Search

The initial flow field of an impulsively started incompressible viscous flow over an elliptic cylinder is analyzed by the method of matched asymptotic expansions. Analytical solutions for the outer and inner flow fields are obtained to the third order. The Symbolic Manipulation Program is used to facilitate the work. Flow separation is studied in some detail, as are the surface quantities such as the vorticity, the pressure distribution, the drag, and the lift coefficients.

Direct digital mammography (DDM) is currently undergoing clinical trial in the United States. The likely outcome of the current clinical trials is that DDM will be found to have appropriate diagnostic accuracy for clinical use for both screening and diagnostic use. Telemammography and the electronic storage of breast images will be shown to be feasible. Computer readable images will be found to help the further implementation of computer assisted breast cancer detection and in the classification of lesions in the breast that could be cancer into benign and malignant categories. Over the next five years new machine and software developments will show that DDM has diagnostic advantages over conventional screen film mammography, especially when coupled with computer aided detection and classification.

Storage phosphor imaging is one of the first clinically applied technologies for full breast digital mammography and is a potential new method for screening and diagnostic mammography [1],[2]. In this study, we compared how well radiologists were able to detect small breast cancers on digital mammograms using storage phosphor (SP) plate technology versus on conventional film-screen (FS) mammograms.

A recently completed ROC study of digital mammography using a 100 micron pixel storage phosphor receptor showed that digital mammography and conventional screen film mammography were essentially equivalent in areas under the ROC curve. In this study, there were 24 biopsy proven breast cancer cases, 25 benign biopsy cases and 48 clinically normal breast images each with matched screen film and storage phosphor images. Fifteen of the 24 cancer cases were 10 mm or less in size. Of these 10 presented with microcalcifications as the sign of disease. Six radiologists not involved with the research program and without prior experience with digital mammography and who met qualification criteria under the Mammography Quality Standards Act of 1992 served as readers. This poster looks at the cases in which there was variance between the radiologists ROC classification system for the digital and screen film system in order to analyze case specific discrepancies that may indicate benefits or deficits of the digital system. Aspects of the ROC ratings are also analyzed including an evaluation of the different thresholds used by radiologists on the digital and screen film systems, the distribution of ROC ratings in normal and abnormal cases, the effect of using different gold standards of proof on the results and the effect of substituting an ACR BIRADS category agreement study as proposed by the FDA compared to the ROC study outcome.

The sensitivity of mammography for the detection of breast cancer is decreased in the radiodense breast. Storage phosphor digital radiographic systems have a wider latitude of exposure than conventional mammographic screen film systems. By using low resolution histogram equalization one can produce a mammographic image of the breast that retains the high frequency information that defines the edges of microcalcifications, architectural distortion and some masses but which, at the same time, allows one to look through into regions of increased breast radiodensity and identify microcalcifications within them. This paper demonstrates the effect of this special form of image processing.

