bluuebunny/crossref_metadata_2025_split huggingface dataset:
Method for evaluating bow tie filter angle‐dependent attenuation in CT: Theory and simulation results
Authors: Boone John M. 
Published: December 2009

PurposeDosimetry in computed tomography (CT) is increasingly based on Monte Carlo studies that define the dose in the patient (in mGy) as a function of air kerma (free in air) at isocenter (mGy). The accuracy of Monte Carlo studies depends in part on the accuracy of the characterization of the bow tie filter for a given CT scanner model. A simple method for characterizing the bow tie filter attenuation profile in CT scanners would therefore be very useful. The theory behind such a method is proposed.MethodsA measurement protocol is discussed mathematically and demonstrated using computer simulation. The proposed method requires the placement of a radiation monitor at the periphery of the CT field, and the time domain signal (kerma rate versus time) is measured with good temporal resolution ( or better) and with all other objects (e.g., patient couch) retracted from the field of view. Knowledge of the source to isocenter distance (or alternately, the isocenter to probe distance) is required. The stationary detector records the kerma rate versus time signal as the gantry rotates through several revolutions. From this temporal data, signal processing techniques are used to extract in‐phase peaks, as well as out‐of‐phase kerma rate levels. From these data, the distance from isocenter to the probe can be determined (or, alternatively, the source to isocenter distance), and the angle‐dependent bow tie filter attenuation can be computed. By measuring the angle‐dependent bow tie filter attenuation at several kVp settings, the bow tie composition versus fan angle can be computed using basis decomposition techniques.ResultsThe simulations illustrated that with 2% added noise in the kerma rate versus time signal, the attenuation properties of a hypothetical two component (aluminum and polymethyl methacrylate) bow tie filter could be determined . Although the computed basis material thicknesses were not exactly equal to the actual thicknesses, their combined attenuation factors matched that of the actual filter across kVp's to within an average of 0.057%.ConclusionsIt is concluded that the proposed method may provide a simple noninvasive approach to characterizing the performance of bow tie filters in CT systems; however, experimental validation is necessary.
Photon beam quality variations of a flattening filter free linear accelerator
Authors: Georg Dietmar , Kragl Gabriele , af Wetterstedt Sacha , McCavana Patrick , McClean Brendan , Knöös Tommy 
Published: December 2009

PurposeRecently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off‐axis ray angle for unflattened beams. In addition, a comparison was made with the off‐axis energy variation in flattened beams.MethodsTwo Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in‐house developed device that can be easily mounted on the gantry. At St. Luke's Hospital (Dublin, Ireland), measurements were performed at 6 MV in narrow beam geometry with the gantry tilted around 270° with pinhole collimators, an attenuator, and the chamber positioned on the table. All attenuation measurements were performed with ionization chambers and a buildup cap (2 mm brass) or a PMMA mini phantom (diameter 3 cm, measurement depth 2.5 cm).ResultsFor flattened 6 and 10 MV photon beams from the Elekta linac the relative varies by about 11% for an off‐axis ray angle . These results agree within ±2% with a previously proposed generic off‐axis energy correction. For unflattened beams, the variation was less than 5% in the whole range of off‐axis ray angles up to 10°. The difference in relative HVL data was less than 1% for unflattened beams at 6 and 10 MV.ConclusionsOff‐axis energy variation is rather small in unflattened beams and less than half the one for flattened beams. Thus, ignoring the effect of off‐axis energy variation for dose calculations in unflattened beams can be clinically justified.
Lung perfusion imaging in small animals using 4D micro‐CT at heartbeat temporal resolution
Authors: Badea Cristian T. , Johnston Samuel M. , Subashi Ergys , Qi Yi , Hedlund Laurence W. , Johnson G. Allan 
Published: December 2009

