Accurate mapping of current flows in the human brain is important for many neuroscientific applications. MRCDI is an emerging method, which combines MRI with externally applied alternating currents to derive current flow distributions based on measurements of the current-induced magnetic fields. However, inaccurate and inconsistent measurements occur unless the stray magnetic fields ca used by the currents flowing in the feeding cables are corrected [1] . Here, we explore the influences of the stray magnetic fields due to the cable - currents in realistic experimental MRCDI set - ups

Göksu, Cihan

Hanson, Lars G.

Scheffler, Klaus

Siebner, Hartwig R.

Thielscher, Axel

Göksu, C.

Scheffler, K.

Siebner, H.

Hanson, L.

Thielscher, A.

MPG.PuRe

Correction of stray magnetic fields caused by cable currents is essential for human in-vivo brain magnetic resonance current density imaging (MRCDI)  Cihan Göksu1, Klaus Scheffler2,3, Hartwig R. Siebner1,4, Lars G. Hanson1,5 and Axel Thielscher1,5   1Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Denmark. 2High-Field Magnetic Resonance Center, Max-Planck-Institute for Biological Cybernetics, Tübingen, Germany. 3Department of Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany. 4Department of Neurology, Copenhagen University Hospital, Bispebjerg, Denmark. 5Center for Magnetic Resonance, DTU Health Technology, Technical University of Denmark, Kgs Lyngby, Denmark.  Introduction: Accurate mapping of current flows in the human brain is important for many neuroscientific applications. MRCDI is an emerging method, which combines MRI with externally applied alternating currents to derive current flow distributions based on measurements of the current-induced magnetic fields. However, inaccurate and inconsistent measurements occur unless the stray magnetic fields caused by the currents flowing in the feeding cables are corrected [1]. Here, we explore the influences of the stray magnetic fields due to the cable-currents in realistic experimental MRCDI set-ups. Theory and Methods: Alternating electrical currents in synchrony with the MR sequence are generated by a current source, filtered to avoid RF interference, and injected into the human head via scalp electrodes (Fig. 1). The injected currents create a magnetic field distribution, and the field component ∆Bz,c that is parallel to the scanner field modulates the measured MR signal phase accordingly. The measured phase difference for opposite currents can thus be used for current-induced field ∆Bz,c = (∡μ+ − ∡μ−)/mseq calculations, where mseq is the phase sensitivity of the sequence that can be simulated by Bloch equations and rotation matrices [1,2]. The measured ∆Bz,c can then be used for current flow reconstructions Jrec =  J0 +1μ0(∂(∆Bz,c−∆Bz,c0)∂y, −∂(∆Bz,c−∆Bz,c0)∂x, 0), where ∆Bz,c0 and J0 are the magnetic field and current flow simulations for a uniform conductivity distribution [3]. Recently, we have achieved consistent ∆Bz,c measurements in the human brain with 0.1nT sensitivity (Fig. 2; for a total scan time of ~9 mins) by means of an optimized sequence (Fig. 1b), including a correction for the stray magnetic fields of the cables. Here, we explore the importance of correcting these fields for MRCDI measurements for a range of cable configurations. We performed Biot-Savart simulations to observe the field changes due to wire segments that are orthogonal to the scanner field and placed d = [4‒16] cm away, and due to a misalignment by θ = [0‒40] degrees of the cable segments that are ideally parallel to the scanner field (Fig. 3). The field change simulations were performed to imitate stray fields in an MRCDI experiment for Ic = 1 mA currents. Here, we use the current-induced field simulations based on the realistic head model of the first subject in [1], and reconstruct the current flows from the sum of simulated relevant and stray current-induced fields, and from the relevant fields only, to demonstrate the deviations for various realistic d and θ values. Results: Figure 2 shows the recently achieved unambiguous ∆Bz,c measurements with 0.1 nT sensitivity, and corresponding current flow reconstructions. The reconstruction method does not preserve sharp changes occurring e.g. in sulci, but it still performs well in preserving higher currents in well-conductive regions, e.g. along the longitudinal fissure.  Biot-Savart simulations (Fig. 4) demonstrate that the current-induced fields with stray fields for d<12 cm severely deviate from the ideal situation (d→ ∞; first row and first column). A misalignment angle θy in the y-z plane influences the stray fields similar to the distant wire case: R2 drastically drops to near zero for θy = 15°. A misalignment angle θx in the x-z plane causes the stray field to shift spatially in the x-direction. Even a small misalignment of θx ~10° can cause severe stray fields with a more than 20% drop in R2.  Current flow simulations (Figure 5) show no significant loss in reconstruction accuracy for d >8 cm. However, R2 drops drastically for d < 8 cm, and erroneously high current density estimates appear in the entire region between the electrodes (red dashed rectangle). Cable misalignment mostly influences the field estimates near electrodes and causes misestimation of the tissue currents. Significant drops in R2 (>10%) occur for misalignments more than 20° for d = 16 cm.  Discussion and Conclusion: Field deviations that exceed the so far best demonstrated sensitivity level of 0.1 nT in vivo (corrected measurements, [1]) can occur due to a 17 cm long wire placed as far as 20 cm away from the region of interest, or due to a slight cable misalignment of θ >3°. In an example that mimics a realistic experimental set-up with d = 12 cm, θx =10° and θy =20°, current flows are misestimated approximately 45% near electrodes and 10% in inner brain regions, which demonstrates the need for correction. References:  [1] Göksu et al, Neuroimage 2018;171:26–39. [2] Jaynes et al, Phys Rev 1955;98:1099–105. [3] Park et al, Phys Med Biol 2007;52:3001–13. Acknowledgements: This study was supported by the Lundbeck foundation (grant R244-2017-196 to Axel Thielscher). Figures  

