Homogeneous Phantom Research Articles

Inhomogeneity detection within a head-sized phantom using tracking of charged nuclear fragments in ion beam therapy

Objective.The highly conformal carbon-ion radiotherapy is associated with an increased sensitivity of the dose distributions to internal changes in the patient during the treatment course. Hence, monitoring methodologies capable of detecting such changes are of vital importance. We established experimental setup conditions to address the sensitivity of a monitoring approach based on secondary-fragment tracking for detecting clinically motivated air cavity dimensions in a homogeneous head-sized PMMA phantom in 40 mm depth.Approach.The air cavities were positioned within the entrance channel of a treatment field of 50 mm diameter at three lateral positions. The measured secondary-fragment emission profiles were compared to a reference measurement without cavities. The experiments were conducted at the Heidelberg Ion-Beam Therapy Center in Germany at typical doses and dose rates.Main results.Significances above a detectability threshold of 2σfor the larger cavities (20 mm diameter and 4 mm thickness, and 20 mm diameter and 2 mm thickness) across the entire treatment field. The smallest cavity of 10 mm diameter and 2 mm thickness, which is on the lower limit of clinical interest, could not be detected at any position. We also demonstrated that it is feasible to reconstruct the lateral position of the cavity on average within 2.8 mm, once the cavity is detected. This is sufficient for the clinicians to estimate medical effects of such a cavity and to decide about the need for a control imaging CT.Significance.This investigation defines well-controlled reference conditions for the evaluation of the performance of any kind of treatment monitoring method and its capability to detect internal changes within head-sized objects. Four air cavities with volumes between 0.31 cm3and 1.26 cm3were narrowed down around the detectability threshold of this secondary-fragment-based monitoring method.

Optimal weighting strategies for maximizing contrast-to-noise ratio in photon counting CT images.

Photon counting detectors (PCDs) with energy discrimination capabilities have the potential to generate grayscale CT images with improved contrast-to-noise ratio (CNR) through optimal weighting of their spectral measurements. This study evaluates the CNR performance of grayscale CT projections and images generated from spectral measurements of PCDs using three energy-weighting strategies: pre-log weighting, post-log weighting, and material decomposition (MD) weighting. This study provides the expressions of optimal weights and maximum achievable CNR of these energy-weighting strategies, which only require the knowledge of detected bin counts and do not require information of PCD energy responses or imaging techniques. We defined and solved a generalized eigenvalue problem to obtain the maximum achievable CNR in the projection domain for low-contrast tasks using three energy-weighting strategies: pre-log weighting (weighted sum of energy bin counts), post-log weighting (weighted sum of line integrals), and MD weighting (weighted sum of basis material thicknesses, which is equivalent to virtual monoenergetic images [VMIs]). These expressions only contain energy bin counts from PCD measurements. We used a realistic PCD energy response model to simulate the detected bin counts and conducted Monte Carlo simulations of different contrast tasks and phantoms to evaluate the projection- and image-domain CNR performance of these energy-weighting strategies. Additionally, the total counts method (a special case of pre-log weighting with unity weights) was included for comparison. We also conducted Gammex head and body phantom scans on an edge-on-irradiated silicon PCCT prototype to evaluate the image-domain CNR performance of these energy-weighting strategies. The results show that pre-log, post-log, and MD weighting strategies generate approximately equal projection-domain maximum achievable CNR, with a difference of less than 2%, and outperform the total counts method. These three energy-weighting strategies also generate approximately equal image-domain maximum CNR when the contrast task is located at the center of a homogeneous phantom. Pre-log weighting generates the highest image-domain CNR for an off-center contrast task location or inhomogeneous phantoms while also outperforming the total counts method. We derived the expression of projection-domain maximum achievable CNR using three energy-weighting strategies. Our results suggest that using pre-log weighting strategies enables fast grayscale CT image generation with high CNR from spectral PCD measurements for inhomogeneous phantoms and off-center region of interests (ROIs).

