Gary Virshup

Gary Virshup
Varian Medical Systems · Ginzton Technology Center

About

120
Publications
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Publications

Publications (120)
Article
Full-text available
Purpose: Electronic portal imagers (EPIDs) with high detective quantum efficiencies (DQEs) are sought to facilitate the use of the megavoltage (MV) radiotherapy treatment beam for image guidance. Potential advantages include high quality (treatment) beam’s eye view imaging, and improved cone-beam computed tomography (CBCT) generating images with m...
Article
Flat panel imagers based on amorphous silicon technology (a-Si) for digital radiography are accepted by the medical and industrial community as having several advantages over radiographic film-based systems. Use of Mega-voltage x-rays with these flat panel systems is applicable to both portal imaging for radiotherapy and for nondestructive testing...
Patent
The claimed subject matter describes a novel technique to measure the beam profile using an area detector. In one embodiment, a set of one-dimensional beam profile measurements is performed by taking two images under the same source conditions but at two different positions of the detector, with each position of the detector shifted by a certain di...
Article
Purpose: This work studies the clinical utility of dual energy (DE) subtraction fluoroscopy for fiducial-free tumor tracking in lung radiation therapy (RT). Improvement in tumor visualization and quantification of tumor shift within a breathing cycle were analyzed.
Conference Paper
Scintillating Fiber Optic Plates (SFOP) or Fiber Optic Scintillator (FOS) made with scintillating fiber-glass, were investigated for x-ray imaging. Two different samples (T x W x L = 2cm x 5cm x 5cm) were used; Sample A: 10μm fibers, Sample B: 50μm fibers both with statistically randomized light absorbing fibers placed in the matrix. A customized h...
Conference Paper
Electronic portal mega-electronvolt (MV) monitoring for radiation therapy is very important for tumor localization. Furthermore, in the latest systems, before the radiation treatment, the patient is also positioned on the treatment bed with a flat-panel detector (FPD) based kilo-electronvolt (kV) CT scan for better soft tissue visibility. In the ar...
Article
Purpose: For portal imaging, high DQE detectors can be constructed from thick pixelated scintillator arrays that absorb MV x-rays. However, due to beam divergence, MTF and DQE losses can be significant for off-axis elements not focused towards the source. We present a novel focusing approach based on situating a shaped fiber optic plate (FOP) betw...
Article
Purpose: Detector lag, or residual signal, in a-Si flat-panel (FP) detectors can cause significant shading artifacts in cone-beam computed tomography reconstructions. To date, most correction models have assumed a linear, time-invariant (LTI) model and correct lag by deconvolution with an impulse response function (IRF). However, the lag correctio...
Article
Full-text available
The purpose of this study was to analyze factors affecting proton stopping-power-ratio (SPR) estimations and range uncertainties in proton therapy planning using the standard stoichiometric calibration. The SPR uncertainties were grouped into five categories according to their origins and then estimated based on previously published reports or meas...
Article
Purpose: Thick pixilated scintillators can offer significant improvements in quantum efficiency over phosphor screen megavoltage (MV) detectors. However spatial resolution can be compromised due to the spreading of light across pixels within septa. Of particular interest are the lower energy x-ray photons and associated light photons that produce h...
Article
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...
Article
Flat panels imagers based on amorphous silicon technology (a-Si) for digital radiography have been accepted by the medical community as having several advantages over film-based systems. Radiotherapy treatment planning systems employ computed tomographic (CT) data sets and projection images to delineate tumor targets and normal structures that are...
Article
Full-text available
Conventional kilovoltage (kV) x-ray-based dual-energy CT (DECT) imaging using two different x-ray energy spectra is sensitive to image noise and beam hardening effects. The purpose of this study was to evaluate the theoretical advantage of the DECT method for determining proton stopping power ratios (SPRs) using a combination of kV and megavoltage...
