Line Spread Function Measurements Research Articles

Calibration of the Marshall Grazing Incidence X-Ray Spectrometer Experiment. II. Flight Instrument Calibration

Abstract The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) is a sounding rocket experiment that observes the soft X-ray spectrum of the Sun from 6.0–24 Å (0.5–2.0 keV), successfully launched on 2021 July 30. End-to-end alignment of the flight instrument and calibration experiments are carried out using the X-ray and Cryogenic Facility at NASA Marshall Space Flight Center. In this paper, we present the calibration experiments of MaGIXS, which include wavelength calibration, measurement of line spread function, and determination of effective area. Finally, we use the measured instrument response function to predict the expected count rates for MaGIXS flight observation looking at a typical solar active region.

Consistency of total column ozone measurements between the Brewer and Dobson spectroradiometers of the LKO Arosa and PMOD/WRC Davos

Abstract. Total column ozone measured by Brewer and Dobson spectroradiometers at Arosa and Davos, Switzerland, have systematic seasonal variations of around 1.5 % using the standard operational data processing. Most of this variability can be attributed to the temperature sensitivity of approx. +0.1 %/K of the ozone absorption coefficient of the Dobson spectroradiometer (in this study D101). While the currently used Bass and Paur ozone absorption cross-sections produce inconsistent results for Dobson and Brewer, the use of the ozone absorption cross-sections from Serdyuchenko et al. (2014) in conjunction with an effective ozone temperature dataset produces excellent agreement between the four Brewers investigated (of which two are double Brewers) and Dobson D101. Even though other ozone absorption cross-sections available in the literature are able to reduce the seasonal variability as well, all of those investigated produce systematic biases in total column ozone between Brewer and Dobson of +2.1 % to −3.2 %. The highest consistency in total column ozone from Brewers and Dobson D101 at Arosa and Davos is obtained by applying the Rayleigh scattering cross-sections from Bodhaine et al. (1999), the ozone absorption cross-sections from Serdyuchenko et al. (2014), the effective ozone temperature from either ozone-sondes or the European Centre for Medium-Range Weather Forecasts (ECMWF), and the measured line spread functions of Brewer and Dobson. The variability of 0.9 % between Brewer and Dobson for single measurements can be reduced to less than 0.1 % for monthly means. As shown here, the applied methodology produces consistent total column ozone datasets between Brewer and Dobson spectroradiometers, with average differences of 0.0 % and a remaining seasonal variability of 0.11 %. For collocated Brewer and Dobson spectroradiometers, as is the case for the Arosa and Davos total column ozone times series, this allows for the merging of these two distinct datasets to produce a homogeneous time series of total column ozone measurements. Furthermore, it guarantees the long-term future of this longest total column ozone time series, by proposing a methodology for how to eventually replace the ageing Dobson spectroradiometer with the state-of-the art Brewer spectroradiometer.

Fabrication of Si photonic waveguides by electron beam lithography using improved proximity effect correction

In this work, electron beam lithography proximity effect correction (PEC) was experimentally studied for patterning of Si photonic waveguides with a relatively thick resist mask. Beam’s energy density distribution (EDD) was experimentally extracted by the line exposure method; however, exposure lines in this work were developed after cleavage with a high-contrast process to reduce developer-related effects. The measured line spread function was fitted to a 4-Gaussian function to model mid-range energy densities accurately. The extracted EDD showed less proximity effects compared to conventional Monte-Carlo simulation performed by a commercial software. PEC processes with both techniques were experimentally compared for a Si photonic waveguide pattern with different side-cladding trench widths. Microscopic images confirmed that the presented calibration method could achieve better development conditions near the required clearance dosage. Single-mode propagation loss for a 500 × 220 nm Si wire waveguide was reduced from 3.2 to 2.4 dB cm−1 using the presented process.

