Improved Myocardial Sodium Quantification at 7 T Using Interleaved ²³Na/¹H pTx MRI With Motion and Anatomy-Based B₁...

To improve accuracy and repeatability of myocardial ²³Na quantification in cardiac MRI at 7 T by combining retrospective respiratory and cardiac motion correction with a novel anatomy-based B₁ bias field correction.

In this study, a dual-nuclear interleaved ²³Na/¹H MRI sequence at 7 T was applied. Here, ¹H MR data enabled automated segmentation of myocardium and blood pool via nnUNet and facilitated respiratory and cardiac motion correction for both contrasts using ¹H navigators. For ²³Na MRI, a novel anatomy-based B₁ bias field correction was developed, estimating the transmit and receive (B₁) field bias from ¹H-derived segmentations and synthetic ²³Na images. All correction methods were validated through realistic simulations and in vivo studies. Repeatability was assessed in 10 healthy subjects, and the proposed B₁ correction was compared to phantom-based methods.

Respiratory and cardiac motion correction reduced quantification errors and improved SNR compared to conventional gating. The anatomy-based B₁ correction effectively mitigated errors caused by B₁ inhomogeneities and outperformed phantom-based methods in terms of repeatability. The measured apparent tissue sodium concentration of the myocardium after all corrections (49.5 $\pm$ 4.7 mM) was consistent with literature values. Combined motion and B₁ corrections improved repeatability, reducing the coefficient of repeatability from 4.9 mM (11.2%) without corrections to 2.0 mM (4.0%) after corrections.

The combination of retrospective motion correction and anatomy-based B₁ bias field correction enables repeatable myocardial sodium quantification at 7 T. The interleaved acquisition facilitates robust segmentation and efficient use of ¹H MRI for motion and B₁ correction, providing a scalable framework for future clinical research studies.