Evaluation of the Performance of the Fast Scanning Platform of an OCT SystemMalcolm Heard1, Miguel Herrera1, Geoffrey Ibbott1

1Department of Radiation Physics, The University of Texas, M.D. Anderson Cancer Center, Houston, Texas

Introduction

Optical CT systems (OCT) are used to readout the dose distributions measured by 3D dosimeters. The system measures the change in optical density of irradiated dosimeters. The first model of the OCT system developed by MGS Research Inc. employed a series of mirrors and a stepping motor to translate the laser across a dosimeter to acquire a projection [1]. An additional stepping motor would turn the dosimeter after the projection was complete. This process would repeat until adequate projections were acquired about 360°. Recently, MGS Research Inc. made available a new, fast scanning model that used a different technique to acquire a projection. The laser is translated across the sample using a rotating mirror which significantly reduces the time to acquire a projection. Other modifications of the system include the uses a system of Fresnel lenses to focus the attenuated laser onto a diffuser. A photodetector was placed immediately behind the diffuser to measure the laser intensity. The new fast scanning platform was recently installed at the Radiological Physics Center (RPC). While the advantages of the upgrades are clear, it is necessary to demonstrate the performance of the system is not compromised.

Materials and Methods

The reproducibility of the original configuration of the OCT system was determined by imaging the same plane of a polymer gel dosimeter repeatedly over a 3 hour period. Percent difference images were computed by comparing the first image with the other images that were acquired. In addition, the uncertainty of the OCT system in measuring optical density values was determined from the mean value of the percent difference images. The same procedure was used to evaluate the fast scanning platform.

Results

Discussion

Due to differences in scan speed, the fast-scanning platform was able to acquire a significant larger number of images during a 3 hour period. For both platforms, there was not a decline in the performance of the scanner over time.

Comparisons between images of the rod phantom acquired with OCT and x-ray CT show very little difference. Overlapping of the images revealed no discrepancies in the rod position larger than 1 mm throughout the entire volume.

At each dose level, the OD values measured with the 180° mode were within one standard deviation of the OD values measured with the 360°. As shown in Figure 7, the uncertainties at low doses (therefore, low OD) increase. The larger uncertainties are attributed to concentric rings that appear in these images. Improvements in the reconstruction algorithm are needed to remove these structures from images. At the 1 Gy dose level, the uncertainty is lower when using the 360° mode which suggest that the 360° mode is preferable for low optical density dosimeters.

Comparison between the 180° and 360° for gels irradiated with steep dose gradients also show very little difference. Agreement between the profiles shown in Figure 8 is better than 5% for 97% of the points. The 80%-20% penumbra width is within 1 mm for both scanning modes.

The fast-scanning platform of the OCT system shows consistent performance with the previous model. The system is capable of significantly reducing scan time making it possible to image an entire 3D volume in 1.5 hours.

References1. Gore, J.C., M. Ranade, M.J. Maryanski, and R.J. Schulz,

Radiation dose distributions in three dimensions from tomographic optical density scanning of polymer gels: I. Development of an optical scanner. Physics in Medicine & Biology, 1996. 41(12): p. 2695-704.

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Fig. 1 Picture of the OCT system with the fast-scanning platform.

The spatial accuracy of the system were also evaluated in this study. A gelatin phantom 17 cm in diameter was manufactured and 4 Teflon® PFA rods 1/8” in diameter were positioned within the phantom. The phantom was imaged with x-ray CT and OCT. The two datasets were registered using external fiducials and overlapped to identify differences in the locations of the rods.

The fast scanning platform has two modes of scanning, a 360° mode and a 180° mode. The 180° mode reduces the scan time in half by acquiring half the number of projections. This study evaluated the differences in the performance of the system when using 180° mode versus the 360° mode. A batch of polymer gels irradiated to doses from 1 Gy up to 6 Gy were scanned using both modes. Dose response curves were plotted for the gels. In addition, polymer gels were irradiated with a half-blocked field to create a steep dose gradient and images were acquired using both modes. Profiles were taken across the steep dose gradient to evaluate the spatial accuracy of both modes.

Fig. 2 X-ray CT image of the phantom used to evaluate the spatial accuracy of the fast-scanning platform.

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Fig. 3 Percent difference between the first scan and successive scans acquired over a 3 hour period using the original configuration of the OCT system.

Fig. 4 Percent difference between the first scan and successive scans acquired over a 3 hour period using the fast-scanning platform of the OCT system.

The uncertainty in measuring optical densities was 1.0% for the original configurations and 0.9% for the fast-scanning platform.

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Fig. 5 Images of the 4 rod phantom acquired with OCT and x-ray CT.

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Fig. 6 Dose response of a polymer gel scanned with the 180° and 360° modes of the fast scanning platform. The error bars represent one standard deviation.

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Fig. 7 Relative uncertainty in the determination of the OD using 180° and 360° mode of the fast scanning platform.

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Fig. 8 Profiles through gels irradiated with half-blocked field and scanned with the 180° and 360° modes of the fast scanning platform.