Noninvasive magnetocardiography of a living rat based on a diamond quantum sensor (2024)

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  Figure 1

  Introduction of the magnetocardiography system based on the $\mathrm{N}$-$V$ magnetometer. (a) 3D schematic of the diamond-based MCG system. The diamond-based MCG system detects the magnetic cardiac signal generated by the experimental animal, which is significantly amplified by a pair of flux concentrators. An optical system was established for $\mathrm{N}$-$V$ center excitation and red fluorescence collection, where the PL was sensed by a photodiode (PD) and readout by electronic devices. (b) Electronic structure and energy-level diagram of the $\mathrm{N}$-$V$ center in diamond. The zero-field splitting $D$ refers to the energy difference between the ground electronic spin state $|m_s=0⟩$ and $|m_s=\pm 1⟩$. When external field parallel to the $\mathrm{N}$-$V$ symmetry axis $B_{\parallel }$ exist, there will be a Zeeman split between the $|m_s=\pm 1⟩$ energy level, which allows for the vector magnetic measurement. Green laser can be used to excite the $\mathrm{N}$-$V$ center from the ground state to the excited state, while the resultant red PL from spontaneous radiation serves as an optical readout for magnetic resonance. (c) Simulated cardiac measurement experiment using the diamond-based MCG system. Here is the waveform of a simulated cardiac measurement using this diamond-based MCG system. A coil with two inversed ring radii $r_1=5$ mm and $r_2=6$ mm was fabricated with copper wire to approximate the rat heart at a speculated heart rate 12 Hz [38]. The red line represents the simulated MCG waveform detected by this system, which evaluates the efficacy of this MCG system in detecting cardiac signal.

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  Figure 2

  Performance of the dual-resonance spin magnetometry based on the $\mathrm{N}$-$V$ center for the diamond-based MCG system. (a) Dual-resonance magnetometry calibration. The lines display the demodulated PL signal in response to varying external magnetic fields for both single- and dual-resonance methods. The red line illustrates the PL response in dual-resonance situation, resonating simultaneously with both $f_{\pm }$ frequency with a linear response 72.6 mV/$\text{μ}\mathrm{T}$. The two gray lines represent the single-resonance situations, each resonating with either $f_{-}$ or $f_{+}$ with linear responses 45.3 $\mathrm{mV}/\text{μ}\mathrm{T}$ and 49.0 $\mathrm{mV}/\text{μ}\mathrm{T}$. (b) 2D optical detection magnetic resonance (ODMR) spectrum. The 2D ODMR spectrum of the $\mathrm{N}$-$V$ centers in diamond demonstrates how the demodulated PL signal varies with an externally applied magnetic field along the flux concentrator and a changing microwave field. The solid line denotes the linear relationship between resonant frequency $f_{\pm }$ and $B_{\mathrm{ext}}$ with magnetic flux concentration $f_{\pm }\approx D\pm \alpha G\gamma B_{\mathrm{ext}}$, and the dotted dashed line represents the theoretical relationship without the flux concentrators $f_{\pm }\approx D\pm \alpha \gamma B_{\mathrm{ext}}$. Here $G$ is the enhancement factor of the magnetic flux concentrator, $\gamma$ is the gyromagnetic ratio of the $\mathrm{N}$-$V$ center, and $\alpha \sim 0.6$ is the angle factor from the misalignment between external magnetic field and the $\mathrm{N}$-$V$ center’s symmetry axis. As the slope of $\alpha G\gamma$ is fitted to be $1027\pm 50$ GHz/T and $\gamma =28$ GHz/T, the amplification factor is estimated to be approximately $60$. (c) Diamond $\mathrm{N}$-$V$ magnetometer sensitivity. This is the sensitivity spectrum of the $\mathrm{N}$-$V$ magnetometer, where the red line represents the sensitivity of the $\mathrm{N}$-$V$ magnetometer, and the gray line indicates the background noise in a nonresonant situation. The dotted green line represents the averaged sensitivity $9\mathrm{pT}{\mathrm{Hz}}^{-1/2}$ ranging from 10 to 200 Hz, in accordance with the frequency range of cardiac signal. where the PL signal is sensitive to the environmental magnetic field. The experiment was carried out in the magnetic shielded drum to ensure minimized environmental electromagnetic interference.

