neuroguide™ tubes improve the accuracy and safety of DBS surgery

Professor Steven Gill at Frenchay Hospital, Bristol, has worked closely with Renishaw to develop implantable guide tubes,neuroguide(tm) for Deep Brain Stimulation (DBS) surgery under an exclusively MRI-directed method, that provides an alternative to electrophysiological monitoring.

Professor Gill explains, “In my view the greatest value of the neuroguide™ system is perioperative confirmation of accurate targeting, since minor displacement of Deep Brain Stimulation (DBS) leads from the target position can result in significant side effects.  Therefore confidence in positioning is essential.  The use of the Renishaw neuroguide electrode introducer kit enables us to carry out functional neurosurgery entirely under general anaesthesia, with an operational accuracy within a millimetre of a Magnetic Resonance Image (MRI) selected target.”

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He continues, “The technique minimises surgical time, providing robust fixation for DBS leads, and the surgeon can directly correlate DBS contact location with clinical outcome.  This has enabled us to continually improve our target selection and confidently target novel structures such as the caudal Zona Incerta (Zi), the Mammilothalamic Tract (MMT) and the Pedunculo Pontine Nucleus (PPN).”

Using the neuroguide system has advantages over attempting to confirm placement of DBS electrodes in situ.  When MRI scans are taken of DBS electrodes it can result in local image distortion such that positioning data becomes unreliable, also there is a risk to the patient of localised heating, that significantly increases with 3 tesla MRI scanners.

Professor Gill compares the neuroguide method to Microelectrode Recording (MER) techniques, which can require multiple passes through the brain and are able to define target boundaries with a spatial resolution no better than the distance between passes, approximately 2 mm.  He considers MER provides insufficient resolution, whilst also presenting a greater risk to the patient's safety.  The neuroguide method, combined with high resolution distortion-corrected MRI images, achieves accurate targeting in a single pass.  MER also relies on recording a known signal, so cannot be used on new targets like the MMT or PPN.

neuroguide™ tube and stylette system

The neuroguide system comprises two radio-opaque elements, namely a guide tube and a stylette. A rigid tungsten carbide probe is used to deliver the neuroguide tube, ensuring it passes through the brain tissue without distortion of the tube, which can be problematic when using a semi-rigid DBS electrode alone.  During a procedure the guide tubes and stylettes are inserted first, which allow perioperative verification of the target localisation with MRI; once this is confirmed the guide tubes then act as a conduit for the delivery of the DBS electrodes, which are finally connected to a DBS generator.

One problem encountered in DBS surgery is that of lead migration, which is reported to be between 1% to 3%.  The neuroguide tubes are ideal in this situation, allowing leads to be replaced easily without the extra time and effort of re-targeting the position within the brain⁹. 

A procedure that uses neuroguide enables a significantly shorter operating time and enhanced safety; reducing the overall complication risk from infection and Cerebral Vascular Accident (CVA) to between 0.5% to 1%⁹ .

The imaging and planning process

Deep brain targets are defined in long-acquisition, high-resolution images acquired under general anaesthetic in both the axial and the coronal planes, with a slice thickness of 2 mm.  Different sequences are used to optimise visualisation of the different targets. For example the Subthalamic Nucleus (STN) is best seen on high resolution T2 images (1.5 T, TR2500, TE 150, TSE11, NSA 12). By combining the data from axial, coronal and sagittal scans, a resolution of less than 0.4 mm can be achieved in all three dimensions. 

Within the Renishaw neuroinspire™ software, used by Professor Gill to plan neuroguide procedures, the images are used to create a 3-dimensional model of the STN by plotting multiple coordinates, with interpolation between them to form a best fit volume.  The target position for the electrode is then chosen after carefully selecting a trajectory that avoids sulci and brain vessels.

The planning includes setting a path for the guide tube to follow to the target position, which is simulated in the software.  The choices of path are far greater, including the option to go through the ventricle; conventionally, transventricular routes are associated with high rates of ventricular lead migration and haemorrhage.

The surgical procedure

In preparation for surgery, the patient is fitted with a Leksell frame under general anaesthetic, followed by a series of MRI scans. A pre-frontal incision allows two burr holes to be made for the introduction of the guide tubes. 

Thus, with the patient in a semi-sitting position, the burrholes are uppermost – along with continuous saline irrigation this minimises Cerebrospinal Fluid (CSF) loss and brain shift when the dura is opened.

The distance from the burr hole to the target is determined and the guide tube, cut 12 mm short of this length, is mounted in the stereoguide.  A tungsten carbide probe is now guided to the target and the guide tube then advanced over the probe with its proximal hub bonded in the burr hole.  The distance from the hub to the target is measured and the stylette, cut to this length, is inserted into the guide tube after removal of the probe.  This part of the procedure takes about 30 minutes, before it is repeated on the other side.

With both sides completed the patient is taken back to the MRI or CT scanner for the peri-operative imaging, to confirm that the stylette tips are in the correct positions.

The patient is taken back into the operating theatre and, after the stylette is withdrawn, the DBS lead is introduced along the guide tube to its final target position.  The method used here has the distal end of the DBS lead extending 12 mm beyond the guide tube, thus ensuring that all contacts are exposed.  The DBS extensions are tunnelled subcutaneously via a post-auricular incision; these are then connected to the DBS generator which may be placed in the chest or the abdomen.

Conclusions and potential for the future

When the neuroguide tube method was first trialled at Bristol, 96.3% of the initial implantations were placed within 1.5 mm of the target⁹. However with more experience and the use of the neuroinspire™ surigcal planning software, repositioning is now very rare indeed and the procedure is currently accurate to within 1 mm, negating the need for MER.