Microcalcifications can be identified on mammograms in approximately 50 - 55% of breast cancer cases. Three factors affect the ability to use the presence of microcalcifications as a sign of cancer. They must be seen (conspicuity), their shape must be assessed (to differentiate benign and malignancy associated shapes) and they should be countable since the greater the number of clustered calcifications, the more likely they are malignancy associated. Concern has been expressed that digital systems with their inherently worse resolution would not allow adequate shape information to be captured. Using a 100 micron pixel size storage phosphor system we randomly selected 20 cases, 10 benign and 10 showing malignancy on biopsy and asked four radiologists to assess the calcifications present comparing the original screen film and the digital images and using the screen film biopsy specimen radiograph as ground truth. The preferences were mixed with some radiologists preferring screen film and others the digital images. Whatever their preferences, the radiologists were unable to use the shape criteria to distinguish benign and malignant cases in this case sample.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We are examining the feasibility of performing digital mammography by combining a storage- phosphor image receptor with a highly efficient x-ray system. The image receptor consists of Fuji series HR-V high resolution imaging plates and a Fuji 9000 reader. The x-ray system was developed using multiparameter optimization techniques, with the goal of reducing patient dose as much as possible while retaining acceptable imaging performance. We have measured sensitometric properties, modulation transfer function (MTF), and noise power spectrum (NPS) of the Fuji plates with low-energy x-ray spectra. We have used the measurements, along with information about the x-ray system, to estimate signal-to-noise ratios (SNRs) for objects in a contrast-detail (C-D) phantom. We present the results of our measurements on the Fuji plates, comparisons of calculated and observed C-D diagrams for this system and a conventional system, and comparisons of phantom images and doses for this system to images and doses for a conventional system. We conclude that digital mammography with the system studied is at least feasible since phantom image quality is comparable to that of a conventional system at dose levels that are somewhat lower.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The purpose of this work is to examine the feasibility of dual-energy techniques to enhance the detection of microcalcifications in digital mammography. The digital mammography system used in this study consists of two different mammography systems; one is the conventional mammography system with molybdenum target and Mo filtration and the other is the clinical version of a low dose x-ray system with tungsten target and aluminum filtration. The low dose system is optimized for screen-film mammography with a highly efficient scatter rejection device built by Fischer Imaging Systems for evaluation at NIH. The system was designed by the University of Southern California based on multiparameter optimization techniques. Prototypes of this system have been constructed and evaluated at the Center for Devices and Radiological Health. The digital radiography system is based on the Fuji 9000 computed radiography (CR) system which uses a storage phosphor imaging plate as the receptor. High resolution plates (HR-V) are used in this study. Dual-energy is one technique to reduce the structured noise associated with the complexity of the background of normal anatomy surrounding a lesion. This can be done by taking the advantage of the x-ray attenuation characteristics of two different structures such as soft tissue and bone in chest radiography. We have applied this technique to the detection of microcalcifications in mammography. The overall system performance based on this technique is evaluated. Results presented are based on the evaluation of phantom images.

It has been stated that digital mammography will reduce the exposure required for mammography. This poster explores the effects of decreased exposure on the information present in digital mammography. In general, the digital system performed better than screen film mammography with lower exposures. With the usual exposures used for screen film mammography, performance was equal. With high exposures sufficient to result in a dark film (OD 1.5), the digital system performed better than screen film with very small test objects. Proposals have been made to decrease the tube loading required for slot scanning devices by increasing KVP. This would result in their being less object contrast due to the decreases in the absorption coefficient of calcium compared to water at higher KVP. This poster looks at the potential for correcting the loss in object contrast that would result from the use of high contrast look up tables. It was found that in the tested system, one could restore the information in one of the two test objects used (but not the other), but that the image processing methods used would result in an image that radiologists would probably find inadequate for interpretation.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Projection radiography, based on the storage phosphor (SP) imaging medium is a promising but challenging technology. SP plates consist of phosphor particles that are embedded in a polymer binder like film and then coated onto a flexible backing. Image quality is dependent not only on the performance of the plate, but also on the handling of the plate-cassette combination by the user and the reading system itself. The imaging plates included in this study are the 14' X 17' and 10' X 12' standard plates (ST-V) used for general radiography and the high resolution (HR-V) plates used in mammography and for extremity exams. The cassettes or image plate holders correspond to the plate sizes with the 14' X 17' being lead backed. Recorded downtime of the reader due to plate-cassette operation failures, diagnostic image quality affected by plate artifacts, and plate and cassette replacement rates are for a period of twelve months. Records are maintained through a detailed maintenance log, the image reader's internal record for plate use, and the system's internal error log for physical malfunctions. By keeping a detailed log on maintenance and performance of specific system components, communication to the vendor has been timely and effective in solving equipment failures. Plate-cassette maintenance must be an integral part of overall quality control (QC) that includes technologist training, physical plant (clinical environment) changes, and routine evaluation of the operation and performance of CR system components. Quality control combined with equipment improvements have minimized the downtime of the system, image artifacts and replacement rates of plates and cassettes.

