Semiconductor-to-Insulator Transition in Inter-Electrode Bridge-like Ensembles of Anatase Nanoparticles under a Long-Term Action of the Direct Current.

The results of experimental studies of ohmic conductivity degradation in the ensembles of nanostructured anatase bridges under a long-term effect of direct current are presented. Stochastic sets of partially conducting inter-electrode bridges consisting of close-packed anatase nanoparticles were formed by means of the seeding particles from drying aqueous suspensions on the surfaces of silica substrates with interdigital platinum electrodes. Multiple-run experiments conducted at room temperature have shown that ohmic conductivity degradation in these systems is irreversible. It is presumably due to the accumulated capture of conduction electrons by deep traps in anatase nanoparticles. The scaling analysis of voltage drops across the samples at the final stage of degradation gives a critical exponent for ohmic conductivity as ≈1.597. This value satisfactorily agrees with the reported model data for percolation systems. At an early stage of degradation, the spectral density of conduction current fluctuations observed within the frequency range of 0.01-1 Hz decreases approximately as 1/ω, while near the percolation threshold, the decreasing trend changes to ≈1/ω2. This transition is interpreted in terms of the increasing contribution of blockages and subsequent avalanche-like breakdowns of part of the local conduction channels in the bridges into electron transport near the percolation threshold.

Appliance of Navigated Transcranial Magnetic Stimulation in Radiosurgery for Brain Metastases.

PURPOSE: Navigated transcranial magnetic stimulation (nTMS) provides noninvasive visualization of eloquent brain areas. The nTMS is usually applied in presurgical planning to minimize the risk of surgery-related neurological deterioration. The aim of this study was to evaluate the usefulness of nTMS data for GammaKnife treatment planning for patients suffering from brain metastases. METHODS: Motor cortex mapping with nTMS was performed in eight patients with brain metastases within or adjacent to the precentral gyrus. The nTMS data set was imported into the planning software and fused with anatomical MRI. Then contouring of the target and critical structures was performed. Treatment plans with and without visualization of the functional structures by nTMS were analyzed and compared by neurosurgeon and medical physicist. RESULTS: The primary motor cortex was successfully delineated even in all cases despite significant peritumoral edema. Beam shaping and combined isocenters were used for conformal dose distribution and steeper dose fall-off near the identified eloquent zone. Compared with plans without nTMS data, treatment plans with integration of cortical nTMS mapping data showed a 2% to 78% (mean, 35.2% ± 22.7%) lower 12-Gy volume within the motor cortex without reduction of the dose applied to the tumor. CONCLUSIONS: The presented approach allows the easy and reliable integration of neurophysiological mapping data into GammaKnife treatment plans by the standard GammaPlan software. Diminishing the dose to critical structures might help to minimize side effects and therefore improve quality of life for brain metastasis patients.