure 1

A schematic diagram of a prototype optical-CT scanner. The mirrors translate left to right to obtain projections of the optical attenuation through the gel as described in the text. (From Gore et al 1996)

Figure 2

Optically scanned 2D dose distribution of irradiated polymer gels (a) the calculated dose map of a cylindrical sample of radius 10cm in which four rectangular fields of different doses were placed; (b) the relationship of optical attenuation to dose. (From Gore et al 1996)

Figure 3

Optical density (turbidity) spectra of BANG gels irradiated to different doses. This gel formulation contained 50% weighting of Bis to acrylamide. (From Maryanski et al 1996)

Figure 4

The dose dependence of the refractive index of BANG polymer gels. This gel formulation contained 50% weighting of Bis to acrylamide. (From Maryanski et al 1996)

Figure 5

Axial images through the same plane of the same gel-flask irradiated with 3 radiosurgery beams (a) MRI image of the R2 distribution, (b) optical-CT image of optical attenuation coefficient. The corresponding profiles along the dashed lines are shown in (c) and (d) respectively. (From Oldham et al 2001)

Figure 6

Illustration of the use of polymer gel-dosimetry in conjunction with optical-CT scanning to verify a 3 isocenter radiosurgery treatment. The left image is the computer modeled dose distribution (20, 50 and 80% isodose lines). The right images shows the corresponding dose map obtained from the optical-CT scanner. (From Oldham et al 2001)

Figure 7

Optical-CT images of the relative dose-distribution delivered in an 8 cm diameter polymer gel after irradiation by focused 40–80 keV x-rays. The in-plane resolution was 1×1 mm² and the images were taken 1 mm apart. The comprehensive 3D measurement presented here would be very difficult to achieve with conventional dosimeters because of the low energy of the radiation and the small dimensions of the field. (From Oldham et al 2003)

Figure 8

Superposition of pre- and post-irradiation optical-CT images of two polymer gel needle phantoms. (A) a high dose of radiation was delivered by a circular 1.5 cm diameter field of 6 MV photons axially through the flask. (B) a rind of dose was delivered by a 5 cm circular field containing a 2 cm central circular lead block. The irradiated regions appear only lightly attenuating because the image is windowed to show needle positions. In reality the irradiated gel was highly attenuating. Irradiated regions are highlighted with dashed line. (From Oldham et al 2004)

Figure 9

(a) Curved slab has same refractive index as surrounding gel, (b) uniform refractive index, (b) curved slab has 2% higher than the refractive index of the surrounding gel.

Figure 10

(a,b) 12 cm diameter gelatin finger phantom containing four variably attenuating dyed-blue gelatin fingers. (c,d) optical-CT images of the same plane through the finger phantom but acquired with different laser step spacing across each projection. Both scans consisted of 100 projections at 1.8 degrees. The laser step sizes across each projection were 2 mm (c) and 0.5 mm (d). When the laser is stepped coarsely, the fingers appear narrower and less attenuating in places due to under-sampling. (From Oldham et al 2003)

Figure 11

Reconstructed image for 6×6 cm² field irradiation. The gel appears less attenuating in a cross shaped region aligned with the diagonals across the field. The cross only appeared in highly scattering gel where the maximum OD was >2.5. Xu et al discuss the possibility that the artifact might arise from the detection of scattered light although the specific mechanisms were not addressed. (From Xu et al 2003)

Figure 12

Top and angled view of the optical-CT simulation geometry. The laser source (right side) and 2×2 cm detector (left side) are visible at opposite sides of the water bath (green square cube of side 20×20 cm). A PET cylindrical gel-flask of radius 8.6 cm is visible submerged in the water-bath. The blue cube located centrally in the flask represents the region of gel irradiated with a square 5×5 cm field of 6 MV radiation. The refractive indices of water-bath fluid, PET flask, unirradiated gel, and irradiated gel were 1.358, 1.52, 1.36, 1.36. Scattered light for 100 photon histories at an arbitrary laser position incident centrally on the waterbath are illustrated. The dimensions and materials are given in the text.

Figure 13

(a) Optical Monte Carlo simulation of an optical-CT gel-dosimetry scan of a 5×5cm square field irradiation inside a cylindrical BANG gel-dosimeter of radius 8.6 cm. The simulation geometry is given in figure 12. The gel was set to be purely absorbing with no scattering. The projections were truncated to 15 cm to minimize wall effects. The greybar scale indicates reconstructed attenuation values per pixel. (b) profiles across the square field in (a) showing excellent reconstruction of the uniformly attenuating square field. The blue profiles are horizontal and vertical profiles through the center of the field. The red outer profile is along the diagonal as illustrated.

Figure 14

(a) Optical Monte Carlo simulation of an optical-CT gel-dosimetry scan of a 5×5 cm square field irradiation inside a cylindrical BANG gel-dosimeter of radius 8.6 cm. This time full Mie scattering was assumed in the irradiated square field portion of the gel. (b) profiles across the square field in (a) showing non-uniform reconstruction of attenuation coefficients. As in figure 13, the inner blue profiles are horizontal and vertical profiles through the center of the field. The red outer profile is along the diagonal. Scattering artifacts are clearly visible as the depressed coefficients centrally in the field.