Spherical Phantom Research Articles

Dielectric-Loaded Conformal Microstrip Antennas for Versatile In-Body Applications

Wireless implantable bioelectronics and accurate in vivo characterization of path loss require efficient and robust in-body antennas. Loading electric antennas with the effective permittivities higher than that of surrounding tissues are a promising solution. In this letter, proof-of-concept conformal microstrip antennas are proposed. The antennas are optimized for a standard impedance of 50 $\Omega$ and operate in all high-water-content tissues. Integral ground plane shields the antenna from inner circuitry. The radiation efficiencies are 0.4%, 2.2%, and 1.2% for the (434, 868, and 1400) MHz designs, respectively, when computed in a $\varnothing$ 100 mm spherical phantom with muscle-equivalent properties. The efficiencies are compared to the fundamental limitations and closely approach them. Specific absorption rates are evaluated, and the corresponding maximum input power levels are established. Prototypes are fabricated and characterized to validate the design.

Electric field simulations for transcranial brain stimulation using FEM: an efficient implementation and error analysis

Objective. Transcranial magnetic stimulation (TMS) and transcranial electric stimulation (TES) modulate brain activity non-invasively by generating electric fields either by electromagnetic induction or by injecting currents via skin electrodes. Numerical simulations based on anatomically detailed head models of the TMS and TES electric fields can help us to understand and optimize the spatial stimulation pattern in the brain. However, most realistic simulations are still slow, and the role of anatomical fidelity on simulation accuracy has not been evaluated in detail so far. Approach. We present and validate a new implementation of the finite element method (FEM) for TMS and TES that is based on modern algorithms and libraries. We also evaluate the convergence of the simulations and estimate errors stemming from numerical and modelling aspects. Main results. Comparisons with analytical solutions for spherical phantoms validate our new FEM implementation, which is three to six times faster than previous implementations. The convergence results suggest that accurately capturing the tissue geometry in addition to choosing a sufficiently accurate numerical method is of fundamental importance for accurate simulations. Significance. The new implementation allows for a substantial increase in computational efficiency of FEM TMS and TES simulations. This is especially relevant for applications such as the systematic assessment of model uncertainty and the optimization of multi-electrode TES montages. The results of our systematic error analysis allow the user to select the best tradeoff between model resolution and simulation speed for a specific application. The new FEM code is openly available as a part of our open-source software SimNIBS 3.0.

Objective evaluation of imaging performance using a commercially available spherical lesion anechoic phantom

The nature of ultrasound imaging makes it challenging to perform quantitative quality control tests representative of clinical performance. IPEM Report No. 102 outlines several different tests which monitor aspects of the systems performance likely to affect the clinical efficiency following deterioration and change. Such tests include measurement of axial and lateral resolution, slice thickness, contrast resolution and low contrast penetration using ultrasound phantoms. Such phantoms consist of cylindrical objects and wire filaments embedded within homogeneous tissue-mimicking material. The ability of an ultrasound system to resolve the contrast difference between the cylindrical targets and background is regarded as an indicator of imaging performance; however, cylindrical targets are not necessarily representative of the clinical situation where ellipsoidal or spherical lesions may be considered more appropriate. To date, spherical lesion target phantoms have been produced in specialist research laboratories and are, thus, not widely available. Accordingly, this study sought to examine a relatively new commercially available anechoic spherical lesion phantom to determine its efficacy for objective evaluation of performance change. Several modern medical ultrasound systems were tested using the Gammex RMI 408 Spherical Lesion target phantom, containing 2mm and 4mm diameter anechoic spherical targets located within one plane throughout the phantom depth. A Matlab® program was used for automatic objective image quality analysis. Preliminary results will be presented on the automated objective evaluation, metric repeatability and reproducibility. Simulated probe faults will be used to establish test efficacy and the results will be compared to traditional QA testing.

