A computer-implemented technique for calculating a position of a surgical device relative to a patient region imaged by an imaging device is described. One of the surgical device and the imaging device has a predetermined first positional relationship to a marking, and a first camera having a field of view that includes the marking has a predetermined second positional relationship to the other device. The positional relationships may be stored as calibration data. A method realization of the technique comprises the steps of receiving, from the imaging device, intra-operatively taken first image data representative of the imaged patient region, receiving, from the first camera, intra-operatively taken second image data representative of the marking, and intra-operatively calculating, based on the first and second image data and the calibration data, the position of at least a part of the surgical device relative to the imaged patient region.