Optical Computed Tomography System Research Articles

Iterative Image Reconstruction Methodology in Optical CT Radiochromic Gel Dosimetry

Modern advancements in radiation therapy require paralleled advancements in the dosimetric tools used to verify dose distributions. Optical computed tomography (CT) imaged radiochromic gel dosimeters provide comprehensive, tissue equivalent, 3D dosimetric information with high spatial resolution and low imaging times. Traditional CT image reconstruction methods (filtered backprojection) do not account for light refraction within the optical CT system reducing the image quality. Iterative reconstruction methods make use of a system matrix that describes this light refraction thus, improving the reconstructed image quality. However, use of iterative reconstruction methods is not widespread, largely due to the impractical storage size of the required system matrix. Furthermore, current iterative reconstruction methods do not address the issue of image degradation due to a single detector element collecting light from multiple raypaths. For optical CT radiochromic gel dosimetry to be used effectively as a radiation therapy treatment plan verification tool, the system must be both practical and accurate. Thus, this work has two main objectives: (i) reduce the size of system matrices by means of polar coordinate discretization in lieu of the traditional Cartesian coordinate discretization, and (ii) reduce image degradation due to multiple raypaths by a novel approach to populating the system matrix that accounts for multiple raypaths.

MAGIC-f gel dosimeter reading: a comparison between an in-house optical CT and MRI imaging

Introduction: According to new developments in radiation therapy techniques, accurate ‎dose verification in three-dimensions has become more critical. Polymer gel dosimeters ‎‎(PGDs) are valuable tools to be used for this purpose. Nowadays, various imaging ‎modalities are employed to read out the gels. This study was aimed to investigate the ‎measured dose distribution recorded in MAGIC-f PGD with optical computed tomography ‎‎(OCT) by comparison with MRI. ‎Material and methods: We developed an in-house CCD based cone-beam OCT scanner. A ‎phantom of MAGIC-f PGD was used to measure a four-field box dose distribution. MRI and ‎OCT scanners were performed for gel readouts. Both measurement results were compared ‎by gamma index analysis with various acceptance criteria. The temporal stability of the gel ‎was also evaluated with the OCT readout system.‎Results: The percentage of isodose lines from two measured datasets agreed well together. ‎The pass rates were 99.02%, 96.8%, and 89.8% with 5%/5mm, 4%/4mm, and 3%/3mm ‎criteria, respectively, at the phantom's central axial slice. ‎Conclusion: The results indicate that the performance of this OCT system is almost the ‎same with acceptable discrepancies to the MRI as accepted standard readout modality, and ‎it can be used for three-dimensional dose verifications.‎

Stray light in cone beam optical computed tomography: III. Evaluation of a redesigned large-volume commercial scanner based on a convergent light source

Optical cone beam computed tomography (CT), using a digital camera to acquire 2D projection images, provides a fast, mechanically simple method for 3D radiation dosimetry. However, original cone beam designs had poor accuracy as a result of considerable scatter/stray light reaching the camera. Previously, our group presented a redesigned convergent light source for optical cone beam CT that considerably reduced stray light contribution and improved accuracy (Dekker et al 2016 Phys. Med. Biol. 61 2910). Here, we performed an evaluation of a newly updated commercial optical cone beam CT scanner (VistaTM, ModusQA, London, Canada) based on that design. Two different light source configurations were examined: the manufacturer’s default configuration which uses a 10 cm wide, 5 cm high diffuser light source, and a smaller, 1.5 cm diameter diffuser light source that more closely aligns with our previously described design. We imaged large volume (15 cm diameter cylinders) absorbing and scattering solution phantoms as well as a 1.25 cm diameter absorber placed within 15 cm diameter gel-like scattering phantom. Optical CT reconstructions were compared against narrow-beam measurements of attenuation made by placing an aperture in the optical CT system. Our results show that considerable stray light is present when using the manufacturer’s default configuration, as cupping artifacts and large (⩾10%) discrepancies between optical CT and narrow-beam attenuation measurements occur when imaging scattering phantoms. However, when imaging is performed using the 1.5 cm diameter source, optical CT measurements agree with narrow-beam measurements within ∼3% for both absorbing and scattering objects, as well as the small absorber in a scattering medium. Using this light source will require higher optical quality vessels than are currently provided by the manufacturer.

