Search

This study shows that the spatial specificity of MEG beamformer estimates of electrical activity can be affected significantly by the way in which covariance estimates are calculated. We define spatial specificity as the ability to extract independent timecourse estimates of electrical brain activity from two separate brain locations in close proximity. Previous analytical and simulated results have shown that beamformer estimates are affected by narrowing the time frequency window in which covariance estimates are made. Here we build on this by both experimental validation of previous results, and investigating the effect of data averaging prior to covariance estimation. In appropriate circumstances, we show that averaging has a marked effect on spatial specificity. However the averaging process results in ill-conditioned covariance matrices, thus necessitating a suitable matrix regularisation strategy, an example of which is described. We apply our findings to an MEG retinotopic mapping paradigm. A moving visual stimulus is used to elicit brain activation at different retinotopic locations in the visual cortex. This gives the impression of a moving electrical dipolar source in the brain. We show that if appropriate beamformer optimisation is applied, the moving source can be tracked in the cortex. In addition to spatial reconstruction of the moving source, we show that timecourse estimates can be extracted from neighbouring locations of interest in the visual cortex. If appropriate methodology is employed, the sequential activation of separate retinotopic locations can be observed. The retinotopic paradigm represents an ideal platform to test the spatial specificity of source localisation strategies. We suggest that future comparisons of MEG source localisation techniques (e.g. beamformer, minimum norm, Bayesian) could be made using this retinotopic mapping paradigm.

Microwave thermotherapy (MT) is an oncological treatment. At present the invasive thermometer probes are clinically used for temperature measuring during an MT. Any invasive handling of tumors is of high-risk. A new possible method of noninvasive monitoring of temperature distribution in tissue has been developed. An MT treatment of the experimentally induced pedicle-tumors of the rat was prepared. For 100 rat samples a strong correlation between the mean gray level in the ROIs in the ultrasound pictures and the invasively measured temperature in the range 37-44 °C was found. The correlation coefficient of the mean gray level and the invasively measured temperature is 0.96a0.05. A system for representation of changes of spatial temperature distribution of the whole tumor during MT is presented.

The development of a technique for the recording of fetal brain activity has been highly limited by the inaccessibility of the developing human brain in utero. Based on the needs of resolution and a noninvasive nature, only biomagnetic measurements are suitable for the determination of the integrity of the fetal brain on a routine basis. We have built a new biomagnetic measurement device dedicated this task. The main construction concerns were those of patient comfort, ease of performance of standard measurements, and appropriate spatial and temporal resolution. Our initial evaluation of the instrument has shown that it is capable of obtaining signals traditionally regarded as evoked fetal fields. Our results also show that this interpretation has to he made very carefully and that a large scale array is more appropriate for the extraction of fetal brain signals., Presented at Annual International Conference of the IEEE Engineering in Medicine and Biology Society (23rd) held in Istanbul, Turkey on 25-28 Oct 2001. See also ADM001351 for entire conference on CD-ROM. The original document contains color images.