Transfer Function Measurements Research Articles

Endorectal digital prostate tomosynthesis (endoDPT): a proof-of-concept study

In this study we present endorectal digital prostate tomosynthesis (endoDPT), a proposed method of high resolution prostate imaging using an endorectal x-ray sensor and an external x-ray source. endoDPT may be useful for visualizing the fine detail of small structures such as low dose rate brachytherapy (LDRBT) seeds that are difficult to visualize with current methods. The resolution of endoDPT was characterized through measurement of the modulation transfer function (MTF) and artifact spread function (ASF) in computational and physical phantoms. The qualitative resolution of endoDPT was assessed relative to computed tomography (CT) through imaging of LDRBT seeds implanted in ex vivo canine prostates. The x-ray sensor MTF reached 10% at 11.50 mm−1, the reconstruction algorithm MTF reached a maximum at 7.08 mm−1, and the ASF was 2.5 mm (full-width at half-maximum). Fine structures in LDRBT seeds like the 0.05 mm thick shell were visible with endoDPT but not CT. All endoDPT images exhibited an overshoot artifact. The measured MTFs were consistent with other studies using similar x-ray sensors and demonstrated improved resolution compared to digital breast tomosynthesis; this result was due to the smaller endoDPT x-ray sensor detection element size and quantitatively demonstrates the high resolution of endoDPT. The ASF demonstrated worse depth resolution compared to in-plane resolution, due to partial angular sampling; partial angular sampling also caused the observed overshoot artifact in the endoDPT images. However, endoDPT still was able to visualize fine structures such as the LDRBT seed shell to a much higher degree than CT. This high-resolution visualization may be useful for improvements in patient specific LDRBT dosimetry. Overall, these results indicate endoDPT is capable of high in-plane spatial resolution and is thus well poised for optimization and studies assessing clinical utility.

Metrology of Acquisition Chains and Signal Processing of LMJ Experiments

Since the first experiment in 2014, more and more plasma diagnostics are being deployed on the Laser MegaJoule (LMJ) facility manufactured by C.E.A/D.A.M. These diagnostics aim at measuring radiations or particles emitted during laser experiments to study high-energy physics, especially inertial confinement fusion (ICF). Different types of sensors surround the LMJ target chamber and realize the conversion of the quantities of interest to an electric signal. The signal is then transmitted via coaxial cables, acquired by a broadband oscilloscope, and digitally post-processed. Each step of this typical acquisition chain adds measurement errors and increases the global uncertainty. First, a numerical model of the digitizer alongside a specific hardware system designed to perform its metrology in situ will be presented. It computes errors sources such as offset, gain and skew, and provides a measurement of the effective number of bits (ENOB) of the digitizer. The experimental characterization of the electrical chain via its transfer function measurement will also be detailed. Finally, the numerical methods deployed to handle the inverse problem, based on deconvolution processes, will be introduced, including future developments exploiting Bayesian inferences and statistical approaches.

A Transfer Function Measurement Setup With an Electro-Optic Sensor for MR Safety Assessment in Cascaded Media

This article introduces a transfer function (TF) measurement setup for cascaded media (CM) and demonstrates that the full immersion of an implant in a lossy medium might result in an underestimation of the RF-induced heating compared to partial immersion. A CMTF measurement setup was constructed using medium-specific local electric field sources and electro-optical sensor (EOS). TFs of bare and insulated wire samples were measured in the CM air/saline. Finite-difference time-domain (FDTD) simulations were implemented using a reciprocal approach with a monopole excitation source as well as a piecewise plane wave excitation method. As a theoretical guideline for understanding the CM behavior of the devices, an analytical model based on the buried wire models was derived. CMTFs agree with good precision between FDTD simulations, analytical modeling, and the EOS measurements. Measured TFs were calibrated and validated similar to ISO/TS 10974:Ed.2. Calculated tip SAR values under uniform incident field show an increase up to a factor of 5.7 for partially immersed wires compared to the full immersion. On average, FDTD-based TFs overestimate SAR by 25% compared to measurement-based TFs. As safety assessments for fully immersed implant leads or devices might not represent the worst-case scenario, single-medium assessments might underestimate the RF-induced heating.

