Gating Beam Research Articles

Practical considerations for the amplification of electro-optically detected THz signals

Advances in THz methods and applications require the detection of weak THz pulsed signals. One solution to this problem is to amplify weak signals using optically biased electro-optical (EO) techniques. Several different EO amplification schemes are compared. The simplest of these involves using a non-polarizing beam splitter followed by two quarter-wave plates set to small positive and negative angles, respectively. The signals are then passed through polarizers perpendicular to the polarization of the 800 nm EO detector crystal gating beam. As long as sufficient intensity is present in the gate pulse for linear photodiode detection, amplification of up to 10 times with dynamic ranges for single sweep scans of >104 is obtained.

Research on the Safety Assessment of Corbel Beams of Reservoir Radial Gates Based on Real States

The radial gate corbel beam is a kind of gate support structure more often used in large and medium-sized reservoirs, but the current corresponding structural design code does not give a review calculation method for it. Because there are obvious differences between a corbel and a corbel beam in structural form and force, if the corbel beam is just simplified in the calculation as a corbel structure, it can easily lead to misjudgment of insufficient bearing capacity. This misjudgment has a significant impact on later safety assessment, danger removal, and reinforcement. Currently, there are limited studies available on the internal stress distribution of the corbel beam. In this study, taking the danger removal and reinforcement of the radial gate beam of a medium reservoir in Beijing as an example, the concrete quality of the dam was tested by the core drilling method, and two safety review methods of the corbel beam for different types of reservoirs were proposed in combination with the Code for Design of Hydraulic Concrete Structures (SL191-2008). Then, three different types of calculation model were established by the method of theoretical mechanics calculation and finite element simulation. Combined with the safety test data, the stress state of the combined stress structure of the corbel beam and gate pier under the real state was analyzed and the safety evaluation was carried out. The calculation results of these two corbel beam safety review methods were respectively reduced by 32% and 47% compared with the current calculation method. Engineering practice has proved the rationality of the two safety evaluation methods proposed in this paper, which can provide a certain reference for similar engineering reinforcement.

Highly efficient terahertz photoconductive metasurface detectors operating at microwatt-level gate powers.

Despite their wide use in terahertz (THz) research and technology, the application spectra of photoconductive antenna (PCA) THz detectors are severely limited due to the relatively high optical gating power requirement. This originates from poor conversion efficiency of optical gate beam photons to photocurrent in materials with sub-picosecond carrier lifetimes. Here we show that using an ultra-thin (160 nm), perfectly absorbing low-temperature grown GaAs metasurface as the photoconductive channel drastically improves the efficiency of THz PCA detectors. This is achieved through perfect absorption of the gate beam in a significantly reduced photoconductive volume, enabled by the metasurface. This Letter demonstrates that sensitive THz PCA detection is possible using optical gate powers as low as 5 µW-three orders of magnitude lower than gating powers used for conventional PCA detectors. We show that significantly higher optical gate powers are not necessary for optimal operation, as they do not improve the sensitivity to the THz field. This class of efficient PCA THz detectors opens doors for THz applications with low gate power requirements.

Controversies of radiotherapy in human epidermal growth factor receptor (HER)-2 positive breast cancer patients.

Tumor biology plays a crucial role in the systemic treatment, specifically in HER2-positive tumors. Distinct biological behavior of breast cancer subtypes is associated with different rates of locoregional recurrence (LRR). HER2- positive breast cancer patients treated with surgery in combination with radiation, without trastuzumab have poor outcome, including high LRR. The efficacy of radiotherapy in HER-2-positive breast cancer appears to be associated with the expression of estrogen receptors. In patients with HER-2-positive breast cancer, studies conducted before the introduction of trastuzumab indicated higher benefit of adjuvant radiation in patients with hormone receptor-positive tumors compared to patients with tumors not expressing hormone receptors. The introduction of agents targeting HER-2 has transformed the management of these patients, resulting in improved outcomes. The data of clinical studies show that the administration of trastuzumab as part of a multimodality approach (with radiation based on standard guidelines) results in improved outcomes, including lower locoregional recurrence. The risk of cardiac toxicity associated with radiation to the heart and administration of potential cardiotoxic trastuzumab is not clear. In patients treated concomitantly with regional lymph node irradiation and anti-HER-2 agents after prior anthracycline-based chemotherapy minimizing the dose to the myocardium, e.g. respiratory gating or proton beam radiotherapy, have been suggested.

