Efficient Calibration Procedure Research Articles

Hydrological modeling in alpine catchments: sensing the critical parameters towards an efficient model calibration

For the Tyrolean part of the river Inn, a hybrid model for flood forecast has been set up and is currently in its test phase. The system is a hybrid system which comprises of a hydraulic 1D model for the river Inn, and the hydrological models HQsim (Rainfall-runoff-discharge model) and the snow and ice melt model SES for modeling the rainfall runoff form non-glaciated and glaciated tributary catchment respectively. Within this paper the focus is put on the hydrological modeling of the totally 49 connected non-glaciated catchments realized with the software HQsim. In the course of model calibration, the identification of the most sensitive parameters is important aiming at an efficient calibration procedure. The indicators used for explaining the parameter sensitivities were chosen specifically for the purpose of flood forecasting. Finally five model parameters could be identified as being sensitive for model calibration when aiming for a well calibrated model for flood conditions. In addition two parameters were identified which are sensitive in situations where the snow line plays an important role.

MO‐FF‐A1‐02: Calibration of a Novel 4D Diode Array for IMRT and VMAT QA

Purpose: To develop calibration procedures for a novel 4D diode array for IMRT as well as arc therapy quality assurance (QA). Method and Materials: A novel 4D diode array (ArcCHECK) was designed for rotational therapy QA. Its cylindrical and isotropic design presents a consistent detector image in beam's eye view for all gantry angles. Signals were acquired every 50 ms, allowing for real‐time per‐beam QA as well as measuring composite dose distributions. An efficient calibration procedure was developed to obtain diode sensitivity and directional response dependence with one full gantry rotation. In this process, each diode was in turn irradiated under the same beam condition on beam central axis. The directional response was obtained using TPS‐calculated beam profiles, which were validated in rectangular geometry. A real time algorithm to derive beam angle based on beam edge back projections was developed to apply the corresponding directional response factors. Its clinical application for IMRT QA was demonstrated with 17 IMRT head and neck beams delivered with planned beam angles. Measured dose distribution was compared to TPS calculation in three dimensions with %diff and distance‐to‐agreement analysis. Its feasibility for VMAT QA was investigated with a prostate conformal arc plan. Results: For the IMRT beams, average passing rate was 94.3%±2.1% and 99.6%±0.8% with 1%/2mm and 1%/3mm, respectively. For the conformal arc plan, the passing rate was 97.7% with 1%/2mm and 100% with 1%/3mm. Conclusion: An efficient calibration procedure was developed to obtain both the diode sensitivity and directional response dependence. Real‐time gantry angles were derived accurately based on beam edge back projections, which were used to apply directional response factors. Excellent agreement with TPS calculation was achieved for both IMRT and VMAT plans. ArcCHECK is an efficient and valuable tool for both IMRT and VMAT QA.Research sponsored by Sun Nuclear Corporation.