Определение коэффициентов затухания по измеренным частотным характеристикам обмоток силовых трансформаторов. Ч. 1. Теоретическое рассмотрение

To model high-frequency processes and determine the stresses on the internal insulation of transformer windings, reliable high-frequency models of power transformers are required. The accuracy of modeling high-frequency resonance processes in the windings depends on how correctly the model reproduces the natural frequencies and damping of free oscillations in the windings. To construct and verify high-frequency models of power transformers, it is necessary to have experimentally obtained data on the values of damping factors. There is a known method for determining the winding damping factors based on measurements of the voltage transfer functions at the internal points of the windings and their subsequent processing using the vector fitting technique, but its application is not always possible in practice. The article presents the results of theoretical studies performed for a simplified transformer winding equivalent circuit. It is shown that the damping factors can be estimated from the width of the resonance peaks of the frequency responses of the voltage modulus and reactive component at the midpoint of the equivalent circuit, and from the input admittance resistive component and current in the neutral of the considered resonance circuit.

Cross-site investigation on head-related and headphone transfer functions: variabilities in relation to loudness balancing

Headphone transfer function (HpTF) and head-related transfer function (HRTF) measurements are crucial in acoustic science and in binaural virtual acoustic applications. Yet, their measurement set-up, procedure or post-processing is different for nearly every lab, especially for the HRTF measurements. To compare findings between different labs, these measurement deviations have to be quantified alongside with their influence on perceptual aspects. In the scope of a cross-site investigation on loudness balancing between headphone and loudspeaker listening, a set of HpTFs with three different headphones (open, closed, insert earphones) and HRTF close to the eardrum were measured in 14 participants travelling to two different measurement sites at Aachen and Oldenburg. Though set-ups for measuring the HRTF are very different between sites, the gathered HRTFs are quite consistent across them. For the measured HpTFs, across sites the open headphones consistently yield a slightly lower variability in the range from 70 to 5000 Hz than the closed one while the insert earphones exhibit much higher variabilities and a limited range of reproducible results. The difference in loudness balancing across labs could well be predicted by site-specific systematic differences in HpTFs with the exception of 1 kHz narrowband stimulus. This clearly indicates the limits in comparability of HpTFs and loudness balancing across labs and the importance of using headphones with high repeatability like the open ones used in this investigation.

Influence of spectral characteristics of light sources on measuring space camera modulation transfer function

Modulation transfer function (MTF) measurement is a major means to evaluate the imaging quality of a space camera. The influence caused by the spectral characteristic of light source on the MTF results is not negligible, because the transmittance and color aberration of optical systems, and quantum efficiency of the space camera detectors are all spectrally related. Thus, MTF results tested by different light sources are different from each other. To address this problem, we propose a method to analyze the influence of spectral characteristics of light sources on measuring the MTF of space cameras. In addition, the devices and methods are designed to calibrate the spectral response and monochrome point spread function (PSF) of space camera. A Sigma lens (focal length: 1000mm, F number: 5.6) and a Cannon EOS 5DSR camera (pixel size: 4.14 μm) are combined into an experimental space camera, whose spectral response is calibrated with a monochromator (Omno30300, NBeT) and a spectral radiometer (FieldSpec, ASD). We calibrate the monochrome PSF of the Sigma lens with the same monochromator and a CCD (PIXIS 1024, Princeton Instruments, pixel size: 13 μm) micro-measuring system (20X objective). During the calibration of spectral response and monochrome PSF, the same collimator (focus: 5000 mm, F number: 10) is used. With using the proposed method and those calibrating data, we compute the theoretical values of the MTF of a space camera measured separately with five different light sources. The results indicate that MTF measured by a xenon lamp is greatly different from those MTFs measured by the other four light sources. Comparisons of those theoretically calculated MTFs, separately, show that the MTF measured by a tungsten halogen lamp is greater than the MTF measured by a xenon lamp at each spatial frequency. The deviation between those two lamps reaches a maximum value of 0.075 in the medium-high frequency zone. Furthermore, in order to verify those theoretical conclusions, a platform including a collimator and the previous space camera is constructed. The MTFs measured by a tungsten halogen lamp and a xenon lamp are computed with the slanted-edge method respectively. The results demonstrate that the distributions and deviations of the MTFs tested by those two lamps are identical to those theoretical results at each spatial frequency, with the maximum deviation being 0.057. The theoretical and experimental results demonstrate that the suggested method can accurately calculate the influence of spectral characteristics of light sources on measuring MTF of space cameras. The proposed method can also be adopted to investigate the influence of spectral characteristics of light sources on MTF of optical systems in the design or test stages.

