What does it mean to be 'amyloid-positive'?

Braak H, Rüb U, Gai WP, Del Tredici K. Idiopathic Parkinson’s disease: possible routes by which vulnerable neuronal types may be subject to neuroinvasion by an unknown pathogen. J Neural Transm 2003; 110: 517–36. Garnett ES, Firnau G, Nahmias C. Dopamine visualized in the basal ganglia of living man. Nature 1983; 305: 137–8. Gjerløff T, Fedorova T, Knudsen K, Munk O, Nahimi A, Jakobsen S, et al. Imaging acetylcholinesterase density in peripheral organs in Parkinson’s disease with C-donepezil positron emission tomography. Brain 2015; 138: 638–48. Harada Y, Hara H, Sukamoto T. Characterization of specific (+ )-[3H]N-allylnormetazocine and [3H]1,3-di(2-tolyl)guanidine binding sites in porcine gastric fundic mucosa. J Pharmacol Exp Ther 1994; 269: 905–10. Hiraoka K, Okamura N, Funaki Y, Watanuki S, Tashiro M, Kato M, et al. Quantitative analysis of donepezil binding to acetylcholinesterase using positron emission tomography and [5-(11)C-methoxy]donepezil. Neuroimage 2009; 46: 616–23. Iranzo A, Valldeoriola F, Lomena F, Molinuevo JL, Serradell M, Salamero M, et al. Serial dopamine transporter imaging of nigrostriatal function in patients with idiopathic rapid-eye-movement sleep behaviour disorder: a prospective study. Lancet Neurol 2011; 10: 797–805. Ishikawa M, Sakata M, Ishii K, Kimura Y, Oda K, Toyohara J, et al. High occupancy of sigma1 receptors in the human brain after single oral administration of donepezil: a positron emission tomography study using [11C]SA4503. Int J Neuropsychopharmacol 2009; 12: 1127–31. Kashihara K, Imamura T, Shinya T. Cardiac 123I-MIBG uptake is reduced more markedly in patients with REM sleep behavior disorder than in those with early stage Parkinson’s disease. Parkinsonism Relat Disord 2010; 16: 252–5. Nandhagopal R, Mak E, Schulzer M, McKenzie J, McCormick S, Sossi V, et al. Progression of dopaminergic dysfunction in a LRRK2 kindred: a multitracer PET study. Neurology 2008; 71: 1790–5. Raffel DM, Koeppe RA, Little R, Wang CN, Liu S, Junck L, et al. PET measurement of cardiac and nigrostriatal denervation in Parkinsonian syndromes. J Nucleic Med 2006; 47: 1769–77.