Digital Mammography Applications Research Articles

Pulse pileup analysis for a double-sided silicon strip detector using variable pulse shapes.

Due to pulse pileup, photon counting detectors (PCDs) suffer from count loss and energy distortion when operating in high count rate environments. In this paper, we studied the pulse pileup of a double-sided silicon strip detector (DSSSD) to evaluate its potential application in a mammography system. We analyzed the pulse pileup using pulses of varied shapes, where the shape of the pulse depends on the location of photon interaction within the detector. To obtain the shaped pulses, first, transient currents for photons interacting at different locations were simulated using a Technology Computer-Aided Design (TCAD) software. Next, the currents were shaped by a CR-RC2 shaping circuit, calculated using Simulink. After obtaining these pulses, both the different orders of pileup and the energy spectrum were calculated by taking into account the following two factors: 1) spatial distribution of photon interactions within the detector, and 2) time interval distribution between successive photons under a given photon flux. We found that for a DSSSD with thickness of 300 μm, pitch of 25 μm and strip length of 1 cm, under a bias voltage of 50 V, the variable pulse shape model predicts the fraction free of pileup can be > 90 % under a photon flux of 3.75 Mcps/mm2. The double-sided silicon-strip detector is a promising candidate for digital mammography applications.

Validation of a modified PENELOPE Monte Carlo code for applications in digital and dual-energy mammography

The presence and morphology of microcalcification clusters are the main point to provide early indications of breast carcinomas. However, the visualization of those structures may be jeopardized due to overlapping tissues even for digital mammography systems. Although digital mammography is the current standard for breast cancer diagnosis, further improvements should be achieved in order to address some of those physical limitations. One possible solution for such issues is the application of the dual-energy technique (DE), which is able to highlight specific lesions or cancel out the tissue background. In this sense, this work aimed to evaluate several quantities of interest in radiation applications and compare those values with works present in the literature to validate a modified PENELOPE code for digital mammography applications. For instance, the scatter-to-primary ratio (SPR), the scatter fraction (SF) and the normalized mean glandular dose (DgN) were evaluated by simulations and the resulting values were compared to those found in earlier studies. Our results present a good correlation for the evaluated quantities, showing agreement equal or better than 5% for the scatter and dosimetric-related quantities when compared to the literature. Finally, a DE imaging chain was simulated and the visualization of microcalcifications was investigated.

Automated Percentage of Breast Density Measurements for Full-field Digital Mammography Applications

Increased mammographic breast density is a significant risk factor for breast cancer. A reproducible, accurate, and automated breast density measurement is required for full-field digital mammography (FFDM) to support clinical applications. We evaluated a novel automated percentage of breast density measure (PDa) and made comparisons with the standard operator-assisted measure (PD) using FFDM data. We used a nested breast cancer case-control study matched on age, year of mammogram and diagnosis with images acquired from a specific direct x-ray conversion FFDM technology. PDa was applied to the raw and clinical display (or processed) representation images. We evaluated the transformation (pixel mapping) of the raw image, giving a third representation (raw-transformed), to improve the PDa performance using differential evolution optimization. We applied PD to the raw and clinical display images as a standard for measurement comparison. Conditional logistic regression was used to estimate the odd ratios (ORs) for breast cancer with 95% confidence intervals (CI) for all measurements; analyses were adjusted for body mass index. PDa operates by evaluating signal-dependent noise (SDN), captured as local signal variation. Therefore, we characterized the SDN relationship to understand the PDa performance as a function of data representation and investigated a variation analysis of the transformation. The associations of the quartiles of operator-assisted PD with breast cancer were similar for the raw (OR: 1.00 [ref.]; 1.59 [95% CI, 0.93-2.70]; 1.70 [95% CI, 0.95-3.04]; 2.04 [95% CI, 1.13-3.67]) and clinical display (OR: 1.00 [ref.]; 1.31 [95% CI, 0.79-2.18]; 1.14 [95% CI, 0.65-1.98]; 1.95 [95% CI, 1.09-3.47]) images. PDa could not be assessed on the raw images without preprocessing. However, PDa had similar associations with breast cancer when assessed on 1) raw-transformed (OR: 1.00 [ref.]; 1.27 [95% CI, 0.74-2.19]; 1.86 [95% CI, 1.05-3.28]; 3.00 [95% CI, 1.67-5.38]) and 2) clinical display (OR: 1.00 [ref.]; 1.79 [95% CI, 1.04-3.11]; 1.61 [95% CI, 0.90-2.88]; 2.94 [95% CI, 1.66-5.19]) images. The SDN analysis showed that a nonlinear relationship between the mammographic signal and its variation (ie, the biomarker for the breast density) is required for PDa. Although variability in the transform influenced the respective PDa distribution, it did not affect the measurement's association with breast cancer. PDa assessed on either raw-transformed or clinical display images is a valid automated breast density measurement for a specific FFDM technology and compares well against PD. Further work is required for measurement generalization.

