Beam Spot Profile Research Articles

Simulation study of protoacoustics as a real-time in-line dosimetry tool for FLASH proton therapy.

Applying ultra-high dose rates to radiation therapy, otherwise known as FLASH, has been shown to be just as effective while sparing more normal tissue compared to conventional radiation therapy. However, there is a need for a dosimeter that is able to detect such high instantaneous dose, particularly in vivo. To fulfill this need, protoacoustics is introduced, which is an in vivo range verification method with submillimeter accuracy. The purpose of this work is to demonstrate the feasibility of using protoacoustics as a method of in vivo real-time monitoring during FLASH proton therapy and investigating the resulting protoacoustic signal when dose per pulse and pulsewidth are varied through multiple simulation studies. The dose distribution of a proton pencil beam was calculated through a Monte Carlo toolbox, TOPAS. Next, the k-Wave toolbox in MATLAB was used for performing protoacoustic simulations, where the initial proton dose deposition was inputted to model acoustic propagations, which were also used for reconstructions. Simulations involving the manipulation of the dose per pulse and pulsewidth were performed, and the temporal and spatial resolution for protoacoustic reconstructions were investigated as well. A 3D reconstruction was performed with a multiple beam spot profile to investigate the spatial resolution as well as determine the feasibility of 3D imaging with protoacoustics. Our results showed consistent linearity in the increasing dose-per-pulse, even up to rates considered for FLASH. The simulations and reconstructions were performed for a range of pulsewidths from 0.1 to 10μs. The results show the characteristics of the proton beam after convolving the protoacoustic signal with the varying pulsewidths. 3D reconstruction was successfully performed with each beam being distinguishable using an 8cm×8cm planar array. These simulation results show that measurements using protoacoustics has the potential for in vivo dosimetry in FLASH therapy during patient treatments in real time. Through this simulation study, the use of protoacoustics in FLASH therapy was verified and explored through observations of varying parameters, such as the dose per pulse and pulsewidth. 2D and 3D reconstructions were also completed. This study shows the significance of using protoacoustics and provides necessary information, which can further be explored in clinical settings.

Improved Intra-Pixel Sensitivity Characterization Based on Diffusion and Coupling Model for Infrared Focal Plane Array Photodetector.

The high-precision characterization of the intra-pixel sensitivity (IPS) for infrared focal plane array (FPA) photodetector is of great significance to high-precision photometry and astrometry in astronomy, as well as target tracking in under-sampled remote sensing images. The discrete sub-pixel response (DSPR) model and fill factor model have been used for IPS characterization in some studies. However, these models are incomplete and lack the description of physical process of charge diffusion and capacitance coupling, leading to the inaccuracy of IPS characterization. In this paper, we propose an improved IPS characterization method based on the diffusion and coupling physical (DCP) model for infrared FPA photodetector, which considering the processes of generation and collection of the charge, can improve the accuracy of IPS characterization. The IPS model can be obtained by convolving the ideal rectangular response function with the charge diffusion function and the capacitive coupling function. Then, the IPS model is convolved with the beam spot profile to obtain the beam spot scanning response model. Finally, we calculate the parameters of IPS by fitting the beam spot scanning response map with the proposed DCP model based on the Trust-Region-Reflective algorithm. Simulated results show that when using a 3 μm beam spot to scan, the error of IPS characterization based on DCP model is 0.63%, which is better than that of DSPR model’s 3.70%. Experimental results show that the fitting error of the beam spot scan response model based on DCP model is 4.29%, which is better than that of DSPR model’s 8.31%.

Development of a storage phosphor imaging system for proton pencil beam spot profile determination.