Detectability of pulmonary nodules by deep learning: results from a phantom study

To investigate how nodule size, nodule density, scan dose, slice thickness and reconstruction methods affect the performance of a deep learning (DL) model for detection of pulmonary nodules in phantom CT scans. Spherical lung nodule phantoms of two different densities (− 630 HU and + 100 HU) and five different sizes (3, 5, 8, 10, and 12 mm) were inserted into an anthropomorphic chest phantom. CT data were scanned and reconstructed using three different tube current (10, 50, 200 mAs), two different slice thickness of 1 and 2 mm, four reconstruction methods (FBP-standard (FBP-STD), FBP-Y sharp kernel (FBP-YA), iDose4-standard kernel (iDose4-STD) and iDose4-Y sharp kernel (iDose4-YA). Evaluation of deep learning model focused on detection sensitivity and precision. According to the statistical results from the study, we found that the sensitivity and precision performance depends on the nodule sizes, nodule type, tube current, reconstruction methods and image thickness. Comparing the solid (100 HU) and ground-glass (GGO, − 630 HU) nodule phantoms, solid nodule phantom predictions are rarely affected by tube current, reconstruction methods and nodule sizes. Both sensitivity and precision are close to 100% in all solid nodule phantom prediction cases. While the sensitivity and precision metrics of GGO nodule phantoms change in a wide range from 42.9 to 100%. Larger nodule size and higher tube current gives a better sensitivity and precision for GGO nodule phantoms in most cases. We also analyze the relationships between the image thickness and the reconstruction methods. For 1-mm thickness images, iDose4-STD and FBP-STD shows a better result in both sensitivity and precision metrics. As for 2-mm thickness images, iDose-YA and FBP-YA gain a better performance. The results of this phantom study demonstrated that high stability and flexibility of deep learning model can be used in daily clinical and screening practice.

MRI quality control for low-field MR-IGRT systems: Lessons learned.

PurposeTo present lessons learned from magnetic resonance imaging (MRI) quality control (QC) tests for low‐field MRI‐guided radiation therapy (MR‐IGRT) systems.MethodsMRI QC programs were established for low‐field MRI‐60Co and MRI‐Linac systems. A retrospective analysis of MRI subsystem performance covered system commissioning, operations, maintenance, and quality control. Performance issues were classified into three groups: (a) Image noise and artifact; (b) Magnetic field homogeneity and linearity; and (c) System reliability and stability.ResultsImage noise and artifacts were attributed to room noise sources, unsatisfactory system cabling, and broken RF receiver coils. Gantry angle‐dependent magnetic field inhomogeneities were more prominent on the MRI‐Linac due to the high volume of steel shielding in the gantry. B0 inhomogeneities measured in a 24‐cm spherical phantom were <5 ppm for both MR‐IGRT systems after using MRI gradient offset (MRI‐GO) compensation on the MRI‐Linac. However, significant signal dephasing occurred on the MRI‐Linac while the gantry was rotating. Spatial integrity measurements were sensitive to gradient calibration and vulnerable to shimming. The most common causes of MR‐IGRT system interruptions were software disconnects between the MRI and radiation therapy delivery subsystems caused by patient table, gantry, and multi‐leaf collimator (MLC) faults. The standard deviation (SD) of the receiver coil signal‐to‐noise ratio was 1.83 for the MRI‐60Co and 1.53 for the MRI‐Linac. The SD of the deviation from the mean for the Larmor frequency was 1.41 ppm for the MRI‐60Co and 1.54 ppm for the MRI‐Linac. The SD of the deviation from the mean for the transmitter reference amplitude was 0.90% for the MRI‐60Co and 1.68% for the MRI‐Linac. High SDs in image stability data corresponded to reports of spike noise.ConclusionsThere are significant technological challenges associated with implementing and maintaining MR‐IGRT systems. Most of the performance issues were identified and resolved during commissioning.

