On-board Imaging System Research Articles

Planar and volumetric imaging dosimetry for longitudinal projection extended cone beam CT based adaptive radiotherapy

BackgroundThe objective of this study is to provide a detailed insight into the broad beam dosimetric characteristics of two commercial cone beam computed tomography (CBCT) imaging systems to complement the longitudinal projection extended CBCT based image guided adaptive radiotherapy (IGART). Methods and materialsThe absolute dose calibration, planar and volumetric dosimetric characteristics of two CBCT imaging systems viz., the Clinac 2100 C/D On-Board Imager (OBI) and the TrueBeam X-ray Imaging system (XI) (Varian Medical Systems, Palo Alto, CA) were investigated. Reference dosimetry specific to volumetric CBCT imaging mode was performed as per American Association of Physicists in Medicine (AAPM) task group (TG) 61 protocol using calibrated 0.6 cc Farmer ionization chamber (30010, PTW Dosimetry Company, Germany). The rotational planar and volumetric dosimetry was performed after calibrating the OCTAVIUS 1500 detector array as a mere photon detector, with and without the detachable hemisphere of 4D Modular phantom. ResultsThe absolute dose of half-fan XI CBCT was found to be 1.2 times lower when compared to OBI CBCT. The abutting kV cone beam dosimetry specific to dual-scan CBCT fusion technique showed a wider in-plane profile for the OBI system when compared to the XI system, due to an additional field opening of 3.4 cm used for half-fan mode in the former. The reconstructed volumetric imaging dose distribution specific to half-fan geometry and dynamic movement of gantry, brought to light valuable details about the kV dose distribution. ConclusionThe planar and 3D imaging dosimetry reported in this study paves the way for the integration of volumetric imaging dose reconstruction capabilities during high precision radiotherapy to demonstrate a complete picture of the total dose delivered to the patient during the planned course of IGART.

Image quality characterization of an ultra-high-speed kilovoltage cone-beam computed tomography imaging system on an O-ring linear accelerator.

The quality of on-board imaging systems, including cone-beam computed tomography (CBCT), plays a vital role in image-guided radiation therapy (IGRT) and adaptive radiotherapy. Recently, there has been an upgrade of the CBCT systems fused in the O-ring linear accelerators called HyperSight, featuring a high imaging performance. As the characterization of a new imaging system is essential, we evaluated the image quality of the HyperSight system by comparing it with Halcyon 3.0 CBCT and providing benchmark data for routine imaging quality assurance. The HyperSight features ultra-fast scan time, a larger kilovoltage (kV) detector, a more substantial kV tube, and an advanced reconstruction algorithm. Imaging protocols in the two modes of operation, treatment mode with IGRT and the CBCT for planning(CBCTp) mode were evaluated and compared with Halcyon 3.0 CBCT. Image quality metrics, including spatial resolution, contrast resolution, uniformity, noise, computed tomography (CT) number linearity, and calibration error, were assessed using a Catphan and an electron density phantom and analyzed with TotalQA software. HyperSight demonstrated substantial improvements in contrast-to-noise ratio and noise in both IGRT and CBCTp modes compared to Halcyon 3.0 CBCT. CT number calibration error of HyperSight CBCTp mode (1.06%) closely matches that of a full CT scanner (0.72%), making it suitable for adaptive planning. In addition, the advanced hardware of HyperSight, such as ultra-fast scan time (5.9s) or 2.5 times larger heat unit capacity, enhanced the clinical efficiency in our experience. HyperSight represented a significant advancement in CBCT imaging. With its image quality, CT number accuracy, and ultra-fast scans, HyperSight has a potential to transform patient care and treatment outcomes. The enhanced scan speed and image quality of HyperSight are expected to significantly improve the quality and efficiency of treatment, particularly benefiting patients.

Real-Time Gated Proton Therapy: Commissioning and Clinical Workflow for the Hitachi System

PurposeTo describe the commissioning of real-time gated proton therapy (RGPT) and the establishment of an appropriate clinical workflow for the treatment of patients. Materials and MethodsHitachi PROBEAT provides pencil beam scanning proton therapy with an advanced onboard imaging system including real-time fluoroscopy. RGPT utilizes a matching score to provide instantaneous system performance feedback and quality control for patient safety. The CIRS Dynamic System combined with a Thorax Phantom or plastic water was utilized to mimic target motion. The OCTAVIUS was utilized to measure end-to-end dosimetric accuracy for a moving target across a range of simulated situations. Using this dosimetric data, the gating threshold was carefully evaluated and selected based on the intended treatment sites and planning techniques. An image-guidance workflow was developed and applied to patient treatment. ResultsDosimetric data demonstrated that proton plan delivery uncertainty could be within 2 mm for a moving target. The dose delivery to a moving target could pass 3%/3 mm gamma analysis following the commissioning process and application of the clinical workflow detailed in this manuscript. A clinical workflow was established and successfully applied to patient treatment utilizing RGPT. Prostate cancer patients with implanted platinum fiducial markers were treated with RGPT. Their target motion and gating signal data were available for intrafraction motion analysis. ConclusionReal-time gated proton therapy with the Hitachi System has been fully investigated and commissioned for clinical application. RGPT can provide advanced and reliable real-time image guidance to enhance patient safety and inform important treatment planning parameters, such as planning target volume margins and uncertainty parameters for robust plan optimization. RGPT improved the treatment of patients with prostate cancer in situations where intrafraction motion is more than defined tolerance.

