  We developed a source detection algorithm based on the Minimal Spanning Tree
(MST), that is a graph-theoretical method useful for finding clusters in a
given set of points. This algorithm is applied to gamma-ray bidimensional
images where the points correspond to the arrival direction of photons, and the
possible sources are associated with the regions where they clusterize. Some
filters to select these clusters and to reduce the spurious detections are
introduced. An empirical study of the statistical properties of MST on random
fields is carried in order to derive some criteria to estimate the best filter
values. We introduce also two parameters useful to verify the goodness of
candidate sources. To show how the MST algorithm works in the practice, we
present an application to an EGRET observation of the Virgo field, at high
galactic latitude and with a low and rather uniform background, in which
several sources are detected.
