1. Apparatus for estimating diameter of an artery having a centerline and an inner wall, comprising: (a) means for acquiring an ultrasonic image of the artery, the image-acquiring means having an imaging plane, the image-acquiring means being positioned so as to orient the imaging plane skew to the artery, whereby it intersects the centerline of the artery; (b) edge-detecting means, coupled to the image-acquiring means, for detecting a plurality of near-edge points and a plurality of far-edge points on the acquired image of the artery, (c) curve-fitting means, coupled to the edge-detecting means, for fitting a first curve to the plurality of near-edge points and a second curve to the plurality of far-edge points; and (d) means, coupled to the curve-fitting means, for measuring at least one distance between the first and second curves and for estimating diameter of the artery from the at least one distance.