Toward Free-Breathing Renal CEST MRI: Preclinical Evaluation of a Novel Prospective Respiratory Triggering Approach

Endogenous renal chemical exchange saturation transfer (CEST) imaging enables contrast-agent-free assessment of renal metabolism but is highly sensitive to respiratory motion. Existing approaches, such as timed breathing, require high patient compliance or preclinical mechanical ventilation, limiting clinical translation. We evaluated a free-breathing prospective respiratory triggering method that allows continuous saturation during active breathing while adaptively aligning image acquisition with the expiratory phase.

Prospective triggering was based on a modified real-time respiratory signal, with a temporal shift continuously updated according to the current respiratory rate. Simulations assessed its performance across saturation durations of 1-6 s, respiratory rates representative of preclinical and clinical settings (f = 8-90 bpm), and varying respiratory frequency (CV = 0%-25%). Results were compared to conventional real-time triggering. For in vivo validation, prospective triggering was implemented on a preclinical 7 T MRI system using a Raspberry Pi-based hardware and software. Six mice underwent CEST imaging with saturation durations of 1, 3, and 5 s using both real-time and prospective triggering, and the Mean Squared Error of Lorentzian fit was used to compare the two approaches.

The real-time trigger error shows a periodic dependence on respiratory frequency, which diminishes with increasing variability, whereas the prospective trigger reduces this periodicity. In vivo, prospective triggering significantly (p < 0.05) reduced motion-induced scattering in the Z-spectrum for saturation times up to 3 s and enabled the detection of distinct metabolic contrasts across renal compartments.

Prospective triggering effectively reduces motion artifacts in preclinical renal CEST imaging.