A computer-assisted system (100) for guiding a surgical instrument (25) in the body of a patient comprising a first marker device (22) configured to be arranged so as to be integral with a patient's body region and including first and second marker elements (26, 27) having a given reciprocal arrangement； a second marker device (24) configured to be coupled to the surgical instrument (25) and including third marker elements (28)； a locating sensor (20) configured to position the second and third marker elements (27, 28) in a first reference system； and a processing unit (4) configured to acquire at least one tomography image of the patient's body comprising the first marker elements (26) in a second reference system different from the first, acquiring the position of the second and third marker elements (27, 28) in the first reference system, determine the position of said third marker elements (28) in the second reference system on the basis of a correlation between the first and the second reference systems.