Comparisons of computer simulations and experimental data for capacitive hyperthermia using different split-phantoms

Introduction Several positive clinical trials have demonstrated that capacitive hyperthermia (CHT) improves the effectiveness of radiation therapy for the treatment of various cancer entities. However, the ability of CHT to induce significant heating throughout the body is under debate. Objectives To perform a pilot study involving comparisons of computer simulations and experimental data using different split-phantoms to validate hyperthermia treatment modeling for pre-planning for a clinical CHT system and to investigate the feasibility of split-phantom measurements in capacitive hyperthermia. Materials and methods The CHT system EHY-2030 (Oncotherm, Budapest, Hungary) was used. The system provides two electrode sizes, but only the smaller electrode, indicated as D200 electrode, was investigated in this pilot study. Horizontally and vertically splittable, different multi-slice phantoms with dielectric material properties simulating muscle and electrically low conductive fat were produced and heated. During the heating procedure, temperature-time curves were measured, and thermal images were captured. Specific absorption rate values were derived from the temperature rise (TR) values. Concomitantly, computer field simulations utilizing a detailed CAD-based model of the CHT system were performed using the simulation platform Sim4Life and compared with measurements. Results For the investigated electrode D200 the system power of 75 W was applied, which is half of the maximum power of 150 W and lies in the range of usual values for this electrode applied in patient treatments in our clinic. For 75 W, a heating of 3.6 °C in 6 min in a depth of 1 cm in an agar-based, muscle tissue-equivalent phantom was achieved. The addition of a 1 cm thick, synthetic, low dielectric fat layer reduced the TR up until a depth of 8.5 cm by on average around 38% (from 8.5 cm onwards the absolute local TR is similar, deviations are ≤0.1 °C). In terms of point-to-point absolute SAR comparison (without any normalization), up to a depth of 11 cm in the phantoms central vertical plot, the simulation differs from the measured TR points by on average 25% (ranging from 7% to 36%) for the homogeneous phantom and by on average 43% (ranging from 26% to 60%) for the inhomogeneous phantom. Conclusion Computer simulations and experimental data were compared for the CHT system EHY-2030 using the D200 electrode, applying a thermal imaging technique for different vertically splittable phantoms. This pilot study data can be used as a guidance regarding the expected heating for this commonly used electrode size but also to further elucidate the significance of non-thermal anticancer effects. Further studies are needed for different sizes and geometries of electrodes and phantoms.

Evaluation of automatic tube current modulation in a CT scanner using a customised homogeneous phantom

Objective.The introduction of automatic tube current modulation (ATCM) has resulted in complex relationships between scanner parameters, patient body habitus, radiation dose, and image quality. ATCM adjusts tube current based on x-ray attenuation variations in the scan region, and overall patient dose depends on a combination of factors. This work aims to develop mathematical models that predict CT radiation dose and image noise in terms of attenuating diameter and all relevant scanner parameters.Approach.A homogenous phantom, equipped with the features to conduct discrete and continuous adaption tests, was developed to model ATCM in a Philips CT scanner. Scanner parameters were varied based on theoretical dose relationships, and a MATLAB script was developed to extract data from DICOM images. R statistical software was employed for data analysis, plotting, and regression modelling.Main Results.Phantom data provided the following insights: Median tube current decreased by 81% as tube potential varied from 80 kVp to 140 kVp. Doubling the DoseRight Index (DRI) from 12 to 24, at 24 cm diameter, produced a 294% increase in mA and a 46% decrease in noise. Mean mA increased by 53% whilst mean noise increased by 5.7% as helical pitch increased from 0.6 to 0.925. Changing rotation time from 0.33s to 0.75s gave a 56% reduction in mean mA and no change in image noise. Increasing detector collimation (n×T) resulted in higher tube currents and lower output image noise values, asnandTwere varied independently. Interpreting these results to apply transformations relevant to each independent variable produced models for tube current and noise with adjusted R-squared values of 0.965 and 0.912, respectively.Significance.The models developed more accurately predict radiation dose and image quality for specific patients and scanner settings. They provide imaging professionals with a practical tool to optimize scan protocols according to patient diameters and clinical objectives.

Development of an algorithm for proton dose calculation in magnetic fields.

