Cardiac MRI at Low Field Strengths

Cardiac MR imaging is well established for assessment of cardiovascular structure and function, myocardial scar, quantitative flow, parametric mapping, and myocardial perfusion. Despite the clear evidence supporting the use of cardiac MRI for a wide range of indications, it is underutilized clinically. Recent developments in low‐field MRI technology, including modern data acquisition and image reconstruction methods, are enabling high‐quality low‐field imaging that may improve the cost–benefit ratio for cardiac MRI. Studies to‐date confirm that low‐field MRI offers high measurement concordance and consistent interpretation with clinical imaging for several routine sequences. Moreover, low‐field MRI may enable specific new clinical opportunities for cardiac imaging such as imaging near metal implants, MRI‐guided interventions, combined cardiopulmonary assessment, and imaging of patients with severe obesity. In this review, we discuss the recent progress in low‐field cardiac MRI with a focus on technical developments and early clinical validation studies.

and musculoskeletal MRI, cardiac MRI exams tend to be complicated to perform, and require technologists and physicians with a high level of specialized expertise.The time-consuming nature of cardiac MRI makes it challenging to justify in environments where MRI resources are limited.In the United States in particular, this challenge is compounded by the fact that cardiac MRI reimbursement rates are unfavorable relative to echocardiography and single photon emission tomography (SPECT), despite the significantly higher costs to purchase, install, maintain, and operate an MRI scanner compared to these other cardiac imaging modalities. 14Additionally, compared to echocardiography and cardiac SPECT, MRI poses greater challenges for severely obese and claustrophobic patients, those unable to breath-hold repeatedly, and for the growing number of patients with cardiac implanted electronic devices (CIED).These factors and others have limited cardiac MRI utilization in the United States to represent only a small fraction of all MRI scans performed (<5%), and an even smaller fraction of cardiac imaging performed using all modalities (<3%).
][20] Additionally, modern data acquisition and model-based or machine learning driven image reconstruction methods are becoming more widely available and further enable low-field MRI.In addition to cost-savings, low-field MRI offers several other potential benefits, for example in patients with implants, for MRI-guided interventional procedures, for examining cardiopulmonary interactions, and for obese patients.Moreover, the deployment of low-field MRI directly in patient care environments such as cardiology, intensive care, emergency departments, and community-based centers may be achievable.The opportunities and challenges of low-field cardiac MRI are outlined in Table 1.
Low-field cardiac MRI is actively under development, with the common goal to expand the availability of imaging to patient groups and underserved geographical regions with limited access to MRI, and to develop applications that are better suited to low field.This article will review the current state of low-field cardiac MRI, defined here as <1.0 T, with a focus on recent and emerging technical developments and early clinical validation studies.

History of Low Field Cardiac MRI
Beginning in the early 1980s, the first commercial MRI units operated at field strengths <0.5 T. Cardiac imaging was attempted very early on, with the first description of clinical cardiac MRI published in 1983 21 using a 0.35 T system (Oxford Instruments).Gated and ungated spin echo imaging was used in 244 subjects, and clear delineation of cardiac chambers and pathologies of the aorta including dissection, aneurysm, and atheroma were reported.
The magnetic field strength (B0) used for clinical MRI immediately started on an upward trajectory, with the first 1.5 T commercial system introduced by General Electric in 1983.By the 1990's, 1.5 T quickly became the clinical standard and dominated the MRI market.Over the next two decades, the pursuit of higher signal-to-noise ratio (SNR) and increased image resolution drove B0 to 3.0 T and higher.While high-field magnets have proven beneficial for many neurological and musculoskeletal imaging applications, there have been no clear direct benefits for most cardiac imaging applications.In addition, higher B0 brings increased system cost, safety concerns, image artifacts, and restrictions on magnet configuration.As such, 1.5 T remains the predominant field strength for cardiac MRI.
While lower field units remained on the market during this period, these were viewed as inferior to high-field MRI and were marketed primarily for their lower cost and "open" bore configuration.Minor efforts have been ongoing to explore the potential for cardiac imaging on the existing low-field scanners.3][24][25] For example, a 0.5 T system (Surrey Medical Imaging Systems Ltd., Surrey, U.K.) was used to demonstrate first-pass perfusion and for cardiac imaging in patients with pacemakers 26,27 ; a 0.5 T double-donut interventional MRI system (GE Signa SP, GE Medical Systems, Milwaukee, WI, USA) was used for upright exercise stress during flow quantification 28 ; an open 0.7 T system (Altaire, Hitachi Medical Corporation) was used for cine imaging with parallel imaging 23 ; an open 0.35 T system (MAGNETOM C!, Siemens Healthcare, Erlangen, Germany) was used for cine, perfusion and late enhancement on a cohort of patients 22 ; and a 0.2 T system (Signa Profile version 7.6; GE-Yokogawa Medical Systems, Tokyo, Japan) was used to develop interventional device tracking techniques. 29long with the move to 1.5 T and 3 T fields in the 1990's and early 2000's, respectively, there have been concurrent advances in RF and gradient systems.Until recently, these advances have not been combined with lower field strength magnets.Contemporary clinical gradient systems have maximum amplitude and slew rates in the range of 45 mT/m and 200 T/m/s respectively, facilitating the implementation of short repetition (TR) and echo times (TE) that are critical for cardiac pulse sequences.Advances in multi-channel receiver coil design have facilitated the development of parallel imaging acceleration techniques that were instrumental in the application of rapid, segmented k-space, breath-held imaging, and singleshot/real-time free-breathing cardiac imaging techniques.GeneRalized Autocalibrating Partial Parallel Acquisition (GRAPPA) and Sensitivity Encoding (SENSE)-based parallel imaging techniques are routinely used in cardiac MRI to reconstruct under-sampled data, although these methods incur an SNR penalty that is not always compatible with low field.In the past decade, compressed sensing (CS) inspired recovery methods, and more recently machine learning-based methods, have altered the traditional tradeoffs between scan time and SNR, opening the possibility for high quality routine and advanced cardiac imaging techniques at low field.
Over the past 6 years, cardiac applications have been explored by several research groups using both commercial and prototype whole-body low-field systems equipped with superconducting magnets, contemporary hardware, and contemporary software; these systems are summarized in Table 2. Two higher field commercial scanners have been ramped down to use a unique combination of lower main magnetic field (0.55 T and 0.75 T) with high performance gradient and RF systems. 16,30Commercially available low-field systems with demonstrated cardiac imaging capabilities include the 0.35 T ViewRay MRIdian (ViewRay Inc.Oakwood, USA), and the 0.55 T Siemens MAGNETOM Free.Max (Siemens Healthcare, Erlangen, Germany).This Siemens system is considerably lighter in weight ($3.2 tons) than higher field magnets, and uses very little helium (<1 liter), eliminating the need for a quench pipe and reducing site preparation, installation, and maintenance costs. 17,31

