Biomedical subjects

Peter R Snoeren

Publications and source records attributed to Peter R Snoeren.

6 recordsLinked to original sources

PubMed2007

Gray-scale and geometric registration of full-field digital and film-screen mammograms.

During the transition from traditional film-screen (FS) mammography to full-field digital (FFD) mammography, images from both modalities are used in hospitals and in mammography screening centers, as comparison of mammograms from subsequent examinations of a client is an important part of the diagnostic procedure. A parametric method is presented to register a FS mammogram and a FFD mammogram of the same woman with respect to geometry and gray-scales. The main motivation for the study is to lessen irrelevant differences between mammograms due to acquisition. First, a technique like this might increase the radiologist's ability to detect relevant differences like abnormal growth in breast tissue that signal breast cancer. Second, applications may be found in subtraction radiology or in computer-aided detection of abnormalities in temporal mammograms. The proposed method is based on a parametric model of the most important aspects of acquisition, which relates the pixel values of two images. This encompasses (1) breast positioning; (2) breast compression; (3) exposure time; (4) incident radiation intensity; and (5a) film properties and digitization for FS mammograms, or (5b) detector response for FFD mammograms. The method does not require a priori knowledge about specific settings of acquisition; the parameters are estimated from the two mammograms themselves.

Female↗

PubMed2007

Importance of comparison of current and prior mammograms in breast cancer screening.

PURPOSE: To retrospectively determine the influence of comparing current mammograms with prior mammograms on breast cancer detection in screening and to investigate a protocol in which prior mammograms are viewed only when necessary. MATERIALS AND METHODS: Institutional review board approval was not required. Participants gave written informed consent. Twelve experienced screening radiologists read 160 soft-copy screening mammograms twice, once with and once without prior mammograms. Eighty mammograms were obtained in women in whom breast cancer was diagnosed later; the other 80 mammograms had been reported as normal or benign. All cancers were visible in retrospect. Readers located potential abnormalities, estimated likelihood of malignancy for each finding, and indicated whether prior mammograms were considered necessary. The effect of prior mammograms on detection was determined by computing the mean lesion localized fraction in a range of low fractions of nonlesion locations corresponding to operating points in screening. Scores for both reading sessions were combined to assess the effect of making prior mammograms available only when requested. Data were analyzed by comparing the number of localized lesions between the two reading conditions with a paired two-tailed Student t test and applying a linear mixed model to test differences in average mean lesion localized fraction between reading conditions. P values less than .05 indicated statistical significance. RESULTS: Without prior mammograms, significantly more annotations were made. When only positive cases were considered, no difference was observed. Reading performance was significantly better when prior screening mammograms were available. At fixed lesion localized fraction, nonlesion localized fraction was reduced by 44% (P<.001) on average when prior mammograms were read. Performance was also increased for combined reading mode (ie, when prior mammograms were available on request only). However, this increase was smaller than that when prior mammograms were always available. Prior mammograms were requested in 24%-33% of all cases and were requested more often in positive cases. CONCLUSION: Comparison with prior mammograms significantly improves overall performance and can reduce referrals due to nonlesion locations. Limiting the availability of prior mammograms to cases selected by the reader reduces the beneficial effect of prior mammograms.

Aged↗

PubMed2006

Volumetric breast density estimation from full-field digital mammograms.

A method is presented for estimation of dense breast tissue volume from mammograms obtained with full-field digital mammography (FFDM). The thickness of dense tissue mapping to a pixel is determined by using a physical model of image acquisition. This model is based on the assumption that the breast is composed of two types of tissue, fat and parenchyma. Effective linear attenuation coefficients of these tissues are derived from empirical data as a function of tube voltage (kVp), anode material, filtration, and compressed breast thickness. By employing these, tissue composition at a given pixel is computed after performing breast thickness compensation, using a reference value for fatty tissue determined by the maximum pixel value in the breast tissue projection. Validation has been performed using 22 FFDM cases acquired with a GE Senographe 2000D by comparing the volume estimates with volumes obtained by semi-automatic segmentation of breast magnetic resonance imaging (MRI) data. The correlation between MRI and mammography volumes was 0.94 on a per image basis and 0.97 on a per patient basis. Using the dense tissue volumes from MRI data as the gold standard, the average relative error of the volume estimates was 13.6%.

Adult↗

PubMed2005

Interactions between binocular rivalry and Gestalt formation.

A question raised a long time ago in binocular rivalry research is whether the phenomenon of binocular rivalry is purely determined by local stimulus properties or that global stimulus properties also play a role. More specifically: do coherent features in a stimulus influence rivalrous behavior? After decades of underexposure of the subject, recently this question seemed to be answered in the affirmative. This paper presents additional evidence for an influence of coherent features. In an experiment in which eye movements cannot bias conclusions it is demonstrated that Gestalt formation influences binocular rivalry positively, i.e., stronger Gestalts have longer total dominance times. Gestalt formation appears to intervene in the states of dominance ("what"), not directly in the dominance durations ("how long"). This generates questions about the nature of interactions between binocular rivalry and Gestalt formation. Gestalt formation seems to be fed by signals that are generated after binocular convergence and only leaves its mark on binocular rivalry by feedback to monocular channels, a conclusion which has been drawn before by Alais and Blake [Alais, D., & Blake, R. (1998). Interaction between global motion and local binocular rivalry. Vision research 38, 637-644].

Adult↗

PubMed2004

Thickness correction of mammographic images by means of a global parameter model of the compressed breast.

Peripheral enhancement and tilt correction of unprocessed digital mammograms was achieved with a new reversible algorithm. This method has two major advantages for image visualization. First, the display dynamic range can be relatively small, and second, adjustment of the overall luminance to inspect details is not required in most cases. The correction is useful for preprocessing in computer-aided detection/diagnosis algorithms. The method is based on knowledge of the three-dimensional compressed breast shape to equalize thickness by adding virtual tissue, which results in intensity equalization for the mammographic image. Previously described methods implicitly estimate the contribution of thickness variations to image intensity, usually by nonparametric methods. The proposed method employs a global parametic breast shape model, which is advantageous for visualization and CAD.

Breast↗

PubMed2003

Gray scale registration of mammograms using a model of image acquisition.

A parametric technique is proposed to match the pixel-value distributions of two mammograms of the same woman. It can be applied to mammograms of the left and the right breast, or, more effectively, to temporal mammograms, e.g., from two screening rounds. The main reason to match mammograms is to lessen irrelevant differences between images due to acquisition: by varying breast compression, different film types, et cetera. Firstly, a technique like this might reduce the radiologist's efforts to detect relevant differences like abnormal growth in breast tissue that signals breast cancer. Secondly, though not the aim of this study, applications might be found in subtraction radiology or in the computer aided detection of abnormalities in temporal mammograms. Instead of arbitrarily shifting and/or scaling the pixel-values of one image to match the other or directly mapping one histogram to the other, the proposed method is based on general aspects of acquisition. This encompasses (1) breast compression; (2) exposure time; (3) incident radiation intensity; and, (4a) film properties and digitization for screen-film mammograms, or (4b) detector response for unprocessed digital mammograms. The method does not require a priori knowledge about specific settings of acquisition to match histograms; the degrees of freedom are estimated from the pixel-value distributions of the two mammograms themselves. By the method it is possible to match digitized screen-film mammograms (in the next also referred to as analog mammograms) as well as unprocessed digital mammograms in any of the four possible combinations: analog to analog, analog to digital, digital to analog, and digital to digital.

Algorithms↗