PET ligand-based imaging has begun to try to address this question through tau imaging in FTD (Dani et al., 2016; Tsai et al., 2019) and translocator protein (TSPO) PET imaging, a neuroinflammation biomarker, in ALS (Van Weehaeghe et al., 2020). Here, TSPO is linked to amyotrophic lateral sclerosis.