**9. Conclusions**

Ultrasound has been used for many years as a diagnostic and interventional imaging modality, and the use is increasing in a number of different clinical areas. It is often conceded that the image quality of ultrasound is inferior to that attainable with MR or CT, but the rapid devel‐ opment of new ultrasound technology (scanners, transducers, specialized probes, etc.) has resulted in significantly improved image quality and make ultrasound the modality of choice for several applications. Some of the obvious advantages being real-time imaging even for blood flow, portability, flexibility, safety and low cost. In addition, ultrasound images can be acquired in the coordinate system of a patient when combined with a tracking system without any need for registration. This makes surgical guidance based on intra-operative ultrasound highly accurate. The combination of several image modalities such as MR, CT and ultrasound registered to each other and to the patient make the interpretation of the individual images easier and enables the surgeon to take advantage of the complimentary information contained in each image. In this context, the ultrasound images provide real-time information in the region of interest, while MR and CT provide anatomical overview facilitating the interpretation of the ultrasound data. The use of contrast agents enhance the visualization of vessels and increase the number and types of lesions that can be detected using ultrasound. New technol‐ ogies such as high-intensity focused ultrasound and the use of microbubbles for targeted drug delivery are examples of non-invasive therapeutic applications where ultrasound will play an increasingly important role in the future.