Research on improving measurement accuracy of acoustic transfer function of underwater vehicle

To address the problem that acoustic transfer functions with underwater platforms cannot be measured accurately, this paper presents a method based on phase compensation to improve the accuracy of acoustic transfer function measurements on underwater platforms. The time-domain impulse response signals with multiple cycles are first collected and intercepted, and then their phase differences are estimated using the least-squares method, and phase compensation is used to align the phases of all the signals, and then the impulse response signals are weighted and averaged over all the impulse response signals to cancel out the random noise. The water pool test proves that this method reduces the measurement random noise while obtaining a high-fidelity time domain transfer function, which effectively improves the signal-to-noise ratio of the measurement. The method adopts only one measurement signal, and without changing the measurement system, the random noise is cancelled out by the in-phase superposition of the multi-cycle impulse response signals to avoid the nonlinear distortion of the measurement results.

Estimating Central Blood Pressure From a Single Peripheral Pressure Measurement

Abstract This paper presents a set of criteria used to identify the parameters describing a tube-load model of the arterial system to estimate central blood pressure (BP). The criteria are generalizable to accommodate for inter- and intra-subject variability encountered in the ICU. The proposed single measurement transfer function (SMTF) requires only a single peripheral pressure measurement, commonly available in the ICU, for central pressure estimation, removing the need for an additional measurement of pulse transit time, common to other central BP estimation models. The method was tested using data from six (6) porcine experiments where septic shock was induced and subsequent treatment was performed. Systolic pressure (SP), pulse pressure (PP) and root-mean-squared (RMSE) errors relative to invasive measurements of aortic pressure were used to assess accuracy. The SMTF method produced mean errors ≤5mmHg across all metrics with 84.4, 88.7 and 63.2% of SP, PP, and RMSE, respectively, within this bound of measured central pressure. Peripheral BP accuracy was also assessed as it is commonly used as a surrogate for central BP in clinical settings. Finally, two alternate methods utilizing the same model equations with additional inputs, one using the measured pulse transit time and the other minimizing the RMSE with measured aortic pressure, were implemented to compare the SMTF accuracy to best case outputs given the model equations.

Intelligent Classifiers in Distinguishing Transformer Faults Using Frequency Response Analysis

With the expansion of the use of frequency response analysis (FRA) as a reliable tool for fault detection in transformers, more capabilities of this method are discovered every day. So that today the number of transformer faults that can be identified by FRA method has also increased. One of the most critical steps in fault detection with FRA is to distinguish faults and classify them in different classes. In this paper, well-known intelligent classifiers (probabilistic neural network, decision tree, support vector machine, and k-nearest neighbors) are used to classify transformer faults. For this purpose, the necessary measurements are performed on the model transformers under the healthy condition and under different fault conditions (axial displacement, radial deformation, disc space variation, short-circuits, and core deformation). Then, by dividing the frequency ranges of the measured transfer functions of the transformer, a new feature based on numerical and statistical indices for training and validation of classifiers is proposed. After completing the training process, the performance of the classifiers is evaluated and compared by applying the data obtained from real transformers.

