Gamma-ray Computed Tomography Research Articles

Examination of corrosion under insulation using gamma ray computed tomography

In this paper, we propose a screening and scanning approach using gamma ray transmission measurements and computed tomography (CT) to detect and locate corrosion areas in a pipeline. Screening obtains interval measurements of collimated gamma ray along the length of pipe, while scanning gives cross sectional images at the investigated area. The CT scanner consists of a clamped on mechanical gantry, a 1” × 1” NaI(Tl) scintillation detector, 11mCi Ba-133 source, 15mm x 5mm lead collimators and Ludlum scaler ratemeter. This technique was shown to be an effective way to determine the wall loss and erosion areas of at 2.5mm thickness. Several locations of an insulated pipe were screened and scanned. Using a portable gamma ray CT scanner we present the results, where the cross sectional image can indicate the occurrence of corrosion under insulator.

Development of data acquisition and control software for gamma-ray process tomography system

Data acquisition (DAQ) and control software has been developed for gamma-ray process tomography system in Malaysian Nuclear Agency. The gamma-ray computed tomography (CT) system uses Computer Automated Measurement and Control (CAMAC) standard for nuclear counting and data acquisition instead of the common Nuclear Instrumentation Module (NIM) standard. For scanning configuration, the system consists of an array of NaI(Tl) scintillation detectors that moves in a small degree of angle by using a stepper motor. The gamma-ray CT software comprises of graphical user interface (GUI) for the system automation that includes CAMAC modules setup, motor and scanning control, data acquisition from CAMAC crate controller via ethernet interface and data recording. The software is developed by using National Instruments LabView development package.

Simulation of a Gamma-Ray Computed Tomography System Using Two Radioisotopes for Structural Inspections: A Preliminary Study

Gamma-ray computed tomography (CT) has widely been used to inspect cracks, obstructions, and fluid flow in pipes. Previous studies have shown that the inspection accuracy is limited when materials in the pipe have a similar density. To overcome this drawback, we investigated a method to discriminate small density differences using a dual-energy gamma-ray CT. In this paper, the performance of a gamma-ray CT system was simulated using Geant4 Application for Tomographic Emission with three types of sources (Am-241, Co-57, and Cs-137). A polyvinyl chloride (PVC) pipe containing oil and sludge was modeled as a phantom. Sparsely acquired data were reconstructed using filtered back-projection and iterative total variation (TV) denoising algorithms. Each material discrimination procedure was performed with a postreconstruction technique. Our results clearly showed that the gamma-ray CT imaging with Am-241 and Co-57 sources and the TV denoising reconstruction algorithm can be used to inspect the internal structures based on discriminated images. The calculated areas of discriminated oil and sludge were within an error margin of 2% (oil: 78.4% and sludge: 21.6%) compared with the simulated phantom (oil: 80% and sludge: 20%). For both gamma sources, the calculated contrast-to-noise ratio (CNR) values with the TV denoising algorithm were the highest values for all structures (Am-241/Co-57: 18.03/28.95 for oil versus sludge, 41.67/31.53 for oil versus PVC pipe, and 23.64/31.53 for sludge versus PVC pipe). The feasibility of our approach and results indicates it usefulness for industrial fields that require structural inspections.

Gamma-ray CT from incomplete projections for two-phase pipe flow.