Online Correction of Intrafraction Motion during Volumetric Modulated Arc Therapy for Prostate Radiotherapy: A Cohort Study Quantifying the Frequency of Shifts and Analysis of Men at Highest Risk.

We have previously described our early experience using fiducial markers to correct for intrafraction motion during radiation therapy (RT) to the prostate using the TrueBeam Advanced imaging package. We sought to further characterize the utility of this approach in a larger cohort, and analyze factors associated with intrafraction motion. A total of 132 men with fiducial markers treated with RT for intact prostate cancer at a single center were treated with a VMAT technique using 2-3 arcs. All patients underwent planning CT after a rectal enema and same day placement of 3 fiducial markers (Gold Anchor). Triggered kV images were acquired every 10 seconds using an onboard imaging system. Intrafraction motion correction was considered if any two fiducial markers were observed beyond a 3 mm tolerance margin. A manual 2D-3D match was performed using the fiducial markers from the single triggered kV image to obtain a couch shift. Shift data were extracted from the record and verify system and expressed as a single 3-dimensional translation. Shift percent was defined as the number of instances of a >3 mm intrafraction correction divided by the total number of fractions for a given patient. Clinical variables were evaluated, including body mass index, hormone therapy (ADT), prostate, rectal bladder volumes, and rectal width (transverse dimension of rectum at the mid-gland of prostate on simulation CT). Across 2659 fractions, intrafraction motion correction was performed 582 times, in 463 fractions (17%). 101/132 patients (77%) had at least one shift during their treatment course, and 48/132 patients (36%) had shifts with an average magnitude of at least 5 mm. The median shift was 3.6 mm (range, 0-2.4 cm; IQR, 1.5-5.4 mm). 25% of men had a shift percent >20% (SP>20%). Univariate analysis revealed that only larger rectal volume or width, smaller prostate size, and use of ADT were associated with SP>20% (p<0.05). Men with rectal width in the top quartile (>3.6 cm) were more likely to have intrafraction motion corrected with SP>20% (47% vs 18%, p = 0.0016), and similarly men with rectal volume in the top quartile (>112 cc) were more likely to have SP>20% (44% vs 19%, p = 0.0067). On multivariate analysis, only rectal parameters (e.g., top quartile rectal width, HR 3.9, p = 0.0024) were correlated with a higher frequency of intrafraction motion. Intrafraction motion occurs in a significant percentage of men undergoing prostate RT with VMAT, and is correctable utilizing a common feature. On multivariate analysis, rectal volume and width were associated with larger shift percent. Treatment approaches which do not account for intrafraction motion should consider including methods of immobilization, or larger PTV margins in order to avoid marginal miss of the prostate.

Towards Clinical Proton Minibeam Radiation Therapy (pMBRT): Development of Clinical pMBRT System Prototype and pMBRT-Specific Treatment Planning Method

Proton minibeam radiation therapy (pMBRT) is an emerging spatially fractionated RT (SFRT) modality that can provide very high therapeutic index compared to conventional radiotherapy methods and clinically-available SFRT methods (GRID and LATTICE). The biological data collected thus far encourage the preparation of clinical trials in pMBRT. This work is to facilitate the clinical translation of pMBRT by developing (1) the first clinical pMBRT system prototype worldwide, readily available for both small-animal biology studies and large-animal pMBRT trials; (2) pMBRT-specific treatment planning method with peak-valley dose ratio (PVDR) optimization capability for large animal and patient pMBRT trials, which is currently unavailable. The pMBRT system is based on clinically-used pencil-beam-scanning proton machine equipped with large-field clinical-size pMBRT collimator, pMBRT-dedicated treatment planning system, and KV/CBCT imaging guidance. The multi-slit brass collimator has 10 × 10 cm field size, 0.4mm width per slit and 4 mm center-to-center distance. The divergence of slits is tailed to the divergence of the proton beam. A unique universal collimator design is implemented, so that we can keep the outer fitting to the snout and conveniently inter-change collimators as needed. The pMBRT-specific treatment planning method jointly optimizes PVDR and dose objectives, to meet a minimal PVDR threshold and maximize PVDR, to avoid the situations where meeting dose objectives can compromise PVDR when PVDR were not optimized. In addition, the survival fraction for organs at risk is also optimized. The dose calculation engine is based on the Monte Carlo method using TOPAS. The optimization algorithm utilizes total variation and L1 sparsity regularization to maximize PVDR and iterative convex relaxation method to solve the optimization problem. Monte Carlo simulations via TOPAS were performed to design this large-field multi-slit collimator, using the beam structure and the beam data specific to our proton system, with mean dose rate of 8 Gy/min under clinical condition with the collimator in place. The feasibility of using this pMBRT system for small-animal studies has been demonstrated, with customized 3D printed holder for immobilizing small animals, on-board KV imaging system for accurate small-animal positioning, and the GAFchromic film for verifying radiation dose and PVDR. On the other hand, the efficacy of pMBRT-specific treatment planning method (with PVDR optimization capability) to improve PVDR has been demonstrated using retrospective patient planning studies in comparison with standard proton treatment planning method (without PVDR optimization capability). The initial development of a clinical pMBRT system prototype and pMBRT-specific treatment planning method of PVDR optimization capability has been completed with ongoing efforts to make this system ready for large-animal pMBRT studies.

