bluuebunny/crossref_metadata_2025 huggingface dataset:
Massive training artificial neural network (MTANN) for reduction of false positives in computerized detection of lung nodules in low‐dose computed tomography
Authors: Suzuki Kenji , Armato Samuel G. , Li Feng , Sone Shusuke , Doi Kunio
Published: July 2003

In this study, we investigated a pattern‐recognition technique based on an artificial neural network (ANN), which is called a massive training artificial neural network (MTANN), for reduction of false positives in computerized detection of lung nodules in low‐dose computed tomography (CT) images. The MTANN consists of a modified multilayer ANN, which is capable of operating on image data directly. The MTANN is trained by use of a large number of subregions extracted from input images together with the teacher images containing the distribution for the “likelihood of being a nodule.” The output image is obtained by scanning an input image with the MTANN. The distinction between a nodule and a non‐nodule is made by use of a score which is defined from the output image of the trained MTANN. In order to eliminate various types of non‐nodules, we extended the capability of a single MTANN, and developed a multiple MTANN (Multi‐MTANN). The Multi‐MTANN consists of plural MTANNs that are arranged in parallel. Each MTANN is trained by using the same nodules, but with a different type of non‐nodule. Each MTANN acts as an expert for a specific type of non‐nodule, e.g., five different MTANNs were trained to distinguish nodules from various‐sized vessels; four other MTANNs were applied to eliminate some other opacities. The outputs of the MTANNs were combined by using the logical AND operation such that each of the trained MTANNs eliminated none of the nodules, but removed the specific type of non‐nodule with which the MTANN was trained, and thus removed various types of non‐nodules. The Multi‐MTANN consisting of nine MTANNs was trained with 10 typical nodules and 10 non‐nodules representing each of nine different non‐nodule types (90 training non‐nodules overall) in a training set. The trained Multi‐MTANN was applied to the reduction of false positives reported by our current computerized scheme for lung nodule detection based on a database of 63 low‐dose CT scans (1765 sections), which contained 71 confirmed nodules including 66 biopsy‐confirmed primary cancers, from a lung cancer screening program. The Multi‐MTANN was applied to 58 true positives (nodules from 54 patients) and 1726 false positives (non‐nodules) reported by our current scheme in a validation test; these were different from the training set. The results indicated that 83% (1424/1726) of non‐nodules were removed with a reduction of one true positive (nodule), i.e., a classification sensitivity of 98.3% (57 of 58 nodules). By using the Multi‐MTANN, the false‐positive rate of our current scheme was improved from 0.98 to 0.18 false positives per section (from 27.4 to 4.8 per patient) at an overall sensitivity of 80.3% (57/71).
A fluence convolution method to account for respiratory motion in three‐dimensional dose calculations of the liver: A Monte Carlo study
Authors: Chetty Indrin J. , Rosu Mihaela , Tyagi Neelam , Marsh Lon H. , McShan Daniel L. , Balter James M. , Fraass Benedick A. , Ten Haken Randall K.
Published: July 2003

We describe the implementation of a fluence convolution method to account for the influence of superior‐inferior (SI) respiratory induced motion on a Monte Carlo‐based dose calculation of a tumor located in the liver. This method involves convolving the static fluence map with a function describing the SI motion of the liver—the motion function has been previously derived from measurements of diaphragm movement observed under fluoroscopy. Significant differences are noted between fluence‐convolved and static dose distributions in an example clinical treatment plan; hot and cold spots (on the order of 25%) are observed in the fluence‐convolved plan at the superior and inferior borders of the liver, respectively. This study illustrates that the fluence convolution method can be incorporated into Monte Carlo dose calculation algorithms to account for some of the effects of patient breathing during radiotherapy treatment planning, thus leading to more accurate dose calculations.
Computer aided detection of masses in mammography using subregion Hotelling observers
Authors: Baydush Alan H. , Catarious David M. , Abbey Craig K. , Floyd Carey E.
Published: July 2003

We propose to investigate the use of the subregion Hotelling observer for the basis of a computer aided detection scheme for masses in mammography. A database of 1320 regions of interest (ROIs) was selected from the DDSM database collected by the University of South Florida using the Lumisys scanner cases. The breakdown of the cases was as follows: 656 normal ROIs, 307 benign ROIs, and 357 cancer ROIs. Each ROI was extracted at a size of pixels and subsampled to pixels. For the detection task, cancer and benign cases were considered positive and normal was considered negative. All positive cases had the lesion centered in the ROI. We chose to investigate the subregion Hotelling observer as a classifier to detect masses. The Hotelling observer incorporates information about the signal, the background, and the noise correlation for prediction of positive and negative and is the optimal detector when these are known. For our study, 225 subregion Hotelling observers were set up in a grid across the center of the ROIs. Each separate observer was designed to “observe,” or discriminate, an pixel area of the image. A leave one out training and testing methodology was used to generate 225 “features,” where each feature is the output of the individual observers. The 225 features derived from separate Hotelling observers were then narrowed down by using forward searching linear discriminants (LDs). The reduced set of features was then analyzed using an additional LD with receiver operating characteristic (ROC) analysis. The 225 Hotelling observer features were searched by the forward searching LD, which selected a subset of 37 features. This subset of 37 features was then analyzed using an additional LD, which gave a ROC area under the curve of and a partial area of 0.6728. Additionally, at 98% sensitivity the overall classifier had a specificity of 55.9% and a positive predictive value of 69.3%. Preliminary results suggest that using subregion Hotelling observers in combination with LDs can provide a strong backbone for a CAD scheme to help radiologists with detection. Such a system could be used in conjunction with CAD systems for false positive reduction.
Dosimetric properties of an amorphous silicon electronic portal imaging device for verification of dynamic intensity modulated radiation therapy
Authors: Greer Peter B. , Popescu Carmen C.
Published: July 2003

