Browsing by Author "Kataja, Juhani"

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  • In vivo and dosimetric investigation on electrical vestibular stimulation with frequency- and amplitude-modulated currents
    (2024-08-01) Nissi, Janita; Kangasmaa, Otto; Kataja, Juhani; Bouisset, Nicolas; Laakso, Ilkka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Objective. Normal function of the vestibular system can be disturbed using a noninvasive technique called electrical vestibular stimulation (EVS), which alters a person’s sense of balance and causes false sensations of movement. EVS has been widely used to study the function of the vestibular system, and it has recently gained interest as a therapeutic tool to improve postural stability and help those suffering from vestibular dysfunction. Yet, understanding of how EVS stimulates the vestibular system, the current intensity needed to produce an effect and the frequencies at which it occurs have remained unclear. Approach. The effect of EVS on postural sway was examined in five participants using sinusoidal alternating current with time-varying amplitude from 0 to 1.5 mA and frequency from 0.1 to 10 Hz for three electrode configurations. Dosimetry of the current flow inside the head was conducted using anatomically realistic computational models created individually for each subject based on magnetic resonance imaging data. An estimate for the minimal field strength capable of affecting the vestibular system was calculated with the finite element method. Main results. Bipolar EVS at frequencies up to 10 Hz caused harmonic full-body swaying, and the frequency of the sway was the same as that of the stimulation current. The size of the sway was amplified by increasing the current intensity. Dosimetry modeling indicated that, for 0.2 mA current, the average electric field strength in the vestibular system was approximately 10-30 mV m−1, depending on the electrode montage. The size of the measured postural sway was proportional to the montage-specific electric field strength in the vestibular system. Significance. The results provide insight to EVS’s working mechanisms and improve its potential as a tool to study the sense of balance.
  • Influence of magnetic forces and magnetostriction on the vibration behavior of an induction motor
    (2019-01-01) Sathyan, Sabin; Aydin, Ugur; Lehikoinen, Antti; Belahcen, Anouar; Vaimann, Toomas; Kataja, Juhani
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This paper presents the vibration analysis and results of an induction motor through two kinds of magneto-mechanical coupling methods. The computation results are validated by comparing them with the vibration measurements of the motor. The role of both the magnetic forces and magnetostriction are examined and distinguished based on their contribution to the vibration behavior of the machine. It was found that the pole pair number of an induction machine can affect the way the vibrations caused by magnetostriction and magnetic forces either add up or oppose each other.
  • Material Coarsening Strategy for Structured Meshless Multigrid Method for Dosimetry in Anisotropic Human Body Models
    (2023-12-01) Kataja, Juhani; Nissi, Janita; Roine, Timo; Laakso, Ilkka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this article, a novel multigrid-based method is developed for modeling of electric fields and currents in electrically anisotropic and heterogeneous media. The method is useful for numerical assessment of human exposure to low-frequency electromagnetic fields, which has been typically performed using isotropic models of the human body. The method is matrix-free and based on structured voxelized grids. Numerical tests indicate that the method has all typical benefits of multigrid methods, i.e., convergence in a constant number of iterations and linear complexity in terms of the number of degrees of freedom. The developed method is applied to the dosimetry of the induced electric field in ten anatomically realistic models of the human head exposed to uniform 50-Hz magnetic fields. The models are constructed from magnetic resonance images and account for the electrical anisotropy of the brain tissues via diffusion weighted images. The dosimetry modeling shows that the tissue anisotropy leads to a small but significant increase (14% on average) of the 99th percentile induced electric field strength in the white matter of the brain. However, isotropic models can still provide sufficient accuracy for dosimetry if this slight underestimation of the 99th percentile electric field strength is accounted for.
  • Measurements and shimming for a low-field magnetic resonance imaging system
    (2022-08-22) Streng, Tony
     Sähkötekniikan korkeakoulu | Master's thesis
    Low field magnetic resonance imaging(MRI) is being researched as an accessible, low-cost imaging method for point-of-care applications. By default, the base magnetic field of an MRI system requires corrective measures to increase homogeneity. The corrective measures, known as shimming, can be done passively by adding ferromagnetic pieces or permanent magnets or actively by running current through coils. Additionally, the base field of the MRI system must be known to shim the magnetic field accurately. Here, the process of shimming a low-field magnetic resonance imaging prototype is presented. Using a modified CNC machine combined with a magnetic field sensor, a measurement setup was created to measure the field at Lebedev quadrature points on a sphere. The measurement data is decomposed into spherical harmonic function coefficients used to design an active shimming solution. Using COMSOL Multiphysics coils are designed that affect two of the most prominent non-linear spherical harmonic function coefficients. The effect on spherical harmonic function coefficients of the two shim coil setups and the three gradient coils is measured. The measured coil coefficients are used to linearly solve for the required currents in each coil to eliminate unwanted coefficients. The result is that the inhomogeneity in a spherical region of interest of diameter 15 cm was reduced to below 500 parts per million(PPM) from above 5800 PPM. Removing the targeted spherical harmonic coefficients showed that a rudimentary setup could be used to shim a low-field MRI device. The homogeneity of the device's magnetic field can be further improved with additional active and passive shimming measures.