Article
Purpose: To analyze the uncertainties in proton stopping?power?ratios (SPRs) calculation using a kV?MV DECT and evaluate the potential for margin reduction for proton therapyMethods: Sources of uncertainties in SPR estimation were broken into five categories: CTimaging uncertainties, CT modeling error, uncertainties in the mean?excitation?energy, S...
Article
Detector lag, or residual signal, in amorphous silicon (a-Si) flat-panel (FP) detectors can cause significant shading artifacts in cone-beam computed tomography (CBCT) reconstructions. To date, most correction models have assumed a linear, time-invariant (LTI) model and lag is corrected by deconvolution with an impulse response function (IRF). Howe...
Article
Purpose: In image-guidedradiotherapy, an artifact typically seen in axial slices of x-ray cone-beam computed tomography(CBCT)reconstructions is a dark region or “black hole” situated below the scan isocenter. The authors trace the cause of the artifact to scattered radiation produced by radiotherapy patient tabletops and show it is linked to the us...
Article
Atomic Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE) of high Tc superconducting films can be used to grow defect-free and flat multi-layer structures in which superconducting molecular layers are stacked with molecular layers having other electronic properties. In particular, tri-layers consisting of c-axis Bi2Sr2CaCu2O8 base and counter electrod...
Article
Purpose: To evaluate different combinations of x?ray energies when using Dual?Energy CT (DECT) method for calculating protonstopping power ratio (SPR) in the presence of random noise and beam hardening effects. Method and Materials: We selected three representing x?ray energy pairs: 100kVp and 140kVp for KV?KV, 100kVp and 1MV for KV?MV, and two 1MV...
Article
Purpose: Many digital x‐ray imaging applications require knowledge of the spatial distribution of the incident x‐ray flux intensity (beam profile), which is often obtained through Monte Carlo simulations or dedicated measuring equipment such as point detector that is raster scanned. Direct measurement of flux intensities using an area detector is d...
Article
Full-text available
We discovered an empirical relationship between the logarithm of mean excitation energy (ln Im) and the effective atomic number (EAN) of human tissues, which allows for computing patient-specific proton stopping power ratios (SPRs) using dual-energy CT (DECT) imaging. The accuracy of the DECT method was evaluated for 'standard' human tissues as wel...
Article
Purpose: To introduce a new method based on dual‐energy CT (DECT) for determining patient‐specific proton stopping power ratios (SPRs) required for accurate computation of proton dose distributions. Accurate determination of SPRs requires atomic number information, which cannot be deduced from conventional CT scans. Method and Materials: We have de...
Article
The use of flat panels based on amorphous silicon technology (a-Si) for digital radiography has been accepted by the medical community as having advantages over film-based systems. Radiation treatment planning employs computed tomographic (CT) data sets and projection images to delineate tumor targets and normal structures that are to be spared fro...
Article
Full-text available
X-ray cone-beam (CB) projection data often contain high amounts of scattered radiation, which must be properly modeled in order to produce accurate computed tomography (CT) reconstructions. A well known correction technique is the scatter kernel superposition (SKS) method that involves deconvolving projection data with kernels derived from pencil b...
Article
The goal of this study was to evaluate the improvement in electron density measurement and metal artifact reduction using orthovoltage computed tomography (OVCT) imaging compared with conventional kilovoltage CT (KVCT). For this study, a bench-top system was constructed with adjustable x-ray tube voltage up to 320 kVp. A commercial tissue-character...
Article
The use of flat panels based on amorphous silicon technology for digital radiography has been accepted in the medical community due to their advantages over film for several applications. For industrial applications similar advantages for panels exist. Digital radiography gives the user improved sensitivity, better contrast resolution and enables s...
Article
Full-text available
Digital flat panel a-Si x-ray detectors can exhibit image lag of several percent. The image lag can limit the temporal resolution of the detector, and introduce artifacts into CT reconstructions. It is believed that the majority of image lag is due to defect states, or traps, in the a-Si layer. Software methods to characterize and correct for the i...