Low Field Imaging Properties and Imaging Technology
Favorable Imaging Properties MRI at field strengths <1 T provides many favorable properties for cardiac imaging.As briefly described in this section, CARDIAC GATING.Most cardiac MRI methods rely on synchronization with the cardiac cycle using in-bore electrocardiography (ECG) or photo plethysmography.ECG is preferred because triggering on the sharp and high-amplitude R-wave enables prospective capturing of systolic contraction.ECG signals obtained at lower B0 field strengths are less noisy and more reliable, primarily due to the reduced magnetohydrodynamic (MHD) effect that is proportional to B0.However, ECG signals may still be susceptible to interference from RF and imaging gradients across field strengths.At higher field strengths such as 3 T, the MHD artifact is a significant source of mis-gating, requiring more sophisticated multi-channel solutions like vector-cardiogram gating. 32Low-field imaging can thus provide reliable cardiac ECG gating with reduced complexity and cost.Figure 1 contains illustrative ECG samples from 0.75 T to 1.5 T.
IMPLANT ARTIFACTS.Metallic implants are extremely common in cardiac patients, and include sternal wires, stents, valve clips, prosthetic valves, occlusion devices, and CIEDs.The increasing prevalence of these implants has created a growing patient population in whom cardiac MRI can be extremely challenging or infeasible.Although MRI safety in patients with CIEDs has now been well documented, [34][35][36] cardiac MRI of patients with CIEDs can be challenging due to metal-induced image artifacts. 37,38The presence of these foreign metals disturbs the homogeneity of the B0 field, producing significant artifacts. 39As the degree of signal distortion is proportional to B0, all artifacts (signal loss, banding, spatial distortions) are dramatically reduced at lower field strengths.In short, low-field imaging is expected to provide improved quality near the metallic hardware site (see "Advanced Applications" section).Moreover, RF-induced heating scales quadratically with B0, and additionally depends on the coupling between the RF field and conductor device to form standing waves.At low field, the RF wavelength is longer, potentially preventing coupling with short conductive leads.Therefore, low-field MRI possibly allows for safer imaging in patients with broken or abandoned leads, which remain a contraindication to MRI due to the risk of heating. 40TIENT COMFORT.Two of the most frequent MRI subject complaints are acoustic noise, and claustrophobia, both of which can be reduced at low field. 41Acoustic noise (and system vibration) is generated by vibration of the gradient coil when the gradient changes (dG/dt) in the presence of the static field (B0).It is, therefore, proportional to both dG/dt and B0.The noise a patient hears and the vibration they feel is also highly dependent on the mechanical engineering of the entire system, which includes sophisticated vibration dampening.However, with all else constant, the acoustic noise from an MRI sequence is indeed linear with B0, meaning that low field reduces acoustic noise. 42Regarding claustrophobia, lower B0 changes the constraints on the superconducting magnet design, which makes it feasible to produce wider bore low-field systems.This reduces claustrophobia, and improves overall comfort for larger subjects, including patients who are obese or pregnant.

Image Contrast
Endogenous NMR relaxation parameters vary with field strength. 43Generally, at low field, T1 relaxation times are shorter, and T2 and T2* relaxation times are longer.Table 3 provides reported T1, T2, and T2* values from 0.35 T to 0.55 T compared to 1.5 T and 3 T for myocardium and blood. 16,31For cardiac imaging, the shorter T1 relaxation times result in more signal recovery during rapid imaging sequences that have short TR, but also result in slightly reduced blood-myocardium contrast for balanced steady state free precession (bSSFP).The relaxivities of exogenous agents have been measured at 0.55 T 16 and are reported in Table S1 in the Supplemental Material.Generally, gadolinium-based contrast agent relaxivity at 0.55 T is similar to 1.5 T, whereas large molecular weight contrast agents such as ferumoxytol have increased relaxivity at lower field strength.Current publications on low-field cardiac MRI have used standard dosing of gadolinium-based contrast agents and have speculated that lower doses of ferumoxytol may be feasible. 45

Image Acquisition
Low field systems provide greater flexibility in pulse sequence design for cardiac MRI.Specific absorption rate (SAR) is a major constraint and is quadratic with field strength.This means that low-field MRI can utilize higher flip angles and more sophisticated RF pulse designs while staying within safety limits. 46In a well-shimmed system, B0 inhomogeneity is dominated by susceptibility effects from the lungs and draining veins, and varies linearly with field strength. 47The improved B0 homogeneity at low field enables the broad use of bSSFP, which suffers from banding artifacts at higher field strengths.Specifically, bSSFP acquisitions with longer TRs and lower receiver bandwidth are feasible without substantial banding and can be used to mitigate SNR loss.
Cardiac MRI generally benefits from motion-robust, efficient, data sampling.At conventional field strengths, spiral and echo-planar imaging have been employed, but their use is limited by off-resonance, which causes spatial blurring for spiral imaging, and warping for echo-planar imaging, and to a lesser extent, T2* signal decay during long readouts.At low field strengths, with more homogenous B0 and elongated TABLE 3. Reported Cardiac T1, T2, and T2* Relaxation Times in Healthy Volunteers From 0.35 T, 44 0.55 T, 16 1.5 T, 16,44 16 or Look-Locker 44 methods, T2 with T2-prepared bSSFP, and T2* with a multi-echo gradient echo sequence.Note that the relaxation times specified here are from the authors' study data and not meant to be interpreted as clinical reference values.
T2*, spiral and echo-planar readouts can be utilized to their full potential. 48On modern 0.55 T systems, it is possible to use 6-8 msec spiral readouts with negligible blurring, a substantial improvement upon the 2-3 msec that has been used at 1.5 T. Spiral and echo-planar readouts can also be combined with bSSFP, with appropriate first moment nulling, to simultaneously capture high SNR efficiency, and high scan efficiency.High duty-cycle spiral acquisitions can also be combined with spectral-spatial excitations for fat suppression. 49These RF pulses use spiral gradient waveforms for spatial and spectral encoding during excitation.The benefits of improved B0 and B1 homogeneity using a contemporary low field MRI system could therefore be applied for optimization of such RF pulses.

