A computer-assisted ultrasound navigation system comprising an instrumental unit cooperating with: - an ultrasound device provided with an ultrasound probe to capture a plurality of two-dimensional ultrasound images of a concerned region; - an infrared or electromagnetic tracking sensor adapted to detect the position of a sensor coupled to the ultrasound probe and provided with infrared marker elements (M-I-U) or electromagnetic marker elements (M-E-U); said instrumental unit being configured to detect the position of the sensor in the reference system of the tracking sensor and to orient the captured ultrasound images (C-2D image), the system being characterized by implementing: a) importing means which import a plurality of two-dimensional images, obtained by means of X-ray tomography CT or magnetic resonance MR techniques and related to a full scan of a concerned anatomic region, into the memory of the computer; said two-dimensional images are preferably in DICOM (Digital Imaging and COmmunications in Medicine) format; b) first reconstruction means, which starting from imported two-dimensional images, form a three-dimensional reconstruction of the entire concerned anatomic region by generating a reconstructed three-dimensional image A; c) transformation means, which transform said three-dimensional image A into a three-dimensional pseudo ultrasound image B formed by a plurality of two-dimensional pseudo ultrasound images (B-2D); d) scanning means by means of which at least one two-dimensional ultrasound image (e) of the concerned anatomic region is captured by manually moving said ultrasound probe with respect to said concerned region; said scanning means being further adapted to detect the position of the sensor coupled to the probe in the reference system of the tracking sensor by orienting the captured two-dimensional ultrasound image in space, which is stored; e) comparison means configured to extract the two-dimensional pseudo ultrasound image (image B-2D) most similar to said stored ultrasound image (C-2D); f) selection means of characteristic points on said previously captured, detected and stored two-dimensional ultrasound image (C-2D) and on the extracted two-dimensional pseudo ultrasound image (B-2D); g) alignment means (250,260) that, according to the selected characteristic points, perform a realignment operation between the reference system of the three-dimensional pseudo ultrasound image (image B) and thus of image A and that of the operative space in which the ultrasound probe operates, so that the identified characteristic points coincide in the respective reference systems of the two two-dimensional images. - an ultrasound device provided with an ultrasound probe to capture a plurality of two-dimensional ultrasound images of a concerned region; - an infrared or electromagnetic tracking sensor adapted to detect the position of a sensor coupled to the ultrasound probe and provided with infrared marker elements (M-I-U) or electromagnetic marker elements (M-E-U); a) importing means which import a plurality of two-dimensional images, obtained by means of X-ray tomography CT or magnetic resonance MR techniques and related to a full scan of a concerned anatomic region, into the memory of the computer; said two-dimensional images are preferably in DICOM (Digital Imaging and COmmunications in Medicine) format; b) first reconstruction means, which starting from imported two-dimensional images, form a three-dimensional reconstruction of the entire concerned anatomic region by generating a reconstructed three-dimensional image A; c) transformation means, which transform said three-dimensional image A into a three-dimensional pseudo ultrasound image B formed by a plurality of two-dimensional pseudo ultrasound images (B-2D); d) scanning means by means of which at least one two-dimensional ultrasound image (e) of the concerned anatomic region is captured by manually moving said ultrasound probe with respect to said concerned region; said scanning means being further adapted to detect the position of the sensor coupled to the probe in the reference system of the tracking sensor by orienting the captured two-dimensional ultrasound image in space, which is stored; e) comparison means configured to extract the two-dimensional pseudo ultrasound image (image B-2D) most similar to said stored ultrasound image (C-2D); f) selection means of characteristic points on said previously captured, detected and stored two-dimensional ultrasound image (C-2D) and on the extracted two-dimensional pseudo ultrasound image (B-2D); g) alignment means (250,260) that, according to the selected characteristic points, perform a realignment operation between the reference system of the three-dimensional pseudo ultrasound image (image B) and thus of image A and that of the operative space in which the ultrasound probe operates, so that the identified characteristic points coincide in the respective reference systems of the two two-dimensional images.