Experimental and numerical investigations of a small animal coil for ultra-high field magnetic resonance imaging (7T)

Abstract The purpose of this work was to develop and investigate a radiofrequency (RF) coil to perform image studies on small animals using the 7T magnetic resonance imaging (MRI) system, installed in the imaging platform in the autopsy room (Portuguese acronym PISA), at the University of Sao Paulo, Brazil, which is the unique 7T MRI scanner installed in South America. Due to a high demand to create new specific coils for this 7T system, it is necessary to carefully assess the distribution of electromagnetic (EM) fields generated by the coils and evaluate the patient/object safety during MRI procedures. To achieve this goal 3D numerical methods were used to design and analyse a 8-rungs transmit/receive linearly driven birdcage coil for small animals. Calculated magnetic field (B1) distributions generated by the coil were crosschecked with measured results, indicating good confidence in the simulated results. Electric field results were post-processed and predictions of local specific absorption rate (SAR) values were achieved for a spherical phantom filled with muscle-like tissue, indicating that the sample would not suffer any unsafe deposition of energy. Post mortem abdomen images obtained from a rat presented good image quality and no artifacts related to field non-homogeneity were observed.

Fabrication of a spherical inclusion phantom for validation of magnetic resonance-based magnetic susceptibility imaging

Fabrication of a spherical multi-compartment MRI phantom is demonstrated that can be used to validate magnetic resonance (MR)-based susceptibility imaging reconstruction. The phantom consists of a 10 cm diameter gelatin sphere that encloses multiple smaller gelatin spheres doped with different concentrations of paramagnetic contrast agents. Compared to previous multi-compartment phantoms with cylindrical geometry, the phantom provides the following benefits: (1) no compartmental barrier materials are used that can introduce signal voids and spurious phase; (2) compartmental geometry is reproducible; (3) spherical susceptibility boundaries possess a ground-truth analytical phase solution for easy experimental validation; (4) spherical geometry of the overall phantom eliminates background phase due to air-phantom boundary in any scan orientation. The susceptibility of individual compartments can be controlled independently by doping. During fabrication, formalin cross-linking and water-proof surface coating effectively blocked water diffusion between the compartments to preserve the phantom's integrity. The spherical shapes were realized by molding the inner gel compartments in acrylic spherical shells, 3 cm in diameter, and constructing the whole phantom inside a larger acrylic shell. From gradient echo images obtained at 3T, we verified that the phantom produced phase images in agreement with the theoretical prediction. Factors that limit the agreement include: air bubbles trapped at the gel interfaces, imperfect magnet shimming, and the susceptibility of external materials such as the phantom support hardware. The phantom images were used to validate publicly available codes for quantitative susceptibility mapping. We believe that the proposed phantom can provide a useful testbed for validation of MR phase imaging and MR-based magnetic susceptibility reconstruction.

Fundamental Limits for Implanted Antennas: Maximum Power Density Reaching Free Space

Fundamental limits on antenna performances are of key interest to the antenna designer, as they allow fast assessment of the feasibility of specific antenna requirements. These limits are defined by the key performance indicators (KPI) of specific antennas, such as the directivity for large aperture antennas or the achievable bandwidth on electrically small antennas. These limits have been obtained considering that the antenna radiates into free space. In this contribution, we develop the fundamental limits for implanted antennas, which thus radiate first into a lossy medium. KPIs assessing the quality of a specific antenna radiating into lossy medium are the total radiated power reaching free space (out of the lossy host medium) and the maximum power density obtained at the surface of the lossy host medium. The fundamental limits for implanted antennas proposed in this paper yield upper bounds for both KPIs and have been obtained considering elementary sources radiating into a spherical phantom. The spherical wave expansion of the electromagnetic fields was used to determine all the fields, and the limits obtained yield a useful upper bound for more complex scenarios.