Image Reconstruction
A variety of modern denoising approaches can be employed at low field, including CS and artificial intelligence-based reconstruction.This is an area of active investigation across all field strengths, and denoising solutions are application specific.For example, ventricular function assessment should be done in a fashion that preserves fidelity of moving boundaries (eg, temporal finite difference constraint), and first-pass perfusion should be done in a fashion that preserves signal intensity time-course (eg, fit to a tracer-kinetic model).
Reduced artifacts at low field, eg, reduced blurring in spiral imaging, reduced banding in bSSFP and reduced susceptibility artifacts, can enable simpler reconstruction pipelines due to minimized correction steps.One exception is concomitant fields, which produce undesired spatially varying phase that is proportional to gradient amplitude and is inversely proportional to B0, making them more significant for low-field systems, particularly with high-performance gradients.The additional phase accumulation can be mitigated during image acquisition and/or corrected in reconstruction. 50,51t is expected that artificial intelligence-based image reconstruction and image enhancement will have a significant impact on image quality as low field MRI technology is further developed.For example, reconstruction with deep images priors can be used improve image quality for dynamic applications, 52 denoising and super-resolution have been demonstrated to enhance images, 53,54 and AUTOMAP has had significant success for image reconstruction at ultra-low field. 55

Routine Cardiac MRI Sequences at Lower Field
A cardiac MRI exam typically consists of several pulse sequences depending on clinical indication, and this section will review the current literature and describe the ongoing efforts to translate the routine sequences used at 1.5 T to low-field MRI.Figures 2 and 3 provide example images of these routine cardiac MRI sequences from prototype and commercial 0.55 T systems, respectively.A comprehensive protocol for a commercial 0.55 T system with reduced gradient performance has also been described by Varghese et al. 20

Cine Imaging
The measurement of chamber volume, systolic function, and ventricular mass using bSSFP cine imaging is a cornerstone of cardiac MRI exams and is clinically indicated in 92% of patients undergoing cardiac MRI. 56Significant work has been done to develop and validate cine imaging for low field strengths.These studies suggest that, despite the lower SNR, low-field MRI can be accurately used for quantitative diagnostic evaluation of cardiac volumes and function and for assessment of regional wall motion abnormalities.
Bandettini et al studied a population of 65 participants, 44 clinically referred patients and 21 healthy volunteers, at 0.55 T (prototype system) and 1.5 T 19 (Fig. 2a; Video S1a in the Supplemental Material). 57Quantitative comparisons of chamber volumes, ejection fraction and LV mass showed excellent correspondence between field strengths.A sector-wise comparison of regional wall motion abnormalities showed excellent agreement (kappa = 0.99).Varghese et al compared quantitative cine imaging, flow quantification and parametric mapping between 0.35 T, 1.5 T, and 3 T in healthy volunteers. 44No significant difference in quantitative cine measurements of LV volume and function was reported.Zu et al also compared an AI-based analysis pipeline for endocardial and epicardial image contouring between 0.35 T, 1.5 T, and 3 T and found that the automated software performed well across field strengths. 58ther studies have focused on improving image acquisition strategies for cine imaging at lower field.Rashid et al explored high flip-angle bSSFP cine at 0.35 T and reported feasibility of flip angles up to 150 , with peak blood-myocardium CNR at 130 . 59Restivo et al developed SNR-efficient spiral inout and EPI bSSFP cine acquisitions for 0.55 T with high acquisition duty cycle (sampling time per TR). 48They showed a 79% increase in myocardial SNR by moving from Cartesian bSSFP to spiral-in-out bSSFP imaging, with no change in breath-hold length or spatiotemporal resolution.Tian et al demonstrated a contrast-optimal simultaneous multi-slice (SMS) acquisition for breath-held cine. 46They developed a spiral-out bSSFP sequence with SMS factors 2 or 3, and experimentally found peak blood-myocardium contrast at a flip angle of 160 for a single band and 120 for SMS acquisition.
CS has also been applied to improve cine image quality.Bandettini et al used an L1-SPiRIT image reconstruction for free-breathing cine and image quality was remarkably similar between 0.55 T and 1.5 T when using the same algorithm. 19imonetti et al used Sparsity adaptive COmpressive REcovery (SCoRe) for improved cine imaging at 0.35 T. 15 Vishnevskiy et al explored cine imaging at 0.75 T and retrospectively sparsely undersampled breath-held Cartesian cine data to evaluate high acceleration factors. 33They were able to achieve acceleration rates of 7 without image degradation.More recently, deep learning-based denoising has been applied to breath-held cine images with GRAPPA reconstruction.Specifically, a g-factor-savvy transformer convolution neural network model was used for GRAPPA reconstruction with acceleration rate of 3 on a commercial 0.55 T system, with more than double the original SNR. 54,60Future work on deep learning-based denoising and imaging reconstruction is expected to further improve image quality.
Real-time cine, which is neither ECG gated nor breath-held, has also been developed for low field strengths.Hamilton et al used a deep image prior reconstruction based on two U-nets to generate spatial and temporal basis functions for real-time cine images from highly undersampled spiral acquisitions on the commercial 0.55 T system. 52Their method used a spatial resolution of 2.2 mm and they compared five acceleration rates from R = 4 (76 msec temporal resolution) to R = 24 (13 msec temporal resolution).A spiral in-out bSSFP acquisition has been paired with a low rank + sparse image reconstruction on a prototype 0.55 T for real-time cine with a spatial resolution of 1.7 mm and temporal resolution of 36 msec. 61Yagiz et al developed real-time cine with SMS factor 3 and a flip angle of 100 , paired with a constrained image reconstruction. 62A realistic simulation framework, based on the XCAT phantom, has also been proposed and used to simulate real-time volumetric (3D) cine at 0.55 T with a stack-of-spirals trajectory. 63inally, Piccini et al performed a proof-of-concept freebreathing motion-resolved 3D imaging study at 0.55 T. 64 This acquisition used a 3D radial trajectory with superiorinferior readouts for extraction of cardiac and respiratory self-navigation signals that were used to sort data into 20 cardiac and 4 respiratory bins.Isotropic spatial resolutions of 1.1 to 2.0 mm 3 were visually compared and 3D images were re-sliced into cardiac views.

