Low-field MRI

Note: The Earth's magnetic field strength ranges from 25 to 65 μT (microteslas) or 0.25 to 0.65 gauss (G). Its average value is therefore 45 μΤ or 0.45 G. A MRI scanner used in the clinical setting today may operate at approximately 1.0 T* meaning at approximately 20.000 (40.000-15.000) times larger magnetic fields.

*(usual values are 1.5 - 3 T)

However, it is possible to perform MRI and specifically brain MRI in low magnetic fields. An approach mentioned in this section uses a magnetic field strength of 6.5 mT (0,0065 Tesla) or 65 G or 6500 μΤ, which is approximately only 140 times larger than the Earth's magnetic field.

Low-field and ultra low-field MRI

Low-field MRI may include a pre-polarization step which uses a pulsed magnetic field to generate increased nuclear polarization 

Reading notes from Sarracanie et al (2015)* and additional references.

Low-field MRI presents significant advantages. It is known that the relaxation times are dependent on the magnetic field strength. In low fields, the relaxation time, especially T1, is shorter and the relative differences of T1 between different tissues are larger (Sepponen RE et al 1985).

The potential of low-field MRI had been initially demonstrated by the pioneering work of Sepponen RE et al (1985) who conducted brain MRI in 20 mT. 

In 1993, the concept of pre-polarized MRI (PMRI) was introduced which consists of the use of a strong, inhomogeneous pulsed magnetic field to generate increased nuclear polarization and a much weaker homogeneous magnetic field for signal detection. The PMRI technique was utilized for all low field MRI systems since then.

Ultra-low field MRI consists of the use of a detection field below 10 mT. Numerous PRMI ultra-low field MRI studies have been conducted using SQUID-magnetometers for detection. A significant effort is represented by the ULF MRI project of the Los Alamos National Laboratory ("Portable MRI developed at Los Alamos" (2015), Los Alamos Science and Technology Magazine | January 2015 (PDF p.24) cf. Zotev et al (2009) and media article) which performs brain MRI with prepolarization at 0.01–0.1 T followed by SQUID-detection.

A Berkeley National Lab/Berkeley University group conducted in 2013 brain MRI using proton prepolarization at 80 mT and SQUID magnetometers for detection at 130 μT (Inglis et al 2013). An atomic magnetometer study in the PMRI regime was published by the Los Alamos National Laboratory (Savukov I et al 2013) with pre-polarization at 80 mT and detection at 4 mT.

In 2015, researchers from the Low-Field MRI lab of the MGH Martinos Center for Biomedical Imaging, a leading center which introduced fMRI and also MRI contrast agents, presented brain MRI in the ULF regime without prepolarization or cryogenics by combining undersampling strategies and a high performance fully refocused steady-state-based acquisition i.e. specific MRI sequences (b-SSFP) (Sarracanie et al (2015)).

* Cited in https://www.nmr.mgh.harvard.edu/news/20151015/low-cost-high-performance-mri-opens-new-opportunities-brain-imaging

Low-field portable MRI scanner (0.064T)

A low-field portable MRI scanner by Hyperfine Research uses a permanent magnet of 0.064T, which is a magnetic field strength that 100 times that of the Earth's magnetic field, while being 20-50 times lower than that of current clinical MRI (1,5-3 Tesla). It was inspired by research from the Athinoula Martinos Center of the Massachusetts General Hospital (MGH) (Harvard's main teaching hospital), a center which has significant contributions in the introduction of functional magnetic resonance imaging (fMRI) and also of contrast agents and low-field MRI. After having being tested at five U.S. teaching hospitals including those of Yale and University of Pennsylvania (ref.) the scanner received FDA approval in 2020.

Video featuring the portable MRI scanner (A metallic key ring is approached to the permanent magnet of the scanner and attracted by it in a small distance).  Additional reference

Smaller, lighter, cheaper: A serial entrepreneur wants his portable MRI to transform medicine (2019) 

Statement by the entrepreneur and additional reference.

(It is noted, as mentioned in the above reference, that the system relies on high-powered computing for image creation as opposed to a powerful magnet. Also noise-canceling technology (cf. headphones) was adapted to block electromagnetic frequency noise (cf. X post), thereby minimizing the need of shielding.)

