Scatter contamination of projection images in cone-beam computed tomography (CT) degrades the image quality. The use of bowtie filters in dedicated breast CT can decrease this scatter contribution. Three bowtie filter designs that compensate for one or more aspects of the beam-modifying effects due to differences in path length in a projection were studied. These designs have been investigated in terms of their ability to reduce the scatter contamination in projection images acquired in a dedicated breast CT geometry. The scatter magnitude was measured as the scatter-to-primary ratio (SPR) using experimental and Monte Carlo techniques for various breast phantom diameters and tube voltages. The results show that a 55% reduction in the center SPR value could be obtained with the bowtie filter designs. On average, the bowtie filters reduced the center SPR by approximately 18% over all breast diameters. The distribution of the scatter was calculated at a range of different locations to produce scatter distribution maps for all three bowtie filter designs. With the inclusion of the bowtie filters, the scatter distribution was more uniform for all breast diameters. The results of this study will be useful in designing scatter correction methods and understanding the benefits of bowtie filters in dedicated breast CT.
The scatter contamination of projection images in cone-beam computed tomography (CT) degrades image quality. The use of bowtie filters in dedicated breast CT can decrease this scatter contribution. Three bowtie filter designs that compensate for one or more aspects of the beam-modifying effects due to the differences in path length in a projection have been studied. The first produces the same beam-hardening effect as breast tissue with a single-material design. The second produces the same beam quality and intensity at the detector with a two-material design and the third eliminates the beam-hardening effect by adjusting the bowtie filter thickness such that the same effective attenuation is produced at the detector. We have selected aluminum, boron carbide/beryllium oxide, and PMMA as the materials for the previously described designs, respectively. These designs have been investigated in terms of their ability to reduce the scatter contamination in projection images acquired in a dedicated breast CT geometry. The magnitude of the scatter was measured as the scatter-to-primary ratio using experimental and Monte Carlo techniques. The distribution of the scatter was also measured at different locations in the scatter image to produce scatter distribution maps for all three bowtie filter designs. The results of this study will be useful in designing scatter correction methods and understanding the benefits of bowtie filters in dedicated breast CT.
For the last few years, development and optimization of three-dimensional (3D) x-ray breast imaging systems,
such as breast tomosynthesis and computed tomography, has drawn much attention from the medical imaging
community, either academia or industry. However, the trade offs between patient safety and the efficacy of the
devices have yet to be investigated with use of objective performance metrics. Moreover, as the 3D imaging
systems give depth information that was not available in planar mammography, standard mammography quality
assurance and control (QA/QC) phantoms used for measuring system performance are not appropriate since they
do not account for background variability and clinically relevant tasks. Therefore, it is critical to develop QA/QC
methods that incorporate background variability with use of a task-based statistical assessment methodology.1
In this work, we develop a physical phantom that simulates variable backgrounds using spheres of different
sizes and densities, and present an evaluation method based on statistical decision theory,2 in particular, with
use of the ideal linear observer, for evaluating planar and 3D x-ray breast imaging systems. We demonstrate
our method for a mammography system and compare the variable phantom case to that of a phantom of the
same dimensions filled with water. Preliminary results show that measuring the system's detection performance
without consideration of background variability may lead to misrepresentation of system performance.
A common method for evaluating projection mammography is Contrast-Detail (CD) curves derived from the CD
phantom for Mammography (CDMAM). The CD curves are derived either by human observers, or by automated
readings. Both methods have drawbacks which limit their reliability. The human based reading is significantly
affected by reader variability, reduced precision and bias. On the other hand, the automated methods suffer from
limited statistics. The purpose of this paper is to develop a simple and reliable methodology for the evaluation
of mammographic imaging systems using the Signal Known Exactly/Background Known Exactly (SKE/BKE)
detection task for signals relevant to mammography. In this paper, we used the spatial definition of the ideal,
linear (Hotelling) observer to calculate the task-specific SNR for mammography and discussed the results. The
noise covariance matrix as well as the detector response H matrix of the imaging system were estimated and
used to calculate the SNRSKEBKE for the simulated discs of the CDMAM. The SNR as a function of exposure,
disc diameter and thickness were calculated.
