SLC6A3 and Parkinson disease: Increased substantia nigra (SN) echogenicity, likely due to accumulation of nigral iron, is observed in PD,308–310 although a proportion of healthy controls and essential tremor patients also exhibit this.311 This sign can, however, differentiate PD from PSP and MSA with good sensitivity (91%) and specificity (82–96%).308 Hyper-echogenicity remains unchanged over follow-up312 and does not correlate with disease severity310,313 or presynaptic DAT loss,314 thus limiting use as a progression marker.