Dosimetric properties of an amorphous silicon electronic portal imaging device (EPID) for verification of dynamic intensity modulated radiation therapy (IMRT) delivery were investigated. The EPID was utilized with continuous frame‐averaging during the beam delivery. Properties studied included effect of buildup, dose linearity, field size response, sampling of rapid multileaf collimator (MLC) leaf speeds, response to dose‐rate fluctuations, memory effect, and reproducibility. The dependence of response on EPID calibration and a dead time in image frame acquisition occurring every 64 frames were measured. EPID measurements were also compared to ion chamber and film for open and wedged static fields and IMRT fields. The EPID was linear with dose and dose rate, and response to MLC leaf speeds up to 2.5 cm s−1 was found to be linear. A field size dependent response of up to 5% relative to ion‐chamber measurement was found. Reproducibility was within 0.8% (1 standard deviation) for an IMRT delivery recorded at intervals over a period of one month. The dead time in frame acquisition resulted in errors in the EPID that increased with leaf speed and were over 20% for a 1 cm leaf gap moving at 1.0 cm s−1. The EPID measurements were also found to depend on the input beam profile utilized for EPID flood‐field calibration. The EPID shows promise as a device for verification of IMRT, the major limitation currently being due to dead‐time in frame acquisition.
Flat panel detector‐based cone beam computed tomography with a circle‐plus‐two‐arcs data acquisition orbit: Preliminary phantom study
Authors: Ning Ruola , Tang Xiangyang , Conover David , Yu Rongfeng
Published: July 2003

Cone beam computed tomography (CBCT) has been investigated in the past two decades due to its potential advantages over a fan beam CT. These advantages include (a) great improvement in data acquisition efficiency, spatial resolution, and spatial resolution uniformity, (b) substantially better utilization of x‐ray photons generated by the x‐ray tube compared to a fan beam CT, and (c) significant advancement in clinical three‐dimensional (3D) CT applications. However, most studies of CBCT in the past are focused on cone beam data acquisition theories and reconstruction algorithms. The recent development of x‐ray flat panel detectors (FPD) has made CBCT imaging feasible and practical. This paper reports a newly built flat panel detector‐based CBCT prototype scanner and presents the results of the preliminary evaluation of the prototype through a phantom study. The prototype consisted of an x‐ray tube, a flat panel detector, a GE 8800 CT gantry, a patient table and a computer system. The prototype was constructed by modifying a GE 8800 CT gantry such that both a single‐circle cone beam acquisition orbit and a circle‐plus‐two‐arcs orbit can be achieved. With a circle‐plus‐two‐arcs orbit, a complete set of cone beam projection data can be obtained, consisting of a set of circle projections and a set of arc projections. Using the prototype scanner, the set of circle projections were acquired by rotating the x‐ray tube and the FPD together on the gantry, and the set of arc projections were obtained by tilting the gantry while the x‐ray tube and detector were at the 12 and 6 o'clock positions, respectively. A filtered backprojection exact cone beam reconstruction algorithm based on a circle‐plus‐two‐arcs orbit was used for cone beam reconstruction from both the circle and arc projections. The system was first characterized in terms of the linearity and dynamic range of the detector. Then the uniformity, spatial resolution and low contrast resolution were assessed using different phantoms mainly in the central plane of the cone beam reconstruction. Finally, the reconstruction accuracy of using the circle‐plus‐two‐arcs orbit and its related filtered backprojection cone beam volume CT reconstruction algorithm was evaluated with a specially designed disk phantom. The results obtained using the new cone beam acquisition orbit and the related reconstruction algorithm were compared to those obtained using a single‐circle cone beam geometry and Feldkamp's algorithm in terms of reconstruction accuracy. The results of the study demonstrate that the circle‐plus‐two‐arcs cone beam orbit is achievable in practice. Also, the reconstruction accuracy of cone beam reconstruction is significantly improved with the circle‐plus‐two‐arcs orbit and its related exact CB–FPB algorithm, as compared to using a single circle cone beam orbit and Feldkamp's algorithm.
Elimination of importance factors for clinically accurate selection of beam orientations, beam weights and wedge angles in conformal radiation therapy
Authors: Bedford James L. , Webb Steve
Published: July 2003

