Shoujun Xu

Magnetic resonance imaging with an optical atomic magnetometer.

We report an approach for the detection of magnetic resonance imaging without superconducting magnets and cryogenics: optical atomic magnetometry. This technique possesses a high sensitivity independent of the strength of the static magnetic field, extending the applicability of magnetic resonance imaging to low magnetic fields and eliminating imaging artifacts associated with high fields. By coupling with a remote-detection scheme, thereby improving the filling factor of the sample, we obtained time-resolved flow images of water with a temporal resolution of 0.1 s and spatial resolutions of 1.6 mm perpendicular to the flow and 4.5 mm along the flow. Potentially inexpensive, compact, and mobile, our technique provides a viable alternative for MRI detection with substantially enhanced sensitivity and time resolution for various situations where traditional MRI is not optimal.

Zero-field remote detection of NMR with a microfabricated atomic magnetometer

We demonstrate remote detection of nuclear magnetic resonance (NMR) with a microchip sensor consisting of a microfluidic channel and a microfabricated vapor cell (the heart of an atomic magnetometer). Detection occurs at zero magnetic field, which allows operation of the magnetometer in the spin-exchange relaxation-free (SERF) regime and increases the proximity of sensor and sample by eliminating the need for a solenoid to create a leading field. We achieve pulsed NMR linewidths of 26 Hz, limited, we believe, by the residence time and flow dispersion in the encoding region. In a fully optimized system, we estimate that for 1 s of integration, 7 × 1013 protons in a volume of 1 mm3, prepolarized in a 10-kG field, can be detected with a signal-to-noise ratio of ≈3. This level of sensitivity is competitive with that demonstrated by microcoils in 100-kG magnetic fields, without requiring superconducting magnets.

Construction and applications of an atomic magnetic gradiometer based on nonlinear magneto-optical rotation

We report on the design, characterization, and applications of a sensitive atomic magnetic gradiometer. The device is based on nonlinear magneto-optical rotation in alkali-metal (Rb87) vapor and uses frequency-modulated laser light. The magnetic field produced by a sample is detected by measuring the frequency of a resonance in optical rotation that arises when the modulation frequency equals twice the Larmor precession frequency of the Rb atoms. The gradiometer consists of two atomic magnetometers. The rotation of light polarization in each magnetometer is detected with a balanced polarimeter. The sensitivity of the gradiometer is 0.8nG∕Hz1∕2 for near-dc (0.1Hz) magnetic fields, with a base line of 2.5cm. For applications in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), a long solenoid that pierces the magnetic shields provides an ∼0.5G leading field for the nuclear spins in the sample. Our apparatus is particularly suited for remote detection of NMR and MRI. We demonstrate a poi...

Novel Detection Schemes of Nuclear Magnetic Resonance and Magnetic Resonance Imaging: Applications from Analytical Chemistry to Molecular Sensors

Nuclear magnetic resonance (NMR) is a well-established analytical technique in chemistry. The ability to precisely control the nuclear spin interactions that give rise to the NMR phenomenon has led to revolutionary advances in fields as diverse as protein structure determination and medical diagnosis. Here, we discuss methods for increasing the sensitivity of magnetic resonance experiments, moving away from the paradigm of traditional NMR by separating the encoding and detection steps of the experiment. This added flexibility allows for diverse applications ranging from lab-on-a-chip flow imaging and biological sensors to optical detection of magnetic resonance imaging at low magnetic fields. We aim to compare and discuss various approaches for a host of problems in material science, biology, and physics that differ from the high-field methods routinely used in analytical chemistry and medical imaging.

Remote detection of nuclear magnetic resonance with an anisotropic magnetoresistive sensor

We report the detection of nuclear magnetic resonance (NMR) using an anisotropic magnetoresistive (AMR) sensor. A “remote-detection” arrangement was used in which protons in flowing water were prepolarized in the field of a superconducting NMR magnet, adiabatically inverted, and subsequently detected with an AMR sensor situated downstream from the magnet and the adiabatic inverter. AMR sensing is well suited for NMR detection in microfluidic “lab-on-a-chip” applications because the sensors are small, typically on the order of 10 μm. An estimate of the sensitivity for an optimized system indicates that ≈6 × 1013 protons in a volume of 1,000 μm3, prepolarized in a 10-kG magnetic field, can be detected with a signal-to-noise ratio of 3 in a 1-Hz bandwidth. This level of sensitivity is competitive with that demonstrated by microcoils in superconducting magnets and with the projected sensitivity of microfabricated atomic magnetometers.

Application of atomic magnetometry in magnetic particle detection

We demonstrate the detection of magnetic particles carriedby water in a continuous flow using an atomic magnetic gradiometer.Studies on three types of magnetic particles are presented: a singlecobalt particle (diameter ~;150 mum, multi-domain), a suspension ofsuperparamagnetic magnetite particles (diameter ~;1 mum), andferromagnetic cobalt nanoparticles (diameter ~;10 nm, 120 kA/mmagnetization). Estimated detection limits are 20 mum diameter for asingle cobalt particle at a water flow rate 30 ml/min, 5x103 magnetiteparticles at 160 ml/min, and 50 pl for the specific ferromagnetic fluidat 130 ml/min. Possible applications of our method arediscussed.

Flow in porous metallic materials: A magnetic resonance imaging study

To visualize flow dynamics of analytes inside porous metallic materials with laser‐detected magnetic resonance imaging (MRI).

Relaxivity of gadolinium complexes detected by atomic magnetometry

Laser atomic magnetometry is a portable and low‐cost yet highly sensitive method for low magnetic field detection. In this work, the atomic magnetometer was used in a remote‐detection geometry to measure the relaxivity of aqueous gadolinium‐diethylenetriamine pentaacetic acid Gd(DTPA) at the Earths magnetic field (40 μT). The measured relaxivity of 9.7 ± 2.0 s−1 mM−1 is consistent with field‐cycling experiments measured at slightly higher magnetic fields, but no cryogens or strong and homogeneous magnetic field were required for this experiment. The field‐independent sensitivity of 80 fT Hz–1/2 allowed an in vitro detection limit of ∼ 10 μM Gd(DTPA) to be measured in aqueous buffer solution. The low detection limit and enhanced relaxivity of Gd‐containing complexes at Earths field motivate continued development of atomic magnetometry toward medical applications. Magn Reson Med 66:603–606, 2011.

Fluid-flow characterization with nuclear spins without magnetic resonance

A technique for noninvasive monitoring of flow inside metallic enclosures using laser-based atomic magnetometry is introduced. The analyte is labeled via nuclear magnetization by magnets, thereby combining the polarization and encoding steps. No radiofrequency or audiofrequency pulses are involved. We demonstrate detection of flow inside an aluminum pipe with an inner diameter of 4.9 mm that has a constriction with a diameter of 1.6 mm and a length of 6.4 mm. The results agree with a model of spin density and relaxation indicating that our technique allows for fast, quantitative, and noninvasive diagnostics of flow with potential applications discussed below.

Optical microchip detection of nuclear magnetic resonance

We demonstrate optical detection of nuclear magnetic resonance on a microchip. A theoretical optimization indicates detection limits that are competitive with that demonstrated by microcoils in high magnetic fields, without requiring superconducting magnets.