노이즈 영상으로부터 모아레 기준 위상의 강인 자동 생성 방법

본 연구는 위상천이 영사식 모아레 측정기의 기준 위상 측정단계에서 기준 위상의 검출 정확도를 높이기 위한 방안으로, 일반적인 N-bucket 방법이 아닌, 줄무늬 패턴 투사 영상의 직접적인 영상처리를 기반으로, 노이즈가 포함된 영상으로부터 기준 위상을 자동으로 검출하고 보정하는 방법을 다룬다. 일반적으로 기준위상을 정확히 구하기 위해서는 상대적으로 노이즈가 원천적으로 적은 고정밀도의 교정 시편과 이에 맞는 세분화된 교정방법이 필요하다. 하지만, 고가의 시편 제작비용 절감과 고정도 교정을 위한 교정시간의 단축을 위해, 상대적으로 일반 정밀도등급의 제작된 시편으로부터 획득된 영상을 이용하여, 효율적인 기준위상 교정 방법을 제안한다. 투사된 평행 라인 띠가 시편에서 반사되어 카메라로 입력되는 영상으로부터, 영상처리 라인마스크를 이용하여 격자 라인들의 중심점을 정밀하게 추출하고, 각 라인들에서 추출된 샘플링점 기반의 최소자승법을 이용하여, 라인 특징치인 기울기와 절편을 추출함으로써 실제 관측되는 격자 라인의 중심선을 획득하였으며, 획득된 중심선들을 이용하여 N-bucket 알고리즘에 적용할 수 있도록 위상을 추출한 후, 기준위상을 자동으로 생성하였다. 본 연구를 통하여 고가의 시편 제작 없이 빠르게 자동으로 기준위상을 추출할 수 있었다. This paper presents the automatic vision algorithm to generate and calibrate reference phase plane to improve the accuracy of 3D measuring machine of using phase shifting projection moire method, which is not traditional N-bucket method, but is based on direct image processing method to the pattern projection image. Generally, to acquire accurate reference phase plane, the calibration specimen with well treated surface is needed, and detailed calibration method should be performed. For the cost reduction of specimen manufacturing and the calibration time reduction, on the specimen, not specially designed, with general accuracy level, an efficient calibration procedure for the reference phase generation is proposed. The proposed vision algorithm is developed to extract the line center points of the projected line pattern from acquired images, derive the line feature information consisting of its slope and intercept by using sampled feature points, and finally generate the related reference phase between line pairs. Experimental results show that the proposed method make reference phase plane with a good accuracy under noisy environment and the proposed algorithm can reduce the total cost to make high accurate calibration specimen, also increase the accuracy of reference phase plane, and reduce the complex calibration procedure to move grid via N-bucket algorithm precisely.

Validation of a Monte Carlo efficiency calibration procedure for a partial body counter system with a voxel model of the LLNL torso phantom

Virtual models of real phantoms used with Monte Carlo methods facilitate the calibration and other studies associated with whole-body and partial-body counting systems. In this investigation, a voxel model of an LLNL torso phantom, available physically in the in vivo laboratory at KIT, was created from computed tomography scans. Series of measurements with a high-purity germanium detector and the real torso phantom, loaded with different radioactive organs, have been carried out. Computer simulations of these measurement setups were performed with the aid of MCNPX, using a coarsened voxel phantom and a validated model of the germanium detector. The results of simulations were compared with data from the measurements and an agreement within the uncertainties was found. The voxel model could therefore be validated. The results of the simulations were then used to quantify the activity of (241)Am impurities detected in the liver loaded with (239)Pu.

Vectorial combination of signals in Fourier transform spectroscopy

In an ideal Fourier transform spectrometer the measured spectrum is a scalar combination of two input sources, while, more generally, a vectorial combination occurs in a real instrument. The vectorial combination is caused by the absorption of the beamsplitter which introduces a deviation from π of the relative phase between the two input sources, as well as a third source. Moreover, the asymmetry of the beamsplitter can cause a frequency dependent phase delay adding complexity to the system. A general model of a Fourier transform spectrometer is presented and it is shown that such a complex behaviour can be characterized through the “composite phase parameter” of the beamsplitter. Furthermore, some general rules are identified that can be used to devise more efficient calibration procedures and strategies for the minimization of the distortions caused by the vectorial combination of the spectra.

SU‐GG‐T‐286: Performance of An Improved Optical Computed Tomography Polymer Gel Dosimeter System for 3‐D Dose Verification of Static and Dynamic Phantom Deliveries