Development of a CMOS-based optical computed tomography system (CMOS-OCT) for 3D radiotherapy dosimetry

Optical Computed Tomography (OCT) was developed as a 3D imaging system for measuring radiotherapy dose delivered to radiochromic dosimeters. The imaging system reconstructs dose distributions delivered to a radiochromic dosimeter such as PRESAGE that changes its colour when irradiated. Current optical CT systems in the market are based on CCD image sensor and have slow imaging speed. The study exploited recent advances in CMOS image sensor (CIS) technology to improve the imaging speed of the OCT system. The study characterised the components in the in-house developed CMOS-based Optical CT (CMOS-OCT) imaging system and investigates the feasibility of the system for imaging 3D radiochromic dosimeters. A green dye solution of various concentration was used to mimic colour variation in a PRESAGE radiochromic dosimeter from its response to radiation dose. The solution was filled in a plastic cylinder to study the linearity and uniformity of the system. A rotary stage was constructed using a stepper motor to hold and rotate the dosimeter between the CIS and a large area LED. The components of the imaging system were integrated and controlled using LabVIEW (National Instrument, Austin, TX). A graphical user interface (GUI) was also developed to acquire projection images of the dosimeter. The measured field of view (FOV) of the CIS is 125 mm by 90 mm that can cover the whole PRESAGE dosimeter. A projection image is captured at every 1.8 degree rotation of the dosimeter, at every second, that amounted to 200 projection images for a 360 degree rotation. Current limitation of the imaging speed is the rotation speed of the motor of 1.8 degree per second which can be improved with upgrading the stepper motor. At an imaging speed only limited by the maximum sensor frame rate of 28.5 fps, the scanning time will be reduced to 7 s compared to 200 s. The results show that the system is capable of capturing the projection images of a 3D translucent object with good linearity and uniformity. Further experiments will be carried out to optimise the development of the system to reconstruct dose distributions in 3D from a PRESAGE dosimeter.

Recent advances in Optical Computed Tomography (OCT) imaging system for three dimensional (3D) radiotherapy dosimetry

Radiotherapy delivery techniques for cancer treatment are becoming more complex and highly focused, to enable accurate radiation dose delivery to the cancerous tissue and minimum dose to the healthy tissue adjacent to tumour. Instrument to verify the complex dose delivery in radiotherapy such as optical computed tomography (OCT) measures the dose from a three-dimensional (3D) radiochromic dosimeter to ensure the accuracy of the radiotherapy beam delivery to the patient. OCT measures the optical density in radiochromic material that changes predictably upon exposure to radiotherapy beams. OCT systems have been developed using a photodiode and charged coupled device (CCD) as the detector. The existing OCT imaging systems have limitation in terms of the accuracy and the speed of the measurement. Advances in on-pixel intelligence CMOS image sensor (CIS) will be exploited in this work to replace current detector in OCT imaging systems. CIS is capable of on-pixel signal processing at a very fast imaging speed (over several hundred images per second) that will allow improvement in the 3D measurement of the optical density. The paper will review 3D radiochromic dosimeters and OCT systems developed and discuss how CMOS based OCT imaging will provide accurate and fast optical density measurements in 3D. The paper will also discuss the configuration of the CMOS based OCT developed in this work and how it may improve the existing OCT system.

Scanning laser optical computed tomography system for large volume 3D dosimetry

Stray light causes artifacts in optical computed tomography (CT) that negatively affect the accuracy of radiation dosimetry in gels or solids. Scatter effects are exacerbated by a large dosimeter volume, which is desirable for direct verification of modern radiotherapy treatment plans such as multiple-isocenter radiosurgery. The goal in this study was to design and characterize an optical CT system that achieves high accuracy primary transmission measurements through effective stray light rejection, while maintaining sufficient scan speed for practical application. We present an optical imaging platform that uses a galvanometer mirror for horizontal scanning, and a translation stage for vertical movement of a laser beam and small area detector for minimal stray light production and acceptance. This is coupled with a custom lens-shaped optical CT aquarium for parallel ray sampling of projections. The scanner images 15 cm diameter, 12 cm height cylindrical volumes at 0.33 mm resolution in approximately 30 min. Attenuation coefficients reconstructed from CT scans agreed with independent cuvette measurements within 2% for both absorbing and scattering solutions as well as small 1.25 cm diameter absorbing phantoms placed within a large, scattering medium that mimics gel. Excellent linearity between the optical CT scanner and the independent measurement was observed for solutions with between 90% and 2% transmission. These results indicate that the scanner should achieve highly accurate dosimetry of large volume dosimeters in a reasonable timeframe for clinical application to radiotherapy dose verification procedures.