Flow Quantification
Phase-contrast flow imaging is commonly used clinically for quantitative metrics such as cardiac output and for characterizing valvular heart disease.Flow imaging uses a T1-weighted  spoiled gradient-echo sequence and inflow enhancement during systole, and benefits from the shorter T1 at lower field.
The effect of concomitant fields can result in additional phase distributions across the imaging plane. 65However, as these fields are predictable, and can be corrected using several approaches, this should not lead to quantification errors. 3omparison studies have shown accurate measurements of cardiac output for both aortic and pulmonary artery measurements at low field.Varghese et al compared breath-held aortic flow measurements and found no statistically significant differences in cardiac output measurements at 0.35 T, 1.5 T, and 3 T in six healthy volunteers. 44The 0.35 T:3 T relative SNR was higher than would be predicted from polarization alone.Shanbhag et al studied pulmonary and systemic flow in 10 healthy subjects and 8 patients referred for valvular or shunt evaluations at 0.55 T (Fig. 2b; Video S1b in the Supplemental Material). 66A high correlation between flow measured at 0.55 T and 1.5 T was measured across all subjects, and Bland-Altman analysis showed reasonable agreement in cardiac output, and the ratio of pulmonary-to-systemic flow.
Research studies have explored using a bSSFP acquisition paired with flow-encoding, exploiting low-field's improved field homogeneity for relatively long-TR steadystate imaging.Using bSSFP eliminates the inflow-driven contrast of GRE and therefore provides more consistent SNR across the cardiac cycle.However, this technique can present difficulties in quantification due to spatial phase errors induced by bSSFP acquisitions. 67Ramasawmy et al demonstrated a bSSFP-flow acquisition using a spiral-readout to maximize the SNR at 0.55 T in 11 volunteers 68 and Peper et al compared a Cartesian bSSFP-flow acquisition to gradient-echo at 0.75 T in 6 volunteers. 69

Late Gadolinium Enhancement
LGE imaging is the standard for noninvasive detection of scar tissue and the assessment of myocardial viability, 70 and is an important component of most clinical cardiac MRI exams.
LGE images are acquired at end-diastole, and can either involve segmented breath-held acquisitions, or fast "snapshot" imaging for free-breathing approaches.Snapshot imaging speed is limited at low field due to reduced SNR, reduced gradient performance on some commercial systems, and low acceleration factors from limited coil arrays.
Bandettini et al compared breath-held, phase-sensitive inversion recovery LGE image quality and diagnostic assessment between a high-performance 0.55 T and 1.5 T in 16 patients with myocardial infarction (MI). 18A bSSFP acquisition was used at low-field to improve SNR (Fig. 2c).A strong correlation was measured between field strengths for absolute LGE mass and percentage MI, and no significant bias was found between measurements of MI mass from Bland-Altman analysis.A recent case study demonstrated LGE imaging on a commercial, lower gradient-performance 0.55 T scanner in a case of nonischemic fibrosis. 71In addition, Ding et al presented a CS based technique that incorporates motion fields along with the reconstruction, leading to increased sharpness in free-breathing LGE acquisitions in 12 volunteers on a commercial 0.55 T system. 72rametric Mapping and ECV Fraction MRI parametric mapping of T1, T2, and T2* relaxation times and ECV fraction of the myocardium facilitates characterization and monitoring of diseases such as fibrosis, amyloidosis and iron overload. 73Parametric mapping is typically acquired during the end-diastolic window with a "snapshot" imaging technique.Though the relaxation rates will vary, with T1 being shorter, and T2 and T2* times being longer at lower field strength, the magnetization preparation schemes used for parametric mapping are not significantly altered for imaging across different field strengths.
Campbell-Washburn et al measured tissue T1, T2, and T2* relaxation times including myocardium and arterial blood in 39 subjects using breath-held imaging at 0.55 T, 16 and Varghese et al reported T1, T2, and T2* at 0.35 T, compared to 1.5 T and 3 T, in six healthy volunteers. 44ancini et al measured native and post gadolinium contrast T1, and ECV in 27 subjects, including 13 patients with MI, across 0.55 T and 1.5 T (Fig. 2d). 74Native and post-contrast T1 relaxation times were shorter at 0.55 T, as expected, with the relative gadolinium induced T1 shortening being approximately 20% greater at 0.55 T. Both T1 relaxation times and ECV had a fair correlation between 0.55 T and 1.5 T across the regions of interest including remote myocardium and infarcted tissue.Crabb et al assessed a prototype 3D wholeheart spiral sequence for joint T1/T1ρ mapping and water-fat imaging on the commercial 0.55 T system and demonstrated promising in vivo results. 75arghese et al demonstrated myocardial T2 mapping in a porcine model of ischemia-reperfusion induced MI in five animals at 0.55 T. 76 The authors demonstrated the breathheld acquisition, and measured significant elevation in T2 within the infarct regions compared to remote myocardium.T2* and R2* have gained acceptance for noninvasive assessment of iron overload in the liver and in the heart. 77,78ue to the improved field homogeneity of contemporary low-field MRI systems, T2* relaxation times are expected to be longer, and R2* values smaller, which could potentially offer improved sensitivity in patients with severe iron overload that is difficult to accurately quantify at 1.5 T or 3 T. Campbell-Washburn et al performed a comparison study of hepatic R2* mapping between 0.55 T and 1.5 T in patients with iron overload, and showed significantly smaller R2* at 0.55 T as expected, reasonable measurement precision, and accurate assessment of liver iron content. 79A predictive model was used to predict R2* across field strengths, which could be extended to other field strengths as well. 80at-Water Separation Myocardial fat infiltration, and epicardial adipose tissue has been linked to an increased risk of cardiovascular disease, including sudden cardiac death and coronary artery disease. 81eparating fat and water peaks is more difficult at lower fields due to the converging chemical frequencies.In addition, the shorter T1 relaxation times at low field will mean that fat, typically the shortest T1 species found in the body, will have increased signal amplitude and will require a narrow signal null period for inversion-recovery-based methods for fatsuppression, though these have been demonstrated at 0.55 T for abdominal imaging. 82Dixon-based methods require longer time intervals between echoes with smaller chemical shift effects, which can limit the sampling frequency for cardiac imaging, but have been successfully demonstrated at 0.55 T for both cardiac and abdominal applications (Fig. 2e).
Non-Cartesian trajectories that can resolve the spectra have been explored, such as by Franson et al, who demonstrated a rosette trajectory, 83 and Tian et al, who proposed a novel multi-echo spiral acquisition. 84Both these trajectories demonstrated good fat-water separation using relatively long TRs (10-12 msec) possible with bSSFP at 0.55 T.

