Random Position Errors Research Articles

Characterization of positional accuracy of MLC using an ionization chamber array

PurposeThe purpose of this study is to characterize the positional accuracy of the multileaf collimator (MLC) using an ionization chamber array and develop an MLC quality assurance (QA) method. MethodsHalf of the volume for each ionization chamber located at the lower layer of the device was occluded by a specific leaf using the half-beam block (HBB) method, and the corresponding ionization chamber response was used as a baseline for the subsequent MLC QA study. The sensitivity of the method was assessed by introducing known leaf positional errors, varying from 0.5 to 2.5 mm in 0.5 mm increments in the positive and negative directions, into the baseline. The relative outputs of the detectors were linearly corrected according to the leaf position error recorded. The relationship between detector response variation relative to the baseline and the introduced leaf error was determined by function. The usefulness of the MLC QA technique developed in this study was verified weekly over 5 months. ResultsDuring the development of the algorithm, the maximum leaf absolute random position error was 0.3 mm. A strong linear relationship was observed between the relative output of the detector and the MLC positional error introduced, and the slope and coefficient of determination were −0.05159 and 0.99928, respectively. The mean MLC positional accuracy across all leaves for the error field and HBB baseline field over the 5-month assessment was 0.0649 ± 0.1340 mm and 0.0002± 0.0835 mm, respectively. ConclusionsThe automated MLC QA procedure we proposed has proven to be effective and robust, and satisfies the American Association of Physicists in Medicine (AAPM) Task Group Report 198 guideline criteria of ± 1 mm.

Sub‐array level structural compensation method for radiating and scattering performance of array antennas

AbstractThe detection and stealth abilities of array antennas depend mainly on the antennas' radiating and scattering performance, respectively. However, the operating environmental loads and assembly lead to serious structural errors, including deformation and random errors, which affect both the radiating and scattering performance. As the demand to guarantee high performance in detection as well as in stealth, a new sub‐array level structural compensation method is presented to simultaneously guarantee the radiating and scattering performance of array antennas in service. First, a statistical model of scattering performance with structural deformation and random position error is established to quickly evaluate the impact of a random structural error on scattering performance. Then, the effects of structural errors in different directions on radiating and scattering performance are analysed to determine the structural adjustment direction. Moreover, the multi‐objective problem considering the comprehensive compensation of radiating and scattering performance is converted into a single‐object problem by constructing a fitness function to realise the sub‐array level structural compensation. Finally, a typical case is used to verify the effectiveness of the compensation method. The results show that the presented method can guarantee both the radiating and scattering performance effectively, providing advantageous guidance for structural design and performance compensation for array antennas.

A random error estimation method for satellites being launched into orbit based on ellipsoid theory

Accurate and fast random error estimation is essential to improve satellite control. In this paper, the random error of the satellite being launched into orbit is explored. According to the uncertainty matrix of six satellite orbit parameters associated with the project, the uncertainty of the orbit position in the three axes of the geocentric inertial frame of reference is calculated via the propagation rate of the covariance matrix. Then, the random error probability density of the satellite position is constructed for normal distribution. The surface of the error with the same probability density is approximated as an ellipsoid, the size is deduced along with the ellipsoid's shape and direction, and the satellite's spatial position probability is obtained. Monte Carlo method and STK software were employed to simulate low-earth orbit micro-nano satellite launching into the same orbital position. It is confirmed that the results obtained via the theoretical model are in line with practice. Therefore, the ellipsoid theory proposed in this paper can be used to describe the satellite random position error.

Application of error classification model using indices based on dose distribution for characteristics evaluation of multileaf collimator position errors

This study aims to evaluate the specific characteristics of various multileaf collimator (MLC) position errors that are correlated with the indices using dose distribution. The dose distribution was investigated using the gamma, structural similarity, and dosiomics indices. Cases from the American Association of Physicists in Medicine Task Group 119 were planned, and systematic and random MLC position errors were simulated. The indices were obtained from distribution maps and statistically significant indices were selected. The final model was determined when all values of the area under the curve, accuracy, precision, sensitivity, and specificity were higher than 0.8 (p < 0.05). The dose–volume histogram (DVH) relative percentage difference between the error-free and error datasets was examined to investigate clinical relations. Seven multivariate predictive models were finalized. The common significant dosiomics indices (GLCM Energy and GLRLM_LRHGE) can characterize the MLC position error. In addition, the finalized logistic regression model for MLC position error prediction showed excellent performance with AUC > 0.9. Furthermore, the results of the DVH were related to dosiomics analysis in that it reflects the characteristics of the MLC position error. It was also shown that dosiomics analysis could provide important information on localized dose-distribution differences in addition to DVH information.