The application of 3D-printed oral stents in intensity-modulated radiotherapy for oropharyngeal cancer and their dosimetric effect on organs at risk

BackgroundThis study investigates the accuracy of 3D-printed dental stents in intensity-modulated radiotherapy (IMRT) for oropharyngeal cancer (OPC) and their dosimetric effects on normal tissues.MethodsWe selected 60 patients with OPC who underwent IMRT in the Department of Oncology, Special Medical Center of Army Medical University. These patients were randomly assigned into 3D-printed oral stent, simple glass bottle, and nonstent groups (20 patients/group). The positioning error was analyzed with the onboard imaging system once a week after 5 fractions of IMRT. The conformity index (CI), homogeneity index (HI), radiation dose of organs at risk (OARs), and oral mucosal reaction were compared among the three groups.ResultsNo significant difference was observed in the conformity and uniformity of the target dose and the dose received by the spinal cord, larynx, and bilateral parotid glands among the three groups (P > 0.05). Meanwhile, the dose received by the upper cheek, hard palate, and soft palate of patients was significantly lower in the 3D-printed oral stent group than in the nonstent group (P < 0.05) but insignificantly different between the 3D-printed oral stent and simple glass bottle groups (P > 0.05). When compared with the nonstent group, the simple glass bottle group showed a markedly lower dose received by the upper cheek (P < 0.05) and an insignificantly different dose received by the hard palate and soft palate (P > 0.05). According to Common Terminology Criteria for Adverse Events v.5.0, the adverse response rate of the hard palate mucosa was lower in the 3D-printed oral stent group than in the simple glass bottle and nonstent groups (P < 0.05).ConclusionsFor OPC patients undergoing IMRT, the application of 3D-printed oral stents can significantly reduce the exposure dose of the upper cheek and hard palate and decrease the occurrence of adverse events such as oral mucositis although it cannot affect the positioning error.

Spine Stereotactic Body Radiation Therapy Without Immobilization: Detailed Analysis of Intrafraction Motion Using High-Frequency kV Imaging During Irradiation

Spine SBRT requires high positioning accuracy and a stable patient to maximize target coverage and reduce excessive irradiation to organs at risk (OARs). Positional verification during spine SBRT delivery helps to ensure accurate positioning for all patients. We report our experience with non-invasive 3-dimensional (3D) target position monitoring during volumetric modulated arc therapy (VMAT) of spine metastases, in non-immobilized patients positioned using only a thin mattress and simple arm and knee supports. Fluoroscopic planar kV images were acquired at 7 frames/second using the on-board imaging system during VMAT spine SBRT. Template matching and triangulation were used to track the target in vertical, longitudinal and lateral direction. If the tracking trace deviated >1mm from the planned position in ≥1 direction, treatment was manually interrupted and 6D CBCT-based couch correction was performed. Tracking data was used to retrospectively analyze the target position. Positional data, agreement with CBCT, correlation between position of the couch and direction of any positional correction, and treatment times were analyzed. 175 fractions were analyzed. Delivery was interrupted 83 times in 66 fractions for a deviation >1mm. In 97% of cases the difference between tracking data and subsequent clinical shift performed after the CBCT match was ≤0.5mm. Lateral/longitudinal shift performed after intervention correlated with the couch roll/pitch at the start of treatment (correlation coefficient: -0.63/0.53). Mean (SD; range) time between start of first imaging and end of the last arc was 15.2 minutes (5.1;7.6-36.3). Spine tracking during irradiation can be used to prompt an intervention CBCT scan and repositioning, so that a spine SBRT target deviates by ≤1mm from the planned position, even in non-immobilized patients. kV tracking and CBCT are in good agreement. The data support verification CBCT after all 6 degrees-of-freedom positional corrections in non-immobilized spine SBRT patients.

