ClearPoint is a second-generation iMRI implantation system that provides a method for iMRI-guided access to deep brain structures. The system is more intuitive, appears more accurate in the small number of experiments performed here and is easier to implement than the earlier technique based on the Nexframe MR14, 17
. These improvements were achieved by designing a skull mounted aiming system specifically with iMRI applications in mind, as well as designing a software environment that automates many steps of the procedure and reduces the likelihood of inaccuracies due to human influences.
The Nexframe MR is widely used in standard operating rooms for implantation of DBS electrodes using frameless stereotaxy. There are elements of its design, however, that limit its functionality in the MRI environment. Its base is funnel-shaped and has a widest diameter of 88 mm, which can present a challenge when trying to mount two devices side-by-side for bilateral simultaneous implantations. The fluid filled MRI visible alignment stem that is placed in the Nexframe MR to align the device to the target requires replacement with a multi-lumen insert for subsequent mandrel and DBS lead placement. Placing the multi-lumen insert correctly is difficult because the patient’s head is a considerable distance from the opening of the bore. Finally, the Nexframe MR is aligned by reaching into the magnet bore while simultaneously watching live MR images, which is physically awkward. The software component of the Nexframe MR technique also has significant shortcomings. It relies on the MR scanner console software to perform all of the imaging, targeting, trajectory planning, alignment and confirmation steps of the procedure. This software was not designed with neurosurgical interventions in mind; therefore, many steps of the procedure require manual entry of scan parameters and imaging planes as well as operator-controlled graphical reconstruction of the three dimension coordinates of implantation hardware.
The ClearPoint system addresses many of these shortcomings. The SMARTFrame is narrow in profile such that there is no interference between two of them during a bilateral simultaneous implantation, yet it exceeds the angular range of potential targets that can be reached by the Nexframe MR18
. In real-world applications, the achievable angle is now likely limited by the diameter of the burr hole and the thickness of the skull. The targeting cannula acts as both an MR-visible aiming device and the guide for mandrel and lead insertion, eliminating mechanical errors that may occur during exchange of device inserts. Finally, the SMARTFrame can be steered remotely (with feedback from the SW), making the process of alignment much easier, faster and more comfortable.
The ClearPoint workflow is quite similar to that of existing stereotactic workstations. It accepts DICOM imaging data from any MR scanner and presents the scans in a consistent format independent of the scanning platform used. Many steps are now automated but can always be edited by the user, saving time while maintaining flexibility and control over the procedure. The procedure times with the Nexframe MR system were strongly dependent on the MR operator, while the ClearPoint system is significantly easier to follow with minimal training. Finally, the software’s ability to localize and segment objects in the surgical field and predict error based on their position reduces the number of steps subject to human error.
The phantom and cadaver studies in this report were designed to determine the capability, accuracy and workflow efficiency of the new system. The SW prediction feature in particular is new to the second-generation device and required validation. The phantom target conditions used to generate the SW predictions were designed to replicate angles and depths that would be encountered clinically. This series of experiments were unique in that the mandrel was placed in the phantom and the SW was then used to target the mandrel itself. The predicted error should therefore be zero, so any predicted error reported by the SW must arise from either non-linearities in MR space or the SW prediction paradigm, and not the aiming accuracy of the SMARTFrame. With pitch and roll adjustment, the mean error was 0.9±0.5 mm with no bias in either the pitch or roll directions (both having magnitude of 0.2±0.7 mm). With X-Y stage translation, the mean error was lower (0.7±0.3 mm) but with a tendency of the software to systematically predict errors in the anterior and lateral directions by 0.5±0.3 mm and 0.4±0.3mm, respectively. This directional bias may be specific to field inhomogeneities and image distortion in our particular MR scanner and not the ClearPoint system itself, or could represent a bias in the SW prediction paradigm specific to the X-Y stage translation step. We used the same scanner for these phantom studies as we do for clinical iMRI DBS implantations using the first-generation system, and have noted similar direction and magnitude specific biases during those procedures. Thus, slight non-linearities in MR space may be the dominant contributor to the errors that were detected in this analysis.
The phantom accuracy testing was a more traditional phantom experiment where a MR visible target was localized using ClearPoint and a mandrel was passed to the target as directed by the SW. In addition to determining the accuracy of ClearPoint itself, a preliminary comparison could be made with the Nexframe MR, as the same phantom, same MRI scanner and same investigators were used for both the current experiments and experiments previously reported using the Nexframe MR. The ClearPoint system was found to have a radial error of 0.5±0.3 mm. In addition to providing submillimetric accuracy, ClearPoint appears to improve on a radial error of 0.8±0.5 mm reported for the Nexframe MR13
. The apparent improvement in accuracy may be attributable to improvements in the software environment as noted above, although improvements in the aiming device due to the integrated targeting cannula may also play a part.
The cadaver comparisons studies were designed to be a head-to-head comparison of the Nexframe MR and ClearPoint with regards to both workflow efficiency and to some degree accuracy. These differed from the phantom testing in that the actual entry point planning and burr hole placement portion of the workflow was done for each system in these tests. This was not done in the phantom accuracy testing, where the same pre-drilled burr holes were used repeatedly. It was also the best opportunity to preliminarily compare the accuracy of the two systems, as one of each aiming system was mounted on the same cadaver head and tested in the same session with the same surgeon. The average procedure time with three surgeons of varying levels of experience was only 4 minutes shorter using ClearPoint; however, given the large amount of clinical experience we have with the Nexframe MR and the virtual lack of clinical experience with ClearPoint, the fact that the new generation system is already comparable or slightly faster is likely significant. Unfortunately with a small sample size (N=3 for each system) it is not possible to draw firm conclusions on relative accuracy, but these initial findings with ClearPoint are encouraging.
Finally, while we have been clinically focused on the use of iMRI for implantation of DBS electrodes, this technology has other potential applications including depth electrode placement for epilepsy monitoring, brain biopsy and catheter or cannula placement. Our group has already used a slightly modified version of ClearPoint to perform iMRI guided infusions in non-human primates, with the goal of using this technology to guide convection enhanced delivery of therapeutics to the brain and monitor them using real time MRI. Such a technique would be a natural extension of gene therapy trials that have already been done for Parkinson’s disease, and would have further applications in other neurodegenerative disorders or in the chemotherapeutic treatment of neoplasms19–23
. Although ClearPoint can be used for a variety of different iMRI-guided interventions, we tested it specifically with DBS implantation in mind. Given the fact that the safety of imaging patients with implanted DBS electrodes at 3T is not established, we only tested the system in a 1.5T environment. For procedures such as brain biopsy or catheter placement, higher field strengths could be considered, although the improvements in tissue discrimination with stronger magnets must be weighted against the greater potential for field inhomogeneity.