A low-energy low-dose γ-ray computed tomography (CT) system used in the gas-liquid two-phase pipe flow measurement has been studied at Tianjin University in recent years. The γ-ray CT system, having a third-generation X-ray CT scanning configuration, is comprised of one 300mCi 241Am source and 17 CdZnTe detector units and achieves a spatial image resolution of about 7 mm. It is primarily intended to measure the two-phase pipe flow and provide improvement suggestions for industrial CT system. Recently we improve the design for image reconstruction from incomplete projection to optimize the scanning parameters and reduce the radiation dose. First, tomographic problem from incomplete projections is briefly described. Next, a system structure and a hardware circuit design are listed and explained, especially on time parameter setting of the pulse shaper. And then a detailed system analysis is provided in Section II, mainly focusing on spatial resolution, temporal resolution, system noise, and imaging algorithm. Finally, we carry on necessary static and dynamic experiments in a full scan (360°) and two sets of partial scan reconstruction tests to determine the feasibility of this γ-ray CT system for reconstructing the images from insufficient projections. And based on an A-variable algebraic reconstruction technique method, a specially designed algorithm, we evaluate the system performance and noise level of this CT system working quantitatively and qualitatively. Results of dynamic test indicate that the acceptable results can be acquired using a multi-source γ-ray CT system with the same parameters when the flow rate is less than 0.04 m/s and the imaging speed is slower than 33 frames/s.

Acoustic fluidized bed hydrodynamics characterization using X-ray computed tomography

Acoustic vibration in a fluidized bed can be used to enhance the fluidization quality of particulate matter; this particular noninvasive method provides no internal changes to the bed material structure. However, due to the complexity of this multiphase flow system, characterizing the hydrodynamics of a fluidized bed has become critical in understanding this system behavior. The local void fraction behavior in a cold flow 3D fluidized bed with and without acoustic intervention is investigated in this research. Several noninvasive techniques like gamma-ray computed tomography (GRT), X-ray computed tomography (XCT), or electrical capacitance tomography (ECT), can be used to study the void fraction distribution in multiphase flow systems. In this study, XCT imaging is used to determine the time-average local void fraction or gas holdup. Experiments are implemented in a 10.2cm ID fluidized bed filled with glass beads or ground walnut shell, having a material density of 2500kg/m3 and 1440kg/m3, respectively, and particle size ranges between 212 and 600μm. In this study, three different bed height-to-diameter ratios are examined: H/D=0.5, 1 and 1.5. The loudspeaker’s frequency, used as the acoustic source, is fixed at 150Hz with a sound pressure level of 120dB for glass beads, and 200Hz and 110dB for ground walnut shell. Local time-average gas holdup results show that the fluidized bed under the presence of an acoustic field provides a more uniform fluidization, the bed exhibits less channeling, and the jetting phenomena produced by the distributor plate is less prominent when compared to no acoustic field. Thus, acoustic intervention affects the local hydrodynamic behavior of the fluidized bed.

Computed Tomography to Estimate the Representative Elementary Area for Soil Porosity Measurements

Computed tomography (CT) is a technique that provides images of different solid and porous materials. CT could be an ideal tool to study representative sizes of soil samples because of the noninvasive characteristic of this technique. The scrutiny of such representative elementary sizes (RESs) has been the target of attention of many researchers related to soil physics field owing to the strong relationship between physical properties and size of the soil sample. In the current work, data from gamma-ray CT were used to assess RES in measurements of soil porosity (ϕ). For statistical analysis, a study on the full width at a half maximum (FWHM) of the adjustment of distribution of ϕ at different areas (1.2 to 1162.8 mm2) selected inside of tomographic images was proposed herein. The results obtained point out that samples with a section area corresponding to at least 882.1 mm2 were the ones that provided representative values of ϕ for the studied Brazilian tropical soil.

A local reconstruction method for low-energy gamma-ray computed tomography system

In the application of computed tomography (CT) system to industrial multiphase flow visualization, the number of projections is usually limited, and the detector elements are sparsely arranged, the limited number of measurement data has put the validity of the conventional reconstruction algorithms in jeopardy. Although improved iterative reconstruction methods have been developed to compensate for the lack of projection data, the imaging quality is still inadequate for the reconstruction of relatively complicated flow regime. To overcome this problem, we propose a novel local reconstruction method based on the Landweber algorithm and apply it to a low-energy gamma-ray CT system for two-phase flow measurement. The object to be reconstructed is partitioned into a number of sub-regions and local reconstruction is implemented on each sub-region utilizing only gamma-rays passing through that region. The feasibility is demonstrated in both numerical simulations and prototype experiments. Comparison between the proposed method and the Landweber iterative algorithm shows that the former has obviously improved the imaging quality.