  • Myelinoitujen aksonien kaapeliyhtälön parametrien mittaukset
    (2022-12-11) Eklund, Eelis
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Predicting the hotspot location and motor threshold prior to transcranial magnetic stimulation using electric field modelling
    (2024-01-07) Matilainen, Noora; Kataja, Juhani; Laakso, Ilkka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Objective. To investigate whether the motor threshold (MT) and the location of the motor hotspot in transcranial magnetic stimulation (TMS) can be predicted with computational models of the induced electric field. Approach. Individualized computational models were constructed from structural magnetic resonance images of ten healthy participants, and the induced electric fields were determined with the finite element method. The models were used to optimize the location and direction of the TMS coil on the scalp to produce the largest electric field at a predetermined cortical target location. The models were also used to predict how the MT changes as the magnetic coil is moved to various locations over the scalp. To validate the model predictions, the motor evoked potentials were measured from the first dorsal interosseous (FDI) muscle with TMS in the ten participants. Both computational and experimental methods were preregistered prior to the experiments. Main results. Computationally optimized hotspot locations were nearly as accurate as those obtained using manual hotspot search procedures. The mean Euclidean distance between the predicted and the measured hotspot locations was approximately 1.3 cm with a 0.8 cm bias towards the anterior direction. Exploratory analyses showed that the bias could be removed by changing the cortical target location that was used for the prediction. The results also indicated a statistically significant relationship (p < 0.001) between the calculated electric field and the MT measured at several locations on the scalp. Significance. The results show that the individual TMS hotspot can be located using computational analysis without stimulating the subject or patient even once. Adapting computational modelling would save time and effort in research and clinical use of TMS.
  • A probabilistic transcranial magnetic stimulation localization method
    (2021-08) Kataja, Juhani; Soldati, Marco; Matilainen, Noora; Laakso, Ilkka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Objective. Transcranial magnetic stimulation (TMS) can be used to safely and noninvasively activate brain tissue. However, the characteristic parameters of the neuronal activation have been largely unclear. In this work, we propose a novel neuronal activation model and develop a method to infer its parameters from measured motor evoked potential signals. Approach. The connection between neuronal activation due to an induced electric field and a measured motor threshold is modeled. The posterior distribution of the model parameters are inferred from measurement data using Bayes' formula. The measurements are the active motor thresholds obtained with multiple stimulating coil locations, and the parameters of the model are the location, preferred direction of activation, and threshold electric field value of the activation site. The posterior distribution is sampled using a Markov chain Monte Carlo method. We quantify the plausibility of the model by calculating the marginal likelihood of the measured thresholds. The method is validated with synthetic data and applied to motor threshold measurements from the first dorsal interosseus muscle in five healthy participants. Main results. The method produces a probability distribution for the activation location, from which a minimal volume where the activation occurs with 95% probability can be derived. For eight or nine stimulating coil locations, the smallest such a volume obtained was approximately 100 mm3. The 95% probability volume intersected the pre-central gyral crown and the anterior wall of the central sulcus, and the preferred direction was perpendicular to the central sulcus, both findings being consistent with the literature. Furthermore, it was not possible to rule out if the activation occurred either in the white or grey matter. In one participant, two distinct activations sites were found while others exhibited a unique site. Significance. The method is both generic and robust, and it lays a foundation for a framework that enables accurate analysis and characterization of TMS activation mechanisms.
