Image-guided surgery(IGS) is the general term used for any surgical procedure where the surgeon employs tracked surgical instruments in conjunction with preoperative or intraoperative images in order to indirectly guide the procedure. Most image-guided surgical procedures are minimally invasive.
Image Guided Surgery/ Brain Mapping 
The technology was originally developed for treatment of brain tumors, but has found widest application when applied to surgery of the sinuses, where image-guided surgery helps avoid damage to brain and nervous system.
A hand-held surgical probe is an essential component of any image-guided surgery system. During the surgical procedure, the IGS tracks the probe position and displays the anatomy beneath it as, for example, three orthogonal image slices on a workstation-based 3D imaging system. Existing IGS systems use different tracking techniques including mechanical, optical, ultrasonic, and electromagnetic.
The Medtronic Stealth Station is the most widely used IGS system on the market, and utilizes both electromagnetic and optical tracking technology.
IGS are difficult often to compare on reports one might easily find. These factors should be considered in comparing systems:
- Preoperative vs. intraoperative images for navigation. In some cases navigation can be done only with preoperative images and current position of surgical instruments, which is sometimes based on the assumption that the region of interest (ROI) is not moving, such as when a head is held in a rigid frame, or when markers on the anatomy, such as fiducials or spine clamps, are moving in synchronization with the ROI. This assumption is not always valid, such as when force applied to one body part changes its relationship to the markers on the anatomy, or between one body part and another. Each manufacturer's instructions will have detailed instructions that need to be understood to maintain the amount of accuracy estimated at the beginning of the procedure. Intraoperative imaging improves the accuracy of navigation by getting fresh anatomic data and images of surgical hardware attached. Some intraoperative imaging comes at a cost of increased radiation doses, which can be reduced by reducing dose to the level needed for navigation as compared to that for diagnosis. Other modalities of imaging have less inherent adverse effect, such as low-field MRI or ultrasound. Many systems will offer both preoperative as well as intraoperative in the same case.
- Tracking techniques. Optical tracking is the most widely used currently. Typically an infrared camera is aimed at the surgical site and gathers input from reference markers or surgical instruments using active or passive tracking. Active tracking is when the instrument can generate its own signal such as with a LED to signal the camera. Passive tracking is a marker with reflective material to reflect back to the camera the infrared signal. The surgeon and equiptment cannot get in the line of sight more than momentarily. In magnetic tracking, a magnetic field generator emits a field much lower in intensity than an MRI. Another associated device scans the magnetic field in a manner than can detect anatomy and instruments in the field.
- Integration with other products. A full-featured system focuses at least on accuracy of navigation, and can communicate with other devices in the operating room. Examples include:
- navigation systems that integrate with operative microscopes, such as by projecting the preoperative scan into the visual field of the camera for comparison to the visual image.
- systems that merge different kinds of scans, such as preoperative CT with intraoperative fMDI (functional MRI), or intraoperative ultrasound.
Market share facts are hard to come by for free, but it is clear that amongst the companies that provide a variety of products for image guided surgery, the top three are:
- On their website, look especially at applications in Neurological, Spinal and Orthopaedic and Ear, Nose, and Throat
- A good starting page for biomed is [http://www.stryker.com/en-us/products/SurgicalNavigationSoftware/index.htm
- Brainlab 
In a SEC filing 02/18/2011 10-K for STRYKER CORP it states: "In the surgical navigation segment, Stryker is one of six principal competitors, including Medtronic Surgical Navigation Technologies (a division of Medtronic, Inc.), BrainLAB Inc. (a subsidiary of BrainLAB AG), AESCULAP AG & Co. KG (a division of B. Braun Melsungen AG), Radionics, Inc. (a subsidiary of Integra LifeSciences Corporation), and GE Medical Systems Navigation and Visualization, Inc. (a subsidiary of General Electric Company). "
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- ↑ Foley, Kevin T. MD., Image-Guided Surgery: Space Age Technology Enters the Operating Room. 
- ↑ Eric L. Grimson, Ron Kikinis, Ferenc A. Jolesz and Peter McL. Black. Virtual-reality technology is giving surgeons the equivalent of x-ray vision, helping them to remove tumors more effectively, to minimize surgical wounds and to avoid damaging critical tissues. 
- ↑ Alexandra J. Golby M.D., Image-Guided Surgery and Brain Mapping. 2011. http://www.brainsciencefoundation.org/