Optical Computed Tomography for Polymer Gel Dosimetry

This article describes an optical computed tomography (OCT) for polymer gel dosimetry, focusing on two systems recently constructed by us. The first OCT system, which is categorized as a first-generation system, comprised a single He-Ne laser, photodiode, and mechanical stages for moving and turning gel dosimeter. Projection data per angle are acquired from turned gel dosimeter. In this system, the reconstructed image is obtained using filtered back-projection (FBP) method from projection data. The second OCT system is a 2D-OCT scanner that utilizes a light panel and a camera detector. The dose-response relationship between the optical density and radiation dose reconstructed from the scanning images is comparable to that obtained using magnetic resonance imaging. Although there is much room for improvement in the image artifacts due to reflection and refraction of light and so on, OCT is expected as a modality for the future polymer gel dosimetry as the rapid, high-resolution, highly accurate evaluation tool.

Optical CT imaging of solid radiochromic dosimeters in mismatched refractive index solutions using a scanning laser and large area detector.

The practical use of the PRESAGE® solid plastic dosimeter is limited by the inconvenience of immersing it in high-viscosity oils to achieve refractive index matching for optical computed tomography (CT) scanning. The oils are slow to mix and difficult to clean from surfaces, and the dosimeter rotation can generate dynamic Schlieren inhomogeneity patterns in the reference liquid, limiting the rotational and overall scan speed. Therefore, it would be beneficial if lower-viscosity, water-based solutions with slightly unmatched refractive index could be used instead. The purpose of this work is to demonstrate the feasibility of allowing mismatched conditions when using a scanning laser system with a large acceptance angle detector. A fiducial-based ray path measurement technique is combined with an iterative CT reconstruction algorithm to reconstruct images. A water based surrounding liquid with a low viscosity was selected for imaging PRESAGE® solid dosimeters. Liquid selection was optimized to achieve as high a refractive index as possible while avoiding rotation-induced Schlieren effects. This led to a refractive index mismatch of 6% between liquid and dosimeters. Optical CT scans were performed with a fan-beam scanning-laser optical CT system with a large area detector to capture most of the refracted rays. A fiducial marker placed on the wall of a cylindrical sample occludes a given light ray twice. With knowledge of the rotation angle and the radius of the cylindrical object, the actual internal path of each ray through the dosimeter can be calculated. Scans were performed with 1024 projections of 512 data samples each, and rays were rebinned to form 512 parallel-beam projections. Reconstructions were performed on a 512 × 512 grid using 100 iterations of the SIRT iterative CT algorithm. Proof of concept was demonstrated with a uniformly attenuating solution phantom. PRESAGE® dosimeters (11 cm diameter) were irradiated with Cobalt-60 irradiator to achieve either a uniform dose or a 2-level "step-dose" pattern. With 6% refractive index mismatching, a circular field of view of 85% of the diameter of a cylindrical sample can be reconstructed accurately. Reconstructed images of the test solution phantom were uniform (within 3%) inside this radius. However, the dose responses of the PRESAGE® samples were not spatially uniform, with variations of at least 5% in sensitivity. The variation appears as a "cupping" artifact with less sensitivity in the middle than at the periphery of the PRESAGE® cylinder. Polarization effects were also detected for these samples. The fiducial-based ray path measurement scheme, coupled with an iterative reconstruction algorithm, enabled optical CT scanning of PRESAGE® dosimeters immersed in mismatched refractive index solutions. However, improvements to PRESAGE® dose response uniformity are required.

