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

doi:10.1016/j.jmr.2013.10.013

Journal of Magnetic Resonance

Volume 237, December 2013, Pages 169-174

https://doi.org/10.1016/j.jmr.2013.10.013 Get rights and content

Highlights

•
SNR theory of hyperpolarized MR as a function of B₀ and frequency optimized RF coil.
•
Low-field MRI sensitivity approaches and even rivals that of high-field MRI.
•
RF coil development for low-field MR.
•
Multi-nuclear imaging and spectroscopy of hyperpolarization at low B₀ field.

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 those at 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.

Graphical abstract

Introduction

The superlative factor governing MRI image quality is the signal-to-noise ratio (SNR). High-field MRI using superconductive magnets has revolutionized medical diagnostics, with ever-increasing magnetic fields fostering sensitivity improvements via higher SNR. Yet low magnetic field strengths offer many attractive advantages such as reduced magnet size and cost, greater subject safety due to lower absorption of radio-frequency energy, and negligible subject induced magnetic field inhomogeneities [1]. These low-field MRI advantages can potentially be truly transformative, permitting performance of the entire MRI exam in seconds [2] provided sufficient SNR is available. The SNR of conventional, higher field detection is a complex equation of nuclear spin polarization, detection frequency, and other factors [3] arising from Faraday inductive detection. Nuclear spin polarization is a key factor contributing to this SNR. It is a relative measure of nuclear spin alignment with the applied magnetic field B₀. Equilibrium nuclear spin polarization, which is only 10⁻⁶–10⁻⁵ at conditions of human body temperature and B₀ of several Tesla, scales linearly with B₀ and, therefore, SNR decreases at low field. However, non-equilibrium ‘hyperpolarization’ schemes make polarization independent of the detection field, providing a unique opportunity for high SNR and image quality at low field.

Hyperpolarization techniques temporarily increase polarization by several orders of magnitude to unity, or 10⁰, referred to as the hyperpolarized state. These techniques include dissolution-Dynamic Nuclear Polarization (DNP) [4], Para-Hydrogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment (PASADENA) [5], Spin Exchange Optical Pumping (SEOP) [6] and others. Regardless of the hyperpolarization approach used, the main goal in the context of biomedical applications is preparation of exogenous contrast agents with high polarization to enable molecular imaging of relatively dilute spin systems otherwise not amenable by conventional MRI. Examples of such contrast agents include hyperpolarized noble gases for lung imaging and ¹³C-labeled metabolites. The latter can be out of balance due to abnormal metabolism, and therefore act as reporters or biomarkers of diseases including those of cancer. ¹³C-pyruvate contrast agent reporting on elevated rate of glycolysis in cancer is one such example already in clinical trial [7] due to recognized status as a promising molecular imaging agent for prostate cancer.

While low-field MRI has been shown useful for hyperpolarized states of noble gases in lung imaging [8] and much progress has been made for utilizing hyperpolarized contrast agents in molecular imaging [4], [7], the B₀ field independent nature of polarization in hyperpolarized contrast agents has not been fully taken advantage of. Maximizing imaging detection sensitivity as a function of both detection field B₀ [9] and frequency ω₀ still remains a challenge. Prior systematic efforts to develop a theoretical SNR foundation for MRI neglected optimization of the Faraday induction coils to the detection frequency ω₀ [3], [9], [10], [11]. We recently demonstrated ¹³C hyperpolarized signals can be nearly field independent [12]. Here, we present a general theoretical description of hyperpolarized MRI sensitivity in the form of SNR as a function of detection frequency ω₀ and experimental validation at four frequencies: 0.5 MHz, 2.0 MHz, 50 MHz, and 200 MHz. It is concluded that low-field MRI can be significantly more sensitive than high-field MRI for detection of hyperpolarized spin states. This is contrary to conventional wisdom that high-field MRI is always more sensitive.

Section snippets

Sample phantoms and preparation of nuclear spin polarizations

¹H and ¹³C spectroscopic and imaging comparisons utilized two spherical phantoms of sodium 1-¹³C-acetate. The phantom for ¹H studies was 1.0 g of sodium 1-¹³C-acetate (product #279293, Isotec–Sigma–Aldrich) dissolved in 99.8% D₂O resulting in 2.8 mL total volume. A larger sample for ¹³C studies consisted of 5.18 g of sodium 1-¹³C-acetate dissolved in 99.8% D₂O resulting in 17.5 mL total volume. High-field data were acquired on a 4.7 T Varian small animal MRI scanner with a multi-nuclear RF probe

