Simulation of B₀ magnetic field conditions in the human heart for improved diagnostic MRI

Cardiac balanced steady-state free precession (bSSFP) MRI suffers from dark band artifacts due to severe B₀ inhomogeneity induced by air-tissue interfaces in the human body. Those artifacts remain uncontrollable despite scanner-provided B₀ shimming based on low-order spherical harmonic shapes. To mitigate those artifacts for higher image fidelity, the development of advanced B₀ shimming techniques is demanding and typically requires knowledge of cardiac B₀ conditions across patient populations. We recently proposed a new simulation approach to derive cardiac B₀ distributions from readily available chest-abdomen-pelvis CT structural images based on first principles of classical electrodynamics. This approach is able to calculate the air-tissue interface-induced magnetic field variation across the body from diverse subjects using regular CT images. While the computational method has been step-by-step validated in previous work, a comprehensive demonstration in patients has been missing.

The aim of this study is to validate our proposed structural CT-derived cardiac B₀ magnetic field computation approach using a side-by-side comparison between simulation and in vivo measurement obtained from the same cohort of clinical subjects.

Six subjects (female: 3, male: 3, age: 54.5 ± 17.2 years) with aortic stenosis or cancer who underwent clinically-indicated CT scans of the chest, abdomen, and pelvis were consented to undergo additional in vivo cardiac B₀ measurement on a 3T MRI scanner. B₀ distributions in the heart were computed based on CT images using our recently proposed approach and in vivo B₀ maps acquired experimentally. CT images and MRI-based anatomical images were co-registered before field calculation, followed by the side-by-side field comparison after computation using correlation analysis of inhomogeneous field values, as well as the comparison of first-order spherical harmonic coefficients after decomposing field maps and the residual B₀ inhomogeneities after shim from first to fifth order.

Simulated B₀ maps show excellent agreement with in vivo measured B₀ maps and strong average correlation (r = 0.92). After first and second-order shim analyses, both field maps show highly similar residual local B₀ field patterns, especially near cardiophrenic angles and in the heart's inferior region, demonstrating the authenticity of localized features beyond overall field congruency. Both groups also demonstrate the trend of decreasing residual B₀ inhomogeneity after shimming from first to fifth order, and their absolute differences in the mean of all subjects range from 0.25 to 2.3 Hz across different SH orders.

The consistency between simulation and in vivo measurements of B₀ field conditions in the human heart from patients exhibiting various cardiac conditions fully validates the established B₀ simulation approach. This approach can accurately compute the air-tissue interface-induced inhomogeneous magnetic field shapes and amplitudes from readily available structural CT images, setting the stage for the development of tailored and advanced cardiac B₀ magnetic field correction methods and more robust cardiac MRI.