Off-axis Field Research Articles

大相对孔径大视场紫外告警相机光学系统

For the special use demands of ultraviolet warning cameras, the structures, distortion balances and the consistency of image plane illumination of ultra wide-angle optical systems were analyzed. An ultraviolet optical system with a large field of view of 120 and a large relative aperture of 1/2 was designed. The system is a double imaging configuration with the spectral range from 0.254 m to 0.272 m. By using inverse telephoto structure, its back working length is 7.48 mm. A quasi-telecentric optical system was selected to improve the illumination uniformity for off-axis and on-axis view fields. Then, by choosing optical materials and distributing the focal powers reasonably, the optical system was optimized. The experiments demonstrate that the system is characterized by smaller imaging distortion and uniform image plane illumination, the dispersion spot diameter is smaller than 63.5 m, and the relative distortion at 0.71 field of view, the relative illumination uniformity for off-axis and on-axis view fields are less than 20% and 10%, respectively. The structure of the whole system is compact and feasible, and is suitable for ultraviolet warning cameras.

Characteristics of a ridge-transform inside corner intersection and associated mafic-hosted seafloor hydrothermal field (14.0°S, Mid-Atlantic Ridge)

Morphotectonic analysis of the inside corner intersection (14.0°S) between the southern Mid-Atlantic Ridge and the Cardno fracture zone indicate a young rough massif emerging after the termination of a previous oceanic core complex. The massif, which hosts an off-axis hydrothermal field, is characterized by a magmatic inactive volcanic structure, based on geologic mapping and sample studies. Mineralogical analyses show that the prominent hydrothermal deposit was characterized by massive pyrite-marcasite breccias with silica-rich gangue minerals. Geochemical analyses of the sulfide breccias indicate two element groups: the Fe-rich ore mineral group and silica-rich gangue mineral group. Rare earth element distribution patterns showing coexistence of positive Eu anomalies and negative Ce anomalies suggest that sulfides were precipitated from diffused discharge resulted from mixing between seawater and vent fluids. Different from several low temperature hydrothermal systems occurring on other intersection dome-like massifs that are recognized as detachment fault surfaces associated with variably metamorphosed ultramafic rocks, the 14.0°S field, hosted in gabbroic-basaltic substrate, is inferred to be of a high temperature system and likely to be driven by deep high temperature gabbroic intrusions. Additionally, the subsurface fossil detachment fault is also likely to play an important role in focusing hydrothermal fluids.

Field mapping of focused ion beam prepared semiconductor devices by off-axis and dark field electron holography

Off-axis electron holography is a unique technique in that it can be used to provide maps of the electrostatic potentials and strain in semiconductor specimens with nm-scale resolution. In this paper, we show that if sufficient care is taken, focused ion beam milling can be used to prepare electrically tested devices from a precise location on a wafer for studies of their electrostatic and strain fields as well as their structure and composition. We have compared the physical properties of several devices with process simulations and electrical test results which were measured over a time period of several months. We believe that electron holography can now be used to measure the positions of the electrical junctions and also quantitative values of the strain in an industrial environment.

Design of an off-axis helmet-mounted display with freeform surface described by radial basis functions

An off-axis optical system with a freeform surface described by Gaussian radial basis functions for a see-through wide field of view helmet-mounted display (HMD) is presented. By using a freeform surface, an off-axis see-through HMD system with one tilted freeform combiner and four relay optical lenses is designed. An off-axis see-through wide field of view HMD system with a 100mm eye relief, 15mm pupil, 45°×32° FOV, and 60° combiner tilt angle is achieved. For the purpose of comparison, two other off-axis see-through HMDs which have the same specifications, but different surface type of combiner and different use count of relay lenses are designed too. One of the two contributions in this paper is the application of radial basis functions to describe optical freeform surface in a wide field of view off-axis helmet-mounted displays, and the other is a way used to determine a starting point of optimization quickly while designing.

