bluuebunny/crossref_metadata_2025 huggingface dataset:
A real‐time dynamic‐MLC control algorithm for delivering IMRT to targets undergoing 2D rigid motion in the beam's eye view
Authors: McMahon Ryan , Berbeco Ross , Nishioka Seiko , Ishikawa Masayori , Papiez Lech
Published: September 2008

An MLC control algorithm for delivering intensity modulated radiation therapy (IMRT) to targets that are undergoing two‐dimensional (2D) rigid motion in the beam's eye view (BEV) is presented. The goal of this method is to deliver 3D‐derived fluence maps over a moving patient anatomy. Target motion measured prior to delivery is first used to design a set of planned dynamic‐MLC (DMLC) sliding‐window leaf trajectories. During actual delivery, the algorithm relies on real‐time feedback to compensate for target motion that does not agree with the motion measured during planning. The methodology is based on an existing one‐dimensional (1D) algorithm that uses on‐the‐fly intensity calculations to appropriately adjust the DMLC leaf trajectories in real‐time during exposure delivery [McMahon et al., Med. Phys. 34, 3211–3223 (2007)]. To extend the 1D algorithm's application to 2D target motion, a real‐time leaf‐pair shifting mechanism has been developed. Target motion that is orthogonal to leaf travel is tracked by appropriately shifting the positions of all MLC leaves. The performance of the tracking algorithm was tested for a single beam of a fractionated IMRT treatment, using a clinically derived intensity profile and a 2D target trajectory based on measured patient data. Comparisons were made between 2D tracking, 1D tracking, and no tracking. The impact of the tracking lag time and the frequency of real‐time imaging were investigated. A study of the dependence of the algorithm's performance on the level of agreement between the motion measured during planning and delivery was also included. Results demonstrated that tracking both components of the 2D motion (i.e., parallel and orthogonal to leaf travel) results in delivered fluence profiles that are superior to those that track the component of motion that is parallel to leaf travel alone. Tracking lag time effects may lead to relatively large intensity delivery errors compared to the other sources of error investigated. However, the algorithm presented is robust in the sense that it does not rely on a high level of agreement between the target motion measured during treatment planning and delivery.
Discrete‐space versus continuous‐space lesion boundary and area definitions
Authors: Sensakovic William F. , Starkey Adam , Roberts Rachael Y. , Armato Samuel G.
Published: September 2008

Measurement of the size of anatomic regions of interest in medical images is used to diagnose disease, track growth, and evaluate response to therapy. The discrete nature of medical images allows for both continuous and discrete definitions of region boundary. These definitions may, in turn, support several methods of area calculation that give substantially different quantitative values. This study investigated several boundary definitions (e.g., continuous polygon, internal discrete, and external discrete) and area calculation methods (pixel counting and Green's theorem). These methods were applied to three separate databases: A synthetic image database, the Lung Image Database Consortium database of lung nodules and a database of adrenal gland outlines. Average percent differences in area on the order of 20% were found among the different methods applied to the clinical databases. These results support the idea that inconsistent application of region boundary definition and area calculation may substantially impact measurement accuracy.
—A potent Auger emitter for targeted radionuclide therapy
Authors: Thisgaard H. , Jensen M.
Published: September 2008

Auger electron emitting radionuclides in cancer therapy offer the opportunity to deliver a high radiation dose to the tumor cells with high radiotoxicity while minimizing toxicity to normal tissue. We have in this study identified the Auger emitter as a potent nuclide for targeted radionuclide therapy based on theoretical dosimetry calculations at a subcellular scale. From these calculations we have determined the cellular S‐values for this therapeutic isotope. Moreover, we have demonstrated the possibility of producing this isotope and also the SPECT‐analogue for patient‐specific dosimetry, by measuring the proton irradiation yields for both isotopes using a low‐energy cyclotron. The excellent SPECT imaging properties of the radionuclide have been shown by scanning a Jaszczak SPECT Phantom.
Shimming with permanent magnets for the x‐ray detector in a hybrid x‐ray/MR system
Authors: Wen Zhifei , Fahrig Rebecca , Williams Scott T. , Pelc Norbert J.
Published: September 2008

In this x‐ray/MR hybrid system an x‐ray flat panel detector is placed under the patient cradle, close to the MR volume of interest (VOI), where the magnetic field strength is . Immersed in this strong field, several electronic components inside the detector become magnetized and create an additional magnetic field that is superimposed on the original field of the MR scanner. Even after linear shimming, the field homogeneity of the MR scanner remains disrupted by the detector. The authors characterize the field due to the detector with the field of two magnetic dipoles and further show that two sets of permanent magnets (NdFeB) can withstand the main magnetic field and compensate for the nonlinear components of the additional field. The ideal number of magnets and their locations are calculated based on a field map measured with the detector in place. Experimental results demonstrate great promise for this technique, which may be useful in many settings where devices with magnetic components need to be placed inside or close to an MR scanner.
A NTCP approach for estimating the outcome in radioiodine treatment of hyperthyroidism
Authors: Strigari L. , Sciuto R. , Benassi M. , Bergomi S. , Nocentini S. , Maini C. L.
Published: September 2008