Electromechanical Coupling and Application of High-Frequency Communication Antenna Channel Capacity

The next-generation communication base station antennas represented by phased array antennas are towards high frequency, high gain, high density, and high pointing accuracy. The influence of mechanical structure factors on communication system channel quality is obviously increasing, and the electromechanical coupling problem is becoming more prominent. To effectively guarantee the realization of 5G/6G communication in complex working environments and accelerate the commercial process of future communication systems, an electromechanical coupling channel capacity model is established in comprehensive consideration of the positional shift, attitude deflection, and temperature change of the communication base station phased array antennas. It can be used to rapidly evaluate the communication index degradation of RF devices within the heating environment. Moreover, a sensitivity model of the electric field strength and array antenna channel capacity to the random position error of each element is constructed. The influence of the random positioning error of each element on the communication indicators is analyzed and compared under different working conditions. The simulation results show that the proposed model can effectively provide a theoretical basis and guiding role for the design and manufacture of high-frequency array base station antennas.

Comparison of pretreatment VMAT quality assurance with the integral quality monitor (IQM) and electronic portal imaging device (EPID).

The purpose of this study was to compare pretreatment volumetric modulated arc therapy (VMAT) quality assurance (QA) measurements and evaluate the multileaf collimator (MLC) error sensitivity of two detectors: the integral quality monitor (IQM) system (iRT systems IQM) and the electronic portal imaging device (EPID) (Varian PortalVision aS1200). Pretreatment QA measurements were performed for 20 retrospective VMAT plans (53 arcs). A subset of ten plans (23 arcs) was used to investigate MLC error sensitivity of each device. Eight MLC error plans were created for each VMAT plan. The errors included systematic opening/closing (±0.25, ±0.50, ±0.75 mm) of the MLC and random positional errors (1 mm) for individual/groups of leaves. The IQM was evaluated using the percent error of the measured cumulative signal relative to the calculated signal. The EPID was evaluated using two methods: a novel percent error of the measured relative to the predicted cumulative signals, and gamma (γ) analysis (1%/1 mm, 2%/2 mm, 3%/3 mm and 3%/1 mm for Stereotactic Body Radiation Therapy plans). The average change in maximum dose obtained from dose‐volume histogram (DVH) data and change in detector signals for different systematic MLC shifts was also compared. Cumulative signal differences showed similar levels of agreement between measured and expected detector signals (IQM: 1.00 ± 0.55%; EPID: 1.22 ± 0.92%). Results from γ analysis lacked specificity. Only the 1%/1 mm criteria produced data with remarkable differences. A strong linear correlation was observed between IQM and EPID cumulative signal differences with MLC error magnitude (R = 0.99). Likewise, results indicate a strong correlation between the cumulative signal for both detectors and DVH dose (RIQM = 0.99; REPID = 0.97). In conclusion, use of cumulative signal differences could be more useful for detecting errors in treatment delivery in EPID than γ analysis.

Calibrating the error from sensor position uncertainty in TDOA-AOA localization

The performance of unknown source localization degrades significantly when random sensor position errors exist. In this paper, a calibration source, whose position is known precisely or imprecisely, is introduced into the hybrid TDOA-AOA localization to alleviate the accuracy loss caused by erroneous sensor positions. The performance improvement, attributed to the use of a calibration source, is derived in terms of Cramér–Rao lower bound (CRLB). To get insight into the use of a calibration source, the optimum placement of a calibration source for TDOA-AOA localization in the presence of sensor position errors is investigated. Two closed-form localization algorithms, both composed of two stages, are proposed for the two different cases. In both algorithms, the sensor position errors are corrected through the calibration source in the first stage, and the location of the unknown source is then estimated from the TDOA-AOA measurements in the second stage. The theoretical analysis shows the proposed algorithms attain the CRLB accuracy when sensor position errors and measurement noises are relatively small. Simulations validate the analytical results and the performance of the proposed algorithms.