A proof of concept kilovoltage intensity modulated radiotherapy platform for the treatment of glioblastoma multiforme

Purpose: To develop a compact, proof-of-concept kilovoltage intensity modulated radiotherapy platform to deliver contrast-enhanced radiotherapy (CERT). Methods: The proof-of-concept requirements were threefold: (i) develop a compact means to generate a kilovoltage x-ray broadbeam capable of targeting the K-edge of heavy metal contrast, (ii) investigate the potential clinical application of such a beam in silico, and (iii) construct a limited working prototype of the platform to identify weaknesses in the approach. The x-ray broadbeam was created with a standard kilovoltage x-ray tube heavily filtered with 0.156 mm tungsten. Depth dose and treatment characteristics of the beam were investigated using GEANT4 Monte Carlo framework. The working prototype included a miniaturized multi-rod collimator (MRC) constructed with tungsten carbide rods, 3D-printed gear rack, and an in-house GUI/microprocessor assembly. Measurements of simulated treatments were made with EBT3 radiochromic film. Results: Monte Carlo simulations of pseudo-clinical treatments on a homogeneous spherical Lucite phantom produced dose distributions that indicated surface dose was less than target dose. When simulated-bone was added to the calculations, dose-to-bone was found to be the probable limiting factor in clinical treatment. Nevertheless, when iodine contrast enhancement was accounted for, dose was escalated in the tumor compared to standard megavoltage treatments even with dose-to-bone limitations. Experimentally, after commissioning the MRC and calibrating the radiochromic film, surface dose was compared to maximum dose measured on each film. Surface dose measurements were higher than those in a megavoltage beam, but below the skin toxicity threshold. Dose at depth was reasonable despite increased beam attenuation at these energies from photoelectric interactions in bone. IMRT and non-coplanar dose falloffs around the target were steeper than kV 3D conformal dose falloff. Conclusions: The compact kilovoltage treatment platform presented here proved clinically feasible in both simulation and measurement. It is reasonable to pursue this modality further as a potential CERT platform.

Development of an IAEA phase-space dataset for the Leksell Gamma Knife® Perfexion™ using multi-threaded Geant4 simulations

This study was conducted to develop a phase-space dataset in the International Atomic Energy Agency (IAEA) format for Monte Carlo (MC) simulations of the Leksell Gamma Knife® (LGK, Elekta Instrument AB, Stockholm, Sweden) Perfexion™ (PFX). An open-source MC code, namely, the Geant4 toolkit with a recently updated multi-threaded mode, was used to maximize the efficiency of the developed IAEA phase-space dataset. The absorbed dose profiles for single shots of the LGK PFX were calculated using the developed dataset and compared with those from radiochromic film measurements and Leksell GammaPlan® version 11.0.3 (LGP, Elekta Instruments) for verification. The mean relative absorbed dose differences in all single shots were less than 3.6% compared with the films and less than 4.0% compared with LGP. The collimator output factors were also calculated for all single shots and compared with the LGP results. The simulated collimator output factor was 0.816 ± 0.003 for a 4-mm shot and 0.903 ± 0.001 for an 8-mm shot in a spherical water phantom. The efficiency of the developed dataset was evaluated by comparing the times required for various simulations. Simulations with the phase-space dataset ran 25, 8.2 and 3.2 times faster than simulations without the phase-space dataset for 4-, 8-, and 16-mm shots, respectively. Using the dataset developed in this study, MC simulations of the LGK PFX can be performed more efficiently for various purposes, such as treatment plan verification and beam quality factor calculations.

Influence of image noise and object size on segmentation accuracy of FDG-PET imaging: a phantom experiment.

We aimed to evaluate the influence of noise and object size on segmentation accuracy of fluorodeoxyglucose positron emission tomography (FDG-PET) imaging. The scanned data of spherical phantoms were used. For the gradient method, 40% maximum standardized uptake value (SUVmax) method, and SUV of 2.5 threshold method, we evaluated the correlation between segmentation accuracy and background variability and that between segmentation accuracy and sphere diameters. For the gradient method, background variability did not affect segmentation accuracy, but sphere diameters had a small effect. As for the 40% SUVmax threshold method, both sphere diameters and background variability affected the segmentation accuracy. In the SUV of 2.5 threshold method, segmentation accuracy was affected by sphere diameters but not by background variability. With regard to segmentation accuracy of FDG-PET imaging, the gradient method may be more accurate and reliable compared to threshold methods when applied to images with varying noise or object size.

