Errors of Image Coregistration may Necessitate Intraoperative Refinement in Functional Neurosurgery

We read with interest the paper by Reck et al “Clinical Outcome of Subthalamic Stimulation in Parkinson's Disease is Improved by Intraoperative Multiple Trajectories Microelectrode Recording” (MER).[1] This study used the “on medication, on stimulation” condition as the outcome criteria. Most outcome studies report “off medication, on stimulation” data, since this is more representative of the effect that deep brain stimulation (DBS) has on Parkinson's disease (PD) symptoms. “On medication, on stimulation” data are logistically easier to obtain; however, such data are also open to critique about variation in medication state between patients.

An important and often overlooked aspect of the surgical technique used by the authors also deserves emphasis. The initial surgical plan was performed using “fused stereotactic computed tomography (CT) and magnetic resonance (MR) images.” When magnetic resonance imaging (MRI) was first introduced to clinical practice, the ability to directly visualize deep-seated structures within the brain was offset by concerns of MR distortion. Initial attempts to circumvent such problems involved coregistration of a nonstereotactic MRI scan to a stereotactic CT scan. However, magnetic inhomogeneities are nonlinear, whereas most fusion algorithms are linear.[2] In addition, coregistration between CT and MR images is open to significant errors; mean errors of between 1.2 and 1.7 mm have been reported with errors of over 3 mm expected in individual patients; this ultimately dwarfs the spatial errors introduced by MRI distortion.[3] Although large fusion errors will be noticed by experienced neurosurgeons, smaller errors may go undetected unless both datasets contain fiducial markers that can be accurately validated.

MR distortion can be addressed more directly and effectively. Manufacturers of MR scanners provide software solutions that correct for distortion, and geometric errors can be reduced to below the size of a voxel.[4] Using stereotactic MRI to both guide and verify the surgical intervention avoids errors of image registration and has several theoretical and practical advantages. Image-guided and image-verified DBS without MER is associated with a significantly lower risk of deficit or death secondary to hemorrhage.[5] Moreover, clinical outcome is comparable to that of other surgical techniques.[6] [7] [8]

Rech et al use an image-guidance technique that introduces a random targeting error to the central trajectory. Therefore, some other intraoperative method of target refinement is likely to be required. This may explain why systematic analysis of multiple surrounding trajectories during surgery (MER group) resulted in a superior outcome measure than the method that did not (non MER group). The authors' conclusion: “using multiple trajectories, MER results in a better clinical outcome … than implantation without MER” only applies within the context of errors introduced by their particular method of anatomical targeting. Therefore, the study results do not justify the use of MER over a meticulous MRI-guided and MRI-verified approach.

• References

• 1 Reck C, Maarouf M, Wojtecki L , et al. Clinical outcome of subthalamic stimulation in Parkinson's disease is improved by intraoperative multiple trajectories microelectrode recording. J Neurol Surg A Cent Eur Neurosurg 2012; 73: 377-386
• 2 Sankar T, Lozano AM. Magnetic resonance imaging distortion in functional neurosurgery. World Neurosurg 2011; 75 (1) 29-31
• 3 O'Gorman RL, Jarosz JM, Samuel M, Clough C, Selway RP, Ashkan K. CT/MR image fusion in the postoperative assessment of electrodes implanted for deep brain stimulation. Stereotact Funct Neurosurg 2009; 87: 205-210
• 4 Doran SJ, Charles-Edwards L, Reinsberg SA, Leach MO. A complete distortion correction for MR images: I. Gradient warp correction. Phys Med Biol 2005; 50: 1343-1361
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• 6 Foltynie T, Zrinzo L, Martinez-Torres I , et al. MRI-guided STN DBS in Parkinson's disease without microelectrode recording: efficacy and safety. J Neurol Neurosurg Psychiatry 2011; 82: 358-363
• 7 Krack P. Subthalamic stimulation for Parkinson's disease: a new benchmark. J Neurol Neurosurg Psychiatry 2011; 82: 356-357
• 8 Starr PA, Martin AJ, Ostrem JL, Talke P, Levesque N, Larson PS. Subthalamic nucleus deep brain stimulator placement using high-field interventional magnetic resonance imaging and a skull-mounted aiming device: technique and application accuracy. J Neurosurg 2010; 112: 479-490