Monte Carlo calculation of the spatial response (Modulated Transfer Function) of a scintillation flat panel and comparison with experimental results

Abstract Phosphor screens are commonly used in many X-ray imaging applications. The design and optimization of these detectors can be achieved using Monte Carlo codes to simulate radiation transport in scintillation materials and to improve the spatial response. This work presents an exhaustive procedure to measure the spatial resolution of a scintillation flat panel image and to evaluate the agreement with data obtained by simulation. To evaluate the spatial response we have used the Modulated Transfer Function (MTF) parameter. According to this, we have obtained the Line Spread Function (LSF) of the system since the Fourier Transform (FT) of the LSF gives the MTF. The experimental images were carried out using a medical X-ray tube (Toshiba E7299X) and a flat panel (Hammamatsu C9312SK). Measurements were based on the slit methodology experimental implementation, which measures the response of the system to a line. LSF measurements have been performed using a 0.2 mm wide lead slit superimposed over the flat panel. The detector screen was modelled with MCNP (version 6) Monte Carlo simulation code in order to analyze the effect of the acquisition setup configuration and to compare the response of scintillator screens with the experimental results. MCNP6 offers the possibility of studying the optical physics parameters (optical scattering and absorption coefficients) that occur in the phosphor screen. The study has been tested for different X-ray tube voltages, from 100 to 140 kV. An acceptable convergence between the MTF results obtained with MCNP6 and the experimental measurements have been obtained.

Comparison of 3-D synthetic aperture phased-array ultrasound imaging and parallel beamforming.

This paper demonstrates that synthetic aperture imaging (SAI) can be used to achieve real-time 3-D ultrasound phased-array imaging. It investigates whether SAI increases the image quality compared with the parallel beamforming (PB) technique for real-time 3-D imaging. Data are obtained using both simulations and measurements with an ultrasound research scanner and a commercially available 3.5- MHz 1024-element 2-D transducer array. To limit the probe cable thickness, 256 active elements are used in transmit and receive for both techniques. The two imaging techniques were designed for cardiac imaging, which requires sequences designed for imaging down to 15 cm of depth and a frame rate of at least 20 Hz. The imaging quality of the two techniques is investigated through simulations as a function of depth and angle. SAI improved the full-width at half-maximum (FWHM) at low steering angles by 35%, and the 20-dB cystic resolution by up to 62%. The FWHM of the measured line spread function (LSF) at 80 mm depth showed a difference of 20% in favor of SAI. SAI reduced the cyst radius at 60 mm depth by 39% in measurements. SAI improved the contrast-to-noise ratio measured on anechoic cysts embedded in a tissue-mimicking material by 29% at 70 mm depth. The estimated penetration depth on the same tissue-mimicking phantom shows that SAI increased the penetration by 24% compared with PB. Neither SAI nor PB achieved the design goal of 15 cm penetration depth. This is likely due to the limited transducer surface area and a low SNR of the experimental scanner used.

Assessment of image quality in x-ray radiography imaging using a small plasma focus device

This paper offers a comprehensive investigation of image quality parameters for a small plasma focus as a pulsed hard x-ray source for radiography applications. A set of images were captured from some metal objects and electronic circuits using a low energy plasma focus at different voltages of capacitor bank and different pressures of argon gas. The x-ray source focal spot of this device was obtained to be about 0.6mm using the penumbra imaging method. The image quality was studied by several parameters such as image contrast, line spread function (LSF) and modulation transfer function (MTF). Results showed that the contrast changes by variations in gas pressure. The best contrast was obtained at a pressure of 0.5mbar and 3.75kJ stored energy. The results of x-ray dose from the device showed that about 0.6mGy is sufficient to obtain acceptable images on the film. The measurements of LSF and MTF parameters were carried out by means of a thin stainless steel wire 0.8mm in diameter and the cut-off frequency was obtained to be about 1.5cycles/mm.

Circular multilayer zone plate for high-energy x-ray nano-imaging

A circular multilayer zone plate (MZP) was fabricated and its focusing performance was evaluated using 20-keV x-rays. MoSi(2) and Si layers were alternately deposited by DC magnetron sputtering on a wire core; all the interfaces satisfied the Fresnel zone condition. The measured line spread function was converted to a point spread function by tomographic reconstruction. The results suggest that the MZP has the potential to realize the diffraction-limited resolving power, which is calculated to be 35 nm using the diffraction integral. Furthermore, scanning transmission microscopy using the MZP could resolve a 50-nm line-and-space pattern.