Cardiac and Vascular Morphology
Cardiac morphology is assessed using black-blood T2-weighted imaging, especially for the evaluation of tumors and masses.Varghese et al optimized T2-weighted turbo spin echo (TSE) sequences for the commercial 0.55 T system (Fig. 3a). 20TSE performed well with little modification since it is a high SNR technique and is not particularly demanding on system performance.
Three-dimensional thoracic vascular morphology is typically imaged using contrast-enhanced or non-contrast MR angiography (MRA).Varghese et al demonstrate non-contrast MRA using a ECG-triggered 3D bSSFP with T2 magnetization preparation and fat suppression, and contrast-enhanced MRA (Fig. 3b) with a CS-accelerated breath-held 3D spoiled gradient echo sequence at 0.55 T. 20 Castillo-Passi et al evaluated a proof-of-concept non-contrast motion-corrected 3D whole-heart MRA sequence and achieved reasonable image quality in 6 minutes at 0.55 T. 85 Perfusion First-pass perfusion imaging of the myocardium during rest and pharmacological stress has excellent sensitivity for detecting myocardial ischemia and there is growing evidence supporting the use of this technique for diagnostic cardiac MRI.Contrast-enhanced perfusion relies on fast, saturationrecovery, gradient-echo, and bSSFP snapshot acquisitions, ideally with a temporal footprint <80 msec.Rapid imaging is especially crucial for performing perfusion imaging during pharmacological stress at higher heart rates.For quantitative imaging, these snapshot acquisitions can be additionally paired with a low-resolution acquisition which measures the arterial-input function. 86The translation of this technique to low field requires care as this technique is already in the low SNR regime at 1.5 T. Simulations regarding perfusion have demonstrated viability of this technique at 0.55 T. 63 Varghese et al demonstrated the use of a CS reconstruction with acceleration rate 5 to achieve snapshot gradient echo images with a <110 msec temporal footprint for perfusion imaging at rest at 0.55 T (Fig. 3c). 20The authors illustrated the successful depiction of a resting perfusion defect in a patient with known MI.

Strain-Encoded Imaging
Left and right ventricular strain characterizes myocardial contractility, and may provide early detection of cardiac dysfunction, prior to overt functional or structural changes. 87train-encoded MRI (SENC) is a reproducible method of directly measuring strain, 88 but as the encoding method results in an inherently low SNR image, its feasibility is questionable at low field.A prototype sequence has been validated with the commercial 0.55 T system using a dynamic gel deformation phantom, and feasibility has been successfully assessed in a small cohort of healthy volunteers and in a porcine ischemia-reperfusion infarct model (Fig. 4). 90

Advanced Cardiac MRI Applications
There are several new opportunities to leverage the unique properties of low field strength to expand the application of cardiac MRI beyond the current clinical routine.This Journal of Magnetic Resonance Imaging section will describe example advanced applications of lowfield cardiac MRI.