Modulation Transfer Function Measurements Using a Learning Approach From Multiple Diffractive Grids for Optical Cameras

Remote sensing images are easily degraded because the imaging process is influenced by the camera itself link (CIL), such as optical systems, electronic systems, and image sensors. The invariant modulation transfer function (IMTF) of a remote sensing camera can represent the degeneration produced by the CIL. Thus, the IMTF compensation can remove the degeneration of the CIL to improve the image quality. However, conventional IMTF measurement methods using a single feature (e.g., edge or pulse) have low measurement accuracy due to the influence of noises, resulting in low image compensation quality. Here, we propose an efficient measurement method of the IMTF in conjunction with a learning approach to compensate for the imaging degeneration. The IMTF from the CIL is measured using multiple diffractive grids, which are generated by two 1-D gratings. A learning approach is proposed to estimate the IMTF from the diffractive grids, where diffractive grids are not directly used as a point spread function (PSF) because grids are easily affected by various noises and result in low reconstruction accuracy of the IMTF. We experimentally confirm that the proposed method is useful for the IMTF estimations and compensation for image quality degeneration of remote sensing cameras under noise conditions. The proposed methods can double the MTF reconstruction accuracy and yield higher image compensation quality compared with traditional methods.

Fabrication-error analysis of injection-molded aspheric elements using typical aberration terms in transmitted wavefront with Shack–Hartmann wavefront-sensing measurement

Aspheric lenses help meet the most demanding optical requirements while the precision injection molding technique hits the target for precision and cost. We developed a method of analyzing aberration terms in the transmitted “wavefront measurement,” determined by Shack–Hartmann wavefront sensing to estimate the fabrication errors of injection-molded aspheric elements. Considering aspheric element fabrication using a small training data set and F-measure fuzzy cluster analysis, an unsupervised learning method was applied to extract typical aberration terms from the wavefront polynomial. The experimental results suggest that these aberration terms, which are related to spherical (third-, fifth-, and seventh-order) and coma (third-order) aberration terms in the transmitted wavefront polynomial expansion, can be employed to estimate the surface error and decenter, respectively, of a lens from a specific mold cavity. The sampling lenses evaluated in the proposed measuring process were collected from different mold cavities according to their total working performance in the modulated transfer function measurement for the whole camera module. The performances of the typical aberration terms were discussed by comparing to the ones obtained from an interferometer and a profilometer. The proposed method could provide high detection efficiency and can thus be applied for the quality control of aspheric elements for mobile phones, where the existing errors are mainly spherical, coma, and astigmatism aberrations.

Piezoelectric mirror shifter transfer function measurement, modelling, and analysis using feedback based synthetic-heterodyne Michelson interferometry

Laser vibrometry has many applications in non-contact dynamic displacement and vibration measurement. A test beam reflected from a target and a reference beam are combined and detected by a photodiode; the photodetected signal is then processed to determine the target displacement and vibration. This paper describes the use of a 9 kHz measurement bandwidth system, consisting of a Michelson interferometer and self-correcting feedback synthetic-heterodyne signal processing technique, to measure the displacement impulse response of a commercial piezoelectric mirror shifter (PMS), consisting of a mirror mounted on a Piezoelectric transducer and a connecting 50 Ω electrical coaxial cable. The actual non-ideal applied impulse and measured impulse response data were used in conjunction with the instrument variable method to determine a Laplace domain linear transfer function approximation to the actual PMS transfer function. The best transfer function fitting, having a 84% normalized root mean square goodness of fit, was obtained using a 5-th order transfer function having two complex conjugate pole pairs, with associated natural frequencies of 6.29 and 6.79 kHz, and a single real pole. The transfer function zeros consisted of a single complex conjugate zero pair, having an antiresonance frequency of 6.38 kHz and a single real zero. Knowing the analytic transfer function of PMS based nanopositioners is useful for example in the design of closed-loop phase-locked interferometers for wideband sensing.

A deep convolutional neural network for simultaneous denoising and deblurring in computed tomography