Assessment of organ and size-specific effective doses from cone beam CT (CBCT) in image-guided radiotherapy (IGRT) based on body mass index (BMI)

The ability of radiation therapy to deliver radiation doses accurately to tumour targets, while sparing normal tissues, has advanced remarkably over recent decades. A development that has been key to achieving this improvement has been the application of image-guidance that facilitates more precise delivery of the prescribed dose to the treatment site. A recent international survey showed that kV cone beam CT (CBCT) is the imaging modality used most widely for image guided radiation therapy (IGRT) procedures. However, IGRT will deliver additional radiation doses to patients that should be considered. The aims of this study were to (1) estimate organ and size-specific effective doses resulting from kV-CBCT chest and pelvis scans based on body mass index (BMI) of the patient, and (2) investigate the influence of patient size on imaging dose. A validated Monte Carlo model developed with BEAMnrc/EGSnrc was employed to simulate kV spectra generated by Varian on-board imaging (OBI) system. Organ and size-specific effective doses were assessed for adult phantoms in the National Cancer Institute (NCI) phantom library using DOSXYZnrc/EGSnrc code. The library has been reconstructed from images of CT patients, and contains 100 male and 93 female phantoms of a wide range of sizes. The phantoms were grouped into six categories based on body mass index (BMI) as classified by the world health organization. For all sizes, average doses to organs that lay fully or partially inside the field of the chest and pelvic scans were in the range of (0.20–3.06 mGy/100 mAs) and (0.17–4.47 mGy/100 mAs), and size-specific effective doses were (0.63 – 1.78 mSv/100 mAs) and (0.30 – 1.17 mSv/100 mAs), respectively. Patient size played a significant role in determining the imaging doses resulting from both scans. In general, organ and effective doses of chest and pelvic scans of thin patients were about 2–3 times larger than those of obese patients, with the impact of patient size on pelvis doses being slightly higher than on chest ones. Results of this study suggests that patient size should be taken into consideration when estimation and optimization of the imaging doses resulting from CBCT. Although development of patient size-specific CBCT protocols is a practical approach that can account for patient size, it has not been implemented worldwide. Results from this investigation might be used as a simple approach to obtain an estimation of the imaging doses for patients of various sizes based on BMI.

Evaluating on-board kVCT- and MVCT-based dose calculation accuracy using a thorax phantom for helical tomotherapy treatments

Purpose. To evaluate the impact of CT number calibration and imaging parameter selection on dose calculation accuracy relative to the CT planning process in thoracic treatments for on-board helical CT imaging systems used in helical tomotherapy. Methods and Materials. Direct CT number calibrations were performed with appropriate protocols for each imaging system using an electron density phantom. Large volume and SBRT treatment plans were simulated and optimized for planning CT scans of an anthropomorphic thorax phantom and transferred to registered kVCT and MVCT scans of the phantom as appropriate. Relevant DVH metrics and dose-difference maps were used to evaluate and compare dose calculation accuracy relative to the planning CT based on a variation in imaging parameters applied for the on-board systems. Results. For helical kVCT scans of the thorax phantom, median differences in DVH parameters for the large volume treatment plan were less than ±1% with dose to the target volume either over- or underestimated depending on the imaging parameters utilized for CT number calibration and thorax phantom acquisition. For the lung SBRT plan calculated on helical kVCT scans, median dose differences were up to –2.7% with a more noticeable dependence on parameter selection. For MVCT scans, median dose differences for the large volume plan were within +2% with dose to the target overestimated regardless of the imaging protocol. Conclusion. Accurate dose calculations (median errors of <±1%) using a thorax phantom simulating realistic patient geometry and scatter conditions can be achieved with images acquired with a helical kVCT system on a helical tomotherapy unit. This accuracy is considerably improved relative to that achieved with the MV-based approach. In a clinical setting, careful consideration should be made when selecting appropriate kVCT imaging parameters for this process as dose calculation accuracy was observed to vary with both parameter selection and treatment type.

On line-of-sight navigation for deep-space applications: A performance analysis

Line-of-Sight (LoS) navigation is an optical navigation technique that exploits the direction to visible celestial bodies, obtained from an onboard imaging system, to estimate the position and velocity of a spacecraft. The directions are fed to an estimation filter, where they are matched with the actual position of the observed bodies, retrieved from onboard stored ephemerides. As LoS navigation represents a really promising option for the next-generation deep-space spacecraft, the objective of this work is to provide new insights into the performance. First, the information matrix is analyzed to show the influence of the geometry between the spacecraft and the observed planet(s). Then, a Monte Carlo approach is used to investigate the influence of measurement error (ranging from 0.1 to 100 arcsec), and tracking frequency (ranging from four observations per day to one observation every two days). The effect on navigation performance is quantified by two indicators. The first is the 3D position and velocity Root-Mean-Square-Errors, computed once the estimation is considered to be steady-state. The second is the convergence time, which quantifies the required time for the estimation to reach the steady-state behaviour. The simulation is based on a set of four planets, which do not follow the common heliocentric dynamics but rotate around the Sun with the same (distance-independent) angular velocity of the spacecraft. This approach allows the separation of scenario-dependent behaviours from navigation intrinsic properties, as the same relative geometry between observer and observed objects is maintained during the whole simulation. The results provide a useful guide for the next-generation autonomous navigation system, for both the definition of hardware requirements and the design of an appropriate navigation strategy. Considerations are then applied to Near-Earth Asteroid fly-by mission scenarios for the definition of the navigation strategy and hardware requirements. It is shown the importance of relative angles between the spacecraft and the planets. In the single-planet observation scenario, when the angle between the position vectors of the spacecraft and planet approaches a null value, the estimation error decreases. In the double-planets observation scenario, when the separation angle between the two LoS directions gets close to 90°, the estimation error decreases. The main influence on the performance is driven by the measurement error, which with current technologies is shown to be able to provide a position error in the order of a few hundred kilometers, while with a lower measurement error (0.1 arcsec) it would be possible to have a position error below 100 km. Finally, it is demonstrated that tracking frequency plays a secondary role in the performance, and only influences tangibly the convergence time.