Purpose: Performance of an improved 3D dosimetry system consisting of polymer gel dosimeters and a next‐generation optical CT scanner (OCT) is characterized. Following improvements are discussed: increased acquisition speed of the scanner; increased useful dosimeter volume achieved by reducing image reconstruction artifacts through background subtraction; efficient calibration procedure using single high‐energy electron field. The characteristics of dose response (range, linearity, saturation) are determined. Feasibility of high‐resolution (<1mm) 3‐D dosimetry is demonstrated under stationary and dynamic delivery conditions. Method and Materials: Large‐volume (2.2 L) polymer gel dosimeters (BANG®‐3) with specially designed TomoBANG™ Virtual Water™ phantom were used to record cumulative, volumetric dose distributions from single‐ and multi‐field deliveries. Calibration procedures using photon (6 MV) and electron (18 MeV) beams were based on using a depth‐dose relationship to correlate delivered dose to the change in optical density of the dosimeter. The accuracy of this approach was verified by delivering a conformal photon plan and comparing the dose distribution to that calculated by the treatment planning system. The feasibility of use for motion (4‐D) dosimetry was demonstrated by comparing dose distributions from static and dynamic deliveries (using a motion phantom) with radiochromic film measurements. Results: Both calibration procedures yielded equivalent results, with the electron‐based method deemed preferable because of its higher efficiency. Dose distributions derived from optical density measurements show good agreement with the treatment planning system and radiographic film measurements. The average distance to agreement between the pairs of isodose curves is below 1 mm for stationary as well as dynamic deliveries. The capability to visualize three‐dimensional dose distributions is also illustrated. Conclusion: In addition to its ability to provide high‐resolution, reliable dosimetric information, the improved gel/OCT dosimetry system allows for 3D visualization and analysis of motion‐induced dosimetric effects, which is unique to this type of dosimeter.

Parameter estimation in distributed hydrological modelling: comparison of global and local optimisation techniques

Much research has been spent in the last three decades in developing more effective and efficient automatic calibration procedures and in demonstrating their applicability to hydrological problems. Several problems have emerged when applying these procedures to calibration of conceptual rainfall–runoff and groundwater (GW) models, such as computational time, large number of calibration parameters, parameter identifiability, model response surface complexity, handling of multiple objectives and parameter equifinality. All these are expected to be much more severe for more complex models, for which comprehensive calibration studies have not so far been conducted. The scope of this paper is to investigate the performance of a global and a local optimisation technique, respectively, the Shuffled Complex Evolution algorithm and the gradient-based Gauss–Marquard–Levenberg algorithm, in calibration of physically based distributed models of different complexity. The models considered are a steady-state GW model, a transient GW model and a fully integrated model of the same catchment. The calibration is conducted in a multi-objective framework where two different aspects of the model response, the simulated runoff and the groundwater elevation are aggregated and simultaneously optimised. Different aggregated objective functions are used to give different weights to the calibration criteria. The results of the calibration procedures are compared in terms of effectiveness and efficiency and demonstrate the different performance of the methods. Moreover, a combination of the global and local techniques is investigated as an attempt to exploit the advantages of both procedures, while overcoming their drawbacks.

SU‐FF‐T‐466: A Single Dose Calibration Method for IMRT QA Film Dosimetry Using Gafchromic® EBT Film

Purpose: In order to decrease the workload for IMRT QA film dosimetry we propose a simple method for Gafchromic® EBT film calibration using a pre‐measured batch calibration curve (BCC) which is mathematically scaled with a single dose exposure (SDE) for each subsequent IMRT QA session. We evaluate how the SDE method corrects for post exposure density growth and temperature dependence in addition to inter film response homogeneity. Method and Materials:Each of five EBT films, from a single production lot, were exposed to doses of 22.5, 43.1, 84.4, 147.4, 209.1, and 291.2 cGy. Measurement of the resulting response curves were performed at times ranging from 0.7 to 52 hours post exposure on a Macbeth TD932 point densitometer customized with a 636nm band pass filter and 16 bit digitizer for increased sensitivity, yielding 35 curves in all. The BCC was generated from a film measured at 0.7 hours and fit to the equation . To test the method, net optical density (NOD) data for the remaining 34 curves was scaled to fit the BCC using 209.1 cGy as the SDE. The scaling factor is the ratio of the actual measured NOD of the SDE, and the NOD predicted by the BCC for this dose. The new scaled calibration is used to predict all 204 test doses, which were then compared to the actual delivered doses. Results: Using the single uncorrected BCC resulted in an average error for the 204 dose points of 11.9 cGy. However, scaling according the method described reduced the average measured dose error to 1.1 cGy. Conclusion: The single dose exposure method described here is an accurate and time efficient calibration procedure for IMRT QA film dosimetry. Conflict of Interest: Partially supported by International Specialty Products.