Computed tomography (CT) has been commonly used for providing medical images. However, CT scans have a potential risk of side-effect due to high radiation dose. In order to overcome this limitation, low-dose CT imaging techniques have been developed by using novel scanners, reconstruction algorithms and adjusted scanning protocols. In spite of these strategies, the conventional low-dose CT imaging techniques are hard to compromise between radiation dose and image quality, leading to noise, blur and artifacts in CT images. These drawbacks of the conventional techniques degrade diagnostic accuracy. In this study, we proposed a deep convolutional neural network (CNN)-based low-dose CT imaging technique for simultaneously reducing noise and blur at once. The proposed method, called denoising and deblurring CNN (DnDbCNN), consisted of two networks, and each network was separately trained for reducing noise and blur in CT images. For combining the trained networks, a convolutional layer was added between the two trained networks, and the combined network was re-trained to update the weights of each layer. Denoising convolutional neural network (DnCNN) and super-resolution convolutional neural network (SRCNN) were also implemented for comparison. We evaluated the performance of the DnDbCNN in terms of mean-square error (MSE), peak signal-to-noise ratio (PSNR), structural similarity index measure (SSIM), coefficient-of-variation (COV) and modulation transfer function (MTF). The results showed that the DnDbCNN improved the noise property, spatial resolution and quantitative accuracy of CT images, and the performance of the DnDbCNN was superior to the conventional methods. As a result, the DnDbCNN can simultaneously reduce noise and blur by combining two networks through three training steps. Therefore, the DnDbCNN has a potential for improving CT image quality with low radiation dose and enables precise diagnosis.

Acoustic characterisation of towing tanks

Large water tanks known as towing tanks, wave basins and cavitation tunnels, are used to determine the performance of ships, boats and submarines at model-scale. They may also be used for noise measurements but suffer from the effects of reverberation, which makes it difficult to capture the free field conditions necessary for complete characterisation of the noise. This paper describes an adapted Image Source Model and its use to understand these reverberation effects. It extends the conventional Image Source Model by the use of spherical wave reflection coefficients at the tank boundaries, in addition to the inclusion of volumetric absorption. Validation of the Image Source Model is undertaken by comparison with published transfer function measurements made in the depressurized wave basin at the Maritime Research Institute Netherlands (MARIN). The relative contribution to the tank response from the direct field and free surface is examined. The tank response is found to vary with three frequency-scales (periodicities) resulting from interactions with the free surface and axial modes.

Single-plate method for practical modulation transfer function measurement in magnetic resonance imaging.

A new "single-plate method" is presented for measuring the modulation transfer function (MTF) in magnetic resonance imaging (MRI). This method sets a slice plane perpendicular to a single-plate phantom to eliminate contamination effects from the direction perpendicular to the measurement direction in the image plane, which occur with a conventional ramp method. As no practical method for measuring the MTF has been established for MRI, we examined whether the MTF can be measured practically using the single-plate method for various fast imaging sequences. Furthermore, the MTFs of T1-weighted (T1W) fast spin echo (FSE) and conventional spin echo (CSE) images obtained using the single-plate method and ramp method were compared. The measured MTFs of T1W CSE images revealed rectangular shapes with a sharp decrease near the Nyquist cutoff frequency in both phase-encoding (PE) and frequency-encoding (readout, RO) directions. The measured MTFs of T1W FSE images obtained with centric-order acquisition showed symmetric step-function shapes reflecting k-space segmentation determined by the echo train length (ETL). The measured MTFs of T2-weighted (T2W) FSE images showed asymmetric step-function shapes reflecting differences in T2 decay of signals from samples. The MTFs obtained using the single-plate method significantly reduced the collapse caused by the contamination effect, which is observed in all the MTF measurements of the ramp method. The proposed "single-plate method" simplified the complicated MTF measurement procedure and eliminated the contamination effect. This method is expected to be useful for evaluating the resolution properties of MR fast imaging techniques with a complicated k-space trajectory.

Uncorrelated blocked force determination on plate and shell structures

Inverse force determination is commonly used to determine input forces when they are not directly measurable. If transfer functions are measured with the source component attached, the inversely determined forces are, by definition, blocked forces. The primary advantage of using blocked forces is that the receiving structure may be modified and blocked forces, in theory, are unchanged. In this research, blocked forces are determined for a plastic engine cover connected to a base plate and a compressor attached to two different structures. At lower frequencies, blocked forces are determined using routine approaches where phase is included in both transfer function and operational response measurements. At high frequencies, it is demonstrated that predictions are improved if phase is ignored and blocked forces are assumed to be uncorrelated with respect to each other. It is also shown that the uncorrelated blocked forces are still valid even when changes are made to the receiving structure.

