Axon hillock currents enable single-neuron-resolved 3D reconstruction using diamond nitrogen-vacancy magnetometry

At a Glance

Metadata	Details
Publication Date	2020-10-02
Journal	Communications Physics
Authors	Madhur Parashar, Kasturi Saha, Sharba Bandyopadhyay
Institutions	Indian Institute of Technology Bombay, Indian Institute of Technology Kharagpur
Citations	5
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond NV Magnetometry for Single-Neuron 3D Reconstruction

Source Paper: Parashar et al., Axon hillock currents enable single-neuron-resolved 3D reconstruction using diamond nitrogen-vacancy magnetometry (Communications Physics, 2020).

Executive Summary

This research validates the feasibility of achieving single-neuron-resolved 3D functional brain mapping using widefield diamond Nitrogen-Vacancy (NV) center magnetometry. The core findings and value proposition for advanced diamond material procurement are summarized below:

Dominant APMF Signature: The Action Potential Magnetic Field (APMF) generated by intra-axonal currents in the mammalian axon hillock is two orders of magnitude larger (36 pT peak-to-peak) than other neuronal regions.
Simplified Inverse Problem: This dominant, localized signature allows the complex 3D source reconstruction problem to be simplified, enabling single-cell resolution (10-20 µm lateral separation).
Algorithm Resilience: A dictionary-based matching pursuit algorithm was developed, demonstrating high resilience to Gaussian noise, requiring a minimum Signal-to-Noise Ratio (SNR) as low as -13.9 dB (2D case).
Material Requirement: Achieving the necessary sensitivity to detect the 36 pT APMF signal requires high-density NV diamond samples with high intrinsic coherence (long T₂ time) to reach sub-picotesla DC sensitivity.
Widefield Imaging Potential: Simulations confirm the ability to reconstruct spiking activity for hundreds to thousands of neurons in 2D layers and 3D volumes, positioning NV diamond as a potential replacement for current functional brain mapping techniques.

Technical Specifications

The following hard data points were extracted from the simulation and analysis of the diamond NV magnetometry system performance:

Parameter	Value	Unit	Context
Mammalian APMF Magnitude	36	pT	Peak-to-peak magnetic field (Y component) below axon hillock
Target DC Field Sensitivity (Inferred)	~1	pT µm^-1 Hz^-1/2	Required for single-cell resolution spike detection
Minimum SNR (2D Lateral Case)	-13.9	dB	Required for correct reconstruction (Gaussian Noise)
Minimum SNR (3D Axial Case)	-10.2	dB	Required for correct reconstruction (Gaussian Noise)
Lateral Spatial Resolution Achieved	10 - 20	µm	Resolvability limit for nearby, near-simultaneously spiking neurons
Reconstruction Accuracy (2D, No Noise)	83.61 ± 2.17	%	Population performance
Reconstruction Accuracy (3D, Gaussian Noise)	71.7281 ± 1.1886	%	Population performance (SNR -9.46 dB)
Simulated NVMM Size	50 x 100	pixels	2D map size
Simulated Pixel Size	20 x 20	µm	Spatial resolution of the NVC sensor
AP Magnetic Field Timescale	~2	ms	Falls within the DC signal range of NV protocols

Key Methodologies

The experimental feasibility was established through computational modeling and algorithm development, relying heavily on accurate simulation of current flow within the neuron and subsequent magnetic field generation.

Neuronal Modeling: A realistic cortical pyramidal neuron model (comprising soma, dendrites, axon hillock, and myelinated/unmyelinated axon segments) was implemented using the NEURON simulation environment.
Voltage Propagation: Membrane potential dynamics were solved using the cable theory equation with a temporal resolution of 10 µs.
Intra-Axonal Current Calculation: Intra-axonal currents (I_{compartment,i}(t)) were calculated based on the discrete version of the cable equation, confirming that the axon hillock current is two orders of magnitude higher than other segments.
APMF Simulation: The vector magnetic field B at a measurement point r was calculated by summing the contributions of current flow across all neuronal segments using the Biot-Savart law.
NVMM Generation: 2D time-series magnetic field maps (NVMMs) were simulated on a plane corresponding to the diamond NVC layer, incorporating B_x, B_y, and B_z components.
Reconstruction Algorithm: A dictionary-based matching pursuit algorithm was adapted for spatiotemporal AP reconstruction.
Dictionary Construction: Dictionary elements were constructed primarily from the dominant axon-hillock NVMM signatures at three successive timepoints (3.5, 4.0, and 4.5 ms) to maximize detection accuracy.

6CCVD Solutions & Capabilities

This research highlights the critical need for high-quality, customized MPCVD diamond substrates to advance widefield NV magnetometry toward clinical and research applications. 6CCVD is uniquely positioned to supply the necessary materials and engineering support for replicating and extending this work.

Applicable Materials

To achieve the required sub-picotesla sensitivity and high coherence (long T₂) necessary for detecting 36 pT APMF signals, researchers require specialized diamond substrates:

High-Purity Single Crystal Diamond (SCD): Essential for maximizing NV center coherence time (T₂). 6CCVD provides high-purity SCD wafers, ideal for precise, shallow NV implantation or in situ growth techniques, ensuring optimal quantum performance.
Polycrystalline Diamond (PCD) Substrates: For widefield imaging applications requiring large sensor areas (e.g., the simulated 1mm x 2mm NVC map, scalable to larger arrays), 6CCVD offers Inch-Size PCD wafers up to 125mm in diameter.
Custom Thickness Control: We supply SCD and PCD wafers with thickness control from 0.1 µm up to 500 µm, allowing researchers to precisely define the active NV layer depth and substrate bulk required for specific magnetometry setups.

Customization Potential

The success of widefield NV magnetometry depends on integrating the diamond sensor into complex optical and electrical systems. 6CCVD offers comprehensive customization services:

Requirement from Research	6CCVD Customization Capability	Technical Advantage
Large Area Widefield Sensors	Custom dimensions for PCD plates/wafers up to 125mm	Enables scaling from simulated 1mm x 2mm maps to practical widefield imaging arrays.
Sensor Proximity/Optical Quality	Ultra-low roughness polishing: Ra < 1nm (SCD), Ra < 5nm (PCD)	Minimizes scattering losses and allows for maximum proximity to biological tissue, crucial for detecting weak APMF signals.
Integrated Control Circuitry	Custom metalization services: Au, Pt, Pd, Ti, W, Cu	Allows for the deposition of microwave strip lines or electrical contacts directly onto the diamond surface, necessary for NV spin manipulation (Ramsey protocol).
Substrate Geometry	Custom laser cutting and shaping services	Provides precise geometries for integration into complex cryogenic or ambient temperature magnetometry setups.

Engineering Support

The paper notes that achieving the target sensitivity requires “engineered diamond” and “optimized collection methods.” 6CCVD’s in-house PhD team specializes in the material science of MPCVD diamond and can assist researchers in:

Material Selection: Guiding the choice between SCD (for highest T₂ coherence) and PCD (for largest area) based on specific experimental constraints (e.g., noise regime, spatial resolution targets).
NV Layer Optimization: Consulting on optimal substrate preparation (polishing, purity) for subsequent high-density NV implantation or delta-doping growth, critical for maximizing signal collection efficiency.

Call to Action: For custom specifications or material consultation regarding high-coherence diamond substrates for quantum sensing and functional brain mapping projects, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s42005-020-00439-6