Dead detector element detection in flat panels using convolutional neural networks.

Independent testing of image quality metrics is important to provide an unbiased determination of medical imaging performance. Due to the underreporting by vendors of dead detector elements, which are elements that do not function but may be corrected using information from surrounding pixels, the health of all imaging detector elements is infrequently reported and extraordinarily difficult to independently determine or verify through traditional means without vendor largesse. In many instances, dead detector data, or dead pixel maps, are only available at the discretion of the vendors, which renders these data inaccessible to many medical physicists. We provide a mechanism to predict which detector elements are dead from a single flat field image and describe a convolutional neural network (CNN) to address this task. This technique provides a vendor-independent and potential correction algorithm-independent means for obtaining this information, which directly relates to the image quality of the diagnostic imaging system. A data set of 61 flat field images was obtained from two Varian on-board imaging (OBI) systems. These images were taken with a range of kVp and mAs settings and with differing levels of copper filtration in the beam path. The dead pixel maps for these detectors were also acquired and used to label the ground truth subimages-or small square images from the original, larger image-from the flat field images. This data set was then used to train, test, and validate five unique CNNs on the task of identifying dead detector elements from subimages derived from the flat field images. The CNNs were validated with an average precision of 0.96, an average recall of 0.48, and an average F1 score of 0.55 attributed to the scarcity and imbalanced nature of available data, and the difficulty in training neural networks using flat-field images. However, performance drastically improved when utilizing a subset of the data with a high dynamic range. This work shows the feasibility of using a CNN to detect dead detector elements from flat field images taken on a detector. Robust training of this type of algorithm could lead to a generalized model that may provide an independent evaluation of dead detectors for a wide range of vendors and models. Ultimately, this method can be a valuable tool for physicists performing quality assurance procedures on all digital diagnostic imaging systems, including X-ray, mammography, and fluoroscopy.

Considerations for CT Number Calibration and Imaging Parameter Selection for Adaptive Planning on a Novel On-Board Helical kVCT System

<h3>Purpose/Objective(s)</h3> To evaluate the impact of CT number calibration and imaging parameters on adaptive dose calculation accuracy relative to the CT planning process for an on-board helical kVCT imaging system used in helical tomotherapy treatments. <h3>Materials/Methods</h3> Helical kVCT calibrations were performed with thorax protocols at 120 kVp for <i>Fine</i> and <i>Normal</i> imaging modes using a standard electron density phantom. For this system, <i>Mode</i> determines the longitudinal beam width at isocenter, couch speed, and views per rotation, which impact the scatter signal, scan time, and patient dose, respectively, and offers <i>Fine</i> (50 mm beam width, long relative scan time, high relative mAs/rotation), <i>Normal</i> (100 mm, moderate scan time, moderate mAs/rotation), and <i>Coarse</i> (∼140 mm, short scan time, low mAs/rotation) mode options. An SBRT treatment plan was simulated for CT simulation scans of an anthropomorphic lung phantom and transferred to on-board helical kVCT scans with varying imaging and calibration protocols, and clinically relevant DVH metrics and dose-difference maps were used to compare the calculated dose distributions. <h3>Results</h3> Use of <i>Fine</i> mode for helical kVCT calibration resulted in smaller dose-volume deviations relative to the planning CT, with the magnitude of these deviations dependent on the mode used to acquire the scan of the lung phantom. For scans acquired under <i>Fine</i> mode, use of <i>Fine</i> mode calibration showed average differences in target volume DVH metrics of ± 1.0% (-1.7% max difference) compared to ± 2.5% (-3.6% max difference) with application of <i>Normal</i> mode calibration for the same images. Likewise, for phantom scans acquired under <i>Normal</i> mode, these differences were ± 1.2% (+ 2.1%) for <i>Fine</i> calibration compared to ± 1.4% (-2.1%) for <i>Normal</i> calibration, while for phantom scans acquired under <i>Coarse</i> mode, differences were ± 2.0% (-4.1%) and ± 3.3% (-6.2%), respectively. Dose-difference maps confirmed the greatest deviations in the calculated dose distributions occurred in the high-dose, target volume region, though some differences were also observed at the tissue-air interface of the chest wall. Visual differences relative to the planning CT were lessened with use of <i>Fine</i> mode acquisitions of both the calibration and lung phantoms. <h3>Conclusion</h3> Use of <i>Fine</i> mode for helical kVCT number calibration and image acquisition resulted in dose calculations most consistent with those calculated on diagnostic-quality CT images. If excess patient dose from additional imaging is a concern, <i>Normal</i> or <i>Coarse</i> mode image acquisitions with application of <i>Fine</i> mode calibration provided dose calculation accuracies within a reasonable tolerance as well. Also, while <i>Fine</i> mode increases scan times relative to other modes, this additional time is still significantly lessened compared to on-board CBCT or MVCT systems currently used for CT-based online adaptive planning.

