Evaluation of an Isosceles-Triangle-Coil Phantom for Magnetoencephalography

Oyama, Daisuke; Adachi, Yoshiaki; Hatsusaka, Natsuko; Kasahara, Tomoko; Yumoto, Masato; Hashimoto, Isao; Uehara, Gen

doi:10.1109/tmag.2011.2148694

Cited by 8 publications

(5 citation statements)

References 7 publications

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“…To quantify localization accuracy of the MEG system, measurements were made using a custom-built current dipole phantom. We designed a “dry phantom” [21], which uses wire triangles to generate short tangential current segments at known positions and orientations. Figures 1b and 1c show the phantom, which consists of a platform and clamps, an arc, and dipole clips.…”

Section: Methodsmentioning

confidence: 99%

“…MEG measurements are made as the wire segments are activated, and the calculated and known positions are compared to establish variability and bias. Previous phantom studies establish the localization accuracy of MEG recordings at 1-5 mm with no systematic bias, using a range of phantom designs and current dipole positions and orientations [10,[20][21][22][23]. However, phantom design, fabrication and calibration is challenging.…”

Section: Introductionmentioning

confidence: 99%

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Noise Reduction and Localization Accuracy in a Mobile Magnetoencephalography System

Bardouille,

Smith,

Vajda

et al. 2024

Preprint

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Magnetoencephalography (MEG) non-invasively provides important information about human brain electrophysiology. The growing use of optically pumped magnetometers (OPM) for MEG, as opposed to fixed arrays of cryogenic sensors, has opened the door for innovation in system design and use cases. For example, cryogenic MEG systems are housed in large shielded rooms to provide sufficient space for the system dewar. Here, we investigate the performance of OPM recordings inside of a cylindrical shield with a 1×2 m2footprint. The efficacy of shielding was measured in terms of field attenuation and isotropy, and the value of post hoc noise reduction algorithms was also investigated. Localization accuracy was quantified for 104 OPM sensors mounted on a fixed helmet array based on simulations and recordings from a bespoke current dipole phantom. Passive shielding attenuated direct current (DC) and power line (60 Hz) fields by approximately 60 dB, and all other frequencies by 80-90 dB. Post hoc noise reduction provided an additional 5-15 dB attenuation. Substantial field anisotropy remained in the volume encompassing the sensor array. The consistency of the anisotropy over months suggests that a field nulling solution could be readily applied. A current dipole phantom generating source activity at an appropriate magnitude for the human brain generated field fluctuations on the order of 0.5-1 pT. Phantom signals were localized with 3 mm localization accuracy and no significant bias in localization was observed, which is in line with performance for cryogenic and OPM MEG systems. This validation of the performance of a small footprint MEG system opens the door for lower cost MEG installations in terms of raw materials and facility space, as well as mobile imaging systems (e.g., truck-based). Such implementations are relevant for global adoption of MEG outside of highly resourced research and clinical institutions.

show abstract

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Noise Reduction and Localization Accuracy in a Mobile Magnetoencephalography System

Bardouille,

Smith,

Vajda

et al. 2024

Preprint

View full text Add to dashboard Cite

show abstract

“…To quantify localization accuracy of the MEG system, measurements were made using a custom-built current dipole phantom. We designed a “dry phantom” [ 17 ], which uses wire triangles to generate short tangential current segments at known positions and orientations. Figure 1 b,c show the phantom, which consists of a platform and clamps, an arc, and dipole clips.…”

Section: Methodsmentioning

confidence: 99%

“…MEG measurements are made as the wire segments are activated, and the calculated and known positions are compared to establish variability and bias. Previous phantom studies established the localization accuracy of MEG recordings at 1–5 mm with no systematic bias, using a range of phantom designs and current dipole positions and orientations [ 5 , 16 , 17 , 18 , 19 ]. However, phantom design, fabrication and calibration is challenging.…”

Section: Introductionmentioning

confidence: 99%

Noise Reduction and Localization Accuracy in a Mobile Magnetoencephalography System

Bardouille,

Smith,

Vajda

et al. 2024

Sensors

View full text Add to dashboard Cite

Magnetoencephalography (MEG) non-invasively provides important information about human brain electrophysiology. The growing use of optically pumped magnetometers (OPM) for MEG, as opposed to fixed arrays of cryogenic sensors, has opened the door for innovation in system design and use cases. For example, cryogenic MEG systems are housed in large, shielded rooms to provide sufficient space for the system dewar. Here, we investigate the performance of OPM recordings inside of a cylindrical shield with a 1 × 2 m2 footprint. The efficacy of shielding was measured in terms of field attenuation and isotropy, and the value of post hoc noise reduction algorithms was also investigated. Localization accuracy was quantified for 104 OPM sensors mounted on a fixed helmet array based on simulations and recordings from a bespoke current dipole phantom. Passive shielding attenuated the vector field magnitude to 50.0 nT at direct current (DC), to 16.7 pT/√Hz at power line, and to 71 fT/√Hz (median) in the 10–200 Hz range. Post hoc noise reduction provided an additional 5–15 dB attenuation. Substantial field isotropy remained in the volume encompassing the sensor array. The consistency of the isotropy over months suggests that a field nulling solution could be readily applied. A current dipole phantom generating source activity at an appropriate magnitude for the human brain generated field fluctuations on the order of 0.5–1 pT. Phantom signals were localized with 3 mm localization accuracy, and no significant bias in localization was observed, which is in line with performance for cryogenic and OPM MEG systems. This validation of the performance of a small footprint MEG system opens the door for lower-cost MEG installations in terms of raw materials and facility space, as well as mobile imaging systems (e.g., truck-based). Such implementations are relevant for global adoption of MEG outside of highly resourced research and clinical institutions.

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“…The field has to produce a measurable signal at the sensor which is typically a magnetometer or a gradiometer. For high precision, so-called isosceles-triangular-coil phantoms may be applied, but their structure has to be crafted to precision better than 1 mm, with more than one electromagnet [5]. An array of small circular coils on a sphere can also be used for a successful calibration [6].…”

Section: Introductionmentioning

confidence: 99%

SQUID-sensor-based ultra-low-field MRI calibration with phantom images: Towards quantitative imaging

Dabek

Vesanen

Zevenhoven

et al. 2012

Journal of Magnetic Resonance

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Evaluation of an Isosceles-Triangle-Coil Phantom for Magnetoencephalography

Cited by 8 publications

References 7 publications

Noise Reduction and Localization Accuracy in a Mobile Magnetoencephalography System

Noise Reduction and Localization Accuracy in a Mobile Magnetoencephalography System

Noise Reduction and Localization Accuracy in a Mobile Magnetoencephalography System

SQUID-sensor-based ultra-low-field MRI calibration with phantom images: Towards quantitative imaging

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