950 resultados para explicit instruction


Relevância:

20.00% 20.00%

Publicador:

Resumo:

Given the adverse impact of image noise on the perception of important clinical details in digital mammography, routine quality control measurements should include an evaluation of noise. The European Guidelines, for example, employ a second-order polynomial fit of pixel variance as a function of detector air kerma (DAK) to decompose noise into quantum, electronic and fixed pattern (FP) components and assess the DAK range where quantum noise dominates. This work examines the robustness of the polynomial method against an explicit noise decomposition method. The two methods were applied to variance and noise power spectrum (NPS) data from six digital mammography units. Twenty homogeneously exposed images were acquired with PMMA blocks for target DAKs ranging from 6.25 to 1600 µGy. Both methods were explored for the effects of data weighting and squared fit coefficients during the curve fitting, the influence of the additional filter material (2 mm Al versus 40 mm PMMA) and noise de-trending. Finally, spatial stationarity of noise was assessed.Data weighting improved noise model fitting over large DAK ranges, especially at low detector exposures. The polynomial and explicit decompositions generally agreed for quantum and electronic noise but FP noise fraction was consistently underestimated by the polynomial method. Noise decomposition as a function of position in the image showed limited noise stationarity, especially for FP noise; thus the position of the region of interest (ROI) used for noise decomposition may influence fractional noise composition. The ROI area and position used in the Guidelines offer an acceptable estimation of noise components. While there are limitations to the polynomial model, when used with care and with appropriate data weighting, the method offers a simple and robust means of examining the detector noise components as a function of detector exposure.

Relevância:

20.00% 20.00%

Publicador:

Resumo:

History of the Office of Superintendent of Public Instruction in Iowa from when Iowa was a Territory to 1938

Relevância:

20.00% 20.00%

Publicador:

Relevância:

20.00% 20.00%

Publicador:

Resumo:

Although functional neuroimaging studies have supported the distinction between explicit and implicit forms of memory, few have matched explicit and implicit tests closely, and most of these tested perceptual rather than conceptual implicit memory. We compared event-related fMRI responses during an intentional test, in which a group of participants used a cue word to recall its associate from a prior study phase, with those in an incidental test, in which a different group of participants used the same cue to produce the first associate that came to mind. Both semantic relative to phonemic processing at study, and emotional relative to neutral word pairs, increased target completions in the intentional test, but not in the incidental test, suggesting that behavioral performance in the incidental test was not contaminated by voluntary explicit retrieval. We isolated the neural correlates of successful retrieval by contrasting fMRI responses to studied versus unstudied cues for which the equivalent "target" associate was produced. By comparing the difference in this repetition-related contrast across the intentional and incidental tests, we could identify the correlates of voluntary explicit retrieval. This contrast revealed increased bilateral hippocampal responses in the intentional test, but decreased hippocampal responses in the incidental test. A similar pattern in the bilateral amygdale was further modulated by the emotionality of the word pairs, although surprisingly only in the incidental test. Parietal regions, however, showed increased repetition-related responses in both tests. These results suggest that the neural correlates of successful voluntary explicit memory differ in directionality, even if not in location, from the neural correlates of successful involuntary implicit (or explicit) memory, even when the incidental test taps conceptual processes.