Chemical exchange saturation transfer imaging in hepatic encephalopathy.

Hepatic encephalopathy (HE) is a common complication in liver cirrhosis and associated with an invasion of ammonia into the brain through the blood-brain barrier. Resulting higher ammonia concentrations in the brain are suggested to lead to a dose-dependent gradual increase of HE severity and an associated impairment of brain function. Amide proton transfer-weighted (APT) chemical exchange saturation transfer (CEST) imaging has been found to be sensitive to ammonia concentration. The aim of this work was to study APT CEST imaging in patients with HE and to investigate the relationship between disease severity, critical flicker frequency (CFF), psychometric test scores, blood ammonia, and APT signals in different brain regions. Whole-brain APT CEST images were acquired in 34 participants (14 controls, 20 patients (10 minimal HE, 10 manifest HE)) on a 3 T clinical MRI system accompanied by T mapping and structural images. T normalized magnetization transfer ratio asymmetry analysis was performed around 3 ppm after B and B correction to create APT images. All APT images were spatially normalized into a cohort space to allow direct comparison. APT images in 6 brain regions (cerebellum, occipital cortex, putamen, thalamus, caudate, white matter) were tested for group differences as well as the link to CFF, psychometric test scores, and blood ammonia. A decrease in APT intensities was found in the cerebellum and the occipital cortex of manifest HE patients. In addition, APT intensities in the cerebellum correlated positively with several psychometric scores, such as the fine motor performance scores MLS1 for hand steadiness / tremor (r = 0.466; p = .044) and WRT2 for motor reaction time (r = 0.523; p = .022). Moreover, a negative correlation between APT intensities and blood ammonia was found for the cerebellum (r = -0.615; p = .007) and the occipital cortex (r = -0.478; p = .045). An increase of APT intensities was observed in the putamen of patients with minimal HE and correlated negatively with the CFF (r = -0.423; p = .013). Our findings demonstrate that HE is associated with regional differential alterations in APT signals. These variations are most likely a consequence of hyperammonemia or hepatocerebral degeneration processes, and develop in parallel with disease severity.