Markov Functional Modeling of Equity, Commodity and Other Assets

In this short note we show how to setup a one dimensional single asset model, e.g. equity model, which calibrates to a full (two dimensional) implied volatility surface. We show that the efficient calibration procedure used in LIBOR Markov functional models may be applied here too. In a addition to the calibration to a full volatility surface the model allows the calibration of the joint asset-interest rate movement (i.e. local interest rates) and forward volatility. The latter allows the calibration of compound or Bermudan options. The Markov functional modeling approach consists of a Markovian driver process x and a mapping functional representing the asset states S(t) as a function of x(t). It was originally developed in the context of interest rate models, see [Hunt Kennedy Pelsser 2000]. Our approach however is similar to the setup of the hybrid Markov functional model in spot measure, as considered in [Fries Rott 2004]. For equity models it is common to use a deterministic Numeraire, e.g. the bank account with deterministic interest rates. In our approach we will choose the asset itself as Numeraire. This is a subtle, but crucial difference to other approaches considering Markov functional modeling. Choosing the asset itself as Numeraire will allow for a very efficient numerically calibration procedure. As a consequence interest rates have to be allowed to be stochastic, namely as a functional of x too. The Black-Scholes model with deterministic interest rates is a special case of such a Markov functional model. The most general form of this modeling approach will allow for a simultaneous calibration to a full two dimensional volatility smile, a prescribed joint movement of interest rates and a given forward volatility structure.

Some considerations on how to solve the Sherman equation in practice

This article discusses the true nature of the problems involved in the solution of the Sherman equation in practice and demonstrates that the Fundamental Algorithm (FA) method, in association with its efficient calibration procedure, proposed by Rousseau in 1982, is still a valid solution. However, the author recognizes that the FA method is not the only valid one, and others have also been proposed. This article does not propose a new analytical method, but describes the three essential characteristics that any valid solution must have. It also suggests some mathematical search avenues to examine for discovering any new valid solution in the future.

Cross-Currency and Hybrid Markov-Functional Models

In this paper we consider cross-currency Markov-functional models and their calibration under the spot measure. Hunt, Kennedy and Pelsser introduced a single-currency Markov-functional interest rate model in the terminal measure and showed how to efficiently calibrate it to LIBOR or swaprate options. Building upon their work we will present a multi-factor cross-currency LIBOR model under different measures. We see zero correlated FX spot and LIBOR rates as a natural starting point. Under this assumption we don't need a change of numeraire drift correction and the functionals of the foreign currency rates under the domestic numeraire are identical to the functionals of the foreign currency model under its (foreign) spot measure. This provides the motivation for first deriving a spot measure version of single-currency Markov-functional model. We will show that in the spot measure it is possible to formulate and implement a very efficient calibration procedure comparable to that provided by Hunt, Kennedy and Pelsser for the terminal measure. Combining single-currency Markov-functional interest rate models with a Markov-functional FX spot model we build two and three factor cross-currency models. Relaxing the zero correlation assumption is technically quite simple, but it entails considerable additional computational costs, mainly for the calibration of the model to FX options. To circumvent this problem we suggest a more efficient approximate procedure, which seems to work quite well for low correlations.

GEANT4 code for simulation of a germanium gamma-ray detector and its application to efficiency calibration

The GEANT4 software was developed by RD44, a world-wide collaboration of national institutes, laboratories and large High-Energy Physics experiments. GEANT4 is a public software package composed of tools which can be used to accurately simulate the passage of particles through matter. In this article, the first attempt to use GEANT4 to model a reverse electrode germanium detector (REGe), and to improve also its efficiency calibration procedure, is presented. A variance reduction algorithm based on a directional bias scheme is implemented into GEANT4 in order to accelerate the efficiency computations. A fast optimisation method to model the detector geometry using standard point sources is also presented and validated for point, Marinelli and air filter sources. The simulated full-energy peak efficiencies agreed with the measured values to within 1% between 36 and 1460 keV for these three counting geometries.

Development of an internal monostandard instrumental neutron activation analysis method based on in situ detection efficiency for analysis of large and nonstandard geometry samples.