Investigation of the operating conditions influence on the optical telescope performance when capturing star images

We propose combining the functionality of star sensors and the optical system of a nanosatel-lite optical-electronic telescopic module (OETM) for Earth remote sensing (ERS) in order to re-duce the influence of thermoregulation inaccuracy on the accuracy of linking the star sky and the Earth's surface images. The main difference from the previous works is that for the thermoregula-tion inaccuracy to be reduced, we use controlled local film electric heaters with moderate energy consumption located on the periphery of optical elements of the OETM system. High quality of the star sky images is sought to be attained for a nanosatellite-borne telescope in a circular sun-synchronous orbit (SSO) and includes solving a number of tasks discussed below. Arguments are given in favor of using the proposed OETM, which is capable of fully complementing / replacing the onboard astronomical imaging system of spacecraft.

Technical note: Characterization of novel iterative reconstructed cone beam CT images for dose tracking and adaptive radiotherapy on L-shape linacs.

Cone-beam computed tomography (CBCT) allows for patient setup and positioning, and potentially dose verification or adaptive replanning prior to each treatment delivery. Poor CBCT image quality due to scatter artifacts and patient motion has been a major limiting factor. A new image reconstruction algorithm was recently clinically implemented for improving image quality through iterative reconstruction (iCBCT). This study aims to characterize iCBCT image quality, establish image value (HU)-to-relative electron density (RED) calibration curves for dose calculation, and assess the dosimetric accuracy for different anatomical sites. Both conventional CBCT and iCBCT scans were acquired from a Varian TrueBeam On-Board Imager system. A Catphan 604 phantom was scanned to compare image quality between the traditional Feldkamp-Davis-Kress (FDK) and novel iterative reconstruction techniques. Computerized Imaging Reference Systems (CIRS) electron density phantom was used to construct site-specific HU-RED curves corresponding to various scan settings. The CIRS Dynamic Thorax phantom, Rando pelvis phantom, and BrainLab head phantom were used for assessing dosimetric accuracy calculated on iCBCT images, compared to that on traditional FDK-based CBCT images. All phantoms were scanned on a computed tomography (CT) to obtain baseline HU values for comparison. Test results obtained from Catphan showed statistically significant improvement with iCBCT, compared to FDK CBCT. Average HU differences from the baseline CT values were improved to within ±30HU for iCBCT, compared to FDK CBCT for phantom studies. Dose calculated on iCBCT for both phantoms and patient cases directly using baseline HU-RED calibration from CT showed 0.5%-2.0% accuracy from the baseline dose calculated on CT, which is comparable to doses calculated using site-specific HU-RED calibration curves. iCBCT provides improved image quality, improved HU accuracy compared to CT baseline, and has potential to provide online dose verification as part of the adaptive radiotherapy workflow directly using the baseline HU-RED calibration curve from CT.

Diamond Detectors for Radiotherapy X-Ray Small Beam Dosimetry

Many new X-Ray treatment machines using small and/or non-standard radiation fields, e.g., Tomotherapy, Cyber-knife, and linear accelerators equipped with high-resolution multi-leaf collimators and on-board imaging system, have been introduced in the radiotherapy clinical routine within the last few years. The introduction of these new treatment modalities has led to the development of high conformal radiotherapy treatment techniques like Intensity Modulated photon Radiation Therapy, Volumetric Modulated Arc Therapy, and stereotactic radiotherapy. When using these treatment techniques, patients are exposed to non-uniform radiation fields, high dose gradients, time and space variation of dose rates, and beam energy spectrum. This makes reaching the required degree of accuracy in clinical dosimetry even more demanding. Continuing to use standard field procedures and detectors in fields smaller than 3 × 3 cm2, will generate a reduced accuracy of clinical dosimetry, running the risk to overshadowing the progress made so far in radiotherapy applications. These dosimetric issues represent a new challenge for medical physicists. To choose the most appropriate detector for small field dosimetry, different features must be considered. Short- and long-term stability, linear response to the absorbed dose and dose rate, no energy and angular dependence, are all needed but not sufficient. The two most sought-after attributes for small field dosimetry are water equivalence and small highly sensitive (high sensitivity) volumes. Both these requirements aim at minimizing perturbations of charged particle fluence approaching the Charged Particle Equilibrium condition as much as possible, while maintaining high spatial resolution by reducing the averaging effect for non-uniform radiation fields. A compromise between different features is necessary because no dosimeter currently fulfills all requirements, but diamond properties seem promising and could lead to a marked improvement. Diamonds have long been used as materials for dosimeters, but natural diamonds were only first used for medical applications in the 80 s. The availability of reproducible synthetic diamonds at a lower cost compared to natural ones made the diffusion of diamonds in dosimetry possible. This paper aims to review the use of synthetic poly and single-crystal diamond dosimeters in radiotherapy, focusing on their performance under MegaVoltage photon beams. Both commercial and prototype diamond dosimeters behaviour are described and analyzed. Moreover, this paper will report the main related results in literature, considering diamond development issues like growth modalities, electrical contacts, packaging, readout electronics, and how do they affect all the dosimetric parameters of interest such as signal linearity, energy dependence, dose-rate dependence, reproducibility, rise and decay times.

