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Real-time ultrasound imaging of the lumbar multifidus muscle was performed in 48 normal subjects (21 males, 27 females) aged 18-35 ycars. Measuremetits of multifidus cross-sectional area (CSA) and shape were symmetrical between the right and left sides of the spine. Muscle shape differed between the males and females. Two measurements (linear dimensions) of the muscle cross-section were closely correlated with CSA in both groups (males r=0.98; females r = 0 . 9 3 ) , but this relationship needs to be determined in wasted muscles where changes in shape may occur. Correlations between CSA and height and weight differed between males and females. Measurements were repeatable between days (CV= 6%) and between scans (CV=4.9%). The present study indicates that real-time ultrasound may be clinically useful for measuring multifidus muscle wasting, but larger scale studies are required to establish definitive reference ranges of data in different age groups of normal subjects. Documentation of changes in symmetry of multifidus muscle size and shape with back pain could then be performed.

A method for building flexible shape models is presented in which a shape is represented by a set of labelled points. The technique determines the statistics of the points over a collection of example shapes. The mean positions of the points give an average shape and a number of modes of variation are determined describing the main ways in which the example shapes tend to deform from the average. In this way allowed variation in shape can be included in the model. The method produces a compact flexible 'Point Distribution Model' with a small number of linearly independent parameters, which can be used during image search. We demonstrate the application of the Point Distribution Model in describing two classes of shapes.