Functional MRI for presurgical planning: problems, artefacts, and solution strategies

Focused high frequency repetitive transcranial magnetic stimulation for localisation of the unexposed primary motor cortex during brain tumour surgery

OBJECTIVES

To investigate if intraoperative focused high frequency repetitive transcranial magnetic stimulation (rTMS) can localise the primary motor cortex without exposure of the cortical surface.

METHODS

A high frequency train (357 Hz) of four suprathreshold magnetic stimuli was delivered transcranially to the region of the rolandic area during brain tumour operations in 12 patients. To induce a focal magnetoelectric field, the flat figure of eight coil (outer diameter of each loop 7 cm) was used. Motor evoked potentials (MEP) were recorded in eight muscles of the upper and lower contralateral extremities. The first stimulation site was 2.5 cm behind the bregma, the second site 2 cm, and the third site 4 cm dorsal to the first stimulation site. If no MEP were obtainable, stimulation was repeated in anteroposterior direction at more laterally located sites. Using neuronavigation, each positive stimulation site was correlated with the underlying cortical anatomy.

RESULTS

Stimulation was performed at a total of 42 sites (in two patients, maximum stimulation at the three initial sites failed to evoke a motor response). In four patients, MEP were obtained only from one stimulation site. This site exactly overlayed the primary motor cortex. In eight patients, MEP could be elicited from more than one stimulation site. In seven of the eight patients, the site from which MEP with peak amplitudes were elicited, corresponded to the primary motor cortex. In total, the primary motor cortex was correctly identified on the basis of electrophysiological findings in 11 of 12 patients (92 %). In two patients, only the more lateral stimulation sites permitted MEP recording.

CONCLUSION

Intraoperative focused rTMS is highly sensitive for localisation of the primary motor cortex. Focused rTMS as a localising instrument alleviates the need of motor cortex exposure and, thereby, can contribute to minimise the surgical approach to brain tumours in the rolandic area.

[Physiological and technical limitations of functional magnetic resonance imaging (fMRI)--consequences for clinical use]

Functional magnetic resonance imaging (fMRI) is the most common noninvasive technique in functional neuroanatomy. The capabilities and limitations of the method will be discussed based on a short review of the current knowledge about the neurovascular relationship. The focus of this article is on current methodical and technical problems regarding fMRI-based detection and localization of neuronal activity. Main error sources and their influence on the reliability and validity of fMRI-methods are presented. Appropriate solution strategies will be proposed and evaluated. Finally, the clinical relevance of MR-based diagnostic methods are discussed.

Presurgical fMRI evaluation of cerebral reorganization and motor deficit in patients with tumors and vascular malformations

The aim of this fMRI study was to evaluate the motor reorganization (cerebral plasticity) and the risk of motor deficit in patients presenting with tumors and vascular malformations, within the framework of their pre-surgical planning. Functional MR imaging data was obtained from 17 patients. Ten patients (seven with tumors and three with vascular malformations) presented with motor reorganization. The results of the present study suggest that the evaluation of the cerebral reorganization is an essential step in predicting the risk of motor deficit in patients having surgical indication for treatment. Furthermore, the cerebral reorganization constitutes an important factor for surgical decision.

The spatial extent of the BOLD response

Functional magnetic resonance imaging is routinely used to localize brain function, with multiple brain scans averaged together to reveal activation volumes. In this study, we examine the seldom-studied effect of multiple scan averaging on the extent of activation volume. Using restricted visual field stimulation, we obtained a large number of scan repetitions and analyzed changes in activation volume with progressively increased averaging and across single scans. Activation volume increased monotonically with averaging and failed to asymptote when as many as 22 scans were averaged together. Expansions in the spatial extent of activation were not random; rather, they were centered about activation loci that appear with little or no averaging. Using empirical and simulated data, changes with averaging in activation volumes and cross correlation coefficient distributions revealed the presence of considerably more activated voxels than commonly surmised. Many voxels have low SNR and remain undetected without extensive averaging. The primary source of such voxels was not downstream venous drainage since there was no significant and consistent delay difference between voxels activated at different averaging levels. Voxels with low SNR may reflect a diffuse subthreshold activity centered about spiking neurons, dephasing gradients from distal veins, or simply a blood flow response extending beyond the locus of neuronal firing. Across single scans, as much as twofold changes in activation volume were observed. These changes were not correlated with the order of scan acquisition, subject task performance, or signal and noise properties of activated voxels. Instead, they may reflect subtle changes between overlapping noise and signal frequency components.

Motor functional MRI for pre-operative and intraoperative neurosurgical guidance

Functional MRI (fMRI) may provide a means of locating areas of eloquent cortex that can be used to guide neurosurgeons in their quest to maximize intracerebral tumour resection whilst minimizing post-procedural neurological deficits. This work aimed to develop and provide an initial assessment of such a technique. 19 patients with mass lesions close to the primary motor cortex underwent fMRI at 1.5T. A single shot echo planar technique was used to acquire data corresponding to right and left hand movement. Resultant activation maps were used to aid pre-surgical planning. Data was used in conjunction with an intraoperative navigation system in 13 cases. Activation was attributed to primary motor, primary somatosensory or supplementary motor cortex in 17 of 19 subjects. No permanent changes in motor deficit were detected post surgery. The additional information provided by fMRI, particularly when incorporated into a neuronavigation guided craniotomy, was deemed highly valuable to the neurosurgeon as it enabled safe resection of tumour in anatomical locations previously deemed to be too high risk for safe resection using conventional (non-fMRI-guided) technique. This observation is reinforced by the fact that no patients suffered permanent neurological deficit after radical tumour debulking (surgical estimates >90% tumour resection).