PurposeQuantitativein vivo imaging of lung perfusion in rodents can provide critical information for preclinical studies. However, the combined challenges of high temporal and spatial resolution have made routine quantitative perfusion imaging difficult in small animals. The purpose of this work is to demonstrate 4D micro‐CT for perfusion imaging in rodents at heartbeat temporal resolution and isotropic spatial resolution.MethodsWe have recently developed a dual tube/detector micro‐CT scanner that is well suited to capture first pass kinetics of a bolus of contrast agent used to compute perfusion information. Our approach is based on the paradigm that similar time density curves can be reproduced in a number of consecutive, small volume injections of iodinated contrast agent at a series of different angles. This reproducibility is ensured by the high‐level integration of the imaging components of our system with a microinjector, a mechanical ventilator, and monitoring applications. Sampling is controlled through a biological pulse sequence implemented inLABVIEW. Image reconstruction is based on a simultaneous algebraic reconstruction technique implemented on a graphic processor unit. The capabilities of 4D micro‐CT imaging are demonstrated in studies on lung perfusion in rats.ResultsWe report 4D micro‐CT imaging in the rat lung with a heartbeat temporal resolution (approximately 150 ms) and isotropic 3D reconstruction with a voxel size of based on sampling using 16 injections of each. The total volume of contrast agent injected during the experiments (0.8 mL) was less than 10% of the total blood volume in a rat. This volume was not injected in a single bolus, but in multiple injections separated by at least 2 min interval to allow for clearance and adaptation. We assessed the reproducibility of the time density curves with multiple injections and found that these are very similar. The average time density curves for the first eight and last eight injections are slightly different, i.e., for the last eight injections, both the maximum of the average time density curves and its area under the curve are decreased by 3.8% and 7.2%, respectively, relative to the average time density curves based on the first eight injections. The radiation dose associated with our 4D micro‐CT imaging is 0.16 Gy and is therefore in the range of a typical micro‐CT dose.Conclusions4D micro‐CT‐based perfusion imaging demonstrated here has immediate application in a wide range of preclinical studies such as tumor perfusion, angiogenesis, and renal function. Although our imaging system is in many ways unique, we believe that our approach based on the multiple injection paradigm can be used with the newly developed flat‐panel slip‐ring‐based micro‐CT to increase their temporal resolution in dynamic perfusion studies.
Automated estimation of progression of interstitial lung disease in CT images
Authors: Arzhaeva Yulia , Prokop Mathias , Murphy Keelin , van Rikxoort Eva M. , de Jong Pim A. , Gietema Hester A. , Viergever Max A. , van Ginneken Bram 
Published: December 2009

PurposeA system is presented for automated estimation of progression of interstitial lung disease in serial thoracic CT scans.MethodsThe system compares corresponding 2D axial sections from baseline and follow‐up scans and concludes whether this pair of sections represents regression, progression, or unchanged disease status. The correspondence between serial CT scans is achieved by intrapatient volumetric image registration. The system classification function is trained with two different feature sets. Features in the first set represent the intensity distribution of a difference image between the baseline and follow‐up CT sections. Features in the second set represent dissimilarities computed between the baseline and follow‐up images filtered with a bank of general purpose texture filters.ResultsIn an experiment on 74 scan pairs, the system classification accuracies were 76.1% and 79.5% for the two feature sets, respectively, while the accuracies of two observer radiologist were 78.5% and 82%, respectively. The agreements of the system with the reference standard, measured by weighted kappa statistics, were 0.611 and 0.683 for the two feature sets, respectively.ConclusionsThe system employing the second feature set showed good agreement with the reference standard, and its accuracy approached that of two radiologists.
Accurate color measurement methods for medical displays
Authors: Saha Anindita , Kelley Edward F. , Badano Aldo 
Published: December 2009

PurposeThe necessity for standard instrumentation and measurements of color that are repeatable and reproducible is the major motivation behind this work. Currently, different instrumentation and methods can yield very different results when measuring the same feature such as color uniformity or color difference. As color increasingly comes into play in medical imaging diagnostics, display color will have to be quantified in order to assess whether the display should be used for imaging purposes. The authors report on the characterization of three novel probes for measuring display color with minimal contamination from screen areas outside the measurement spot or from off‐normal emissions. They compare three probe designs: A modified small‐spot luminance probe and two conic probe designs based on black frusta.MethodsTo compare the three color probe designs, spectral and luminance measurements were taken with specialized instrumentation to determine the luminance changes and color separation abilities of the probes. The probes were characterized with a scanning slit method, veiling glare, and a moving laser and LED arrangement. The scanning slit measurement was done using a black slit plate over a white line on an LCD monitor. The luminance was measured in increments from the center of the slit to above and below the slit at different distances between the probe and the slit. The veiling glare setup consisted of measurements of the luminance of a black spot pattern with a white disk of radius of as the black spot increases in radius increments. The moving LED and laser method consisted of a red and green light orthogonal to the probe tip for the light to directly shine into the probe. The green light source was moved away from the red source in increments to measure color stray‐light contamination at different probe distances.ResultsThe results of the color testing using the LED and laser methods suggest a better performance of one of the frusta probes at shorter distances between the light sources, which translates to less contamination. The tails of the scans indicate the magnitude of the spread in signal due to light from areas outside the intended measurement spot. The measurements indicate a corresponding glare factor for a large spot of 140, 500, and 2000 for probe A, B1, and B2, respectively. The dual‐laser setup suggests that color purity can be maintained up to a few tens of millimeters outside the measurement spot.ConclusionsThe comparison shows that there are significant differences in the performance of each probe design, and that those differences have an effect on the measured quantity used to quantify display color. Different probe designs show different measurements of the level of light contamination that affects the quantitative color determination.
Computerized lesion segmentation of breast ultrasound based on marker‐controlled watershed transformation
Authors: Gómez W. , Leija L. , Alvarenga A. V. , Infantosi A. F. C. , Pereira W. C. A. 
Published: December 2009