Coupling of separate solitons in a series circuit of two photon photorefractive crystals exhibiting simultaneous quadratic and linear nonlinearities

Separately coupled optical spatial solitons are shown to exist in a biased series two-photon photorefractive(TPPR) crystal circuit consisting of two TPPR crystals which have the quadratic and linear electro-optic(EO) effect present simultaneously. The space charge field in each photorefractive(PR) crystal in the circuit has been derived. We find that the space charge field in each crystal is coupled with the space charge field in the other crystal. We consider the cases of separately coupled bright, dark and grey solitons and obtain the numerical solutions for the spatial solitons’ profile. The coupling effect due to the soliton peak intensity in each crystal and the gating beam intensity is studied in detail. The light induced current causes a change in the spatial profile of the dark or grey soliton in one of the crystal which results in a change in the spatial profile and propagation of the soliton in the other crystal. We also find that the bright soliton intensity in one crystal does not affect the soliton in the other crystal. A change in the gating beam intensity in one crystal influences the soliton in the other crystal and this is true for all the three cases of dark, grey and bright separate soliton pairs.

Real-Time Infrared Motion Tracking Analysis for Patients Treated With Gated Frameless Image Guided Stereotactic Radiosurgery

The transition from frame-based brain stereotactic radiosurgery (SRS) to frameless delivery is supported by real-time intrafraction monitoring to ensure accurate delivery. The purpose of this study is to characterize these real-time motion traces in a large cohort of patients treated with frameless gated brain SRS and to develop patient-specific predictions of tolerance violations. SRS patients treated on the Gamma Knife Icon were immobilized using a device-specific thermoplastic head mask. A motion marker was fixed to the patient's nose, with gating and cone beam computed tomography (CBCT)-based corrections to the treatment at excursions from baseline exceeding 1.5 mm. The traces of 1446 fractions were analyzed according to magnitude (932 unique treatment plans for 462 unique individual patients), directional distribution of displacement, and stability. A neural network model was developed to predict interruptions based on a subset of trace data. The average displacement of the marker in the first fraction of all patients was 0.62 ± 0.25 mm with beam CBCT corrections, which would otherwise be modeled at 0.96 ± 0.96 mm without intrafraction motion correction (P < .0001). Twenty-nine percent of fractions delivered were interrupted, of which the Z-axis (superoinferior) motion was the largest contributor to excursion. Baseline corrections significantly compensated for the magnitude of motion in all 3 dimensions (P < .01). The motion relative to the first acquired CBCT was on average seen to consistently increase with treatment time, with the minimum P value occurring at 61.3 minutes. The neural network prediction model was able to predict treatment interruptions with 84% sensitivity on the first 5-minute sample of the trace. Corrections to marker position significantly decreased marker excursions in all 3 axes compared with a single CBCT alignment. Patient-specific modeling may aid in the optimization of cases selected for frameless radiosurgery to increase the accuracy of planned delivery.

Directional gated imaging in a turbid medium using an ultrafast optical Kerr gate

Directional gated imaging of hidden objects in scattering media is demonstrated using an ultrafast optical Kerr gate (OKG), in which the gating and imaging beams are focused by utilizing a cylindrical lens and a convergent lens, respectively. Owing to the spatial transmission characteristics of the directional gate, a rectangular aperture stop is formed, which leads to different resolution limits for vertical and horizontal structures of an object. By implementing directional gated imaging, the directional feature can be extracted from an original image.

A Monte Carlo approach to the microdosimetric kinetic model to account for dose rate time structure effects in ion beam therapy with application in treatment planning simulations.

