TY - JOUR
AU - Havlová,, Václava
AB - Abstract Unique 3D tomography apparatus was built and successfully tested in Research Centre Rez. The apparatus allows three-dimensional view into the interior of low-dimension radioactive samples with a diameter up to several tens of millimeters with a betterresolution then 1 mm3 and is designed to detect domains with different levels of radioactivity. Structural inhomogeneities such as cavities, cracks or regions with different chemical composition can be detected using this equipment. The SPECT scanner has been successfully tested on several samples composed of a 3-mm radionuclide source located eccentrically within homogeneous steel bushings. To detect fine cracks inside a small sample, an ultrafine scan of the sample was carried out in the course of 24 hours with a 0.5-mm longitudinal and transverse step and 18° angular step. The exact location and orientation of a fine crack artificially formed inside a sample has been detected. INTRODUCTION Unique 3D tomography apparatus was built and successfully tested in Research Centre Rez(1)—Figure 1. The apparatus allows for 3D view into the interior of low-dimension radioactive samples with the diameter up to several tens of millimeters. Structural inhomogeneities such as cavities, cracks or regions with different chemical composition can be detected using this equipment. The unique collimator design, the use of stepper motors for fine and accurate sample scanning, along with the advanced 3D image reconstruction software developed at Research Centre Rez, enables for achieving resolution better than 1 mm3. Devices working on a similar principle have been used for decades, e.g. in nuclear medicine for the diagnosis of malignant tumors(3), and are increasingly being applied in the nuclear industry(4–8). HARDWARE PART The scanner consists of a rotary sample holder and a scanning device to enable scanning across the transverse plane of the sample and move the gamma detector in the longitudinal direction of the active sample. In 2017, the original G-M counter was replaced by a scintillation probe working in the current mode, i.e. without dead time effect. The REP171-ISD probe is controlled by a processor and contains a CdWO4 cadmium tungstate scintillation detector, photomultiplier, regulated high-voltage source and current/frequency converter. The gamma energy range from 50 keV, the dynamic response of pulse counting is 5 orders without HV switching, the linearity of response is <2%. The CdWO4 scintillator was chosen for its high density (7.9 g cm−2), high atomic number and relatively high light yield. Good detection efficiency for the 60Co gamma radiation is achieved due to the length of the scintillator 30 mm as a result of optimization. The small scintillator diameter of 10 mm and the tungsten shield of 40 mm thickness around the scintillator then allow good ratio of the response from the activated sample to both the gamma background and undesirable scattered gamma radiation from the sample. The gamma ray energy spectrum emitted from the activated sample differs only in intensity and therefore the use of non-spectrometric scintillation probe gives good results. SOFTWARE PART Development of a specialized 3D image reconstruction software based on a method known as the filtered back projection, developed in 1963 by Allan McLeod Cormack, began in 2015. Initially, the software was created for recording signal timing from the detector and for converting this record into sinograms(9,10). Figure 1 Open in new tabDownload slide SPECT scanner—real device manufactured by ALVAT company.(2) Figure 1 Open in new tabDownload slide SPECT scanner—real device manufactured by ALVAT company.(2) The 3D reconstruction of the object space from the sinograms was performed using Python programming language, which allows to obtain a high-quality 3D image(11). FUNCTION In March 2017, when the device was optimally tuned, the 3D scanning was carried out. The SPECT scanner has been successfully tested on several samples consisting of a radionuclide source 137Cs with diameter and height of 3 mm and of 10 MBq activity that was eccentrically stored within homogeneous 10-mm-thick steel capsule(12). To get complex information about 3D distribution of radioactivity within the sample, the sample has to be scanned in several steps. The radiation emitted by the sample is collimated by a series of interchangeable tungsten collimators with the pinhole of different diameter and the aperture profile allows to flexibly optimize the ratio between the required resolution and uncertainty of response depending on the sample activity, time of one point measurement, gamma background and undesirable scattered gamma radiation from sample. Then, the whole sample is rotated by a fixed angle and the transverse motion of the detector is repeated. The sample is gradually rotated by 360° and, thus, the general image can be reconstructed. Each scan (a set of values obtained for one particular sample rotation angle) forms one line of the so-called sinogram—the Fourier image of the scanned slice, in the so-called Radon space (developed by Johan Radon in 1917)(13). The individual rows (for different angles of sample rotation) then form a matrix of values constituting the entire sinogram. Before the inverse Radon transformation of the Fourier images of the scanned object (sinograms) is made, a low-pass filter convolution was used to eliminate the statistical fluctuations