Florian Willomitzer

Consequences of EEG electrode position error on ultimate beamformer source reconstruction performance

Inaccuracy of EEG electrode coordinates forms an error term in forward model generation and ultimate source reconstruction performance. This error arises from the combination of both intrinsic measurement noise of the digitization apparatus and manual coregistration error when selecting corresponding points on anatomical MRI volumes. A common assumption is that such an error would lead only to displacement of localized sources. Here, we measured electrode positions on a 3D-printed full-scale replica head, using three different techniques: a fringe projection 3D scanner, a novel “Flying Triangulation” 3D sensor, and a traditional electromagnetic digitizer. Using highly accurate fringe projection data as ground truth, the Flying Triangulation sensor had a mean error of 1.5 mm while the electromagnetic digitizer had a mean error of 6.8 mm. Then, again using the fringe projection as ground truth, individual EEG simulations were generated, with source locations across the brain space and a range of sensor noise levels. The simulated datasets were then processed using a beamformer in conjunction with the electrode coordinates registered with the Flying Triangulation and electromagnetic digitizer methods. The beamformers output SNR was severely degraded with the digitizer-based positions but less severely with the Flying Triangulation coordinates. Therefore, the seemingly innocuous error in electrode registration may result in substantial degradation of beamformer performance, with output SNR penalties up to several decibels. In the case of low-SNR signals such as deeper brain structures or gamma band sources, this implies that sensor coregistration accuracy could make the difference between successful detection of such activity or complete failure to resolve the source.

Single-shot three-dimensional sensing with improved data density

We introduce a novel concept for motion robust optical 3D sensing. The concept is based on multiline triangulation. The aim is to evaluate a large number of projected lines (high data density), in a large measurement volume, with high precision. Implementing all those attributes at the same time principally allows for the “perfect” single-shot 3D movie camera (our long-term goal). The key problem toward this goal is ambiguous line indexing: we will demonstrate that the necessary information for unique line indexing can be acquired by two synchronized cameras and a back projection scheme. The introduced concept preserves high lateral resolution, since the lines are as narrow as the sampling theorem allows. No spatial bandwidth is consumed by encoding of the lines. In principle, the distance uncertainty is only limited by shot noise and coherent noise. The concept can be also advantageously implemented as a hand-guided sensor with real-time registration, for a complete and dense 3D acquisition of complicated scenes.

Single-shot 3D motion picture camera with a dense point cloud

We discuss physical and information theoretical limits of optical 3D metrology. Based on these principal considerations we introduce a novel single-shot 3D movie camera that almost reaches these limits. The camera is designed for the 3D acquisition of macroscopic live scenes. Like a hologram, each movie-frame encompasses the full 3D information about the object surface and the observation perspective can be varied while watching the 3D movie. The camera combines single-shot ability with a point cloud density close to the theoretical limit. No space-bandwidth is wasted by pattern codification. With 1-megapixel sensors, the 3D camera delivers nearly 300,000 independent 3D points within each frame. The 3D data display a lateral resolution and a depth precision only limited by physics. The approach is based on multi-line triangulation. The requisite low-cost technology is simple. Only two properly positioned synchronized cameras solve the profound ambiguity problem omnipresent in 3D metrology.

Flying triangulation - A motion-robust optical 3D sensor for the real-time shape acquisition of complex objects

The three-dimensional shape acquisition of objects has become more and more important in the last years. Up to now, there are several well-established methods which already yield impressive results. However, even under quite common conditions like object movement or a complex shaping, most methods become unsatisfying. Thus, the 3D shape acquisition is still a difficult and non-trivial task. We present our measurement principle “Flying Triangulation” which enables a motion-robust 3D acquisition of complex-shaped object surfaces by a freely movable handheld sensor. Since “Flying Triangulation” is scalable, a whole sensor-zoo for different object sizes is presented. Concluding, an overview of current and future fields of investigation is given.

Flying Triangulation - Acquiring the 360 Topography of the Human Body on the Fly

We introduce a novel optical measurement principle: “Flying Triangulation”. It fills an important gap in 3D metrology because it enables an acquisition of the topography of moving objects. The immunity against relative motion between object and sensor also allows for medical applications. An easy acquisition of complex objects is possible – just by freely hand guiding the sensor around the object. No tracking is necessary. We will present a “Flying Triangulation” sensor for the intraoral measurement of teeth and a sensor realization for the full 360° 3D acquisition of a person’s head. Parts of the body can be captured with high precision by comfortably guiding the sensor, with real-time control of the result.