Implementation and commissioning of an integrated micro-CT/RT system with computerized independent jaw collimation

To design, construct, and commission a set of computer-controlled motorized jaws for a micro-CT∕RT system to perform conformal image-guided small animal radiotherapy. The authors designed and evaluated a system of custom-built motorized orthogonal jaws, which allows the delivery of off-axis rectangular fields on a GE eXplore CT 120 preclinical imaging system. The jaws in the x direction are independently driven, while the y-direction jaws are symmetric. All motors have backup encoders, verifying jaw positions. Mechanical performance of the jaws was characterized. Square beam profiles ranging from 2×2 to 60×60 mm2 were measured using EBT2 film in the center of a 70×70×22 mm3 solid water block. Similarly, absolute depth dose was measured in a solid water and EBT2 film stack 50×50×50 mm3. A calibrated Farmer ion chamber in a 70×70×20 mm3 solid water block was used to measure the output of three field sizes: 50×50, 40×40, and 30×30 mm2. Elliptical target plans were delivered to films to assess overall system performance. Respiratory-gated treatment was implemented on the system and initially proved using a simple sinusoidal motion phantom. All films were scanned on a flatbed scanner (Epson 1000XL) and converted to dose using a fitted calibration curve. A Monte Carlo beam model of the micro-CT with the jaws has been created using BEAMnrc for comparison with the measurements. An example image-guided partial lung irradiation in a rat is demonstrated. The averaged random error of positioning each jaw is less than 0.1 mm. Relative output factors measured with the ion chamber agree with Monte Carlo simulations within 2%. Beam profiles and absolute depth dose curves measured from the films agree with simulations within measurement uncertainty. Respiratory-gated treatments applied to a phantom moving with a peak-to-peak amplitude of 5 mm showed improved beam penumbra (80%-20%) from 3.9 to 0.8 mm. A set of computer-controlled motorized jaws for a micro-CT∕RT system were constructed with position reliably better than a tenth of a millimeter. The hardware system is ready for image-guided conformal radiotherapy for small animals with capability of respiratory-gated delivery.

A source model for modulated electron radiation therapy using dynamic jaw movements

The development of fast and accurate source models (SMs) might be of crucial importance for the future clinical implementation of modulated electron radiation therapy (MERT). In this study, a SM is presented for reconstructing phase-space information of modulated electron beams using a few-leaf electron collimator (FLEC) and the photon jaws. During a FLEC-based delivery, two collimation devices (jaws and FLEC) modulate the electron beam characteristics dynamically. The SM separates the beam into a primary and a scattered component. The primary component is derived by a fast Monte Carlo (MC) transport calculation in air using the EGSnrc/BEAMnrc code. The scattered beam is modeled analytically. The accelerator was decomposed into its individual leaf components and the scattered beam was characterized at various levels of the accelerator. Scattered particles are assigned an energy and position by sampling pre-calculated probability distributions. The direction is estimated by geometrical arguments. Particles were assumed to emerge from tunable virtual sources on the side of each collimator leaf. A leaf-hit algorithm was developed to dynamically reject particles that are incident on any collimating leaf. Electron transport in air between the two collimation levels was calculated based on a MC-modified version of the Fermi-Eyges scattering theory. Correlations between direction and position were observed and taken into account at the final collimation level. To validate the model, reconstructed phase-space data were compared with the full accelerator MC phase-space data. The model accurately reproduced the beam characteristics and preserved important correlations. Depth and profile dose distributions in water were derived for square, rectangular, and off-axis field sizes and for a range of clinical energies. Discrepancies in the dose distributions and dose output were within 3% in all cases. Fast and accurate SMs open the possibility for fast treatment planning in MERT, based on an inverse optimization MC treatment planning scheme.

