Highly Sensitive Detection of Bio-magnetic Fields and Relevant Applications of Diamond Quantum Sensors
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2023-12-05 |
| Journal | The Brain & Neural Networks |
| Authors | Masaki Sekino |
| Institutions | The University of Tokyo |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This paper reviews the status and future potential of Diamond Nitrogen-Vacancy (NV) quantum sensors for ultra-highly sensitive bio-magnetic field measurement, positioning them as a critical successor to traditional SQUID technology.
- Cryogen-Free Operation: NV sensors are solid-state and operate effectively at room temperature, eliminating the need for bulky, expensive, and resource-intensive cryogenic cooling (a major limitation of SQUID).
- Ultra-High Sensitivity & Dynamic Range: The technology achieves high sensitivity (down to 10 pT Hz-1/2) combined with a wide dynamic range, allowing for the detection of faint bio-magnetic signals (pT/fT level) even amidst ambient environmental magnetic noise.
- Millimeter-Scale MCG Demonstrated: The research successfully demonstrated millimeter-scale Magnetocardiography (MCG) mapping in living rats, achieving world-class spatial resolution for estimating internal cardiac current distribution.
- Non-Invasive Diagnostics: NV sensors are being developed for non-radioactive detection of superparamagnetic iron oxide nanoparticles (SPIONs) used as tracers for sentinel lymph node biopsy, offering a safer and more accessible diagnostic tool.
- Miniaturization and Wearability: The solid-state nature facilitates miniaturization and integration, enabling the development of compact, wearable devices for continuous monitoring of brain (MEG) and heart (MCG) activity in real-world settings (e.g., monitoring drivers or remote patients).
- Broad Applicability: The sensor system is scalable, capable of measuring fields from the atomic level (single NV centers for cellular imaging) up to the millimeter scale (bulk diamond for organ-level mapping).
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Center Excitation Wavelength | 532 | nm | Green laser pumping for spin initialization. |
| NV Center Fluorescence Wavelength | 638-800 | nm | Detected signal (Red fluorescence). |
| Zero-Field Splitting (ODMR) | 2.87 | GHz | Microwave frequency for magnetic resonance. |
| DC Sensitivity (Reported) | 10 | pT Hz-1/2 | Sensitivity achieved in recent diamond quantum magnetometers (Ref 23). |
| Spontaneous MEG Field Strength | PicoTesla | pT | Required sensitivity for detecting alpha waves and other spontaneous brain activity. |
| Evoked MEG Field Strength | Hundreds of FemtoTesla | fT | Required sensitivity for detecting evoked responses. |
| MCG Field Strength | Tens of PicoTesla | pT | Required sensitivity for heart activity measurement. |
| MCG Spatial Resolution (Achieved) | Millimeter | mm | Spatial resolution demonstrated in rat heart mapping. |
| SQUID Operating Temperature | Cryogenic | K | Requires liquid helium or nitrogen cooling. |
| NV Sensor Operating Temperature | Room | °C | Cryogen-free operation. |
Key Methodologies
Section titled “Key Methodologies”The core methodology relies on Optically Detected Magnetic Resonance (ODMR) using the NV center’s spin state, which is sensitive to external magnetic fields.
- Optical Pumping and Initialization: NV centers, which exist in a triplet ground state (S=1, 3A2), are initialized into the ms = 0 state by illuminating the diamond with a 532 nm green laser.
- Zeeman Splitting: The external magnetic field (B0) causes the ms = ±1 spin sublevels to split in energy relative to the ms = 0 state (Zeeman splitting). The energy difference is proportional to the magnetic field strength.
- Microwave (MW) Resonance: Microwaves are applied near the 2.87 GHz zero-field splitting frequency. When the MW frequency matches the energy difference (Zeeman splitting), transitions occur between ms = 0 and ms = ±1.
- Fluorescence Readout (ODMR): Transitions to the ms = ±1 states increase the probability of non-radiative decay via singlet states, resulting in a measurable decrease in the red fluorescence intensity (638-800 nm).
- Field Determination: By monitoring the shift in the MW resonance frequency that causes the fluorescence dip, the magnitude of the external magnetic field is accurately determined.
- MCG Mapping Setup: For rat MCG, a bulk polycrystalline diamond sensor is positioned within millimeters of the heart. The sensor is scanned across the heart area using XY stages to generate a 2D map of the magnetic field distribution, which is then used to estimate internal current flow.
- Magnetic Tracer Detection: For sentinel lymph node biopsy, a specialized probe uses a solenoid coil to magnetize injected superparamagnetic iron oxide nanoparticles (SPIONs). The resulting local magnetic field generated by the magnetized tracers is then detected by the NV sensor at the probe tip.
Commercial Applications
Section titled “Commercial Applications”The unique advantages of diamond NV sensors—room temperature operation, high sensitivity, and miniaturization potential—open up several high-impact commercial and medical applications.
- Clinical Diagnostics (MEG/MCG):
- High-resolution mapping of cardiac current distribution for diagnosing arrhythmias and ischemic heart disease.
- Clinical MEG for localizing abnormal electrical activity (e.g., epilepsy) without the need for large, cryogenically cooled SQUID systems.
- Surgical Guidance and Oncology:
- Development of compact, handheld magnetic probes for non-radioactive sentinel lymph node biopsy using magnetic nanoparticle tracers (SPIONs).
- Intraoperative tracking of magnetic markers for precise tumor excision.
- Wearable and Remote Monitoring:
- Integration into compact, helmet-based systems for continuous, non-contact monitoring of brain function (MEG) in daily life.
- Monitoring of cognitive states (e.g., driver fatigue, attention) in vehicles.
- Enabling remote medical diagnostics and home monitoring of neurological and cardiac conditions, reducing the burden on patients and specialized facilities.
- Fundamental Neuroscience Research:
- High spatial resolution imaging of neural network activity in vitro, overcoming the spatial constraints of traditional multi-electrode arrays.
- Advanced Sensing Technology:
- Development of integrated solid-state quantum magnetometers with high dynamic range, suitable for operation in magnetically noisy, unshielded environments.
View Original Abstract
本稿では，生体磁気計測のための次世代型量子センサとして期待されているダイヤモンド窒素—空孔中心について，計測技術の現状と今後の応用可能性について述べる．心臓の電気活動に由来する微弱な磁場のマッピングから心臓内の電流分布を推定して機能的評価を行う心磁図や，同様に脳の機能的評価を行う脳磁図は，基礎研究から臨床の検査まで幅広い応用を有している．これらを中心とする超高感度の生体磁気計測には，超伝導量子干渉計が長く用いられてきたが，冷媒が必要なため普及に課題があった．近年，冷媒を必要としない超高感度磁気センサが急速に発達しており，中でもダイヤモンド窒素—空孔中心は，究極的には原子レベルの高分解能を有し，固体であることから集積化に向いており，ダイナミックレンジが高くリアルワールドでの応用に適するなどの特徴から，注目を集めている．生体計測の具体的事例として，動物の心磁図の計測や，リンパ節へ取り込まれた微量の磁性ナノ粒子の検出などが報告されている．機器のコンパクト性を活かして，今後は自動車のドライバーの脳機能計測や遠隔医療など，応用の開拓が期待されている．