The advantages of proton therapy can be further enhanced with online magnetic resonance imaging (MRI) guidance. One of the challenges in the realization of MRI-guided proton therapy (MRPT) is accurately calculating the radiation dose in the presence of magnetic fields. This study aims to develop an efficient and accurate proton dose calculation algorithm adapted to the presence of magnetic fields. An analytical-numerical radiation dose calculation algorithm, Proton and Ion Dose Engine (PRIDE), was developed. The algorithm combines the pencil beam algorithm (PBA) with a novel iterative voxel-based ray-tracing algorithm. The new ray-tracing method uses fewer assumptions and ensures broader applicability for proton beam trajectory prediction in magnetic fields, and has been compared to Wolf's method and Schellhammer's method. The accuracy of PRIDE algorithm was validated on three phantoms and two practical plans (one single-field water plan and one prostate tumor plan) in different magnetic field strengths up to 3.0 T. The validation was performed by comparing the results against the Monte Carlo (MC) simulations, using the global gamma index criteria of 2%/2mm and 3%/3mm with a 10% threshold. PRIDE showed good agreement with MC in homogeneous and slab heterogeneous phantom, achieving gamma passing rates (%GPs) above 99% for 2%/2mm criteria when magnetic field strength is not greater than 1.5 T. Although the agreement decreased for scenarios involving high proton energy (240 MeV) and strong magnetic field (3.0 T), the 2%/2mm %GPs still remained above 98%. In lateral heterogeneous phantom, the accuracy of PRIDE decreased due to the PBA's limitation. For the two practical plans in different magnetic fields, %GPs exceeded 98% and 99% for 2%/2mm and 3%/3mm criteria, respectively. PRIDE can perform efficient and accurate proton dose calculation in magnetic fields up to 3.0 T, and is expected to work as a useful tool for proton dose calculation in MRPT.

RayStation/GATE Monte Carlo simulation framework for verification of proton therapy based on the 12N imaging

Objective.12N, having a half-life of 11 ms, is a highly effective positron emitter that can potentially provide near real-time feedback in proton therapy. There is currently no framework for comparing and validating positron emission imaging of12N. This work describes the development and validation of a Monte Carlo (MC) framework to calculate the images of12N, as well as long-lived isotopes, originating from activation by protons.Approach. The available dual-panel Biograph mCT PET scanner was modeled in GATE and validated by comparing the simulated sensitivity map with the measured one. The distributions of12N and long-lived isotopes were calculated by RayStation and used as the input of GATE simulations. The RayStation/GATE combination was verified using proton beam irradiations of homogeneous phantoms. A 120 MeV pulsed pencil beam with 108protons per pulse was used. Two-dimensional images were created from the GATE output and compared with the images based on the measurements and the 1D longitudinal projection of the full 2D image was used to calculate the12N activity range.Main results. The simulated sensitivity in the center of the FoV (5.44%) agrees well with the measured one (5.41%). The simulated and measured 2D sensitivity maps agree in good detail. The relative difference between the measured and simulated positron activity range for both12N and long-lived isotopes is less than 1%. The broadening of the12N images relative to those of the longer-lived isotopes can be understood in terms of the large positron range of12N.Significance. We developed and validated a MC framework based on RayStation/GATE to support the in-beam PET method for quality assurance of proton therapy. The inclusion of the very short-lived isotope12N makes the framework useful for developing near real-time verification. This represents a significant step towards translating12N real-time in vivo verification to the clinic.

Comparative assessment and QA measurement array validation of Monte Carlo and Collapsed Cone dose algorithms for small fields and clinical treatment plans.

Many studies have demonstrated superior performance of Monte Carlo (MC) over type B algorithms in heterogeneous structures. However, even in homogeneous media, MC dose simulations should outperform type B algorithms in situations of electronic disequilibrium, such as small and highly modulated fields. Our study compares MC and Collapsed Cone (CC) dose algorithms in RayStation 12A. Under consideration are 6 MV and 6 MV flattening filter-free (FFF) photon beams, relevant for VMAT plans such as head-and-neck and stereotactic lung treatments with heterogeneities, as well as plans for multiple brain metastases in one isocenter, involving highly modulated small fields. We aim to investigate collimator angle dependence of small fields and performance differences between different combinations of ArcCHECK configuration and dose algorithm. Several verification tests were performed, ranging from simple rectangular fields to highly modulated clinical plans. To evaluate and compare the performance of the models, the agreements between calculation and measurement are compared between MC and CC. Measurements include water tank measurements for test fields, ArcCHECK measurements for test fields and VMAT plans, and film dosimetry for small fields. In very small or narrow fields, our measurements reveal a strong dependency of dose output to collimator angle for VersaHD with Agility MLC, reproduced by both dose algorithms. ArcCHECK results highlight a suboptimal agreement between measurements and MC calculations for simple rectangular fields when using inhomogeneous ArcCHECK images. Therefore, we advocate for the use of homogeneous phantom images, particularly for static fields, in ArcCHECK verification with MC. MC might offer performance benefits for more modulated treatment fields. In ArcCHECK results for clinical plans, MC performed comparable to CC for 6 MV. For 6 MV FFF and the preferred homogeneous phantom image, MC resulted in consistently better results (13%-64% lower mean gamma index) compared to CC.