A two‐dimensional matrix correction for off‐axis portal dose prediction errors

This study presents a follow-up to a modified calibration procedure for portal dosimetry published by Bailey et al. ["An effective correction algorithm for off-axis portal dosimetry errors," Med. Phys. 36, 4089-4094 (2009)]. A commercial portal dose prediction system exhibits disagreement of up to 15% (calibrated units) between measured and predicted images as off-axis distance increases. The previous modified calibration procedure accounts for these off-axis effects in most regions of the detecting surface, but is limited by the simplistic assumption of radial symmetry. We find that a two-dimensional (2D) matrix correction, applied to each calibrated image, accounts for off-axis prediction errors in all regions of the detecting surface, including those still problematic after the radial correction is performed. The correction matrix is calculated by quantitative comparison of predicted and measured images that span the entire detecting surface. The correction matrix was verified for dose-linearity, and its effectiveness was verified on a number of test fields. The 2D correction was employed to retrospectively examine 22 off-axis, asymmetric electronic-compensation breast fields, five intensity-modulated brain fields (moderate-high modulation) manipulated for far off-axis delivery, and 29 intensity-modulated clinical fields of varying complexity in the central portion of the detecting surface. Employing the matrix correction to the off-axis test fields and clinical fields, predicted vs measured portal dose agreement improves by up to 15%, producing up to 10% better agreement than the radial correction in some areas of the detecting surface. Gamma evaluation analyses (3 mm, 3% global, 10% dose threshold) of predicted vs measured portal dose images demonstrate pass rate improvement of up to 75% with the matrix correction, producing pass rates that are up to 30% higher than those resulting from the radial correction technique alone. As in the 1D correction case, the 2D algorithm leaves the portal dosimetry process virtually unchanged in the central portion of the detector, and thus these correction algorithms are not needed for centrally located fields of moderate size (at least, in the case of 6 MV beam energy). The 2D correction improves the portal dosimetry results for those fields for which the 1D correction proves insufficient, especially in the inplane, off-axis regions of the detector. This 2D correction neglects the relatively smaller discrepancies that may be caused by backscatter from nonuniform machine components downstream from the detecting layer.

METALLICITY DISTRIBUTION FUNCTIONS, RADIAL VELOCITIES, AND ALPHA ELEMENT ABUNDANCES IN THREE OFF-AXIS BULGE FIELDS

We present radial velocities and chemical abundance ratios of [Fe/H], [O/Fe], [Si/Fe], and [Ca/Fe] for 264 red giant branch (RGB) stars in three Galactic bulge off-axis fields located near (l,b)=(-5.5,-7), (-4,-9), and (+8.5,+9). The results are based on equivalent width and spectrum synthesis analyses of moderate resolution (R~18,000), high signal-to-noise ratio (S/N~75-300) spectra obtained with the Hydra spectrographs on the Blanco 4m and WIYN 3.5m telescopes. The targets were selected from the blue side of the giant branch to avoid cool stars that would be strongly affected by CN and TiO; however, a comparison of the color-metallicity distribution in literature samples suggests our selection of bluer targets should not present a significant bias against metal-rich stars. We find a full range in metallicity that spans [Fe/H]\approx-1.5 to +0.5, and that, in accordance with the previously observed minor-axis vertical metallicity gradient, the median [Fe/H] also declines with increasing Galactic latitude in off-axis fields. The off-axis vertical [Fe/H] gradient in the southern bulge is estimated to be ~0.4 dex/kpc. The (+8.5,+9) field exhibits a higher than expected metallicity, with a median [Fe/H]=-0.23, that might be related to a stronger presence of the X--shaped bulge structure along that line-of-sight. All fields exhibit an identical, strong decrease in velocity dispersion with increasing metallicity that is consistent with observations in similar minor-axis outer bulge fields. Additionally, the [O/Fe], [Si/Fe], and [Ca/Fe] versus [Fe/H] trends are identical among our three fields, and are in good agreement with past bulge studies. [abridged]

Retinoblastoma external beam photon irradiation with a special ‘D’-shaped collimator: a comparison between measurements, Monte Carlo simulation and a treatment planning system calculation