Rapid acquisition of the 3D MRI gradient impulse response function using a simple phantom measurement.

To provide a simple tool for rapid measurement of the 3D gradient modulation transfer function (GMTF) of clinical MRI systems using a phantom. Knowledge of the transfer function is useful for gradient chain characterization, system calibration, and improvement of image reconstruction results. Starting from the well-established thin slice method used for phantom-based measurement of the 1D GMTF, we add phase encoding to partition the thin slices into voxels that act as localized field probes. From the signal phase evolution measured at the 3D voxel positions, the GMTF can be derived for cross and higher order spatial terms represented by spherical harmonics up to 3rd order. Using spherical phantoms, 16 GMTFs representing all terms up to 3rd order harmonics can be determined in a scan time of <2 min. A large voxel volume of >1 mL yields high SNR, enabling signal acquisition using the system's body coil. The method is applied for improving system calibration and for characterizing the effect of additional hardware in the bore. The presented method seems well-suited for rapid measurement of the GMTF of a clinical system, as it delivers high-quality results in a short scan time.

Response of a nanoDot OSLD system in megavoltage photon beams

PurposeThe aim of this study was to investigate the response of a nanoDot optically stimulated luminescence dosimeter (OSLD) system in megavoltage photon beams. MethodsThe nanoDot response was compared with the ionization chamber measurements for 4–18-MV photons in a plastic water phantom. The response was also calculated by the Monte Carlo method. In addition, the perturbation correction factor, PQ, in the nanoDot cavity was calculated according to the Burlin’s cavity theory. The angular dependence of the nanoDot was evaluated using a spherical phantom. ResultsThe calculated and measured nanoDot responses at a 10-cm depth and 10 × 10-cm2 field were in agreement within 1% for 4–18-MV. The response increased by 3% at a 20 × 20-cm2 field for the lower energy of 4 MV; however, it was constant within ±1% for 6–18 MV. The response was in a range from 1.0 to 0.99 for mean photon energy of more than 1.0 MeV but it increased with less than the 1.0 MeV. PQ for the nanoDot cavity was approximately constant at 0.96–0.97 for greater than and equal to 10 MV. The angular dependence decreased by 5% and 3% for 6 and 15 MV, respectively. ConclusionsThe nanoDot was energy-independent in megavoltage photon beams.

Remote afterloading patient-specific brachytherapy with liquid radioisotope for irradiation of extensive scalp lesions: A Monte Carlo study.

The aim of this study is to propose a remote afterloading patient-specific brachytherapy technique for total scalp irradiation by utilizing liquid radioisotope as well as a three-dimensional (3D) printer and to find an optimal radioisotope for the suggested technique. We designed a brachytherapy device composed of liquid radioisotope tank, tube, patient-specific applicator, and a thin flexible pouch. The liquid radioisotope tank, tube, and the flexible pouch are interconnected one another to constitute a closed loop system. The pouch is located inside the solid patient-specific applicator; therefore, when the liquid radioisotope is injected into the pouch, the pouch is inflated and fills the space inside the applicator. The 3D-printed patient-specific applicator keeps the uniform thickness of the liquid radioisotope conforming patient's contour. To investigate an optimum condition for the suggested system, we performed Monte Carlo simulation with the GEANT4 simulation toolkit. To find the optimal radioisotope, percent depth doses (PDDs) of P-32, Sr-89, Y-90, and I-125 solutions were acquired in a rectangular parallelepiped phantom. For the selected radiation source, PDDs as well as dose rates in spherical phantoms with radii of 7.7cm (infant head size) and 9.1cm (adult head size) were acquired. To deliver prescription doses at 4-mm depth regions (scalp region), 1-mm-thick Y-90 and 5-mm-thick I-125 in liquid form were found to be feasible for the suggested technique. For both spherical phantoms with radii of 7.7 and 9.1cm, when delivering 2Gy at the 4-mm depth region with the 1-mm-thick Y-90 and 5-mm-thick I-125 sources, 53.3 and 3.8Gy were delivered at the surface regions, respectively (delivery time=111.1 and 3.5min with 1GBq/ml solutions). The PDDs of Y-90 and I-125 became less than 1% at depths greater than 8 and 50mm, respectively. The remote afterloading patient-patient specific brachytherapy with I-125 or Y-90 in liquid form seems feasible for total scalp irradiation.