Epilepsy and surgical mapping

Drug treatment resistant epilepsy is an important public health problem. Patients with epilepsy of focal origin may have an excellent outcome following surgery that removes the source of seizures. Identification of the precise cortical region producing seizures is crucial to a good outcome; additionally, identification of eloquent cortical areas near the region to be resected is essential to prevent postoperative neurological deficit. A wide range of imaging techniques is valuable for imaging the epileptogenic zone, including high-resolution T1 MRI, T2 signal quantitation, MR spectroscopy, diffusion imaging, PET, SPECT and simultaneous EEG-fMRI. Eloquent cortex has in the past been mapped using highly invasive techniques; fMRI of motor and cognitive tasks holds great promise for future non-invasive mapping strategies.

Ethical issues in the clinical application of fMRI: factors affecting the validity and interpretation of activations

The ability of functional magnetic resonance imaging (fMRI) to localize activations in a single patient, along with the safety and widespread availability of this methodology, has lead to an increasing use of fMRI for clinical purposes such as pre-surgical planning. As methodology continues to improve and more experience with fMRI in the clinical setting is acquired, clinical functional neuroimaging will likely have an increasing influence over patient care. Therefore, ethical use of fMRI, as with other medical techniques, requires understanding the factors impacting the interpretation of the methodology. Issues affecting the validity and interpretation of clinical functional neuroimaging, including effects of altered hemodynamic response function, head motion, and structural changes in the brain, are reviewed. The distinction between correlated and necessary activation in a clinical context is discussed. Different types of statistical errors in fMRI analysis are described, along with their consequences to the patient. Finally, for the future of clinical fMRI development, the need for normative patient data, as well as standardized tasks, scan protocols, and data analyses, is discussed.

Preoperative assessment of motor cortex and pyramidal tracts in central cavernoma employing functional and diffusion-weighted magnetic resonance imaging

BACKGROUND

Functional MRI (fMRI) combines anatomic with functional information and has therefore been widely used for preoperative planning of patients with mass lesions affecting functionally important brain regions. However, the courses of functionally important fiber tracts are not visualized. We therefore propose to combine fMRI with diffusion-weighted MRI (DWI) that allows visualization of large fiber tracts and to implement this data in a neuronavigation system.

METHODS

DWI was successfully performed at a field strength of 1.5 Tesla, employing a spin-echo sequence with gradient sensitivity in six noncollinear directions to visualize the course of the pyramidal tracts, and was combined with echo-planar T2* fMRI during a hand motor task in a patient with central cavernoma.

RESULTS

Fusion of both data sets allowed visualization of the displacement of both the primary sensorimotor area (M1) and its large descending fiber tracts. Intraoperatively, these data were used to aid in neuronavigation. Confirmation was obtained by intraoperative electrical stimulation. Postoperative MRI revealed an undisrupted pyramidal tract in the neurologically intact patient.

CONCLUSION

The combination of fMRI with DWI allows for assessment of functionally important cortical areas and additional visualization of large fiber tracts. Information about the orientation of fiber tracts in normal appearing white matter in patients with tumors within the cortical motor system cannot be obtained by other functional or conventional imaging methods and is vital for reducing operative morbidity as the information about functional cortex. This technique might, therefore, have the prospect of guiding neurosurgical interventions, especially when linked to a neuronavigation system.

Metabolic and electrophysiological validation of functional MRI

OBJECTIVES

Although functional MRI is widely used for preoperative planning and intraoperative neuronavigation, its accuracy to depict the site of neuronal activity is not exactly known. Experience with methods that may validate fMRI data and the results obtained when coregistering fMRI with different preoperative and intraoperative mapping modalities including metabolically based (18)F-fluorodeoxyglucose PET, electrophysiologcally based transcranial magnetic stimulation (TMS), and direct electrical cortical stimulation (DECS) are described.

METHODS

Fifty patients were included. PET was performed in 30, TMS in 10, and DECS in 41 patients. After coregistration using a frameless stereotactic system, results were grouped into overlapping (<1 cm distance), neighbouring (<2 cm), or contradictory (>2 cm).

RESULTS

Comparing fMRI with PET, 18 overlapping, seven neighbouring, and one contradictory result were obtained. In four patients no comparison was possible (because of motion artefacts, low signal to noise ratio, and unusual high tumour metabolism in PET). The comparison of TMS and fMRI showed seven overlapping and three neighbouring results. In three patients no DECS results could be obtained. Of the remaining 38 patients, fMRI hand motor tasks were compared with DECS results of the upper limb muscles in 36 patients, and fMRI foot motor tasks were compared with DECS results of the lower limb on 13 occasions. Of those 49 studies, overlapping results were obtained in 31 patients, and neighbouring in 14. On four occasions fMRI did not show functional information (because of motion artefacts and low signal to noise).

CONCLUSIONS

All validation techniques have intrinsic limitations that restrict their spatial resolution. However, of 50 investigated patients, there was only one in whom results contradictory to fMRI were obtained. Although it is not thought that fMRI can replace the intraoperatively updated functional information (DECS), it is concluded that fMRI is an important adjunct in the preoperative assessment of patients with tumours in the vicinity of the central region.