A method of simultaneously optimizing beam orientations, beam weights, and wedge angles for conformal radiotherapy is presented. This method removes the need for importance factors by optimizing one objective only, subject to a set of rigid constraints. This facilitates the production of inverse solutions which, without trial‐and‐error modification of importance factors, precisely satisfy the specified constraints. The algorithm minimizes an objective function which is based upon the single objective to be optimized, but which is forced to an artificially high value when the constraints are not met, so that only satisfactory solutions are allowed. Due to the complex nature of the objective function space, including multiple local minima separated by large regions of plateau, a random search technique equivalent to fast simulated annealing is used for producing inverse plans. To illustrate the novel features of the new algorithm, a simulation is first presented, for the case of a cylindrical phantom. The morphology of the objective function space is shown to be significantly different for the new algorithm, compared to that for a conventional quadratic objective function. Clinical cases for prostate and craniopharyngioma are then presented. For the prostate case, the objective is to reduce irradiated rectal volume. Three‐field, four‐field, and six‐field optimizations, with or without orientation optimization, are shown to provide solutions which are consistent with previously reported plans and class solutions. For the craniopharyngioma case, which involves the use of a high‐precision stereotactic conformal technique, the objective is to reduce the irradiated volume of normal brain. Practically feasible beam angles are produced which, compared to a standard plan, provide a small but worthwhile sparing of normal brain. The algorithm is thereby shown to be robust and suitable for clinical application.
Implementation of nuclear magnetic resonance polymer‐gel dosimetry into clinical practice
Authors: Novotny Josef
Published: July 2003

The aim of this thesis research was to evaluate some basic properties of the polymer‐gel dosimeter and test it in clinical practice. Dependence of polymer‐gel dosimeter sensitivity on different photon and electron energies as well as on different mean dose rates of standard clinically used linear accelerator was studied. A physical/mathematical description of the effect of temperature on the polymer‐gel dosimeter, able to correct a dosimeter 1/T2 response for different temperatures during the NMR measurement, was developed. The polymer‐gel dosimeter was tested for the verification of stereotactic radiosurgery techniques with the Leksell Gamma Knife. There was an observed trend in polymer‐gel dosimeter sensitivity dependence on quality index for the high‐energy x ray beams. Similarly, there was an observed trend in dosimeter sensitivity dependence on mean electron energy. It was demonstrated that these trends were statistically significant. Polymer‐gel dosimeter sensitivity was decreased with increased photon or electron energy. There was observed no trend in polymer‐gel dosimeter sensitivity dependence on mean dose rates for both photon and electron beams within experimental errors. It was demonstrated in this study that the NMR 1/T2‐dose response is increased for lower temperatures of the evaluated polymer‐gel samples. The relaxation rate response can be expressed as a function consisting of two terms, one denoted for the temperature dependence and the other one for the dose dependence. Two clinical applications of the polymer‐gel dosimeter were demonstrated in this study. The first experiment allowed a check of the entire stereotactic procedure with the Leksell gamma knife including target stereotactic localization on NMR, treatment planning, and irradiation. The second experiment evaluated dosimetric and geometric inaccuracies associated with the stereotactic irradiation of the experimental rat brain. It was demonstrated that polymer‐gel can be effectively used in evaluation of relative dose distribution obtained in stereotactic radiosurgery procedures.
Integrated density of a lesion: A quantitative, mammographically derived, invariable measure
Authors: Chang Yuan‐Hsiang , Good Walter F. , Leader Joseph K. , Wang Xiao‐Hui , Zheng Bin , Hardesty Lara A. , Hakim Christiane M. , Gur David
Published: July 2003

A method for quantitatively estimating lesion “size” from mammographic images was developed and evaluated. The main idea behind the measure, termed “integrated density” (ID), is that the total x‐ray attenuation attributable to an object is theoretically invariant with respect to the projected view and object deformation. Because it is possible to estimate x‐ray attenuation of a lesion from relative film densities, after appropriate corrections for background, the invariant property of the measure is expected to result in an objective method for evaluating the “sizes” of breast lesions. ID was calculated as the integral of the estimated image density attributable to a lesion, relative to surrounding background, over the area of the lesion and after corrections for the nonlinearity of the film characteristic curve. This effectively provides a measure proportional to lesion volume. We computed ID and more traditional measures of size (such as “mass diameter” and “effective size”) for 100 pairs of ipsilateral mammographic views, each containing a lesion that was relatively visible in both views. The correlation between values calculated for each measure from corresponding pairs of ipsilateral views were computed and compared. All three size‐related measures (mass diameter, effective size, and ID) exhibited reasonable linear relationship between paired views Specifically, the ID measures for the 100 masses were found to be highly correlated between ipsilateral views of the same mass. The correlation increased substantially when a measure with linear dimensions of length was defined as the cube root of ID. There is a high degree of correlation between ID‐based measures obtained from different views of the same mass. ID‐based measures showed a higher degree of invariance than mass diameter or effective size.
Dose distribution close to metal implants in Gamma Knife Radiosurgery: A Monte Carlo study
Authors: Cheung Joel Y. C. , Yu K. N. , Chan Josie F. K. , Ho Robert T. K. , Yu C. P.
Published: July 2003

