Target Audience

Students and researchers interested in identifying the opportunities for CMR at low field.

Objective

The attendees will be able to comprehend the challenges and opportunities of utilizing low-field MRI scanners for CMR.

Purpose

Cardiac Magnetic Resonance (CMR) research and development continues at a rapid pace. Recently developed advanced data acquisition and processing methods have expanded the capability of CMR to enable higher spatial and temporal resolutions and reduced acquisition time. Despite the advances in CMR technology over the past decades, the utility of CMR as a routine diagnostic cardiovascular imaging tool remains limited. Ease of use and cost, which scales with the field strength, are important considerations for routine clinical utilization of CMR. Unfortunately, current MRI equipment prices put CMR at a cost disadvantage to other commonly used cardiac imaging modalities such as echocardiography and nuclear SPECT. With recent optimization of MRI hardware as well as development of new image reconstruction methods, the time is ripe to reconsider the future of CMR at low field (< 1T) [1].

Methods

Six volunteers (33.2 ± 8.2 yrs) were each imaged at 0.35 T (MRIdian, ViewRay, Cleveland, OH), 1.5T (Magnetom Avanto, Siemens Healthcare, Erlangen, Germany), and 3T (Magnetom Prisma, Siemens Healthcare, Erlangen, Germany) systems. Breath-held segmented cine images were acquired in a short axis view covering the left ventricle (LV), as well as in two-chamber and four-chamber views. Breath-held 2D phase contrast (PC-MRI) cine images were acquired in an axial view of the aorta. LV volumes and function from cine, and flow quantification from PC-MRI was performed using SuiteHeart software (Neosoft, Pewaukee, WI). Myocardial T1, T2 and T2* relaxation times were also determined using quantitative parametric mapping sequences. Bonferroni corrected pairwise comparison was performed to compare quantitative cine and flow measurements from the three systems (p < 0.05 significant). Cine images were scored for image quality by two blinded observers on a scale of 1 to 5 (1 very poor, 2 poor, 3 adequate, 4 good and 5 excellent). Other qualitative measures such as blood-myocardium contrast and contrast-to-noise ratio (CNR) for cine, phase signal-to-noise ratio (PSNR) for PC-MRI and coefficient of variation (CV) within the myocardium for parametric maps were also determined. From one of the volunteers, prospectively undersampled free-breathing real-time cine data was collected at 0.35T. This dataset was reconstructed using a compressed sensing (CS) method [2].

Results

Example cine and flow images, and parametric maps at three field strengths will be shown. The mean and standard deviation, and ranges of cardiac function and flow quantification results will be provided. Across the different field strengths, there were no significant differences between quantitative cine and flow measurements. All 0.35T cine image series received scores of 3 (acceptable) or better. Contrast was similar across field strengths for cine, while CNR was lowest at 0.35T. PSNR was comparable between 0.35T and 1.5T but significantly lower to 3T. Myocardial T1 decreased and T2 and T2* increased with decreasing field strength. CV was higher at 0.35T for T1 and T2 maps, but improved with increased averages. For real-time, free-breathing cine dataset collected at 0.35T, diagnostic quality images were reconstructed using advanced image recovery methods.

Discussion

The preliminary results of this study using standard cardiovascular imaging sequences indicate that CMR evaluation of cine and flow, and myocardial tissue characterization can be successfully performed at low-field systems. By optimizing the acquisition protocol, e.g., increasing the flip angle for balanced steady state free precession (bSSFP) to 110 degrees, and by utilizing advanced data processing methods, e.g., compressed sensing or machine learning based image recovery methods, some of the SNR loss at low field can be successfully offset.

Conclusion

The use of lower magnetic field strength magnets is worth revisiting in the interest of cost-savings, patient safety, and ease of use. The prospect for excellent field homogeneity, virtually no limits on Specific Absorption Rate (SAR), and the effectiveness of CS [1] may open up the potential for new applications of techniques such as bSSFP, echo planar imaging (EPI), and spiral imaging that can be problematic at higher field strength.

Acknowledgements

No acknowledgement found.