MRI B0 homogeneity and geometric distortion with continuous linac gantry rotation on an Elekta Unity MR-linac

This work aimed to quantify any principal magnetic field (B0) inhomogeneity and changes in MR image geometric distortion with continuous linac gantry rotation on an Elekta Unity MR-linac. This situation occurs for around a second between treatment beams during current image guided radiotherapy treatment and would occur frequently in foreseeable real-time adaptive radiotherapy treatment.Pixel by pixel maps of B0 inhomogeneity were obtained via repeated high temporal resolution pulse sequences with the linac gantry static at 36 gantry angles spaced ten degrees apart, and in continuous rotation at both 1 and 2 rpm. Individual B0 maps were subtracted from average maps across all data and the residual peak to peak inhomogeneity was calculated for each. The bulk geometric shift and change in physical extent of a 10 cm diameter spherical flood phantom during continuous linac gantry rotation at 1 and 2 rpm was compared to the static gantry case for two pulse sequences: the real-time clinical monitoring bFFE sequence and a non-clinical EPI sequence, chosen for its susceptibility to geometric distortion.The peak to peak inhomogeneity in the deviation-from-average ppm maps, plotted against gantry angle with the gantry in continuous rotation at 1 and 2 rpm were negligibly different from equivalent data obtained with the gantry static. The real-time clinical monitoring pulse sequence was shown to give negligible geometric distortion during continuous gantry motion, whilst a non-clinical EPI sequence showed bulk shifts of the order of one pixel and gantry angle dependent changes in extent, demonstrating the sensitivity of the chosen method.MR imaging on the Elekta Unity MR-Linac with the gantry in continuous motion is negligibly different from the static gantry case with current clinical pulse sequences. Real-time tracking and treatment plan adaptation using MR images obtained with the linac gantry in motion is possible.

Ray-tracing simulation of the radiation dose distribution on the surface of the spherical phantom of the MATROSHKA-R experiment onboard the ISS

Space radiation is one of the main concerns for human space flights. The prediction of the radiation dose for the actual spacecraft geometry is very important for the planning of long-duration missions. We present a numerical method for the fast calculation of the radiation dose rate during a space flight. We demonstrate its application for dose calculations during the first and the second sessions of the MATROSHKA-R space experiment with a spherical tissue-equivalent phantom. The main advantage of the method is the short simulation time, so it can be applied for urgent radiation dose calculations for low-Earth orbit space missions.The method uses depth-dose curve and shield-and-composition distribution functions to calculate a radiation dose at the point of interest. The spacecraft geometry is processed into a shield-and-composition distribution function using a ray-tracing method. Depth-dose curves are calculated using the GEANT4 Monte-Carlo code (version 10.00.P02) for a double-layer aluminum-water shielding. Aluminum-water shielding is a good approximation of the real geometry, as water is a good equivalent for biological tissues, and aluminum is the major material of spacecraft bodies.The method is applied to model the dose distribution on the surface of the spherical phantom in the MATROSHKA-R space experiment. The experiment has been carried out onboard the ISS from 2004 to the present. The absorbed dose was determined in 32 points on the phantom's surface. We find a good agreement between the data obtained in the experiment and our calculation results. The simulation method is thus applicable for future radiation dose predictions for low-Earth orbit missions and experiments.

