Many different imaging modalities have been used for image-guided navigation, including ultrasound, x-ray imaging such as fluoroscopy and computed tomography (CT), and magnetic resonance imaging (MRI). Most image-guided navigation systems, particularly those systems that have been commercialized by the major vendors, rely on preoperative tomographic imaging such as CT or MRI. Tomographic imaging can provide an exquisite three-dimensional map of the anatomy, and these images can be displayed to the physician as described in the visualization section below.

While most image-guided navigation systems are based on preoperative imaging, there is an increasing interest in using intraoperative imaging for navigation. One trend in this direction has been the development of cone-beam CT imaging (CBCT), which provides CT-like capabilities using a fluoroscopy system. A fluoroscopy system consists of an x-ray source and detector, which are placed on opposite sides of a patient, and provides projection images of the patient. By rotating the C-arm on which the source and detector are mounted, multiple projections from different angles can be obtained and used to reconstruct a tomographic data set. CBCT has become a mainstay of many interventional suites, and commercial vendors such as Philips and Siemens have introduced navigation systems based on this technology. CBCT can also be used with mobile C-arms that can be brought into the operating room to provide navigation for surgical procedures. This concept was pioneered by Medtronic Inc., (Minneapolis, MN) with the O-arm system, which consists of a CBCT and associated navigation hardware and software [12].

In the future, we can expect to see more use of multiple imaging modalities and image fusion to aid in navigation. This trend is already seen in the clinical environment with the widespread acceptance of PET-CT imaging, which combines positron emission tomography (PET) with CT imaging. The main application area of PET-CT has been in oncology, and it has also been incorporated in image-guided navigation for that purpose. A group in Austria recently reported on a system for image-guided navigation in head and neck cancer using PET-CT image fusion [13]. A study of 6 patients found an unsafe resection margin in 4 of the patients, and additional image-guided resection allowed local control of the tumor to be achieved in 3 patients.

Tracking devices are an essential component of any image-guided navigation system. Their primary function is to track the position of instruments relative to patient anatomy. Improvements in sophistication and accuracy of tracking systems in the last 10 years have been critical to the development of image-guided systems. The earliest tracking systems used were mechanical digitizers. While highly accurate, their main limitation was the large physical footprint and cumbersome use in an operating environment. Addressing this concern, optical tracking systems with high measurement accuracy and large field of view found quick adoption in the early image-guided navigation space. However, optical tracking systems require that a line-of-sight be maintained between the tracking device and instrument to be tracked. This is not always convenient and furthermore precludes tracking of flexible instruments inside the body. This challenge led to the development of electromagnetic tracking systems, which have no line-of-sight requirement and are able to track instruments such as catheters and the tips of needles inside the body.

Today, the choice of what tracking system to incorporate within an IGT solution is application dependent and variant on factors such as cost, desired work volume, and clinical accuracy constraints. Table 6.2 provides a list of tracking systems. We then give a brief synopsis of the different technologies in use: (a) optical videometric, (b) optical active and passive infrared, and (c) electromagnetic.