Abstract: Apparatus for determining a proper insertion depth of a biopsy needle so that a sampling end of the needle just reaches to a designated target area within the body comprises at least one straight calibrated pointing device aligned to point through the selected point in a straight line passing through the designated target region. The pointing device exhibits first and second markers along its length such that respective images are formed on a first image plane by utilizing radiation from a radiation source, along with images corresponding to the selected point and the target area. The images are formed along a straight line in this order: (A) the first marker, (B) the second marker, (C) the selected point, and (D) the target region. The apparatus includes an arrangement for measuring distances on the image plane between images (A), (B), (C), and (D) and a calculator for calculating the cross ratio of the distances, whereby the proper insertion depth of the biopsy needle is determined.