Radioiodine has been in use for over as a treatment for hyperthyroidism. Major changes in clinical practice have led to accurate dosimetry capable of avoiding the risks of adverse effects and the optimization of the treatment. The aim of this study was to test the capability of a radiobiological model, based on normal tissue complication probability (NTCP), to predict the outcome after oral therapeutic administration. Following dosimetric study, 79 patients underwent treatment for hyperthyroidism using radioiodine and then 67 had at least a one‐year follow up. The delivered dose was calculated using the MIRD formula, taking into account the measured maximum uptake of administered iodine transferred to the thyroid, U0, and the effective clearance rate, and target mass. The dose was converted to normalized total dose delivered at per fraction . Furthermore, the method to take into account the reduction of the mass of the gland during radioiodine therapy was also applied. The clinical outcome and dosimetric parameters were analyzed in order to study the dose‐response relationship for hypothyroidism. The and parameters of the NTCP model approach were then estimated using the likelihood method. The , expressed as , resulted in (95% C.I.: ) and (95% C.I.: ) for patients affected by Graves or autonomous/multinodular disease, respectively. This supports the clinical evidence that Graves’ disease should be characterized by more radiosensitive cells compared to autonomous nodules. The parameter for all patients was 0.27 (95% C.I.: 0.22–0.36). These parameters were compared with those reported in the literature for hypothyroidism induced after external beam radiotherapy. The NTCP model correctly predicted the clinical outcome after the therapeutic administration of radioiodine in our series.
Volumetric breast density evaluation from ultrasound tomography images
Authors: Glide‐Hurst Carri K. , Duric Neb , Littrup Peter
Published: September 2008

Previous ultrasound tomography work conducted by our group showed a direct correlation between measured sound speed and physical density in vitro, and increased in vivo sound speed with increasing mammographic density, a known risk factor for breast cancer. Building on these empirical results, the purpose of this work was to explore a metric to quantify breast density using our ultrasound tomography sound speed images in a manner analogous to computer‐assisted mammogram segmentation for breast density analysis. Therefore, volumetric ultrasound percent density (USPD) is determined by segmenting high sound speed areas from each tomogram using a k‐means clustering routine, integrating these results over the entire volume of the breast, and dividing by whole‐breast volume. First, a breast phantom comprised of fat inclusions embedded in fibroglandular tissue was scanned four times with both our ultrasound tomography clinical prototype (with spatial resolution) and CT. The coronal transmission tomograms and CT images were analyzed using semiautomatic segmentation routines, and the integrated areas of the phantom's fat inclusions were compared between the four repeated scans. The average variability for inclusion segmentation was and , respectively, and a close correlation was observed in the integrated areas between the two modalities. Next, a cohort of 93 patients was imaged, yielding volumetric coverage of the breast (45–75 sound speed tomograms/patient). The association of USPD with mammographic percent density (MPD) was evaluated using two measures: (1) qualitative, as determined by a radiologist's visual assessment using BI‐RADS Criteria and (2) quantitative, via digitization and semiautomatic segmentation of craniocaudal and mediolateral oblique mammograms. A strong positive association between BI‐RADS category and USPD was demonstrated [Spearman ], with significant differences between all BI‐RADS categories as assessed by one‐way ANOVA and Scheffé posthoc analysis. Furthermore, comparing USPD to calculated mammographic density yielded moderate to strong positive associations for CC and MLO views ( and 0.59, respectively). These results support the hypothesis that utilizing USPD as an analogue to mammographic breast density is feasible, providing a nonionizing, whole‐breast analysis.
Air‐kerma strength determination of a high dose rate brachytherapy source
Authors: VanDamme J. J. , Culberson W. S. , DeWerd L. A. , Micka J. A.
Published: September 2008

The increased demand for high dose rate (HDR) brachytherapy as an alternative to external beam radiotherapy has led to the introduction of a HDR brachytherapy isotope . This source offers a dose rate similar to HDR sources, at about one fourth the effective photon energy. This work presents the calibration of this source in terms of air‐kerma strength, based on an adaptation of the current, National Institute of Standards and Technology traceable, in air measurement technique currently used for HDR sources. Several additional measurement correction factors were required, including corrections for air scatter, air attenuation, and ion recombination. A new method is introduced for determining the ion chamber calibration coefficient . An uncertainty analysis was also performed, indicating an overall measurement expanded uncertainty in the air‐kerma strength of 2.2%.
More accurate fitting of and radial dose functions
Authors: Taylor R. E. P. , Rogers D. W. O.
Published: September 2008

