True Beam Research Articles

Design and Finite Element Method Analysis of Laterally Actuated Multi-Value Nano Electromechanical Switches

We report on the design and modeling of novel nano electromechanical switches suitable for implementing reset/set flip-flops, AND, NOR, and XNOR Boolean functions. Multiple logic operations can be implemented using only one switching action enabling parallel data processing; a feature that renders this design competitive with complementary metal oxide semiconductor and superior to conventional nano-electromechanical switches in terms of functionality per device footprint. The structural architecture of the newly designed switch consists of a pinned flexural beam structure which allows low strain lateral actuation for enhanced mechanical integrity. Reliable control of on-state electrical current density is achieved through the use of metal-metal contacts, true parallel beam deflection, and lithographically defined contact area to prevent possible device welding. Dynamic response as a function of device dimensions numerically investigated using ANSYS and MatLab Simulink.

A deconvolution map-making method for experiments with circular scanning strategies

Aims. To investigate the performance of a deconvolution map-making algorithm for an experiment with a circular scanning strategy, specifically in this case for the analysis of Planck data, and to quantify the effects of making maps using simplified approximations to the true beams. Methods. We present an implementation of a map-making algorithm which allows the combined treatment of temperature and polarisation data, and removal of instrumental effects, such as detector time constants and finite sampling intervals, as well as the deconvolution of arbitrarily complex beams from the maps. This method may be applied to any experiment with a circular scanning-strategy. Results. Low-resolution experiments were used to demonstrate the ability of this method to remove the effects of arbitrary beams from the maps and to demonstrate the effects on the maps of ignoring beam asymmetries. Additionally, results are presented of an analysis of a realistic full-scale simulated data-set for the Planck LFI 30 GHz channel. Conclusions. Our method successfully removes the effects of the beams from the maps, and although it is computationally expensive, the analysis of the Planck LFI data should be feasible with this approach.

SU-E-T-555: Small Field Dosimetry of the Flattening Filter Free (FFF) X-Ray Photons

Purpose: The dosimetry of small field is critical in high‐resolution photonradiation therapy, such as the highly conformal stereotactic radiosurgery(SRS) and stereotactic body radiotherapy(SBRT). The flattening filter free (FFF) linac as an emerging technique can offer highly efficient photon use and enhanced dose rate, which benefits SRS and SBRT. This study investigates the small field photondosimetry of the FFF X‐rays using diode detectors and compares results to the conventional flattened X‐ rays. Methods: A Varian's TrueBeam (TB) linac with 6X‐FFF and 6X‐flat X‐rays and a Novalis TX (NTX) linac with 6X‐flat X‐rays have been fully commissioned. Output factors (6X‐flat and 6X‐FFF of TB and 6 X‐flat NTX) were measured by a diode detector (Edge detector, Sun Nuclear) and solid water phantom with different field sizes (FS) created in Eclipse Treatment Planning System. Different FS were formed by MLC patterns rather than jaws (FS=5, 10, 20, 30, 40, 60, 100mm by MLC and the corresponding jaw size=7, 12, 22, 32, 42, 60, 100mm). A regular SSD=100cm and extended SSD=210cm with dmax=1.5cm buildup were setup for the small field dosimetry measurement and all these relative dosimetry was normalized to FS=10cm.Results: Good repeatability of the measurement was demonstrated. The regular and extended SSD results were comparable, which showed the OF of 6X‐flat was smaller than the OF of 6X‐FFFfor FS=1–10cm, likely due to the absence of flattening filter. Moreover, the OF of TB's 6X‐flat was generally lower than the OF of NTX's 6X‐flat. One possible cause is that leaf thickness of NTX is thinner than TB. Conclusions: The 6X‐FFF photons have higher output than the 6X‐ flat in the TrueBeam linac, maybe due to the removal of flattening filter. The TB's 6X‐flat photons have lower output than NTX's 6X‐flat X‐ray photons probably because NTX has thinner leaves.

