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Topic Name: NIST Researchers developed super-sensitive mini-sensor can detect nuclear magnetic resonance
Category: Nuclear Magnetic Resonance
Research persons: NIST Research Team
Location: National Institute of Standards and Technology (NIST), United States
Details
A super-sensitive mini-sensor developed at the National
Institute of Standards and Technology (NIST) can detect nuclear magnetic
resonance (NMR) in tiny samples of fluids flowing through a novel microchip. The
prototype chip device, developed in a collaboration between NIST and the University
of California, may have wide application as a sensitive chemical analyzer,
for example in rapid screening to find new drugs.
As described in Proceedings of the National Academy of Sciences (PNAS),* the
NMR chip detected magnetic signals from atomic nuclei in tap water flowing
through a custom silicon chip that juxtaposes a tiny fluid channel and the NIST
sensor. The Berkeley group recently co-developed this “remote NMR” technique
for tracking small volumes of fluid or gas flow inside soft materials such as
biological tissue or porous rock, for possible applications in industrial
processes and oil exploration. The chip could be used in NMR spectroscopy, a
widely used technique for determining physical, chemical, electronic and
structural information about molecules. NMR signals are equivalent to those
detected in MRI (magnetic resonance imaging) systems
Berkeley scientists selected the NIST sensor, a type of atomic magnetometer,
for the chip device because of its small size and high sensitivity, which make
it possible to detect weak magnetic resonance signals from a small sample of
atoms in the adjacent microchannel. Detection is most efficient when the sensor
and sample are about the same size and located close together, lead author Micah
Ledbetter says. Thus, when samples are minute, as in economical screening of
many chemicals, a small sensor is crucial, Ledbetter says.
Its small size and extreme sensitivity make the NIST sensor ideal for the
microchip device, in contrast to SQUIDs (superconducting quantum interference
devices) that require bulky equipment for cooling to cryogenic temperatures or
conventional copper coils that need much higher magnetic fields (typically
generated by large, superconducting magnets) like those in traditional MRI.
The results reported in the PNAS demonstrate another use for the NIST
mini-sensor, a spin-off of NIST’s miniature atomic clocks. The sensor already
has been shown to have biomedical imaging applications.
Note for Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the quantum mechanical magnetic properties of an atom's nucleus. NMR also commonly refers to a family of scientific methods that exploit nuclear magnetic resonance to study molecules.
All nuclei that contain odd numbers of protons or neutrons have an intrinsic magnetic moment and angular momentum. The most commonly measured nuclei are hydrogen-1 (the most receptive isotope at natural abundance) and carbon-13, although nuclei from isotopes of many other elements (e.g. 15N, 14N 19F, 31P, 17O, 29Si, 10B, 11B, 23Na, 35Cl, 195Pt) can also be observed.
NMR resonant frequencies for a particular substance are directly proportional to the strength of the applied magnetic field, in accordance with the equation for the Larmor precession frequency.
NMR studies magnetic nuclei by aligning them with an applied constant magnetic field and perturbing this alignment using an alternating magnetic field, those fields being orthogonal. The resulting response to the perturbing magnetic field is the phenomenon that is exploited in NMR spectroscopy and magnetic resonance imaging, which use very powerful applied magnetic fields in order to achieve high resolution spectra, details of which are described by the chemical shift and the Zeeman effect.
NMR phenomena are also utilised in low field NMR and Earth's field NMR spectrometers, and some kinds of magnetometer.
NMR spectroscopy is one of the principal techniques used to obtain physical, chemical, electronic and structural information about molecules due to the chemical shift and Zeeman effect on the resonant frequencies of the nuclei. It is a powerful technique that can provide detailed information on the topology, dynamics and three-dimensional structure of molecules in solution and the solid state. Also, nuclear magnetic resonance is one of the techniques that has been used to build elementary quantum computers.
Note for Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is primarily used in medical imaging to visualize the structure and function of the body. It provides detailed images of the body in any plane. MR has much greater soft tissue contrast than Computed tomography (CT) making it especially useful in neurological, musculoskeletal, cardiovascular and oncological diseases. Unlike CT it uses no ionizing radiation. The scanner creates a powerful magnetic field which aligns the magnetization of hydrogen atoms in the body. Radio waves are used to alter the alignment of this magnetization. This causes the hydrogen atoms to emit a weak radio signal which is amplified by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body.
Magnetic resonance spectroscopy is used to measure the levels of different metabolites in body tissues. The MR signal produces a spectrum of different resonances that correspond to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the
brain, as well as to provide information on tumor metabolism.