MRI-Guided Invasive Procedures
MRI-guided cardiac catheterization procedures use real-time MRI to guide catheter-based devices in the heart.8][99] There have also been many pre-clinical studies exploring the advantages of MRI-guidance for other procedures such as endomyocardial biopsy, stenting, ventricular tachycardia (VT) ablation, extra-anatomic bypass, etc. [100][101][102][103] Most off-the-shelf catheterization devices contain long metallic components and are susceptible to RF-induced heating, and there are few custom-built devices approved for human use.Patient MRI-guided catheterization has been accomplished mainly with polymer catheters.The paucity of devices that are MR-safe and mechanically adequate has hampered the clinical translation of many of these procedures.
Lower fields offer reduced RF-induced heating of metallic devices, and therefore may enable new procedures using off-the-shelf devices or simplify the design of custom-built devices.Most work on low-field MRI-guided catheterization procedures has been performed on a prototype 0.55 T system with high performance gradients.Heating is quadratically related to field strength, meaning that 0.55 T offers 7.5-fold less device heating than 1.5 T, and 30-fold less device heating than 3 T. 16,104 Campbell-Washburn et al demonstrated MRI-guided right heart catheterization at 0.55 T in seven patients. 16Importantly, they used off-the-shelf metallic guidewires (fully insulated nitinol guidewires) without modification to the devices or to the real-time bSSFP imaging parameters (flip angle = 45 , TR = 4 msec).They measured <1 C of heating during 2 minutes of real-time imaging in 9 of 16 test nitinol guidewires and stainless-steel braided catheters.Recently, Özen et al systematically evaluated device safety for several offthe-shelf devices (guidewires, catheters, needles, and microwave applicator) and found negligible heating at 0.55 T. 104 Kolandaivelu et al assessed the visibility of chemoablation lesion and RF-ablation lesions at 0.55 T in a porcine model as a precursor to MRI-guided ablation at low field (Fig. 5). 105he ability to visualize and assess ablation lesions is a key advantage of MRI-guidance over X-Ray guidance of these procedures. 106Seemann et al performed invasive pressure-volume loop measurements at 0.55 T during dynamic inferior vena cava occlusion to alter preload conditions. 107The technique combines simultaneous real-time imaging to measure cardiac volumes and invasive measurement of pressure to generate pressure-volume loops.This work was performed in three pig models: naïve, ischemic cardiomyopathy, aortic banding to increase afterload, and illustrated lower cardiac contractility and higher compliance in cardiomyopathy.
Visualization of metallic devices using susceptibility artifacts alone can be challenging at 0.55 T. The artifact profile of low-susceptibility materials (eg, nitinol, stainless steel 316) are reduced at 0.55 T, whereas those of high-susceptibility materials (eg, stainless steel 304) are consistent across field strengths because they are already saturated below 0.55 T. 108 Pilot studies have investigated computer vision methods to improve the detection of nitinol devices at 0.55 T. 109,110 Custom devices with built-in receiver electronics designed for "active" visualization are attractive to improve sensitivity and specificity of device visualization.Design constraints related to RF-induced heating are eased at 0.55 T, allowing for increased flexibility in device configuration.A safe-by-design active guidewire with continuous shaft-to-tip profile that is mechanically comparable to off-the-shelf devices has been demonstrated for 0.55 T. 111 Additionally, a new technique of thin-film printed circuitry which can fabricate RF antenna components direction onto metallic surfaces with conductive ink has been demonstrated for needle devices at 0.55 T. 112 Most recently, interventional studies on a commercial 0.55 T system has demonstrated promising real-time imaging results despite the reduced gradient performance.Armstrong et al demonstrated cardiac catheterization, angioplasty, and stenting on the commercial 0.55 T (Fig. 6; Video S2 in the Supplemental Material). 113Mooiweer et al demonstrated the feasibility of real-time proton resonance frequency shiftbased thermometry.MRI thermometry is a common method to assess thermal ablation treatments in real-time, and therefore is an essential tool for application of low-field MRI for cardiac ablation procedures. 114

MRI-Guided Radiotherapy
Hybrid MRI-guided radiotherapy systems (MR-linacs) have also been applied for cardiac interventional procedures.Specifically, a 0.35 T ViewRay MRI-linac system was used for MRI-guided stereotactic ablative radiotherapy of ventricular tachycardia.The first-in-human procedure was performed in a patient with dilated cardiomyopathy and recurrent sustained ventricular tachycardia with a cardiac implantable cardioverter-defibrillator (Fig. 7). 115Motion tracking was performed with the upper liver and procedural planning used invasive electroanatomical mapping and prior cardiac MRI.The patient received a single fraction of 25 Gy, with cine-tracking time of 46 minutes and beam-on time of 24 minutes.Since the initial case report, additional studies have explored cardiorespiratory motion management for MRI-guided radioablation procedures a 1.5 T that may also have applicability at lower field. 116mbined Cardiopulmonary Assessment Contemporary low-field MRI systems using superconducting magnets can provide improved B0 field homogeneity compared to 1.5 T or 3 T, which translates to reduced susceptibility gradients in anatomy with air-tissue interfaces, such as the lung, thereby providing an attractive environment for pulmonary imaging.Imaging the heart and lung in the same setting can be valuable for assessment of cardiopulmonary interactions, especially when paired with exercise and hemodynamic catheterization as appropriate.][124] For the assessment of patients with heart failure, Seemann et al optimized and validated a stack-of-spiral ultrashort echo time (UTE) sequence for the quantification of lung water density with an automated inline image processing pipeline at 0.55 T (Fig. 8). 125Cardiogenic pulmonary edema is the pressure-driven accumulation of fluid in the pulmonary interstitium which causes breathlessness and is a key feature of heart failure.Moreover, this technique was extended to measure dynamic changes in lung water during exercise stress, since exercise intolerance is an early symptom of cardiogenic pulmonary edema. 126Measurements of lung water can be combined with the routine assessment of heart failure by cardiac MRI.

Imaging Patients With Implants
The reduced susceptibility artifacts of contemporary low-field MRI systems can also be leveraged for reduced artifacts in the growing population of patients with implants and devices.Bandettini et al performed a pilot study comparing image artifacts caused by implants between 1.5 T and 0.55 T (Fig. 9). 127s expected, they observed smaller and less disruptive image artifacts at 0.55 T for most devices, however, the amount of artifact reduction depends on the implant material.Keskin et al demonstrated the feasibility of gradient-echo-based sequences, including bSSFP, for near-metal imaging at 0.55 T. 128 Van Speybroeck  et al demonstrated susceptibility artifacts for a 50 mT permanent magnet system in phantoms. 129To date, few devices are labeled for low-field MRI.While the general assumption is that low field is safer than higher field strengths, safety has a complex dependence on device length/RF wavelength and system geometry and thus safety testing is still required. 130,131aging Obese Patients Contemporary cardiovascular medicine heavily relies on noninvasive imaging by echocardiography, CT, SPECT, and MRI.Unfortunately, severely obese patients are difficult and sometimes impossible to assess by any cardiovascular imaging modality for a variety of reasons. 132,133In a recent study in patients with a mean BMI = 43 undergoing SPECT myocardial perfusion imaging, 32.6% of the studies were non-diagnostic. 134While CT scanners with 80 cm and larger bore diameter are available, CT of severely obese patients can suffer from truncation and cropping artifacts. 135CT radiation dose may be increased by factors of 3 or more in morbidly obese patients, 136 and five times higher for fluoroscopy. 137Furthermore, it may not be practical or safe to administer contrast agents or radioisotope dose based on body weight in the largest patients. 138,139While echocardiography has no table or bore restrictions, it may be the modality most limited by severe obesity. 133,138,139Increased body thickness decreases beam penetration, and thick layers of fat that attenuate the signal at a rate of 0.63 dB/cm further reduce signal-to-noise. 133,139n MRI, larger patients who fit tightly into the scanner may not be properly insulated from the magnet bore to prevent burns. 140,141Severely obese patients may be unable to lie supine for extended periods, may have significant trouble with extended or repeated breath-holding, and ECG signal may be attenuated. 142Thus, the potential benefits of MRI with its versatile diagnostic and prognostic capabilities 143,144 are unavailable to a large segment of the population at high risk for CVD.
The greater flexibility in magnet design at low field makes wider, more open bore configurations possible, eliminating the primary barrier to MRI for these patients.][147] Varghese et al 20 demonstrated the potential clinical utility of cardiac imaging on an 80 cm bore commercial 0.55 T system in two patients unable to be scanned on a standard 70 cm bore system due to their large body habitus; image results are shown in Fig. 10 and Video S3 in the Supplemental Material.Patient A, having a weight of 350 lbs and BMI = 48 kg/m 2 , was unable to be scanned in a 70 cm bore due to physical discomfort and anxiety.As shown in the figure, prior echocardiography images were of poor quality, even with the use of contrast.Conversely, the bSSFP cine, breath-held segmented LGE, and contrast-enhanced aortic MRA demonstrated good quality and diagnosis of cardiomyopathy and dilated aorta.In the second patient shown (Patient B), weighing 410 lbs and having BMI > 57 kg/m 2 , cine, flow, and LGE images were successfully acquired on the 80 cm bore 0.55 T system and revealed normal biventricular systolic function but also identified nonischemic fibrosis.Both patients reported being comfortable throughout the exam and demonstrate how valuable diagnostic information could be provided by low-field MRI when other cardiac imaging was not an option.