PurposeThis paper presents a computerized segmentation method for breast lesions on ultrasound (US) images.MethodsIt consists of first applying a contrast‐enhanced approach, i.e., a contrast‐limited adaptive histogram equalization. Then, aiming at removing speckle and enhancing the lesion boundary, an anisotropic diffusion filter, guided by texture descriptors derived from a set of Gabor filters, is applied. To eliminate the distant pixels that do not belong to the tumor, the resulting filtered image is multiplied by a constraint Gaussian function. By doing so, both the segmentation and the marker functions are generated and could be used in the marker‐controlled watershed transformation algorithm to create potential lesion boundaries. Finally, to determine the lesion contour, the average radial derivative function is evaluated. The proposed method was tested with 50 breast US images and 60 simulated “ultrasound‐like” images. Accuracy and precision of the segmentation method were then assessed. For the accuracy, three parameters were used: Overlap ratio (OR), normalized residual value (nrv), and proportional distance (PD) between contours.ResultsThe average results for US images were , , and . For simulated ultrasound‐like images, a better performance (, , and ) was achieved.ConclusionsThe segmentation method proposed was capable of delineating the lesion contours with high accuracy in comparison to both the radiologists’ delineations and the true delineations of simulated images. Moreover, this method was also found to be robust to human‐dependent parameters variations.
Effective point of measurement of thimble ion chambers in megavoltage photon beams
Authors: Tessier Frédéric , Kawrakow Iwan 
Published: December 2009

PurposeDetermine the effective point of measurement (EPOM) of 12 thimble ion chambers, including miniature chambers and three models widely used for clinical reference dosimetry. The EPOM is the point at which the measured dose would arise in the measurement medium in the absence of the probe: For cylindrical chambers, it is shifted upstream relative to the central axis of the chamber. Although current dosimetry protocols prescribe a blanket upstream EPOM shift of , with as the chamber cavity radius, it has been shown in recent years that the EPOM does, in fact, depend on every detail of the chamber design and on the beam characteristics. In the wake of this finding, the authors undertake a comprehensive study of the EPOM for a series of chambers in water.MethodsThis work relies on EGSnrc Monte Carlo calculations for the central axis depth dose in a water phantom and in ion chambers. They use a full Elekta Precise linac treatment head simulation to generate realistic photon beams with nominal energies of 6 and and fields sizes of and . The correct EPOM shift for the 12 ion chambers, modeled in realistic detail, is taken as the one minimizing the deviation of the ratio between the dose to water and the dose to the gas of the chamber cavity, according to a method proposed and validated in previous work.ResultsThe analysis reveals that the actual EPOM shift is significantly smaller than the recommended value in current dosimetry protocols, by up to 25% for reference‐class chambers and 80% for miniature chambers. The location of the EPOM also depends on the characteristics of the incident beam and varies in a well‐defined way with the cavity length, the central electrode radius, and the thimble wall thickness.ConclusionsThe authors confirm that an upstream EPOM shift of is too large for thimble ion chambers in high energy photon beams. Proper values for the EPOM shift could be tabulated per beam and per chamber, but they envisage that a single shift for all practical beams may prove sufficient. Moreover, the systematic dependence on chamber characteristics provides evidence that a universal parametrization in terms of a few design parameters is conceivable and has implication for the calculation of chamber correction factors.
Monte Carlo based, patient‐specific RapidArc QA using Linac log files
Authors: Teke Tony , Bergman Alanah M. , Kwa William , Gill Bradford , Duzenli Cheryl , Popescu I. Antoniu 
Published: December 2009