Assessment of using an Octavius 4D Measuring System for Patient-specific VMAT Quality Assurance in Togo

Purpose and objective: Quality assurance (QA) programs are designed to improve the quality and the safety of radiation treatments, including machine- and patient-specific QA (PSQA). The objective of this study was to evaluate the current state of PSQA practice and identify the area for potential improvement for VMAT delivery. Materials and methods: The Octavius 4D (O4D) system accuracy was evaluated using an O4D homogeneous phantom for different field sizes. The system response to dose linearity, field sizes, and PDD difference tests were performed against the calculated dose of the treatment planning system (TPS) for a 6 MV photon beam. The deviation of the delivered dose was evaluated at the isocenter and different depths. Moreover, pretreatment verification of 40 VMAT plans was performed including prostate cancer (PC), head and neck cancer (HNC), uterine and cervical cancer (UCC), and breast cancer (BC). The PTW VeriSoft software was used to perform the local and global 3D gamma analysis by comparing the reconstructed 3D dose against the calculated dose using criteria 2%/2 mm and 3%/3 mm, 20% of low-dose threshold, and 95% of gamma passing rate (%GP) tolerance level. In the clinical scenario, the sensitivity of the O4D system in detecting VMAT delivery and setup errors has been investigated by measuring the variation of %GP values before and after the simulated errors using one of the VMAT plans related to each treatment site. Results: The O4D system reported good agreement for linearity, field size, and PDD differences with TPS dose being within ± 2% tolerance for a 6 MV photon beam. Output factors were consistent between the ionization chamber and the O4D detector 1500 array down to 4 x 4 cm2 field size with a maximum deviation of less than 1%. The introduction of deliberate errors caused the decrease of %GP values. In most scenarios, the %GP value of simulated errors was detected with 2%/2 mm and ranged between the detection threshold and gamma passing threshold. Conclusion: The results indicate that the O4D system is sensitive to detect delivery and setup errors with restrictive criteria of 2%/2 mm for routine pretreatment verification. Moreover, this system should be used in combination with kV-CBCT to improve dosimetry accuracy and treatment reproducibility.

A novel stress sensor enables accurate estimation of micro-scale tissue mechanics in quantitative micro-elastography.

Quantitative micro-elastography (QME) is a compression-based optical coherence elastography technique enabling the estimation of tissue mechanical properties on the micro-scale. QME utilizes a compliant layer as an optical stress sensor, placed between an imaging window and tissue, providing quantitative estimation of elasticity. However, the implementation of the layer is challenging and introduces unpredictable friction conditions at the contact boundaries, deteriorating the accuracy and reliability of elasticity estimation. This has largely limited the use of QME to ex vivo studies and is a barrier to clinical translation. In this work, we present a novel implementation by affixing the stress sensing layer to the imaging window and optimizing the layer thickness, enhancing the practical use of QME for in vivo applications by eliminating the requirement for manual placement of the layer, and significantly reducing variations in the friction conditions, leading to substantial improvement in the accuracy and repeatability of elasticity estimation. We performed a systematic validation of the integrated layer, demonstrating >30% improvement in sensitivity and the ability to provide mechanical contrast in a mechanically heterogeneous phantom. In addition, we demonstrate the ability to obtain accurate estimation of elasticity (<6% error compared to <14% achieved using existing QME) in homogeneous phantoms with mechanical properties ranging from 40 to 130 kPa. Furthermore, we show the integrated layer to be more robust, exhibiting increased temporal stability, as well as improved conformity to variations in sample surface topography, allowing for accurate estimation of elasticity over acquisition times 3× longer than current methods. Finally, when applied to ex vivo human breast tissue, we demonstrate the ability to distinguish between healthy and diseased tissue features, such as stroma and cancer, confirmed by co-registered histology, showcasing the potential for routine use in biomedical applications.