Accurate two-dimensional (2D) profile measurements at submillimeter precision are necessary for proton beam commissioning and periodic quality assurance (QA) purposes and are currently performed at our institution with a commercial scintillation detector (Lynx PT) with limited means for independent checks. The purpose of this work was to create an independent dosimetry system consisting of an in-house optical scanner and a BaFBrI:Eu2+ storage phosphor dosimeter by: (a) determining the optimal settings for the optical scanner, (b) measuring 2D proton spot profiles with the storage phosphors, and (c) comparing them to similar measurements using a commercial scintillation detector. An in-house 2D laboratory optical scanner was constructed and spatially calibrated for accurate 2D photostimulated luminescence (PSL) dosimetry. Square 5×5cm2 BaFBrI:Eu2+ dosimeter samples were uniformly irradiated with line scans performed to determine the physical and electronic scanner settings resulting in the highest signal-to-noise ratios (SNR) at a sub-millimeter spatial resolution. The resultant spatial resolution of the scanner was then quantitatively assessed by measuring (a) line pairs on a standard X-ray lead bar phantom and (b) modulation transfer functions. Following this, 2D proton spot profiles from a Mevion S250i Hyperscan proton unit were obtained at 1, 10, 20, 30, 40, and 50 monitor unit (MU) settings at maximum energy (E0 =227.1MeV) and compared to baseline profiles from a commercial scintillation detector, where 1MU is calibrated to deliver 1Gy absolute proton dose-to-water under reference conditions, that is, 41×41 proton spots uniformly spaced by 0.25cm within a 10×10cm2 square field size at maximum energy (227.1MeV) in water at depth of 5cm at isocenter. The dosimetric system's sensitivities to (a) ±1mm positional shifts and (b) ±0.3mm beam lateral spread changes were quantitatively evaluated through a Gaussian fitting of the crossline and inline plots of the respective artificially shifted beam profiles. The physical scanner settings of (a) Δτ=27ms time interval between data samples, (b) vx =1.235cm/s scanning speed, (c) 1% laser transmission (0.02mW power) and (d) (Δx, Δy)=(0.33, 0.50mm) pixel sizes with electronic settings of (a) 300 microseconds time constant, (b) normal dynamic reserve, (c) 24dB/oct low pass filter slope, and (d) 160Hz chopping frequency resulted in the highest SNR while maintaining sub-millimeter spatial resolution. The BaFBr0.85 I0.15 :Eu2+ storage phosphor dosimeters were linear from 1 to 50MU and their profiles did not saturate up to 150MU. The scanner was able to detect lateral displacements of ±1mm in both the crossline and inline directions and ±0.3mm beam spread changes that were artificially introduced by varying the incident proton energy. Specific to our proton unit, proton energy changes of ±1MeV can also be detected indirectly via beam spread measurements. Our combined dosimetric system including an in-house laboratory optical scanner and reusable BaFBr0.85 I0.15 :Eu2+ storage phosphors demonstrated a sufficient spatial resolution and dosimetric accuracy to support its use as an independent proton spot measurement dosimeter system. Its wide dynamic range allows for other versatile applications such as proton halo measurements.

Beam characteristics of the first clinical 360° rotational single gantry room scanning pencil beam proton treatment system and comparisons against a multi-room system.

PurposeThe purpose of this study was to present the proton beam characteristics of the first clinical single‐room ProBeam Compact™ proton therapy system (SRPT) and comparison against multi‐room ProBeam™ system (MRPT).Materials and MethodsA newly designed SRPT with proton beam energies ranging from 70 to 220 MeV was commissioned in late 2019. Integrated depth doses (IDDs) were scanned using 81.6 mm diameter Bragg peak chambers and normalized by outputs at 15 mm WET and 1.1 RBE offset, following the methodology of TRS 398. The in‐air beam spot profiles were acquired by a planar scintillation device, respectively, at ISO, upper and down streams, fitted with single Gaussian distribution for beam modeling in Eclipse v15.6. The field size effect was adjusted for the best overall accuracy of clinically relevant field QAs. The halo effects at near surface were quantified by a pinpoint ionization chamber. Its major dosimetric characteristics were compared against MRPT comparable beam dataset.ResultsContrast to MRPT, an increased proton straggling in the Bragg peak region was found with widened beam distal falloffs and elevated proximal transmission dose values. Integrated depth doses showed 0.105–0.221 MeV (energy sigma) or 0.30–0.94 mm broader Bragg peak widths (Rb80–Ra80) for 130 MeV or higher energy beams and up to 0.48–0.79 mm extended distal falloffs (Rb20–Rb80). Minor differences were identified in beam spot sizes, spot divergences, proton particles/MU, and field size output effects. High passing scores are reported for independent end‐to‐end dosimetry checks by IROC and for initial 108 field‐specific QAs at 3%/3 mm Gamma index with fields regardless with or without range shifters.ConclusionsThe author highlighted the dosimetry differences in IDDs mainly caused by the shortened beam transport system of SRPT, for which new acceptance criteria were adapted. This report offers a unique reference for future commissioning, beam modeling, planning, and analysis of QA and clinical studies.