Figure 1: Demonstration of scanning of objects (fruit and vegetable) with the Hyperfine portable MRI scanner (reference).

Earth's field NMR/MRI scanner "Terranova-MRI"

MRI and NMR in the Earth's magnetic field can be conducted using the Terranova-MRI scanner by Magritek. The EFNMR/MRI system is described in this presentation and this student guide. The system probe consists of three concentric coils, a polarizing coil, an excitation/detection gradient coil and a gradient coil. A demonstration is provided in a series of videos starting with the one entitled "Introductory NMR & MRI: Video 03: How the Terranova-MRI works". 

(Please note that the experiment itself, initiated by the prepolarization pulse and followed by the acquisition of the FID is shown in Video 04: Acquiring a Free Induction Decay (FID)).

Figure 2: Terranova-MRI system by Magritek.

The system with certain modifications has been used in the study entitled "A practical and flexible implementation of 3D MRI in the Earth’s magnetic field" by Halse M.E. et al (2006) which images a specific object (red capsicum). The study uses a polarizing time of 2 seconds with an average field strength of approximately 20 mT (Fig.1 of Halse M.E. et al (2006). Following this, we have the application of a broadband pulse of 1.5 ms to excite the sample. The repetition time was 5.5 seconds and the acquisition time was 2 seconds. A related study by Mohoric A. et al (2004) images similar objects.

Brain Ultra-Low Field (ULF) MRI at 6.5 mT using balanced Steady-State-Free-Precession (b-SSFP)* sequences and undersampling

Investigation of algorithmic pattern recognition for matching to predicted signal evolutions (Magnetic Resonance Fingerprinting - MRF)

(Sarracanie et al (2015))

Reading notes from above and references therein

The technique uses an ultra-low field of 6.5 mT, while no pre-polarization or cryogenics are utilized. It is noted that the Larmor frequency at this magnetic field strength it 276 KHz. It also uses balanced Steady-State-Free-Precession (b-SSFP)* sequences, which dynamically refocus the spins after measurement, thereby eliminating the delays associated with T2 decay and T1 recovery (Sarracanie et al (2015). This reduces acquisition time and provides the highest signal-to-noise ratio (SNR) per unit time of all imaging sequences. These sequences are very sensitive to the spin dephasing which occurs between consecutive RF pulses and therefore are highly susceptible to inhomogeneities of the magnetic field, major cause of spin decoherence. A significant technical requirement associated with high magnetic fields is the need for high homogeneity as even small heterogeneities can introduce very important artifacts. This requirement is less important at low magnetic fields and as a result at 6.5 mT, b-SSFP sequences are largely immune to heterogeneity artifacts.

Additionally, an undersampling stategy is pursued meaning that sparse samples are obtained for the reconstruction of the image. This favors the selection of samples representing image features (large coefficients) while reducing the samples representing noise and artifacts (small coefficients).

Also, a technique termed "Magnetic Resonance Fingerprinting", which is similar to "compressed sensing" is investigated (Ma D et al 2013). This consists of pseudorandomized acquisition which causes signals from different tissues to have a unique signal evolution of "fingerprint" that is simultaneously a function of the multiple constituents properties. Following acquisition, processing with a pattern recognition algorithm allows for matching of the fingerprints to a predefined dictionary of predicted signal evolutions. These can be translated to quantitative maps of MR parameters. It must be mentioned that the technique allows for the simultaneous examination of many MR parameters thereby enabling computer-aided multiparametric MR analyses, similar to genomic or proteomic analyses, which can detect important complex changes across a large set of MR parameters simultaneously. Also, use of appropriate pattern recognitions algorithms allows to minimize the effect of noise and artifacts, practically suppressing these factors.

Furthermore, there are theoretical frameworks for image reconstruction of highly undersampled datasets using multiple channel acquisition and parallel imaging i.e. different coils. Finally, parallelized computing enables to address computationally demanding tasks.

Zero-field NMR

  One implementation of zero-field NMR is the use of atomic magnetometers or optically pumped magnetometers (e.g. with rubidium vapor cells).