Article
Full-text available
Purpose: To develop an efficient analytical scatter correction algorithm for the On‐Board Imager (OBI) for both the center‐detector and offset‐detector geometries used for cone‐beam computed tomography (CBCT). The offset‐detector geometry is used for larger transaxial field‐of‐views and is particularly challenging due to the asymmetric nature of th...
Article
An iodine contrast agent, in conjunction with an X-ray cone beam CT imaging system, was used to clearly image three, biopsy verified, cancer lesions in two patients. The lesions were approximately in the 10 mm to 6 mm diameter range. Additional regions were also enhanced with approximate dimensions down to 1 mm or less in diameter. A flat panel det...
Article
Purpose: X‐ray scatter significantly degrades the quality of cone‐beam CT (CBCT) reconstructions by introducing cupping and streaking artifacts. Simple correction techniques, based on subtracting a constant value across a projection, fail when the flat‐panel detector is transaxially offset as is required to increase the reconstruction field‐of‐view...
Article
Purpose: To investigate the source of and possible corrections for a new artifact seen in cone‐beam CT(CBCT)images acquired using an a‐Si flat panel imager(FPI) (Varian 4030CB). The new artifact is a bright circular region, tangent to the phantom edge, in images of elliptical and off‐center phantoms. Methods and Materials: Temporal response of the...
Article
Full-text available
In contrast to the narrow fan of clinical Computed Tomography (CT) scanners, Cone Beam scanners irradiate a much larger proportion of the object, which causes additional X-ray scattering. The most obvious scatter artefact is that the middle area of the object becomes darker than the outer area, as the density in the middle of the object is underest...
Article
Purpose: We have developed a method of geometrically calibrating the MV and kV radiographic isocenters of a Varian linear accelerator equipped with an on‐board kV imaging system. Method and Materials: The calibration uses a cylindrical phantom containing 16 BB's placed in a spiral pattern and ∼400–600 radiographs of the phantom acquired using...
Article
Full-text available
The dynamic range of many flat panel imaging systems are fundamentally limited by the dynamic range of the charge amplifier and readout signal processing. We developed two new flat panel readout methods that achieve extended dynamic range by changing the read out charge amplifier feedback capacitance dynamically and on a real-time basis. In one met...
Article
Vapor deposited lead iodide films show a wide range of physical attributes dependant upon fabrication conditions. High density is most readily achieved with films less than 100 mum. Thicker films, with lessening density, often show lower response (gain) as charge collection becomes less efficient. Lack of consistency in density throughout a deposit...
Article
Photoconductive polycrystalline mercuric iodide deposited on flat panel thin film transistor (TFT) arrays is one of the best candidates for direct digital X-ray detectors for radiographic and fluoroscopic medical imaging. The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This deposition technology has been scaled up to th...
Article
Full-text available
We previously reported on 2"x 2" and 4" x 4" size imagers, direct digital radiography X-ray detectors, based on photoconductive polycrystalline mercuric iodide deposited on a flat panel thin film transistor (TFT) array, as having great potential for use in medical imaging, NDE, and security applications. Recently we successfully upgraded our mercur...
Article
Full-text available
Mercuric iodide (HgI2) and lead iodide (PbI2) have been under development for several years as direct converter layers in digital x-ray imaging. Previous reports have covered the basic electrical and physical characteristics of these and several other materials. We earlier reported on 5cm x 5cm and 10cm x 10cm size imagers, direct digital radiograp...
Article
Full-text available
Photoconductive polycrystalline mercuric iodide deposited on flat panel thin film transistor (TFT) arrays is one of the best candidates for direct digital X-ray detectors for radiographic and fluoroscopic medical imaging. The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This deposition technology has been scaled up to the...
Article
Full-text available
Polycrystalline Mercuric Iodide coated TFT arrays are now the best candidates for direct digital radiographic detectors for both static and dynamic (fluoroscopic) applications. Their high X-ray sensitivity, high resolution, low dark current, low voltage operation, and good lag characteristics meet the required imaging performance parameters. Small...