Materials with high atomic numbers favor the occurrence of the photoelectric effect when they are irradiated with gamma rays. Therefore, the photoelectric effects of metal implants within the target regions in Gamma Knife Radiosurgery are worth studying. In the present work, Monte Carlo simulations using EGS4 were employed to investigate the resulting dose enhancements. A dose enhancement as high as 10% was observed close to a platinum implant along the and axes, while no significant dose enhancements were observed for silver, stainless steel 301, and titanium ones. A dose enhancement as high as 20% was observed close to the platinum implant along the axis at the superior position of the metal–phantom interface and was 10% higher for other metal implants.
A dynamic approach to identifying desired physiological phases for cardiac imaging using multislice spiral CT
Authors: Vembar M. , Garcia M. J. , Heuscher D. J. , Haberl R. , Matthews D. , Böhme G. E. , Greenberg N. L.
Published: July 2003

In this investigation, we describe a quantitative technique to measure coronary motion, which can be correlated with cardiac image quality using multislice computed tomography (MSCT) scanners. MSCT scanners, with subsecond scanning, thin‐slice imaging (sub‐millimeter) and volume scanning capabilities have paved the way for new clinical applications like noninvasive cardiac imaging. ECG‐gated spiral CT using MSCT scanners has made it possible to scan the entire heart in a single breath‐hold. The continuous data acquisition makes it possible for multiple phases to be reconstructed from a cardiac cycle. We measure the position and three‐dimensional velocities of well‐known landmarks along the proximal, mid, and distal regions of the major coronary arteries [left main (LM), left anterior descending (LAD), right coronary artery (RCA), and left circumflex (LCX)] during the cardiac cycle. A dynamic model (called the “delay algorithm”) is described which enables us to capture the same physiological phase or “state” of the anatomy during the cardiac cycle as the instantaneous heart rate varies during the spiral scan. The coronary arteries are reconstructed from data obtained during different physiological cardiac phases and we correlate image quality of different parts of the coronary anatomy with phases at which minimum velocities occur. The motion characteristics varied depending on the artery, with the highest motion being observed for RCA. The phases with the lowest mean velocities provided the best visualization. Though more than one phase of relative minimum velocity was observed for each artery, the most consistent image quality was observed during mid‐diastole (“diastasis”) of the cardiac cycle and was judged to be superior to other reconstructed phases in 92% of the cases. In the process, we also investigated correlation between cardiac arterial states and other measures of motion, such as the left ventricular volume during a cardiac cycle, which earlier has been demonstrated as an example of how anatomic‐specific information can be used in a knowledge‐based cardiac CT algorithm. Using these estimates in characterizing cardiac motion also provides realistic simulation models for higher heart rates and also in optimizing volume reconstructions for individual segments of the cardiac anatomy.
Dose distributions in bifurcated coronary vessels treated with catheter‐based photon and beta emitters in intravascular brachytherapy
Authors: Yue Ning , Roberts Kenneth B. , Pfau Steven E. , Nath Ravinder
Published: July 2003

The dose distributions in the bifurcated vessels treated with intravascular brachytherapy line sources are complicated and depend on the bifurcation geometry consisting of a main and a branch vessel at different angles. To investigate the dosimetric effects at the bifurcation, calculations were performed on branching vessels of various bifurcation angles ranging from 20° to 90°. Two catheter based delivery systems were considered in the calculations using a 40 mm long radioactive sources of or It was assumed that the bifurcated vessel was treated in two separate source insertions, once for the main vessel and later for the branch vessel. Calculations were performed for different values of source gap from 0 to 9 mm, at the junction of main and branch vessels. Our results indicate that main vessel always receives a higher dose (up to 200%) when the branch vessel is also treated. Hot spots at portions of the main vessel near the junction cannot be totally avoided without severely underdosing the branch vessel. For bifurcation angle greater than 45°, a 4 mm source gap can almost ensure that overdosing of the main vessel does not exceed 60% and underdosing of the branch vessel does not exceed 10% for However, for the same is not possible unless the bifurcation angle is larger than 70°. Dose heterogeneity using is more sensitive to the value of source gap than because source provides a sharper dose‐fall‐off than For both photon and beta emitters, there is no acceptable solution for bifurcation angles less than 30°, where the activity of the line source has a uniform distributions. Appropriate choice of gap at the junction can only help to reduce either overdosing of the main vessel or underdosing of the branch vessel, but not both.
Computerized analysis of shadowing on breast ultrasound for improved lesion detection
Authors: Drukker Karen , Giger Maryellen L. , Mendelson Ellen B.
Published: July 2003