Design of a modular signal processing board (MSPB) for gamma-ray imaging applications

Abstract In this paper a new modular signal processing board (MSPB) for high-resolution gamma-ray computed tomography (GCT) is presented. The MSPB is optimised for parallel signal processing of eight detector channels operating in pulse counting mode. Signal processing stages comprise of variable gain amplifiers, pulse height discrimination stages, 13-bit counters with corresponding latches as well as logic circuitry for coordinated data transfer with a multitude of MSPBs. The digital signal processing units are realised in commercially available complex programmable logic devices (CPLD). Each MSPB is addressable by an 8-bit DIP-switch, which allows the use of up to 256 modules or 2048 detector pixels within one detector system. The geometry of the MSPB allows a multiple and seamless detector module arrangement, which eases the adaptation of a given gamma-ray detector system to specific industrial and laboratory applications. The choice of the electronic devices and the thermal design was optimised for low power consumption in order to minimise internal heat production, which would affect the characteristics of the detector's intrinsic gain strongly. Thermal measurements have been executed to prove the functionality of the thermal design.

Void measurement using high-resolution gamma-ray computed tomography

We present a high-resolution gamma-ray computed tomography (CT) measurement system for the determination of cross-sectional time-averaged void distributions in thermo hydraulic facilities. The system has been carefully designed for harsh operating conditions, such as varying temperature fields and strong magnetic fields, typically produced by thermal hydraulic test loops with direct electric bundle heating. Measurements are non invasive, thus the two-phase flow in the test section is not influenced. The gamma-ray CT system consists of a collimated 137Cs isotopic source, a gamma radiation detector arc including 320 single elements, a pulse processing unit and a thermal stabilisation unit. The spatial resolution of the CT system is about 2 mm in plane. Recently, the thermal design of the detector arc is improved to secure maintenance of constant temperature of thermally sensitive components under changing environmental conditions. This turned out to be a key issue for achieving accurate quantitative measurements. First results of laboratory measurements on a bundle mock-up with this improved system are presented.

Study of beam width on reconstructed image contrast using the first generation gamma ray CT

Different factors may influence the image quality of the first generation computed tomography (CT) system single-source-single-detector. One of the factors in the improvement of the reconstructed image quality is the related characteristics of the beam width. These characteristics depend on the shape of collimator and dimension of collimation apertures. For studying the effect of beam width an industrial CT system was designed and developed. The CT scanner consists of a 5.08 cm NaI(Tl) detector in diameter and a 137Cs (30 mCi) radioactive source. The position of phantom was defined by three motors. The CT scans were taken out by scanning 180° to collect attenuation beams. The images are reconstructed from the measured projections by the filtered back projection method to perform the inverse Radon transform. In this study we have investigated the beam width on reconstructed image contrast. To go through the process, several experiments were performed with different collimation apertures. Finally, the contrast of different images is compared by computing the RMS contrast of each image.

Gamma ray computed tomography to evaluate wetting/drying soil structure changes

Wetting and drying (W–D) cycles can cause strong modifications of the structure of a soil, especially in pore distribution, which reflects the temporal and spatial distribution of soil water and, consequently, these processes can affect soil water and nutrient retention and movement. These alterations have important practical consequences when calculating soil water storages and matric potentials, widely used in irrigation management. The present paper has as objective to use gamma ray computed tomography (GCT) as a tool to investigate possible modifications in soil structure induced by W–D cycles and to analyze how these alterations can affect soil water retention. The GCT scanner used was a first generation system with a fixed source–detector arrangement, with a radioactive gamma ray source of 241Am. Soil samples were taken from profiles of three different soils characterized as Xanthic Ferralsol (Fx), Eutric Nitosol (Ne) and Rhodic Ferralsol (Fr). Eighteen samples (50cm3), six from each soil, were submitted to none (T0), three (T1) and nine (T2) wetting/drying cycles. Based on image analysis it was possible to detect modifications in soil structure in all samples after wetting/drying cycles for all treatments. Tomographic unit profiles of the samples permitted to identify an increase on soil porosity with the increase in the number of wetting/drying cycles and it was also possible to quantify the average porosity values. The statistical test (Duncan test) indicates that there are significant differences between treatments for all samples at the 5% significance level.