The scanners used in medicine have a typical magnetic field strength of 0.2 to 3 teslas. Construction costs approximately US$ 1 million per tesla and maintenance an additional several hundred thousand dollars per year. Research using MRI scanners operating at ultra high field strength (up to 21.1 tesla) can produce images of the mouse brain with a resolution of 18 micrometres.
MRI works by making certain atoms in the body emit radiowaves by using a powerful magnet. When a person is in the scanner some of the hydrogen atoms, which are mainly found in water, align with this magnetic field. A radiowave at just the right frequency makes these atoms resonate and absorb energy. The atoms then release this energy in the form of a very weak radiowave that can be amplified and measured by the scanner. Extra magnetic fields are used to constantly change the magnetic field to allow images of the body to be reconstructed. These fields are created by gradient coils which make the familiar banging sounds of an MRI scan. Contrast agents may be injected to demonstrate blood vessels or inflammation in the tissues. Unlike CT scanning MRI uses no ionizing radiation and is generally a very safe procedure. Patients with some metal implants and cardiac pacemakers are prevented from having an MRI due to effect of the powerful magnetic field.
MRI is used to image every part of the body, but is particularly useful in neurological conditions, disorders of the muscles and joints, for evaluating tumors and showing abnormalities in the heart and blood vessels.
Note for Superconducting Quantum Interference Devices
Superconducting Quantum Interference Devices (SQUID) are very sensitive magnetometers used to measure extremely small magnetic fields, based on superconducting loops containing Josephson junctions. They have noise levels as low as 3 fT·Hz−½. For comparison, a typical refrigerator magnet produces 0.01 tesla (10−2 T), and some processes in animals produce very small magnetic fields between 10−9 T to 10−6 T. Recently invented SERF atomic magnetometers are more sensitive but are physically huge and power intensive to operate compared to SQUIDs. For decades SQUID sensors were the only way to measure very small magnetic fields.
The extreme sensitivity of SQUIDs makes them ideal for studies in biology. Magnetoencephalography (MEG), for example, uses measurements from an array of SQUIDs to make inferences about neural activity inside brains. Because SQUIDs can operate at acquisition rates much higher than the highest temporal frequency of interest in the signals emitted by the brain (kHz), MEG achieves good temporal resolution. Another area where SQUIDs are used is magnetogastrography, which is concerned with recording the weak magnetic fields of the stomach. A novel application of SQUIDs is the magnetic marker monitoring method, which is used to trace the path of orally applied drugs.
Probably the most common use of SQUIDs is in magnetic property measurement systems. These are turn-key systems, made by several manufacturers, that measure the magnetic properties of a material sample. This is typically done over a temperature range from that of liquid helium (~4K), to a couple of hundred degrees above room temperature.
For example, UC Berkeley Physics Professor John Clarke has been using SQUIDs as a detector to perform Magnetic Resonance Imaging. While high field MRI uses precession fields of one to several tesla, SQUID-detected MRI uses measurement fields that lie in the microtesla regime. Since the NMR signal drops off as the square of the magnetic field, a SQUID is used as the detector because of its extreme sensitivity. The SQUID coupled to a second-order gradiometer and input circuit, along with the application of gradients are the fundamental entities which allows his research group to retrieve noninvasive images. SQUID-detected MRI has many advantages such as the low cost required to build such a system, its compactness, the ability to image human extremities, and its application for tumor screening.
Another application is the scanning SQUID microscope, which uses a SQUID immersed in liquid helium as the probe. The use of SQUIDs in oil prospecting, mineral exploration, earthquake prediction and geothermal energy surveying is becoming more widespread as superconductor technology develops; they are also used as precision movement sensors in a variety of scientific applications, such as the detection of gravity waves. Four SQUIDs were employed on Gravity Probe B in order to test the limits of the theory of general relativity.
The principal investigator for the study described in PNAS is Alexander
Pines, a leading authority on NMR. A joint university/NIST patent application is
being filed for the microchip device. The research was supported by the Office
of Naval Research, U.S.
Department of Energy, a CalSpace Minigrant and the Defense
Advanced Research Projects Agency.
* M.P. Ledbetter, I.M. Savukov, D. Budker, V. Shah, S. Knappe, J. Kitching,
D. Michalak, S. Xu , and A. Pines. Zero-field remote detection of NMR with a
microfabricated atomic magnetometer. Proceedings of the National Academy of
Sciences, posted online Feb. 6, 2008.
In figure 1, Modern 3 tesla clinical MRI scanner
In figure 2, Prototype microchip device combining NIST's miniature atomic magnetometer with a fluid channel for studies of tiny samples
In figure 3, A prototype of a Semiconductor Superconducting Quantum Interference Device (SQUID)
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