Sonography is being considered for the screening of women at high risk for breast cancer. We are developing computerized detection methods to aid in the localization of lesions on breast ultrasound images. The detection scheme presented here is based on the analysis of posterior acoustic shadowing, since posterior acoustic shadowing is observed for many malignant lesions. The method uses a nonlinear filtering technique based on the skewness of the gray level distribution within a kernel of image data. The database used in this study included 400 breast ultrasound cases (757 images) consisting of complicated cysts, solid benign lesions, and malignant lesions. At a false‐positive rate of 0.25 false positives per image, a detection sensitivity of 80% by case (66% by image) was achieved for malignant lesions. The performance for the overall database (at 0.25 false positives per image) was less at 42% sensitivity by case (30% by image) due to the more limited presence of posterior acoustic shadowing for benign solid lesions and the presence of posterior acoustic enhancement for cysts. Our computerized method for the detection of lesion shadows alerts radiologists to lesions that exhibit posterior acoustic shadowing. While this is not a characterization method, its performance is best for lesions that exhibit posterior acoustic shadowing such as malignant and, to a lesser extent, benign solid lesions. This method, in combination with other computerized sonographic detection methods, may ultimately help facilitate the use of ultrasound for breast cancer screening.
Testing and optimization of a semiautomatic prostate boundary segmentation algorithm using virtual operators
Authors: Ladak Hanif M. , Wang Yunqiu , Downey Dónal B. , Fenster Aaron
Published: July 2003

Image analysis tasks such as size measurement and landmark‐based registration require the user to select control points in an image. The output of such algorithms depends on the choice of control points. Since the choice of points varies from one user to the next, the requirement for user input introduces variability into the output of the algorithm. In order to test and/or optimize such algorithms, it is necessary to assess the multiplicity of outputs generated by the algorithm in response to a large set of inputs; however, the input of data requires substantial time and effort from multiple users. In this paper we describe a method to automate the testing and optimization of algorithms using “virtual operators,” which consist of a set of spatial distributions describing how actual users select control points in an image. In order to construct the virtual operator, multiple users must repeatedly select control points in the image on which testing is to be performed. Once virtual operators are generated, control points for initializing the algorithm can be generated from them using a random number generator. Although an initial investment of time is required from the users in order to construct the virtual operator, testing and optimization of the algorithm can be done without further user interaction. We illustrate the construction and use of virtual operators by testing and optimizing our prostate boundary segmentation algorithm. The algorithm requires the user to select four control points on the prostate as input.
The value of scatter removal by a grid in full field digital mammography
Authors: Veldkamp Wouter J. H. , Thijssen Martin A. O. , Karssemeijer Nico
Published: July 2003

Our objective in this study was to investigate the usefulness of an anti‐scatter grid in digital mammography using a contrast detail phantom. The mammography system we investigated was a GE Senographe 2000D. We carried out phantom measurements under various conditions with and without using the anti‐scatter grid. A new version of the CDMAM phantom (version 3.4) was used. This phantom consists of a matrix of square cells with disks of varying size and contrast. For given exposure conditions detectability of these disks can be determined and used for construction of contrast detail curves. Previously, a computer program was developed at our institute that performs a fully automatic analysis of the phantom recordings using the ideal observer model. Breast thickness was simulated by a homogeneous layer of PMMA in the range of 1 to 7 cm. Series of images were recorded for different KeV and target–filter combinations depending on the simulated thickness. The dose was kept constant for each thickness with and without using a grid. It appeared that image quality improved for simulated breast thickness below 5 cm when the grid was removed. In the range from 5 to 7 cm contrast detail curves obtained with or without a grid were similar. Results suggest that for compressed breast thickness in the range of 1 to 7 cm a grid might not be needed in the digital mammography system we investigated. Below 5 cm, omitting the grid may allow lower dose to the patient without losing image quality.
Performance evaluation of a “dual‐side read” dedicated mammography computed radiography system
Authors: Fetterly Kenneth A. , Schueler Beth A.
Published: July 2003

The image quality of a dedicated mammography computed radiography (CR) system was characterized. A unique feature of this system is that it collects image signals from both sides of the storage phosphor. Measurements of the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) were made. This work included improvements in our measurement methods to specifically account for the detrimental effects of system glare on the MTF and to accurately characterize the low‐frequency NPS components. Image quality measurements were performed using a 25 kVp beam filtered with 2 mm Al and an exposure range of 1 to 100 mR (87 to 870 μGy). The DQE was found to decrease with increasing exposure due to an increased contribution of storage phosphor structure noise. The DQE of this system was compared to similar measurements made using a standard CR system. The dual‐side read system demonstrated superior DQE compared to the standard system. The decrease in DQE with increasing exposure was more severe for the standard system than the dual‐side read system. This finding suggests that the CR system noise was reduced for the dual‐side read system compared to the standard system.
Effect of applicator diameter on lesion size from high temperature interstitial ultrasound thermal therapy
Authors: Tyréus Per Daniel , Nau William H. , Diederich Chris J.
Published: July 2003

