The most commonly used tracers for SLN mapping are radioactive isotopes such as technetium-99m, which allows lymphatic mapping using 2D planar scintigraphy and/or 3D SPECT/CT imaging. Unfortunately, these images are mostly available only preoperatively and cannot be used directly during surgery. Nevertheless, small, hand-held one-dimensional gamma-probes [1, 13] or two-dimensional gamma cameras can be used during the SLNB procedure [25, 21]. However, intra-operative 3D nuclear imaging would improve the overall quality of the surgery and the SLN detecting rates by providing depth information in contrast to the two hand-held radiation detecting devices [18].