A method for the characterization of intra-pixel response of infrared sensor

Due to the response variations within a pixel, response of sensor varies with the positions of the point target. The accurate measurement of intra-pixel response of the sensor is the key to high-precision centroid estimation and radiometric calibration. In this paper, we establish an intra-pixel response model based on the pixel structure and use Gauss function to describe the beam spot profile. We further calculate the full width at half maximum (FWHM) of the Gauss function and the fill factor of the infrared sensor using the grid search algorithm. Experimental results show that the Gauss function FWHM differences among different pixels using the same radius could be less than 6%. The fill factors are not constant in different pixels because of errors in the process of manufacture and assembly, and the error between model and measured values is less than 3.6%. This work could be useful for stellar calibration and centroid estimation.

Improvement of imaging characteristics by pupil apodization in a laser scanning system for electro-photography

It is well known that imaging characteristics such as beam spot size and focal depth can be improved by applying apodization of amplitude and phase at the entrance pupil of an optical system. As an optical system for the electro-photography continuously requires highly resolved dot image and extended focal length to obtain more delicate expression with adequate production stability. Advantages from apodization technique can improve system performance and supply high degree of reliability of the optical system. However, issues on reduction in absolute optical power and on side-lobe of beam spot profile have been technical barrier for its useful implementation to imaging optics for commercial devices. Thorough this paper, theoretical feasibility of application of a pupil apodization to a laser scanning optical unit is mainly discussed in the aspects of optical power reduction and side-lobe of spot profile, as well as enhancement of optical resolution and focal depth. In addition, fundamental experimental results on beam spot profile through focal region are followed to uphold feasibility analysis.

Optical characterization of high speed microscanners based on static slit profiling method

Optical characterization of high-speed microscanners is a challenging task that usually requires special high speed, extremely expensive camera systems. This paper presents a novel simple method to characterize the scanned beam spot profile and size in high-speed optical scanners under operation. It allows measuring the beam profile and the spot sizes at different scanning angles. The method is analyzed theoretically and applied experimentally on the characterization of a Micro Electro Mechanical MEMS scanner operating at 2.6kHz. The variation of the spot size versus the scanning angle, up to ±15°, is extracted and the dynamic bending curvature effect of the micromirror is predicted.

SU‐E‐CAMPUS‐T‐04: Measurement of Proton Pencil Beam Spot Profile Using Cherenkov Radiation in Two Dimensional Optical Fiber Arrays

Purpose:Proton therapy aims to deliver a high dose in a well‐defined target volume while sparing the healthy surrounding tissues thanks to their inherent depth dose characteristic (Bragg peak). In proton therapy, several techniques can be used to deliver the dose into the target volume. The one that allows the best conformity with the tumor, is called PBS (Pencil Beam Scanning). The measurement of the proton pencil beam spot profile (spot size) and position is very important for the accurate delivery of dose to the target volume with a good conformity.Methods:We have developed a fine segmented detector array to monitor the PBS. A prototype beam monitor using Cherenkov radiation in clear plastic optical fibers (cPOF) has been developed for continuous display of the pencil beam status during the therapeutic proton Pencil Beam Scanning mode operation. The benefit of using Cherenkov radiation is that the optical output is linear to the dose. Pedestal substraction and the gain adjustment between channels are performed. Spot profiles of various pencil beam energies(100 MeV to 226 MeV) are measured. Two dimensional gaussian fit is used to analyze the beam width and the spot center. The results are compared with that of Lynx(Scintillator‐based sensor with CCD camera) and EBT3 Film.Results:The measured gaussian widths using fiber array system changes from 13 to 5 mm for the beam energies from 100 to 226 MeV. The results agree well with Lynx and Film within the systematic error.Conclusion:The results demonstrate good monitoring capability of the system. Not only measuing the spot profile but also monitoring dose map by accumulating each spot measurement is available. The x‐y monitoing system with 128 channel readout will be mounted to the snout for the in‐situ real time monitoring.