Simulations of digital imaging systems based on scintillator screens usually employ a Poisson model for the phosphor conversion gain. However, the statistics of the scintillation output are determined by complex phenomena that involve many sources of variability including inhomogeneities in the crystalline and screen structure, variations in the deposited energy for each primary quantum available for excitation, variations in the relationship between radiate and non-radiative decay processes, energy dependencies in the conversion gain variance, and spread of secondary quanta. We use a combined x-ray/electron/optical Monte Carlo code to study the statistics of the scintillation output in columnar phosphors. The simulation code is the result of merging the x-ray transport code PENELOPE and the optical transport code DETECT-II. Using an improved geometric model for the columnar structure, we present results concerning pulse-height spectra of the scintillation output (and corresponding Swank factors) as a function of x-ray energy. This study improves our understanding of the underlying causes of conversion gain variations and should facilitate more accurate simulation efforts for the investigation and optimization of image acquisition systems based on scintillator screens.
Our work investigates the effect on the zero-spatial-frequency DQE of indirect mammographic image receptors of the combined effects of x-ray beam hardening and light photon transport within the x-ray sensitive medium of the image receptor. Beam hardening, in this context, refers to the preferential absorption of low-energy x-ray photons at the surface of the detector on which the x-ray beam is incident, with deeper layers of the detector absorbing higher and higher average energies. The light photon transport properties of both powder and crystalline/columnar phosphors favor the collection at the light-sensitive element of the detector of light photons generated closest to that element. The net result of these two effects is to perform a weighting of the detected x-ray spectrum. For the standard back-screen configuration used in conventional mammography, low-energy x rays are more heavily weighted, matching the energy dependence of the signal to be detected. For a front-screen configuration, used in all existing indirect-detection digital mammographic systems, just the opposite is true. We have used the optical Monte Carlo simulation code DETECT-II to determine average light collection efficiency and optical pulse distributions for a Min-R screen in both front- and back-screen configurations, and for two thicknesses of CsI, as a function of x-ray energy. These data, along with appropriately hardened x-ray spectra for several anode and filter combinations and a range of tube voltages, were used as input to the task-dependent DQE theory of Tapiovaara and Wagner.
The measurement of image quality in diagnostic monochrome display devices has been the focus of recent efforts. However, there has not been a complete understanding of the capabilities of such methods to appropriately characterize display resolution and noise. In addition to being influenced by the luminance level of the scene being displayed, both properties are affected by the intrinsic details of the pixel emissive structures. In this paper, we report on the resolution and noise properties of cathode-ray tubes (CRTs) and active-matrix liquid crystal displays (AMLCDs). We present results on the resolution and noise characteristics of 5-megapixel monochrome CRTs and a 3-megapixel monochrome AMLCD with dual-domain in-plane switching pixel design. The CCD camera was calibrated using a uniform light source constructed specifically for excellent spatial uniformity. The methods employed for resolution are based on either the measurement of line patterns or the broadband transfer method. Noise is analyzed in terms of 2-dimensional noise power and signal-to-noise ratios. We found that all 4 methods considered are affected by residual deterministic structure in the recorded images. Especially in AMLCDs, the MTFs computed by the line method and broadband response method do not agree, due to contributions of the non-stochastic component that remain in the analyzed images after subtraction of a representative background.