A k0-based internal monostandard instrumental neutron activation analysis method for determination of relative elemental concentration in samples of large size and irregular geometry has been developed. In this method, one of the elements present in the sample is used as comparator. A priori knowledge of the concentration of one of the constituents is required to convert the relative concentration into absolute values. The problems of gamma-ray self-attenuation and geometrical effects that arise in the assay of large and nonstandard geometry samples were overcome by an in situ relative detection efficiency calibration procedure, which requires one or more activation products emitting gamma-rays over a wide range of the spectrum. To minimize the problem of neutron flux perturbations that may arise in large samples, irradiations were carried out using a thermal column with thermal neutron component of more than 99.9%. The method has been standardized with samples of silica (approximately 0.5 kg) and water (0.5 L) spiked with known amounts of different elements and has been advantageously applied to some alloy and metal samples of irregular geometry, where complete compositional characterization was carried out using mass balance. This approach is highly valuable for analysis of large, irregularly shaped samples if not too high demands are set to the degree of accuracy.

Calibration of an UTW Si(Li) detector in the 0.28–22.1 keV energy range at a nuclear microprobe

The efficiency of an ultra thin windowed Si(Li) detector has been determined experimentally in the photon energy range of 0.28–22.1 keV at the Debrecen scanning nuclear microprobe facility. The calibration technique was based on the measurement of thick targets made from pure elements and chemical compounds by PIXE technique. Characteristic X-ray spectra were fitted and PIXE cross-sections were calculated by the new version of the PIXEKLM program package. The efficiency function was determined by fitting the detector parameters in order to minimize the difference between the measured and calculated efficiency values. The accuracy of the method is estimated to be 20–10% in the 0.28–1 keV energy range and 10–5% above 1 keV. An overview of theoretical and experimental aspects of the efficiency calibration procedure will be presented.

Novel coordinate mapping algorithm for three-dimensional profile noncontact measurement

A novel approach to a measuring algorithm and calibration method for 3-D coordinate noncontact measurement has been developed. This algorithm introduces variables that relate the output digital image directly to the viewed 3-D object. A simple and efficient calibration procedure is achieved using the least-squares technique. It takes advantage of a 2-D coordinate mapping relation, along with dual CCD cameras and a 3-D calibration grid consisting of a series of characteristic points with known coordinates in real space. The space coordinates of these characteristic points are compared to their image coordinates in the CCDs. A 3-D mapping function between the actual space and its image planes in the dual CCD cameras is then constructed. The relationship between the two is determined accordingly and can be applied to extract 3-D information from a tested object sitting in a specific space with satisfactory accuracy. This algorithm not only significantly reduces the 3-D measurement computations, but also increases the measurement precision and speed. Current experimental results show that the measurement standard deviation is under 0.06 mm in a bounded space of 80 x60x60 mm. This result reveals that this measurement algorithm has the potential to be an effective positioning method for rapid and precise noncontact coordinate measuring systems.

Calibrating a Coupled SVAT–Vegetation Growth Model with Remotely Sensed Reflectance and Surface Temperature—A Case Study for the HAPEX-Sahel Grassland Sites

