Visualization\nof Unconjugated Bilirubin In Vivo with\na Novel Approach Using Chemical Exchange Saturation Transfer Magnetic\nResonance Imaging in a Rat Model

Unconjugated bilirubin (UCB) visualization is valuable\nfor early\nbilirubin encephalopathy (BE) diagnosis and management. UCB neurotoxicity\nis a challenge, necessitating improved imaging modalities for precise\nlocalization and characterization. This study developed a noninvasive\nmethod for UCB imaging in the brain using chemical exchange saturation\ntransfer (CEST) magnetic resonance imaging, which visualizes UCB distribution\nthrough amide–bulk water proton exchange, a process termed\nbilirubin CEST (Bil-CEST) imaging. Bil-CEST imaging parameters were\ninitially optimized; the exchange rate of the amide protons of UCB\nwas calculated. Bil-CEST imaging characteristics and specificity were\nassessed using in vitro images of UCB solutions under different conditions\nand images of other brain metabolites. Bil-CEST maps of the rat brain\nwere collected at the baseline and dynamically, postinjection of the\nUCB solution or vehicle into lateral ventricles of Sprague–Dawley\nrats. The model was validated using a water maze and pathological\nstaining. In vitro, the Bil-CEST effect was observed at approximately\n5.5 ppm downfield from bulk water. This effect was proportional to\nthe UCB concentration and B1 amplitude. In vivo, Bil-CEST imaging\nrevealed a progressive enhancement following a lateral ventricular\nUCB injection. Conversely, no significant imaging changes were observed\nin the vehicle group. Compared with the vehicle group, the UCB group\nhad more hippocampal neuronal apoptosis and worse cognitive function.\nThese findings highlight the utility of Bil-CEST in direct UCB imaging,\nindicating its potential as a clinically valuable biomarker for BE\ndiagnosis and management.