## 1 Introduction  
_Fritillaria thunbergii_ Miq. is a kind of lilliaceous plant available in China. _Fritillaria thunbergii_ Miq. is well known as an ornamental plant, and more importantly, a medicinal plant. _Fritillaria thunbergii_ Bulbus has been used as an important Chinese medicinal plant for over 2000 years. _Fritillaria thunbergii_ Bulbus shows special curative effect on clearing heat, resolving phlegm, relieving cough, detoxification, and disperse abscesses and nodules by a sulfur-fumigated process using chromatographic fingerprinting analysis [1]. Processing and trading of _Fritillaria thunbergii_ Bulbus as a medicine is popular, which has great economic value.  
_Fritillaria thunbergii_ Bulbus are dried to be stored, transported, and traded. Traditional methods to dry _Fritillaria thunbergii_ Bulbus are natural drying methods, drying instruments, and sulfur fumigation. Sulfur fumigation (SF) is a traditional preservation method in Chinese traditional medicine [2]. Sulfur fumigation could be efficiently used to prolong the shelf life of Chinese traditional medicine.  
However, increasing concerns of SF in Chinese medicine have been discussed due to its uncertain safety. Studies have been reported that SF could damage the bioactive compounds, change chemical profiles, and generate detrimental exogenous materials of Chinese medicine [3; 4; 5]. The Chinese government has set strict rules to end the use of SF in Chinese traditional medicine [6]. Unfortunately, the benefits of SF, such as easy operation and high cost-effectiveness, drives the producers to use SF in Chinese traditional medicine for seeking higher profits. Sulfur dioxide (SO\({}_{2}\)) residuals could be detected in the SF-treated Chinese medicines, which is generally the index to detect SF-treated Chinese medicine.  
The traditional methods to detect the SF-treated Chinese medicine are human eyes or experience-based detection and laborious methods [6]. The accuracy of human eyes or experience-based detection could not be guaranteed, due to the subjectiveness of the detection results. The laborious methods are accurate and used as standard methods. However, these methods are time-consuming, reagent consuming, expensive, and require complex sample preparation and operation. Thus, rapid and accurate detection techniques are needed.  
Near-infrared spectroscopy has been used as a non-destructive, rapid, and accurate technique to detect SF-treated Chinese traditional medicines [7]. However, near-infrared spectra are acquired from quite a small area of the sample. Hyperspectral imaging (HSI), integrating both spectroscopy technique and imaging technique, provides spectral and spatial information simultaneously. In the acquired hyperspectral images, there is a spectrum of each pixel, and there is a grey-scale image at each wavelength. Hyperspectral images provides comprehensively external and internal information related to quality parameters, and the advantage of hyperspectral images makes it feasible to predict the quality parameters of each pixel to form a prediction map. The prediction map provides the direct visual information distribution of the quality parameters. The use of HSI has been studied to identify sulfur-fumigated Chinese medicines [8], but the studies of SO\({}_{2}\) residual content detection by HSI are rarely reported.  
The main objective of this study was to detect and visualize SO\({}_{2}\) residuals in SF-treated _Fritillaria thunbergii_ Bulbus using NIR-HSI combined with chemometric methods.