Dynamics of dark hollow Gaussian laser pulses in relativistic plasma

Optical beams with null central intensity have potential applications in the field of atom optics. The spatial and temporal evolution of a central shadow dark hollow Gaussian (DHG) relativistic laser pulse propagating in a plasma is studied in this article for first principles. A nonlinear Schrodinger-type equation is obtained for the beam spot profile and then solved numerically to investigate the pulse propagation characteristics. As series of numerical simulations are employed to trace the profile of the focused and compressed DHG laser pulse as it propagates through the plasma. The theoretical and simulation results predict that higher-order DHG pulses show smaller divergence as they propagate and, thus, lead to enhanced energy transport.

TH‐C‐BRA‐12: Tomographic Measurement of X‐Ray Beam Spot Profiles Using a Rotating Edge

Purpose: X‐ray beam spot size and shape are critical performance determinants of an imaging or treatment system. However, quantitative assessment of such spot profiles can prove difficult, particularly at MV energies. We have developed a novel and convenient tomographic spot measurement technique that uses a rotating edge phantom. The method can be applied to x‐ray systems equipped with flat panel imagers. Methods: Data were acquired at 10MV and 6MV on a Varian TrueBeam system. A 0.5 mm thick tantalum sheet (attenuation ∼5%) was placed on the system's rotatable treatment head with an edge abutting the axis of rotation. A total of 144 projections, each 10MU, were acquired at 2.5 degree steps. For each projection, a line‐spread function (LSF) is generated by differentiating the measured edge spread function. For sufficiently high source magnifications, the LSF, taken at a given rotation angle, is a tomographic projection of the x‐ray beam spot at that angle. The LSF's were then assembled into a sinogram and the corresponding spot profile was reconstructed using a parallel‐beam CT algorithm. The reconstructed profiles were compared to those measured using a film‐based ‘spot camera’ made from a 15 cm thick tungsten cylinder pierced with an array of small holes. Monte Carlo simulations of the edge and the spot camera experiments were also performed. Results: The edge technique produced profiles similar to those from the spot camera yet with higher resolution (0.17mm vs. 0.25mm). These results were confirmed by Monte Carlo simulations. The measured FWHM of the 10 MV spot was 1.6 mm. The 6 MV spot was slightly asymmetric with an average FWHM of 1.5 mm. Conclusions: A thin rotating edge with low attenuation can be used to accurately and conveniently measure x‐ray beam spot profiles. NIH NIH 1R01CA138426 Employees of Varian Medical Systems

Lateral dose profile characterization in scanning particle therapy

In light-ion beam dose delivery with the scanning technique the spacing between adjacent spots is an important parameter during treatment planning. In order to study the effect of spot spacing on dose conformity and robustness for single field uniform dose configurations, fundamental geometrical properties of placement of Gaussian beamlets are explored. In particular, the dependence of penumbra width and flatness on spot width and spot spacing is investigated. Infinitesimal calculus and analytical methods are used to derive simple expressions for the lateral penumbra and the flatness of one-dimensional dose profiles in continuous scanning and uniform discrete spot scanning. In the same way expressions for the fundamental modes of perturbation of the spot sequence are developed. A numerical, matrix-based approach is followed to optimize weights spot-by-spot. Generally the lateral penumbra widths lie between 1.13 sigma(b) and 1.68 sigma(b) with sigma(b) being the standard deviation of the beam spot profile. For regularly placed spots of equal weight with spot spacing lambda the lateral penumbra is given by 1.68 sigma' where sigma' results from quadratic subtraction of lambda/square root of 12 from sigma(b). The quantization error is identified as additional parameter describing the lateral dose conformity. It's variance is given by lambda2/12 for a bunch of spots with uniform weights. The matrix-based optimization of weights for a one-dimensional dose box results in a lateral penumbra of typically 1.4 sigma(b). This value reduces to about 1.3 sigma(b) if also the positions of the beam spots are optimized for the considered field size. The analytical formulas for uniform discrete scanning can be used as rough approximations of the best-case scenarios for weight-optimized dose profiles if the spot spacing is defined as effective spot spacing. The trade-off between flatness, quantization error, and robustness on the one side and penumbra width on the other side can be described analytically for equally weighted spots. Treatment planning systems often perform a least-squares optimization of the individual spot weights which results in smaller lateral penumbras and smaller quantization errors than for uniform discrete scanning. However, the benefit of this weight optimization decreases with increasing lambda (in the regime lambda > sigma(b)). The spot spacing, which is obtained from the scenario that the optimization objective is met by uniform discrete scanning, poses a sharp upper limit for the spot spacing lambda in weight optimization methods.

