Proton change of parotid glands after gustatory stimulation examined by magnetic resonance imaging

The aim of this study was to investigate proton changes of the parotid gland after gustatory stimulation by semi‐quantitative parameters and an empirical mathematical model (EMM) using high‐temporal‐resolution, double‐echo, echo‐planar imaging (EPI). Approved by a local institutional review board, this study examined 20 parotid glands from 10 healthy volunteers (male:female = 6: 4; age ± standard deviation =35.1 ± 14.1 years) with written informed consent obtained. All participants underwent 1.5‐T, double‐echo EPI with gustatory stimulation. Semi‐quantitative parameters, including maximal drop ratio (MDR), time to peak (TTP), drop slope (DS), recovery slope (RS) and recovery ratio (RR), were calculated. The effect of temporal resolution on parotid functional parameters was evaluated. An EMM comprising an output function ( Sot=Aoe−kot+B) and an input function ( Sint=Ain1−e−kint) was also applied to fit all dynamic curves. Kruskal–Wallis test, Wilcoxon test, linear regression analysis and goodness of fit were used for statistical analysis. p < 0.05 was considered to be statistically significant. The signal intensity dropped significantly after gustatory stimulation on the proton density (PD) image (p < 0.01). MDR was 8.26% in the PD image. MDR and RR were negatively associated with time interval, whereas DS and TTP were significantly positively associated with time interval (all p < 0.05). EMM parametric values derived from PD–time curves of parotid glands were 12.04 ± 6.81%, 6.43 ± 4.23 min–1, 88.73 ± 6.18%, 8.41 ± 4.86 min–1 and 1.09 ± 1.35 for Ao, ko, B, Ain and kin, respectively. Semi‐quantitative functional parameters and EMM parameters using high‐temporal‐resolution, double‐echo EPI allow the quantification of parotid proton changes after gustatory stimulation. The water change in parotid glands is similar to that in a flush tank. In parotid glands, it can be conceptually expressed by an input via the artery supply and output via the parotid duct drainage. Intraoral injection of lemon juice stimulates the autonomic nervous system, which, in turn, triggers the parotid glands to empty saliva immediately. After stimulation, parotid glands refill water slowly and continuously. Echo‐planar magnetic resonance imaging (MRI) allows the measurement of the proton density, which is the main component of saliva, of parotid glands with high temporal resolution. The washout phase and refill phase observed on MRI are similar to those observed on sialoscintigraphy, wh