An efficient error estimation method for antenna distortions in space based radar

The phased array antenna plays an important role in determining the performance achievable in a space based radar system. However, the array position errors caused by the antenna distortions in the complex cosmic vacuum environment will distort the antenna pattern, which significantly degrades the performance of the radar systems. Most of the conventional estimation methods are proposed based on the random position errors without the consideration of the distortion character for spaceborne arrays, resulting in a large estimation error. In this paper, to address this issue, an efficient array error estimation method is proposed to compensate the antenna distortion errors. In the proposed method, the initial sensor positions can be obtained based on the measured results from the units distributed over the array surface. Then the distortion model parameters can be estimated by solving an optimization problem. Based on this, the sensor positions can be corrected with high accuracy at the lth iteration. In this way, the proposed method can improve the accuracy and the robustness of the estimation simultaneously. Several simulated results are presented to validate the proposed method for the position error estimation in a space based radar system.

Bias in particle tracking acceleration measurement

We investigate sources of systematic error (bias) in acceleration statistics derived from Lagrangian particle tracking data and demonstrate techniques to eliminate or minimise these bias errors introduced during processing. Numerical simulations of particle tracking experiments in isotropic turbulence show that the main sources of bias error arise from noise due to random position errors and selection biases introduced during numerical differentiation. We outline the use of independent measurements and filtering schemes to eliminate these biases. Moreover, we test the validity of our approach in estimating the statistical moments and probability densities of the Lagrangian acceleration. Finally, we apply these techniques to experimental particle tracking data and demonstrate their validity in practice with comparisons to available data from the literature. The general approach, which is not limited to acceleration statistics, can be applied with as few as two cameras and permits a substantial reduction in the position accuracy and sampling rate required to adequately measure the statistics of Lagrangian acceleration.Graphical abstractSources of bias error in Lagrangian Particle Tracking measurements are explored. Methods are presented and validated to correct acceleration statistics for the main sources of systematic errors introduced by random position error and filtering, allowing for a substantial improvement in the effective temporal resolution of particle tracking measurements.

A service evaluation of on-line image-guided radiotherapy to lower extremity sarcoma: Investigating the workload implications of a 3 mm action level for image assessment and correction prior to delivery

IntroductionAlthough all systematic and random positional setup errors can be corrected for in entirety during on-line image-guided radiotherapy, the use of a specified action level, below which no correction occurs, is also an option. The following service evaluation aimed to investigate the use of this 3 mm action level for on-line image assessment and correction (online, systematic set-up error and weekly evaluation) for lower extremity sarcoma, and understand the impact on imaging frequency and patient positioning error within one cancer centre. MethodsAll patients were immobilised using a thermoplastic shell attached to a plastic base and an individual moulded footrest. A retrospective analysis of 30 patients was performed. Patient setup and correctional data derived from cone beam CT analysis was retrieved. The timing, frequency and magnitude of corrections were evaluated. The population systematic and random error was derived. Results20% of patients had no systematic corrections over the duration of treatment, and 47% had one. The maximum number of systematic corrections per course of radiotherapy was 4, which occurred for 2 patients. 34% of episodes occurred within the first 5 fractions. All patients had at least one observed translational error during their treatment greater than 0.3 cm, and 80% of patients had at least one observed translational error during their treatment greater than 0.5 cm. The population systematic error was 0.14 cm, 0.10 cm, 0.14 cm and random error was 0.27 cm, 0.22 cm, 0.23 cm in the lateral, caudocranial and anteroposterial directions. The required Planning Target Volume margin for the study population was 0.55 cm, 0.41 cm and 0.50 cm in the lateral, caudocranial and anteroposterial directions. ConclusionThe 3 mm action level for image assessment and correction prior to delivery reduced the imaging burden and focussed intervention on patients that exhibited greater positional variability. This strategy could be an efficient deployment of departmental resources if full daily correction of positional setup error is not possible.

