Compounds useful for the early diagnosis of malignant tumors, multiple sclerosis, and especially Alzheimer's Disease and related dementias; especially compounds of Formula (I) wherein R1 is selected from the group consisting of hydrogen, cyano, fluoro, iodo, alkyl, and aryl; R2 is selected from the group consisting of hydrogen, alkyl, and aryl; X is selected from the group consisting of CH2, CH2O, oxygen, OCH2, CH2S, SCH2, NH, N-alkyl, and N-aryl; Y is an optional spacer group, absent or selected from the group consisting of oxygen, sulfur, NH, N-alkyl, and N-aryl; and Z is selected from the group consisting of alkyl substituted with cyano, fluoro, or iodo and aryl substituted with cyano, fluoro, or iodo. In a preferred embodiment, R1 and R2 are hydrogen, X is CH2O, Y is absent, and Z is phenyl substituted with fluoro, cyano, or iodo. In some embodiments, Z is more specifically 18F-phenyl or 123I-phenyl or 131I-phenyl. Other compounds are also provided.