Retinoblastoma is the most common eye tumour in childhood. According to the available long-term data, the best outcome regarding tumour control and visual function has been reached by external beam radiotherapy. The benefits of the treatment are, however, jeopardized by a high incidence of radiation-induced secondary malignancies and the fact that irradiated bones grow asymmetrically. In order to better exploit the advantages of external beam radiotherapy, it is necessary to improve current techniques by reducing the irradiated volume and minimizing the dose to the facial bones. To this end, dose measurements and simulated data in a water phantom are essential. A Varian Clinac 2100 C/D operating at 6 MV is used in conjunction with a dedicated collimator for the retinoblastoma treatment. This collimator conforms a ‘D’-shaped off-axis field whose irradiated area can be either 5.2 or 3.1 cm2. Depth dose distributions and lateral profiles were experimentally measured. Experimental results were compared with Monte Carlo simulations’ run with the penelope code and with calculations performed with the analytical anisotropic algorithm implemented in the Eclipse treatment planning system using the gamma test. penelope simulations agree reasonably well with the experimental data with discrepancies in the dose profiles less than 3 mm of distance to agreement and 3% of dose. Discrepancies between the results found with the analytical anisotropic algorithm and the experimental data reach 3 mm and 6%. Although the discrepancies between the results obtained with the analytical anisotropic algorithm and the experimental data are notable, it is possible to consider this algorithm for routine treatment planning of retinoblastoma patients, provided the limitations of the algorithm are known and taken into account by the medical physicist and the clinician. Monte Carlo simulation is essential for knowing these limitations. Monte Carlo simulation is required for optimizing the treatment technique and the dedicated collimator.

Magnetic-field-dependent photodynamics of single NV defects in diamond: an application to qualitative all-optical magnetic imaging

Magnetometry and magnetic imaging with nitrogen–vacancy (NV) defects in diamond rely on the optical detection of electron spin resonance (ESR). However, this technique is inherently limited to magnetic fields that are weak enough to avoid electron spin mixing. Here, we focus on the high off-axis magnetic field regime where spin mixing alters the NV defect spin dynamics. We first study, in a quantitative manner, the dependence of the NV defect optical properties on the magnetic field vector B. Magnetic-field-dependent time-resolved photoluminescence (PL) measurements are compared to a seven-level model of the NV defect that accounts for field-induced spin mixing. The model reproduces decreases in (i) ESR contrast, (ii) PL intensity and (iii) excited level lifetime with an increasing off-axis magnetic field. We next demonstrate that these effects can be used to perform all-optical imaging of the magnetic field component |B⊥| orthogonal on the NV defect axis. Using a scanning NV defect microscope, we map the stray field of a magnetic hard disc through both PL and fluorescence lifetime imaging. This all-optical method for high magnetic field imaging at the nanoscale might be of interest in the field of nanomagnetism, where samples producing fields in excess of several tens of milliteslas are typically found.

Sci-Thur AM: Planning - 07: A fast and accurate source model for energy and intensity modulated electron beams.

The purpose of this study is to develop a highly accurate and fast method for calculating electron beam dose distributions in Modulated Electron Radiation Therapy (MERT). An algorithm has been developed for creating phase-space files at the exit of a linear accelerator for any arbitrary intensity and energy electron beam without the need of full Monte Carlo simulations. The model assigns each particle to one of the 3 following sources: primary, secondary collimator and electron collimator scatter. The primary component is derived by fast MC transport in air. The scatter components are derived by the use of MC pre-calculated leaf kernels. Each kernel includes the fluence distribution, energy distribution and scatter probability of generating an electron from a leaf. The original position is sampled from tunable Gaussian or uniform distributions. The direction is estimated by geometrical means. According to the projection of the direction a particle is rejected if it is expected to suffer a leaf-hit. A leaf-hit counter is used to calculate the output of scatter particles based on the pre-calculated scatter probabilities. To account for multiple coulomb scattering in air a MC-corrected version of the Fermi-Eyges scattering theory was implemented. Depth and profile dose distributions were derived for the largest and smallest square field sizes, as well as for irregular and off-axis fields. The model agreed with full MC dose distributions within 3 % in all cases. Output at the depth of maximum dose exhibited discrepancies less than 2.6 % in all cases. The model was 16-22 times faster in generating a phase-space file than a full MC simulation with the BEAMnrc code.