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Correction of stray magnetic fields caused by cable currents is essential for human in-vivo brain magnetic resonance current density imaging (MRCDI)

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Downloaded from orbit.dtu.dk on: Mar 30, 2019Correction of stray magnetic fields caused by cable currents is essential for human in-vivo brain magnetic resonance current density imaging (MRCDI)Göksu, Cihan; Scheffler, Klaus; Siebner, Hartwig R. ; Hanson, Lars G. ; Thielscher, AxelPublication date:2019Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Göksu, C., Scheffler, K., Siebner, H. R., Hanson, L. G., & Thielscher, A. (2019). Correction of stray magneticfields caused by cable currents is essential for human in-vivo brain magnetic resonance current density imaging(MRCDI). Abstract from 2nd International workshop on MR-based Electrical Properties mapping, Utrecht,Netherlands.Correction of stray magnetic fields caused by cable currents is essential for human in-vivo brain magnetic resonance current density imaging (MRCDI)  Cihan Göksu1, Klaus Scheffler2,3, Hartwig R. Siebner1,4, Lars G. Hanson1,5 and Axel Thielscher1,5   1Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Denmark. 2High-Field Magnetic Resonance Center, Max-Planck-Institute for Biological Cybernetics, Tübingen, Germany. 3Department of Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany. 4Department of Neurology, Copenhagen University Hospital, Bispebjerg, Denmark. 5Center for Magnetic Resonance, DTU Health Technology, Technical University of Denmark, Kgs Lyngby, Denmark.  Introduction: Accurate mapping of current flows in the human brain is important for many neuroscientific applications. MRCDI is an emerging method, which combines MRI with externally applied alternating currents to derive current flow distributions based on measurements of the current-induced magnetic fields. However, inaccurate and inconsistent measurements occur unless the stray magnetic fields caused by the currents flowing in the feeding cables are corrected [1]. Here, we explore the influences of the stray magnetic fields due to the cable-currents in realistic experimental MRCDI set-ups. Theory and Methods: Alternating electrical currents in synchrony with the MR sequence are generated by a current source, filtered to avoid RF interference, and injected into the human head via scalp electrodes (Fig. 1). The injected currents create a magnetic field distribution, and the field component ∆Bz,c that is parallel to the scanner field modulates the measured MR signal phase accordingly. The measured phase difference for opposite currents can thus be used for current-induced field ∆Bz,c = (∡μ+ − ∡μ−)/mseq calculations, where mseq is the phase sensitivity of the sequence that can be simulated by Bloch equations and rotation matrices [1,2]. The measured ∆Bz,c can then be used for current flow reconstructions Jrec =  J0 +1μ0(∂(∆Bz,c−∆Bz,c0)∂y, −∂(∆Bz,c−∆Bz,c0)∂x, 0), where ∆Bz,c0 and J0 are the magnetic field and current flow simulations for a uniform conductivity distribution [3]. Recently, we have achieved consistent ∆Bz,c measurements in the human brain with 0.1nT sensitivity (Fig. 2; for a total scan time of ~9 mins) by means of an optimized sequence (Fig. 1b), including