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  Figure 3

  Results of the noninvasive MCG experiment on rat. (a) Cardiac waveform acquired by diamond-based MCG system. The red line demonstrates the rat MCG waveform detected by the $\mathrm{N}$-$V$ diamond sensor, while the gray line represents the background magnetic noise without the rat. The identified peak of typical $Q$, $R$, $S$, $T$ waves are marked in the figure. The orange area demonstrates the 15 ms $QRS$ complex with amplitude approximately $21$ pT in the rat MCG and the yellow area demonstrates the 53-ms $T$ wave with approximately $15$ pT due to the repolarization of the ventricles. (b) Comparison of rat MCG waveform and ECG waveform. The subplots with red lines represent the MCG waveforms acquired repeatedly in different experiments as in (a). And the green lines on the right represent the concurrently acquired rat ECG waveforms, which show high conformity with the pertinent MCG waveforms. (c) Schematic of the MCG and ECG measurement. The MCG signal and ECG signal were captured simultaneously in the experiment. The left figure demonstrates the setup of $\mathrm{N}$-$V$ diamond sensor in the MCG experiment. The rat $QRS$ complex in ECG waveform is approximately $0.62\pm 0.7$ mV. The diamond sensor clamped with the FCs was positioned right above the chest of the rat for 5 mm from skin, 10 cm on the spinal line from its nose to its root of the tail. Meanwhile, as depicted in the right figure, the ECG signal acquired by three electrode probes form the lead II configuration of ECG measurement.

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  Figure 4

  A comparative demonstration of different magnetometers by scale and sensitivity [48, 52, 53], each type of sensor are denoted by different symbols. The work related to biomagnetic measurements are depicted in solid lines, while the unrelated works are marked with dotted lines. Among all these sensors, the diamond $\mathrm{N}$-$V$ center is notable for its high spatial resolution and high theoretical potential in full-vector magnetometry for millimeter-scale MCG [26, 36, 37, 46]. The blue dotted line represents the sensitivity of diamond $\mathrm{N}$-$V$ magnetometers applied in biomagnetic measurements. One of the highest resolution MCG system was developed for mice cardiac measurement [47], but the minimum detection range limited by dewar thickness restricted the application of SQUID in submillimeter-scale biomagnetic measurement [12]. This work extends the application realm of $\mathrm{N}$-$V$ diamond magnetometer in macroscopic living animal level.

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  Figure 5

  Setup diagram of the diamond-based MCG system. The system consisted of three primary modules. The first was a microwave module that generates the microwave signal to manipulate the $\mathrm{N}$-$V$ spins in diamond via the dual-resonance method. The second was an optical and sensing module tasked with exciting the $\mathrm{N}$-$V$ centers and collecting the PL emitted from them, together with auxiliary electrodes for ECG measurement. The third was an electrical readout module to capture and synchronize both the MCG signal and ECG signal. Two home-built trans-impedance amplifiers (TIA) were employed to extract and amplify the ac fluorescence photocurrent by a factor of 2000 $\mathrm{V}/\mathrm{A}$.

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  Figure 6

  Simulation of the potential sensitivity enhancement. This figure demonstrates the relationship between diamond thickness’s relationship, PL collection efficiency and magnetic flux concentrator’s amplification factor $G$. The blue line graphically denotes the expected sensitivity enhancement from simulation, while the green and red lines represent $G$ and the PL collection efficiency, respectively. Here the sensitivity enhancement is directly proportional to $G$ and relates to the square root of the phonon number $\sqrt{N}$. The current system parameters are indicated in the graph by the inverted triangles. To augment the signal strength, the simulations included a reflective coating on the clenching face of the flux concentrator and an additional reflective mirror for fluorescence reflection [28, 55]. The best expected enhancement about threefold is achieved at 40-$\text{μ}\mathrm{m}$ gap width, where the magnetic amplification factor $G\approx 120$ and the fluorescence collection efficiency approximately equal to $1.5\mathrm{\%}$. Combining the 280% improvement from the system optimization and the 350% potential improvement from the background noise reduction (2.6 $\mathrm{pT}/\mathrm{Hz}{}^{-1/2}$), we anticipate an approximate tenfold enhancement in magnetometry sensitivity based on the system’s current performance.

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