The Effect of CT Scan Parameters on the Measurement of CT Radiomic Features: A Lung Nodule Phantom Study.

The purpose of this study was to explore the effects of CT slice thickness, reconstruction algorithm, and radiation dose on quantification of CT features to characterize lung nodules using a chest phantom. Spherical lung nodule phantoms of known densities (−630 and + 100 HU) were inserted into an anthropomorphic thorax phantom. CT scan was performed ten times with relocations. CT data were reconstructed using 12 different imaging settings; three different slice thicknesses of 1.25, 2.5, and 5.0 mm, two reconstruction kernels of sharp and standard, and two radiation dose of 30 mAs and 12 mAs. Lesions were segmented using a semiautomated method. Twenty representative CT quantitative features representing CT density and texture were compared using multiple regression analysis. In 100 HU nodule phantoms, 18 and 19 among 20 computer features showed significant difference between different mAs and reconstruction algorithms, respectively (p ≤ 0.05). 20, 19, and 19 computer features showed difference between slice thickness of 5.0 vs 1.25, 5.0 vs 2.5, and 2.5 vs 1.25 mm, respectively (p ≤ 0.05). In −630 HU nodule phantoms, 18 and 19 showed significant difference between different mAs and reconstruction algorithms, respectively (p ≤ 0.05). 18, 11, and 17 computer features showed difference between slice thickness of 5.0 vs 1.25, 5.0 vs 2.5, and 2.5 vs 1.25 mm, respectively (p ≤ 0.05). When comparing the absolute value of regression coefficient, the effect of slice thickness in 100 HU nodule and reconstruction algorithm in −630 HU nodule was greater than the effect of remaining scan parameters. The slice thickness, mAs, and reconstruction algorithm had a significant impact on the quantitative image features. In clinical studies involving deep learning or radiomics, it should be noted that differences in values can occur when using computer features obtained from different CT scan parameters in combination. Therefore, when interpreting the statistical analysis results, it is necessary to reflect the difference in the computer features depending on the scan parameters.

Microwave Emissions From Cell Phones Exceed Safety Limits in Europe and the US When Touching the Body

In our publications, we have shown both from measurements and computer modeling that the specific absorption rate (SAR) reduces by 10%-15% for every millimeter separation of the cell phone on account of rapidly diminishing EM fields in the near-field region of the cell phone antenna. This rapid reduction of SAR depending on the antenna and its location on the handset has been shown, both computationally and experimentally, regardless of the phantom model such as a flat phantom suggested for SAR compliance testing of devices in contact with the body, for a sphere phantom, and for head-shaped models used for SAR compliance testing of cell phones. Unfortunately, our observations in the past were based on SARs of only three cell phones. Expecting that the SARs for cell phones may exceed the safety limits for body contact, cell phone manufacturers have started to recommend that the devices can be used at 5-25 mm from the body even though it is difficult to see how to maintain this distance correctly under mobile conditions. The National Agency ANFR of France recently released the cell phone SAR test data for 450 cell phones that measure 10-g SARs reducing by 10%-30% for each millimeter distal placement from the planar body phantom. Their data corroborate our findings that most cell phones will exceed the safety guidelines when held against the body by factors of 1.6-3.7 times for the European/ICNIRP standard or by factors as high as 11 if 1-g SAR values were to be measured as required by the U.S. FCC.

Influence of field-of-view and section thickness of diagnostic imaging on thermal neutron flux estimation in dose-planning for boron neutron capture therapy.