Point-spread function of the ocean color bands of the Moderate Resolution Imaging Spectroradiometer on Aqua

The Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua platform has nine spectral bands with center wavelengths from 412 to 870 nm that are used to produce the standard ocean color data products. Ocean scenes usually contain high contrast due to the presence of bright clouds over dark water. About half of the MODIS Aqua ocean pixels are flagged as spatial stray light contaminated. The MODIS has been characterized for stray light effects prelaunch. In this paper, we derive point-spread functions for the MODIS Aqua ocean bands based on prelaunch line-spread function measurements. The stray light contamination of ocean scenes is evaluated based on artificial test scenes and on-orbit data.

An effective method to verify line and point spread functions measured in computed tomography

This study describes an effective method for verifying line spread function (LSF) and point spread function (PSF) measured in computed tomography (CT). The CT image of an assumed object function is known to be calculable using LSF or PSF based on a model for the spatial resolution in a linear imaging system. Therefore, the validities of LSF and PSF would be confirmed by comparing the computed images with the images obtained by scanning phantoms corresponding to the object function. Differences between computed and measured images will depend on the accuracy of the LSF and PSF used in the calculations. First, we measured LSF in our scanner, and derived the two-dimensional PSF in the scan plane from the LSE Second, we scanned the phantom including uniform cylindrical objects parallel to the long axis of a patient's body (z direction). Measured images of such a phantom were characterized according to the spatial resolution in the scan plane, and did not depend on the spatial resolution in the z direction. Third, images were calculated by two-dimensionally convolving the true object as a function of space with the PSF. As a result of comparing computed images with measured ones, good agreement was found and was demonstrated by image subtraction. As a criterion for evaluating quantitatively the overall differences of images, we defined the normalized standard deviation (SD) in the differences between computed and measured images. These normalized SDs were less than 5.0% (ranging from 1.3% to 4.8%) for three types of image reconstruction kernels and for various diameters of cylindrical objects, indicating the high accuracy of PSF and LSF that resulted in successful measurements. Further, we also obtained another LSF utilizing an inappropriate manner, and calculated the images as above. This time, the computed images did not agree with the measured ones. The normalized SDs were 6.0% or more (ranging from 6.0% to 13.8%), indicating the inaccuracy of the PSF and LSE We could verify LSFs and PSFs for three types of reconstruction kernels, and demonstrated differences between modulation transfer functions (MTFs) derived from validated LSFs and inaccurate LSFs. Our technique requires a simple phantom that is suitable for clinical scanning, and does not require a particular phantom containing some metals or specific fine structures, as required in methods previously used for measurements of spatial resolution. Therefore, the scanned image of the phantom will be reliable and of good quality, and this is used directly as a confident reference image for the verification. When one obtains LSF, PSF or MTF values, verification using our method is recommended. Further, when another method for the measurement of LSF and PSF is developed, it could be validated using our technique, as illustrated in the method proposed by Boone [Med. Phys. 28, 356-360 (2001)] and used in this paper.

SU‐FF‐I‐47: A Method of the Pixelized Detectors Coordinate Resolution and MTF Estimation

Purpose: We propose a simple method to estimate MTF and coordinate resolution of pixelized detectors used in digital imaging systems. Method and Materials: The traditional determination of coordinate resolution for pixelized detectors is mainly based on the use of line spread function measurements. The proposed method is based on the determination of the statistical correlation between neighbour elements in the detector. The correlation is evaluated by means of statistical noise measurement of the isolated elements and the linear combination of neighbour elements. To suppress the possible contribution of the beam spatial variation, the difference between values of neighbour elements is used. The distribution of differences between the detector neighbour pixels is constructed for the pixels sharing an edge (either in X or Y directions), or having one common vertex (U or V). For the validation of the method, we have studied the GE Senographe 2000D mammography unit digital detector. The detector has been irradiated by X‐rays across its entire surface. The dependence of the resolution on the photon intensity value and the pixel coordinate has been studied. The resolution function has been considered as a result of convolution of Gaussian and uniform distributions. Results: The coordinate resolution of the studied detector, having pixel size of 100 μ, is of the order of 50 –55 μ, that is, almost twice the value conditioned by pixel geometrical size. A small asymmetry between the X and Y directions has been detected. The reconstructed modulation transfer function agrees with MTF measurements of the studied detector, within a few percent. Conclusion: The proposed method can be useful for the estimation of coordinate resolution, line spread function and modulation transfer function of the detectors used in the systems of medical digital imaging.

