Eleanor L. Carter, Virginia F. J. Newcombe, Robert C. Hawkes and Jonathan P. Coles
Derek L. G. Hill, Andrew D. Castellano Smith, Andrew Simmons, Calvin R. Maurer Jr., Timothy C. S. Cox, Robert Elwes, Michael Brammer, David J. Hawkes and Charles E. Polkey
Object. Several authors have recently reported studies in which they aim to validate functional magnetic resonance (fMR) imaging against the accepted gold standard of invasive electrophysiological monitoring. The authors have conducted a similar study, and in this paper they identify and quantify two characteristics of these data that can make such a comparison problematic.
Methods. Eight patients in whom surgery for epilepsy was performed and five healthy volunteers underwent fMR imaging to localize the part of the sensorimotor cortex responsible for hand movement. In the patient group subdural electrode mats were subsequently implanted to identify eloquent regions of the brain and the epileptogenic zone. The fMR imaging data were processed to correct for motion during the study and then registered with a postimplantation computerized tomography (CT) scan on which the electrodes were visible. The motion during imaging in the two groups studied, and the deformation of the brain between the preoperative images and postoperative scans were measured.
The patients who underwent epilepsy surgery moved significantly more during fMR imaging experiments than healthy volunteers performing the same motor task. This motion had a particularly increased out-of-plane component and was significantly more correlated with the stimulus than in the volunteers. This motion was especially increased when the patients were performing a task on the side affected by the lesion. The additional motion is hard to correct and substantially degrades the quality of the resulting fMR images, making it a much less reliable technique for use in these patients than in others. Also, the authors found that after electrode implantation, the brain surface can shift more than 10 mm relative to the skull compared with its preoperative location, substantially degrading the accuracy of the comparison of electrophysiological measurements made in the deformed brain and fMR studies obtained preoperatively.
Conclusions. These two findings indicate that studies of this sort are currently of limited use for validating fMR imaging and should be interpreted with care. Additional image analysis research is necessary to solve the problems caused by patients' motion and brain deformation.
Virginia F. J. Newcombe, Robert C. Hawkes, Sally G. Harding, Roslyn Willcox, Sarah Brock, Peter J. Hutchinson, David K. Menon, T. Adrian Carpenter and Jonathan P. Coles
Magnetic resonance imaging and spectroscopy may provide important clinical information in the acute stages of brain injury. For this to occur it must be ensured that intracranial pressure (ICP) monitoring devices are safe to bring into the MR imaging suite. The authors tested a Codman MicroSensor ICP Transducer (Codman & Shurtleff, Inc.) within a 3-T MR imaging system using the transmit body coil and receive-only coils and the transmit-and-receive head coil. Extreme and rapid heating of 64°C was noted with the transducer wire in certain positions when using the transmit body coil and receive-only head coil. This is consistent with the phenomenon of resonance, and the probe was shown to have a distinct resonant response when coupled to HP 4195A Network Analyzer (Hewlett Packard). Coiling some of the transducer wire outside of the receive-only head coil reduced the generated current and so stopped the thermogenesis. This may be due to the introduction of a radiofrequency choke. The ICP transducer performed within clinically acceptable limits in both the static magnetic field and during imaging with high radiofrequency power when the excess wire was in this configuration. No heating was observed when a transmit-and-receive head coil was used. This study has shown when using a high-field magnet, the Codman ICP probe is MR conditional. That is, in the authors' system, it can be safely used with the transmit-and-receive head coil, but when using the transmit body coil the transducer wire must be coiled into concentric loops outside of the receive-only head coil.