Advanced ion beam therapeutic techniques, such as hypofractionation, respiratory gating, or laser-based pulsed beams, have dose rate time structures which are substantially different from those found in conventional approaches. The biological impact of the time structure is mediated through the β parameter in the linear quadratic (LQ) model. The aim of this study was to assess the impact of changes in the value of the β parameter on the treatment outcomes, also accounting for noninstantaneous intrafraction dose delivery or fractionation and comparing the effects of using different primary ions. An original formulation of the microdosimetric kinetic model (MKM) is used (named MCt-MKM), in which a Monte Carlo (MC) approach was introduced to account for the stochastic spatio-temporal correlations characteristic of the irradiations and the cellular repair kinetics. A modified version of the kinetic equations, validated on experimental cell survival invitro data, was also introduced. The model, trained on the HSG cells, was used to evaluate the relative biological effectiveness (RBE) for treatments with acute and protracted fractions. Exemplary cases of prostate cancer irradiated with different ion beams were evaluated to assess the impact of the temporal effects. The LQ parameters for a range of cell lines (V79, HSG, and T1) and ion species (H, He, C, and Ne) were evaluated and compared with the experimental data available in the literature, with good results. Notably, in contrast to the original MKM formulation, the MCt-MKM explicitly predicts an ion and LET-dependent β compatible with observations. The data from a split-dose experiment were used to experimentally determine the value of the parameter related to the cellular repair kinetics. Concerning the clinical case considered, an RBE decrease was observed, depending on the dose, ion, and LET, exceeding up to 3% of the acute value in the case of a protraction in the delivery of 10min. The intercomparison between different ions shows that the clinical optimality is strongly dependent on a complex interplay between the different physical and biological quantities considered. The present study provides a framework for exploiting the temporal effects of dose delivery. The results show the possibility of optimizing the treatment outcomes accounting for the correlation between the specific dose rate time structure and the spatial characteristic of the LET distribution, depending on the ion type used.

Speckle Suppression of OKG Imaging in Highly Turbid Medium Using SC-Assisted Fundamental Frequency

We propose a method to suppress the speckle of optical Kerr gated (OKG) imaging in highly turbid medium using an 800-nm femtosecond laser, in which, the 800-nm fundamental frequency light and the supercontinuum (SC) containing the fundamental frequency are used as the gating beam and imaging beam, respectively. A sequence of images of a test chart hidden behind a highly turbid medium is obtained using the OKG technique. Compared with images obtained with OKG imaging using only the fundamental frequency, the image resolution and contrast of images obtained through OKG imaging using the SC-assisted fundamental frequency are both significantly enhanced.

Single-shot terahertz time-domain spectroscopy in pulsed high magnetic fields

We have developed a single-shot terahertz time-domain spectrometer to perform optical-pump/terahertz-probe experiments in pulsed, high magnetic fields up to 30 T. The single-shot detection scheme for measuring a terahertz waveform incorporates a reflective echelon to create time-delayed beamlets across the intensity profile of the optical gate beam before it spatially and temporally overlaps with the terahertz radiation in a ZnTe detection crystal. After imaging the gate beam onto a camera, we can retrieve the terahertz time-domain waveform by analyzing the resulting image. To demonstrate the utility of our technique, we measured cyclotron resonance absorption of optically excited carriers in the terahertz frequency range in intrinsic silicon at high magnetic fields, with results that agree well with published values.

Optimization and evaluation of multiple gating beam delivery in a synchrotron-based proton beam scanning system using a real-time imaging technique

PurposeTo find the optimum parameter of a new beam control function installed in a synchrotron-based proton therapy system. MethodsA function enabling multiple gated irradiation in the flat top phase has been installed in a real-time-image gated proton beam therapy (RGPT) system. This function is realized by a waiting timer that monitors the elapsed time from the last gate-off signal in the flat top phase. The gated irradiation efficiency depends on the timer value, Tw. To find the optimum Tw value, gated irradiation efficiency was evaluated for each configurable Tw value. 271 gate signal data sets from 58 patients were used for the simulation. ResultsThe highest mean efficiency 0.52 was obtained in TW=0.2s. The irradiation efficiency was approximately 21% higher than at TW=0s, which corresponds to ordinary synchrotron operation. The irradiation efficiency was improved in 154 (57%) of the 271 cases. The irradiation efficiency was reduced in 117 cases because the TW value was insufficient or the function introduced an unutilized wait time for the next gate-on signal in the flat top phase. In the actual treatment of a patient with a hepatic tumor at Tw=0.2s, 4.48GyE irradiation was completed within 250s. In contrast, the treatment time of ordinary synchrotron operation was estimated to be 420s. ConclusionsThe results suggest that the multiple gated-irradiation function has potential to improve the gated irradiation efficiency and to reduce the treatment time.

