Jürgen Weidemann

Sequential visualization of brain and fiber tract deformation during intracranial surgery with three-dimensional ultrasound: an approach to evaluate the effect of brain shift.

OBJECTIVE: We present a technique that allows intraoperative display of brain shift and its effects on fiber tracts. METHODS: Three patients had intracranial lesions (one malignant glioma, one metastasis, and one cavernoma) in contact with either the corticospinal or the geniculostriate tract that were removed microneurosurgically. Preoperatively, magnetic resonance diffusion-weighted imaging (DWI) was performed to visualize the fiber tract at risk. DWI data were fused with those obtained from anatomic T1-weighted magnetic resonance imaging. A single-rack three-dimensional ultrasound neuronavigation system, which simultaneously displays the MRI scan and the corresponding ultrasound image, was used intraoperatively for 1) navigation; 2) definition of fixed and potentially shifting ultrasound landmarks near the fiber tract; and 3) sequential image updating at different steps of resection. The result was time-dependent brain deformation data. With a standard personal computer equipped with standard image software, the brain shift-associated fiber tract deformation was assessed by use of sequential landmark registration. After surgery, DWI was performed to confirm the predicted fiber tract deformation. RESULTS: The lesions were removed without morbidity. Comparison of three-dimensional ultrasound with DWI and T1-weighted magnetic resonance imaging data allowed us to define fixed and potentially shifting landmarks close to the respective fiber tract. Postoperative DWI confirmed that the actual fiber tract position at the conclusion of surgery corresponded to the sonographically predicted fiber tract position. CONCLUSION: By definition and sequential intraoperative registration of ultrasound landmarks near the fiber tract, brain shift-associated deformation of a tract that is not visible sonographically can be assessed correctly. This approach seems to help identify and avoid eloquent brain areas during intracranial surgery.

Involvement of the Superior Temporal Cortex and the Occipital Cortex in Spatial Hearing: Evidence from Repetitive Transcranial Magnetic Stimulation

The processing of auditory spatial information in cortical areas of the human brain outside of the primary auditory cortex remains poorly understood. Here we investigated the role of the superior temporal gyrus (STG) and the occipital cortex (OC) in spatial hearing using repetitive transcranial magnetic stimulation (rTMS). The right STG is known to be of crucial importance for visual spatial awareness, and has been suggested to be involved in auditory spatial perception. We found that rTMS of the right STG induced a systematic error in the perception of interaural time differences (a primary cue for sound localization in the azimuthal plane). This is in accordance with the recent view, based on both neurophysio-logical data obtained in monkeys and human neuroimaging studies, that information on sound location is processed within a dorsolateral where stream including the caudal STG. A similar, but opposite, auditory shift was obtained after rTMS of secondary visual areas of the right OC. Processing of auditory information in the OC has previously been shown to exist only in blind persons. Thus, the latter finding provides the first evidence of an involvement of the visual cortex in spatial hearing in sighted human subjects, and suggests a close interconnection of the neural representation of auditory and visual space. Because rTMS induced systematic shifts in auditory lateralization, but not a general deterioration, we propose that rTMS of STG or OC specifically affected neuronal circuits transforming auditory spatial coordinates in order to maintain alignment with vision.

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.

The neural correlate of very-long-term picture priming.

Repetition priming denotes a behavioural change caused by prior exposure to a stimulus. The effect is known to last for weeks. This study addresses the underlying neural mechanisms for very‐long‐term picture priming by using event‐related functional magnetic resonance imaging complemented by a behavioural paradigm. Previous functional imaging studies with shorter retention intervals have shown that priming is associated with changes in the activity of both the occipital and posterior temporal cortex. In this study we compared retention intervals of 1 day and 6 weeks after initial exposure to a picture stimulus. Priming‐related decreases in cortical activity in posterior extrastriate and dorsal left inferior frontal areas were found only for the shorter retention interval. In contrast, fMRI activation in the inferior posterior temporal and anterior left inferior frontal cortex was reduced following priming for both retention intervals. In the behavioural paradigm, the priming effect was stable over time. We conclude that the left inferior frontal and inferior posterior temporal cortex play a key role in the very‐long‐term priming effect.

