Visualization of the corticospinal tract pathway using magnetic resonance axonography and magnetoencephalography for stereotactic irradiation planning of arteriovenous malformations

Increased muscle fiber fractional anisotropy value using diffusion tensor imaging after compression without fiber injury

BACKGROUND

Recent studies have indicated that injuries such as muscle tears modify the microstructural integrity of muscle, leading to substantial alterations in measured diffusion parameters. Therefore, the fractional anisotropy (FA) value decreases. However, we hypothesized that soft tissue, such as muscle tissue, undergoes reversible changes under conditions of compression without fiber injury.

PURPOSE

To evaluate the FA change due to compression in muscle tissue without fiber injury.

MATERIAL AND METHODS

Diffusion tensor imaging (DTI) was performed on both feet of 10 healthy volunteers (mean age = 35.0 ± 10.39 years; age range = 23-52 years) using a 3.0-T magnetic resonance imaging (MRI) scanner with an eight-channel phased array knee coil. An MRI-compatible sphygmomanometer was applied to the individuals' lower legs and individuals were placed in a compressed state. Then, rest intervals of 5 min were set in re-rest state after compression. The FA value, apparent diffusion coefficient (ADC), and eigenvalues (λ1, λ2, λ3) of the gastrocnemius and soleus muscle were measured at each state.

RESULTS

The mean FA values increased in all muscles in a compressed state, while the mean λ3 decreased. In all muscles, significant differences were found between the rest and compressed states in terms of mean FA and λ3 (P < 0.0001).

CONCLUSION

We confirmed the reversibility of the DTI metrics, which suggests that there was no muscle injury during this study. In cases of compression without fiber injury, the FA value increases, because fibers are strongly aligned in the longitudinal direction.

Presurgical Functional Cortical Mapping Using Electromagnetic Source Imaging

Preoperative localization of functionally eloquent cortex (functional cortical mapping) is common clinical practice in order to avoid or reduce postoperative morbidity. This review aims at providing a general overview of magnetoencephalography (MEG) and high-density electroencephalography (hdEEG) based methods and their clinical role as compared to common alternatives for functional cortical mapping of (1) verbal language function, (2) sensorimotor cortex, (3) memory, (4) visual, and (5) auditory cortex. We highlight strengths, weaknesses and limitations of these functional cortical mapping modalities based on findings in the recent literature. We also compare their performance relative to other non-invasive functional cortical mapping methods, such as functional Magnetic Resonance Imaging (fMRI), Transcranial Magnetic Stimulation (TMS), and to invasive methods like the intracarotid Amobarbital Test (WADA-Test) or intracranial investigations.

Radiation sparing of cerebral cortex in brain tumor patients using quantitative neuroimaging

BACKGROUND AND PURPOSE

Neurocognitive decline in brain tumor patients treated with radiotherapy (RT) may be linked to cortical atrophy. We developed models to determine radiation treatment-planning objectives for cortex, which were tested on a sample population to identify the dosimetric cost of cortical sparing.

MATERIAL AND METHODS

The relationship between the probability of cortical atrophy in fifteen high-grade glioma patients at 1-year post-RT and radiation dose was fit using logistic mixed effects modeling. Cortical sparing was implemented using two strategies: region-specific sparing using model parameters, and non-specific sparing of all normal brain tissue.

RESULTS

A dose threshold of 28.6 Gy was found to result in a 20% probability of severe atrophy. Average cortical sparing at 30 Gy was greater for region-specific dose avoidance (4.6%) compared to non-specific (3.6%). Cortical sparing resulted in an increase in heterogeneity index of the planning target volume (PTV) with an average increase of 1.9% (region-specific) and 0.9% (non-specific).

CONCLUSIONS

We found RT doses above 28.6 Gy resulted in a greater than 20% probability of cortical atrophy. Cortical sparing can be achieved using region-specific or non-specific dose avoidance strategies at the cost of an increase in the dose heterogeneity of the PTV.

Semiautomatic tractography: motor pathway segmentation in patients with intracranial vascular malformations

Object

The visualization of white matter tracts using tractography has previously been achieved by displaying streamlines that pass between regions of interest (ROIs). These techniques require a significant amount of user interaction, and their results are entirely dependent on the positioning of the ROIs. Furthermore, in patients with intracerebral hemorrhage secondary to intracranial vascular malformation, there is often significant cerebral edema and susceptibility artifact from the hematoma, which degrade the reliability of tractography. In this paper, the authors' objectives were to visualize the motor pathways of patients with hemorrhagic and nonhemorrhagic vascular malformations by using a novel semiautomated technique that functions without the need for multiple ROIs.

