Adding Curvature to Minimum Description Length Shape Models

The Minimum Description Length (MDL) approach to shape modelling seeks a compact description of a set of shapes in terms of the coordinates of marks on the shapes. It has been shown that the mark positions resulting from this optimisation to a large extent solve the so-called point correspondence problem: How to select points on shapes defined as curves so that the points correspond across a data set. However, this MDL approach does not capture important shape characteristics related to the curvature of the curves, and occasionally it places marks in obvious conflict with the human notion of point correspondence. This paper shows how the MDL approach can be fine-tuned by adding a term to the cost function expressing the mismatch of curvature features across the data set. The method is illustrated on silhouettes of adult heads. The MDL method is able to solve the point correspondence problem and a classification of the heads into male and female improves dramatically when using the MDL-generated marks

Automatic determination of Greulich and Pyle bone age in healthy Dutch children

Background: Bone age (BA) assessment is a routine procedure in paediatric radiology, for which the Greulich and Pyle (GP) atlas is mostly used. There is rater variability, but the advent of automatic BA determination eliminates this. Objective: To validate the BoneXpert method for automatic determination of skeletal maturity of healthy children against manual GP BA ratings. Materials and methods: Two observers determined GP BA with knowledge of the chronological age (CA). A total of 226 boys with a BA of 3-17 years and 179 girls with a BA of 3-15 years were included in the study. BoneXpert's estimate of GP BA was calibrated to agree on average with the manual ratings based on several studies, including the present study. Results: Seven subjects showed a deviation between manual and automatic BA in excess of 1.9 years. They were re-rated blindly by two raters. After correcting these seven ratings, the root mean square error between manual and automatic rating in the 405 subjects was 0.71 years (range 0.66-0.76 years, 95% CI). BoneXpert's GP BA is on average 0.28 and 0.20 years behind the CA for boys and girls, respectively. Conclusion: BoneXpert is a robust method for automatic determination of BA

Ace of Bayes: Application of Neural Networks with Pruning

MacKay's Bayesian framework for backpropagation is a practical and powerful means of improving the generalisation ability of neural networks. The framework is reviewed and extended in a pedagogical way. The notation is simplified using the ordinary weight decay parameter, and the noise parameter fi is shown to be nothing more than an overall scale. A detailed and explicit procedure for adjusting several weight decay parameters is given. Pruning is incorporated into the Bayesian framework. Appropriate symmetry factors on sparse architectures are deduced. Bayesian weight decay is demonstrated using artificial data generated by a sparsely connected network. Pruning yields computational advantages: by removing unimportant weights the posterior weight distribution becomes Gaussian, and pruning removes zero-modes of the Hessian and redundant hidden units. In addition, pruning improves generalisation. The Bayesian evidence is used as a stop criterion for pruning. Bayesian backprop is applied ..