“…Different strategies were investigated to improve the performance of the external surrogate, either making use of audio-visual biofeedback (To et al 2016b) or advanced sorting (Liu et al 2015, Liang et al 2016, Tryggestad et al 2013d, Du et al 2015. As previously mentioned however, the use of internal breathing surrogates directly extracted from the acquired 2D images has been shown to increase robustness in organ motion description with respect to external surrogates (Stemkens et al 2015, Liu et al 2016a, Li et al 2017. Two main methods based on navigator sequences (Von Siebenthal et al 2007, Tokuda et al 2008, Wachinger et al 2012 or image-derived approaches (Cai et al 2011, Fontana et al 2016, Paganelli et al 2015c, Liu et al 2014a, Hui et al 2016, van de Lindt et al 2018b, van de Lindt et al 2018a, Uh, Khan and Hua 2016 are reported in the literature, relying on the acquisition of a navigator for sorting data, or on the derivation of the information directly from the data itself, respectively.…”