Distributed Compressed Sensing of Hyperspectral Images According to Spectral Library Matching

The ever-increasing resolution puts tremendous pressure to the onboard hyperspectral imaging system. Compressed sensing technology is one of the important ways to solve this problem. Distributed compressed sensing was proposed to exploit both intra- and inter-correlation structures of hyperspectral images. However, the implementation method of distributed compressed sampling has not been reported, and the joint sparsity reconstruction algorithm cannot achieve excellent image reconstruction performance. In this paper, a distributed compressed sampling strategy inspired by distributed compressed video sensing and optical implementation model are proposed to collect compressed hyperspectral data. In the image reconstruction process, we discard the direct application of the joint sparsity constraint on the data itself. Instead, we explore the estimation method of abundance and endmember with the help of the existing spectral library. Then, the images are recovered by applying the linear mixing model of hyperspectral. The comparison experiments of various schemes show that the proposed compressed sensing scheme has an obvious advantage in reconstruction performance under the low sampling rate. The proposed compressed sensing scheme has great potential in a high-compression ratio onboard hyperspectral imaging system.

A Novel Use of Hydrogel as a Dual-Buffer in Stereotactic Body Proton Therapy for Locally Advanced Pancreatic Cancer

To evaluate the dosimetric efficacy of using hydrogel as a dual buffer, namely to suppress the dose to duodenum and boost the plan robustness in stereotactic body proton therapy for locally advanced pancreatic cancer. The dosimetric advantages of hydrogel spacer is quantitatively evaluated by a proposed quantity, full-width-half-maximum (FWHM) of dose spread function (DSF), which is defined as the distance needed to dwindle dose 50% off to the duodenum. The dose spread function is the derivative of dose falloff profile along the neighboring of 100% isodose line and critical organ. A proof-of-concept study was implemented on one of the previously treated pancreatic patients with prescribed dose of 33 Gy (RBE) in 5 fractions using spot scanning proton beam. The gross target volume is expanded with 2 mm using on-board volumetric imaging system to an optimization target volume (OTV). The proton plan dose profile in the abutment region between 100% isodose line and duodenum is sampled. The dose falloff is differentiated to obtain the FWHM distance to lower the dose to a 50% level. For all voxels representing the surface of duodenum, the mean values of FWHMs is therefore determined as the required spacing from hydrogel injection. An organ deformation method is developed to simulate the hydrogel spacer included in the post-injection plan, which is compared dosimetrically with pre-injection plan. The plan robust evaluation of the target coverage was assessed with 2 mm setup and 3.5% range uncertainties, defined as the percentage of the disturbed scenarios meeting the goal of more than 95% of target covered by at least 100% of the prescribed dose. The target coverages of both pre-injection and post-injection plans are able to achieve D98% with about 99% of the prescription. The required hydrogel spacing FWHM (mean distance for duodenum dose dropping to 50%) is determined as 14.1 mm. The average dose for duodenum in the hydrogel pre-injection plan is 7.35 Gy (RBE), while for post-injection plan is 4.61 Gy (RBE). By simulating the organ deformation induced by hydrogel spacing using the pre-determined value of FWHM, the dose covering 50% volume of the duodenum is lowered from 3.67 Gy (RBE) of pre-injection plan to 1.77 Gy (RBE) in the simulated post-injection plan, which is consistent with estimation using FWHM value. The high dose region to duodenum is also lowered for D2cc from 28 Gy (RBE) to 20.9 Gy (RBE). The robustness evaluation indicates that the passing rate of pre-injection plan is only 7%, while for the simulated post-injection plan is 21%. This study indicates that a hydrogel in SBPT can function dual roles as decreasing the dose to duodenum and improving the robustness of the post-injection pancreatic plan. With the knowledge of FWHM from DSF, the required spacing from hydrogel is determined. The dose level in post-injection plan match the desired dose level. The post-injection plan also demonstrates the improved immunity to range uncertainties and setup errors.