of the signal(14). RESULTS Each of the sinograms carries complete information about the scanned cross section of a sample in the Radon space. To get the 3D information about the whole specimen, several series of measurements in different cross sections are necessary. For this purpose, further movement of the detector in the longitudinal direction (perpendicular to the transverse plane of the sample) is required. This information can be reconstructed through a number of different mathematical algorithms. In this case, the algebraic iteration method known as Kaczmarz method(15) and the mathematically more demanding algorithm known as the filtered back projection were used(16). ULTRAFINE SCANNING To detect a fine crack (about 0.5 mm) artificially formed inside a small prism (10 × 10 × 15 mm) of activity 1 MBq of 60Co, the ultrafine scan of the sample was carried out with a 0.5-mm-longitudinal and transverse step and 18° angular step. The scanning lasted for 26 hours. The results can be seen in Figure 2. For our presentation, we have chosen the most appropriate viewing angle in which the area of low activity, possibly indicating a crack or cavity, is best visible. Figure 2 Open in new tabDownload slide ultrafine 3D scan of the small sample. The figure depicts the fine crack artificially formed inside the sample. Darker colors show places of higher source emission rate. The crack appears as a light line marked with white arrows. The grayscale shows the average source emission rate per voxel at each point of the 2D projection, measured across the entire sample. Figure 2 Open in new tabDownload slide ultrafine 3D scan of the small sample. The figure depicts the fine crack artificially formed inside the sample. Darker colors show places of higher source emission rate. The crack appears as a light line marked with white arrows. The grayscale shows the average source emission rate per voxel at each point of the 2D projection, measured across the entire sample. Our reconstruction method allows to display of samples in 3D and partially transparent. The distribution of activity within the sample is represented by scale of different colors. In addition, samples can be rotated in the space and viewed from different sides. Fine structures that are aligned with each other at certain angles of view are mutually reinforcing, while from other angles, they may not be nearly visible. For our presentation, we have chosen the most appropriate viewing angle in which the crack is best visible. Collimators The replacing of the original lead collimator(12) by a set of tungsten collimators of 50–150 mm thickness (Figures 3 and 4), interchangeable according to the type of scanning sample, allowed for a flexible optimization of the ratio between the resolution and signal-to-noise ratio (SNR) depending on the specific physical characteristics of the sample. The new collimators provide a resolution of up to 0.5 mm. Figure 3 Open in new tabDownload slide stacked tungsten collimators, |$\varnothing $| 100 mm. Height of every collimator is 50 mm. Throw the rotary axe of every collimator going pinhole |$\varnothing $| 1 or 2 mm or slit 20 mm × 1 mm. Figure 3 Open in new tabDownload slide stacked tungsten collimators, |$\varnothing $| 100 mm. Height of every collimator is 50 mm. Throw the rotary axe of every collimator going pinhole |$\varnothing $| 1 or 2 mm or slit 20 mm × 1 mm. Figure 4 Open in new tabDownload slide stacked tungsten collimators—one of the six possible complete assemblies, and the scintillation probe with small CdWO4 scintillator. Figure 4 Open in new tabDownload slide stacked tungsten collimators—one of the six possible complete assemblies, and the scintillation probe with small CdWO4 scintillator. Figure 5 Open in new tabDownload slide 3D reconstruction of the volume distribution of the source emission rate inside the rock sample. A side profile (left) and face profile (right) of 3D scan of a granite sample, with a deposited activity of 160 kBq 134CsCl diffusing through the sample from left to right. Resolution: 8 mm3 per voxel, scanning time: 36 hours. We adjust the irradiation time to the dimensions and activity of the scanned samples so that the overall uncertainty does not exceed 3%. The grayscale shows the average source emission rate per voxel at each point of the 2D projection, measured across the entire sample. Figure 5 Open in new tabDownload slide 3D reconstruction of the volume distribution of the source emission rate inside the rock sample. A side profile (left) and face profile (right) of 3D scan of a granite sample, with a deposited activity of 160 kBq 134CsCl diffusing through the sample from left to right. Resolution: 8 mm3 per voxel, scanning time: 36 hours. We adjust the irradiation time to the dimensions and activity of the scanned samples so that the overall uncertainty does not exceed 3%. The grayscale shows the average source emission rate per voxel at each point of the 2D projection, measured across the entire sample. From the point of view of good detection efficiency, it is advantageous to use a collimator as thin as possible so that the detector can be as close as possible to the sample. At the same time, however, it is necessary for the collimator to shield well even the gamma radiation to eliminate the gamma background and undesirable scattered gamma radiation. The calculations performed in the MCNP program have shown that the original lead collimator(12) is sufficient for 