Risto J Ilmoniemi

Publications (232) View all

• Article: Efficient concomitant and remanence field artifact reduction in ultra-low-field MRI using a frequency-space formulation.

  Yi-Cheng Hsu, Panu T Vesanen, Jaakko O Nieminen, Koos C J Zevenhoven, Juhani Dabek, Lauri Parkkonen, I-Liang Chern, Risto J Ilmoniemi, Fa-Hsuan Lin
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  ABSTRACT: PURPOSE: For ultra-low-field MRI, the spatial-encoding magnetic fields generated by gradient coils can have strong concomitant fields leading to prominent image distortion. Additionally, using superconducting magnet to pre-polarize magnetization can improve the signal-to-noise ratio of ultra-low-field MRI. Yet the spatially inhomogeneous remanence field due to the permanently trapped flux inside a superconducting pre-polarizing coil modulates magnetization and causes further image distortion. METHOD: We propose a two-stage frequency-space (f-x) formulation to accurately describe the dynamics of spatially-encoded magnetization under the influence of concomitant and remanence fields, which allows for correcting image distortion due to concomitant and remanence fields. RESULTS: Our method is computationally efficient as it uses a combination of the fast Fourier transform algorithm and a linear equation solver. With sufficiently dense discretization in solving the linear equation, the performance of this f-x method was found to be stable among different choices of the regularization parameter and the regularization matrix. CONCLUSION: We present this method together with numerical simulations and experimental data to demonstrate how concomitant and remanence field artifacts in ultra-low-field MRI can be corrected efficiently. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
  Magnetic Resonance in Medicine 05/2013; · 2.96 Impact Factor
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  Available from: Tiina Näsi

  Article: Effect of task-related extracerebral circulation on diffuse optical tomography: experimental data and simulations on the forehead.

  Tiina Näsi, Hanna Mäki, Petri Hiltunen, Juha Heiskala, Ilkka Nissilä, Kalle Kotilahti, Risto J Ilmoniemi
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  ABSTRACT: The effect of task-related extracerebral circulatory changes on diffuse optical tomography (DOT) of brain activation was evaluated using experimental data from 14 healthy human subjects and computer simulations. Total hemoglobin responses to weekday-recitation, verbal-fluency, and hand-motor tasks were measured with a high-density optode grid placed on the forehead. The tasks caused varying levels of mental and physical stress, eliciting extracerebral circulatory changes that the reconstruction algorithm was unable to fully distinguish from cerebral hemodynamic changes, resulting in artifacts in the brain activation images. Crosstalk between intra- and extracranial layers was confirmed by the simulations. The extracerebral effects were attenuated by superficial signal regression and depended to some extent on the heart rate, thus allowing identification of hemodynamic changes related to brain activation during the verbal-fluency task. During the hand-motor task, the extracerebral component was stronger, making the separation less clear. DOT provides a tool for distinguishing extracerebral components from signals of cerebral origin. Especially in the case of strong task-related extracerebral circulatory changes, however, sophisticated reconstruction methods are needed to eliminate crosstalk artifacts.
  Biomedical Optics Express 03/2013; 4(3):412-26. · 2.33 Impact Factor
• Article: TMS-evoked changes in brain-state dynamics quantified by using EEG data.

  Tuomas Mutanen, Jaakko O Nieminen, Risto J Ilmoniemi
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  ABSTRACT: To improve our understanding of the combined transcranial magnetic stimulation (TMS) and electroencephalography (EEG) method in general, it is important to study how the dynamics of the TMS-modulated brain activity differs from the dynamics of spontaneous activity. In this paper, we introduce two quantitative measures based on EEG data, called mean state shift (MSS) and state variance (SV), for evaluating the TMS-evoked changes in the brain-state dynamics. MSS quantifies the immediate TMS-elicited change in the brain state, whereas SV shows whether the rate at which the brain state changes is modulated by TMS. We report a statistically significant increase for a period of 100-200 ms after the TMS pulse in both MSS and SV at the group level. This indicates that the TMS-modulated brain state differs from the spontaneous one. Moreover, the TMS-modulated activity is more vigorous than the natural activity.
  Frontiers in Human Neuroscience 01/2013; 7:155. · 2.34 Impact Factor
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  Available from: Fa-Hsuan Lin

  Article: Suppressing multi-channel ultra-low-field MRI measurement noise using data consistency and image sparsity.

  Fa-Hsuan Lin, Panu T Vesanen, Yi-Cheng Hsu, Jaakko O Nieminen, Koos C J Zevenhoven, Juhani Dabek, Lauri T Parkkonen, Juha Simola, Antti I Ahonen, Risto J Ilmoniemi
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  ABSTRACT: Ultra-low-field (ULF) MRI (B 0 = 10-100 µT) typically suffers from a low signal-to-noise ratio (SNR). While SNR can be improved by pre-polarization and signal detection using highly sensitive superconducting quantum interference device (SQUID) sensors, we propose to use the inter-dependency of the k-space data from highly parallel detection with up to tens of sensors readily available in the ULF MRI in order to suppress the noise. Furthermore, the prior information that an image can be sparsely represented can be integrated with this data consistency constraint to further improve the SNR. Simulations and experimental data using 47 SQUID sensors demonstrate the effectiveness of this data consistency constraint and sparsity prior in ULF-MRI reconstruction.
  PLoS ONE 01/2013; 8(4):e61652. · 4.09 Impact Factor
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  Available from: Fa-Hsuan Lin

  Article: Noise Amplification in Parallel Whole-Head Ultra-Low-Field Magnetic Resonance Imaging Using 306 Detectors

  Fa-Hsuan Lin, Panu T. Vesanen, Jaakko O. Nieminen, Yi-Cheng Hsu, Koos C.J. Zevenhoven, Juhani Dabek, Lauri T. Parkkonen, Andrey Zhdanov, Risto J. Ilmoniemi
  Magnetic Resonance in Medicine 09/2012; · 2.96 Impact Factor