Fetal Cardiac Imaging
Fetal imaging at low field appears promising for several reasons.The potential for a wider bore is beneficial for maternal  comfort.The reduced acoustic noise and reduced SAR is beneficial from the fetal motion and safety perspective, and the improved real-time MRI is beneficial since most imaging is performed without fetal cardiac gating.Hutter et al reported on the first 150 fetal MRI scans at 0.55 T for noncardiac imaging. 149Recently, it has been shown that fetal cardiac function and anatomy can be captured in real-time at 0.55T without requiring retrospective gating that is typical at 1.5 T and 3 T. 150 Furthermore, metric optimized retrospective gating can successfully recover SNR at 0.55T when targeting finer spatial resolutions, or in low SNR scenarios (eg, underlying maternal obesity). 151Finally, 4D reconstructions can be generated from these data. 152At this point, feasibility has been demonstrated, and larger numbers of cases are needed to determine clinical potential.

Future Advanced Cardiac MRI Applications
Three-dimensional coronary imaging with MRI is of great interest to evaluate coronary artery disease, and to identify coronary anomalies in congenital heart disease.Coronary imaging relies on sub-millimeter spatial resolution, which presents a challenge for low field MRI.Several recent publications have demonstrated whole-heart free-breathing imaging with reasonable acquisition and reconstruction times. 153The capability of low field MRI to identify coronary arteries and characterize coronary anomalies remains of-interest, and advanced data sampling and image reconstruction techniques will likely be required to address this demanding application.Multi-dimensional multi-contrast techniques, such as MRI fingerprinting 154 and MR multitasking, 155 are attractive for low field cardiac MRI.These techniques may allow "push button" imaging that generates several parameters of interest from a single acquisition, thereby making accessible low field MRI easier to use.MR fingerprinting typically relies on spiral imaging which performs well at low field, and dictionary matching which tends to be less sensitive to low-SNR regimes.MR fingerprinting has been demonstrated for noncardiac applications at field strengths from 6.5 mT to 0.55 T. [156][157][158][159] While there are no publications on cardiac MR fingerprinting or multi-tasking to-date, there remains significant interest in this area.

Discussion
Low-field MRI systems offer significant promise to enable routine cardiac MRI in underserved geographical regions and patient groups with limited access to cardiac MRI previously, as well as to reduce siting requirements for point-of-care imaging.Studies to-date confirm that low-field MRI offers high measurement concordance and consistent interpretation with clinical 1.5 T imaging.Moreover, low-field MRI may enable specific new clinical imaging opportunities, beyond what is possible at 1.5 T, due to certain favorable imaging properties.
Increased accessibility by virtue of lower system cost is one of the key advantages of low-field MRI, however the exact cost can be hard to predict.The cost to manufacture a lower field superconducting magnet for whole body MRI is inherently lower since less material is required.Siting costs such as floor reinforcements are also inherently lower because these systems are lighter weight.Low-field systems can be equipped with cooling technology that uses low levels of helium and thus avoid a quench pipe. 31However, other costs such as gradient hardware, system electronics, room shielding, computers, and operating costs are expected to be similar for current whole-body system designs.If other compromises are made, costs could be further reduced, but may sacrifice performance for cardiac imaging applications.Overall, low-field MRI systems designed for cardiac imaging will be of lower cost than the current clinical standard, but the exact cost will depend on system specifications and design.
This article has focused on superconducting cylindrical whole-body MRI systems used for cardiac MRI, however operating a lower field enables more flexibility in magnet design.Outside the heart, low-field and ultra-low field systems have been designed with varied bore geometries, including vertical bores, single-sided systems, and planar systems.Unfortunately, these non-standard geometry systems often lack the field homogeneity and gradient performance needed for cardiac applications.Hypothetically, one could imagine low-field MRI designed with a short and wide bore for patients with claustrophobia undergoing cardiac MRI, or a vertical bore system for upright and/or exercise imaging.
Low-field whole-body MRI systems can also be used for other applications within the radiology setting including routine body imaging, musculoskeletal imaging, and neuroimaging, and specialized tasks such as pulmonary imaging, and imaging near metal implants.The availability of low-field MRI systems may alter how systems are selected and deployed within radiology departments. 160Alternatively, large cardiology centers may opt to install low-field MRI systems directly in cardiology departments.
Increased system accessibility could increase the demand for trained cardiac MRI technologists or radiographers to operate the scanners.In parallel, accessibility could drive improvements in automated slice planning and quality assurance.Advanced technology that is under development, such as push-button isotropic multi-contrast 3D scanning, could also simplify cardiac imaging in the hands of non-experts.
Low SNR due to the reduced equilibrium polarization proportional to B0, is an obvious disadvantage of low field.As discussed here, low SNR can be partially compensated by switching from GRE to bSSFP and moving from Cartesian to spiral or echo-planar readouts and by incorporating advanced image reconstruction techniques.However, these solutions may not be applicable or adequate for some cardiac imaging techniques, especially SNR-starved ones such as cardiac diffusion imaging, and cardiac spectroscopy.Low field is additionally problematic for advanced applications that benefit from spectral shifts, short T2* relaxation times and/or long T1 relaxation times at conventional field strengths.This includes proton spectroscopy that can assess myocardial triglycerides using chemical shift, although this has been demonstrated at 0.75 T. 30 This also includes arterial spin labeling (ASL), 161 and blood-oxygen level dependent (BOLD) imaging, 162 both nascent approaches for detecting ischemia without contrast agents at conventional field strengths.
Another challenge of low-field MRI is concomitant field effects.For gradient-echo imaging, these effects can be mitigated with frequency-segmented deblurring or an expanded signal model reconstruction. 50,163Effects have been mitigated for rapid acquisition of recalled echoes (RARE, also known as fast spin echo or TSE), using compensatory gradient pulses. 164,165This issue is not fully resolved for bSSFP and mitigation approaches are necessary, such as decreasing gradient amplitudes. 51ow-field MRI offers significant promise for cardiac MRI, both to reduce cost and enable new applications.Basic cardiac sequences are ready to be used for clinical evaluation at low field, while more advanced cardiac imaging techniques need further optimization and development.Further clinical studies establishing reference parametric values and assessing the diagnostic accuracy of low-field MRI across a range of techniques and patient populations are warranted.Additionally, techniques designed for challenging patients, like those with arrhythmia or those unable to hold their breath, require evaluation and validation before low-field cardiac MRI can be broadly adopted.There are still significant opportunities to improve image acquisition and reconstruction techniques to leverage the specific physical properties of low-field MRI, and there are opportunities to translate computational methods developed for higher field strengths to be applied at lower field strengths.Substantial advancements in both imaging methods and clinical applications are anticipated for low-field cardiac MRI over the next several years.