Modulating carrier and sideband coupling strengths in a standing-wave gate beam

We control the relative coupling strength of carrier and first order motional sideband interactions of a trapped ion by placing it in a resonant optical standing wave. Our configuration uses the surface of a microfabricated chip trap as a mirror, avoiding technical challenges of in-vacuum optical cavities. Displacing the ion along the standing wave, we show a periodic suppression of the carrier and sideband transitions with the cycles for the two cases $180^\circ$ out of phase with each other. This technique allows for suppression of off-resonant carrier excitations when addressing the motional sidebands, with applications in quantum simulation and quantum control. Using the standing wave fringes, we measure the relative ion height as a function of applied electric field, allowing for a precise measurement of ion displacement and, combined with measured micromotion amplitudes, a validation of trap numerical models.

Spontaneous breathing vs. mechanical ventilation for respiratory-gated MR-HIFU ablation in the liver

Magnetic resonance-guided High Intensity Focused Ultrasound (MR-HIFU) ablation in the liver is complicated by the continuous motion of the target due to the respiratory cycle. Several motion compensation strategies have been proposed in the past, such as breath-holding, respiratory gating and dynamic beam steering. Respiratory gating for sonication and MR thermometry uses a pencil beam navigator on the diaphragm to limit power output and image acquisition to the resting phase of the diaphragm. Previously, we have used General Anesthesia with mechanical ventilation (GA) to obtain a long and reproducible resting phase of the diaphragm. From a patient’s perspective however, Procedural Sedation and Analgesia (PSA) has several advantages over GA such as a lower risk of complications and shorter recovery. In addition, it has lower associated costs and can be performed by non-anesthesiologists. The purpose of this animal study was to investigate the feasibility of respiratory-gated MR-HIFU ablation in the liver under PSA with spontaneous breathing.

SU‐E‐T‐439: Fundamental Verification of Respiratory‐Gated Spot Scanning Proton Beam Therapy

Purpose:The spot‐scanning proton beam irradiation with respiratory gating technique provides quite well dose distribution and requires both dosimetric and geometric verification prior to clinical implementation. The purpose of this study is to evaluate the impact of gating irradiation as a fundamental verification.Methods:We evaluated field width, flatness, symmetry, and penumbra in the gated and non‐gated proton beams. The respiration motion was distinguished into 3 patterns: 10, 20, and 30 mm. We compared these contents between the gated and non‐gated beams. A 200 MeV proton beam from PROBEAT‐III unit (Hitachi Co.Ltd) was used in this study. Respiratory gating irradiation was performed by Quasar phantom (MODUS medical devices) with a combination of dedicated respiratory gating system (ANZAI Medical Corporation). For radiochromic film dosimetry, the calibration curve was created with Gafchromic EBT3 film (Ashland) on FilmQA Pro 2014 (Ashland) as film analysis software.Results:The film was calibrated at the middle of spread out Bragg peak in passive proton beam. The field width, flatness and penumbra in non‐gated proton irradiation with respiratory motion were larger than those of reference beam without respiratory motion: the maximum errors of the field width, flatness and penumbra in respiratory motion of 30 mm were 1.75% and 40.3% and 39.7%, respectively. The errors of flatness and penumbra in gating beam (motion: 30 mm, gating rate: 25%) were 0.0% and 2.91%, respectively. The results of symmetry in all proton beams with gating technique were within 0.6%.Conclusion:The field width, flatness, symmetry and penumbra were improved with the gating technique in proton beam. The spot scanning proton beam with gating technique is feasible for the motioned target.

Operation speed of polariton condensate switches gated by excitons

We present a time-resolved photoluminescence (PL) study in real- and momentum-space of a polariton condensate switch in a quasi-1D semiconductor microcavity. The polariton flow across the ridge is gated by excitons inducing a barrier potential due to repulsive interactions. A study of the device operation dependence on the power of the pulsed gate beam obtains a satisfactory compromise for the ON/OFF-signal ratio and -switching time of the order of 0.3 and $\thicksim50$ ps, respectively. The opposite transition is governed by the long-lived gate excitons, consequently the OFF/ON-switching time is $\thicksim200$ ps, limiting the overall operation speed of the device to $\thicksim3$ GHz. The experimental results are compared to numerical simulations based on a generalized Gross-Pitaevskii equation, taking into account incoherent pumping, decay and energy relaxation within the condensate.

Polariton condensate transistor switch

A polariton condensate transistor switch is realized through optical excitation of a microcavity ridge with two beams. The ballistically ejected polaritons from a condensate formed at the source are gated using the 20 times weaker second beam to switch on and off the flux of polaritons. In the absence of the gate beam the small built-in detuning creates a potential landscape in which ejected polaritons are channelled toward the end of the ridge where they condense. The low-loss photonlike propagation combined with strong nonlinearities associated with their excitonic component makes polariton-based transistors particularly attractive for the implementation of all-optical integrated circuits.