A simplified calibration for receiver transfer function and effects of its diffraction correction on the nonlinearity parameter of solids

The determination of the acoustic nonlinearity parameter (β) for fluids or solids is composed of two-step measurements: receiver calibration and harmonic generation. The purpose of calibration is to find the receiver transfer function and convert the electrical output of the measured harmonic generation into physical displacements, from which the absolute β value is determined. Therefore, accurate measurement of the receiver transfer function is required for the accurate determination of β This paper presents an accurate and simplified calibration technique for receiving transducers that can eliminate the effects of impedance mismatch and minimize internal reflections from cabling/probe connections. The formula for the receiver transfer function is explicitly derived with the inclusion of diffraction and attenuation effects. After the transfer function is measured with minimal reflections from the cabling/probe connections, its effects on the β determination can be evaluated. Specifically, beam diffraction and material attenuation effects during calibration and harmonic generation measurements are systematically investigated. Diffraction and attenuation corrections were found to have more dominant effects on calibration measurements than on harmonic generation measurements in calculating the β value. After all necessary corrections are made, the β value of 6.46 ± 0.13 is obtained for Al 7075-T6511 samples with five different thicknesses (4, 6, 8, 10, and 12 cm), validating the proposed calibration and β measurement techniques.

Small footprint head-related transfer function measurement apparatus for binaural heads utilizing one loudspeaker

Apparatuses for measuring head-related transfer-functions (HRTFs) often require large anechoic spaces with one or more arcs of loudspeakers. A measurement setup to obtain the HRTF of a Brüel & Kjær HATS 4100-D binaural head was designed and implemented that utilizes one loudspeaker in a 1309 ft3 anechoic space with 18 in. wedges. By rotating the HATS along two axes, the characterization of many look directions can be achieved. The HATS can be rotated about the axis through the center of the ears via a turntable, and the yaw of the HATS can be adjusted via mounting gear on custom framing. HRTF measurements were completed using both maximum-length sequence and sine sweep signals in order to compare the two techniques. Comparisons between the measured 4100-D HRTF and the HRTF of a similar B&K binaural head will be discussed, as well as plans to use the 4100-D HRTF to compare binaural room impulse responses (RIRs) with spatial RIRs obtained with a spherical microphone array. [Work supported by a Graduate Assistance in Areas of National Need (GAANN) Fellowship.]

A frequency band constrained filtered-x least mean square algorithm for feedback active control systems.

The generalized leaky filtered-x least mean square (GLFxLMS) algorithm can reduce the noise amplification caused by the waterbed effect in feedback active control systems effectively; however, it suffers from a high computation complexity. Hence, a frequency band constrained filtered-x least mean square algorithm is proposed to reduce the computation complexity of the GLFxLMS algorithm by replacing the penalty term containing a symmetric Toeplitz matrix in the cost function with the mean square of a penalty signal. The simulation results based on the measured transfer functions of an active headrest system show that the proposed algorithm has the same performance as the GLFxLMS algorithm, but with much lower computation complexity.

Guided Wave Inspection of Bars in Reinforced-Concrete Beams Using Surface-Mounted Vibration Sensors

Steel reinforcement bars (rebars) in concrete structures are inaccessible and not conducive to many inspection methods. This paper proposes a non-invasive technique based on guided waves for detecting localised abnormalities in rebars embedded in concrete beams. The technique is predicated on previously published observations that guided waves are strongly reflected by discontinuities at the frequency at which they begin to propagate, i.e., at cut-on. The reflection coefficient at cut-on is estimated using a simple wave decomposition in which a near-zero wavenumber value is assumed. A simulated study is first carried out to evaluate the technique on a concrete beam featuring four rebars. The wave finite element approach is adopted to model two uniform beams which are coupled via a short, damaged section modelled in conventional finite element analysis. Estimated reflection coefficients arising from the discontinuity are close to the true values at cut-on and independent of frequency elsewhere, so that no prior knowledge of cut-on frequencies is required. Three steel-reinforced concrete beams were fabricated—one uniform and two with localised rebar damage—and reflection coefficients were estimated from measured transfer functions. As predicted, abrupt deviations in the reflection coefficient occurred at cut-on frequencies for both damaged beams.