Methods

The authors investigated the tractography appearance of the descending motor pathways in 6 patients with intracranial vascular malformations. Of these patients 4 presented with a spontaneous intracranial hemorrhage, 2 of whom were clinically hemiparetic. Diffusion tensor imaging was performed using a 1.5-T clinical MR imaging system, and whole-brain tractography was performed after reconstruction of the data. A fractional anisotropy threshold of 0.05 was used to terminate the tractography. The semiautomatic motor pathway segmentation technique required definition of a single voxel within the corticospinal tract of the medulla from which the descending motor pathways were automatically defined by grouping together all streamlines within the entire image with a geometry similar to that of the single streamline generated from this initial voxel. The results of this segmentation were then visually assessed and compared with the patient's motor function.

Results

The authors' semiautomatic algorithm consistently visualized the location of the descending motor pathways in patients with nonhemorrhagic and hemorrhagic vascular malformations. In 1 patient whose complete right hemiplegia (complete paralysis) was caused by a large left frontal hematoma that bisected the descending motor pathways, the authors were unable to reconstruct the motor pathways due to severe tract degeneration. However, in all cases in which motor function was intact or only mildly impaired, the technique clearly delineated the motor pathways, even in the presence of large anatomical displacement by the vascular abnormality or associated hemorrhage.

Conclusions

Semiautomatic tractography allows consistent and rapid demonstration of the descending motor pathways in patients with hemorrhagic and nonhemorrhagic intracranial vascular malformations. The technique allows the use of a comparatively low fractional anisotropy threshold and does not require the definition of multiple ROIs. These techniques may help to improve the clinical feasibility and potentially the reliability of tractography for the evaluation of patients with intracranial vascular malformations as well as other space-occupying lesions of the brain.

Magnetoencephalography in Neurosurgery

OBJECTIVE:

To present applications of magnetoencephalography (MEG) in studies of neurosurgical patients.

METHODS:

MEG maps magnetic fields generated by electric currents in the brain, and allows the localization of brain areas producing evoked sensory responses and spontaneous electromagnetic activity. The identified sources can be integrated with other imaging modalities, e.g., with magnetic resonance imaging scans of individual patients with brain tumors or intractable epilepsy, or with other types of brain imaging data.

RESULTS:

MEG measurements using modern whole-scalp instruments assist in tailoring individual therapies for neurosurgical patients by producing maps of functionally irretrievable cortical areas and by identifying cortical sources of interictal and ictal epileptiform activity. The excellent time resolution of MEG enables tracking of complex spaciotemporal source patterns, helping, for example, with the separation of the epileptic pacemaker from propagated activity. The combination of noninvasive mapping of subcortical pathways by magnetic resonance imaging diffusion tensor imaging with MEG source localization will, in the near future, provide even more accurate navigational tools for preoperative planning. Other possible future applications of MEG include the noninvasive estimation of language lateralization and the follow-up of brain plasticity elicited by central or peripheral neural lesions or during the treatment of chronic pain.

CONCLUSION:

MEG is a mature technique suitable for producing preoperative “road maps” of eloquent cortical areas and for localizing epileptiform activity.

Fractionated Radiotherapy Techniques

A convergence of advances in patient immobilization and localization, patient imaging, beam shaping and delivery, and treatment planning has led to considerable improvement in the ability to deliver highly conformal radiation treatments by radiosurgical or fractionated radiotherapy techniques. The selection of the "best" treatment technique for any given patient needs to consider the morphology of the target and regional organs at risk as well as available technology and institutional expertise.

Utility of three-dimensional anisotropy contrast magnetic resonance axonography for determining condition of the pyramidal tract in glioblastoma patients with hemiparesis

BACKGROUND AND PURPOSE

Three-dimensional anisotropy contrast magnetic resonance axonography (3DAC) is a technique for diffusion weighted magnetic resonance imaging (DWI) that offers reliable visualization of the pyramidal tracts. This study evaluated condition of the pyramidal tract using 3DAC in glioblastoma patients with hemiparesis.

METHODS

In 18 glioblastoma patients before surgery, 3DAC findings of the pyramidal tract responsible for hemiparesis were compared with finding from proton density-weighted imaging (PDWI). To estimate extent of pyramidal tract destruction, fractional anisotropy (FA) values using diffusion tensor magnetic resonance imaging were examined for both the responsible and non-pathological pyramidal tracts.

RESULTS

In all five patients for whom PDWI indicated no hyperintense foci in the responsible pyramidal tract, 3DAC demonstrated no change in color. When PDWI revealed hyperintense foci, 3DAC showed two types of findings: no color change (five patients); or obscured dark area (six patients). When 3DAC showed a dark area, mean FA value in the responsible tract was significantly lower than that for the non-pathological tract.

CONCLUSION

When PDWI indicates hyperintense foci on the pyramidal tract, 3DAC allows prediction of pyramidal tract condition, such as large tumor invasion.