The use of detective quantum efficiency (DQE) in evaluating the performance of gamma camera systems

The imaging properties of an imaging system can be described by its detective quantum efficiency (DQE). Using the modulation transfer function calculated from measured line spread functions and the normalized noise power spectrum calculated from uniformity images, DQE was calculated with the number of photons emitted from a plane source as a measure for the incoming SNR2. Measurements were made with 99mTc, using three different pulse height windows at 2 cm and 12 cm depths in water with high resolution and all purpose collimators and with two different crystal thicknesses. The results indicated that at greater depths a 15% window is the best choice. The choice of collimator depends on the details in the organ being investigated. There is a break point at 0.5 cycles cm−1 and 1.2 cycles cm−1 at 12 cm and 2 cm depths, respectively. A difference was found in DQE between the two crystal thicknesses, with a slightly better result for the thick crystal for measurements at 12 cm depth. At 2 cm depth, the thinner crystal was slightly better for frequencies over 0.5 cm−1. The determination of DQE could be a method to optimize the parameters for different nuclear medicine investigations. The DQE could also be used in comparing different gamma camera systems with different collimators to obtain a figure of merit.

Monte Carlo study of a highly efficient gas ionization detector for megavoltage imaging and image-guided radiotherapy.

The imaging characteristics of an arc-shaped xenon gas ionization chamber for the purpose of megavoltage CT imaging were investigated. The detector consists of several hundred 320 microm thick gas cavities separated by thin tungsten plates of the same thickness. Dose response, efficiency and resolution parameters were calculated using Monte Carlo simulations. The calculations were compared to measurements taken in a 4 MV photon beam, assuming that the measured signal in the chambers corresponds to the therein absorbed dose. The measured response profiles for narrow and broad incident photon beams could be well reproduced with the Monte Carlo calculations. They show, that the quantum efficiency is 29.2% and the detective quantum efficiency at zero frequency DQE(0) is 20.4% for the detector arc placed in focus with the photon source. For a detector placed out of focus, these numbers even increase. The efficiency of this kind of radiation detector for megavoltage radiation therefore surpasses the reported efficiency of existing detector technologies. The resolution of the detector is quantified with calculated and measured line spread functions. The corresponding modulation transfer functions were determined for different thicknesses of the tungsten plates. They show that the resolution is only slightly dependent on the plate thickness but is predominantly determined by the cell size of the detector. The optimal plate thickness is determined by a tradeoff between quantum efficiency, total signal generation and resolution. Thicker plates are more efficient but the total signal and the resolution decrease with plate thickness. In conclusion, a gas ionization chamber of the described type is a highly efficient megavoltage radiation detector, allowing to obtain CT images with very little dose for a sufficient image quality for anatomy verification. This kind of detector might serve as a model for a future generation of highly efficient radiation detectors.

Satellite-Earth remote sensor scatter effects on Earth scene radiometric accuracy

A general modelling formalism is used to extend near-field point spread function (PSF) measurements over a wide-field off-axis angular range, covering a dynamic range of 5 to 6 orders of magnitude, using bidirectional reflectance distribution function (BRDF) measurements of the scan mirror and other key optical elements. Line spread function (LSF) model results were compared with measured near-field LSF measurements for the Moderate Resolution Imaging Spectroradiometer (MODIS) of the Earth Observing System (EOS), demonstrating excellent agreement between model and measurements. When realistic effects of sensor scattered light are taken into account, significant radiometric bias errors are produced near high-contrast structured scenes (e.g. bright clouds over dark oceans and land; broken snow and ice scenes). Image restoration using an asymptotically exact PSF is shown to produce results significantly different from those produced by traditional near-field 5 × 5, 7 × 7, ..., PSF kernel matrix inversion techniques. The results reported have implications for future remote-sensor specifications and testing, in-flight and surface-based calibration comparisons, and the assessment of radiometric bias errors in the presence of moderate- to high-contrast Earth scenes.