SU‐E‐I‐122: Polymer Gel and Magnetic Resonance Relaxometry Based 3D Dose Monitoring

Purpose: Recent advances in Radiation Therapy allow accurate shaping of the dose volume so that normal tissue and critical regions can be spared. However, it is important to monitor the 3D dose distribution of these complex treatment plans to verify the delivered dose distribution. Our aim was to optimize Magnetic Resonance imaging to map the 3D dose distribution of a novel radiation therapy system, TrueBeam sn1018(Varian) using a polymer gel. Methods: The polymer gel (BANG KIT, MGS Research) was prepared under normoxic conditions and stored in glass vials. A calibration was performed in the dose range of 3–18 Gy with irradiation using the True Beam. Several imaging sequences (Fast Spin Echo, Multi (8) Spin Echo, Single Spin Echo) were evaluated for spin‐spin relaxometry (T2) at 1.5 T (GE Medical Systems) with focus on accuracy, high spatial resolution, SNR while minimizing acquisition time. Treatment plan based irradiations (simulating different volumes and shapes) were performed at a dose rate of 600 MU/min for verification of the dose maps. Results: The calibration curve (1/T2 vs Dose) was linear in the range of irradiation. The three imaging sequences were consistent in 1/T2 values in the low dose range with the FSE sequence showing some saturation at high dose values. The treatment plan dose mapping is currently underway and will be aligned with the treatment plan simulation to evaluate the degree of match between the plan and actual irradiation.Conclusion: The polymer gel and MR monitoring can enable generation of 3D dose maps with high accuracy and spatial resolution to verify treatment plans of small volumes and steep dose gradients.

SU‐E‐T‐344: A Study of True Beam Linac Operation in a Varying Magnetic Field Environment

Purpose: To quantify the effects of transient, DC magnetic fields on the normal operation of a True Beam linac and to verify normal and accurate treatment delivery capability. This is necessary to ensure normal linac operation and accurate treatment delivery in such an environment. Methods: A data acquisition system was constructed consisting of a Lake Shore vector magnetometer and 3‐axis probe including custom software to monitor and record the real‐time vector magnetic field value at an in‐air reference point near the linac. This data was correlated with various dosimetric and beam quality measurements taken simultaneously. Long duration measurements (2–5 minutes) of the four steering servo currents (Asymmetry Radial (AR), Position Radial (PR), Asymmetry Transverse (AT), and Position Transverse (PT)) were recorded and correlated with magnetic field measurements. Measurements of machine output, dose rate, and beam asymmetry were also recorded and correlated with magnetic field readings. These measurements were repeated for various combinations of gantry orientation and photon and electron beam energies. Results: Linac output, dose rate, and beam asymmetry versus magnetic field demonstrated no dependence on magnetic field. Correlation plots of the four steering servo currents versus magnetic field indicated a strong dependence (correlation coefficient close to ±1.0) for 3 of the 4 steering servos at a gantry angle of 0° at a photon energy of 6MV. This dependence weakened for one of the servos (AT) and strengthened for one (AR) as the gantry angle was changed to 270°. Extrapolation of the correlation plots of each servo to the engineering limits were used to obtain a maximum permissible magnetic field value for the room Conclusions: Effects of magnetic field on linac operation was systematically quantified. Results indicate that the linac is fully capable of normal and accurate treatment delivery despite the ambient magnetic field environment in the room.

SU-E-T-556: Interplay Effect Between Dynamic MLC and Moving Target for Lung SBRT with IMAT Technique Delivered by Flattening Filter Free Beam of True Beam Machine

Purpose: To investigate the dosimetric impact of the interplay effect between dynamic MLC and moving target on lungSBRT with intensity‐ modulated arc therapy (IMAT) technique delivered by flattening filter free (FFF) beam of True‐Beam machine. Methods: 6 lungcancer patients with 0.5–1.0 cm tumor motions were investigated in this study. All patients underwent 4DCT scan using audio coaching. For each patient, a 2‐arc IMAT plan was retrospectively generated using Varian RapidArc planning system. Based on the total MU, dose rate and respiratory cycle, the planned MLC control points of IMAT plans were assigned into different respiratory phases. Afterward, 10 new IMAT plans related to different respiratory phases were generated and imported back into Eclipse planning system to calculate the radiationdose on the CTimages of different respiratory phases. In‐house 4D dose calculation program with deformable registration capacity was used to calculate the cumulative doses from all respiratory phases. Following parameters were used to evaluate the dosimetric impacts: dose error (DE), i.e., the difference between the 3D dose of original IMAT plan and the calculated 4‐D dose, and the percentage of volume receiving 100% of the prescribed dose (V100).Results: For all patients, the average maximum DE of PTV and CTV are 32.6% and 4.8%. The DE is larger than 5% for 22% of PTV and 0.3% of CTV volume, and the DE is less than 3% for over 88% of the CTV volume. Compared to the original IMAT plans, the V100 of PTV are lower by 15%. However, the changes on V100 of CTV are less than 1%. Conclusions: For lung IMAT treatment delivered by FFF beam of True‐Beam machine, the interplay effect between dynamic MLC and moving target could change the absolute doses within the target, but its impact on CTV dose coverage is insignificant.