High temperature ultrasound thermal therapy using interstitial and external approaches is becoming increasingly acceptable as a minimally invasive clinical treatment for cancerous and benign disease. The diameter of an interstitial applicator can influence its clinical practicality and effectiveness as well as application site. The purpose of this study was to determine whether the use of larger ultrasound transducers and the inherent increase in applicator size could be justified by potentially producing larger lesion diameters. Four applicator configurations and sizes were studied using ex vivo tissue experiments in liver and beef and using acoustic and biothermal simulations. Catheter‐cooled and internally cooled applicators with outer diameters between 2.2 and 4.0 mm produced 3.5 to 5.0 cm diameter lesions in ex vivo liver and 3.0 to 3.5 cm lesions in ex vivo beef muscle with 20–40 W/cm applied for 10 min. Larger applicators produced lesions with radial penetration depths superior to their smaller counterparts at power levels in the 20–40 W/cm range. The higher cooling rates along the outer surface of the larger diameter applicators due to their greater surface area was a dominant factor in increasing lesion size. The higher cooling rates pushed the maximum temperature farther from the applicator surface and reduced the formation of high acoustic attenuation tissue zones. Applicator configuration and frequency (6.7–8.2 MHz) had less influence on lesion size than diameter in the ranges studied. Acoustic and biothermal simulations matched the experimental data well and were applied to model these applicators within sites of clinical interest such as prostate, uterine fibroid, brain, and normal liver. Lesions of 3.9 to 4.7 cm diameter were predicted for moderately perfused tissues such as prostate and fibroid and 2.8 to 3.2 cm for highly perfused tissues such as normal liver. In sites such as uterine fibroid where larger applicators placed using an endoscopic approach could be tolerated, treatment volume increases of 37% were predicted for an applicator diameter increase from 2.4 to 4.0 mm.
X‐ray scattering in full‐field digital mammography
Authors: Nykänen Kirsi , Siltanen Samuli
Published: July 2003

Effects of x‐ray scattering on full‐field digital mammography are analyzed with the scattering model of Seibert and Boone [Med. Phys.15, 567–575 (1988)]. A new method is introduced for the estimation of model parameters from measurements. It is shown that with breasts thinner than a certain threshold, removing the anti‐scatter grid leads to an improved contrast‐to‐noise ratio with a smaller patient dose. A fast approximate algorithm is presented for determining the scattered field in a gridless digital mammogram.
System performance of a prototype flat‐panel imager operated under mammographic conditions
Authors: Jee Kyung‐Wook , Antonuk Larry E. , El‐Mohri Youcef , Zhao Qihua
Published: July 2003

The results of an empirical and theoretical investigation of the performance of a high‐resolution, active matrix flat‐panel imager performed under mammographic conditions are reported. The imager is based upon a prototype, indirect detection active matrix array incorporating a discrete photodiode in each pixel and a pixel‐to‐pixel pitch of 97 μm. The investigation involved three imager configurations corresponding to the use of three different x‐ray converters with the array. The converters were a conventional ‐based mammographic phosphor screen (Min‐R) and two structured CsI:Tl scintillators: one optimized for high spatial resolution (FOS‐HR) and the other for high light output (FOS‐HL). Detective quantum efficiency for mammographic exposures ranging from to at 26 kVp were determined for each imager configuration through measurements of x‐ray sensitivity, modulation transfer function (MTF), and noise power spectrum (NPS). All configurations were found to provide significant presampling MTF at frequencies beyond the Nyquist frequency of the array, consistent with the high spatial resolution of the converters. In addition, the effect of additive electronic noise on the NPS was found to be significantly larger for the configuration with lower system gain (FOS‐HR) than for the configurations with higher gain (Min‐R, FOS‐HL). The maximum DQE values obtained with the CsI:Tl scintillators were considerably greater than those obtained with the Min‐R screen due to the significantly lower Swank noise of the scintillators. Moreover, DQE performance was found to degrade with decreasing exposure, although this exposure‐dependence was considerably reduced for the higher gain configurations. Theoretical calculations based on the cascaded systems model were found to be in generally good agreement with these empirically determined NPS and DQE values. In this study, we provide an example of how cascaded systems modeling can be used to identify factors limiting system performance and to examine trade‐offs between factors toward the goal of maximizing performance.
Generalized monitor unit calculation for the Varian enhanced dynamic wedge field
Authors: Liu Chihray , Kim Siyong , Kahler Darren L. , Palta Jatinder R.
Published: July 2003