In this study an improved functional form for fitting the radial dose functions, , of and brachytherapy seeds is presented. The new function is capable of accurately fitting radial dose functions over ranges as large as for seeds and for seeds. The average discrepancies between fit and calculated data are less than 0.5% over the full range of fit and maximum discrepancies are 2% or less. The fitting function is also capable of accounting for the sharp increase in (upturn) seen for some sources for . This upturn has previously been attributed to the breakdown of the approximation of the sources as a line, however, in this study we demonstrate that another contributing factor is the characteristic x‐rays emitted from the Ti seed casing. Radial dose functions are calculated for 18 seeds and 9 seeds using the EGSnrc Monte Carlo user‐code BrachyDose. Fitting coefficients of the new function are tabulated for all 27 seeds. Extrapolation characteristics of the function are also investigated. The new functional form is an improvement over currently used fitting functions with its main strength being the ability to accurately fit the rapidly varying radial dose function at small distances. The new function is an excellent candidate for fitting the radial dose function of all and brachytherapy seeds and will increase the accuracy of dose distributions calculated around brachytherapy seeds using the TG‐43 protocol over a wider range of data. More accurate values of for may be particularly important in the treatment of ocular melanoma.
Accuracy and sensitivity of finite element model‐based deformable registration of the prostate
Authors: Brock Kristy K. , Nichol Alan M. , Ménard Cynthia , Moseley Joanne L. , Warde Padraig R. , Catton Charles N. , Jaffray David A.
Published: September 2008

Purpose: Evaluate the accuracy and the sensitivity to contour variation and model size of a finite element model‐based deformable registration algorithm for the prostate. Methods and materials: Two magnetic resonance images (MRIs) were obtained for 21 prostate patients with three implanted markers. A single observer contoured the prostate and markers and performed blinded recontouring of the first MRI. A biomechanical‐model based deformable registration algorithm, MORFEUS, was applied to each dataset pair, deforming the second image (B) to the first image (A). The residual error was calculated by comparing the center of mass (COM) of the markers with the predicted COM. Sensitivity to contour variation was calculated by deforming B to the repeat contour of A (A2). The sensitivity to the model size was calculated by reducing the number of nodes (, , ) and repeating the analysis. Results: The average residual error of the registration for B to A and B to A2 was (SD: ) and (SD: ), respectively. The average residual error of the registration of to and to was (SD: ) and (SD: ), respectively. The average time to run MORFEUS on the standard and reduced model was (SD: ) and (SD: ), respectively. Conclusions: The accuracy of the algorithm, equal to the image voxel size, is not affected by intraobserver contour variability or model size. Reducing the model size significantly increases algorithm efficiency.
An EGSnrc Monte Carlo‐calculated database of TG‐43 parameters
Authors: Taylor R. E. P. , Rogers D. W. O.
Published: September 2008

Monte Carlo methods are used to calculate a complete TG‐43 dosimetry parameter data set for 27 low‐energy photon emitting brachytherapy sources (18 and 9 ). All Monte Carlo calculations are performed using the EGSnrc user‐code BrachyDose. TG‐43 dosimetry parameters, including dose rate constants, radial dose functions (with functional fitting parameters), and anisotropy data, are calculated with finer spatial resolution, greater range of distances, and smaller uncertainties than data currently available in the literature for many of these sources. In particular, for most of the seeds, this is the first time that anisotropy data have been tabulated at distances less than from the source. These calculations employ the state‐of‐the‐art XCOM photon cross sections, and detailed source geometries are modeled using Yegin's multigeometry package. This data set serves as a completely independent verification of the currently available dosimetry parameters calculated using other Monte Carlo codes, including MCNP and PTRAN. This report also describes the Carleton Laboratory for Radiotherapy Physics TG‐43 Parameter Database, a publicly accessible web site (at http://www.physics.carleton.ca/clrp/seed_database/) through which all of the data calculated for this study can be accessed. Also available on the web site are descriptions of the methods and Monte Carlo models used in this study and comparisons of data calculated in this study with data calculated by other authors.
Microwave‐induced thermoacoustic scanning CT for high‐contrast and noninvasive breast cancer imaging
Authors: Nie Liming , Xing Da , Zhou Quan , Yang Diwu , Guo Hua
Published: September 2008

A fast thermoacoustic computed tomography system with a multielement linear transducer array was developed to image biological tissues with circular scanning. The spatial resolution of the imaging system and the spectra of the thermoacoustic signals were analyzed. A modified integration backprojection algorithm using velocity potential was employed to recover the direct energy deposition distribution, signal processing methods, and reconstruction algorithms were validated by imaging a phantom. The differences of the microwave‐frequency dielectric properties between malignant and normal adipose‐dominated tissues in the breast are considerable, and the absorption contrast can reach as large as 6:1 at . An experiment of human breast tissue with a tumor was performed with this system; the thermoacoustic images reconstructed by a limited‐field‐filtered backprojection algorithm and a modified integration backprojection algorithm were also compared with a mammogram. Our results show that the system can provide a rapid and noninvasive approach for high‐contrast breast cancer imaging.
Quantifying the effect of off‐focal radiation on the output of kilovoltage x‐ray systems
Authors: Ali E. S. M. , Rogers D. W. O.
Published: September 2008