Effect of Randomness in Element Position on Performance of Communication Array Antennas in Internet of Things

As a critical component for wireless communication, active phased array antennas face the restrictions of creating effective performance with the effect of randomness in the position of the array element, which are inevitably produced in the manufacturing and operating process of antenna. A new method for efficiently and effectively evaluating the statistic performance of antenna is presented, with consideration of randomness in element position. A coupled structural‐electromagnetic statistic model for array antenna is proposed from the viewpoint of electromechanical coupling. Lastly, a 12 × 12 planar array is illustrated to evaluate the performance of antenna with the saddle‐shaped distortion and random position error. The results show that the presented model can obtain the antenna performance quickly and effectively, providing an advantageous guidance for structural design and performance optimization for array antennas in wireless application.

Efficient closed-form solution for target localization using TDOA measurements in the presence of sensor position errors

AbstractAn efficient solution for locating a target was proposed, which by using time difference of arrival (TDOA) measurements in the presence of random sensor position errors to increase the accuracy of estimation. The cause of position estimation errors in two-stage weighted least squares (TSWLS) method is analyzed to develop a simple and effective method for improving the localization performance. Specifically, the reference sensor is selected again and the coordinate system is rotated according to preliminary estimated target position by using TSWLS method, and the final position estimation of the target is obtained by using weighted least squares (WLS). The proposed approach exhibits a closed-form and is as efficient as TSWLS method. Simulation results show that the proposed approach yields low estimation bias and improved robustness with increasing sensor position errors and thus can easily achieve the Cramer-Rao lower bound (CRLB) easily and effectively improve the localization accuracy.

Electromechanical coupling based performance evaluation of distorted phased array antennas with random position errors

Electromechanical coupling based performance evaluation of distorted phased array antennas with random position errors

Analyzing of correlation between the setup error and the couch position in radiotherapy

Objective To investigate the correlation between setup error and couch position error in radiotherapy. Methods A total of 25 patients with thoracic and abdominal tumors who recently finished image-guided radiotherapy were randomly selected. The data on couch position during treatment were obtained through the record validation system, and then the couch position error was calculated. The Pearson correlation analysis was used to investigate the correlation between setup error and couch position error during treatment. Results In the ≥5 setup errors among the 25 patients, the correlation coefficient between random setup error and random couch position error was 0.83(P=0.00), and the correlation coefficient between systematic setup error and systematic couch position error was 0.36(P=0.11). Conclusions In radiotherapy, the random setup error is highly correlated with the random couch position error, while a moderate or low correlation exists between the systematic setup error and the systematic couch position error. Key words: Radiotherapy; Setup error; Couch position; Correlation

WORST-CASE TOLERANCE SYNTHESIS FOR LOW-SIDELOBE SPARSE LINEAR ARRAYS USING A NOVEL SELF-ADAPTIVE HYBRID DIFFERENTIAL EVOLUTION ALGORITHM

A worst-case tolerance synthesis problem for low-sidelobe sparse linear arrays is solved by using a novel self-adaptive hybrid differential evolution (SAHDE) algorithm. First, we establish a worst- case tolerance synthesis model for low-sidelobe sparse linear arrays, in which random position errors are considered and assumed to obey the Gaussian distributions. Through the random sampling, the random model is converted to a deterministic optimization problem. Then, a novel SAHDE algorithm is presented for solving the problem. As a modification to the existing hybrid differential evolution algorithm, a simplified quadratic interpolation (SQI) operator is used to tune the control parameters self-adaptively, establishing the connections between control parameters and the fitness values. In order to determine appropriate control parameter values quickly, a selection operation is also used. Detailed implementation procedure for the SAHDE algorithm is presented, and some numerical results show its effectiveness. Finally, for the deterministic optimization problem, we present a fast way for calculating its fitness values. The SAHDE algorithm is used to obtain optimal nominal element positions. Simulated results illustrate that the worst-case peak sidelobe levels for the sparse linear arrays are improved evidently. The SAHDE algorithm is efficient for solving the worst-case tolerance synthesis problem.