Power grip disinhibits the ipsilateral sensorimotor cortex: a TMS and fMRI study.

Electrophysiological studies have shown that forceful activation of the hand muscles (power grip) is accompanied by an increased excitability of the ipsilateral corticospinal system. This increase in excitability may be due to spinal or cortical mechanisms. Here we show with fMRI that this phenomenon is at least in part mediated at a cortical level. We used TMS to show that the increased ipsilateral excitability during a forceful maneuver leads to enhanced stimulus-response curves. fMRI was used to compare the activation during a repetitive hand movement with or without an accompanying power grip on the opposite site. The power grip reduced movement-related activation in the ipsilateral sensorimotor cortex. Peak deactivation was located in the left postcentral gyrus extending into the adjacent precentral gyrus. This finding suggests that a forceful activation of the hand muscles disinhibits a distinct functional representation in the ipsilateral sensorimotor cortex. Consequently, the excitability of the corticospinal system increases and less neuronal excitatory activity is needed to perform a given task. The results may be important for a variety of studies as they suggest that fMRI may show decreased hemodynamic response under conditions in which other neurophysiological methods have shown increased functional activity.

Diffusion-weighted imaging-guided resection of intracerebral lesions involving the optic radiation

In this paper we report our experience with diffusion-weighted imaging (DWI) for optic radiation (OR) visualization during resection of tumors. We hypothesize that intraoperative OR visualization helps to maintain patients’ visual fields. DWI studies were performed together with T1-weighted postcontrast magnetic resonance imaging (MRI) in four patients with lesions in or adjacent to the OR (glioblastoma, oligo-astrocytoma, cavernoma, and metastasis; n=1 each). The OR was identified from one of six DWI data acquisitions, segmented and reconstructed three-dimensionally. The image data were neuronavigationally transferred into the operative field, and provided the neurosurgeon with information on lesion site and adjacent OR localization. Preoperative and postoperative neuroophthalmological testing included, among others, perimetry to define the value of diffusion-weighted image guidance during OR lesion resection. Three lesions were removed completely. In one case, low-grade tumor parts infiltrating the OR were intentionally left. No persistent visual field deficits were induced. In one patient, a transient homonymous hemianopia attributable to postoperative swelling completely resolved under steroid medication. The authors conclude that intraoperative OR visualization, realized by neuronavigationally displayed DWI data, might prove to be helpful to maintain patients’ visual fields.

Preoperative mapping of cortical motor function: prospective comparison of functional magnetic resonance imaging and [15O]-H2O-positron emission tomography in the same co-ordinate system.

BackgroundTwo of the most widely accepted approaches to map eloquent cortical areas preoperatively are positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). As yet, no study has compared these two modalities within the same frame of reference in tumour patients. AimWe employed [15O]-H2O-PET and fMRI in patients undergoing presurgical evaluation and compared the results with those obtained by direct electrical cortical stimulation (DECS). MethodsTwenty-five patients with tumours of different aetiology near the central region were investigated. fMRI and PET were processed using the same methods, i.e. statistical parametric mapping (SPM) without anatomical normalization, and transformed into the same frame of reference. ResultsfMRI activity was found in more cranial and lateral sections, i.e. closer to the brain surface, in comparison with PET, which demonstrated parenchymal activation. The mean localization difference between fMRI and PET was 8.1±4.6 mm (range, 2–18 mm). fMRI and [15O]-H2O-PET could reliably identify the central sulcus, as demonstrated by DECS. ConclusionsfMRI and [15O]-H2O-PET demonstrate comparable results and are sensitive and reliable tools to map the central region, especially in cases of infiltrating brain tumours. However, fMRI is more prone to artefacts, such as the visualization of draining veins, which may explain the more cranial and lateral activation visualized by fMRI, whereas PET depicts capillary perfusion changes and therefore shows activation closer to the parenchyma.

