Quantitative MR imaging using "LiveWire" to measure tibiofemoral articular cartilage thickness.

OBJECTIVE: To assess the reliability and accuracy of manual and semi-automated segmentation methods for quantifying knee cartilage thickness. This study employed both manual and LiveWire-based semi-automated segmentation methods, ex vivo and in vivo, to measure tibiofemoral (TF) cartilage thickness. METHODS: The articular cartilage of a cadaver knee and a healthy volunteer's knee were segmented manually and with LiveWire from multiple 3T MR images. The cadaver specimen's cartilage thickness was also evaluated with a 3D laser scanner, which was assumed to be the gold standard. Thickness measurements were made within specific cartilage regions. The reliability of each segmentation method was assessed both ex vivo and in vivo, and accuracy was assessed ex vivo by comparing segmentation results to those obtained with laser scanning. RESULTS: The cadaver specimen thickness measurements showed mean coefficients of variation (CVs) of 4.16%, 3.02%, and 1.59%, when evaluated with manual segmentation, LiveWire segmentation, and laser scanning, respectively. The cadaver specimen showed mean absolute errors versus laser scanning of 4.07% and 7.46% for manual and LiveWire segmentation, respectively. In vivo thickness measurements showed mean CVs of 2.71% and 3.65% when segmented manually and with LiveWire, respectively. CONCLUSIONS: Manual segmentation, LiveWire segmentation, and laser scanning are repeatable methods for quantifying knee cartilage thickness; however, the measurements are technique-dependent. Ex vivo, the manual segmentation error was distributed around the laser scanning mean, while LiveWire consistently underestimated laser scanning by 8.9%. Although LiveWire offers repeatability and decreased segmentation time, manual segmentation more closely approximates true cartilage thickness, particularly in cartilage contact regions.

On the psychophysics of the shape triangle.

We earlier introduced an approach to categorical shape description based on the singularities (shocks) of curve evolution equations. We now consider the simplest compositions of shocks, and show that they lead to three classes of parametrically ordered shape sequences, organized along the sides of a shape triangle. By conducting several psychophysical experiments we demonstrate that shock-based descriptions are predictive of performance in shape perception. Most significantly, the experiments reveal a fundamental difference between perceptual effects dominated by when shocks form with respect to one another, versus those dominated by where they form. The shock-based theory provides a foundation for unifying tasks as diverse as shape bisection, recognition, and categorization.

Parts of visual form: psychophysical aspects.

Part-based representations allow for recognition that is robust in the presence of occlusion, movement, growth, and deletion of portions of an object, and play an important role in theories of object categorization and classification. A partitioning theory for visual form is proposed that is based on two types of parts: limb-based parts arise from a pair of negative curvature minima with evidence for "good continuation' of boundaries on one side; neck-based parts arise from narrowings in shape. The motivation for this model is computational requirements for recognition. The psychophysical relevance of this model is addressed by measuring intrasubject and intersubject consistency in partitioning tasks and comparing perceived and computed parts. A series of experiments were performed in which subjects were required to partition a variety of biological and nonsense two-dimensional shapes into perceived components. Specifically, it was examined (1) whether a subject determines components consistently across different trials of the same partitioning task, (2) whether there is evidence for consistency between subjects for the same partitioning task, and (3) how the perceived parts compare with limbs and necks resulting from the computational model. The results are interpreted as suggesting that there are high levels of both intrasubject and intersubject consistency and that a large majority of the perceived parts do in fact correspond to the parts computed on the basis of our model. The implications of our model are discussed in relation to previous experimental results. Intuitive observations concerning the relationship between parts of visual form and their function are then presented. Finally, a role is envisioned for parts in figure/ground segregation; the notion of a "parts receptive field' through which parts can serve as an intermediate representation between local image features, eg edges, and global object models, is suggested.