Low-field MRI can be more sensitive than high-field MRI

Abstract

MRI signal-to-noise ratio (SNR) is the key factor for image quality. Conventionally, SNR is proportional to nuclear spin polarization, which scales linearly with magnetic field strength. Yet ever-stronger magnets present numerous technical and financial limitations. Low-field MRI can mitigate these constraints with equivalent SNR from non-equilibrium ‘hyperpolarization’ schemes, which increase polarization by orders of magnitude independently of the magnetic field. Here, theory and experimental validation demonstrate that combination of field independent polarization (e.g. hyperpolarization) with frequency optimized MRI detection coils (i.e. multi-turn coils using the maximum allowed conductor length) results in low-field MRI sensitivity approaching and even rivaling that of high-field MRI. Four read-out frequencies were tested using samples with identical numbers of 1H and 13C spins. Experimental SNRs at 0.0475 T were ∼40% of those obtained at 4.7 T. Conservatively, theoretical SNRs at 0.0475 T 1.13-fold higher than 4.7 T were possible despite an ∼100-fold lower detection frequency, indicating feasibility of high-sensitivity MRI without technically challenging, expensive high-field magnets. The data at 4.7 T and 0.0475 T was obtained from different spectrometers with different RF probes. The SNR comparison between the two field strengths accounted for many differences in parameters such as system noise figures and variations in the probe detection coils including Q factors and coil diameters.

Electromagnetic interaction simulation of parallel-plate waveguide and Maxwell gradient coils in low field magnetic resonance imaging systems

“…Low-field MRI systems have demonstrated being able to generate good anatomical images of the human body [1,2,3]. Ultra-low magnetic field systems (30µT-60µT) are usually dedicated for teaching MRI principles and can provide images of vegetables and fruits.…”

External high-quality-factor resonator tunes up nuclear magnetic resonance

“…Sillerud et al 5 were the first to address this problem by adding an external inductor to a microcoil for NMR detection at 40 MHz. Coffey et al 6 reported a very weak frequency dependence of the signal-to-noise ratio (SNR) when comparing hyperpolarized NMR/MRI at 0.047 T and 4.7 T (keeping the polarization constant), with a possibly higher SNR of hyperpolarized low-field NMR/MRI. So far, NMR spectroscopy with microcoil detection and a large SNR have not be realized at very low Larmor frequencies -1000 kHz, where  is the gyromagnetic ratio of the nuclear spin species and B0 is the strength of the static magnetic field.…”

“…2ac, the 1 H and 6 Li spins were first prepolarized at Bp = 2 T, then transported into the input coil (Di =1 cm, n = 80, W = 63), and, after /2 pulse excitation, the NMR signal was recorded at 500 kHz ( 1 H) or at 83 kHz ( 6 Li). A large SNR = 172 is obtained at 500 kHz with an external coil having a ferrite core (QE = 218) and for a 1 H prepolarization of P = 1.5 x 10 -6 (Fig.…”

“…The measurement of rare nuclei at low frequencies is difficult owing to the small numbers of spins and the low spin polarization. Nevertheless, 6 LiCl dissolved in H2O…”

“…In the future, the LOD may be extended even further so that 6 Li + ions in batteries or in neurons may become measurable using external resonators with Qred >> 300 and better hyperpolarization technologies [16][17][18][19][20] .…”