Results and discussion

Seminal work by Hoult et al. [3], [15] described the SNR for Faraday inductive detection of the MRI signal in RF coils as $Ψ_{rms} = \frac{1}{\sqrt{2}} \frac{| ε |}{V} = \frac{{KB}_{1} V_{S} ω_{0} μ_{N} PN}{\sqrt{2} {[4 Fk Δ f (T_{C} ζ R_{C} + T_{S} R_{S})]}^{1 / 2}}$

Eq. (1) relates the magnitude of the electromotive force induced in the RF coil |ε|, noise V, oscillating RF field homogeneity over the subject K, oscillating RF field strength per unit current over the subject B₁, subject volume V_S, detection frequency ω₀, nuclear magnetic moment μ_N, nuclear spin polarization P, number of spins N

Conclusion

To summarize, a theoretical basis for SNR of hyperpolarized contrast agents as a function of detection frequency is described and validated experimentally. Low-field MRI can indeed be more sensitive for hyperpolarized contrast agents. Moreover, hyperpolarized low-field MRI in combination with cryogenically cooled RF coils [20], [28] can significantly surpass the sensitivity of hyperpolarized high-field MRI, which contradicts the ‘conventional wisdom’ of high-field MRI SNR superiority. In

Acknowledgments

We thank Professor Boyd M. Goodson for his comments on the writing of the manuscript and gratefully acknowledge funding support from NIH R25 CA136440, 3R00 CA134749, DOD CDMRP W81XWH-12-1-0159/BC112431.

References (32)

D.I. Hoult et al.
The signal-to-noise ratio of the nuclear magnetic resonance experiment
J. Magn. Reson.
(1976)
J. Kurhanewicz et al.
Analysis of cancer metabolism by imaging hyperpolarized nuclei: prospects for translation to clinical research
Neoplasia
(2011)
L.L. Tsai et al.
An open-access, very-low-field MRI system for posture-dependent He-3 human lung imaging
J. Magn. Reson.
(2008)
W. Myers et al.
Calculated signal-to-noise ratio of MRI detected with SQUIDs and Faraday detectors in fields from 10 μT to 1.5 T
J. Magn. Reson.
(2007)
A.M. Coffey et al.
A large volume double channel ¹H-X RF probe for hyperpolarized magnetic resonance at 0.0475 Tesla
J. Magn. Reson.
(2012)
D.I. Hoult et al.
The sensitivity of the zeumatographic experiment involving human samples
J. Magn. Reson.
(1979)
D.G. Gadian et al.
Radiofrequency losses in NMR experiments on electrically conducting samples
J. Magn. Reson.
(1979)
T.W. Redpath et al.
Estimating patient dielectric losses in NMR images
Magn. Reson. Imag.
(1984)
L. Darrasse et al.
Perspectives with cryogenic RF probes in biomedical MRI
Biochimie
(2003)
B. Blasiak et al.
An optimized solenoidal head radiofrequency coil for low-field magnetic resonance imaging
Magn. Reson. Imag.
(2009)

F. Resmer et al.

Cryogenic receive coil and low noise preamplifier for MRI at 0.01 T

J. Magn. Reson.

(2010)

M.E. Hayden et al.

Specific absorption rates and signal-to-noise ratio limitations for MRI in very-low magnetic fields

Concept Magn. Res. A

(2012)

H. Imai et al.

Hyperpolarized ¹²⁹Xe lung MRI in spontaneously breathing mice with respiratory gated fast imaging and its application to pulmonary functional imaging

NMR Biomed.

(2011)

K. Golman et al.

Real-time metabolic imaging

Proc. Natl. Acad. Sci. USA

(2006)

C.R. Bowers et al.

Para-hydrogen and synthesis allow dramatically enhanced nuclear alignment

J. Am. Chem. Soc.

(1987)

T.G. Walker et al.

Spin-exchange optical pumping of noble-gas nuclei

Rev. Mod. Phys.

(1997)

Cited by (102)