Single Wavelength Blue-Laser Optical Head-Like Opto-Mechanical System for Turntable Thermal Mode Lithography and Stamper Fabrication

The characterization of novel small size 405 nm blue-laser optical head-like opto-mechanical system for turntable thermal mode lithography system that can provide high quality beam spot profile and real time high precision servo control and feasibility for thermal mode lithography process application are presented. The optical head-like opto-mechanical system consists of a single wavelength laser direct-write blue-laser opto-mechanical module and related servo control main board. An ideally narrow round shape exposure beam spot about 260 nm and high precision power control, focusing servo control are produced for thermal mode inorganic resist exposure. The small size opto-mechanical system combination with thermal mode lithography technology has been successfully applied to fabricate nickel stamper with submicro/nano structure patterns. The minimum pit (or hole) diameter is about 180 nm and ultra-low surface roughness (2 nm) is achieved after electroforming process.

Improved field scanner incorporating parabolic optics Part 2: experimental verification and potential for volume scanning

We investigated the experimental performance of an afocal scan engine employing two off-axis parabolic reflectors and it was found not to introduce astigmatism when compared to a freely propagated beam. The performance of the new afocal engine is very similar to an ideal single-mirror scan engine in terms of spot size and beam spot profile (or point spread function) and has an improved flatness of field over other two-dimensional laser scan engines. The parabolic scan engine is contrasted with a comparable spherical mirror arrangement and found to produce superior performance at the intermediate image plane when focused through a scan lens. Further modeling and experimentation point toward volume scanning applications. The significant performance improvement provided by this design, now verified experimentally, will result in superior image quality for fast scanning confocal and two-photon microscopy in particular.

A study on the realization of high resolution solid immersion lens-based near-field imaging optics by use of an annular aperture

We report on the realization of solid immersion lens (SIL)-based near-field (NF) optics with an annular aperture, which is targeted to achieve high optical resolution. A numerical aperture (NA) = 1.84 hemisphere SIL-optics with an annular aperture achieves higher optical resolution than the conventional NA = 2.0 SIL-optics. The designed aperture is fabricated by photo-lithography and dry-etching technique. Experimental verification of the designed optics was performed through beam spot profile measurement under NF imaging conditions. A 15% smaller full-width-at-half-maximum spot diameter is obtained by the aperture. We verified that this method gives an improvement of the resolution in the optical imaging systems requiring higher resolution.

Effect of Elliptical Gaussian Beam in Solid-Immersion-Lens-Based Near-Field Recording Optics

In this study, we summarized the characteristics of beam spot profiles, exit pupil images, the gap error signal and the push–pull signal obtained when using a Gaussian beam in solid immersion lens (SIL) based near-field recording (NFR) optics. As the rim intensity of the Gaussian beam decreases and the tracking direction shift of the SIL assembly increases, the intensity of the beam spot is decreases. The exit pupil image is distorted asymmetrically by the tracking direction shift of the SIL assembly. We confirmed that the tracking direction shift of the SIL assembly affects the gap error signal and push–pull signal. When using linearly polarized incident light, it is observed that a tracking direction shift of 500 µm induces a gap error signal reduction of 3.2%. The push–pull signal is offset by 18.7% in the case of a tracking direction shift of when the tracking direction is shifted by 20 µm.