Remote sensing of high energy charged particle current (HEC) for megavoltage therapeutic electron beams

Objective. We demonstrate detection of high energy particle current (HEC) for MeV therapeutic electron beams. Detection of HEC comprises of remote sensing or acquiring information about HEC inside radiation transport medium from a distance outside of the medium. Approach. HEC is self-propelled motion of charged particles through a radiation transport medium. Remote sensing of HEC is embodied in an experimental setup, which includes homogeneous and heterogeneous phantoms irradiated with 4–15 MeV electron beams and two large area parallel-plane electrodes extraneous to the phantoms providing two-parameter detection. We also introduce a new type of scanning method (depth-scan) for probing object properties along the beamline axis. Main Results. Deterministic radiation transport simulations and measurements agree, considering differences in simulation vs experimental geometry and experimental uncertainties. Significance. This method may be suitable for range detection of charged particle beams, or for probing of radiation opaque objects in non-destructive testing.

Acoustomotive diffuse correlation spectroscopy for sensing mechanical stiffness in tissue-mimicking phantoms.

Many diseases, such as inflammation, dropsy, or tumors, often cause alterations in the mechanical stiffness of human tissues. Ultrasound-based techniques are commonly adopted in clinics for stiffness assessment, whereas optical methodologies hold promise for sensing strain changes and providing optical information pertaining to the microcirculatory network, thereby facilitating comprehensive measurements of tissue physiopathology. Diffuse correlation spectroscopy (DCS), an emerging dynamic light scattering technique, has been used to capture the enhanced motion of light scatterers induced by acoustic radiation force (ARF). Theoretically, the amplitude of this enhanced scatterers motion is related to the medium stiffness. Based on this relationship, we report a light coherent technique that combines ARF and DCS to qualitatively evaluate changes in the stiffness of medium. We experimentally demonstrate the accuracy and feasibility of this technique for probing stiffness in homogeneous phantom by comparing it with independent ultrasound methods. Additionally, we explore a potential application of this technique in distinguishing between fluid filled lesion and homogeneous tissue through heterogeneous phantom experiments. This unique combination of ARF and DCS, namely, acoustomotive DCS (AM-DCS), would provide an alternative way to measure particle-motion related stiffness, thereby assisting in the diagnosis and treatment of diseases.

Stopping power and range estimations in proton therapy based on prompt gamma timing: motion models and automated parameter optimization

Objective. Particle therapy treatments are currently limited by uncertainties of the delivered dose. Verification techniques like Prompt-Gamma-Timing-based Stopping Power Estimation (PGT-SPE) may allow for reduction of safety margins in treatment planning. Approach. From Prompt-Gamma-Timing measurements, we reconstruct the spatiotemporal distribution of prompt gamma emissions, which is linked to the average motion of the primary particles. The stopping power is determined by fitting a model of the average particle motion. Here, we compare a previously published implementation of the particle motion model with an alternative formulation and present two formulations to automatically select the hyperparameters of our procedure. The performance was assessed using Monte-Carlo simulations of proton beams (60 MeV–219 MeV) impinging on a homogeneous PMMA phantom. Main results. The range was successfully determined within a standard deviation of 3 mm for proton beam energies from 70 MeV to 219 MeV. Stopping power estimates showed errors below 5% for beam energies above 160 MeV. At lower energies, the estimation performance degraded to unsatisfactory levels due to the short range of the protons. The new motion model improved the estimation performance by up to 5% for beam energies from 100 MeV to 150 MeV with mean errors ranging from 6% to 18%. The automated hyperparameter optimization matched the average error of previously reported manual selections, while significantly reducing the outliers. Significance. The data-driven hyperparameter optimization allowed for a reproducible and fast evaluation of our method. The updated motion model and evaluation at new beam energies bring us closer to applying PGT-SPE in more complex scenarios. Direct comparison of stopping power estimates between treatment planning and measurements during irradiation would offer a more direct verification than other secondary-particle-based techniques.

Using the Probability Density Function-Based Channel-Combination Bloch-Siegert Method Realizes Permittivity Imaging at 3T.

