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.Evidence Level: 5Technical Efficacy: Stage 1

Measurement of blood oxygen saturation (O2 saturation) is of great importance for evaluation of patients with many cardiovascular diseases, but currently there are no established non-invasive methods to measure blood O2 saturation in the heart. While T2-based CMR oximetry methods have been previously described, these approaches rely on technique-specific calibration factors that may not generalize across patient populations and are impractical to obtain in individual patients. We present a solution that utilizes multiple T2 measurements made using different inter-echo pulse spacings. These data are jointly processed to estimate all unknown parameters, including O2 saturation, in the Luz-Meiboom (L-M) model. We evaluated the accuracy of the proposed method against invasive catheterization in a porcine hypoxemia model. Sufficient data diversity to estimate the various unknown parameters of the L-M model, including O2 saturation, was achieved by acquiring four T2 maps, each at a different τ 180 (12, 15, 20, and 25 ms). Venous and arterial blood T2 values from these maps, together with hematocrit and arterial O2 saturation, were jointly processed to derive estimates for venous O2 saturation and other nuisance parameters in the L-M model. The technique was validated by a progressive graded hypoxemia experiment in seven pigs. CMR estimates of O2 saturation in the right ventricle were compared against a reference O2 saturation obtained by invasive catheterization from the right atrium in each pig, at each hypoxemia stage. O2 saturation derived from the proposed technique was also compared against the previously described method of applying a global calibration factor (K) to the simplified L-M model. Venous O2 saturation results obtained using the proposed CMR oximetry method exhibited better agreement (y = 0.84× + 12.29, R2 = 0.89) with invasive blood gas analysis when compared to O2 saturation estimated by a global calibration method (y = 0.69× + 27.52, R2 = 0.73). We have demonstrated a novel, non-invasive method to estimate O2 saturation using quantitative T2 mapping. This technique may provide a valuable addition to the diagnostic utility of CMR in patients with congenital heart disease, heart failure, and pulmonary hypertension.

Objectives While cardiac amyloidosis (CA) classically involves the left ventricle (LV), less is known about its impact on the right ventricle (RV) and pulmonary vasculature. We performed a retrospective analysis to identify the prevalence and types of pulmonary hypertension (PH) profiles in CA and to determine haemodynamic and cardiovascular magnetic resonance (CMR) predictors of major adverse cardiovascular events (MACE). Methods Patients with CA who underwent CMR and right heart catheterisation (RHC) within 1 year between 2010 and 2019 were included. Patients were assigned the following haemodynamic profiles based on RHC: no PH, precapillary PH, isolated postcapillary PH (IPCPH), or combined precapillary and postcapillary PH (CPCPH). The relationship between PH profile and MACE (death, heart failure hospitalisation) was assessed using survival analysis. CMR and RV parameters were correlated with MACE using Cox-regression analysis. Results A total of 52 patients were included (age 69±9 years, 85% men). RHC was performed during biopsy in 44 (85%) and for clinical indications in 8 (15%) patients. Rates of no PH, precapillary PH, IPCPH and CPCPH were 5 (10%), 3 (6%), 29 (55%) and 15 (29%), respectively. Haemodynamic PH profile did not correlate with risk of death (p=0.98) or MACE (p=0.67). Transpulmonary gradient (TPG) (HR 0.88, CI 0.80 to 0.97), RV, (HR 0.95, CI 0.92 to 0.98) and LV ejection fraction (HR 0.95, CI 0.92 to 0.98) were significantly associated with MACE. Conclusions PH is highly prevalent in CA, even at the time of diagnosis. While IPCPH was most common, CPCPH is not infrequent. TPG and RV ejection fraction (RVEF) are prognostic markers in this population.

CMR has not seen widespread adoption beyond large urban academic centers. The reasons for this limited uptake include the cost, time-intensive nature and special expertise of CMR. Self-navigated 5D, x-y-z-cardiac-respiratory, free-funning whole-heart CMR using self-navigation (5D CMR) implemented on a low-field clinical scanner may help bridge this gap for bi-ventricular function assessment and left atrium volume index measurement. Whole-heart 3D radial phyllotaxis bSSFP data were collected in 10 healthy adult subjects. Self-navigation was used to extract respiratory and cardiac motion signals and to generate motion-resolved 5D CMR datasets. The right- and left-ventricular ejection fraction (RVEF and LVEF), left atrial volume index (LAVI), contrast ratio, sharpness, perceived image quality, and total scan durations were ascertained and compared to those obtained with the reference 2D cine images. 5D CMR allowed for time-efficient and concordant measurements when compared to the 2D reference method. The 5D CMR images resulted in lower CR on 5D CMR images (3.3 ± 2.9) than on 2D cines (4.7 ± 1.2), and similarly lower perceived image quality (1.8 ± 0.8 for 5D CMR and 3.6 ± 0.9 for the 2D cines). However, the LVEF measurements were similar with no statistically significant differences (Mean: 58 ± 5% for 5D CMR and 59 ± 5% for 2D cine, p = 0.49) and the LoA were low (-2.81% to 3.81%). For the RVEF, the measurements were also in good agreement when RVEF was measured on the axial views (60 ± 3% for 5D CMR and 60 ± 4% for 2D cine, p = 0.85) and the LoA were also low (-2.87% to 3.07%). 5D CMR without the need for ECG, breath-holding, navigators, or complex scan planning enables a highly simplified and time-efficient assessment of bi-ventricular cardiac function on a 0.55T clinical system in 7:50min.