a correction for the stray magnetic fields of the cables. Here, we explore the importance of correcting these fields for MRCDI measurements for a range of cable configurations. We performed Biot-Savart simulations to observe the field changes due to wire segments that are orthogonal to the scanner field and placed d = [4‒16] cm away, and due to a misalignment by θ = [0‒40] degrees of the cable segments that are ideally parallel to the scanner field (Fig. 3). The field change simulations were performed to imitate stray fields in an MRCDI experiment for Ic = 1 mA currents. Here, we use the current-induced field simulations based on the realistic head model of the first subject in [1], and reconstruct the current flows from the sum of simulated relevant and stray current-induced fields, and from the relevant fields only, to demonstrate the deviations for various realistic d and θ values. Results: Figure 2 shows the recently achieved unambiguous ∆Bz,c measurements with 0.1 nT sensitivity, and corresponding current flow reconstructions. The reconstruction method does not preserve sharp changes occurring e.g. in sulci, but it still performs well in preserving higher currents in well-conductive regions, e.g. along the longitudinal fissure.  Biot-Savart simulations (Fig. 4) demonstrate that the current-induced fields with stray fields for d<12 cm severely deviate from the ideal situation (d→ ∞; first row and first column). A misalignment angle θy in the y-z plane influences the stray fields similar to the distant wire case: R2 drastically drops to near zero for θy = 15°. A misalignment angle θx in the x-z plane causes the stray field to shift spatially in the x-direction. Even a small misalignment of θx ~10° can cause severe stray fields with a more than 20% drop in R2.  Current flow simulations (Figure 5) show no significant loss in reconstruction accuracy for d >8 cm. However, R2 drops drastically for d < 8 cm, and erroneously high current density estimates appear in the entire region between the electrodes (red dashed rectangle). Cable misalignment mostly influences the field estimates near electrodes and causes misestimation of the tissue currents. Significant drops in R2 (>10%) occur for misalignments more than 20° for d = 16 cm.  Discussion and Conclusion: Field deviations that exceed the so far best demonstrated sensitivity level of 0.1 nT in vivo (corrected measurements, [1]) can occur due to a 17 cm long wire placed as far as 20 cm away from the region of interest, or due to a slight cable misalignment of θ >3°. In an example that mimics a realistic experimental set-up with d = 12 cm, θx =10° and θy =20°, current flows are misestimated approximately 45% near electrodes and 10% in inner brain regions, which demonstrates the need for correction. References:  [1] Göksu et al, Neuroimage 2018;171:26–39. [2] Jaynes et al, Phys Rev 1955;98:1099–105. [3] Park et al, Phys Med Biol 2007;52:3001–13. Acknowledgements: This study was supported by the Lundbeck foundation (grant R244-2017-196 to Axel Thielscher). Figures  

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Correction of stray magnetic fields caused by cable currents is essential for human in-vivo brain magnetic resonance current density imaging (MRCDI)

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