Article
Full-text available
Single crystals of mercuric iodide have been studied for many years for nuclear detectors. We have investigated the use of x-ray photoconductive polycrystalline mercuric iodide coatings on amorphous silicon flat panel thin film transistor (TFT) arrays as x-ray detectors for radiographic and fluoroscopic applications in medical imaging. The mercuric...
Article
Full-text available
We report x-ray imaging results on polycrystalline HgI2 detector used for direct x-ray imaging. Due to its good electrical properties and high stopping power for x-rays and gamma rays, the material is a good candidate for many applications in medical imaging. The deposition of the HgI2 thick films is made by hot wall physical vapor deposition, (PVD...
Article
Full-text available
Films of Bi$\rm _2$Sr$\rm _2$CaCu$\rm _2$O$\rm _8$ and Bi$\rm _2$Sr$\rm _2$CuO$\rm _6$ have been grown using Atomic-Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE) on lattice-matched substrates. These materials have been combined with layers of closely-related metastable compounds like Bi$\rm _2$Sr$\rm _2$Ca$\rm _7$Cu$\rm _8$O$\rm _{20}$ (2278) and...
Article
Pressure burst tests gave measured tensile strengths between 230 and 410 MPa for a total of six chemical‐vapor‐deposited (CVD) diamond disks in both transparent ‘‘white’’ and opaque ‘‘black’’ forms obtained from three different sources. The disks were nominally 0.635 cm in diameter and 254 μm thick. These strengths are explained by a theoretical mo...
Article
We have explored two high-transition-temperature Josephson junction technologies for application in voltage standard arrays: step-edge junctions made with YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> and Au normal-metal bridges, and stacked series arrays of Josephson junctions in selectively doped, epitaxially grown Bi<sub>2</sub>Sr<sub>2</sub>CaCu...
Article
We report on the properties of stacked series arrays of trilayer Josephson junctions grown by atomic layer-by-layer molecular beam epitaxy. Trilayer Josephson junctions oriented so that the current travels in the c-axis direction have been described previously. Series arrays are made by placing more than one barrier layer in the Ba<sub>2</sub>Sr<su...
Article
Trilayer tunneling structures consisting of cuprate electrodes and titanate barriers were grown by atomic layer-by-layer molecular beam epitaxy and processed into c-axis transport samples. Barriers of SrTiO<sub>3</sub> and related titanates with thicknesses ranging from 4 Å to 28 Å (one to seven unit cells of the titanate) were grown. While no supe...
Article
Hc2(T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T. Hc2(T) diverged anomalously as the temperature decreased: at the lowest temperature, it was five times that expected for a conventional superconductor. Although deviations from the conventional behavior have been observed in other superconductor...
Article
A technique has been developed for synthesis of heterostructures containing various Bi- and Dy-based cuprates and other complex oxides, with a high level of control of deposition of individual atomic monolayers as well as capability for atomic-layer engineering. It produces multilayers and superlattices which display striking long-range crystalline...
Article
Oxides exhibit most of the interesting phenomena known to occur in solid-state systems. As a class of materials they may be richer in phenomenology than any other comparable class. Oxides can be insulators, semiconductors, or metals. The copperoxide-based compounds we have studied are superconductors with the highest critical temperatures. In some...
Article
Thin bismuth strontium calcium copper oxide (BSCCO) films and BSCCO/insulator/BSCCO trilayers have been prepared on SrTiO3 and MgO substrates by evaporation from elemental sources in ozone atmosphere. Accurate control of the stoichiometry is achieved through monitoring of the atomic fluxes by use of in situ atomic absorption spectroscopy, as well a...
Article
Very precise artificial structuring of high Tc heterostructures is possible using atomic layer-by-layer molecular beam epitaxy. Cuprates are combined with other oxides, such as titanates, to make atomically precise heterostructures for studying transport and interfacial effects. Titanate slabs as thin as one unit cell thick can be grown without pin...