In a typical x‐ray tube, off‐focal radiation is mainly generated by the backscattered electrons that reenter the anode outside the focal spot. In this study, BEAMnrc (an EGSnrc user‐code) is modified to simulate off‐focal radiation. The modified BEAMnrc code is used to study the characteristics of electrons that backscatter from the anode, and to quantify their effect on the output of typical x‐ray systems. Results show that the first generation backscatter coefficient is for tungsten anodes at diagnostic energies, and for molybdenum anodes at mammography energies. Second and higher generations of backscatter have a relatively minor contribution. At the patient plane, our simulation results are in excellent agreement with experimental measurements in the literature for the spectral shape of both the primary and the off‐focal components, and also for the integral off‐focal‐to‐primary ratio. The spectrum of the off‐focal component at the patient plane is softer than the primary, which causes a slight softening in the overall spectrum. For typical x‐ray systems, the off‐focal component increases patient exposure (for a given number of incident primary electrons) by up to 11% and reduces the half‐value layer and the effective energy of the average spectrum by up to 7% and 3%, respectively. The larger effects are for grounded cathode tubes, smaller interelectrode distance, higher tube voltage, lighter filtration, and less collimation. Simulation time increases by when the off‐focal radiation is included, but the overall simulation time remains of the order of a few minutes. This study concludes that the off‐focal radiation can have a non‐negligible effect on the output parameters of x‐ray systems and that it should be included in x‐ray tube simulations for more realistic modeling of these systems.
A survey of approaches for direct parametric image reconstruction in emission tomography
Authors: Tsoumpas Charalampos , Turkheimer Federico E. , Thielemans Kris
Published: September 2008

The quantitative data obtained by emission tomography are decoded using a number of techniques and methods in sequence to provide physiological information. Conventionally, the data are reconstructed to produce a series of static images. Then, pharmacokinetic modeling techniques are applied, and kinetic parameters that have physiological or functional significance are derived. Although it is possible to optimize each estimation step in this process, many simplifying assumptions have to be introduced to make the methods that are used practicable. Published research has shown that if the kinetic parameters are estimated directly from the measured data, the parametric images will have higher quality and lower mean‐squared error than if this was done indirectly. This review highlights some aspects of the methods that have been proposed for such direct estimation of pharmacokinetic information from raw emission data.
Regularized iterative weighted filtered backprojection for helical cone‐beam CT
Authors: Sunnegårdh Johan , Danielsson Per‐Erik
Published: September 2008

Contemporary reconstruction methods employed for clinical helical cone‐beam computed tomography (CT) are analytical (noniterative) but mathematically nonexact, i.e., the reconstructed image contains so called cone‐beam artifacts, especially for higher cone angles. Besides cone artifacts, these methods also suffer from windmill artifacts: alternating dark and bright regions creating spiral‐like patterns occurring in the vicinity of high ‐direction derivatives. In this article, the authors examine the possibility to suppress cone and windmill artifacts by means of iterative application of nonexact three‐dimensional filtered backprojection, where the analytical part of the reconstruction brings about accelerated convergence. Specifically, they base their investigations on the weighted filtered backprojection method [Stierstorfer et al., Phys. Med. Biol. 49, 2209–2218 (2004)]. Enhancement of high frequencies and amplification of noise is a common but unwanted side effect in many acceleration attempts. They have employed linear regularization to avoid these effects and to improve the convergence properties of the iterative scheme. Artifacts and noise, as well as spatial resolution in terms of modulation transfer functions and slice sensitivity profiles have been measured. The results show that for cone angles up to , cone artifacts are suppressed and windmill artifacts are alleviated within three iterations. Furthermore, regularization parameters controlling spatial resolution can be tuned so that image quality in terms of spatial resolution and noise is preserved. Simulations with higher number of iterations and long objects (exceeding the measured region) verify that the size of the reconstructible region is not reduced, and that the regularization greatly improves the convergence properties of the iterative scheme. Taking these results into account, and the possibilities to extend the proposed method with more accurate modeling of the acquisition process, the authors believe that iterative improvement with non‐exact methods is a promising technique for medical CT applications.
Comparison of SNR and CNR for in vivo mouse brain imaging at 3 and using well matched scanner configurations
Authors: DiFrancesco M. W. , Rasmussen J. M. , Yuan W. , Pratt R. , Dunn S. , Dardzinski B. J. , Holland S. K.
Published: September 2008

Signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) for magnetic resonance microimaging were measured using two nearly identical magnetic resonance imaging (MRI) scanners operating at field strengths of 3 and . Six mice were scanned using two imaging protocols commonly applied for in vivo imaging of small animal brain: RARE and FLASH. An accounting was made of the field dependence of relaxation times as well as a small number of hardware disparities between scanner systems. Standard methods for relaxometry were utilized to measure T1 and T2 for two white matter (WM) and two gray matter (GM) regions in the mouse brain. An average increase in T1 between 3 and of 28% was observed in the brain. T2 was found to decrease by 27% at in agreement with theoretical models. The SNR was found to be uniform throughout the mouse brain, increasing at higher field by a factor statistically indistinguishable from the ratio of Larmor frequencies when imaging with either method. The CNR between GM and WM structures was found to adhere to the expected field dependence for the RARE imaging sequence. Improvement in the CNR for the FLASH imaging sequence between 3 and was observed to be greater than the Larmor ratio, reflecting a greater susceptibility to partial volume effects at the lower SNR values at . Imaging at versus in small animals clearly provides advantages with respect to the CNR, even beyond the Larmor ratio, especially in lower SNR regimes. This careful multifaceted assessment of the benefits of higher static field is instructive for those newly embarking on small animal imaging. Currently the number of MRI scanners in use for research in human subjects is increasing at a rapid pace with approximately 30 systems deployed worldwide in 2008. The data presented in this article verify that if system performance and radio frequency uniformity is optimized at , it should be possible to realize the expected improvements in the CNR and SNR compared with MRI at .
The x‐ray light valve: A low‐cost, digital radiographic imaging system—Spatial resolution
Authors: MacDougall Robert D. , Koprinarov Ivaylo , Rowlands J. A.
Published: September 2008

An x‐ray light valve (XLV) coupled with an optical scanner has the potential to meet the need for a low‐cost, high quality digital imaging system for general radiography. The XLV/scanner concept combines three well‐established, and hence, low‐cost technologies: An amorphous selenium (‐Se) layer as an x‐ray‐to‐charge transducer, a liquid crystal (LC) cell as an analog display, and an optical scanner for image digitization. The XLV consists of an ‐Se layer and LC cell in a sandwich structure which produces an optical image in the LC layer upon x‐ray exposure. The XLV/scanner system consists of an XLV in combination with an optical scanner for image readout. Here, the effect of each component on the spatial resolution of an XLV/scanner system is investigated. A theoretical model of spatial resolution of an XLV is presented based on calculations of the modulation transfer function (MTF) for ‐Se and a LC cell. From these component MTFs, the theoretical MTF of the XLV is derived. The model was validated by experiments on a prototype XLV/scanner system. The MTF of the scanner alone was obtained by scanning an optical test target and the MTF of the XLV/scanner system was measured using x rays. From the measured MTF of the scanner, the theoretical MTF of the XLV/scanner system was established and compared with the experimental results. Good general agreement exists between experimental and theoretical results in the frequency range of interest for general radiography, although the theoretical curves slightly overstate the measured MTFs. The experimental MTF of the XLV was compared with the MTF of two clinical systems and was shown to have the capability to exceed the resolution of flat‐panel detectors. From this, the authors can conclude that the XLV has an adequate resolution for general radiography. The XLV/scanner also has the potential to eliminate aliasing while maintaining a MTF that exceeds that of a flat‐panel imager.
Measurement of multiple scattering of 13 and electrons by thin foils
Authors: Ross C. K. , McEwen M. R. , McDonald A. F. , Cojocaru C. D. , Faddegon B. A.
Published: September 2008

To model the transport of electrons through material requires knowledge of how the electrons lose energy and scatter. Theoretical models are used to describe electron energy loss and scatter and these models are supported by a limited amount of measured data. The purpose of this work was to obtain additional data that can be used to test models of electron scattering. Measurements were carried out using 13 and pencil beams of electrons produced by the National Research Council of Canada research accelerator. The electron fluence was measured at several angular positions from 0° to 9° for scattering foils of different thicknesses and with atomic numbers ranging from 4 to 79. The angle, , at which the fluence has decreased to of its value on the central axis was used to characterize the distributions. Measured values of ranged from 1.5° to 8° with a typical uncertainty of about 1%. Distributions calculated using the EGSnrc Monte Carlo code were compared to the measured distributions. In general, the calculated distributions are narrower than the measured ones. Typically, the difference between the measured and calculated values of is about 1.5%, with the maximum difference being 4%. The measured and calculated distributions are related through a simple scaling of the angle, indicating that they have the same shape. No significant trends with atomic number were observed.
Computer‐aided detection of masses in digital tomosynthesis mammography: Comparison of three approaches
Authors: Chan Heang‐Ping , Wei Jun , Zhang Yiheng , Helvie Mark A. , Moore Richard H. , Sahiner Berkman , Hadjiiski Lubomir , Kopans Daniel B.
Published: September 2008

