Polarization enhancement technique for nuclear quadrupole resonance detection.

We demonstrate a dramatic increase in the signal-to-noise ratio (SNR) of a nuclear quadrupole resonance (NQR) signal by using a polarization enhancement technique. By first applying a static magnetic field to pre-polarize one spin subsystem of a material, and then allowing that net polarization to be transferred to the quadrupole subsystem, we increased the SNR of a sample of ammonium nitrate by one-order of magnitude.

Magnetic-resonance imaging of the human brain with an atomic magnetometer.

Magnetic resonance imaging (MRI) is conventionally performed in very high fields, and this leads to some restrictions in applications. To remove such restrictions, the ultra-low field MRI approach has been proposed. Because of the loss of sensitivity, the detection methods based on superconducting quantum interference devices (SQUIDs) in a shielded room were used. Atomic magnetometers have similar sensitivity as SQUIDs and can also be used for MRI, but there are some technical difficulties to overcome. We demonstrate that MRI of the human brain can be obtained with an atomic magnetometer with in-plane resolution of 3 mm in 13 min.

Non-cryogenic ultra-low field MRI of wrist-forearm area.

Ultra-low field (ULF) MRI as an alternative to high field MRI can find some niche applications where high field is a liability. Previously we demonstrated hand images with a non-cryogenic ULF MRI system, but such a system was restrictive to the size of the imaging objects. We have modified the previous setup to increase the imaging volume and demonstrate the image of human hand near the wrist area. One goal for the demonstration is the evaluation of quality of larger bone structure to project image quality to other parts of extremities, such as elbows, shoulders, and knees. We found that after 12 min of acquisition, the image quality was quite satisfactory. To achieve this image quality, several problems were solved that appeared in the new system. The increase in the imaging volume size led to an increase in transient time and various measures were taken to reduce this time. We also explored a method of overcoming the artifacts and image quality reduction arising from field drifts present in the system due to heating of the coils. We believe that our results can be useful for evaluation of diagnostic capability of non-cryogenic ULF MRI of extremities and other parts of the body. The system can be also applied to image animals and tissues.

Method for accurate measurements of nuclear-spin optical rotation for applications in correlated optical-NMR spectroscopy.

The nuclear-spin optical rotation (NSOR) effect recently attracted much attention due to potential applications in combined optical-NMR spectroscopy and imaging. Currently, the main problem with applications of NSOR is low SNR and accuracy of measurements. In this work we demonstrate a new method for data acquisition and analysis based on a low-power laser and an emphasis on software based processing. This method significantly reduces cost and is suitable for application in most NMR spectroscopy laboratories for exploration of the NSOR effect. Despite the use of low laser power, SNR can be substantially improved with fairly simple strategies including the use of short wavelength and a multi-pass optical cell with in-flow pre-polarization in a 7 T magnet. Under these conditions, we observed that NSOR signal can be detected in less than 1 min and discuss strategies for further improvement of signal. With higher SNR than previously reported, NSOR constants can be extracted with improved accuracy. On the example of water, we obtained measurements at a level of accuracy of 5%. We include a detailed theoretical analysis of the geometrical factors of the experiment, which is required for accurate quantification of NSOR. This discussion is particularly important for relatively short detection cells, which will be necessary to use in spectroscopy or imaging applications that impose geometrical constraints.

Anatomical MRI with an atomic magnetometer.

Ultra-low field (ULF) MRI is a promising method for inexpensive medical imaging with various additional advantages over conventional instruments such as low weight, low power, portability, absence of artifacts from metals, and high contrast. Anatomical ULF MRI has been successfully implemented with SQUIDs, but SQUIDs have the drawback of a cryogen requirement. Atomic magnetometers have sensitivity comparable to SQUIDs and can be in principle used for ULF MRI to replace SQUIDs. Unfortunately some problems exist due to the sensitivity of atomic magnetometers to a magnetic field and gradients. At low frequency, noise is also substantial and a shielded room is needed for improving sensitivity. In this paper, we show that at 85 kHz, the atomic magnetometer can be used to obtain anatomical images. This is the first demonstration of any use of atomic magnetometers for anatomical MRI. The demonstrated resolution is 1.1 mm×1.4 mm in about 6 min of acquisition with SNR of 10. Some applications of the method are discussed. We discuss several measures to increase the sensitivity to reach a resolution 1 mm×1 mm.

Non-cryogenic anatomical imaging in ultra-low field regime: hand MRI demonstration.

Ultra-low field (ULF) MRI with a pulsed prepolarization is a promising method with potential for applications where conventional high-, mid-, and low-field medical MRI cannot be used due to cost, weight, or other restrictions. Previously, successful ULF demonstrations of anatomical imaging were made using liquid helium-cooled SQUIDs and conducted inside a magnetically shielded room. The Larmor frequency for these demonstrations was ∼3 kHz. In order to make ULF MRI more accessible, portable, and inexpensive, we have recently developed a non-cryogenic system. To eliminate the requirement for a magnetically shielded room and improve the detection sensitivity, we increased the frequency to 83.6 kHz. While the background noise at these frequencies is greatly reduced, this is still within the ULF regime and most of its advantages such as simplicity in magnetic field generation hardware, and less stringent requirements for uniform fields, remaining. In this paper we demonstrate use of this system to image a human hand with up to 1.5mm resolution. The signal-to-noise ratio was sufficient to reveal anatomical features within a scan time of less than 7 min. This prototype can be scaled up for constructing head and full body scanners, and work is in progress toward demonstration of head imaging.

Investigation of antirelaxation coatings for alkali-metal vapor cells using surface science techniques.

Many technologies based on cells containing alkali-metal atomic vapor benefit from the use of antirelaxation surface coatings in order to preserve atomic spin polarization. In particular, paraffin has been used for this purpose for several decades and has been demonstrated to allow an atom to experience up to 10 000 collisions with the walls of its container without depolarizing, but the details of its operation remain poorly understood. We apply modern surface and bulk techniques to the study of paraffin coatings in order to characterize the properties that enable the effective preservation of alkali spin polarization. These methods include Fourier transform infrared spectroscopy, differential scanning calorimetry, atomic force microscopy, near-edge x-ray absorption fine structure spectroscopy, and x-ray photoelectron spectroscopy. We also compare the light-induced atomic desorption yields of several different paraffin materials. Experimental results include the determination that crystallinity of the coating material is unnecessary, and the detection of C[Double Bond]C double bonds present within a particular class of effective paraffin coatings. Further study should lead to the development of more robust paraffin antirelaxation coatings, as well as the design and synthesis of new classes of coating materials.

Polarized alkali-metal vapor with minute-long transverse spin-relaxation time.

We demonstrate lifetimes of Zeeman populations and coherences in excess of 60 sec in alkali-metal vapor cells with inner walls coated with an alkene material. This represents 2 orders of magnitude improvement over the best paraffin coatings. We explore the temperature dependence of cells coated with this material and investigate spin-exchange relaxation-free magnetometry in a room-temperature environment, a regime previously inaccessible with conventional coating materials.

Coherent population trapping spectra in presence of ac magnetic fields.

Experimental and theoretical investigations are reported on the effects induced by an alternating magnetic field on coherent population trapping resonances. We show that the ac magnetic field produces sidebands of these resonances in such a way that the spectrum observed is similar to those observed via the FM spectroscopic technique. Because of the very narrow linewidth of the resonances, sidebands are resolved even for ac field frequencies as low as a fraction of a kHz. The theoretical model developed, which takes into account a very simple atomic structure, fits the experimental data quite well.