Contents

1. Introduction

Due to the high soft tissue contrast, the absence of ionizing radiation and spatially unrestricted imaging capabilities, MRI has become the predominant imaging modality for neurosurgical interventions. MRI-based navigation techniques assist the neurosurgeon with the orientation at the patient and allow precise instrument guidance. 

Navigation in neurosurgery can be roughly divided into preoperative and intraoperative based approaches. In preoperative based approaches, MRI images are acquired several days prior to surgery and co-registered (i.e., mapped to the spatial coordinates of the patient during surgery in relation to the intervention position). Even though these methods are not restricted in terms of the magnetic field and offer the entire diagnostic capabilities of MR, they do not allow for adjustment of brain shift, that occurs when the cranium is opened. This technical inability has led to intraoperative MR systems, that acquire MRI images during surgery. The specific advantages and limitations of these systems are highly dependent on the magnet design that defines the diagnostic applications and the interventional environment.

Navigation generally means the use of geometrical an anatomical data to approach a target position in or at the patient. For this purpose the position and orientation of a surgical instrument needs to be reliably defined. A stereoscopic camera is typically used to continuously determine the 3D position of an instrument. 

Several navigation techniques and the corresponding MR-systems are briefly discussed bellow:

The original form of static guidance is traditional stereotaxy. Here, geometrical data is used to point along a predefined path or trajectory between the desired entry and the target point.

Frame based stereotaxy [1] without intraoperative control is the simplest form of navigation and is performed in a traditional operating room based on preoperative MRI data. It is most commonly used for interventions, where brain shift is not critical, or for biopsy procedures. 

An MR-compatible frame with MR-visible markers is securely fixed to the skull prior to MR imaging. After scanning, the corresponding data are transferred to the OR, where the surgical procedure is planned. In addition, the MR-visible markers need to be identified on the MR image. After that, the corresponding coordinates at the frame are computed  and the guidance system is set accordingly. The surgical intervention may then be performed in the predetermined manner as long as no intraoperative changes occur.

2.2 Dynamic Guidance

The aim of dynamic guidance is to control the slice orientation in such a way that the instrument is ideally displayed along its entire length, (i.e., on an oblique slice defined by the actual orientation of the instrument).

One common and relatively simple approach is called freehand navigation. However, that term is differently defined for interventions in either closed-bore or open MR scanners. In closed-bore systems the instrument is iteratively advanced to the target by switching between manipulation outside the bore and control imaging inside the bore. In open MR scanners a suitable plane including the target point is defined and continuously scanned. If the orientation of the instrument is tracked, then the selected plane may be adapted automatically.

Another common approach is based on continuous imaging and interactive scan plane orientation. This  method is exclusively performed in open scanners because the target region needs to be near the magnet isocenter and the surgeon has to simultaneously hold the instrument and follow its way to the target on the monitor. In addition the instrument needs to be tracked which is mostly accomplished by optical methods. Such an open intraoperative 0.12 T system where surgery is performed entirely within the magnetic field using standard instrumentation is shown in Fig 1. A very similar system is used at the UCI Medical Center for intraoperative MR-guided neurosurgery as well.

Fig 1.  A 0.12 T intraoperative MR system, where surgery is performed entirely within the magnetic field using standard instrumentation [2].

Another system for navigation, based on continuous imaging, is shown in Fig.2. This vertically open 0.5 T system, does not necessariyily require coregistration between patient and image data, because the stereoscopic camera can be integrated within the MR scanner.

Fig. 2. A 0.5 T double coil design intraoperative MR system, where imaging can be performed continuously while the surgeon operates between the coils [2].

3. Intraoperative MR Systems for Functional Imaging

Many of the MR techniques for functional neuroimaging, including MR venography (MRV), MR angiography (MRA), brain activity studies known as functional magnetic resonance imaging (fMRI), and diffusion-weighted imaging (DWI), require magnetic field strength of at least 1.5 T. High field strengths are especially necessary for applications where quantitative accuracy is important and where signal to noise ratio (SNR) is critical. Since SNR is the limiting factor for DTI measures, a high magnetic field stenght of at least 1.5 T is required for adequate DTI measuremts. At this point, high field strengths of 1.5 T and above, only can be achieved with closed-bore MR systems. Such a intraoperative closed-bore MR systems is shown in Fig. 3.

Fig. 3. A intraoperative 1.5 T closed-bore MR system, that can be used for functional imaging during neurosurgery [2].