An imaging agent comprising a compound of Formula I, or a pharmaceutically acceptable salt thereof, wherein L 1 is -CH=CH- or L 1 is absent; R 1 is chosen from phenyl or heteroaryl, each of which is optionally substituted with one, two, or three groups independently chosen from cyano, halo, heteroaryl, C 1-6 alkyl, C 1-6 alkyl substituted with one or two substituents independently chosen from C 1-6 alkoxy substituted with heteroaryl, -C O- C 1-6 alkyl, hydroxyl, C 1-6 alkynyloxy, C 1-6 alkoxy, and C 1-6 alkoxy substituted with one or two substituents independently chosen from halo, heterocycloalkyl, heteroaryl, heteroaryl substituted with C 1-6 alkoxy, optionally substituted amino, alkyl substituted with heteroaryl, and alkyl substituted with heteroaryl substituted with C 1-6 alkoxy; or R 1 is phenyl substituted with two groups, which taken together with the carbon atoms, to which they are bonded form a heterocycloalkenyl ring wherein said phenyl is further optionally substituted with a substituent chosen from halo, heteroaryl, and optionally substituted amino; L 2 is -N(R 4)-; R 2 is chosen from hydrogen, C 1-6 alkyl, and C 1-6 alkyl substituted with C 1-6 alkoxy, amino, (alkyl)amino, (dialkyl)amino, or hydroxy; for each occurrence, R 3 is independently chosen from halo, cyano, C 1-6 alkoxy, C 1-6 alkyl optionally substituted with amino, (alkyl)amino, or di(alkyl)amino, and ethynyl optionally substituted with tri(alkyl)silyl; R 4 is chosen from hydrogen and C 1-6 alkyl; and m is 0, 1, or 2, wherein the compound of Formula I, or a pharmaceutically acceptable salt thereof, is labeled with one or more positron-emitting radionuclides. L 1 is -CH=CH- or L 1 is absent; R 1 is chosen from phenyl or heteroaryl, each of which is optionally substituted with one, two, or three groups independently chosen from cyano, halo, heteroaryl, C 1-6 alkyl, C 1-6 alkyl substituted with one or two substituents independently chosen from C 1-6 alkoxy substituted with heteroaryl, -C O- C 1-6 alkyl, hydroxyl, C 1-6 alkynyloxy, C 1-6 alkoxy, and C 1-6 alkoxy substituted with one or two substituents independently chosen from halo, heterocycloalkyl, heteroaryl, heteroaryl substituted with C 1-6 alkoxy, optionally substituted amino, alkyl substituted with heteroaryl, and alkyl substituted with heteroaryl substituted with C 1-6 alkoxy; or cyano, halo, heteroaryl, C 1-6 alkyl, C 1-6 alkyl substituted with one or two substituents independently chosen from C 1-6 alkoxy substituted with heteroaryl, -C O- C 1-6 alkyl, hydroxyl, C 1-6 alkynyloxy, C 1-6 alkoxy, and C 1-6 alkoxy substituted with one or two substituents independently chosen from halo, heterocycloalkyl, heteroaryl, heteroaryl substituted with C 1-6 alkoxy, optionally substituted amino, alkyl substituted with heteroaryl, and alkyl substituted with heteroaryl substituted with C 1-6 alkoxy; or halo, heterocycloalkyl, heteroaryl, heteroaryl substituted with C 1-6 alkoxy, optionally substituted amino, alkyl substituted with heteroaryl, and alkyl substituted with heteroaryl substituted with C 1-6 alkoxy; or R 1 is phenyl substituted with two groups, which taken together with the carbon atoms, to which they are bonded form a heterocycloalkenyl ring wherein said phenyl is further optionally substituted with a substituent chosen from halo, heteroaryl, and optionally substituted amino; halo, heteroaryl, and optionally substituted amino; L 2 is -N(R 4)-; R 2 is chosen from hydrogen, C 1-6 alkyl, and C 1-6 alkyl substituted with C 1-6 alkoxy, amino, (alkyl)amino, (dialkyl)amino, or hydroxy; hydrogen, C 1-6 alkyl, and C 1-6 alkyl substituted with C 1-6 alkoxy, amino, (alkyl)amino, (dialkyl)amino, or hydroxy; for each occurrence, R 3 is independently chosen from halo, cyano, C 1-6 alkoxy, C 1-6 alkyl optionally substituted with amino, (alkyl)amino, or di(alkyl)amino, and ethynyl optionally substituted with tri(alkyl)silyl; halo, cyano, C 1-6 alkoxy, C 1-6 alkyl optionally substituted with amino, (alkyl)amino, or di(alkyl)amino, and ethynyl optionally substituted with tri(alkyl)silyl; R 4 is chosen from hydrogen and C 1-6 alkyl; and m is 0, 1, or 2, wherein the compound of Formula I, or a pharmaceutically acceptable salt thereof, is labeled with one or more positron-emitting radionuclides.