Models simulating the seasonal growth of vegetation have been recently coupled to soil–vegetation–atmosphere transfer schemes (SVATS). Such coupled vegetation–SVATS models (V–S) account for changes of the vegetation leaf area index (LAI) over time. One problem faced by V–S models is the high number of parameters that are required to simulate different sites or large areas. Therefore, efficient calibration procedures are needed. This study describes an attempt to calibrate a V–S model with satellite [Advanced Very High Resolution Radiometer (AVHRR)] data in the shortwave and longwave domains. A V–S model is described using ground data collected over three semiarid grassland sites during the Hydrological Atmospheric Pilot Experiment (HAPEX)-Sahel experiment. The effect of calibrating model parameters with time series of normalized difference vegetation index (NDVI) and thermal infrared (TIR) data is assessed by examining the simulated latent heat flux (LE) and LAI for a suite of calibration experiments. A sensitivity analysis showed that the parameters related to plant growth vigor and to soil evaporative resistance were the best candidates for calibration. The NDVI and TIR time series were used to calibrate these parameters, both independently and simultaneously, to assess their synergy. Ground-based, airborne, and satellite sensor (AVHRR) data were successively investigated. Both airborne and AVHRR NDVI data could be used to constrain the vegetation growth vigor. These calibrations significantly improved the simulation of the LAI and LE (rmse decreased by 21% for LE), and the site-to-site variability was greatly enhanced. The soil resistance could also be calibrated with ground-based TIR data, but the effect on the simulated variables was small. Although both NDVI and ground-based TIR data were suitable to constrain the V–S model, the synergy between the two wavelengths was not clearly established. Last, satellite TIR data from the AVHRR proved unsuitable for model calibration. Indeed, the AVHRR surface temperature values were systematically lower than both ground-based data and model outputs. The authors conclude that the calibration of a vegetation–SVAT model with shortwave AVHRR time series can be used to scale the energy and water fluxes up to the regional scale.

Testing of different true coincidence correction approaches for gamma-ray spectrometry of voluminous sources

To achieve the highest possible sensitivity of analysis for environmental samples it is common practice to use both a high efficiency detector and a close measurement geometry with a large sample size (e.g. Marinelli beaker). Under such conditions, the typical efficiency calibration procedure results in a biased activity value for many nuclides due to the true coincidence summing effect. While there are a few methods to correct for this effect with special calibration standards, such calibrations can be both time consuming and expensive. Due to these calibration difficulties, the true coincidence summing effect is often simply ignored. Recently, it has been demonstrated that the coincidence summing correction can be performed mathematically even for voluminous sources. This new method consists of an integration of the coincidence correction factor over the sample volume while taking into account its chemical composition and the container. In this paper, we will discuss the latest approaches for establishing the peak efficiency and peak-to-total efficiency curves, which are required for this method. These approaches have been tested for HPGe detectors of two different relative efficiencies.

Multi-objective global optimization for hydrologic models

The development of automated (computer-based) calibration methods has focused mainly on the selection of a single-objective measure of the distance between the model-simulated output and the data and the selection of an automatic optimization algorithm to search for the parameter values which minimize that distance. However, practical experience with model calibration suggests that no single-objective function is adequate to measure the ways in which the model fails to match the important characteristics of the observed data. Given that some of the latest hydrologic models simulate several of the watershed output fluxes (e.g. water, energy, chemical constituents, etc.), there is a need for effective and efficient multi-objective calibration procedures capable of exploiting all of the useful information about the physical system contained in the measurement data time series. The MOCOM-UA algorithm, an effective and efficient methodology for solving the multiple-objective global optimization problem, is presented in this paper. The method is an extension of the successful SCE-UA single-objective global optimization algorithm. The features and capabilities of MOCOM-UA are illustrated by means of a simple hydrologic model calibration study.

A self‐consistent calibration method for industrial X‐ray spectrometric analyses

AbstractA fast, efficient calibration procedure is proposed based on the accurate analysis of multicomponent systems by means of the influence coefficient method. It relies on a series of well designed additions made to a suitable standard composition, the latter also being used as a reference sample. We show that, for an n element composition, using the hypothesis of constant coefficients, (n + 1) preparations – allowing n(n + 1) intensity measurements – are in principle sufficient to calculate the n2 calibration parameters [i.e. n(n − 1) influence coefficients and n referenc relative intensities] which are necessary for a complete analysis. Actual analyses are carried out by measuring the fluorescent intensities using the ratio (i.e. unknown/standard) method, and then calculating the concentrations from the usual relationships through an iterative calculation, preferably executed by a computer program. A detailed description of a typical application is given, which involved the study of fused specimens (borax fusion) of bauxite and resulted in the very accurate analysis of calcined samples, as well as the direct analysis of raw bauxites following an adequate treatment of the ignition loss problem. While the method is immediately applicable to every system studied in either solid or liquid solution, it clearly extends to compact systems over a suitable compositional range, provided that carefully controlled additions can be achieved and that all prepared samples are homogeneous.