The authors are developing a computer‐aided detection (CAD) system for masses on digital breast tomosynthesis mammograms (DBT). Three approaches were evaluated in this study. In the first approach, mass candidate identification and feature analysis are performed in the reconstructed three‐dimensional (3D) DBT volume. A mass likelihood score is estimated for each mass candidate using a linear discriminant analysis (LDA) classifier. Mass detection is determined by a decision threshold applied to the mass likelihood score. A free response receiver operating characteristic (FROC) curve that describes the detection sensitivity as a function of the number of false positives (FPs) per breast is generated by varying the decision threshold over a range. In the second approach, prescreening of mass candidate and feature analysis are first performed on the individual two‐dimensional (2D) projection view (PV) images. A mass likelihood score is estimated for each mass candidate using an LDA classifier trained for the 2D features. The mass likelihood images derived from the PVs are backprojected to the breast volume to estimate the 3D spatial distribution of the mass likelihood scores. The FROC curve for mass detection can again be generated by varying the decision threshold on the 3D mass likelihood scores merged by backprojection. In the third approach, the mass likelihood scores estimated by the 3D and 2D approaches, described above, at the corresponding 3D location are combined and evaluated using FROC analysis. A data set of 100 DBT cases acquired with a GE prototype system at the Breast Imaging Laboratory in the Massachusetts General Hospital was used for comparison of the three approaches. The LDA classifiers with stepwise feature selection were designed with leave‐one‐case‐out resampling. In FROC analysis, the CAD system for detection in the DBT volume alone achieved test sensitivities of 80% and 90% at average FP rates of 1.94 and 3.40 per breast, respectively. With the 2D detection approach, the FP rates were 2.86 and 4.05 per breast, respectively, at the corresponding sensitivities. In comparison, the average FP rates of the system combining the 3D and 2D information were 1.23 and 2.04 per breast, respectively, at 80% and 90% sensitivities. The difference in the detection performances between the 2D and the 3D approach, and that between the 3D and the combined approach were both statistically significant ( and 0.01, respectively) as estimated by alternative FROC analysis. The combined system is a promising approach to improving automated mass detection on DBTs.
Validity of a closed‐form diffusion solution in approximation for reflectance imaging with an oblique beam of arbitrary profile
Authors: Lu Jun Q. , Chen Cheng , Pravica David W. , Brock R. Scott , Hu Xin‐Hua
Published: September 2008

Determination of optical parameters of turbid media from reflectance image data is an important class of inverse problems due to its potential for noninvasive characterization of materials and biological tissues, which demands rapid modeling tools to generate calculated images. We treat the problem of reflectance imaging with homogeneous semi‐infinite turbid media as a boundary‐value problem of diffusion type in the approximation to the radiative transfer equation. A closed‐form solution has been obtained for an oblique incident beam of arbitrary profile and its accuracy has been examined against a Monte Carlo method and measured data. We find that the diffusion solution provides a sufficiently accurate tool to rapidly calculate reflectance images for samples of large or moderate scattering albedo illuminated by a beam of arbitrary profile as long as the anisotropy factor remains less than 0.7 and single scattering albedo larger than 0.8. The closed‐form solution can thus be used as a part of a forward modeling toolbox to determine optical parameters from reflectance image data in combination with other method such as the Monte Carlo simulation.
Simulated real time image guided intrafraction tracking‐delivery for hypofractionated prostate IMRT
Authors: Hossain Sabbir , Xia Ping , Chuang Cynthia , Verhey Lynn , Gottschalk Alexander R. , Mu Guanwei , Ma Lijun
Published: September 2008

Hypofractionated stereotactic body radiotherapy (SBRT) has been tested for prostate cancer radiotherapy. This study aims to investigate the dosimetric effects of intrafraction prostate motion on the target and the normal structures for SBRT. For prostate cancer patients treated with an image‐tracking CyberKnife system, the intrafraction prostate movements were recorded during treatment time. Based on the recorded intrafraction prostate movements, treatment plans were created for these cases using intensity modulated beams while scaling the average time patterns from the CyberKnife treatment to simulate hypofractionated intensity modulated radiotherapy (IMRT) delivery. The effect of delivery time on the intrafraction organ motion was investigated. For a nominal single fraction delivery of with IMRT, we found that the dosimetric effect of the intrafraction prostate movement is case dependent. For most cases, the dose volume histograms exhibited very small changes from the treatment plans that assumed no intrafractional prostate motion when the maximum intrafraction movements were within . However, when sporadic prostate movements greater than were present in any one direction, significant changes were found. For example, the for the prostate could be reduced by more than 10% to less than 85% of the prostate volume coverage. If these large movements could be excluded by some active correction strategies, then the average for the simulated plan could be restored to within approximately 2% of the ideal treatment plans. On average, the sporadic intrafraction motion has less dosimetric impact on the prolonged treatment delivery versus fast treatment delivery. For example, the average for the clinical target volume was reduced from the original for prolonged treatment, and to when the treatment time was shortened by 50%. Due to the observed large sporadic prostate motions, we conclude that an on‐line target motion monitoring and correction strategy is necessary to implement hypofractionated SBRT with intensity modulated beams for prostate cancer treatments.
Feasibility study for image‐guided kidney surgery: Assessment of required intraoperative surface for accurate physical to image space registrations
Authors: Benincasa Anne B. , Clements Logan W. , Herrell S. Duke , Galloway Robert L.
Published: September 2008