PurposeA Monte Carlo (MC) based QA process to validate the dynamic beam delivery accuracy for Varian RapidArc™ (Varian Medical Systems, Palo Alto, CA) using Linac delivery log files (DynaLog) is presented. Using DynaLog file analysis and MC simulations, the goal of this article is to (a) confirm that adequate sampling is used in the RapidArc optimization algorithm (177 static gantry angles) and (b) to assess the physical machine performance [gantry angle and monitor unit (MU) delivery accuracy].MethodsTen clinically acceptable RapidArc treatment plans were generated for various tumor sites and delivered to a water‐equivalent cylindrical phantom on the treatment unit. Three Monte Carlo simulations were performed to calculate dose to the CT phantom image set: (a) One using a series of static gantry angles defined by 177 control points with treatment planning system (TPS) MLC control files (planning files), (b) one using continuous gantry rotation with TPS generated MLC control files, and (c) one using continuous gantry rotation with actual Linac delivery log files. Monte Carlo simulated dose distributions are compared to both ionization chamber point measurements and with RapidArc TPS calculated doses. The 3D dose distributions were compared using a 3D gamma‐factor analysis, employing a distance‐to‐agreement criterion.ResultsThe dose difference between MC simulations, TPS, and ionization chamber point measurements was less than 2.1%. For all plans, the MC calculated 3D dose distributions agreed well with the TPS calculated doses (gamma‐factor values were less than 1 for more than 95% of the points considered). Machine performance QA was supplemented with an extensive DynaLog file analysis. A DynaLog file analysis showed that leaf position errors were less than for 94% of the time and there were no leaf errors greater than . The mean standard deviation in MU and gantry angle were and 0.355°, respectively, for the ten cases analyzed.ConclusionsThe accuracy and flexibility of the Monte Carlo based RapidArc QA system were demonstrated. Good machine performance and accurate dose distribution delivery of RapidArc plans were observed. The sampling used in the TPS optimization algorithm was found to be adequate.
Low‐ target optimization for spatial resolution improvement in megavoltage imaging
Authors: Connell Tanner , Robar James L. 
Published: December 2009

PurposeRecently, several authors have shown contrast improvements in megavoltage portal imaging and cone‐beam computed tomography using low atomic number targets. This work compliments previous studies by investigating the effects of varying different beam production parameters including target atomic number, target thickness, and incident electron energy on spatial resolution.MethodsTarget materials of beryllium, aluminum, and tungsten were investigated over a range of thicknesses between 10% and 100% of the continuous slowing down approximation range of electrons. Incident electron kinetic energies of 4.5 and were used, in conjunction with custom targets installed above the carousel of a modern radiotherapy linear accelerator. Monte Carlo simulations of the accelerator were constructed and compared to the experimental results.ResultsThe results showed that thinner targets, as well higher incident electron energies, generally produce more favorable modulation transfer function (MTF) curves. Due to an MTF dependence of the detector system on the photon energy, the experimental results showed that low‐ targets produced superior MTF curves. Simulations showed 14.5% and 21.5% increases in for the 7.0 and targets (Al; 60% ), respectively, when moved from the carousel to the location of the clinical target. values for the custom targets were compared to the clinical beam and were found to be between 10.4% lower and 15.5% higher .ConclusionsIntegration of low‐ external targets into the treatment head of a medical linear was achieved with only minor modifications. It was shown that reasonably high resolution images on par or better than those acquired with the clinical beam can be achieved using external low‐ targets.
Changes in optically stimulated luminescent dosimeter (OSLD) dosimetric characteristics with accumulated dose
Authors: Jursinic Paul A. 
Published: December 2009

PurposeA new type ofin vivo dosimeter, an optically stimulated luminescent dosimeter (OSLD), has now become commercially available for clinical use. The OSLD is a plastic disk infused with aluminum oxide doped with carbon . Crystals of , when exposed to ionizing radiation, store energy that is released as luminescence (420 nm) when the OSLD is illuminated with stimulation light (540 nm). The intensity of the luminescence depends on the dose absorbed by the OSLD and the intensity of the stimulation light. The effects of accumulated dose on OSLD response were investigated.MethodsThe OSLDs used in this work were nanodot dosimeters, which were read with a MicroStar reader (Landauer, Inc., Glenwood, IL). Dose to the OSLDs was delivered by 6 MV x rays and gamma rays from Co‐60 and Ir‐192. The signal on the OSLDs after irradiation is removed by optical annealing with a 150 W tungsten‐halogen lamp or a 14 W compact fluorescent lamp was investigated.ResultsIt was found that OSLD response to dose was supralinear and this response was altered with the amount of accumulated dose to the OSLD. The OSLD response can be modeled by a quadratic and an exponential equation. For accumulated doses up to 60 Gy, the OSLD sensitivity (counts/dose) decreases and the extent of supralinear increases. Above 60 Gy of accumulated dose the sensitivity increases and the extent of supralinearity decreases or reaches a plateau, depending on how the OSLDs were optically annealed. With preirradiation of OSLDs with greater than 1 kGy, it is found that the sensitivity reaches a plateau 2.5 folds greater than that of an OSLD with no accumulated dose and the supralinearity disappears. A regeneration of the luminescence signal in the dark after full optical annealing occurs with a half time of about two days. The extent of this regeneration signal depends on the amount of accumulated dose.ConclusionsForin vivo dosimetric measurements, a precision of ±0.5% can be achieved if the sensitivity and extent of supralinearity is established for each OSLD and use. Methods are presented for accomplishing this task.