Demonstration of CEST MRI in an Animal Model of Acute Ischemic Stroke at 15.2 T With Simple Linearization for B₁...

CEST at ultra-high fields such as 15.2 T benefits significantly from increased signal and increased spectral resolution. However, inhomogeneous fields become a confounding issue for B₀ and B₁. While B₀ inhomogeneity correction is widely studied and implemented, methods for B₁ correction have been limited to interpolation, which requires close to ground truth sampling across B₁ or sophisticated methods that increase data acquisition requirements. This work demonstrates a simple and easy-to-employ model-based linearization that allows for reasonable B₁ correction with as low as two B₁ samples for implementation.

Simulation and in vivo rodent brain CEST data at 15.2 T were tested for reasonable B₁ correction using a surface coil to demonstrate an exaggerated situation of B₁ inhomogeneity. Individual exchange fitting was performed, and quantified maps were examined for apparent residual B₁ dependencies, and quantitative values were examined for consistency across animals.

Simulation showed linearization significantly improved linear dependency over simple linear interpolation, even when multiple pools were involved. B₁ corrected Z-spectra demonstrated reasonable consistency across animals, and parameter maps of simple quantifying CEST parameters demonstrated little B₁ dependency. Fitting CEST curves demonstrates similar consistency and independence from B₁. Finally, comparison of parameters across animals demonstrates reasonable consistency after B₁ correction.

The simple linearization described in this paper offers a facile way to rapidly correct for B₁ inhomogeneity with minimally required additional spectral sampling. This method can have significant advantages, particularly in scan-time restricted situations where multiple Z-spectra at more than two power levels are not feasible.