Resolution of polymethyl methacrylate: Molecular weights of 950 000 vs 50 000

The work examines the resolution and line-to-line resolution of two molecular weights of polymethyl methacrylate (PMMA), developed in 25%–50% solutions of methylisobutyl ketone in isopropanol. Both PMMA’s exhibited similar minimum linewidths of 15–18 nm. Measured linespread functions showed primary Gaussian standard deviations of 11.7 and 28 nm for the 950 and 50 K molecular weight PMMAs, respectively. Developer strength produced ⩽1 nm difference in the width of the linespread function. Atomic force microscope images of latent grating patterns show the same line-to-line resolution for the two undeveloped resists. Developed gratings show the 950 K PMMA to have superior line-to-line resolution. The results are analyzed through integration of the measured linespread functions.

Quantification of gamma camera spatial resolution by means of bar phantom images

A method that has previously been described for rapid objective measurement of intrinsic gamma camera spatial resolution using statistical moments has been applied to images of a four-quadrant bar phantom acquired with various collimators. The full-width at half-maximum (FWHM) of the line spread function (LSF) and modulation transfer functions determined by the moments method was compared with those obtained directly from line spread functions. It was found that, for highest accuracy, the formulae originally described for intrinsic measurements have to be modified to take account of a more accurate description of the Fourier components of the input from a bar phantom. When this modification was applied, FWHM measurements agreed to within 4.0% with the LSF measurements in the range 4.5-7.2 mm if well-resolved images of bars wider than 3.0 mm were used. The method is simple to apply and since the coefficient of variation of these measurements was < 2.2%, it is well-suited for rapid objective routine monitoring of gamma camera spatial resolution. For modulation transfer functions > 0.2, bar sizes of 4.0 and 4.5 mm gave values within 4% of the LSF measurements, when the modified formula was applied, with coefficients of variation less than 4%.

Control in sub-100 nm lithography in SAL-601

This article focuses on e-beam nanolithography in SAL-601, a negative tone, chemically amplified resist from the Shipley Corporation. It examines the role of exposure conditions and the postexposure bake (PEB) on the ability to define features of critical dimensions of 100 nm and below. Lithography was performed with a 50 kV e beam and 140 nm thick resist, where forward scattering of the e beam is expected to be minimal. Common PEB conditions of 105 °C for 1, 3, and 10 min and 110 °C for 1 min were investigated. Although the different PEB conditions produced a factor of up to 2.4× increase in sensitivity, no change was observed in the contrast, 5.8±0.1, or the minimum linewidths, ∼60 nm. The measured line spread function forward Gaussian widths were characterized by standard deviations, σ=27–33 nm, depending on PEB conditions. These are compared to σ of 19 nm for PMMA and the expected σ of 11 nm for the incident probe (σ=10 nm) convoluted with a Monte Carlo code generated point spread function. The measured backscatter coefficient was 0.51, in agreement with our Monte Carlo simulation results. In the analysis of dose latitude to form critical dimension gaps (±10% precision) between a series of pads of differing sizes, the different PEB conditions produced no observed difference. The feature size, however, had a large effect on the ability to form the critical dimension gap. Furthermore, the dose latitude was much smaller than calculated by integration of Monte Carlo derived line spread functions. A semiempirical model is introduced in which the forward Gaussian width of a measured line spread function (LSF) in developed resist is convoluted with the Monte Carlo code LSF. The calculated exposure profiles based on the semiempirical LSF show much better agreement with the experiment than those based on the expected, σ=11 nm, forward Gaussian width.