Article
A technique for atomic layer‐by‐layer epitaxy of cuprate superconductors and other complex oxides has been developed at Varian. The samples are engineered by stacking molecular layers of different compounds to assemble multilayers and superlattices, by adding or omitting atomic monolayers to create novel compounds, and by doping within specified at...
Article
H c2(T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T.H c2(T) diverged anomalously as the temperature decreased. At the lowest temperature, it was five times that expected for a conventional superconductor.
Article
A technique for atomic layer-by-layer synthesis of cuprate superconductors and other complex oxides has been developed. Thin films with excellent transport properties and atomically flat surfaces and interfaces are obtained. The samples are engineered by stacking molecular layers of different compounds to assemble multilayers and superlattices, by...
Article
H[sub c2](T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T. H[sub c2](T) diverged anomalously as the temperature decreased. At the lowest temperature, it was five times that expected for a conventional superconductor. This result cannot be explained by any conventional model which relies on magneti...
Article
Hc2(T) has been measured for thin BSCO films over a larger combined range of magnetic field and reduced temperature than for any other superconductor. Hc2(T) diverged anomalously as the temperature decreased: At the lowest temperature, it was 5 times that expected for a conventional superconductor. Such a strong divergence cannot be explained by an...
Article
{\mathit{H}}_{\mathrm{c}2}$(T) has been measured for thin BSCO films over a larger combined range of magnetic field and reduced temperature than for any other superconductor. ${\mathit{H}}_{\mathit{c}2}$(T) diverged anomalously as the temperature decreased: At the lowest temperature, it was 5 times that expected for a conventional superconductor. S...
Article
Optical wavelength shifting of 1.054-nm laser pulses in excess of +or- 10 wavenumbers (+or- 300 GHz) was demonstrated. The wavelength shifter consists of synchronous microwave and optical waveguides fabricated monolithically on LiNbO/sub 3/. An optical pulse experiences a constant refractive index gradient that travels with the pulse and causes the...
Article
The resistive transitions of a wide variety of Bi2Sr2CaCu2O8+delta thin films have been measured in mu0H parallel-to c from 0.01 to 15 T. For all samples, the transitions can be well approximated by the thermally activated form: R (T,H) almost-equal-to R(n)exp{-A(1-T/T(c))/k(B)T square-root H} in the range 10(-6)R(n) < R < 10(-2)R(n). The energy sc...
Article
In atomic layer-by-layer molecular beam epitaxy of high Tc superconducting films, the chemical reaction coordinate of each molecular layer is monitored and controlled during growth. Distinct synthesis routes are characterized by different intermediate states representing different degrees of order or disorder during monolayer growth. Individual mol...
Article
The idea of engineering the detailed microstructure of materials by controlling the chemical synthesis process at an atomic level is particularly interesting for high-temperature superconductors (HTS) and many other oxide compounds. The prospect of integrating such materials as superconductors, ferroelectrics, and compounds with interesting magneti...
Article
Josephson junctions with ultrathin (25–40 Å) barriers were fabricated using high T C , trilayer films grown by atomic layer‐by‐layer molecular beam epitaxy (ALL‐MBE). The films consisted of top and bottom electrodes of Bi 2 Sr 2 CaCu 2 O 8 , separated by a single molecular layer of a metastable compound, Bi 2 Sr 2 (Ca, Sr, Bi, Dy) n-1 Cu n O 2n+4 ,...
Article
Hysteretic Josephson junctions have been fabricated by growing c‐axis oriented trilayer Bi 2 Sr 2 CaCu 2 O 8 /Bi 2 Sr 2 (Ca,Bi, Sr) 7 Cu 8 O 20 /Bi 2 Sr 2 CaCu 2 O 8 single‐crystal thin films and patterning them into devices. To our knowledge, these are the first cuprate superconductor junctions to exhibit current‐voltage characteristics with subst...
