Clinical ROC Studies of Digital Stereo Mammography

The objective of this study was to explore and document the diagnostic utility of digital stereo mammography for the detection of localized breast cancer in women. In it we character­ized the ability of experienced mammographers, general radiologists, and non-radiologists to detect three types of tumor masses embedded within a heterogeneous background of normal tis­sue elements in numerically simulated digital mammograms. The simulated mammograms were displayed to the subjects on a high resolution video display, both in stereo mode and in mono mode. Half of the mammograms contained a single tumor, ranging from 0.3 to 0.8 cm in maxi­mal diameter. Each reader rated 120 images (60 in stereo and 60 in mono) as to the probability of abnormality on scale of 1-5. Observer responses were evaluated using receiver operating characteristic (ROC) analysis to characterize any difference in diagnostic performance between the two viewing modes. The synthesized mammograms and the digital display were highly rated by the participant radiologists as promising tools for future research. The results of ROC analysis, however, indicated no significant difference in tumor detection when the same readers utilized the stereo mode versus the mono mode (Az mono = 0.833 versus, Az stereo = 0.826). The results were similar for readers of all 3 experience levels--mammographers, general radiolo­gists, and non-radiologists

A computer model to predict strength-duration curve parameters from measured data

The threshold strength for an electric or magnetic (electrodeless) stimulus is related to its duration by the strength-duration (s-d) curve. Two common mathematical expressions are used to describe the s-d curve; the first is exponential in form while the second is hyperbolic in form. Classically, the rectangular pulse has been used to determine strength-duration curves. However, recent applications have required the use of more complex waveforms, and the ability of these s-d curves to adequately describe tissue response to more complex waveforms has been questioned. Single-axon and single-cell models, first introduced by Hodgkin and Huxley (1952), were used in several computer models to simulate the process of tissue excitation. The single-cell models contain parameters empirically derived from voltage-clamped measurements on single-cells. It may be inappropriate to use the parameters derived from single-cells to model excitation in more complex geometry tissue. These observations have caused us to re-examine the classical view of stimulation, and in particular, to appreciate the need for a more sophisticated framework in which to interpret results. Our goal is to simplify the existing models based on single-cell and single-axon responses and use it to extend the s-d model in its exponential form to enable prediction of the s-d curve for non-rectangular stimuli. Our new model is called The Extended and Simplified Non-Linear (ESN) model for tissue stimulation . The ESN model contains parameters, analogous to those forming the basis of the single-cell and single-axon models, but differs from the existing models in that the value of key parameters are calculated directly from data measured in complex vital systems. In a subsequent effort we adapt the parameters extracted from the experimental data to build a computer program to predict thresholds for complex stimulus waveforms. A view of the proposed model can be divided in two sections, the first is a backward direction one in which values for the parameters are extracted from measured data. The second part of the model is a forward direction model, in which the parameters from the first part have been adapted in a special formulation to be used in predicting stimulation thresholds for complex waveforms