EPID dosimetry for pretreatment quality assurance with two commercial systems

This study compares the EPID dosimetry algorithms of two commercial systems for pretreatment QA, and analyzes dosimetric measurements made with each system alongside the results obtained with a standard diode array. 126 IMRT fields are examined with both EPID dosimetry systems (EPIDose by Sun Nuclear Corporation, Melbourne FL, and Portal Dosimetry by Varian Medical Systems, Palo Alto CA) and the diode array, MapCHECK (also by Sun Nuclear Corporation). Twenty‐six VMAT arcs of varying modulation complexity are examined with the EPIDose and MapCHECK systems. Optimization and commissioning testing of the EPIDose physics model is detailed. Each EPID IMRT QA system is tested for sensitivity to critical TPS beam model errors. Absolute dose gamma evaluation (3%, 3 mm, 10% threshold, global normalization to the maximum measured dose) yields similar results (within 1%–2%) for all three dosimetry modalities, except in the case of off‐axis breast tangents. For these off‐axis fields, the Portal Dosimetry system does not adequately model EPID response, though a previously‐published correction algorithm improves performance. Both MapCHECK and EPIDose are found to yield good results for VMAT QA, though limitations are discussed. Both the Portal Dosimetry and EPIDose algorithms, though distinctly different, yield similar results for the majority of clinical IMRT cases, in close agreement with a standard diode array. Portal dose image prediction may overlook errors in beam modeling beyond the calculation of the actual fluence, while MapCHECK and EPIDose include verification of the dose calculation algorithm, albeit in simplified phantom conditions (and with limited data density in the case of the MapCHECK detector). Unlike the commercial Portal Dosimetry package, the EPIDose algorithm (when sufficiently optimized) allows accurate analysis of EPID response for off‐axis, asymmetric fields, and for orthogonal VMAT QA. Other forms of QA are necessary to supplement the limitations of the Portal Vision Dosimetry system.PACS numbers: 87.53.Bn, 87.53.Jw, 87.53.Kn, 87.55.Qr, 87.56.Fc, 87.57.uq

SU-E-T-156: Robust Algorithms for Correction of Predicted Electronic Portal Imager Response.

Predicted electronic portal imaging device (EPID) response, as calculated by a commercial treatment planning system (TPS), is up to 15% lower than measured EPID response for off-axis IMRT fields. Two original algorithms are presented to correct for EPID prediction errors. The EPID prediction algorithm and a recent image-to-dose conversion algorithm are each tested for ability to identify TPS dose calculation errors. By comparing test images to respective predictions, correction factors were calculated to modify the EPID diagonal calibration profile (applied via radial symmetry). Secondly, image/prediction comparisons were used to compute a 2D correction matrix for EPID predictions, to account for radially-asymmetric errors. Over 50 IMRT fields of varying complexity were tested with each correction technique, and with a diode array. Absolute dose and beam-profile errors were separately induced into the TPS and a number of IMRT plans were recalculated and measured with three systems - an EPID prediction system, an EPID image-to-dose conversion system, and a diode array - for comparison to verification plans. With the profile correction, TPS predictions agree much better with EPID measurements, yielding improvement in gamma pass rates (3%,3mm) of over 30% on average for off-axis IMRT fields. Since off-axis prediction errors are not radially-symmetric, the matrix correction further improves pass rates by 5% on average (up to 30%) for fields where the profile correction is limited. The EPID prediction system was unable to catch either induced TPS error, while both the image-to-dose conversion system and the diode array indicated both errors. Profile correction is effective and efficient though approximate, due to radial symmetry. The matrix correction is comprehensive but requires computational manipulation of DICOM images. Users must be aware that EPID prediction systems may be unable to catch delivered IMRT inaccuracies due to calculation errors downstream from the actual fluence calculation.

SU-E-T-273: Commissioning Motorized Jaws for a Micro-CT/RT.

To design, construct and commission a set of computer-controlled motorized jaws for a micro-CT to perform conformal image-guided small animal radiotherapy. We report on the design and evaluation of our custom-built motorized orthogonal jaws, which allows the delivery of off-axis rectangular fields, installed on a GE eXplore CT 120 preclinical imaging system. The jaws in the x direction are independently driven, while the y direction jaws are symmetric. All motors have backup encoders, verifying their positions. Measurements were performed using the large focal spot (1.0 mm) at 140 kVp, 50 mA, 25 ms pulse duration, with 220 ms between pulses. Additionally, a Monte Carlo beam model of the micro-CT and jaws has been created in BEAMnrc. Square beam profiles (2 × 2, 5 × 5, 10 × 10, 20 × 20 and 30 × 30 mm) were measured using EBT2 film in the center of a 40 × 40 × 40 mm solid water block. A calibrated Farmer ion chamber in a 50 × 20 × 70 mm solid water block was used to measure the output of three field sizes: 50 × 50, 40 × 40 and 30 × 30 mm. Both setups were simulated in DOSXYZnrc. Films were scanned on a flatbed scanner (Epson 1000XL) and converted to dose using a fitted calibration curve. Relative output factors measured with the ion chamber agree with Monte Carlo simulations within 2%. Penumbra widths from film agree well with simulations. The averaged random error of positioning both jaws is estimated at less than 0.1 mm. Finer calibration points will improve the absolute jaw positioning errors. The computer-controlled motorized jaws can be positioned reliably down to sub-millimeter resolutions. Upon completion of small field dosimetry (< 1 cm), this will be ready for conformal radiotherapy for small animals.