Enhanced Schottky signals from electron-cooled, coasting beams in a heavy-ion storage ring

Measurements at the Test Storage Ring of the Max-Planck-Institut für Kernphysik in Heidelberg (Germany) have shown that the signal amplitude induced in a Schottky-noise pickup electrode by a coasting electron-cooled ion beam can be greatly enhanced by exposure of the latter to a perturbing radiofrequency signal which is detuned from the true beam revolution frequency. The centre frequencies obtained from harmonic analysis of the observed pickup signal closely follow those imposed on the ions by the electron cooling force. The phenomenon can be exploited to measure the true revolution frequency of ion beams of very low intensity, whose pure Schottky noise is too weak to be measurable under normal circumstances.

Detecting photons in the dark region of Laguerre-Gauss beams

We show that a photon detector, sensitive to the magnetic field or to the gradient of electric field, can help to characterize the quantum properties of spatially-dependent optical fields. We discuss the excitation of an atom through magnetic dipole or electric quadrupole transitions with the photons of a Bessel beam or a Laguerre-Gauss (LG) beams. These spiral beams are shown to be not true hollow beams, due to the presence of magnetic fields and gradients of electric fields on beam axis. This approach paves the way to an analysis at the quantum level of the propagating light beams having a complicated spatial structure.

A Comparison of Propagation Between Apertured Bessel and Gaussian beams

A true Bessel beam is a family of diffraction-free beams. Thus the most interesting and attractive characteristic of such beam is non-diffracting propagation. In optics, the comparisons of maximum propagation distance had been done between Bessel and Gaussian beams by Durnin and Sprangle, respectively. However, the results obtained by them are conflict due to the difference between their criteria. Because Bessel beams have many potential applications in millimeter wave bands, therefore, it is necessary and significant that the comparison is carried out at these bands. A new contrast criterion at millimeter wavelengths is proposed in our paper. Under this criterion, the numerical results are presented and a new conclusion is drawn.

SU‐FF‐T‐23: A Monte Carlo Simulation and Deconvolution Study of Detector Response Function

Purpose: The purpose of this study is to determine the detector response function using Monte Carlo simulations of small radiation fields and simulations of detector responses to those fields. The results may be used to deconvolve the effect of finite size detectors on beam broadening in small field measurements. Method and Materials: A Monte Carlo model of a linear accelerator was generated to simulate the characteristics of a clinically used photon beam. These simulations were used to create a true beam profile. Three clinically available radiation detectors and two theoretical detectors were modeled in the beam. Detector specific models of measured beam profiles were generated from simulated detector responses. Detector response functions were deconvolved from the models of simulated measured profiles and the true beam profile. The results were then analyzed using curve fitting method. Results: The penumbra of the beam profiles generated by different cylindrical detectors demonstrated a significant linear relationship with detector radius. Detector response functions of cylindrical detectors can be approximately represented as a Gaussian curve dependent upon the radius of the detector. Fourier transform methods can be used to deconvolve the detector response function for the penumbral areas, but can introduce noise in the non‐penumbral areas. Deconvolution methods can reduce, but cannot remove the effects of detector size. Conclusions: Monte Carlo simulation can be used to model small radiation fields and detector responses. Detector response functions can be determined from the simulated fields and simulated detector responses. A deconvolution method based on a detector specific Gaussian response function can be used to reduce detector size effect in the penumbral areas.