Magnetic resonance electrical properties tomography (MR EPT) can retrieve permittivity from the B1+ magnitude. However, the accuracy of the permittivity measurement using MR EPT is still not ideal due to the low signal-to-noise ratio (SNR) of B1+ magnitude. In this study, the probability density function (PDF)-based channel-combination Bloch-Siegert (BSS) method was firstly introduced to MR EPT for improving the accuracy of the permittivity measurement. MRI experiments were performed using a 3T scanner with an eight-channel receiver coil. The homogeneous water phantom was scanned for assessing the spatial distribution of B1+ magnitude obtained from the PDF-based channel-combination BSS method. Gadolinium (Gd) phantom and rats were scanned for assessing the feasibility of the PDF-based channel-combination BSS method in MR EPT. The Helmholtz-based EPT reconstruction algorithm was selected. For quantitative comparison, the permittivity measured by the open-ended coaxial probe method was considered as the ground-truth value. The accuracy of the permittivity measurement was estimated by the relative error between the reconstructed value and the ground-truth value. The reconstructed relative permittivity of Gd phantom was 52.413, while that of rat leg muscle was 54.053. The ground-truth values of relative permittivity of Gd phantom and rat leg muscle were 78.86 and 49.04, respectively. The relative error of average permittivity was 33.53% for Gd and 10.22% for rat leg muscle. The results indicated the high accuracy of the permittivity measurement using the PDF-based channel-combination BSS method in MR EPT. This improvement may promote the clinical application of MR EPT technology, such as in the early diagnosis of cancers.

Design, construction, and dosimetry of 3D printed heterogeneous phantoms for synchrotron brain cancer radiation therapy quality assurance

Objective. This study aims to design, manufacture, and test 3D printed quality assurance (QA) dosimetry phantoms for synchrotron brain cancer radiation therapy at the Australian synchrotron. Approach. Fabricated 3D printed phantoms from simple slab phantoms, a preclinical rat phantom, and an anthropomorphic head phantom were fabricated and characterized. Attenuation measurements of various polymers, ceramics and metals were acquired using synchrotron monochromatic micro-computed tomography (CT) imaging. Polylactic acid plus, VeroClear, Durable resin, and tricalcium phosphate were used in constructing the phantoms. Furthermore, 3D printed bone equivalent materials were compared relative to ICRU bone and hemihydrate plaster. Homogeneous and heterogeneous rat phantoms were designed and fabricated using tissue-equivalent materials. Geometric accuracy, CT imaging, and consistency were considered. Moreover, synchrotron broad-beam x-rays were delivered using a 3 Tesla superconducting multipole wiggler field for four sets of synchrotron radiation beam qualities. Dose measurements were acquired using a PinPoint ionization chamber and compared relative to a water phantom and a RMI457 Solid Water phantom. Experimental depth doses were compared relative to calculated doses using a Geant4 Monte Carlo simulation. Main results. Polylactic acid (PLA+) shows to have a good match with the attenuation coefficient of ICRU water, while both tricalcium phosphate and hydroxyapatite have good attenuation similarity with ICRU bone cortical. PLA+ material can be used as substitute to RMI457 slabs for reference dosimetry with a maximum difference of 1.84%. Percent depth dose measurement also shows that PLA+ has the best match with water and RMI457 within ±2.2% and ±1.6%, respectively. Overall, PLA+ phantoms match with RMI457 phantoms within ±3%. Significance and conclusion. The fabricated phantoms are excellent tissue equivalent equipment for synchrotron radiation dosimetry QA measurement. Both the rat and the anthropomorphic head phantoms are useful in synchrotron brain cancer radiotherapy dosimetry, experiments, and future clinical translation of synchrotron radiotherapy and imaging.

Meander-Lined Implantable Antenna Design at 2.45 GHz Using Transmission Line Model

Scientists mostly follow a trial-and-error approach using simulators for designing implantable antennas which may increase time complexity and memory usage. In this article, the Transmission Line Model (TLM) is utilized to design a miniaturized meander-line implantable antenna at 2.45 GHz for reducing time and memory requirements. The meander-line antenna is first decomposed into different segments – x-directed, y-directed, and bends. Each segment is considered a transmission line and each line is replaced by its equivalent lumped LC network. The lumped parameters are calculated using the dimensions of the segment and guided wavelength inside the body. Here, a three-layered body model is represented by a T-type resistor, inductor and capacitor (RLC) network. The antenna within the human body is then simulated using Finite Element Method (FEM)-based CST Microwave Studio software. FEM technique is taking 4.94-MB memory space and 13-minutes time to analyze this implanted system whereas TLM is analyzing the same system by considering 726-KB memory within 52 s. TLM is predicting implanted antenna performance considering ∼0.8% error concerning simulated response. The designed antenna is fabricated and measured within a homogeneous body phantom and minced pork to verify the simulated response. The effect of chamfering of the corners of the meander-line antenna is also analyzed here. From this study, it is observed that TLM can predict implantable antenna response efficiently with low memory and less time requirement with respect to FEM which is helpful for antenna engineers. Analysis of implantable antenna with low memory and time requirements using TLM is a novel approach to this work.