We have investigated the transmission characteristics of an alternative all-optical-waveguide system for x-ray delivery to a precise tissue area. The delivery system includes two basic optical elements: a funnel-shaped uncoated hollow glass taper and a flexible hollow delivery waveguide. The hollow taper provides direct launching of the input x-ray radiation into a delivery waveguide. It is an uncoated glass taper whose operating principle is based on the grazing-incidence effect. We investigated both experimentally and theoretically how the transmission properties of the hollow taper depend on its geometrical parameters such as cone shape, length, input and output core diameters. The x-ray-source-to-taper coupling efficiency obtained was about 20-25%. That is relatively low in comparison with typical laser-to-taper coupling efficiencies due to the poorly collimated x-ray beam. Furthermore, we have studied the x-ray beam profile conversion by the grazing-incidence-based hollow taper. The x-ray radiation was launched into the delivery waveguide by a direct taper-to-waveguide coupling. In our experiment, we used both uncoated and metal-coated hollow waveguides with various geometrical parameters. The waveguide transmission characteristics, including the coupling efficiencies and beam profile conversion, were investigated for both straight and bent delivery waveguides. The results obtained as presented in this report give considerable confidence for successful application of the all-waveguide system as an alternative x-ray delivery technique for biomedical use.
Creatv MicroTech is developing two-dimensional, air-core, anti-scatter grids that have the potential to significantly reduce scatter-to-primary ratio and increase primary transmission in mammography. The fabrication method uses x-ray lithography and electroplating, which allows the fabrication of high aspect ratio metal parts. Two unfocused nickel grids were fabricated, one 1.5 cm X 1.5 cm and the other 1.44 cm X 1.44 cm. The grids have 20 micron thick walls and a period of 300 microns. Monte Carlo simulations were performed to predict their performance. The x-ray source was a 30 kVp Mo-anode spectrum and 30 microns of added Mo filtration. Preliminary calculations for a 2 mm-high grid and a 4 cm lucite phantom indicate that a scatter-to-primary ratio less than 3% can be achieved even at 3 cm from the center of the grid. Experiments to test the performance of the grids have been conducted at FDA using a Mo target, 30 micron Mo filter at 30 kVp and a 4 cm thick lucite phantom. A germanium detector was used. Data from a mammographic grid made by Smit Rontgen was taken as a reference. These Ni grids with grid ratios of 6.4 and 7.1 reduce scatter and increase primary transmission compared to the conventional reference grid. This fabrication method is capable of producing focused grids. The demonstration of larger, focused grids is the next step.
The purpose of this work is to evaluate the imaging and dose performance of an x-ray imaging system optimized for mammography. The x-ray system design was developed by the University of Southern California and the Center for Devices and Radiological Health using multiparameter optimization techniques. The prototype was built by Fischer Imaging and is now under evaluation at the National Institutes of Health. While previous reports have concentrated on demonstrating the does reduction potential of the system, for this study the x-ray spectrum was modified to maximize imaging performance. Measurements were made to assess the level of imaging performance achieved and to determine the increase in dose. Contrast-detail analysis along with qualitative evaluation of images of conventional mammography phantoms were used to assess imaging performance. Both imaging performance and the dose delivered by the system were compared to those of a conventional mammography system. Because of the current interest in digital mammography, the performance of the optimized system with a storage phosphor plate image receptor, sometimes referred to as computed radiography (CR), was also studied. The optimized system provided significantly better imaging performance than the conventional system with both film-screen and CR detectors. The dose was increased to a level comparable to the average value for conventional systems using grids.
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.
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.
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.
A computational approach is being developed for the evaluation of mammographic imaging system performance. This approach takes into account both the spatial frequency properties and the x-ray spectral characteristics of the system being evaluated. The initial version of the program that implements the approach has been used to evaluate a conventional mammography source assembly for several breast thicknesses, and to compare the conventional tube and filter combination to alternatives that have been suggested for the imaging of breasts that are thicker or more dense than average. It has also been used to study the effect of varying the thickness of the molybdenum filter in the conventional system. The parameters calculated include contrast, average glandular dose, tube load, and a figure of merit, SNR2/Dose. The calculations confirm the strong dependence of system performance on both tube potential and breast thickness for the standard system, and indicate that alternative designs can improve performance in the imaging of thicker or more dense breasts. The study of filter thickness shows that, of the four parameters calculated, only tube load is strongly affected by filter thickness.