Discrete scintillator coupled mercuric iodide photodetector arrays for breast imaging

Multi-element (4/spl times/4) imaging arrays with high resolution collimators, size matched to discrete CsI(Tl) scintillator arrays and mercuric iodide photodetector arrays (HgI/sub 2/ PDA) were developed as prototypes for larger 16/spl times/16 element arrays for breast imaging. The compact nature of the arrays allows detector positioning in close proximity to the breast to eliminate activity not in the line-of-sight of the collimator, thus reducing image background. Short collimators, size matched to 11.5/spl times/1.5 mm/sup 2/ scintillators show a factor of 2 and 3.4 improvement in spatial resolution and efficiency, respectively, compared to high resolution collimated gamma cameras for the anticipated compressed breast geometries. Monte Carlo simulations, confirmed by measurements, demonstrated that scintillator length played a greater role in efficiency and photofraction for 140 keV gammas than cross sectional area, which affects intrinsic spatial resolution. Simulations also demonstrated that an increase in the ratio of scintillator area to length corresponds to an improvement in light collection. Electronic noise was below 40 e/sup -/ RMS indicating that detector resolution was not noise limited. The high quantum efficiency and spectral match of prototype unity gain HgI/sub 2/ PDAs coupled to 1/spl times/1/spl times/2.5 mm/sup 3/ and 2/spl times/2/spl times/4 mm/sup 3/ CsI(Tl) scintillators demonstrated energy resolutions of 9.4% and 8.8% FWHM at 140 keV, respectively, without the spectral tailing observed in standard high-Z, compound semiconductor detectors. Line spread function measurements matched the scintillator size and pitch, and small, complex phantoms were easily imaged.

Programmable dynamic line phantom control

The dynamic line phantom (DLP) produces various test patterns for the qualitative assessment of gamma camera performance, by stepper motor control of the translatory movement of a radioactive rigid line source. The addition of a V24 serial interface port has allowed external control via an RS-232 connection to a personal computer. Programmability has enabled the DLP to be used as a quantitative test tool. One such program produces a variable contrast 'hot' and 'cold' bar pattern. The addition of a Quick Basic computer program modifies the line source movement to compensate for variation in line source activity. This maintains a test pattern with fixed, absolute contrast levels, giving the potential for the measurement of variations in long-term imaging performance. A modulation transfer function (MTF) pattern has been designed which encompasses a sinusoidally varying intensifying function with a changing spatial frequency component. Repeated measurements on two gamma cameras demonstrated poor reproducibility (CV of up to 50% dependent upon spatial frequency) and significant differences between the MTF obtained from line spread function measurements (percentage deviation of up to 23%, dependent upon spatial frequency). Neither test pattern has been adopted for routine quantitative assessment of gamma camera imaging characteristics. The reproducibility of results is poor (dominated by counting statistic limitations), making the techniques insensitive to small changes in camera performance.

A comparison of PET detector modules employing rectangular and round photomultiplier tubes

We have compared the high resolution BGO detector blocks from the EXACT HR PET system which use two dual-cathode rectangular photomultiplier (PM) tubes with a new block design, the EXACT HR PLUS, which uses four round PM tubes. Despite the lower coupling area between photocathode and scintillator, the HR PLUS block compares favorably with the HR block. The energy resolution averages 20% for the HR PLUS block and 23% for the HR block, with efficiency variations of 17% in both blocks. Additional measurements were carried out on the HR PLUS block to characterize depth of interaction effects and cross-talk between elements. Coincidence line spread function measurements had a FWHM of 3.0 mm in the axial direction and 2.9 mm in the transaxial direction. In light of these results, limitations of the BGO block design are discussed and some solutions proposed. >

Order-sorting filter transmittance measured with an array detector

The simultaneous measurement of the spectrally and spatially variant transmittance of a linear variable order-sorting filter in a manner that closely resembles its conditions of actual use is described. The transmittance of a prototype order-sorting filter was measured in the 400- to 880-nm wavelength region by illuminating it with the output beam of a spectrophotometer while the filter was attached to the front of a 30 x 32 pixel silicon array detector. The filter was designed to be used in the output beam of a grating spectrometer to prevent the dispersal of higher diffracted orders onto an array detector. Areas of the filter that were spatially matched to the corresponding detector pixel column had measured peak transmittances of about 90 percent that were uniform to within +/- 1.5 percent along a given column. Transmittances for incident wavelengths shorter than the desired bandpass, corresponding to the order overlap region, were measured in the 0.003 range. Line spread function measurements made with the array detector indicated no significant beam spreading caused by inserting the filter into the beam.