Boundary element method in spatial characterization of the electrocardiogram

The electrochemical activity of the heart gives rise to an electric field. In electrocardiography, cardiac electrical activity is assessed by analyzing the potential distribution of this field on the body surface. The potential distribution, or the set of measured surface-voltage signals, is called the electrocardiogram (ECG). Spatial properties of the ECG can be captured with body surface potential mapping (BSPM), in which the electrocardiogram is measured using dozens of electrodes. In this Thesis, methods for solving the forward and inverse problems of electrocardiography are developed and applied to characterization of acute myocardial ischemia. The methodology is based on numerical computation of quasi-static electric fields in a volume conductor model. An open-source Matlab toolbox for solving volume conductor problems with the boundary element method (BEM) is presented. The Galerkin BEM and analytical operator-integrals are, for the first time, applied to the epicardial potential problem; the formulation for a piece-wise homogeneous volume conductor is presented in detail, enabling straightforward inclusion of the lungs or other inhomogeneities in the thorax model. The results show that errors due to discretization and forward-computation are smaller with the linear Galerkin (LG) method than with the conventional methods. These benefits do, however, not reflect to the Tikhonov-regularized inverse solution. If the lungs are omitted, as commonly is done, the choice of the computational method is not significant. In a set of 22 patients measured with BSPM during coronary angioplasty (PTCA), the application of a BEM thorax model with dipolar equivalent sources enabled accurate discrimination between occluded coronary arteries: the correct classification was obtained in 21 patients using the BSPM and in 20 patients using a 5-electrode set suggested elsewhere. The ischemic regions could also be localized anatomically correctly with simplified epicardial potential imaging, even though patient-specific thorax models were not used. In another set, comprising 79 acute ischemic patients and 84 controls, dipole-markers performed well in detection and quantification of acute ischemia. These results show that the modeling-approach can provide valuable information also without patient-specific models and complicated protocols

Truncated RAP-MUSIC (TRAP-MUSIC) for MEG and EEG source localization

Electrically active brain regions can be located applying MUltiple SIgnal Classification (MUSIC) on magneto-or electroencephalographic (MEG; EEG) data. We introduce a new MUSIC method, called truncated recursively-applied-and-projected MUSIC (TRAP-MUSIC). It corrects a hidden deficiency of the conventional RAP-MUSIC algorithm, which prevents estimation of the true number of brain-signal sources accurately. The correction is done by applying a sequential dimension reduction to the signal-subspace projection. We show that TRAP-MUSIC significantly improves the performance of MUSIC-type localization; in particular, it successfully and robustly locates active brain regions and estimates their number. We compare TRAP-MUSIC and RAP-MUSIC in simulations with varying key parameters, e.g., signal-to-noise ratio, correlation between source time-courses, and initial estimate for the dimension of the signal space. In addition, we validate TRAP-MUSIC with measured MEG data. We suggest that with the proposed TRAP-MUSIC method, MUSIC-type localization could become more reliable and suitable for various online and offline MEG and EEG applications.Peer reviewe

Cast immobilization in bayonet position versus reduction and pin fixation of overriding distal metaphyseal radius fractures in children under ten years of age : a case control study

Purpose Completely displaced distal radius fractures in children have been traditionally reduced and immobilized with a cast or pin fixed. Cast immobilization leaving the fracture displaced in the bayonet position has been recently suggested as a non-invasive and effective treatment alternative. This is a pilot comparative study between reduction and no reduction. Methods We assessed subjective, functional and radiographic outcome after a minimum 2.5-year follow-up in 12 children under ten years of age who had sustained a completely displaced metaphyseal radius fracture, which had been immobilized leaving the fracture in an overriding position (shortening 3 mm to 9 mm). A total of 12 age-matched patients, whose similar fractures were reduced and pin fixed, were chosen for controls. Results At follow-up none of the 24 patients had visible forearm deformity and the maximal angulation in radiographs was 5 degrees Forearm and wrist movement was restored (<10 degrees of discrepancy) in all 24 patients. Grip strength ratio was normal in all but three surgically treated patients. All patients had returned to their previous activities. One operatively treated boy who was re-operated on reported of pain (visual analogue scale 2). Conclusion The results of this study do not demonstrate the superiority of reduction and pin fixation over cast immobilization in the bayonet position of closed overriding distal metaphyseal radius fractures in children under ten years with normal neurovascular findings.Peer reviewe

Most surgeons still prefer to reduce overriding distal radius fractures in children

