Frameless stereotaxy in the nonhuman primate.

With the advent of magnetic resonance imaging (MRI), it is possible to obtain high-resolution anatomical images of the monkey brain. Accuracy, however, is lost in the laboratory or surgical setting when the localization of brain structures depends on nonstereotaxic tracking methods. Here we present an image-guided stereotaxic system that is able to localize and access anatomical brain structures using the monkey's MRI. This system, which is also known as frameless stereotaxy, is capable of computing the relation of the physical "real space" of the monkey's head to the corresponding image space, while a position sensor enables the tracking of the animal's head and the localization of brain areas and favorable paths to targets within the brain using real time display software. Surgical procedures make use of an adjustable upright chair and a surgical headclamp instead of the traditional restrictive head holder with ear bars. This novel system allows for the flexible positioning of the animal and the ability to reach areas of the brain that were difficult to access in the past. The headclamp also serves as a tool holder, which in the present application guided a cannula of retrograde tracer to the desired location in the frontal lobe. Histological examination of the brain showed that the injection reached the target site, and tests using an MRI compatible phantom demonstrated that the precision of the system in bringing an injection to target is less than 1.2 mm. This system can be used to inject accurately tracers for anatomical tract-tracing, to make precise lesions, and to position electrodes for electrophysiological studies.