“…The obvious advantage of an unsupervised method is that they do not require a training phase. Some initial candidate detection methods that have been proposed are Gaussian filters [4]- [6] or their variants [8], [17], [18], simple thresholding [15], [16], [19], Moat operator [20], double ring filter [9], mixture model-based clustering [21] 1D scan lines [13], [14], extended minima transform [11], [22], Hessian matrix Eigenvalues [7], [23], Frangi-based filters [24] and hit-or-miss transform [10].…”