137Cs (662 keV) radiation energy, e.g. for 60Co (1333 keV), however, a much more dense material would have been preferable. Of the possible variants, finally, tungsten proved to be ideal (due to its high density and other suitable physical properties) and the most affordable. LAST RESULTS—SCANNING OF A GEOLOGICAL SAMPLE The device also appears promising in terms of the possibility of analyzing rock samples from diffusion experiments, that study radionuclide migration through porous rock media. The geological samples (Aare granite) are used for the study of intergrains pore network of granite in order to describe potential migration of radionuclides within rock matrix. The samples with the size of ϕ50 mm and height 15 mm were obtained from borehole 10.001 in Grimsel test site within Long term diffusion experiment(17). The diffusion experiments are based on emplacing rock samples between inlet and outlet reservoirs, filled with 134Cs solution and groundwater solution respectively. The results are then evaluated based on Fick’s law(18). 3D analysis of the spatial distribution of the source emission rate of the tracer in a geological sample after the realization of the diffusion experiment is shown in Figures 5 and 6. The analysis shows the tracer propagation around the disturbance in the granite or the intergrains permeability of the intact granite(19). Figure 6 Open in new tabDownload slide the plot of distribution of average source emission rate per voxel along the sample axis x. In its marginal portions, there is the background radiation data, and the central 15 mm contains a record of the source emission rate distributions along the x-axis of the sample. Figure 6 Open in new tabDownload slide the plot of distribution of average source emission rate per voxel along the sample axis x. In its marginal portions, there is the background radiation data, and the central 15 mm contains a record of the source emission rate distributions along the x-axis of the sample. CONCLUSION The scanner was primarily designed for the 3D imaging of samples activated at nuclear reactor to activity levels from 1 MBq to 1 TBq and will be in operation within the hot cells at Research Centre Rez. It has been optimized by several 3D scans of the radionuclide source 137Cs 3 mm in diameter and of 10 MBq activity, eccentrically stored within homogenous 10-mm-thick steel capsule. It has been successfully scanned and the 3D distribution of activity was computed. To detect a fine crack artificially formed inside a small prism (10 × 10 × 15 mm) of activity 1 MBq of 60Co, an ultrafine scan of the sample was carried out with a 0.5 mm longitudinal and transverse step and 18° angular step. The scanning lasted 26 hours. The crack has been visible on a 3D scan as a dark blue curve roughly in the middle of the image. It seems that the method does not yet provide sufficient spatial resolution to clearly distinguish places with lower activity from very small areas where the material is missing (cracks and caverns). However, in color display with the possibility to rotate by the 3D model of the sample and view it from different angles, we can at least identify different suspicious areas in the samples. The device also appears promising in terms of the possibility of analyzing rock samples from diffusion cells, where the source emission rate along the sample axis is determined, to examine the water permeability of the sample with appropriate radioactive salt. On a 3D scanner, a geological sample has been evaluated after the diffusion experiment where the rock sample is placed between two reservoirs with a suitable solution. The intake reservoir contains the radioactive tracer (134Cs), the outlet reservoir is inactive. The diffusion is then determined based on the increase in the activity of the outlet reservoir. 3D scanning results allow to monitor the eventual sorption of radionuclides in the sample or identify preferential migration paths. ACKNOWLEDGEMENTS The presented work was financially supported by the Ministry of Education, Youth and Sport Czech Republic—project LQ1603 Research for SUSEN. This work has been realized within the SUSEN Project (established in the framework of the European Regional Development Fund (ERDF) in project CZ.1.05/2.1.00/03.0108 and of the European Strategy Forum on Research Infrastructures (ESFRI) in the project CZ.02.1.01/0.0/0.0/15_008/0000293, which is financially supported by the Ministry of Education, Youth and Sports—project LM2015093 Infrastructure SUSEN and with the use of infrastructure Reactors LVR-15 and LR-0, which is financially supported by the Ministry of Education, Youth and Sports—project LM2015074. P. Zháňal also acknowledges the financial support by the Student Project SVV-2018-260442. Diffusion experiments on rock sample were funded by SURAO within Long term diffusion Phase III. Project. The methods describing radionuclide migration and distribution through granite rock sample have been developed within the project RADEMET (FV30430, Ministry of Trade and Industry). References 1. 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TI - 3D RECONSTRUCTION OF INNER STRUCTURE OF RADIOACTIVE SAMPLES UTILIZING GAMMA TOMOGRAPHY
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncz211
DA - 2019-12-31
UR - https://www.deepdyve.com/lp/oxford-university-press/3d-reconstruction-of-inner-structure-of-radioactive-samples-utilizing-PU79DXA6Bz
SP - 1
VL - Advance Article
IS - 
DP - DeepDyve
ER -