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 (i.e., the dissolved-phase signal) may change over time due to the increase in xenon 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 two, three, or four 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 xenon signal was observed to 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 xenon magnetization is only achieved after a series of xenon inhalations and RF excitations, and it requires synchronization of the breathing rate with the MR acquisition.