The generalized monitor unit (MU) calculation equation for the Varian enhanced dynamic wedge (EDW) is derived. The assumption of this MU calculation method is that the wedge factor of the EDW at the center of the field is a function of field size, the position of the center of the field in the wedge direction, and the final position of the moving jaw. The wedge factors at the center of the field in both symmetric and asymmetric fields are examined. The difference between calculated and measured wedge factors is within 1.0%. The method developed here is easy to implement. The only datum required in addition to the standard set of conventional physical wedge implementation data is the off‐axis output factor for the open field in the reference condition. The off‐center point calculation is also examined. For the off‐center point calculation, the dose profile in the wedge direction for the largest EDW field is used to obtain the relative off‐center ratio in any smaller wedge field. The accuracy of the off‐center point calculation decreases when the point of calculation is too close to the field edge.
A systematic study of imaging uncertainties and their impact on prostate brachytherapy dose evaluation
Authors: Lindsay Patricia E. , Van Dyk Jake , Battista Jerry J.
Published: July 2003

In order to calculate the dose distribution delivered by a prostate brachytherapy implant, the seed positions and prostate volume are normally identified on post‐implant CT images. We have systematically considered the impact of uncertainties in contouring the prostate, seed localization, and visualization of all the seeds on the calculated dose distributions, dose–volume histograms, and predicted radiobiological outcome. This study was done for a collection of 27 clinical prostate brachytherapy implants, performed at the London Regional Cancer Centre during our early adoption of this technique. For these clinical dose distributions, the median D90 was 76% of the prescription dose, or 110 Gy, and the median V90 was 80%. We calculated the changes in these dosimetric indices (D90 and V90) and radiobiological outcome TCP) as a function of contouring uncertainty, seed localization uncertainty, inability to localize all of the seeds, and binary combinations of these three. The results are presented for a range of uncertainties, which allows the possible application of these results to a variety of imaging modalities that have differing spatial resolutions. We found that both contouring uncertainties and seed localization uncertainties had a large impact on the predicted radiobiological outcome, but that seed localization uncertainties of 6 mm had the largest impact on the dosimetric indices. We also found that the variability in both the predicted radiobiological and dosimetric outcome was largest for contouring uncertainties of 4–8 mm. We conclude that accounting for contouring uncertainties is crucial in accurately deducing the DVHs for post‐implant prostate brachytherapy, and hence enabling valid correlation with ultimate clinical outcome.
Comparison of IMRT optimization based on a pencil beam and a superposition algorithm
Authors: Scholz Christian , Nill Simeon , Oelfke Uwe
Published: July 2003

To investigate the role of sophisticated dose calculation methods for treatment planning, we compared conventional pencil beam optimized 6 and 15 MV intensity‐modulated treatment plans with optimizations based on the superposition technique. Five lung and five head and neck IMRT cases with spatial resolutions of bixels and dose voxels usually employed in clinical practice were considered for tumor volumes between 15 and 500 cm3. We investigated the systematic error of the pencil beam algorithm and the pencil beam induced error to the optimal solution of bixel weights. For the lung cases, the pencil beam overestimated the mean dose deposited inside the planning target volume (PTV) by about 8%, for small lung tumors even up to 20.6%. In the head and neck cases only a slight overestimation in mean PTV dose of 1.5% was observed. The optimization with the superposition method substantially improved the dose coverage of the considered radiation targets. Additionally, for the head and neck cases, the brainstem was significantly spared by about 4% mean PTV dose through the use of the superposition technique. Our studies showed that, in target regions with intricate tissue inhomogeneities, superposition or Monte Carlo techniques have to be used for the optimization and the final dose calculation of intensity‐modulated treatment plans.
Mammogram synthesis using a three‐dimensional simulation. III. Modeling and evaluation of the breast ductal network
Authors: Bakic Predrag R. , Albert Michael , Brzakovic Dragana , Maidment Andrew D. A.
Published: July 2003

A method is proposed for realistic simulation of the breast ductal network as part of a computer three‐dimensional (3‐D) breast phantom. The ductal network is simulated using tree models. Synthetic trees are generated based upon a description of ductal branching by ramification matrices (R matrices), whose elements represent the probabilities of branching at various levels of a tree. We simulated the ductal network of the breast, consisting of multiple lobes, by random binary trees (RBT). Each lobe extends from the ampulla and consists of branching ductal segments of decreasing size, and the associated terminal ductal‐lobular units. The lobes follow curved paths that project from the nipple toward the chest wall. We have evaluated the RBT model by comparing manually‐ traced ductal networks from 25 projections of ductal lobes in clinical galactograms and manually‐ traced networks from 23 projections of synthetic RBTs. A root‐mean‐square (rms) fractional error of 41%, between the R‐matrix elements corresponding to clinical and synthetic images, was computed. This difference was influenced by projection and segmentation artifacts and by the limited number of available images. In addition, we analyzed 23 synthetic trees generated using R matrices computed from clinical images. A comparison of these synthetic and clinical images yielded a rms fractional error of 11%, suggesting the possibility that a more appropriate model of the ductal branching morphology may be developed. Rejection of the RBT model also suggests the existence of a relationship between ductal branching morphology and the state of mammary development and pathology.
Test of GEANT3 and GEANT4 nuclear models for 160 MeV protons stopping in
Authors: Paganetti H. , Gottschalk B.
Published: July 2003

