7.1 Patient Selection

Intraoperative CT imaging serves two purposes. First, it is used to register and assess an implant location, as in the registration of a stereotactic frame or a bone fiducial. Second, it can act as a stereotactic device in conjunction with a frameless navigation system. For example, it can be used for O-arm navigation of pedicle screws, Vertek arm alignment, or a fiducial-less Nexframe case. The precise location of the positioned device can then be evaluated.

All stereotactic devices have a delivery error in the range of 2 mm as measured with current techniques. Thus, intraoperative imaging provides ongoing accuracy feedback and allows interpretation of physiologic data based on the actual location not the expected location. The intraoperative CT scanner can also be used to verify the correct delivery of a device to a location within the brain whether or not it was placed stereotactically. For example, it can identify the position of a shunt within the ventricles. Intraoperative CT imaging should therefore be performed when stereotactic guidance is required, or intraoperative verification of device location is desired.

7.2 Imaging Device Selection

What are the advantages of CT scans over MRI? CT scans have a high geometric accuracy, but they lack soft tissue contrast needed to identify particular brain structures. ² One method to circumvent this problem is to merge the intraoperative CT images with a preoperative MRI. Doing this creates a risk of coregistration error. On the other hand, CT imaging does have some advantages. It can be performed on larger, heavier patients, those with pacemakers and other electronic implants, with patients awake or asleep, and with the head fixed or free.

Portable intraoperative CT imaging devices are being used more often. A fixed intraoperative diagnostic CT scanner is less commonly present. Portable technologies include O-arm (Medtronic, Inc., Minneapolis, MD), Ceretom (NeuroLogica Corporation Danvers, MA), BodyTom (NeuroLogica Corporation Danvers, MA), and Airo CT (Brainlab Munich Germany). By the time of publication, there may be additional choices as well. The main consideration in choosing an imaging device is its compatibility with other equipment, the level of image quality sought, size limitations, field of view, and of course, availability. Earlier portable CTs had a limited field of view, but this has been corrected with the previously mentioned devices. The O-arm has a large bore to accommodate imaging of microelectrode tracks and the second iteration has a sufficiently large enough field of view to visualize a stereotactic frame localizer. It is also widely available since it is also used in spine surgery. The bore of the Ceretome, however, is much smaller than the others causing the patient to lay completely flat, and this creates limitations on positioning. Nonetheless, it is been put to excellent use during asleep deep brain stimulation (DBS) implantation surgery. ³ The BodyTom is large, which limits its portability. The Airo CT and O-arm can seamlessly integrate with commercially available navigation systems as a stereotactic device. As long as a reference arc is attached, the O-arm can be paired with the Stealth Station to provide registration with a scan. The Brain Lab and Airo CT are similarly connected. Each system has a unique work flow, but the principles are the same.

In this chapter we will describe the use of the O-arm for fiducial-less, frameless DBS placement as an example of using the scanner as a stereotactic device. The updated Stealth software package, Cranial 3.0 is required for this approach.

We will also discuss its use in the intraoperative identification of implant locations.