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Knowledge-based Segmentation and Feature Analysis of Hand-wrist Radiographs 1

Important procedures exist in paediatric radiology for the assessment of skeletal maturity from radiographs of a child's hand. Typically, these procedures involve comparison of a radiograph with textual and pictorial information from an atlas. This is a tedious and time-consuming process, and there has been widespread interest in the possibility of automation using computers. We have devised a segmentation algorithm which makes it possible to automate the TW2 method for skeletal maturity assessment. Our algorithm exploits knowledge of the anatomical structure of the hand and capitalises on the fact that large structures (the radius, the ulna, the hand itself) are easier to detect than smaller, fainter features (e.g., the carpals). We begin by separating the hand from the background in a radiograph and verifying, in as eecient a manner as possible, that it is normal and suitable for further analysis. At this stage, we can also isolate those regions of the image corresponding to the ngers, thereby tightly constraining attempts to segment the associated phalanges. Observables such as nger orientation can be propagated inward to constrain the position and orientation of other bones. Other techniques applied at this early stage include analysis of one-dimensional projections of grey level data within the hand. This yields further information on the location of features such as the metacarpals and the capitate. Detection of bone boundaries is achieved using deformable models. We have developed an approach which uses template matching techniques to initialise a snake, the latter then being deformed subject to externally-imposed constraints. A parametrically deformable model has also been investigated; here, the primary constraints on model deformation are internal, being derived through a process of training. We have shown that many of the written criteria used in the TW2 method can be readily implemented in software, e.g., as rules in an expert system. Ultimately, it may be possible to augment or even replace certain of these criteria with more quantitative tests based on shape analysis of the detailed bone boundary data obtained from segmentation. The measurement of boundary curvature could be particularly useful in this regard.