Agar-based Phantom for Evaluating Targeting of High-intensity Focused Ultrasound Systems for Breast Ablation

Aim: Phantoms are often utilized for the preclinical evaluation of novel high-intensity focused ultrasound (HIFU) systems, serving as valuable tools for validating efficacy. In the present study, the feasibility of a homogeneous agar-based breast-shaped phantom as a tool for the preclinical evaluation of HIFU systems dedicated to breast cancer was assessed. Specifically, the effect of the increased phantom curvature on temperature increase was examined through sonications executed on two sides having varied curvatures. Materials and Methods: Assessment was performed utilizing a 1.1 MHz focused transducer. Sonications on the two phantom sides were executed at varied acoustical power in both a laboratory setting and inside a 1.5 T magnetic resonance imaging scanner. Sonications were independently performed on two identical phantoms for repeatability purposes. Results: Temperature changes between 7.1°C–34.3°C and 5.1°C–21.5°C were recorded within the decreased and increased curvature sides, respectively, for acoustical power of 3.75–10 W. High-power sonications created lesions which were approximately symmetrically formed around the focal point at the decreased curvature side, while they were shifted away from the focal point at the increased curvature side. Conclusions: The present findings indicate that increased curvature of the breast phantom results in deformed focal shapes and decreased temperatures induced at the focal area, thus suggesting treatment correction requirements in the form of focus control or accurate robotic movement. The developed breast-shaped phantom can be utilized as an evaluation tool of HIFU systems dedicated to breast cancer since it can visually verify the efficacy of any system.

Development and verification of a Geant4 model of the electron beam mode in a clinical linear accelerator

At present, a significant number of studies are focused on the development of novel methodologies for the fabrication of dosimetry phantoms. One of these methods is to produce heterogeneous samples by 3D printing. In order to select the most appropriate parameters for such products, it is necessary to conduct numerical simulations. In this work, we developed the model of a beam-forming system using a medical linear accelerator as a reference. This model was used to determine simulation parameters and corresponding dose distributions of an electron beam with nominal energies of 6, 12, and 15 MeV in a homogeneous water phantom. These parameters were, in fact, adapted to provide maximum agreement between simulated distributions and those experimentally obtained with the clinical linear accelerator. The beam simulation was performed using the Geant4 Monte Carlo toolkit. The simulation geometry of the accelerator treatment head includes scattering foil and a flattening filter, which are designed for electron beam broadening. Additionally, the beam-forming system was incorporated to collimate the beam to the required size. A metal applicator was included to reduce the contribution of electron scattering in air. The main simulation parameters were iteratively tuned by comparing simulation results with experimentally obtained data. It is shown that the simulated percentage depth dose and transverse profiles for electron beams in water phantom are in good agreement with the experimental data obtained with a cylindrical ionization chamber. This demonstrates that the methodology employed in the development of the numerical model of the medical linear accelerator is vendor-independent, readily implementable, and allows for rapid calculations. Furthermore, the model can be applied for a variety of purposes, including the selection of parameters for the fabrication of heterogeneous dosimetry phantoms.

An end-to-end deep convolutional neural network-based dose engine for parotid gland cancer seed implant brachytherapy.