Nanoscale magnetic field mapping with a single spin scanning probe magnetometer

We demonstrate quantitative magnetic field mapping with nanoscale resolution, by applying a lock-in technique on the electron spin resonance frequency of a single nitrogen-vacancy defect placed at the apex of an atomic force microscope tip. In addition, we report an all-optical magnetic imaging technique which is sensitive to large off-axis magnetic fields, thus extending the operation range of diamond-based magnetometry. Both techniques are illustrated by using a magnetic hard disk as a test sample. Owing to the non-perturbing and quantitative nature of the magnetic probe, this work should open up numerous perspectives in nanomagnetism and spintronics.

APENELOPE‐based system for the automated Monte Carlo simulation of clinacs and voxelized geometries—application to far‐from‐axis fields

Two new codes, PENEASY and PENEASYLINAC, which automate the Monte Carlo simulation of Varian Clinacs of the 600, 1800, 2100, and 2300 series, together with their electron applicators and multileaf collimators, are introduced. The challenging case of a relatively small and far-from-axis field has been studied with these tools. PENEASY is a modular, general-purpose main program for the PENELOPE Monte Carlo system that includes various source models, tallies and variance-reduction techniques (VRT). The code includes a new geometry model that allows the superposition of voxels and objects limited by quadric surfaces. A variant of the VRT known as particle splitting, called fan splitting, is also introduced. PENEASYLINAC, in turn, automatically generates detailed geometry and configuration files to simulate linacs with PENEASY. These tools are applied to the generation of phase-space files, and of the corresponding absorbed dose distributions in water, for two 6 MV photon beams from a Varian Clinac 2100 C∕D: a 40 × 40 cm(2) centered field; and a 3 × 5 cm(2) field centered at (4.5, -11.5) cm from the beam central axis. This latter configuration implies the largest possible over-traveling values of two of the jaws. Simulation results for the depth dose and lateral profiles at various depths are compared, by using the gamma index, with experimental values obtained with a PTW 31002 ionization chamber. The contribution of several VRTs to the computing speed of the more demanding off-axis case is analyzed. For the 40 × 40 cm(2) field, the percentages γ(1) and γ(1.2) of voxels with gamma indices (using 0.2 cm and 2% criteria) larger than unity and larger than 1.2 are 0.2% and 0%, respectively. For the 3 × 5 cm(2) field, γ(1) = 0%. These figures indicate an excellent agreement between simulation and experiment. The dose distribution for the off-axis case with voxels of 2.5 × 2.5 × 2.5 mm(3) and an average standard statistical uncertainty of 2% (1σ) is computed in 3.1 h on a single core of a 2.8 GHz Intel Core 2 Duo processor. This result is obtained with the optimal combination of the tested VRTs. In particular, fan splitting for the off-axis case accelerates execution by a factor of 240 with respect to standard particle splitting. PENEASY and PENEASYLINAC can simulate the considered Varian Clinacs both in an accurate and efficient manner. Fan splitting is crucial to achieve simulation results for the off-axis field in an affordable amount of CPU time. Work to include Elekta linacs and to develop a graphical interface that will facilitate user input is underway.

Two-dimensional scanning probe driven by a solenoid-based single actuator for optical coherence tomography

We propose the two-dimensional scanning probe operating with a single actuator, which is thought useful as a sample probe for optical coherence tomography (OCT). The probe was designed to use a single-body lensed fiber cantilever loaded with an iron-bead and driven by a single-solenoid actuator. Elliptic spiral trace patterns were achieved using off-axis magnetic fields of the solenoid. A three-dimensional OCT image was obtained for a scanning area of 3.8 mm × 3.4 mm at an acquisition speed of 16.7 s/V. Up to 27 Hz B-scan rate, the proposed probe worked well, and 1000 A-scans were made per each B-scan.

