In vivo hyperCEST imaging: Experimental considerations for a reliable contrast.

PURPOSE: HyperCEST contrast relies on the reduction of the solvent signal after selective saturation of the solute magnetization. The scope of this work is to outline the experimental conditions needed to obtain a reliable hyperCEST contrast in vivo, where the "solvent" signal (ie, the dissolved-phase signal) may change over time due to the increase in xenon (Xe) accumulation into tissue.
METHODS: Hyperpolarized 129 Xe was delivered to mice at a constant volume and rate using a mechanical ventilator, which triggered the saturation, excitation, and acquisition of the MR signal during the exhale phase of the breath cycle-either every breath or every 2, 3, or 4 breaths. Serial Z-spectra and hyperCEST images were acquired before and after a bolus injection of cucurbit[6]uril to assess possible signal fluctuations and instabilities.
RESULTS: The intensity of the dissolved-phase Xe signal was observed to first increase immediately after the beginning of the hyperpolarized gas inhalation and NMR acquisition, and then decrease before reaching a steady-state condition. Once a steady-state dissolved-phase magnetization was established, a reliable hyperCEST contrast, exceeding 40% signal reduction, was observed.
CONCLUSION: A reliable hyperCEST contrast can only be obtained after establishing a steady-state dissolved phase 129 Xe magnetization. Under stable physiological conditions, a steady-state dissolved-phase Xe magnetization is only achieved after a series of Xe inhalations and RF excitations, and it requires synchronization of the breathing rate with the MR acquisition.