Radiation treatment planning for boron neutron capture therapy (BNCT) often uses computed tomography (CT) images reconstructed utilizing various section thickness and field-of-view (FOV) settings. Based on these images, a geometrical model is created by setting material regions manually over the pixel space defined in the treatment planning system. Thus, a setting difference of several pixels inevitably occurs in creation of the model. The influence of different section thicknesses and FOVs on thermal neutron flux estimations using the BNCT planning system was studied here. A virtual phantom was created with six FOV sizes on the planning system. The position of the irradiated side of the phantom surface was shifted by 1-10 pixels along the beam direction or in the opposite direction to simulate the material setting on different pixels in the geometric model. The effect of a one-pixel-difference setting on thermal neutron flux increased with increasing FOV size. Next, a cylindrical and a spherical phantom were scanned, and each CT image was reconstructed with six FOV sizes and seven section thicknesses. The flux changes for all conditions were compared, with an allowable error rate of ± 0.05, as in conventional X-ray radio therapy. The accuracy of neutron flux estimations was also evaluated by repeating the calculation procedures with CT scanning 5 or 10 times, and was found to be mostly within 0.03, except for the FOV-500 condition (0.074). These results suggested that a smaller FOV and section thickness with realistic conditions could improve evaluation accuracy of the thermal neutron flux for BNCT.

Texture analysis for automated evaluation of Jaszczak phantom SPECT system tests.

Routine quarterly quality assurance (QA) assessment of single photon emission computed tomography (SPECT) systems includes analysis of multipurpose phantoms containing spheres and rods of various sizes. When evaluated by accreditation agencies, criteria applied to assess image quality are largely subjective. Determining a quantified image characteristic metric that emulates human reader impressions of image quality could be quite useful. Our investigation was conducted to ascertain whether image texture analysis metrics, such as those applied to PET scans to detect neoplasms, could prove helpful in linking qualitative statements of phantom sphere and rod visibility to quantified parameters. Because it is not obvious whether it is preferable to submit reconstructions to accrediting agencies performed using typical clinical (CLIN) protocol processing parameters or to follow agencies' filtered backprojection (FBP) suggestions, we applied texture analysis metrics to determine the degree to which these choices affect equipment capability assessment. Data were processed retrospectively for 125 different Tc-99m SPECT scans of standardized phantoms for 14 rotating Anger detector systems as part of routine quarterly QA. Algorithms were written to compute several classes of image metrics: quantile curve metrics, image texture analysis gray-level co-occurrence matrix (GLCM) metrics, contrast metrics, and count histogram metrics. For qualitative image scores, two experienced physicists independently graded sphere and rod visibility on a 5-level scale and assigned dichotomous visibility scores, without knowledge of quantified texture analysis metrics or each other's readings. The same phantom was used to collect 15 additional data sets with two dual-detector SPECT/CT systems, reconstructed both by FBP parameters that have been suggested by accrediting agencies and by manufacturers' default settings for CLIN SPECT/CT bone imaging protocols by ordered subsets expectation maximization (OSEM), incorporating attenuation correction using the CT scan. Image characteristics metrics were compared for FBP and CLIN reconstructions. For spheres, the metric with the strongest rank correlation with 5-level scale readings was the quantile curve slope (ρ=0.83, P<0.0001), while for rods it was GLCM Energy normalized to the maximum GLCM Energy value (EnergyNorm) (ρ=-0.88, P<0.0001). Compared to dichotomous readings, the metric with the highest ROC area under curve (AUC) for spheres was the quantile curve slopes (AUC=96±1%, sensitivity=91%, specificity=90%), and for rods was EnergyNorm (AUC=98±1%, sensitivity=92%, specificity=95%). Image contrast was higher for all sphere sizes and rod EnergyNorm was lower for sectors of intermediate-sized rods for FBP compared to CLIN reconstructions, in agreement with more rods judged to be visible from FBP than CLIN reconstructions (47% vs 33%, P=0.002). When preparing to submit quality assurance images of standardized phantoms to accrediting agencies, a reliable gauge of sphere and rod visibility can be predicted accurately using quantified reader-independent image texture analysis metrics, which also provide a useful basis for choosing among alternative image reconstruction options.