Development of a PC interface board for true color control using an Ar–Kr white-light laser

For the optimal laser display, it is crucial to select and control color signals of proper wavelengths in order to construct a wide range of laser display colors. In traditional laser display schemes, color control has been achieved through the mechanical manipulation of red, green, and blue (RGB) laser beam intensities using color filters. To maximize the effect of a laser display and its color contents, it is desirable to generate laser beams with wide selection of wavelengths. We present an innovative laser display control technique, which generates six channel laser wavelengths from a white-light laser using a RF-controlled polychromatic acousto optical modulator (PCAOM). This technique enables us not only to control the intensity of individual channels, but also to achieve true color signals for the laser beam display including RGB, yellow, cyan, and violet (YCV), and other intermediate colors. For the optimal control of the PCAOM and galvano-mirror, we designed and fabricated a PC interface board. Using this PC control, we separated the white-light from an Ar–Kr mixed gas laser into various wavelengths and reconstructed them into different color schemes. Also we demonstrated the effective control and simultaneous display of reconstructed true color laser beams on a flat screen.

Influence of forward and multiple light scatter on the measurement of beam attenuation in highly scattering marine environments.

Using three-dimensional Monte Carlo radiative transfer simulations, we examine the effect of beam transmissometer geometry on the relative error in the measurement of the beam-attenuation coefficient in an aquatic environment characterized by intense light scattering, especially within submerged bubble clouds entrained by surface-wave breaking. We discuss the forward-scattering error associated with the detection of photons scattered at small angles (< 1 degrees) and the multiple-scattering error associated with the detection of photons scattered more than once along the path length of the instrument. Several scattering phase functions describing bubble clouds at different bubble void fractions in the water are considered. Owing to forward-scattering error, a beam-attenuation meter (beam transmissometer) with a half-angle of receiver acceptance of 1.0 degrees and a path length of 0.1 m can underestimate the true beam attenuation within the bubble cloud by more than 50%. For bubble clouds with a beam attenuation of as much as 100 m(-1), the multiple-scattering error is no more than a few percent. These results are compared with simulations for some example phase functions that are representative of other scattering regimes found in natural waters. The forward-scattering error for the Petzold phase function of turbid waters is 16% for a typical instrument geometry, whereas for the Henyey-Greenstein phase function with the asymmetry parameter of 0.7 and 0.9 the error range is 8-28%.

Comparison of electron beam characteristics from multiple accelerators

Purpose To determine the relationships between electron beam depth dose characteristics, depth of maximum dose (d max), depth of 80% dose (d 80), and depth of 50% dose (d 50), and the nominal energy designation of electron beams from multiple linear accelerators for the purpose of electron beam treatment planning and quality assurance. Methods The Radiological Physics Center Staff, during its on-site dosimetry review visits to institutions participating in clinical trials, measured depth dose characteristics for more than 2000 electron beams. Measurements were performed on Varian, Siemens, and Elekta/Philips accelerators generating beams with nominal energy values ranging from 4–22 MeV. The depth dose data were determined at the nominal source-to-skin distance with the reference cone size in accordance with recommendation of the American Association of Physicists in Medicine Task Group 25 report. Results The important depth dose characteristics d max, d 80, and d 50 varied in a predictable fashion when plotted against the true beam quality indicator, R 50. However, d 80 and d 50 values overlapped considerably when plotted against the manufacturers' nominal electron energy values. For a specific nominal electron energy value, the values of d max, d 80, and d 50 varied by as little as 3 mm for low energy levels to nearly as much as 20 mm for high energies. Conclusions The manufacturer's nominal electron energy value does not adequately describe the depth dose characteristics of an electron beam for treatment planning purposes. Clinicians and physicists should determine and use only the specific depth dose data for their clinical beams and not the manufacturer's nominal value.

Three-dimensional simulation code for SPring-8 RF gun system

To analyze the beam characteristics of an RF gun system, we developed a true three-dimensional beam tracking code. All forces imposed on each macrocharged particle are included in this code. Asymmetrical figures of beams and RF fields can be calculated with this code. In this paper, we describe the details of the developed code, and show comparisons between our code and PARMELA. In addition, we compare the results of our code and the experimental results of the SPring-8 S-band single-cell RF gun system. Using this code, significant differences in measured emittance values in horizontal and vertical directions resulted from an oblique incidence of a laser, that is, the difference of arrival time of the laser wavefront on the cathode.

