Robust, susceptibility-matched NMR probes for compensation of magnetic field imperfections in magnetic resonance imaging (MRI) (original) (raw)
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NMR probes for measuring magnetic fields and field dynamics in MR systems
Magnetic Resonance in Medicine, 2008
High-resolution magnetic field probes based on pulsed liquidstate NMR are presented. Static field measurements with an error of 10 nanotesla or less at 3 tesla are readily obtained in 100 ms. The further ability to measure dynamic magnetic fields results from using small (ϳ1 L) droplets of MR-active liquid surrounded by susceptibility-matched materials. The consequent high field homogeneity allows free induction decay signals lasting 100 ms or more to be readily achieved. The small droplet dimensions allow the magnetic field to be measured even in the presence of large gradients. Highly sensitive detection yields sufficient SNR to follow the relevant field evolution without signal averaging and at bandwidths up to hundreds of kHz. Transient, nonreproducible effects and drifts are thus readily monitored. The typical application of k-space trajectory mapping has been demonstrated. Potential further applications include characterization, tuning, and maintenance of gradient systems as well as the mapping of the static field distribution of MRI magnets. Connection of the probes to a standard MR spectrometer is similar to that used for imaging coils. Magn Reson Med 60:176 -186, 2008.
A transmit/receive system for magnetic field monitoring of in vivo MRI
Magnetic Resonance in Medicine, 2009
Magnetic field monitoring with NMR probes has recently been introduced as a means of measuring the actual spatiotemporal magnetic field evolution during individual MR scans. Receiveonly NMR probes as used thus far for this purpose impose significant practical limitations due to radiofrequency (RF) interference with the actual MR experiment. In this work these limitations are overcome with a transmit/receive (T/R) monitoring system based on RF-shielded NMR probes. The proposed system is largely autonomous and protected against RF contamination. As a consequence the field probes can be positioned freely and permit monitoring imaging procedures of arbitrary geometry and angulation. The T/R approach is also exploited to simplify probe manufacturing and remove constraints on material choices. Probe miniaturization permits monitoring imaging scans with nominal resolutions on the order of 400 m. The added capabilities of the new probes and system are demonstrated by first in vivo results, obtained with monitored gradient-echo and spin-echo echo-planar imaging (EPI) scans. Magn Reson Med 62:269 -276, 2009.
Remote detection of nuclear magnetic resonance with an anisotropic magnetoresistive sensor
Proceedings of the National Academy of Sciences, 2008
We report the detection of nuclear magnetic resonance (NMR) using an anisotropic magnetoresistive (AMR) sensor. A ''remotedetection'' 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 m. An estimate of the sensitivity for an optimized system indicates that Ϸ6 ؋ 10 13 protons in a volume of 1,000 m 3 , 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.
Ultra-High Field NMR and MRI—The Role of Magnet Technology to Increase Sensitivity and Specificity
Frontiers in Physics, 2017
Starting with postwar developments in nuclear magnetic resonance (NMR) a race for stronger and stronger magnetic fields has begun in the 1950s to overcome the inherently low sensitivity of this promising method. Further challenges were larger magnet bores to accommodate small animals and eventually humans. Initially, resistive electromagnets with small pole distances, or sample volumes, and field strengths up to 2.35 T (or 100 MHz 1 H frequency) were used in applications in physics, chemistry, and material science. This was followed by stronger and more stable (Nb-Ti based) superconducting magnet technology typically implemented first for small-bore systems in analytical chemistry, biochemistry and structural biology, and eventually allowing larger horizontal-bore magnets with diameters large enough to fit small laboratory animals. By the end of the 1970s, first low-field resistive magnets big enough to accommodate humans were developed and superconducting whole-body systems followed. Currently, cutting-edge analytical NMR systems are available at proton frequencies up to 1 GHz (23.5 T) based on Nb 3 Sn at 1.9 K. A new 1.2 GHz system (28 T) at 1.9 K, operating in persistent mode but using a combination of low and high temperature multi-filament superconductors is to be released. Preclinical instruments range from small-bore animal systems with typically 600-800 MHz (14.1-18.8 T) up to 900 MHz (21 T) at 1.9 K. Human whole-body MRI systems currently operate up to 10.5 T. Hybrid combined superconducting and resistive electromagnets with even higher field strength of 45 T dc and 100 T pulsed, are available for material research, of course with smaller free bore diameters. This rather costly development toward higher and higher field strength is a consequence of the inherently low and, thus, urgently needed sensitivity in all NMR experiments. This review particularly describes and compares the developments in superconducting magnet technology and, thus, sensitivity in three Moser et al. UHF MR-Magnet Technology fields of research: analytical NMR, biomedical and preclinical research, and human MRI and MRS, highlighting important steps and innovations. In addition, we summarize our knowledge on safety issues. An outlook into even stronger magnetic fields using different superconducting materials and/or hybrid magnet designs is presented.
Unilateral NMR with a barrel magnet
Journal of Magnetic Resonance, 2017
Unilateral NMR can examine samples without regard to sample size. It is also an easy path to mobile or portable NMR as well as inexpensive NMR. The objective of this work was to develop unilateral NMR with an improved performance in a sample region that was remote from the apparatus. This was accomplished with the creation of a saddle point where all second derivatives of the main component of the field were nulled. A ∼10cm diameter ∼5cm thick magnet combined with a gradiometer coil on the surface detected signals from a sensitive region that extended ∼2cm from the magnet. The relatively homogeneous field of these unilateral NMR devices allows the measurement of rapidly diffusing spins as well as the use of smaller RF amplifiers, which enhances system mobility.
AIP Advances, 2017
In this work a systematic identification of factors contributing to signal ringing in unilateral nuclear magnetic resonance (NMR) sensors is conducted. Resonant peaks that originate due to multiple factors such as NMR, electrical, magneto-acoustic, core material response, eddy currents and other factors were observed. The peaks caused by the measurement system or electrical resonances and induced magnet vibrations are further analyzed. They appear in every measurement and are considered as interference to signals received from the magnetic core. Forming a distinction between different peaks is essential in identifying the primary contribution to the captured resonant signal. The measurements for the magnetic core indicate that the magnetization induced resonant peaks of the core have relatively higher amplitudes and shorter decay times at low frequencies.