Diagnostics and Upgrade of the DAFNE Beam Test Facility (BTF)

The DAFNE Beam Test Facility (BTF), operational in Frascati LNF since November 2002, is a beam transfer line optimized for single particle production, mainly for high-energy detectors calibration. It can provide e − / e + beams in a wide range of intensity – between single particle and 1010 particles/pulse – and energy, from few tens of MeV up to 800 MeV; the pulse duration can also be adjusted between 1 and 10 ns at a maximum repetition rate of 50 Hz. In the last year many groups have accessed the facility, for testing a number of apparata: silicon detectors, calorimeters, scintillator counters, GEM (in single particle mode); fluorescence counters, crystal and nanotube bendings, Cerenkov counters, thermo-acoustical detectors (at high intensity). The large beam multiplicity range required to implement different diagnostic devices: calorimeters for particle counting at low intensity, fluorescence chambers and Cerenkov detectors for intermediate-high intensity monitor, up to standard beam diagnostic systems. The beam spot profile and position is also measured by means of a x-y scintillating fiber hodoscope with multi-anode PMT readout.

Dysplay-Special Contribution from IWD'02-A 16-cm Dual Neck Diameter, Integrated Component, Projection CRT

A 16cm, ∅29.1mm compatible projection CRT, integrated component system has been developed utilizing an LD-Hi-UPF electron gun with an effective focusing lens of 21mm. This was achieved with a dual diameter neck (∅29.1-36.5mm) integrated component design. The current ∅29.1mm projection CRT deflection power was maintained while reducing the 5% center beam spot profile size by 20% (2mA optimum focus). The integrated components were optimized to provide the system with improved corner focus. This CRT delivers superior image quality and is compatible with current ∅29.1mm neck CRTs in rear projection systems.

Theoretical Analysis of Recording Light Distribution in Phase-Change Near-Field Recording Using Solid Immersion Lens

We investigate the behavior of recording light in a phase-change near-field recording system in the presence of a solid immersion lens (SIL) and an optical disk. The relationship of the recording light density with the air gap and the disk structure is analyzed in detail, together with the beam spot profiles. Moreover, the coupling efficiency in the presence of the optical disk is calculated. It is found that the behavior of the recording light depends strongly on the air gap and the entire optical disk structure.

INITIAL STAGES OF GROWTH OF IRON ON Ag(100) AT LOW TEMPERATURE

The initial stages of growth of Fe on a well-defined single crystal Ag(100) surface have been investigated by means of thermal energy helium diffraction. We report on specular beam spot profiles obtained at 0.5 monolayer for several temperatures of deposition in the range of 135–300 K. Typical sidebands ("satellites") in diffraction patterns allow one to estimate the mean separation <L> between islands. Experimental data in the range of 135–228 K show a linear dependence of ln < L> as a function of the inverse of the temperature of deposition, while at higher temperatures a clear deviation from this law is found, indicating a change in the regime of growth. The nucleation of small islands turns out to be the factor which favors two-dimensional growth at low temperature.

Feasibility of a large area x‐ray sensitive vidicon for medical fluoroscopy: Resolution and lag factors

A large area x-ray-sensitive vidicon utilizing amorphous selenium (a-Se) is being investigated as an alternative to the x-ray image intensifier and television camera combination (XRII/TV) for medical fluoroscopy. The x-ray vidicon is, to a first approximation, a scaled up version of the 1" (2.54 cm) light-sensitive vidicon. The modulation transfer function (MTF) of an x-ray vidicon is mainly determined by the number of lines used, the beam spot profile, and the electronic bandwidth. The x-ray vidicon is expected to have roughly the same MTF as the 1" vidicon from a XRII/TV system referenced to the x-ray input and as such will be superior to the MTF of the complete XRII/TV system especially in high resolution applications such as cardiac cine. Beam discharge lag is expected to be approximately the same as in the 1" vidicon since they have comparable photoconductor layer capacitance. Photoconductive lag is also expected to be low since a-Se is known to have a low trap density.