Background and purpose - Traditionally, overriding distal radius fractures in children have been reduced and immobilized with a cast or treated with percutaneous pin fixation. There is recent evidence that these fractures heal well if immobilized in the bayonet position without reduction. We evaluated the present treatment of these fractures. Methods - A questionnaire including AP and lateral radiographs of overriding distal radius fractures in 3 pre-pubertal children was answered by 213 surgeons from 28 countries. The surgeons were asked to choose their preferred method of treatment (no reduction, reduction, reduction and osteosynthesis), type and length of cast immobilization, and the number of clinical and radiographic follow-ups. Results - Of the 213 participating surgeons, 176 (83%) would have reduced all 3 presented fractures, whereas 4 (2%) would have treated all 3 children with cast immobilization without reduction. Most reductions (77%) would have been done under general anesthesia. Over half (54%) of the surgeons who preferred anesthesia would have fixed (pins 99%, plate 1%) the fractures. An above-elbow splint or circular cast was chosen in 84% of responses, and the most popular (44%) length of immobilization was 4 weeks. Surgeons from the Nordic countries were more eager to fix the fractures (54% vs. 31%, p <0.001) and preferred shorter immobilization and follow-up times and less frequent clinical and radiological follow-ups compared with their colleagues from the USA. Interpretation - Most of the participating surgeons prefer to reduce overriding distal radius fractures in children under anesthesia. There is substantial lack of agreement on the indications for osteosynthesis, type of cast, length of immobilization, and follow-up protocol.Peer reviewe

Coil optimisation for transcranial magnetic stimulation in realistic head geometry

Background: Transcranial magnetic stimulation (TMS) allows focal, non-invasive stimulation of the cortex. A TMS pulse is inherently weakly coupled to the cortex; thus, magnetic stimulation requires both high current and high voltage to reach sufficient intensity. These requirements limit, for example, the maximum repetition rate and the maximum number of consecutive pulses with the same coil due to the rise of its temperature. Objective: To develop methods to optimise, design, and manufacture energy-efficient TMS coils in realistic head geometry with an arbitrary overall coil shape. Methods: We derive a semi-analytical integration scheme for computing the magnetic field energy of an arbitrary surface current distribution, compute the electric field induced by this distribution with a boundary element method, and optimise a TMS coil for focal stimulation. Additionally, we introduce a method for manufacturing such a coil by using Litz wire and a coil former machined from polyvinyl chloride. Results: We designed, manufactured, and validated an optimised TMS coil and applied it to brain stimulation. Our simulations indicate that this coil requires less than half the power of a commercial figure-of-eight coil, with a 41% reduction due to the optimised winding geometry and a partial contribution due to our thinner coil former and reduced conductor height. With the optimised coil, the resting motor threshold of abductor pollicis brevis was reached with the capacitor voltage below 600 V and peak current below 3000 A. Conclusion: The described method allows designing practical TMS coils that have considerably higher efficiency than conventional figure-of-eight coils. (C) 2017 Elsevier Inc. All rights reserved.Peer reviewe

Accuracy and precision of navigated transcranial magnetic stimulation

Objective. Transcranial magnetic stimulation (TMS) induces an electric field (E-field) in the cortex. To facilitate stimulation targeting, image-guided neuronavigation systems have been introduced. Such systems track the placement of the coil with respect to the head and visualize the estimated cortical stimulation location on an anatomical brain image in real time. The accuracy and precision of the neuronavigation is affected by multiple factors. Our aim was to analyze how different factors in TMS neuronavigation affect the accuracy and precision of the coil-head coregistration and the estimated E-field. Approach. By performing simulations, we estimated navigation errors due to distortions in magnetic resonance images (MRIs), head-to-MRI registration (landmark- and surface-based registrations), localization and movement of the head tracker, and localization of the coil tracker. We analyzed the effect of these errors on coil and head coregistration and on the induced E-field as determined with simplistic and realistic head models. Main results. Average total coregistration accuracies were in the range of 2.2-3.6 mm and 1 degrees; precision values were about half of the accuracy values. The coregistration errors were mainly due to head-to-MRI registration with average accuracies 1.5-1.9 mm/0.2-0.4 degrees and precisions 0.5-0.8 mm/0.1-0.2 degrees better with surface-based registration. The other major source of error was the movement of the head tracker with average accuracy of 1.5 mm and precision of 1.1 mm. When assessed within an E-field method, the average accuracies of the peak E-field location, orientation, and magnitude ranged between 1.5 and 5.0 mm, 0.9 and 4.8 degrees, and 4.4 and 8.5% across the E-field models studied. The largest errors were obtained with the landmark-based registration. When computing another accuracy measure with the most realistic E-field model as a reference, the accuracies tended to improve from about 10 mm/15 degrees/25% to about 2 mm/2 degrees/5% when increasing realism of the E-field model. Significance. The results of this comprehensive analysis help TMS operators to recognize the main sources of error in TMS navigation and that the coregistration errors and their effect in the E-field estimation depend on the methods applied. To ensure reliable TMS navigation, we recommend surface-based head-to-MRI registration and realistic models for E-field computations.Peer reviewe