Measurements of the scattering coefficients of intralipid solutions by a femtosecond optical Kerr gate

An ultrafast optical Kerr gate (OKG) in femtosecond time scale was used to determine the scattering coefficients of intralipids, in which the BI2O3-B2O3-SiO2 oxide glass was employed as the Kerr medium. Because of the joint action of the time gate and a transient spatial gate that was induced in the Kerr materials by the gating beam, more precise scattering coefficients could be obtained. Our experimental results show that, for low turbid media, the scattering coefficients measured using the OKG method are similar to those measured using the collimated transmittance (CT) approach, while for highly turbid media, the results obtained using the OKG method are bigger than those using the CT approach.

Poster — Wed Eve—22: Measurement of Imaging and Treatment Time Delays in Gated Radiotherapy

In gated radiotherapy, the accuracy of treatment delivery is determined by the accuracy with which both the imaging and treatment beams are gated. The time delay is the time between when the markers enter or leave the specified position range for amplitude based gating, or phase for phase based gating, and when an image is obtained or linear accelerator beam starts or ceases delivery. We measured time delay for three fluoroscopy systems and three linear accelerator treatment beams, varying gating type (amplitude vs. phase), beam energy, dose rate, and period. In the system we studied, the treatment beam was turned on and off later than expected by 0.08 ± 0.03s (1 SD). Retrospectively gated fluoroscopy systems had time delays in the opposite direction, indicating the images appeared to have been acquired earlier than they had in reality. The average beam on imaging time delays were −0.04 ± 0.05s (amplitude,), −0.11 ± 0.04s (phase); while the average beam off imaging time delays were −0.18 ± 0.08s (amplitude) and −0.15 ± 0.04s (phase). In this case, the asynchronous time delays for the imaging and treatment systems add linearly. We measure and model the effect of asynchronous time delays on gated deliveries and discuss their implications.

Carbon Nanotube Bundle Array Cold Cathodes for THz Vacuum Tube Sources

We present high performance cold cathodes composed of arrays of carbon nanotube bundles that routinely produce > 15 A/cm2 at applied fields of 5 to 8 V/µm without any beam focusing. They have exhibited robust operation in poor vacuums of 10-6 to 10-4 Torr- a typically achievable range inside hermetically sealed microcavities. A new double-SOI process was developed to monolithically integrate a gate and additional beam tailoring electrodes. The ability to design the electrodes for specific requirements makes carbon nanotube field emission sources extremely flexible. The lifetime of these cathodes is found to be affected by two effects: a gradual decay of emission due to anode sputtering, and catastrophic failure because of dislodging of CNT bundles at high fields ( > 10 V/µm).

Time delays and margins in gated radiotherapy

In gated radiotherapy, the accuracy of treatment delivery is determined by the accuracy with which both the imaging and treatment beams are gated. Time delays are of four types: (1) beam on imaging time delay is the time between the target entering the gated region and the first gated image acquisition; (2) beam off imaging time delay is the time between the target exiting a gated region and the last image acquisition; (3) beam on treatment time delay is the time between the target entering the gated region and the treatment beam on; and (4) beam off treatment time delay is the time between the target exiting the gated region and treatment beam off. Asynchronous time delays for the imaging and treatment systems may increase the required internal target volume (ITV) margin. We measured time delay on three fluoroscopy systems, and three linear accelerator treatment beams, varying gating type (amplitude vs. phase), beam energy, dose rate, and period. The average beam on imaging time delays were −0.04±0.05sec (amplitude, 1 SD), −0.11±0.04sec (phase); while the average beam off imaging time delays were −0.18±0.08sec (amplitude) and −0.15±0.04sec (phase). The average beam on treatment time delays were +0.09±0.02sec (amplitude, 1 SD), +0.10±0.03sec (phase); while the average beam off time delays for treatment beams were +0.08±0.02sec (amplitude) and +0.07±0.02sec (phase). The negative value indicates the images were acquired early, and the positive values show the treatment beam was triggered late. We present a technique for calculating the margin necessary to account for time delays. We found that the difference between these imaging and treatment time delays required a significant increase in the ITV margin in the direction of tumor motion at the gated level.PACS number: 87.53.Dq