SU‐F‐T‐434: Development of a Fan‐Beam Optical Scanner Using CMOS Array for Small Field Dosimetry

Purpose:To design and construct a second generation optical computed tomography (OCT) system using a fan‐beam with a CMOS array detector for the 3D dosimetry with polymer gel and radiochromic solid dosimeters. The system was specifically designed for the small field dosimetry.Methods:The optical scanner used a fan‐beam laser, which was produced from a collimated red laser beam (λ=620 nm) with a 15‐degree laser‐line generating lens. The fan‐beam was sent through an index‐matching bath which holds the sample stage and a sample. The emerging laser light was detected with a 2.54 cm‐long CMOS array detector (512 elements). The sample stage rotated through the full 360 degree projection angles at 0.9‐degree increments. Each projection was normalized to the unirradiated sample at the projection angle to correct for imperfections in the dosimeter. A larger sample could be scanned by using a motorized mirror and linearly translating the CMOS detector. The height of the sample stage was varied for a full 3D scanning. The image acquisition and motor motion was controlled by a computer. The 3D image reconstruction was accomplished by a fan‐beam reconstruction algorithm. All the software was developed inhouse with MATLAB.Results:The scanner was used on both PRESAGE and PAGAT gel dosimeters. Irreconcilable refraction errors were seen with PAGAT because the fan beam laser line refracted away from the detector when the field was highly varying in 3D. With PRESAGE, this type of error was not seen.Conclusion:We could acquire tomographic images of dose distributions by the new OCT system with both polymer gel and radiochromic solid dosimeters. Preliminary results showed that the system was more suited for radiochromic solid dosimeters since the radiochromic dosimeters exhibited minimal refraction and scattering errors. We are currently working on improving the image quality by thorough characterization of the OCT system.

Assessment of optical CT as a future QA tool for synchrotron x-ray microbeam therapy

Synchrotron microbeam radiation therapy (MRT) is an advanced form of radiotherapy for which it is extremely difficult to provide adequate quality assurance. This may delay or limit its clinical uptake, particularly in the paediatric patient populations for whom it could be especially suitable. This study investigates the extent to which new developments in 3D dosimetry using optical computed tomography (CT) can visualise MRT dose distributions, and assesses what further developments are necessary before fully quantitative 3D measurements can be achieved.Two experiments are reported. In the first cylindrical samples of the radiochromic polymer PRESAGE® were irradiated with different complex MRT geometries including multiport treatments of collimated ‘pencil’ beams, interlaced microplanar arrays and a multiport treatment using an anthropomorphic head phantom. Samples were scanned using transmission optical CT. In the second experiment, optical CT measurements of the biologically important peak-to-valley dose ratio (PVDR) were compared with expected values from Monte Carlo simulations. The depth-of-field (DOF) of the optical CT system was characterised using a knife-edge method and the possibility of spatial resolution improvement through deconvolution of a measured point spread function (PSF) was investigated.3D datasets from the first experiment revealed excellent visualisation of the 50 μm beams and various discrepancies from the planned delivery dose were found. The optical CT PVDR measurements were found to be consistently 30% of the expected Monte Carlo values and deconvolution of the microbeam profiles was found to lead to increased noise. The reason for the underestimation of the PVDR by optical CT was attributed to lack of spatial resolution, supported by the results of the DOF characterisation.Solutions are suggested for the outstanding challenges and the data are shown already to be useful in identifying potential treatment anomalies.

Validation of a prototype optical computed tomography system

In radiation cancer treatments, the most of the side effects could be minimized using a proper dosimeter. Gel dosimeter is the only three-dimensional dosimeter and magnetic resonance imaging (MRI) is the gold standard method for gel dosimeter readout. Because of hard accessibility and high cost of sample reading by MRI systems, some other alternative methods were developed. The optical computed tomography (OCT) method could be considered as the most promising alternative method that has been studied widely. In the current study, gel dosimeter scanning using a prototype optical scanner and validation of this optical scanner was performed. Optical absorbance of the irradiated gel samples was determined by both of conventional spectrophotometer and the fabricated OCT system at 632 nm. Furthermore, these irradiated vials were scanned by a 1.5 T MRI. The slope of the curves was extracted as the dose-response sensitivity. The R2-dose sensitivity measured by MRI method was 0.1904 and 0.113 for NIPAM and PAGAT gels, respectively. The optical dose sensitivity obtained by conventional spectrophotometer and the fabricated optical scanner was 0.0453 and 0.0442 for NIPAM gels and 0.0244 and 0.0242 for PAGAT gels, respectively. The scanning results of the absorbed dose values showed that the new OCT and conventional spectrophotometer were in fair agreement. From the results, it could be concluded that the fabricated system is able to quantize the absorbed dose values in polymer gel samples with acceptable accuracy.

