Numerous reports on the utility of iUS as a tool for intraoperative navigation exist. Moiydi and Shetty (2011) evaluated the use of conventional 2D iUS in 77 cases of both cranial and spinal lesions. In their hands, iUS was useful in identifying and delineating lesions in 100% of cases, in planning the durotomy in 78% of cases, in planning the corticectomy in 85% of cases and in visualizing adjacent structures in 96% of cases, respectively. They concluded that iUS is a useful tool in planning various stages of tumor resection. In a study comparing conventional iUS and navUS, we could show, that orientation can even be improved by navigating the US probe, thus facilitation anatomical understanding (Miller et al. 2007). Moreover navUS was useful in planning the correct transsulcal approach to subcortical lesions in eloquent areas (Zhou et al. 2009) and in visualizing structures adjacent to tumors. This was most obvious in cystic gliomas (Enchev et al. 2006), where an opening of a cyst during surgery might lead to a local tissue shift. With the aid of navUS, vessels that might be encased or displaced by tumor tissue could be visualized and landmarked as reported by our group (Sure et al. 2005). Rygh et al. (2006) showed that 3D US was helpful in visualizing hidden vessels adjacent to and inside the tumor thus facilitating tailored approaches and increasing the safety of biopsies. Similar to navigated 2D US, navigated 3D US solved the orientation problem experienced with conventional 2D US in neurosurgery as it allowed axial, coronal and sagittal reconstructions and a direct comparison of iUS and preoperative MRI in the same plane of view (Miller et al. 2012; Unsgaard et al. 2002). Three-dimensional US could be used as a navigational tool even without preoperative MRI in cases where no preoperative MRI was available, an intraoperative brain shift occurred or in case of a failure of the navigation system (Miller et al. 2011; Peredo-Harvey et al. 2012).

Deformations of the brain during surgery may be due to loss of cerebro-spinal fluid, swelling or retraction. These deformations may range from 2 to 25 mm depending on the size, the location and the kind of tumor (Lindner et al. 2006).

Several groups worked on compensating brain shift via a co-registration of iUS and preoperative MRI (Coenen et al. 2005; Reinertsen et al. 2007). Coenen et al. (2005) defined ultrasound landmarks in the vicinity of the tumor and the nearby fiber tracts and used a sequential landmark registration to assess fiber tract deformation. They could show on postoperative diffusion-weighted imaging that the actual fiber tract position at the end of surgery corresponded to the sonographically predicted fiber tract position. Reinertsen et al. (2007) validated a vessel-based registration technique for the correction of brain shift. There are multiple reports on other possible co-registration- and deformation algorithms, however large patient trials are still lacking. Further studies are needed to create robust deformation models to allow fusion of MRI and iUS.

Tsutsumi et al. (1989) first achieved real-time imaging in single burr hole procedures in 1989. Histologic diagnosis could be established in 87–95% of cases of US-guided tumor biopsies via a single burr hole trepanation (Benediktsson et al. 1992; Di Lorenzo et al. 1991; Lunardi et al. 1993; Strowitzki et al. 2000).

As shown by our group and others, iUS can be used for a differential diagnosis of different intradural spinal tumors. While intramedullary tumors showed an inhomogeneous pattern, extramedullary tumors were displayed with a homogeneous signal on iUS. Extramedullary tumors were sharply demarcated from the myelon in all cases. Moreover, iUS proved to be useful in displaying the size and exact location of the lesion, thus allowing adjusting the extension of the bony opening (Regelsberger et al. 2005; Zhou et al. 2011).