  • Pulssimuodon säätö ja vaikutukset transkraniaalisessa magneettistimulaatiossa
    (2021-12-17) Peltola, Pauliina
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Red to Near-Infrared Light Stimulation in the Treatment of Alzheimer’s Disease
    (2023-09-24) Lahdensivu, Otto
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Reliable and Efficient Numerical Methods for Time Harmonic Electromagnetic Design Problems
    (2014) Kataja, Juhani
     School of Electrical Engineering | Doctoral dissertation (article-based)
    This dissertation studies numerical methods for solving time-harmonic Maxwell's equations. In this work, boundary element and a least squares type finite element method for time-harmonic Maxwell's equations is analysed. The boundary element method is studied from the point of view of shape optimization and shape sensitivity analysis. The main result of the dissertation is the description how to calculate numerically the derivative of electromagnetic properties of a metallic scatterer with respect to variation of its shape. The motivation of the result is the adjoint variable method aimed at computing the derivative of a cost function of a generic optimization problem. The method has the property that, essentially, it suffices to differentiate the system matrix with respect to shape if the load functional or the cost functional do not depend on the shape explicitly. The boundary element method is also applied in as wide as possible characterisation of the shapes of wire dipoles and that way in shape optimisation. The least-squares finite-element method for time-harmonic Maxwell's equations results in a method where the system matrix is positive definite despite the underlying physics those of time-harmonic fields usually lead to strictly indefinite systems. The positive definiteness of the system matrix is inherited from the ellipticity of the sesquilinear form of the variational method. The implementation of a conformal discretization is not, however, always reasonable for the reason that in certain geometries the finite element solution vanishes on the boundary when only tangential, or normal, component is required to vanish. Therefore, the discrete solution space is widened in such a manner that on mesh dense enough the sesquilinear form is still elliptic and that the tangential, or normal, component vanishes in suitable weak sense.
  • Sähköimpedanssitomografia (EIT) laitteistonäkökulmasta
    (2020-12-07) Sorvoja, Mika
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Sähkömagnettisen sironnan reunaintegraaliyhtälöiden numeriikasta
    (2008) Kataja, Juhani
     Faculty of Electronics, Communications and Automation | Master's thesis
    Tässä työssä johdetaan Stratton-Chu kaavoista ja niihin liittyvien potentiaalien raja-arvokaavoista lähtien EFIE, MFIE, CFIE ja Müllerin formulaatiot sähkömagneettisen sironnan integraaliyhtälöille. Näiden avulla rakennetaan C- ja MATLAB-ympäristössä RWG-elementtejä käyttäen numeeriset ratkaisuohjelmat EFIE, MFIE sekä CFIE-formulaatioille täysjohtavan kappaleen tapauksessa sekä Müllerin formulaatiolle dielektrisen kappaleen tapauksessa. Tämän toteutuksen avulla tutkitaan eri formulaatioiden käyttäytymistä numeerisesti. Erityisesti näytettiin numeerisesti pintaintegraaliyhtälöiden sisäresonanssi- ja matalan taajuuden ongelmien esiintyminen. Lisäksi näytettiin numeerisesti, että nämä ongelmat poistuvat sopivalla formulaatiolla. Lopuksi sirontayhtälön ratkaisijaa käytettiin pitkän kapean levyantennin muodon optimointiin antennin hyvän suuntaavuuden saavuttamiseksi. Tuloksena löydettiin muoto, joka on lähellä aikaisemmin tunnettua Landstorferin lanka-antennia sekä kaksi muuta muotoa, jotka poikkeavat huomattavasti Landstorferin antenneista.
  • Transkraniaalinen staattinen magneettikenttä -stimulaatio (tSMS)
    (2023-12-20) Myllymäki, Tomi
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Transkraniaalisella magneettistimulaatiolla aikaansaadut herätepotentiaalit
    (2020-09-01) Mykkänen, Riina
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Vaimea jakso transkraniaalisessa magneettistimulaatiossa
    (2023-05-05) Toivanen, Laura
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Verification of neuronavigated TMS accuracy using structured-light 3D scans
    (2024-04-21) Matilainen, Noora; Kataja, Juhani; Laakso, Ilkka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Objective. To investigate the reliability and accuracy of the manual three-point co-registration in neuronavigated transcranial magnetic stimulation (TMS). The effect of the error in landmark pointing on the coil placement and on the induced electric and magnetic fields was examined. Approach. The position of the TMS coil on the head was recorded by the neuronavigation system and by 3D scanning for ten healthy participants. The differences in the coil locations and orientations and the theoretical error values for electric and magnetic fields between the neuronavigated and 3D scanned coil positions were calculated. In addition, the sensitivity of the coil location on landmark accuracy was calculated. Main results. The measured distances between the neuronavigated and 3D scanned coil locations were on average 10.2 mm, ranging from 3.1 to 18.7 mm. The error in angles were on average from two to three degrees. The coil misplacement caused on average a 29% relative error in the electric field with a range from 9% to 51%. In the magnetic field, the same error was on average 33%, ranging from 10% to 58%. The misplacement of landmark points could cause a 1.8-fold error for the coil location. Significance. TMS neuronavigation with three landmark points can cause a significant error in the coil position, hampering research using highly accurate electric field calculations. Including 3D scanning to the process provides an efficient method to achieve a more accurate coil position.