Improvements in mammography equipment related to a decrease in pixel size of digital mammography detectors raise questions of the possible effects of these new detectors. Mathematical modeling suggested that the benefits of moving from 100 to 50 micron detectors were slight and might not justify the cost of these new units. Experiments comparing screen film mammography, a storage phosphor 100 micron digital detector, a 50 micron digital breast spot device, 100 micron film digitization and 50 micron film digitization suggests that object conspicuity should be better for digital compared to conventional systems, but that there seemed to be minimal advantage to going from 100 to 50 microns. The 50 micron pixel system appears to provide a slight advantage in object contrast and perhaps in shape definition, but did not allow smaller objects to be detected.

Computed radiography (CR) based on storage phosphor (SP) detector is a new and complex technology for obtaining medical digital imaging. The CR system was developed such that its image quality in diagnosis (wide latitude, certainty of visual diagnosis), speed (image sensitivity), image processing, and the overall imaging performance exceeds or at least equals those of the conventional screen-film (SF) radiography systems which are considered to be `gold standard.' The unique capability of the CR system places a new responsibility on the medical physicist, radiologist, and radiologic technologist to ensure that the digital images give the same information at least as film, and improve rather than degrade the image. For that reason the quality of the image must be ensured and each component of the system must function properly. The clinical knowledge about quality control (QC) and the standard procedures for CR devices have not yet been established and are still under development. This paper presents the acceptance/QC of the CR image reader and laser imager. The procedure includes the acceptance of the equipment for its image quality, image sensitivity, and overall imaging performance. The procedures are currently and routinely being utilized in our institution.

The conventional x-ray source for mammography, with a molybdenum (Mo) anode and Mo filter, works well for breasts of low to moderate x-ray attenuation, but is not readily adaptable to the production of higher x-ray energies that are more suitable for imaging breasts of higher attenuation. Accordingly, alternative sources with anodes of rhodium (Rh) and tungsten (W) have been developed to improve the efficiency of the examination for thick or radiographically dense breasts. We have applied previously developed multiparameter optimization techniques to imaging systems using these alternative x-ray sources. Since these sources are intended to improve mammography of high-attenuation breasts, optimizations were performed for a range of breast thicknesses. Since high attenuation is generally associated with high scatter, optimizations for each source were done with a high-ratio, air-interspace grid similar to the one developed in our previous work. Preliminary results have been obtained for optimized system configurations using a W-anode source with Mo, Rh, and aluminum (Al) filters, and for a Mo-anode source with Rh filtration. These results indicate that the alternative sources studied can significantly improve the efficiency of mammography of high-attenuation breasts.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Digital mammography is likely to replace conventional mammography within a few years. In anticipation of this, our group has been exploring the implications of image processing in digital mammography. Some of our findings are reported here.

Previously in this forum we have reported the application of multiparameter optimization techniques to the design of a minimum dose mammography system. The approach used a reference system to define the physical imaging performance required and the dose to which the dose for the optimized system should be compared. During the course of implementing the resulting design in hardware suitable for laboratory testing, the state of the art in mammographic imaging changed, so that the original reference system, which did not have a grid, was no longer appropriate. A reference system with a grid was selected in response to this change, and at the same time the optimization procedure was modified, to make it more general and to facilitate study of the optimized design under a variety of conditions. We report the changes in the procedure, and the results obtained using the revised procedure and the up- to-date reference system. Our results, which are supported by laboratory measurements, indicate that the optimized design can image small objects as well as the reference system using only about 30% of the dose required by the reference system. Hardware meeting the specification produced by the optimization procedure and suitable for clinical use is currently under evaluation in the Diagnostic Radiology Department at the Clinical Center, NH.

