Modified optical computed tomography scanner for low temperature scanning

Large radiochromic hydrogels, such as ferrous xylenol formulations require several hours to warm from the storage temperature of 278 K to 293 K for optical CT scanning and irradiation. This warm-up time can be avoided if gels are scanned at 278 K, resulting in a more practical 3D dose measurement. In this study a Vista 16 optical CT scanner was modified to allow scanning at 278 K, below the temperature where condensation forms on the aquarium windows. The refractive index matching liquid was first cooled to 276K in order to cool the aquarium as it was filled. The modification involved pumping cool, dry air into the aquarium scanner compartment to provide a positive pressure, preventing condensation on the aquarium windows. A warming rate of 2 K per hour was achieved with passive cooling which relied on the liquid-filled aquarium’s thermal mass. The radiochromic reaction rate decreased with temperature, requiring an increased post-irradiation wait time from 1 hour at 293 K to 1.5 hours at 278 K. In addition, initial sample transmission was greater by avoiding the auto-darkening that occurred as the samples were warmed from 278 K to 293 K over several hours. This increase in transmission resulted in a larger dynamic range for the dosimeter system.


Introduction
Radiochromic hydrogel dosimeter formulations such as ferrous xylenol orange (FX) and leuco crystal violet surfactant (LCV) are stored around 278 K to minimize darkening due to auto-oxidation.Before irradiation they are warmed to a uniform temperature, typically 293 K.For larger dosimeters, such as 15 cm diameter cylinders, this requires more than 5 hours in a water bath.A previous investigation reported reducing this time to 3 hours by active heating with a near infrared incandescent lamp [1].If the gel dosimeter could be optically scanned and irradiated at 278 K this would reduce the time of the overall experiment.It would also result in higher transmission samples prior to irradiation since considerable darkening occurs during the 5-hour warm-up period.Higher pre-irradiation transmission results in a larger dynamic range for dosimeter system.In the case of the FX dosimeter, lowering the temperature also reduces the diffusion rate, which may be an additional advantage of low temperature scanning [2,3].Early investigations, in our laboratory, revealed that condensation on the aquarium windows, at the dew point temperature, was a problem for optical computed tomography (CT) scanning.Practically, this limited scans to temperatures of 283 K or greater.Also, some of the refractive index matching dye solutions that were optimized at 293 K would separate phases at lower temperatures.The earliest laser scanners were slow requiring several minutes per slice.Scan times in excess of one hour were typical.Maintaining a fixed low temperature for longer times was also another level of complexity.For these reasons, low temperature scanning was abandoned in favour of 293 K as a standard.
The Vista16, optical cone beam scanner, from ModusQA, can acquire a scan of 1000 projections in less than 5 minutes.If this scanner can be modified for 278 K scanning, then several hours per experiment could be saved, making 3D dosimetry of large gel dosimeters more practical.This investigation successfully scanned gels at 278 K with minor modifications to a Vista16 scanner which involved precooling the refractive index matching liquid and providing a positive pressure to the interior of the scanner with cool dry air to avoid condensation on the aquarium windows.

Methods
The Vista16 scanner has an aquarium which provides a 16 cm field of view and has a volume of 7.3 litres.The aquarium is constructed of aluminum face and top plates and plastic sides and bottom.It is aligned with the central compartment by sitting on 3 steel pins.Leaving a 0.5 mm gap between the aquarium top plate and scanner housing.There are 3 mm air spaces between the aquarium window face plates and aluminum bulkheads of the central compartment.Insulating foam sheets, 1 cm thick, were taped to the plastic sides and bottom to test if there was an increase in thermal stability.Drain ports in the bottom of the compartment were sealed with tape.Two ports were drilled to attach plastic tubing for pressurizing the compartment.The tubing was positioned to direct cool dry air into the 3 mm air spaces along the entrance and exit windows of the aquarium.Compressed lab air flowed through a heat exchanger constructed with 5 mm OD copper tubing (length = 6.3 m) coiled to fit into an insulated box and packed with ice.After the heat exchanger the tubing was connected to a water trap and routed through 6.5 mm OD nylon tubing to the Vista16 compartment.Pressure was verified by sensing cool air flow exiting the compartment above the aquarium windows.
The refractive index matching liquid was cooled to ~276 K prior to filling the aquarium.After cooling the aquarium, the liquid temperature stabilized at 278 K. Gel samples were transferred from the storage refrigerator to the aquarium for the pre-irradiation reference scan.The liquid temperature was monitored for stability.Following the reference scan, a jar filled with ice-water was substituted for the gel in the aquarium to provide passive cooling while the gel was being irradiated.Samples were irradiated in a 278 K water tank, with a 30 cm circular 60 Co beam at a source to surface distance of 60 cm.Maximum dose was 1 Gy, to ensure sufficient transmission for post irradiation scan.After a wait period for the radiochromic reaction to complete the gel was reinserted in the aquarium for the post scan.The laboratory temperature was maintained at 293 K.
The gel formulation (FX) consisted of 5% gelatin, 60 mM sulfuric acid, 0.5 mM tannic acid, 0.3 mM ferrous ammonium sulfate and 0.05 mM xylenol orange.Custom, cylindrical, polyethylene terephthalate vessels of 10 and 15 cm diameter were used for this study.

