Examining the Effects of One- and Three-Dimensional Spatial Filtering Analyses in Magnetoencephalography

Spatial filtering, or beamforming, is a commonly used data-driven analysis technique in the field of Magnetoencephalography (MEG). Although routinely referred to as a single technique, beamforming in fact encompasses several different methods, both with regard to defining the spatial filters used to reconstruct source-space time series and in terms of the analysis of these time series. This paper evaluates two alternative methods of spatial filter construction and application. It demonstrates how encoding different requirements into the design of these filters has an effect on the results obtained. The analyses presented demonstrate the potential value of implementations which examine the timeseries projections in multiple orientations at a single location by showing that beamforming can reconstruct predominantly radial sources in the case of a multiple-spheres forward model. The accuracy of source reconstruction appears to be more related to depth than source orientation. Furthermore, it is shown that using three 1-dimensional spatial filters can result in inaccurate source-space time series reconstruction. The paper concludes with brief recommendations regarding reporting beamforming methodologies in order to help remove ambiguity about the specifics of the techniques which have been used

Validity of the 3D VECTRA photogrammetric surface imaging system for cranio-maxillofacial anthropometric measurements

PURPOSE: The use of three-dimensional (3D) photography for anthropometric measurements is of increasing interest, especially in the cranio-maxillofacial field. Before standard implementation, accurate determination of the precision and accuracy of each system is mandatory. METHODS: A mannequin head was labelled with 52 landmarks, and 28 three-dimensional images were taken using a commercially available five-pod 3D photosystem (3D VECTRA; Canfield, Fairfield, NJ) in different head positions. Distances between the landmarks were measured manually using a conventional calliper and compared with the digitally calculated distances acquired from labelling by two independent observers. The experimental set-up accounted for clinical circumstances by varying the positioning (vertical, horizontal, sagittal) of the phantom. RESULTS: In the entire calliper measurement data set (n = 410), a significant difference (p = 0.02) between the directly measured and corresponding virtually calculated distances was found. The mean aberration between both modalities covering all data was 7.96 mm. No differences (p = 0.94) between the two groups were found using a cut-off of 10 % (leaving n = 369 distances) due to considerable errors in direct measurements and the necessary manual data translation. The mean diversity of both measurement modalities after cut-off was 1.33 mm (maximum, 6.70 mm). Inter-observer analysis of all 1,326 distances showed no difference (p = 0.99; maximal difference, 0.58 mm) in the digital measurements. CONCLUSION: The precision and accuracy of this five-pod 3D photosystem suggests its suitability for clinical applications, particularly anthropometric studies. Three-hundred-and-sixty degree surface-contour mapping of the craniofacial region within milliseconds is particularly useful in paediatric patients. Proper patient positioning is essential for high-quality imaging