Design and Construction of an Optical Computed Tomography Scanner for Polymer Gel Dosimetry Application

Polymer gel dosimeter is the only accurate three dimensional (3D) dosimeter that can measure the absorbed dose distribution in a perfect 3D setting. Gel dosimetry by using optical computed tomography (OCT) has been promoted by several researches. In the current study, we designed and constructed a prototype OCT system for gel dosimetry. First, the electrical system for optical scanning of the gel container using a Helium-Neon laser and a photocell was designed and constructed. Then, the mechanical part for two rotational and translational motions was designed and step motors were assembled to it. The data coming from photocell was grabbed by the home-built interface and sent to a personal computer. Data processing was carried out using MATLAB software. To calibrate the system and tune up the functionality of it, different objects was designed and scanned. Furthermore, the spatial and contrast resolution of the system was determined. The system was able to scan the gel dosimeter container with a diameter up to 11 cm inside the water phantom. The standard deviation of the pixels within water flask image was considered as the criteria for image uniformity. The uniformity of the system was about ±0.05%. The spatial resolution of the system was approximately 1 mm and contrast resolution was about 0.2%. Our primary results showed that this system is able to obtain two-dimensional, cross-sectional images from polymer gel samples.

TU-FF-A4-02: Evaluation of the Performance of the Fast Scanning Platform of An OCT System

Purpose: To ensure that the performance of a fast optical computed tomography (OCT) scanning system is comparable with previous models. Method and Materials: MGS Research Inc. developed an OCT system based on the translate-rotate method used by early generation x-ray CT scanners. The performance of the system has been investigated and the system has been used in several published studies. Recently, a new OCT system was developed by MGS Research Inc. that reduces scan times by a factor of 10 or more. Several 3D dosimeters were irradiated using 6 MV photons and imaged on both version of the scanner. The image noise, reproducibility, and spatial accuracy were determined and used to evaluate the performance of the system. Results: The new version of the scanner reduced the scan time per plane from 7 minutes to 30 seconds. Preliminary results showed that noise levels in the images from both models were comparable. The uncertainty in the determination of the optical density values from images acquired with both models was ∼2%. The new model uses Fresnel lenses that may need adjustment prior to an imaging session which can affect the reproducibility of the system. Conclusion: The new version of the OCT scanner shows promise as a replacement for the previous version. Continued improvement in the software and hardware are needed to make the system as robust as the previous version. The investigation was supported by PHS grant CA 10953 awarded by the NCI, DHHS.

Continuous Three‐Dimensional Examination of Interior Hair Structure

The ability to continuously examine the interior hair structure throughout a treatment process is very important in designing effective hair products. Microscopy is commonly used to observe the interior of hair, but this method requires a sliced sample, making continuous observation impossible. Use of X‐ray computed tomography (CT) as a non‐destructive measurement has been proposed, but this method has a disadvantage in that it is impossible to obtain full‐color interior images of the sample. Thus, a non‐destructive method for continuous, full‐color examination of the interior hair structure has been lacking. In this study a new method is proposed that enables non‐destructive and continuous measurement of the interior hair structure with color information. In our method, optical CT is used for reconstruction of the interior hair structure. Our new theories enabled us to solve the crucial problem of the large observational error of traditional optical CT systems caused by internal light scattering and to make its practical application possible. A new optical CT system based on our method was implemented. This system displayed sufficient accuracy when the phantom image was measured, and clear and full‐color cross‐sectional images were obtained without destruction of the sample when human hair was observed. When the bleaching and dyeing processes were continuously measured, changes in the interior hair with time could be observed. These results clearly indicate that our new method provides a powerful tool for research and product development.