A notable complication of applying current image‐guided surgery techniques of soft tissue to kidney resections (nephrectomies) is the limited field of view of the intraoperative kidney surface. This limited view constrains the ability to obtain a sufficiently geometrically descriptive surface for accurate surface‐based registrations. The authors examined the effects of the limited view by using two orientations of a kidney phantom to model typical laparoscopic and open partial nephrectomy views. Point‐based registrations, using either rigidly attached markers or anatomical landmarks as fiducials, served as initial alignments for surface‐based registrations. Laser range scanner (LRS) obtained surfaces were registered to the phantom's image surface using a rigid iterative closest point algorithm. Subsets of each orientation's LRS surface were used in a robustness test to determine which parts of the surface yield the most accurate registrations. Results suggest that obtaining accurate registrations is a function of the percentage of the total surface and of geometric surface properties, such as curvature. Approximately 28% of the total surface is required regardless of the location of that surface subset. However, that percentage decreases when the surface subset contains information from opposite ends of the surface and/or unique anatomical features, such as the renal artery and vein.
Accelerator beam data commissioning equipment and procedures: Report of the TG‐106 of the Therapy Physics Committee of the AAPM
Authors: Das Indra J. , Cheng Chee‐Wai , Watts Ronald J. , Ahnesjö Anders , Gibbons John , Li X. Allen , Lowenstein Jessica , Mitra Raj K. , Simon William E. , Zhu Timothy C.
Published: September 2008

For commissioning a linear accelerator for clinical use, medical physicists are faced with many challenges including the need for precision, a variety of testing methods, data validation, the lack of standards, and time constraints. Since commissioning beam data are treated as a reference and ultimately used by treatment planning systems, it is vitally important that the collected data are of the highest quality to avoid dosimetric and patient treatment errors that may subsequently lead to a poor radiation outcome. Beam data commissioning should be performed with appropriate knowledge and proper tools and should be independent of the person collecting the data. To achieve this goal, Task Group 106 (TG‐106) of the Therapy Physics Committee of the American Association of Physicists in Medicine was formed to review the practical aspects as well as the physics of linear accelerator commissioning. The report provides guidelines and recommendations on the proper selection of phantoms and detectors, setting up of a phantom for data acquisition (both scanning and no‐scanning data), procedures for acquiring specific photon and electron beam parameters and methods to reduce measurement errors , beam data processing and detector size convolution for accurate profiles. The TG‐106 also provides a brief discussion on the emerging trend in Monte Carlo simulation techniques in photon and electron beam commissioning. The procedures described in this report should assist a qualified medical physicist in either measuring a complete set of beam data, or in verifying a subset of data before initial use or for periodic quality assurance measurements. By combining practical experience with theoretical discussion, this document sets a new standard for beam data commissioning.
The role of medical physicists in developing stereotactic radiosurgery
Authors: Benedict Stanley H. , Bova Frank J. , Clark Brenda , Goetsch Steven J. , Hinson William H. , Leavitt Dennis D. , Schlesinger David J. , Yenice Kamil M.
Published: September 2008

This article is a tribute to the pioneering medical physicists over the last who have participated in the research, development, and commercialization of stereotactic radiosurgery (SRS) and stereotactic radiotherapy utilizing a wide range of technology. The authors have described the evolution of SRS through the eyes of physicists from its beginnings with the Gamma Knife™ in 1951 to proton and charged particle therapy; modification of commercial linacs to accommodate high precision SRS setups; the multitude of accessories that have enabled fine tuning patients for relocalization, immobilization, and repositioning with submillimeter accuracy; and finally the emerging technology of SBRT. A major theme of the article is the expanding role of the medical physicist from that of advisor to the neurosurgeon to the current role as a primary driver of new technology that has already led to an adaptation of cranial SRS to other sites in the body, including, spine, liver, and lung. SRS continues to be at the forefront of the impetus to provide technological precision for radiation therapy and has demonstrated a host of downstream benefits in improving delivery strategies for conventional therapy as well. While this is not intended to be a comprehensive history, and the authors could not delineate every contribution by all of those working in the pursuit of SRS development, including physicians, engineers, radiobiologists, and the rest of the therapy and dosimetry staff in this important and dynamic radiation therapy modality, it is clear that physicists have had a substantial role in the development of SRS and theyincreasingly play a leading role in furthering SRS technology.
Directional hearing or binaural phenomenon?
Authors: Zaslavski Gennadi L.
Published: September 2008

Binaural phenomenon is known to be responsible for sound localization in humans. A similar mechanism is thought to provide the dolphin with the ability to localize targets and sound source. At the same time, the dolphin possesses quite good directional hearing, let alone very directional click transmission, which is absent in humans. The directional hearing in the dolphin, at least in the horizontal plane, appears to be consistent with two ears’ reception. Does it signify that the binaural phenomenon coexists in dolphins with directional hearing? As far as humans are concerned, the spatial hearing does not involve any substantial hearing directionality. The directionality seems to negate the main advantage of the binaural hearing, that is instantaneous determination of a sound source direction, and complicate the use of binaural cues. It was found that the lesser intensity of high-frequency clicks the smaller an angle between their sources should be for the dolphin to hear them simultaneously. The directivity pattern of the dolphin’s auditory system was measured as a dependence of the threshold angle between the sources on the click’s intensity. At high frequencies, the dolphin’s auditory system performed as a directional system rather than an omnidirectional binaural system known for humans.
Range accuracy: One more peculiarity of the dolphin’s sonar
Authors: Zaslavski Gennadi L.
Published: September 2008