FIGURE 2 :
FIGURE 2: Example images from the prototype 0.55 T MRI system.(a) Free breathing bSSFP cine imaging of cardiac function using a compressed sensing image reconstruction (diastolic frame shown).This patient has a wall motion abnormality post myocardial infarction which can be seen in Video S1 in the Supplemental Material.(b) Flow quantification using phase-contrast MRI in the aorta and main pulmonary artery (MPA) in a patient with a ventricular septal defect.(c) Late gadolinium enhancement and (d) T1 mapping are shown in a patient with hypertrophic cardiomyopathy (white arrows).(e) Fat/water separation using the Dixon method in a patient with myocardial fat infiltration (orange arrows).

FIGURE 3 :
FIGURE 3: Example images from the commercial 0.55 T MRI system.(a) T2-weighted turbo spin echo (TSE) black blood imaging in a healthy volunteer with and without deep learning (DL) image enhancement.DL enhancements were vendor-provided reconstruction methods to densoise and increase image sharpness.(b) A compressed sensing-based ECG-triggered contrast-enhanced (CE) MR angiography (MRA) acquisition in a healthy volunteer.(c) Rest perfusion images and comparison late gadolinium enhancement (LGE) images illustrating a perfusion defect (arrows) in a swine model of myocardial infarction.[Adapted from Varghese et al 20 ]

FIGURE 4 :
FIGURE 4: Strain images from a commercial 0.55 T MRI system.The left panel shows Late Gadolinium Enhanced images in a porcine myocardial infarction model, showing apical, antero-septal infarct caused by 90-minute occlusion followed by reperfusion of the left anterior descending coronary artery.The AHA 16-segment bulls-eye plot in the middle shows longitudinal strain deficit (in yellow and green) in the segments corresponding to the infarct location.The corresponding apical short-axis SENC image in the right panel shows the longitudinal strain deficit in the anterior septal region.[Reproduced from Liu et al. 89 ]

FIGURE 5 :FIGURE 6 :
FIGURE 5: Assessment of RF ablation lesions (green arrows) and chemoablation lesions (orange arrows) on a prototype 0.55 T MRI system in a swine model.In vivo imaging included (a) 3D T2-weighted imaging, (b) 3D T1-weighted imaging, (c) T1 mapping, and ex vivo imaging used (d) 3D T1-weighted imaging to confirm the location of the lesion in fixed tissue.[Reproduced from Kolandaivelu et al. 105 ]

FIGURE 7 :FIGURE 8 :
FIGURE 7: Example images from the first-in-human MRI-guided radioablation at 0.35 T with estimated delivered radiation dose overlaid.[Adapted from Mayinger et al. 115 ]

FIGURE 9 :
FIGURE 9: Comparison of artifacts caused by metallic implants at 1.5 T and 0.55 T. [Adapted from Bandettini et al 127 ]

FIGURE 10 :
FIGURE 10: Example images from a commercial 80 cm bore, 0.55 T system in two obese patients unable to undergo cardiac MRI assessment on 70 cm bore systems due to body habitus.Patient A (Male, 61 y.o., 350 lb, BMI 48 kg/m 2 , body surface area 2.6 m 2 )-Breath-held segmented cine images in four and two-chamber views are shown along with the patient's echocardiographic images, acquired without and with ultrasound contrast.Multiplanar reformatted images of the thoracic aorta acquired with a non-triggered contrast-enhanced MR angiogram depicts a dilated aorta.Patient B (Male, 6.5.y.o., 410 lbs, BMI > 57 kg/m 2 , body surface area 2.86 m 2 )-Breath-held segmented cine (four chamber and short axis view), free-breathing motion-corrected (four chamber) and breath-held segmented (short axis) LGE demonstrating fibrosis, and magnitude and phase images of the aortic root are shown.[Adapted from Varghese et al. 20 and Simonetti et al. 148 ]

TABLE 1 .
Opportunities and Challenges for Cardiac MRI at Low Field, compared to 1.5T or 3T.
these include simplified cardiac gating, reduced artifacts around implant hardware, and improved patient comfort.