Comparison of image quality and diagnostic efficacy of routine clinical lumbar spine imaging at 0.55T and 1.5/3T
2024, European Journal of Radiology
To compare image quality, assess inter-reader variability, and evaluate the diagnostic efficacy of routine clinical lumbar spine sequences at 0.55T compared with those collected at 1.5/3T to assess common spine pathology.
665 image series across 70 studies, collected at 0.55T and 1.5/3T, were assessed by two neuroradiology fellows for overall imaging quality (OIQ), artifacts, and accurate visualization of anatomical features (intervertebral discs, neural foramina, spinal cord, bone marrow, and conus / cauda equina nerve roots) using a 4-point Likert scale (1 = non-diagnostic to 4 = excellent). For the 0.55T scans, the most appropriate diagnosis(es) from a picklist of common spine pathologies was selected. The mean ± SD of all scores for all features for each sequence and reader at 0.55T and 1.5/3T were calculated. Paired t-tests (p ≤ 0.05) were used to compare ratings between field strengths. The inter-reader agreement was calculated using linear-weighted Cohen's Kappa coefficient (p ≤ 0.05). Unpaired VCG analysis for OIQ was additionally employed to represent differences between 0.55T and 1.5/3T (95 % CI).
All sequences at 0.55T were rated as acceptable (≥2) for diagnostic use by both readers despite significantly lower scores for some compared to those at 1.5/3T. While there was low inter-reader agreement on individual scores, the agreement on the diagnosis was high, demonstrating the potential of this system for detecting routine spine pathology.
Clinical lumbar spine imaging at 0.55T produces diagnostic-quality images demonstrating the feasibility of its use in diagnosing spinal pathology, including osteomyelitis/discitis, post-surgical changes with complications, and metastatic disease.
Development of a Helmet-Shape Dual-Channel RF coil for brain imaging at 54 mT using inverse boundary element method
2024, Journal of Magnetic Resonance
Very-low field (VLF) magnetic resonance imaging (MRI) offers advantages in term of size, weight, cost, and the absence of robust shielding requirements. However, it encounters challenges in maintaining a high signal-to-noise ratio (SNR) due to low magnetic fields (below 100 mT). Developing a close-fitting radio frequency (RF) receive coil is crucial to improve the SNR. In this study, we devised and optimized a helmet-shaped dual-channel RF receive coil tailored for brain imaging at a magnetic field strength of 54 mT (2.32 MHz). The methodology integrates the inverse boundary element method (IBEM) to formulate initial coil structures and wiring patterns, followed by optimization through introducing regularization terms. This approach frames the design process as an inverse problem, ensuring a close fit to the head contour. Combining theoretical optimization with physical measurements of the coil's AC resistance, we identified the optimal loop count for both axial and radial coils as nine and eight loops, respectively. The effectiveness of the designed dual-channel coil was verified through the imaging of a CuSO4 phantom and a healthy volunteer's brain. Notably, the in-vivo images exhibited an approximate 16–25 % increase in SNR with poorer B₁ homogeneity compared to those obtained using single-channel coils. The high-quality images achieved by T1, T2-weighted, and fluid-attenuated inversion-recovery (FLAIR) protocols enhance the diagnostic potential of VLF MRI, particularly in cases of cerebral stroke and trauma patients. This study underscores the adaptability of the design methodology for the customization of RF coil structures in alignment with individual imaging requirements.
Breast intervention device for low-field MRI with a customized unilateral coil
2023, Journal of Magnetic Resonance
With the incidence of breast cancer rising to the top among female malignant tumors, magnetic resonance images guided breast biopsy intervention and minimally invasive treatment have developed as a clinically practical research issue. High field studies have shown the diagnostic value of breast MRI, but the examination costs greatly exceed those of competing conventional mammography. In this case, low-field MRI cannot merely provide typical MRI contrast, but also significantly reduce the cost of diagnosis and treatment for breast cancer patients. This work describes a unilateral breast coil and prototype intervention device, which provides a customized solution for low-field MRI-guided breast intervention. Results demonstrate that the low-field MRI breast intervention device facilitates medical intervention procedures. And the designed positioning device can locate the target lesion within 2–3 mm accuracy. Phantom tests with the customized unilateral coil indicate that the open loops perform as well as the 4-channel commercial closed breast coil, presenting a relatively good SNR (signal-to-noise ratio) and uniformity characteristics. MR scanning images of the volunteer breast using the breast intervention coil also show high SNR, which lays a foundation for further implementation of image-guided breast interventional minimally invasive surgery with the low-field MRI system.
Image Quality of Lumbar Spine Imaging at 0.55T Low-Field MRI is Comparable to Conventional 1.5T MRI – Initial Observations in Healthy Volunteers
2023, Academic Radiology
To assess the potential of 0.55T low-field MRI system in lumbar spine imaging with and without the use of additional advanced postprocessing techniques.
