¹⁵N Heteronuclear Chemical Exchange Saturation Transfer MRI

A two-step heteronuclear enhancement approach was combined with chemical exchange saturation transfer (CEST) to magnify ¹⁵N MRI signal of molecules through indirect detection via water protons. Previous CEST studies have been limited to radiofrequency (rf) saturation transfer or excitation transfer employing protons. Here, the signal of ¹⁵N is detected indirectly through the water signal by first inverting selectively protons that are scalar-coupled to ¹⁵N in the urea molecule, followed by chemical exchange of the amide proton to bulk water. In addition to providing a small sensitivity enhancement, this approach can be used to monitor the exchange rates and thus the pH sensitivity of the participating ¹⁵N-bound protons.