Three-Dimensional Visualization of Motor Cortex and Pyramidal Tracts Employing Functional and Diffusion Weighted MRI Methods, Applications and Limitations

AbstractBackground: 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 course of functionally important fiber tracts is not visualized. We therefore propose to combine fMRI with diffusion weighted MRI (DWI) that allows visualization of large fiber tracts and to implement these data in a neuronavigation system. Methods: DWI was successfully performed at a field strength of 1.5 T, employing a spin-echo sequence with gradient sensitivity in six non-colinear directions to visualize the course of the pyramidal tracts and combined with echo-planar T2*fMRI during a hand motor task in 15 patients and 30 healthy controls. Results: Fusion of both data sets allowed visualization of the displacement of both primary sensorimotor area (M1) and corresponding large descending fiber tracts in patients. Intraoperatively, these data were present in a neuronavigation environment. Conclusion: The combination of fMRI with DWI allows for assessment of functionally important cortical areas and additional visualization of large fiber tracts. Information about 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 as vital for reducing operative morbidity as the information about functional cortex. This technique might, therefore, have the prospect of guiding neurosurgical interventions, especially when being linked to a neuronavigation system.ZusammenfassungHintergrund: Ein wesentliches Problem der funktionellen MRT in der präoperativen Risikoabschätzung neurochirurgischer Operationen ist die fehlende Darstellung von funktionell relevanten Bahnsystemen (z. B. Pyramidenbahn, Sehbestrahlung), deren intraoperative Schädigung ebenso wie die Läsion kortikaler Neurone zu einem irreversiblen neurologischen Defizit führen kann. Methoden, die lediglich auf der Darstellung hämodynamischer Veränderungen beruhen, können aufgrund der geringeren Perfusion aufgabenabhängige Blutflussveränderungen in der weißen Substanz nicht darstellen. MRT-Sequenzen, die die gerichtete Diffusion untersuchen, können hingegen die Struktur von Faserverbindungen, nicht jedoch ihre funktionelle Spezifität darstellen. Die Kombination funktionell bildgebender Methoden mit dieser strukturellen Bildgebung könnte deshalb für die präoperative Risikoabschätzung von großem Wert sein. Das Ziel dieser Studie war es, die Kombination beider Methoden bei Patienten mit Tumoren im motorischen System zu untersuchen und die intraoperative Nutzung dieser Daten mit Hilfe eines Neuronavigationssystems zu beschreiben. Methodik: Bei 15 Patienten und 30 Kontrollprobanden wurden bei 1,5 T mittels einer diffusionsgewichteten Spinechosequenz in sechs nicht kolinearen Gradientensensitivierungen der Verlauf der Pyramidenbahn visualisiert und mit T2*-gewichteten funktionellen MRT-Sequenzen, die während der Durchführung motorischer Aufgaben aufgenommen wurden, kombiniert. Die Daten standen nach Segmentierung der Pyramidenbahnen auch dreidimensional zur Verfügung (Abbildungen 1 bis 5). Ergebnisse: Die Fusion beider Datensätze erlaubte Aussagen über den Verlauf bzw. die Verlagerung von dem primär motorischen Kortex und den Pyramidenbahnen (Abbildungen 8 bis 12). Die Daten waren durch das Neuronavigationssystem auch intraoperativ zugänglich (Abbildungen 13 bis 16) und konnten durch neuroanatomische Studien verifiziert werden (Abbildungen 6 und 7). Schlussfolgerung: Die Kombination beider Methoden erlaubt sowohl die Darstellung des aktiven Kortex als auch seiner zugehörigen Faserverbindungen. Die Information über den Faserverlauf kann in den Routinesequenzen nicht erhalten werden, ist jedoch zur Senkung der operationsbedingten Morbidität ebenso wichtig wie das Wissen um den funktionell aktiven Kortex. Diese Technik kann deshalb hilfreich für den Neurochirurgen sein, vor allem wenn die Bilddaten intraoperativ zur Verfügung stehen.