Effects of breast thickness and lesion location on resolution in digital magnification mammography

This study aimed to examine the resolution effects of breast thickness and lesion location in magnification mammography by evaluating generalized modulation transfer function (GMTF) including the effect of focal spot, effective pixel size, and the scatter. Polymethyl methacrylate (PMMA) thicknesses ranging from 10 to 40 mm were placed on a standard supporting platform that was positioned to achieve magnification factors ranging from 1.2 to 2.0.As the magnification increased, the focal spot MTF degraded, while the detector MTF improved. The GMTF depended on the trade-off between the focal spot size and effective pixel size. Breast thickness and lesion location had little effect on the resolution at high frequencies. The resolution of small focal spot did improve slightly with increasing PMMA thickness for magnification factors less than 1.8. In contrast, system resolution decreased with increasing PMMA thickness for magnification factors greater than 1.8 since focal spot blur begins to dominate spatial resolution. In particular, breast thickness had a large effect on the resolution at lower frequencies. A low-frequency drop effect increased with increasing PMMA thickness because of the increase in scatter fraction. Hence, the effect of compressed breast thickness should be considered for the standard magnification factor of 1.8 that is most commonly used in clinical practice. Our results should provide insights for determining optimum magnification in clinical application of digital mammography, and our approaches can be extended to a wide diversity of radiological imaging systems.

Evaluation of the Luminescence Efficiency of YAG:Ce Powder Scintillating Screens for Use in Digital Mammography Detectors

In the present study scintillating screens prepared from Y3Al5O12:Ce (YAG:Ce) powder phosphor were evaluated for use in digital mammography. YAG:Ce has never previously been used in X-ray medical imaging, however since it emits green light (i.e peak at 550 nm), it is expected to match well the spectral sensitivities of most photodetectors (photodiodes, CCDs and amorphous silicon sensors) incorporated in various digital mammography detectors. YAG:Ce was purchased in powder form and was used in order to prepare test screens in the laboratory. Screens were evaluated by determining the quantum detection efficiency (QDE), the energy absorption efficiency (EAE), the absolute luminescence efficiency, the X-ray luminescence efficiency (XLE), the light emission spectrum, the X-ray to light intrinsic conversion efficiency and the spectral compatibility with various photodetectors. Results were compared with phosphor materials commercially employed in X-ray imaging such as CsI:Tl and Gd2O2 S:Tb. Maximum YAG:Ce emission efficiency was observed for the 63 mg/cm2 screen at 49 kVp. The spectral compatibility with amorphous silicon photodiodes (0.93) and CCDs (0.95) was found to be very high, better than the corresponding compatibility of the CsI:Tl screen, mostly used in current digital radiography detectors. Taking into account the YAG:Ce overall performance, its short decay time as well as its spectral compatibility with amorphous silicon detectors and CCDs, YAG:Ce could be of interest for further investigation for applications in digital mammography.

Study of the availability of digital mammography for premenopausal women in the Valencian Breast Cancer Early Detection Program

In 1992, the Valencian Breast Cancer Screening Program (VBCSP) started in the Valencian Community. To date, 24 mammography units have been installed all over the region. There is a health risk in the studied women that has to be estimated and controlled. A methodology to calculate approximately the radiological detriment in the VBCSP has been developed based on Monte Carlo techniques. As qualitative parameters in the program, the average mean glandular doses from representative sample populations undergoing screening mammography (digital or film) from each of the 24 units in operation have been obtained. The American College of Radiology Imaging Network reached the conclusion that digital mammography performed significantly better than film for premenopausal and perimenopausal women younger than 50 years old [1]. Our group uses the software SCREENRISK [2] to estimate the induction/detection rates in order to corroborate American conclusions in a European region. The obtained results confirm the American results about the application of digital mammography in premenopausal and perimenopausal women younger than 50 years old.