Analytical expressions have been written for image quality in mammography. Multiparameter optimizations have been conducted to find mammographysystems providing the lowest patient dose for a given image quality. The optimizations are subject to constraints imposed by technology, such as power limits on the tube focal spot, absorption efficiency related to detector resolution, and others. The optimizations permit system geometry, kVp, filtration, detector resolution, focal spot size, and grid characteristics to vary simultaneously and self‐consistently subject to the constraints. A system configuration approaching a factor of 3 dose reduction has been found without assuming radical technological advances. The system satisfies image quality constraints for both large and small targets and would be possible to implement clinically. The sensitivity of the results to the assumptions made in the modeling has been investigated.

The Navier-Stokes and energy equations for the steady, incompressible, laminar flow past a circular cylinder at constant temperature are solved by expressing the temperature as well as the stream function in truncated Fourier series. The partial differential equations are reduced to a system of simultaneous ordinary differential equations, which are then numerically integrated. The Reynolds numbers Re, based on the diameter, range from 1 to 40; and the Prandtl number Pr for air is taken as 0.72. Navier-Stokes solutions at large distances from the cylinder obtained by coordinate expansion are used as velocity boundary conditions at infinity. This type of boundary condition at infinity is shown to be more appropriate than the free stream or Oseen approximation. Heat transfer in terms of local and mean Nusselt numbers are computed and compared with available numerical and experimental data.

Analytical expressions have been written for image quality in mammography. A multiparameter optimization has been conducted to find the mammography system requiring the lowest patient dose for a given image quality. The optimization is subject to constraints imposed by technology such as; power limits on tube focal spots, absorption efficiency related to detector resolution and others. The optimization permits system geometry, kVp, filtration, detector resolution, focal spot size and grid characteristics to vary simultaneously and self consistently subject to an exposure time constraint. Significant dose reductions compared to current mammography systems have been found without assuming radical technological advances.

We have been considering the question of calculating, conveniently but with precision, significant basic image quality indicators for mammography. As a result of this work, a number of questions have been addressed regarding both the philosophy and the practicability of such calculations. One way to make a prediction is to define a phantom, determine a primary photon fluence, obtain the scatter fluence from empirical correlations, define a target of interest and make a straightforward calculation. The other extreme is to define a phantom and target and do a complete Monte-Carlo calculation. The first is too approximate for making close calls between various system configurations. The second, although it can be very useful, tends to gather in one place the weaknesses of both experiments and theory. On one hand the Monte-Carlo calculations are unwieldy like experiments; on the other, like any theory they can only represent the physics that is included. It is always necessary to verify the calculation by experiments. For example, scatter radiation in mammography has a simple energy dependency but its angular distribution is quite complicated due to interference phenomena and also quite important. The first approach mentioned above would certainly miss the complication, as experiments have in the past. The Monte-Carlo calculations also miss the complication because, for practical reasons, their angular resolution is generally too low and in addition the appropriate physics is not included. We will describe our approach to the modelling of mammography. It is a combination of prediction techniques, based on variables chosen to be convenient for describing the physics, and experiments to verify uncertainties in the modelling. We will also discuss the point of greatest vulnerability of either calculations or experiments - the phantom. Our thesis is that all possible phantoms and targets can and should be treated in generalized combinations, which can be used to raise the level of usefulness of either experiments or predictions.

The application of numerical multiparameter optimization techniques tolthe design of improvqd mammographic imaging systems has been discussed by Jafroudi et al., and by Muntz et al.. The approach taken requires the specification of a reference system for dios/p and imaging performance comparisons. The optimized designs that have been reported are based on a reference system without a grid. Since those designs were developed, it has become common practice to use grids in mammography. Accordingly, the optimization has been repeated with a current state-of-the-art system, including a grid, as the reference. An approximation to the revised optimized design has been assembled and compared to a Thomson-CGR Senographe 500T mammography unit equipped with a grid. When the optimized system is operated under conditions that produce imaging performance comparable to that of the Senographe, the average glandular dose delivered by the optimized system is about 1/3 the dose delivered by the Senographe.