The lumbar spine of 14 volunteers (32.9 ± 3.6 years) was imaged both at 0.55T and 1.5T using sequences from clinical routine. On the 0.55T scanner system, additional sequences with simultaneous multi-slice acquisition and artificial intelligence-based postprocessing techniques were acquired. Image quality of all 28 examinations was assessed by three musculoskeletal radiologists with respect to signal/contrast, resolution, and assessability of the spinal canal and neuroforamina using a 5-point Likert scale (1 = non-diagnostic to 5 = perfect quality). Interrater agreement was evaluated with the Intraclass Correlation Coefficient and the Mann-Whitney U test (significance level: p < 0.05).
Image quality at 0.55T was rated lower on the 5-point Likert scale compared to 1.5T regarding signal/contrast (mean: 4.16 ± 0.29 vs. 4.54 ± 0.29; p < 0.001), resolution (4.07 ± 0.31 vs. 4.49 ± 0.30; p < 0.001), assessability of the spinal canal (4.28 ± 0.13 vs. 4.73 ± 0.26; p < 0.001) and the neuroforamina (4.14 ± 0.28 vs. 4.70 ± 0.27; p < 0.001). Image quality for the AI-processed sagittal T1 TSE and T2 TSE at 0.55T was also rated slightly lower, but still good to perfect with a concomitant reduction in measurement time. Interrater agreement was good to excellent (range: 0.60–0.91).
While lumbar spine image quality at 0.55T is perceived inferior to imaging at 1.5T by musculoskeletal radiologists, good overall examination quality was observed with high interrater agreement. Advanced postprocessing techniques may accelerate intrinsically longer acquisition times at 0.55T.
New challenges and opportunities for low-field MRI
2023, Journal of Magnetic Resonance Open
In this manuscript we deal with recent advances in low-field Magnetic Resonance Imaging (MRI). The development of low-cost MRI solutions allowing portability and trustable diagnosis is a hot topic worldwide by these days. We analyze basic technical issues of recent examples of fixed-field instruments operating at low-field. Then we discuss pros and cons of the pre-polarized approach, from both physical and technical perspectives. Permanent magnet and electromagnet technology are confronted. Finally, magnetic field-cycling is introduced as an alternative MRI technique, where field-dependent experiments can be exploded for the development of new contrast mechanisms that are not feasible for fixed-field MRI instruments. As field cycled machines usually deals with switched currents in electromagnets, magnetic field instability and inhomogeneity are the main limiting factors affecting image quality. We finalize this manuscript discussing how it turns possible to overcome these limitations, thus opening new possibilities for the development of cost effective MRI technology.
Hyperpolarized <sup>129</sup>Xe MRI at low field: Current status and future directions
2023, Journal of Magnetic Resonance
Magnetic Resonance Imaging (MRI) is dictated by the magnetization of the sample, and is thus a low-sensitivity imaging method. Inhalation of hyperpolarized (HP) noble gases, such as helium–3 and xenon-129, is a non-invasive, radiation-risk free imaging technique permitting high resolution imaging of the lungs and pulmonary functions, such as the lung microstructure, diffusion, perfusion, gas exchange, and dynamic ventilation. Instead of increasing the magnetic field strength, the higher spin polarization achievable from this method results in significantly higher net MR signal independent of tissue/water concentration. Moreover, the significantly longer apparent transverse relaxation time T₂* of these HP gases at low magnetic field strengths results in fewer necessary radiofrequency (RF) pulses, permitting larger flip angles; this allows for high-sensitivity imaging of in vivo animal and human lungs at conventionally low (<0.5 T) field strengths and suggests that the low field regime is optimal for pulmonary MRI using hyperpolarized gases.
In this review, theory on the common spin-exchange optical-pumping method of hyperpolarization and the field dependence of the MR signal of HP gases are presented, in the context of human lung imaging. The current state-of-the-art is explored, with emphasis on both MRI hardware (low field scanners, RF coils, and polarizers) and image acquisition techniques (pulse sequences) advancements. Common challenges surrounding imaging of HP gases and possible solutions are discussed, and the future of low field hyperpolarized gas MRI is posed as being a clinically-accessible and versatile imaging method, circumventing the siting restrictions of conventional high field scanners and bringing point-of-care pulmonary imaging to global facilities.

View all citing articles on Scopus

View full text

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

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Sample phantoms and preparation of nuclear spin polarizations

Results and discussion

Conclusion

Acknowledgments

J. Magn. Reson.

Neoplasia

J. Magn. Reson.

J. Magn. Reson.

J. Magn. Reson.

J. Magn. Reson.

J. Magn. Reson.

Magn. Reson. Imag.

Biochimie

Magn. Reson. Imag.

J. Magn. Reson.

Specific absorption rates and signal-to-noise ratio limitations for MRI in very-low magnetic fields

Concept Magn. Res. A

Hyperpolarized 129Xe lung MRI in spontaneously breathing mice with respiratory gated fast imaging and its application to pulmonary functional imaging

NMR Biomed.

Real-time metabolic imaging

Proc. Natl. Acad. Sci. USA

Para-hydrogen and synthesis allow dramatically enhanced nuclear alignment

J. Am. Chem. Soc.

Spin-exchange optical pumping of noble-gas nuclei

Rev. Mod. Phys.

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

Hyperpolarized ¹²⁹Xe lung MRI in spontaneously breathing mice with respiratory gated fast imaging and its application to pulmonary functional imaging