A comprehensive evaluation of treatment accuracy, including end-to-end tests and clinical data, applied to intracranial stereotactic radiotherapy

Background and purposeA methodology is presented to quantify the uncertainty associated with linear accelerator-based frameless intracranial stereotactic radiotherapy (SRT) combining end-to-end phantom tests and clinical data. Methods and materialsThe following steps of the SRT chain were analysed: planning computed tomography (CT) and magnetic resonance (MR) scans registration, target volume delineation, CT and cone beam CT (CBCT) registration and intrafraction-patient displacement. The overall accuracy was established with an end-to-end test. The measured uncertainties were combined, deriving the total systematic (ΣT) and random (σT) error components, to estimate the GTV-PTV margin. ResultsThe uncertainty in the MR-CT registration was on average 0.40mm (averaged over AP, CC and LR directions). Rotational variations were smaller than 0.5° in all directions.Interobser variation in GTV delineation was on average 0.29mm.The uncertainty in the CBCT-CT registration was on average 0.15mm. Again, rotational variations were smaller than 0.5° in all directions.The systematic and random intrafraction displacement errors were on average 0.55mm and 0.45mm, respectively.The systematic and random positional errors from the end-to-end test were on average 0.49mm and 0.53mm, respectively.Combining these uncertainties resulted in an average ΣT=0.9mm and σT=0.7mm and an average GTV-PTV margin of 2.8mm. ConclusionThis comprehensive methodology including end-to-end tests enabled a GTV-PTV margin calculation considering all sources of uncertainties. This generic method can also be used for other treatment sites.

A four-year comprehensive analysis of random and systematic position errors in a RT centre running two linacs equipped with kV-CBCT

A four-year comprehensive analysis of random and systematic position errors in a RT centre running two linacs equipped with kV-CBCT

The influence of random element displacement on DOA estimates obtained with (Khatri-Rao-)root-MUSIC.

Although a wide range of direction of arrival (DOA) estimation algorithms has been described for a diverse range of array configurations, no specific stochastic analysis framework has been established to assess the probability density function of the error on DOA estimates due to random errors in the array geometry. Therefore, we propose a stochastic collocation method that relies on a generalized polynomial chaos expansion to connect the statistical distribution of random position errors to the resulting distribution of the DOA estimates. We apply this technique to the conventional root-MUSIC and the Khatri-Rao-root-MUSIC methods. According to Monte-Carlo simulations, this novel approach yields a speedup by a factor of more than 100 in terms of CPU-time for a one-dimensional case and by a factor of 56 for a two-dimensional case.

Colour matters: the effects of lensing on the positional offsets between optical and submillimetre galaxies in Herschel★-ATLAS

We report an unexpected variation in the positional offset distributions between Herschel-ATLAS sub-millimetre (submm) sources and their optical associations, depending on both 250-{\mu}m signal-to-noise ratio and 250/350-{\mu}m colour. We show that redder and brighter submm sources have optical associations with a broader distribution of positional offsets than would be expected if these offsets were due to random positional errors in the source extraction. The observation can be explained by two possible effects: either red submm sources trace a more clustered population than blue ones, and their positional errors are increased by confusion; or red submm sources are generally at high redshifts and are frequently associated with low-redshift lensing structures which are identified as false counterparts. We perform various analyses of the data, including the multiplicity of optical associations, the redshift and magnitude distributions in H-ATLAS in comparison to HerMES, and simulations of weak lensing, and we conclude that the effects are most likely to be explained by widespread weak lensing of Herschel-SPIRE sources by foreground structures. This has important consequences for counterpart identification and derived redshift distributions and luminosity functions of submm surveys.

TDOA Source Localization in the Presence of Synchronization Clock Bias and Sensor Position Errors

TDOA positioning requires the sensors to be synchronized when sampling the signals to locate an emitting source. It also demands the sensor positions to be accurate. Achieving synchronization may not be feasible when the sensors are widely separated or are many, and precise sensor positions are often not available in practice. This paper investigates TDOA localization using a set of imperfect sensors, where timing synchronization offsets and random position errors exist among different sensor groups. The degradation in localization accuracy caused by synchronization offsets and position errors is examined through the Cramér–Rao lower bound (CRLB) analysis under Gaussian noise. A novel method in obtaining sequentially algebraic solutions of the source location, the sensor positions, and the synchronization offsets is developed. Sequential estimation simplifies the localization task and offers complexity reduction. The performance of the proposed estimates is shown to reach the CRLB accuracy analytically using small noise analysis, although they are estimated sequentially and not jointly. Simulations support and corroborate the theoretical developments.