Motion artifact variability in biomagnetic wearable devices

Abstract

Motion artifacts can be a significant noise source in biomagnetic measurements when magnetic sensors are not separated from the signal source. In ambient environments, motion artifacts can be up to ten times stronger than the desired signals, varying with environmental conditions. This study evaluates the variability of these artifacts and the effectiveness of a gradiometer in reducing them in such settings. To achieve these objectives, we first measured the single channel output in varying magnetic field conditions to observe the effect of homogeneous and gradient background fields. Our analysis revealed that the variability in motion artifact within an ambient environment is primarily influenced by the gradient magnetic field rather than the homogeneous one. Subsequently, we configured a gradiometer in parallel and vertical alignment with the direction of vibration (X-axis). Our findings indicated that in a gradient background magnetic field ranging from 1 nT/mm to 10 nT/mm, the single-channel sensor output exhibited a change of 164.97 pT per mm unit increase, while the gradiometer output showed a change of only 0.75 pT/mm within the same range. Upon repositioning the gradiometer vertically (Y direction), perpendicular to the direction of vibration, the single-channel output slope increased to 196.85 pT, whereas the gradiometer output only increased by 1.06 pT/mm for the same range. Our findings highlight the influence of ambient environments on motion artifacts and demonstrate the potential of gradiometers to mitigate these effects. In the future, we plan to record biomagnetic signals both inside and outside the shielded room to compare the efficacy of different gradiometer designs under varying environmental conditions.

Keywords: biomagnetic measurements; gradient background field; homogeneous background field; motion artifacts; wearable sensors.

Figure 1

Motion/vibration artifacts in wearable MMG with TMR sensors and gradiometer. (A) Signals detected through TMR sensors. (A1) MMG signals detected in a highly shielded environment (10⁶) showing minimal motion artifacts. (A2) MMG signals detected in a less shielded environment (10⁴), exhibiting higher levels of motion artifacts, although the desired signals remain detectable. (A3) MMG signals detected in an ambient environment, where motion artifacts significantly pollute the signals, making the target signals undetectable. (B) Signals detected through gradiometer. (B1) MMG signals detected in a highly shielded environment (10⁶) showing minimal motion artifacts. (B2) MMG signals detected in a less shielded environment (10⁴), exhibiting a comparable level of motion artifacts to B1. (B3) MMG signals are detected in an ambient environment where the target signals are still detectable. Plots (A1–3 and B1–3) are schematic representations based on the literature on EMG and MMG (–11).

Figure 2

Experiment setup. (A) The 3D schematic design of the printed customised model, Magnetised linear motor, Twinleaf-MS2 magnetically shielded chamber, and frontend board, located separately from the sensor part, to avoid any interference from the frontend vibration on the sensor output. (B) Single channel TMR sensor inside the chamber, placed on the white 3D printed supporter coming in from one of the holes on the chamber wall. The sensor and the frontend are placed separately to avoid interference from the frontend vibration. (C) Gradiometer (Parallel configuration) with two sensors placed beside one another with the baseline in the X direction. (D) Gradiometer (Vertical configuration) with two sensors placed on top of one another with the baseline in the Y direction.

Figure 3

Gradiometer characterisation metrics. (A) Gradiometer sensitivity by applying a 10 Hz AC gradient field from 1 to 10 nT/mm. (B) Gradiometer noise level measurements show a 3.88 pT/mm at 8 Hz.

Figure 4

Single channel output in variable homogeneous background magnetic field. (A) Single channel output across the entire frequency bandwidth. (B) Zoomed-in view of the single channel output to enhance visibility at the 8 Hz vibration frequency. (C) Channel output at 8 Hz, illustrating no correlation between the signal output and the homogeneous background magnetic field, as the signal output is in the noise level. (D) Sensor noise level between 1 and 1,000 Hz frequency bandwidth, which is consistent with the signal output at 8 Hz in (C).

Figure 5

Single channel output in gradient background magnetic field. (A) Single channel output across the entire frequency bandwidth. (B) Zoomed-in view of the single channel output to enhance visibility at the 8 Hz vibration frequency. (C) Channel Output at 8 Hz, illustrating a slope of 210.70 pT for each unit increase in the background gradient magnetic field from 1 nT/mm to 10 nT/mm. (D) Fitted data points when adding gradient background magnetic field from 1 nT/mm to 10 nT/mm to three different DC homogeneous background fields (0, 25,50 nT).

Figure 6

Gradiometer output in gradient background magnetic field. (A) Output of gradiometer with parallel configuration across the entire frequency bandwidth. (B) Zoomed-in view of the parallel gradiometer output to enhance visibility at the 8 Hz vibration frequency. (C) Parallel gradiometer configuration: Gradiometer Output at 8 Hz, illustrating a slope of 164.97 m⁻¹ and 0.75 for single channel and gradiometer output, respectively, for each unit increase in the background gradient magnetic field ranging from 1 nT/mm to 10 nT/mm. (D) Vertical gradiometer configuration: Fitted data points at 8 Hz, illustrating a slope of 196.85 m⁻¹ and 1.06 for single channel and gradiometer output, respectively, for each unit increase in the background gradient magnetic field ranging from 1 nT/mm to 10 nT/mm.