原子磁気センサを用いた生体熱雑音磁場計測の理論的検討

Abstract

近年, 原子磁気センサの生体磁気計測やMRIシステムへの応用が注目されている.しかし, 原子磁気センサで計測される生体熱雑音の正確な見積もりは未だ成されていない.従来の生体熱雑音を計算する手法は, センサとなるピックアップコイルに直列に接続された生体と等価な抵抗体を考慮するもので, 原子磁気センサに直接適用できるものではない.今回, 生体を微小体積に分割し, それぞれに配置した, 熱エネルギーによる電流を表すランダム電流素がセンサ部に生じさせる磁束密度の分散を計算することで生体熱雑音の大きさを見積もる新たな手法を提案した.まず, ピックアップコイルをセンサとした系を用いて従来法と本手法を比較し, 本手法の妥当性を検証した.さらに, 原子磁気センサをセンサとする系に本手法を適用し, 実頭部モデルより生じる生体熱雑音の大きさを見積もった.

In recent years, there has been considerable interest in applying atomic magnetometer (AM) to biomagnetic measurement including MRI. However, a precise estimation of the thermal noise generated from biological object has not been carried out. Conventionally, the thermal noise has been estimated by considering a resistor that is electrically equivalent to a biological-conductive sample and is connected in series to a pickup coil. However, this method has major limitations in that the receiver has to be a coil and that it cannot be applied directly to a system using AM. In this paper, we propose a method to estimate the thermal noise measured by AM. We calculate the thermal noise from the variance of the magnetic sensor output produced by current-dipole moments that simulate thermally fluctuating current sources in a biological sample. We validate the proposed method by comparing our results with those obtained by conventional method with the system using the pickup coil as the sensor. Then, we apply the proposed method to the system using AM as the receiver with realistic head model and estimate the thermal noise.