Results
Without cool, dry air to pressurize the scanner's aquarium compartment, condensation developed on the 278 K aquarium windows during summer months when the ambient building humidity was greater.Pressurizing the compartment had no impact on the aquarium warming rate.The insulating foam which covered only the plastic sides and base of aquarium also had no impact on the warming rate.The substitution of a jar of ice-water for the gel sample, when not scanning, was sufficient to maintain aquarium temperature at 278 K for greater than one hour.Sample transmission near the optic axis was 5% initially and 2.5% 85 minutes after 1 Gy irradiation relative to the liquid outside the vessel.A sequence of data scans was recorded to determine when the FX radiochromic reaction was completed.It was found that 1.5 hours was required to reach 95% completion at 278 compared to 1 hour at 293 K.While not the focus of this study, the auto-darkening of the samples was markedly reduced.Once the aquarium was filled with the 276 K solution the temperature stabilized at 298K after quickly cooling the aquarium.Without additional cooling the aquarium temperature increased 2 K per hour.The substitution of a jar of ice-water when not scanning was sufficient to maintain aquarium temperature at 278 K.
The transmission along the 15 cm diameter for a similar sample decreased to 60% compared to initial value when placed in the scanner at 278 K and allowed to warm to 293 K over 6 hours and 1.5 hours before final scan.This decrease in transmission due to auto-oxidation during warm-up and experiment could have been avoided if the 278 K scanning was employed.Figure 1 is a transverse slice from the 3D reconstruction at 1.5 cm depth in gel with dose of ~1 Gy in a 15 cm diameter vessel.

Discussion
When the scanner aquarium was maintained at 278 K, condensation formed on the windows once the humidity level exceeded a threshold value.Specifically, in the warmer summer months the laboratory humidity level was above this threshold.Insulating the plastic sides and base had no effect on the heating rate.In principle, a low temperature aquarium could be designed without metal components and thermally insulated windows.However, increasing the number of glass-to-air interfaces would increase reflections present in the projection data.Degrading the accuracy of the reconstructions.Hydrophobic coatings could be applied to the windows as an alternate method to eliminate condensation.If the scanner was made air-tight and the aquarium re-designed then passively dehumidifying the interior of the scanner would also be an option.
Because the Vista16 scanner requires less than 5 minutes per pre or post irradiation scans, temperature drift during a scan is not an issue, primarily due to the large thermal mass of the 278 K, liquid-filled, aquarium.For this reason, active cooling was not required.Eliminating condensation enabled optical CT scanning at 278 K which avoided the 5-hour gel warm-up period for scanning at 293 K.However, it does introduce additional work and not all irradiation experiments can be easily performed at 278 K. Depending on the dosimeter properties and overall length of an experiment, there could be situations which warrant developing an active cooling approach.For example, flowing liquid from a larger insulated reservoir through the aquarium between scans could be implemented.For this particular gel formulation, the radiochromic reaction rate decreased with lower temperature.With the practical consequence of increasing the post irradiation time required for the reaction to reach IOP Publishing doi:10.1088/1742-6596/2630/1/0120204 95% or greater completion.An additional 0.5 hours was required to exceed this 95% completion period.This slower reaction rate negates the potential advantage of a lower diffusion rate [2].Note, the tannic acid slowly reduces the ferric ions to ferrous, manuscript in preparation.The net effects of the tannic acid are increased time to maximum radiochromic response and lower auto-darkening.The main advantage of 278 K scanning of FX gels is the dramatic reduction in auto-darkening.Previously, the low initial optical transmission of FX gels has limited their use to samples smaller than 10 cm diameter.Coloured, index matching solutions have been used to compensate for the low initial transmission of the 10 cm samples.But often the coloured solution introduces drifts in transmission that become artefacts to the data sets.In this work, 15 cm diameter samples were scanned at 590 nm with a dose of 1 Gy.Typically, a 15 cm diameter, 278 K, FX gel will decrease in transmission more than 60% from initial placement in the 293 K scanner until 6 hours later when it has reached thermal equilibrium.By increasing the initial transmission of the gel samples at scan time, more dynamic range is available for writing dose distributions.

Disclosure
K Jordan has a licensing agreement with ModusQA related to dosimetry applications of optical computed tomography.

Figure 1 .
Figure 1.Optical CT reconstruction of scan at 278 K; transverse slice at 1.5 cm depth, 1 Gy dose, custom vessel diameter 14.7 cm.