Monte Carlo simulations are used for many problems in proton radiation therapy, some of which are sensitive to the nuclear interaction model. The available models have been little tested in the regime of interest, namely in their ability to predict the secondary particle yield, including their angle and energy, when 70–250 MeV protons stop in various materials. The present study provides one such test in carbon, complementing a previous one in copper. Using a multilayer Faraday cup we have measured the projected range distribution of charged nuclear secondaries from 160 MeV protons stopping in polyethylene To test the popular GEANT Monte Carlo we have simulated the experiment with GEANT3 using the “Gheisha” (default) and “Fluka” models and with GEANT4.5 using the “low‐energy” and “precompound” models. The GEANT3/Fluka and GEANT4/precompound simulations agree moderately well with the observed range distribution. The data are given in a convenient form for testing other Monte Carlo programs.
Acoustic wave propagation in double porosity media
Authors: Olny Xavier , Boutin Claude
Published: July 2003

A study of the propagation of waves in porous media with an interconnected network of pores and micropores of very different characteristic sizes, saturated by a compressible Newtonian fluid, is proposed. With this aim, the homogenization technique for periodic separated scales media, is applied to realistic double porosity materials with motionless skeleton. From preliminary explicit estimations of wavelengths in the two fluid networks, it is shown that the macroscopic descriptions depend on the contrast of static permeability between pores and micropores and on frequency. The local equations are solved in the cases of low and high contrasts of permeability, and two main macroscopic behaviors are obtained. In the low contrast situation, the macroscopic flow is given by a kind of generalized Darcy’s law involving both pores and micropores, and their respective characteristic frequencies. Regarding compressibility effects, both pore networks act in parallel. The high permeability contrast reveals that the macroscopic flow law is governed by the pores. The microporous part of the material is submitted to pressure diffusion effects, bringing dissipation, and modifying the dynamic bulk modulus of the material. The two situations of coupling are illustrated for simple geometry of double porosity materials, including perforated-and slits-microporous materials.
Noise duration for a single overflight
Authors: Makarewicz Rufin , Wojciechowska Hanna
Published: July 2003

Overflights in national parks and preserves interfere with communication and sounds of nature. The percentage of time that an aircraft is audible, P, can be used as a noise metric. To calculate P the overflight time for a single aircraft, τ, has to be known. The method of τ calculation is based on the assumption that an aircraft is a point source and the noise propagation is governed by geometrical spreading, air absorption, and refraction. The atmosphere is characterized by the effective sound speed gradient. Analytical formulas for τ are derived for down- and crosswind flights.
The effect of trabecular material properties on the frequency dependence of backscatter from cancellous bone (L)
Authors: Wear Keith A.
Published: July 2003

Previous experimental studies indicate that backscatter coefficient for human calcaneal trabecular bone varies approximately as frequency cubed. This frequency dependence has been shown to be consistent with a model in which trabeculae are thought of as long thin cylinders composed of a substance with the same material properties as hydroxyapatite. The true material properties of human trabecular bone are not known however. Based on reported measurements of material properties of many bones and bonelike substances, it is possible that the density and longitudinal sound speed of trabecular bone material are far lower than the hydroxyapatite model would suggest. In this letter, it is shown that the frequency dependence of backscatter is still expected to be approximately cubic for wide ranges for density and longitudinal sound speed (encompassing the conceivable ranges for trabecular bone).
Experimental subwavelength localization of scatterers by decomposition of the time reversal operator interpreted as a covariance matrix
Authors: Prada Claire , Thomas Jean-Louis
Published: July 2003

The D.O.R.T. method (French acronym for Decomposition of the Time Reversal Operator) is an active remote sensing technique using arrays of antennas for the detection and localization of scatterers [Prada et al., J. Acoust. Soc. Am. 99, 2067–2076 (1996)]. The analogy between the time reversal operator and the covariance matrix used for classical sources separation in passive remote sensing [Bienvenu et al., IEEE Trans. ASSP 31, 1235–1247 (1983)] is established. Then, an experiment of subwavelength detection and localization of point-like scatterers with a linear array of transducers is presented. Using classical estimators in reception like Maximum-Likelihood and Multiple Signal Characterization (MUSIC), two point-like scatterers separated by λ/3 and placed at 100λ from the array of transducers are resolved. In these experiments, the role of multiple scattering and the existence of additional eigenvectors associated with dipolar and monopolar radiation of each scatterer is discussed.
Second derivative analysis of consonant-vowel transition waveforms (L)
Authors: Norian K. H.
Published: July 2003

The subject of this letter is the characterization of consonant-vowel syllables, through waveforms of the second derivative of the transition, to identify novel features within the transition. The second derivative of the first cycle of the transition region results in a modulated sinusoidal waveform with a spectral peak that is a function of the site of articulation of the consonant in the consonant-vowel syllable. The velars and alveolars give the lowest and highest spectral peaks, respectively, while the bilabial peaks occupy the mid-frequency range.