Performance of a medical imaging system for photons in the 60–140 keV energy range

We report the status of the art of a prototype based on a GaAs pixel detector bump-bonded to a dedicated VLSI chip to be possibly used for imaging in the nuclear medicine field. This device, with a 200 μm thick pixel matrix (64×64 square pixels, 170 μm side), has already been tested with very good results for digital mammography applications (mean energy 20 keV). For more energetic photons, as in nuclear medicine, a 600 μm thick detector has been chosen. Using radioactive sources ( 241 Am, 60 keV and 99 m Tc, 140 keV photons) we have measured the performance of our prototype in terms of charge collection and detection efficiency of the detector, discrimination capability of the electronics and imaging properties of the whole system. In particular, we have evaluated the spatial resolution properties measuring the Point Spread Function and the imaging capabilities using a home made thyroid phantom. We present also the comparison between these results and those obtained with a traditional gamma camera and the evaluation, made by both experimental measurements and software simulations, of the geometry related to the use of a collimator.

Wavelets in temporal and spatial processing of biomedical images.

We review some of the most recent advances in the area of wavelet applications in medical imaging. We first review key concepts in the processing of medical images with wavelet transforms and multiscale analysis, including time-frequency tiling, overcomplete representations, higher dimensional bases, symmetry, boundary effects, translational invariance, orientation selectivity, and best-basis selection. We next describe some applications in magnetic resonance imaging, including activation detection and denoising of functional magnetic resonance imaging and encoding schemes. We then present an overview in the area of ultrasound, including computational anatomy with three-dimensional cardiac ultrasound. Next, wavelets in tomography are reviewed, including their relationship to the radon transform and applications in position emission tomography imaging. Finally, wavelet applications in digital mammography are reviewed, including computer-assisted diagnostic systems that support the detection and classification of small masses and methods of contrast enhancement.

Simulation model of mammographic calcifications based on the american college of radiology breast imaging reporting and data system, or BIRADS

The authors developed and evaluated a method for the simulation of calcification clusters based on the guidelines of the Breast Imaging Reporting and Data System of the American College of Radiology. They aimed to reproduce accurately the relative and absolute size, shape, location, number, and intensity of real calcifications associated with both benign and malignant disease. Thirty calcification clusters were simulated by using the proposed model and were superimposed on real, negative mammograms digitized at 30 microns and 16 bits per pixel. The accuracy of the simulation was evaluated by three radiologists in a blinded study. No statistically significant difference was observed in the observers' evaluation of the simulated clusters and the real clusters. The observers' classification of the cluster types seemed to be a good approximation of the intended types from the simulation design. This model can provide simulated calcification clusters with well-defined morphologic, distributional, and contrast characteristics for a variety of applications in digital mammography.

Multiresolution probability analysis of gray-scaled images

Digitized images can be decomposed into a sum of images by use of a multiresolution wavelet expansion. Each image of the expansion can be analyzed with a parametric statistical model for the histogram associated with each expansion component. The statistical analysis of the individual expansion components is relatively simple, whereas the analysis of the original image is complicated. We show that the probability model for each expansion component can be approximated with a Laplace probability-density function for some important applications in digital mammography. An approach to random variable analysis based on the predominant low-frequency characteristics of the random fields provides theoretical support for the approximation. The theoretical framework of multiresolution analysis provides a natural extension for modeling many levels of image detail simultaneously for special cases. We demonstrate this with a noise field simulation and a mammographic application, where the expansion components are treated as independent random variables.

A high-rate X- Y coincidence VLSI system for 2-D imaging detectors

We describe a VLSI-based processing system performing X- Y coincidence at a very high rate in the synchronous mode. The processor has been designed for applications in digital mammography with two-sided μ-strip silicon detectors but can be used to provide the computational section of any imaging system that needs fast coincidence. The VLSI chip has been successfully tested on a 32 + 32 channels DAQ chain.

Expanding the dynamic range of x-ray videodensitometry using ordinary image digitizing devices

While x-ray images on film have a wide dynamic range of up to three optical density units (o.d.) or 1:1000 variation in transparency, many ordinary digitizing video systems have a limited dynamic range of ~1.4 o.d. in actual practice. The digital images thus either lack details in the dark areas or are saturated and bloom in the bright areas and have a poor contrast ratio. We present here an object-dependent digital image processing method to overcome this drawback by appropriately selecting and correcting pixels drawn from two images acquired from the same radiograph at different light levels. A calibrated gray scale is used to compensate for the nonlinearity of the video system and derive the true optical density for each pixel. The method should find applications in digital mammography, teleradiology, archiving of radiographs, and videodensitometry in general.