Article
A 25.2% efficiency measured under 1 sun, air mass 0 illumination has been achieved in a two‐terminal AlGaAs/GaAs/InGaAsP three‐junction solar cell. The cascade cell consists of a monolithic AlGaAs (E g =1.93 eV)/GaAs two‐junction mechanically stacked on an InGaAsP (E g =0.95 eV) single‐junction cell. The component cell of the AlGaAs/GaAs two‐juncti...
Article
We report the use of pseudo‐double‐beam atomic absorption spectroscopy to make very accurate (0.1%–1%) measurements of the beam flux from Knudsen effusion cells in a molecular beam epitaxy system. This system has been used to grow Bi–Sr–Ca–Cu–O–based superconducting thin films and heterostructures in an atomic‐layer‐by‐layer fashion. The resulting...
Article
Utilizing atomic layer-by-layer molecular beam epitaxy (ALL-MBE), we have synthesized a series of high-quality superlattices in which ultrathin slabs (one-half unit cell thick) of the high-T c superconductor Bi2Sr2CaCu2O8 alternate with up to five such layers of the low-T c Bi2Sr2Cu1O6 phase. In all these superlattices we foundT c to be essentially...
Article
Utilizing atomic layer-by-layer molecular beam epitaxy (ALL-MBE), the authors have synthesized a series of high-quality superlattices in which ultrathin slabs (one-half unit cell thick) of the high-{Tc} superconductor Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} alternate with up to five such layers of the low-{Tc} Bi{sub 2}Sr{sub 2}Cu{sub 1}O{sub 6}phase...
Article
Thin films of various Bi-Sr-Ca-Cu-O (2201, 2212,…, 2–2–11–12) compounds have been synthesized using atomic-layer-by-layer molecular beam epitaxy (ALL-MBE). Single-crystal films with excellent transport properties, smooth surfaces and atomically sharp interfaces in multilayer structures have been obtained. That made it possible to deposit virtually...
Conference Paper
A 25.2% efficiency measured under 1-Sun, air mass 0 illumination has been achieved in a two-terminal AlGaAs/GaAs/InGaAsP three-junction solar cell. The cascade cell consists of a monolithic AlGaAs ( E <sub>g</sub>=1.93 eV)/GaAs two-junction mechanically stacked on an InGaAsP ( E <sub>g</sub>=0.95 eV) single-junction cell. The component cell of the...
Article
Full-text available
Component efficiencies of 0.2/sq cm cells at approximately 100x AMO light concentration and 80 C temperatures are not at 15.3 percent for a 1.9 eV AlGaAs top cell, 9.9 percent for a 1.4 eV GaAs middle cell under a 1.9 eV AlGaAs filter, and 2.4 percent for a bottom 1.0 eV InGaAs cell under a GaAs substrate. The goal is to continue improvement in the...
Article
A Ga 0.8 In 0.2 As/GaAs thermally actuated optical switch has been demonstrated for unpolarized 1.08 μm light. Its measured on:off contrast ratio of 4.7 was produced electrically by changing the applied voltage from 0 to 3.2 V on a device with a 2.25‐μm‐thick Ga 0.8 In 0.2 As active layer. The ‘‘on’’ insertion loss was 3.8 dB. The on:off change in...
Conference Paper
A practical commercial design has been developed for a 1000×concentrator module based on a 2×18 layout of high-efficiency, GaAs, single junction solar cells. A 2×1 minimodule section has been fabricated and tested in Palo Alto sunlight and found to have a 22.7% AM1.5D efficiency. It is passively cooled, has a low part count, and uses Fresnel primar...
Conference Paper
To evaluate the effects of the space environment on advanced III-V semiconductor solar cells, 19 experiments were placed on the LIPS III satellite. These experiments consisted of both single-junction GaAs cells and InGaAs and AlGaAs components for monolithic multijunction cells. The LIPS III satellite has been sending data to earth for nearly three...
Conference Paper
Various types of multijunction cells, including monolithic, mechanically stacked, two, three, four, and six-terminal, are reviewed along with various interconnection techniques. Since different measurement techniques are appropriate for accurately measuring the cell performance of each type of multijunction cell, the key issues in obtaining accurat...