A computer-assisted system for guiding a surgical instrument in the body of a patient comprising: • a patient marker device configured to be arranged so as to be integral with a patient's body region to be treated by means of said surgical instrument, and including a tomography-transparent supporting frame fitted with tomography-opaque first marker elements and infrared-reflective, tomography-transparent second marker elements, said first and second marker elements (26, 27) being arranged on the supporting frame in pairs, with the first marker elements arranged within the paired second marker elements, and said first and second marker elements (26, 27) having a given real reciprocal arrangement; • an instrument marker device configured to be coupled to said surgical instrument and including infrared-reflective third marker elements; • an infrared locating sensor configured to locate said infrared-reflective second and third marker elements (27, 28) in a first reference system; and • a processing unit connected to the locating sensor and configured to: - acquire at least one tomography image of the patient's body region where the patient marker device is arranged, said tomography image including a representation of said tomography-opaque first marker elements in a second reference system different from the first reference system; - acquire the position of said infrared-reflective second and third marker elements (27, 28) in the first reference system provided by the infrared locating sensor; - reciprocally correlate the first and second reference systems so that the virtual reciprocal arrangement of the first and second marker elements (26, 27) in the second reference system corresponds to the real reciprocal arrangement thereof; and - determine the position of said infrared-reflective third marker elements in the second reference system on the basis of the correlation between the first and second reference systems. • a patient marker device configured to be arranged so as to be integral with a patient's body region to be treated by means of said surgical instrument, and including a tomography-transparent supporting frame fitted with tomography-opaque first marker elements and infrared-reflective, tomography-transparent second marker elements, said first and second marker elements (26, 27) being arranged on the supporting frame in pairs, with the first marker elements arranged within the paired second marker elements, and said first and second marker elements (26, 27) having a given real reciprocal arrangement; • an instrument marker device configured to be coupled to said surgical instrument and including infrared-reflective third marker elements; • an infrared locating sensor configured to locate said infrared-reflective second and third marker elements (27, 28) in a first reference system; and • a processing unit connected to the locating sensor and configured to: - acquire at least one tomography image of the patient's body region where the patient marker device is arranged, said tomography image including a representation of said tomography-opaque first marker elements in a second reference system different from the first reference system; - acquire the position of said infrared-reflective second and third marker elements (27, 28) in the first reference system provided by the infrared locating sensor; - reciprocally correlate the first and second reference systems so that the virtual reciprocal arrangement of the first and second marker elements (26, 27) in the second reference system corresponds to the real reciprocal arrangement thereof; and - determine the position of said infrared-reflective third marker elements in the second reference system on the basis of the correlation between the first and second reference systems. - acquire at least one tomography image of the patient's body region where the patient marker device is arranged, said tomography image including a representation of said tomography-opaque first marker elements in a second reference system different from the first reference system; - acquire the position of said infrared-reflective second and third marker elements (27, 28) in the first reference system provided by the infrared locating sensor; - reciprocally correlate the first and second reference systems so that the virtual reciprocal arrangement of the first and second marker elements (26, 27) in the second reference system corresponds to the real reciprocal arrangement thereof; and - determine the position of said infrared-reflective third marker elements in the second reference system on the basis of the correlation between the first and second reference systems.