Coupling strength with off-axial external field in magnetic tunnel junction cells

A series of hysteresis loops with off-axial external magnetic fields have been measured to identify magnetization reversal properties as well as coupling effects in magnetic tunnel junctions (MTJs). MTJ films, consisting of a synthetic artificial antiferromagnetic pinned layer of CoFeB/Ru/CoFe/IrMn, were patterned into an elliptical cell array with sizes of 200 × 300 nm and 500 × 750 nm. The hysteresis loops were measured using a vibrating sample magnetometer with various directions of the external magnetic field; the angle θ between the external magnetic field (H→E) and the biasing field of the pinned layer (H→B) was varied from 0° to 90°. The two significant coupling effects, ascribed to exchange bias between CoFe/IrMn and Ruderman–Kittel–Kasuya–Yosida–like coupling in CoFeB/Ru/CoFe, can be observed in these hysteresis loops. The angular dependence of the hysteresis loops is employed to characterize the variation of these coupling effects in extended and patterned MTJ films.

SU-GG-T-595: Implementation of a Near Real-Time Dose Calculation Engine for IMRT

Introduction — IMRT field dose calculation is traditionally implemented through pencil beam convolution algorithm in various treatment planning systems. For quality assurance purpose, an independent patient specific IMRT dose calculation scheme is in need. Such calculation should be performed within a short timeframe in order to be clinically implementable. We develop a dose calculation engine that serves such purpose. Method — Once the treatment planning system finishes the dose optimization and calculation, the patient CT data and the treatment planning data are exported in DICOM format. Our calculation reads in the patient geometry and each IMRT beam/field setup including the MLC fractioned intensity map. The primary dose for each beam is calculated through patient attenuation function, output ratio considering the MLC setup, primary off-axis ratio and IMRT field intensity profile. A fast ray tracing algorithm is applied in calculating attenuation function in the primary dose. It traces the voxel density along the photon beam ray. The voxel densities are derived from patient CT numbers. The primary dose is then used to calculate the scatter dose by convolving a kernel with primary kerma. This convolution is calculated using a Fast Fourier Transform algorithm. The total dose for each beam is the sum of primary and scatter dose and repeated for each beam setup. Results — This IMRT dose calculation is performed within seconds from an IMRT plan. The resultant dose is comparable with the output from treatment planning system. Summary — We implemented a new IMRT dose calculation engine which is able to calculate the dose for a patient with IMRT fields. This engine takes into account of patient inhomogeneity through a precise fast ray tracing algorithm. A fast FFT convolution algorithm is used to obtain scatter dose. The calculation is performed with fast speed which allows potential real-time clinical applications.

Dosimetric validation of a commercial Monte Carlo based IMRT planning system

Recently a commercial Monte Carlo based IMRT planning system (Monaco version 1.0.0) was released. In this study the dosimetric accuracy of this new planning system was validated. Absolute dose profiles, depth dose curves, and output factors calculated by Monaco were compared with measurements in a water phantom. Different static on-axis and off-axis fields were tested at various source-skin distances for 6, 10, and 18 MV photon beams. Four clinical IMRT plans were evaluated in a water phantom using a linear diode detector array and another six IMRT plans for different tumor sites in solid water using a 2D detector array. In order to evaluate the accuracy of the dose engine near tissue inhomogeneities absolute dose distributions were measured with Gafchromic EBT film in an inhomogeneous slab phantom. For an end-to-end test a four-field IMRT plan was applied to an anthropomorphic lung phantom with a simulated tumor peripherally located in the right lung. Gafchromic EBT film, placed in and around the tumor area, was used to evaluate the dose distribution. Generally, the measured and the calculated dose distributions agreed within 2% dose difference or 2 mm distance-to-agreement. But mainly at interfaces with bone, some larger dose differences could be observed. Based on the results of this study, the authors concluded that the dosimetric accuracy of Monaco is adequate for clinical introduction.