Soil bulk density evaluation by conventional and nuclear methods

Among the methods used to measure soil bulk density, the following have been prominent: paraffin sealed clod (PS), volumetric ring (VR), and the modern methods like gamma ray computed tomography (GCT) and the neutron/gamma surface gauge (SG). The objective of this work was to compare soil bulk density values obtained through these methods, with the aim of assisting researchers on the choice of the more appropriate method. For this, a 200-m spatial transect was chosen in an experimental area cultivated with coffee, belonging to ESALQ/USP, Piracicaba, SP, Brazil. The SG readings were first taken in the field and thereafter soil samples were collected at 8 different points, spaced at 25 m, for the other methods. The lowest values of soil bulk density were obtained for the SG method (average 1.468 g/cm3) and the highest for the PS (average 1.685 g/cm3), which was similar to the GCT method (average 1.684 g/cm3). The average soil bulk density for the VR method, which has been used in soil science as a standard method, was 1.544 g/cm3. The Tukey test indicates that the PS and GCT methods do not differ significantly (P > 0.05). They do differ in comparison with VR and SG, which also do not differ among themselves.

QUALITATIVE AND QUANTITATIVE ANALYSIS OF SOIL SAMPLES BY COMPUTERIZED TOMOGRAPHY

There are several methods to measure soil bulk density (rs) such as the paraffin sealed clod (PS), the volumetric ring (VR), the computed tomography (CT), and the neutron-gamma surface gauge (SG). In order to evaluate, in a non-destructive way, the possible modifications in soil structure caused by sampling for the PS and VR methods of rs evaluation we proposed to use the gamma ray CT method. A first generation tomograph was used, consisting of a 241Am source and a 7.62 cm by 7.62 cm NaI(Tl) scintillation crystal detector coupled to a photomultiplier tube. Results confirm the effect of soil sampler devices on the structure of soil samples, and that the compaction caused during sampling causes significant alterations in soil bulk density. Through the use of CT it was possible to determine the level of compaction and to make a detailed analysis of the distribution of the soil bulk density within the soil sample. Key words: soil structure, soil bulk density, sampling, radioisotopes application

Quantitative γ-ray computed tomography of the radius in normal subjects and osteoporotic patients

A new design of gamma-ray computed tomography (CT) scanner was used to measure bone density in 516 normal adults and in 116 female patients with osteoporotic fractures. Trabecular bone density (TBD) was measured for a core of the ultradistal radius; cortical bone mass per unit length (MLS) and mass per unit volume (MVS) were computed for the radius shaft. TBD, MLS and MVS peaked at the age of about 40 in females and fell by 32%, 15% and 24%, respectively, by the age of 65. In males, TBD decreased from the third decade and MLS and MVS peaked in the fourth decade. The corresponding falls by 65 years were 20%, 6% and 7%. In patients with Colles' fracture TBD and MLS were reduced by 13% and 10%, respectively, relative to age matched normal female subjects. For patients with vertebral crush fracture the reductions relative to age matched normal females were 36%, 17% and 16% for TBD, MLS and MVS. In fracture patients the deficit in TBD was significantly greater than that for cortical bone but TBD was no better at discriminating normal and osteoporotic individuals because of its larger normal range. We conclude that gamma-ray CT may best be used for monitoring changes in bone rather than as a diagnostic screening tool.