Refractive-diffractive hybrid optical element for true beam smoothing

A refractive-diffractive hybrid optical element for true beam smoothing is designed with the precise design method. The intensity of any point on the focal plane is fully filled with the required demand, besides the sampling points used in the optimization. An aspheric refrac- tive surface gives a good initial intensity distribution on the focal plane and can decrease the phase depth of the diffractive surface. After the optimization, the phase depth of the diffractive surface is smaller than 1.1p, which is correspondingly easy to fabricate. © 2003 Society of Photo-

Helical cardiac cone beam reconstruction using retrospective ECG gating

In modern computer tomography (CT) systems, the fast rotating gantry and the increased detector width enable 3D imaging of the heart. Cardiac volume CT has a high potential for non-invasive coronary angiography with high spatial resolution and short scan time. Due to the increased detector width, true cone beam reconstruction methods are needed instead of adapted 2D reconstruction schemes. In this paper, the extended cardiac reconstruction method is introduced. It integrates the idea of retrospectively gated cardiac reconstruction for helical data acquisition into a cone beam reconstruction framework. It leads to an efficient and flexible algorithmic scheme for the reconstruction of single- and multi-phase cardiac volume datasets. The method automatically adapts the number of cardiac cycles used for the reconstruction. The cone beam geometry is fully taken into account during the reconstruction process. Within this paper, results are presented on patient datasets which have been acquired using a 16-slice cone beam CT system.

True beam smoothing in the fractional fourier transform domain

The design of a diffractive optical element (DOE) for true beam smoothing in the fractional Fourier transform domain is described. Based on the Fresnel integrals, the intensity distribution on the output plane is calculated accurately and the discretization error of the spherical phase factor is avoided. The ‘fine design' of the DOE for true beam smoothing is completed with the sampling interval chosen as half of the traditional sampling interval. Simulation results show that the intensity at any point on the output plane fully meets the required demands, not just those sampling points used in the optimization.

True beam smoothing in the fractional Fourier transform domain

True beam smoothing in the fractional Fourier transform domain

Faraday cup detector array with electronic multiplexing for multichannel mass spectrometry

A Faraday cup detector array (FCDA) and electronic multiplexing circuit have been developed for position sensitive ion beam detection. The entire FCDA always remains open to intercept the incident ion beam flux, and each cup is periodically and sequentially discharged through the electronic multiplexer. This produces true multichannel ion beam detection since none of the incident ion beam flux is lost, as is the case for scanning position sensitive detectors, and higher sensitivity detection is thus obtained. The FCDA consists of a one-dimensional or two-dimensional array of individual cups which are electrostatically isolated from each other by means of an intervening ground conductor, with resulting fill factors F of 58% to 85%. Each cup acts as a charge collector and integrator which is quickly discharged during the readout to create a time-multiplexed output signal that gives the position distribution of the ion beam. When N cups are sequentially scanned and read out, the ion collection efficiency is F(1−r/N), where r is the fraction of a clock cycle that is used for resetting the integrating capacitor. With a typical r=0.2, a 64 element array thus provides an ion collection efficiency of better than 0.997F. The device measures absolute ion currents, and has a wide dynamic range from less than 1 pA to more than 100 nA with less than 70 dB of cross talk between cups. The integration of the electronic multiplexer with the FCDA allows the combined unit to operate within a vacuum chamber by means of only a six-pin feedthrough. The FDCA and electronic multiplexer have been used as a position sensitive ion detector in a compact mass spectrometer to produce better than 1 amu resolution over a 200 amu span with a sensitivity of less than 0.8 pA at a 10:1 signal-to-noise ratio.

Spatial-frequency spectrum analysis of the performance of diffractive optical element for beam smoothing

In this paper, based on the spatial-frequency spectrum, the intensity distribution of diffractive optical elements (DOE) for beam smoothing is quantitatively analyzed. Two parameters, with which the performance of the DOE can be reliably evaluated, are re-defined. The performances of the designed DOEs for beam smoothing are re-calculated, whether the fine design is adopted or not. The results show that true beam smoothing can be realized with the fine design.