Model for selecting the Earth observation satellite systems by object recognition probability

Model for selecting the Earth observation satellite systems by object recognition probability

Evaluation of four-dimensional cone beam computed tomography ventilation images acquired with two different linear accelerators at various gantry speeds using a deformable lung phantom.

We evaluated four-dimensional cone beam computed tomography (4D-CBCT) ventilation images (VICBCT) acquired with two different linear accelerator systems at various gantry speeds using a deformable lung phantom. The 4D-CT and 4D-CBCT scans were performed using a computed tomography (CT) scanner, an X-ray volume imaging system (Elekta XVI) mounted in Versa HD, and an On-Board Imager (OBI) system mounted in TrueBeam. Intensity-based deformable image registration (DIR) was performed between peak-exhale and peak-inhale images. VICBCT- and 4D-CT-based ventilation images (VICT) were derived by DIR using two metrics: one based on the Jacobian determinant and one on changes in the Hounsfield unit (HU). Three different DIR regularization values (λ) were used for VICBCT. Correlations between the VICBCT and VICT values were evaluated using voxel-wise Spearman's rank correlation coefficient (r). In case of both metrics, the Jacobian-based VICBCT with a gantry speed of 0.6deg/sec in Versa HD showed the highest correlation for all the gantry speeds (e.g., λ=0.05 and r=0.68). Thus, the r value of the Jacobian-based VICBCT was greater or equal to that of the HU-based VICBCT. In addition, the ventilation accuracy of VICBCT increased at low gantry speeds. Thus, the image quality of VICBCT was affected by the change in gantry speed in both the imaging systems. Additionally, DIR regularization considerably influenced VICBCT in both the imaging systems. Our results have the potential to assist in designing CBCT protocols, incorporating VICBCT imaging into the functional avoidance planning process.

A Monte Carlo study to investigate the feasibility of an on-board SPECT/spectral-CT/CBCT imager for medical linear accelerator.

The on-board flat-panel cone-beam computed tomography (CBCT) lacks molecular/functional information for current online image-guided radiation therapy (IGRT). It might not be adequate for adaptive radiation therapy (ART), particularly for biologically guided tumor delineation and targeting which might be shifted and/or distorted during the course of RT. A linear accelerator (Linac) gantry-mounted on-board imager (OBI) was proposed using a single photon counting detector (PCD) panel to achieve single photon emission computed tomography (SPECT), energy-resolved spectral CT, and conventional CBCT triple on-board imaging, which might facilitate online ART with an addition of volumetric molecular/functional imaging information. The system was designed and evaluated in the GATE Monte Carlo platform. The OBI system including a kV-beam source and a pixelated cadmium zinc telluride (CZT) detector panel mounted on a medical Linac orthogonally to the MV beam direction was designed to obtain online CBCT, spectral CT, and SPECT tri-modal imaging of patients in the treatment room. The spatial resolutions of the OBI system were determined by imaging simulated phantoms. The CBCT imaging was evaluated by a simulated contrast phantom. A PMMA phantom containing gadolinium was imaged to demonstrate quantitative imaging of spectral-CT/CBCT of the system. The capability of tri-modal imaging of the OBI was demonstrated using three different spectral CT imaging methods to differentiate gadolinium, gold, calcium within simulated PMMA and the SPECT to image radioactive 99m Tc distribution. The dual-isotope SPECT imaging of the system was also evaluated by imaging a phantom containing 99m Tc and 123 I. The radiotherapy-related parameters of iodine contrast fraction and virtual non-contrast (VNC) tissue electron density in the Kidney1 inserts of a simulated phantom were decomposed using the Bayesian eigentissue decomposition method for contrast-enhanced CBCT/spectral-CT of the OBI in a single scan. The spatial resolutions of CBCT and SPECT of the OBI were determined to be 15.1lp/cm at 10% MTF and 4.8-12mm for radii of rotation of 10-40cm, respectively. In CBCT image of the contrast phantom, most of the soft-tissue inserts were visible with sufficient spatial structure details. As compared to the CBCT image of gadolinium, the spectral CT image provided higher image contrasts. Calcium, gadolinium, and gold were separated well by using the spectral CT material imaging methods. The reconstructed distribution of 99m Tc agreed with the spatial position within the phantom. The two isotopes were separated from each other in dual-isotope SPECT imaging of the OBI. The iodine fractions and the VNC electron densities were estimated in the iodine-enhanced Kidney1 tissue inserts with reasonable RMS errors. The main procedures of the tri-modal imaging guided online ART workflow were presented with new functional features included. Using a single photon counting CZT detector panel, an on-board SPECT, spectral CT, and CBCT tri-modal imaging could be realized in Linacs. With the added online molecular/functional imaging obtained from the new OBI for the online ART proposed, the accuracy of radiation treatment delivery could be further improved.