No abstract

The purpose of these experiments was to estimate basic sensitivity to motion gradients, and to evaluate the evidence for second-order integration and differentiation of motion signals. We measured sensitivity to spatially sinusoidal contrast modulation between two oppositely-moving bandpass-filtered noise images. The motion-contrast sensitivity function, defined as the inverse of threshold modulation amplitude as a function of modulation spatial frequency, was band-pass in shape with declines at both highest and lowest frequencies. The functions for three noise spatial frequencies were approximately the same shape when modulation frequency was expressed as a fraction of the noise frequency. We compared the data to a model in which linear motion filters, whose outputs are squared or rectified, are followed by a second stage of excitatory and/or inhibitory pooling. The data are consistent with a model in which 1) all excitatory pooling occurs at the linear stage, and 2) the second stage contains a large inhibitory pooling area, with a radius about 8 times that of the linear receptive field.

No abstract

The precision of quantitative and subjective evaluations of phantom image quality has been studied. Twenty-seven images of the American College of Radiology (ACR) mammography accreditation phantom were acquired under different x-ray techniques and digitized. Several quantitative image quality measures were obtained from each image by analyzing microcalcification and nodule target objects in the phantom. All images were also scored subjectively by 8 observers, each of whom provided a count of the number of objects seen in each target class (fibrils, microcalcifications, and nodules). An analysis was performed to predict the subjective measurements from the quantitative measurements and to estimate their variabilities. It was found that the subjective measures could be well predicted by the quantitative measures and that the variance of the quantitative measures was significantly smaller than that of the subjective measure, by almost a factor of 10. The implication for the ACR accreditation program for mammography is that a substantial improvement is possible in the image quality evaluation process by performing computerized analysis of the phantom images in addition to subjective analysis.

No abstract