In order to measure range accuracy in the dolphin, a range difference discrimination experiment should be performed. However, when two targets are presented at different distances simultaneously, even at an angle as big as 40–60=B0, one dolphin’s click can cause echoes from both targets. As a result, the targets can be distinguished by the difference in the echo arrival time without measuring the target distance. The best way to prevent the dolphin from simultaneous targets insonifying by each click turned out to be successive submerging of the targets in water at different distances from the dolphin. Nevertheless, it proved to be impossible to force the dolphin to measure the distance from itself to the targets. Two dolphins solved the task by measuring the time interval between the target echo and echo from a reference object; that could be the pool walls or even a nylon net. There was no consistent change in the threshold range difference with the targets’ range, whereas it regularly decreased when the targets were moved closer to the reference object. The dolphins preferred to measure the relative target position even if the reference object was at a much greater distance than the targets.
Anatomy and biomechanics of interaural coupling in a sarcophagid fly
Authors: Robert Daniel , Miles Ronald N. , Hoy Ronald R.
Published: September 2008

In flies, the sense of hearing is related to the parasitic life-style. First reported in the parasitoid family Tachinidae, tympanal hearing is used to acoustically locate singing hosts. Here, the auditory anatomy and biomechanics of another family of parasitoid, the Sarcophagidae, is presented. These small auditory organs are located on the ventral prosternum and in several respects conform to the general Bauplan of a dipteran ear. However, the structural organization of their auditory periphery—the tympanal complex—is quite different: it is conspicuously lacking the central pivoting sclerite that constitutes the key element for directional sensitivity by mechanical coupling found in tachinids [Robert et al., J. Comp. Physiol. 179, 29–44 (1996)]. Based on scanning laser vibrometry, deflection shape analysis shows that, in response to sound, the tympana vibrate asymmetrically but with deflection modes clearly different from those observed in tachinids. The tympanal complex does not oscillate about a fixed central fulcrum, but sways about one of its relatively immobile ends. Interaural coupling is proposed to be caused by he anisotropic tympanal structure. This data suggests the presence of an alternative mechanism for directional hearing in this fly. [Work supported by NIH, NSF, and the Swiss Science Foundation.]
Theoretical and experimental study of the electrostatic transducer with a nonplanar back plate
Authors: Skvor Jan
Published: September 2008

Usually the electrostatic transducer as a microphone or pressure sensor is formed by a circular or rectangular membrane and a planar back electrode with holes. The number, the diameter, and the position of the holes in the back electrode determine the acoustic impedance of the air-gap and especially the damping of the transducer. The aim of this contribution is to show the possibility of the realization of a transducer with a thin diaphragm and a back electrode of convex shape and without holes. The convex shape of the back electrode, because of the nonuniform displacement distribution over the membrane, has a small influence on the transducer sensitivity, but a very high influence on the membrane damping. In the contribution is shown the application of the theory of this type of transducer with a circular membrane vibrating in the fundamental mode with approximate parabolic distribution of membrane displacement and a paraboloidal back electrode without the holes. This transducer modification is applicable for classical and silicon micro-mechanical technologies [Z. Skvor and J. Skvor, Proc. 4th Congress on Acoust., SFA, Marseille (1997), Vol. 1, pp. 63–66].
On the subspace methods for direction-of-arrival estimation in underwater acoustics
Authors: Frikel Miloud , Barroso Victor
Published: September 2008

The problem of bearing estimation appears in a vast number of applications, e.g., radar, sonar, communication systems,... . In this study, an investigation of the passive angular location of sources in underwater acoustics using an array of sensors is presented. First, eigenstructure/noneigendecomposition methods for direction-of-arrival (DOA) estimation of narrow-band/wide-band signals’ wave fronts are studied. The noneigenanalysis-based techniques are emphasized because of their weak computational load. The studied methods are of high resolution such as (music, minimum-norm, their propagator,...). The eigenstructure based techniques on the exploitation of two principal properties: first, the data matrix has (N−P) multiplicity of its smallest eigenvalues, with N and P as the number of sensors and sources, respectively, and the eigenvectors corresponding to the minimal eigenvalues; these methods have better performance than the classical methods. A comparative study of these types of subspace methods is presented. Finally, a robust method to improve the accuracy of the estimation of the direction-of-arrival of the sources is developed. The results of simulations are provided to illustrate the theoretical predictions. [This work was partially funded by FEDER and PRAXIS XXI, under contract PRAXIS/3/3.1/TPR/23/94.]