Seed implant brachytherapy (SIBT) is a promising treatment modality for parotid gland cancers (PGCs). However, the current clinical standard dose calculation method based on the American Association of Physicists in Medicine (AAPM) Task Group 43 (TG-43) Report oversimplifies patient anatomy as a homogeneous water phantom medium, leading to significant dose calculation errors due to heterogeneity surrounding the parotid gland. Monte Carlo Simulation (MCS) can yield accurate dose distributions but the long computation time hinders its wide application in clinical practice. This paper aims to develop an end-to-end deep convolutional neural network-based dose engine (DCNN-DE) to achieve fast and accurate dose calculation for PGC SIBT. A DCNN model was trained using the patient's CT images and TG-43-based dose maps as inputs, with the corresponding MCS-based dose maps as the ground truth. The DCNN model was enhanced based on our previously proposed model by incorporating attention gates (AGs) and large kernel convolutions. Training and evaluation of the model were performed using a dataset comprising 188 PGC I-125 SIBT patient cases, and its transferability was tested on an additional 16 non-PGC head and neck cancers (HNCs) I-125 SIBT patient cases. Comparison studies were conducted to validate the superiority of the enhanced model over the original one and compare their overall performance. On the PGC testing dataset, the DCNN-DE demonstrated the ability to generate accurate dose maps, with percentage absolute errors (PAEs) of 0.67%±0.47% for clinical target volume (CTV) D90 and 1.04%±1.33% for skin D0.1cc. The comparison studies revealed that incorporating AGs and large kernel convolutions resulted in 8.2% (p<0.001) and 3.1% (p<0.001) accuracy improvement, respectively, as measured by dose mean absolute error. On the non-PGC HNC dataset, the DCNN-DE exhibited good transferability, achieving a CTV D90 PAE of 1.88%±1.73%. The DCNN-DE can generate a dose map in less than 10 ms. We have developed and validated an end-to-end DCNN-DE for PGC SIBT. The proposed DCNN-DE enables fast and accurate dose calculation, making it suitable for application in the plan optimization and evaluation process of PGC SIBT.

MR Safety of Inductively Coupled and Conventional Intraoral Coils.

Intraoral coils (IOCs) in magnetic resonance imaging (MRI) significantly improve the signal-to-noise ratio compared with conventional extraoral coils. To assess the safety of IOCs, we propose a 2-step procedure to evaluate radiofrequency-induced heating of IOCs and compare maximum temperature increases in 3 different types of IOCs. The 2-step safety assessment consists of electric field measurements and simulations to identify local hotspots followed by temperature measurements during MRI. With this method, 3 different coil types (inductively coupled IFC, transmit/receive tLoop, and receive-only tLoopRx) were tested at 1.5 T and 3 T for both tuned and detuned coil states. High SAR and regular MRI protocols were applied for 2 coil positions. The measured E field maps display distinct hotspots for all tuned IOCs, which were reduced by at least 40-fold when the IOCs were detuned. Maximum temperature rise was higher when the coils were positioned at the periphery of the phantom with the coil planes parallel to B0. When neither active nor passive detuning was applied, maximum temperature increase of ΔT = 1.3/0.5/1.8 K was found for IFC/tLoop/tLoopRx coils. Hotspots detected by E field measurements, and simulations were consistent. In the simulations, the results were different for homogeneous phantoms compared with full anatomical models. The 2-step test procedure is applicable to different coil types. The results indicate that a risk for radiofrequency-induced heating exists for tuned IOCs, so that adequate detuning circuits need to be integrated in the coils to ensure safe operation.

Ultrasound Shear Elastography With Expanded Bandwidth (USEWEB): A Novel Method for 2D Shear Phase Velocity Imaging of Soft Tissues.

Ultrasound shear wave elastography (SWE) is a noninvasive approach for evaluating mechanical properties of soft tissues. In SWE either group velocity measured in the time-domain or phase velocity measured in the frequency-domain can be reported. Frequency-domain methods have the advantage over time-domain methods in providing a response for a specific frequency, while time-domain methods average the wave velocity over the entire frequency band. Current frequency-domain approaches struggle to reconstruct SWE images over full frequency bandwidth. This is especially important in the case of viscoelastic tissues, where tissue viscoelasticity is often studied by analyzing the shear wave phase velocity dispersion. For characterizing cancerous lesions, it has been shown that considerable biases can occur with group velocity-based measurements. However, using phase velocities at higher frequencies can provide more accurate evaluations. In this paper, we propose a new method called Ultrasound Shear Elastography with Expanded Bandwidth (USEWEB) used for two-dimensional (2D) shear wave phase velocity imaging. We tested the USEWEB method on data from homogeneous tissue-mimicking liver fibrosis phantoms, custom-made viscoelastic phantom measurements, phantoms with cylindrical inclusions experiments, and in vivo renal transplants scanned with a clinical scanner. We compared results from the USEWEB method with a Local Phase Velocity Imaging (LPVI) approach over a wide frequency range, i.e., up to 200-2000 Hz. Tests carried out revealed that the USEWEB approach provides 2D phase velocity images with a coefficient of variation below 5% over a wider frequency band for smaller processing window size in comparison to LPVI, especially in viscoelastic materials. In addition, USEWEB can produce correct phase velocity images for much higher frequencies, up to 1800 Hz, compared to LPVI, which can be used to characterize viscoelastic materials and elastic inclusions.