A polyrotaxane comprising a poloxamer core and at least one cyclodextrin comprising a nuclide chelating moiety, wherein the polyrotaxane has the general formula: or a salt thereof, wherein: n is an integer from 1 to 30; C and C' are the same or different and represent endcapping groups of the formula: wherein L 1 is a (C 1 -C 6)hydrocarbylene group, t is an integer from 2 to 5, G 1 and G 2, together, form a radical having the formula: wherein each L 2 is independently a bond or acyl; and each G 3 is a -O-cholesteryl group; and each s is independently an integer from 1 to 5; B represents a polymer chain of the formula: wherein each R 5 is independently a substituted or unsubstituted (C 1 -C 20)hydrocarbyl group, d is an integer from 100 to 800, and q is an integer from 100 to 800; and A represents the macrocyclic host molecule of the general formula: wherein each Z is independently O or NH, each R 1 is independently hydrogen or a substituted or unsubstituted (C 1 -C 20)hydrocarbyl group, interrupted by 0 to 5 groups chosen from -O-, -NH-, and -S-; x is an integer from 1 to 3; and R 2 is substituted or unsubstituted (C 1 -C 20)hydrocarbyl group, interrupted by 0 to 5 groups chosen from -O-,-NH-, and -S-; with the proviso that at least one R 2 is (C 1 -C 20)hydrocarbyl group, interrupted by 0 to 5 groups chosen from -O-, -NH-, and -S-, substituted with a group C N 2, wherein one R is hydrogen and the other is an aryl group substituted with a radionuclide chelating moiety; wherein a radionuclide chelating moiety is selected from diethylenetriamine pentaacetic acid (DTPA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), triethylenetetramine (TETA) and 1,4,7-triazacyclononane-triacetic acid (NOTA); wherein hydrocarbylene refers to alkylene, alkenylene, alkynylene, arylene, cycloalkylene, divalent acyl, or a combination thereof; wherein hydrocarbyl refers to alkyl, alkenyl, aryl, cycloalkyl, acyl or any combination thereof; and substituents in substituted hydrocarbyl groups include halogen (e.g., F, Cl, Br, and I); hydroxyl groups, alkoxy groups, aryloxy groups, aralkyloxy groups, oxo(carbonyl) groups, carboxyl groups including carboxylic acids, carboxylates, and carboxylate esters; thiol groups,.alkyl and aryl sulfide groups, sulfoxide groups, sulfone groups, sulfonyl groups, and sulfonamide groups; amines, hydroxylamines, nitriles, nitro groups, N-oxides, hydrazides, azides, and enamines. wherein the polyrotaxane has the general formula: or a salt thereof, wherein: n is an integer from 1 to 30; C and C' are the same or different and represent endcapping groups of the formula: wherein L 1 is a (C 1 -C 6)hydrocarbylene group, t is an integer from 2 to 5, L 1 is a (C 1 -C 6)hydrocarbylene group, t is an integer from 2 to 5, G 1 and G 2, together, form a radical having the formula: wherein each L 2 is independently a bond or acyl; and each G 3 is a -O-cholesteryl group; and each s is independently an integer from 1 to 5; wherein each L 2 is independently a bond or acyl; and each G 3 is a -O-cholesteryl group; and each s is independently an integer from 1 to 5; B represents a polymer chain of the formula: wherein each R 5 is independently a substituted or unsubstituted (C 1 -C 20)hydrocarbyl group, d is an integer from 100 to 800, and q is an integer from 100 to 800; and A represents the macrocyclic host molecule of the general formula: wherein each Z is independently O or NH, each R 1 is independently hydrogen or a substituted or unsubstituted (C 1 -C 20)hydrocarbyl group, interrupted by 0 to 5 groups chosen from -O-, -NH-, and -S-; x is an integer from 1 to 3; and R 2 is substituted or unsubstituted (C 1 -C 20)hydrocarbyl group, interrupted by 0 to 5 groups chosen from -O-,-NH-, and -S-; each Z is independently O or NH, each R 1 is independently hydrogen or a substituted or unsubstituted (C 1 -C 20)hydrocarbyl group, interrupted by 0 to 5 groups chosen from -O-, -NH-, and -S-; x is an integer from 1 to 3; and R 2 is substituted or unsubstituted (C 1 -C 20)hydrocarbyl group, interrupted by 0 to 5 groups chosen from -O-,-NH-, and -S-; with the proviso that at least one R 2 is (C 1 -C 20)hydrocarbyl group, interrupted by 0 to 5 groups chosen from -O-, -NH-, and -S-, substituted with a group C N 2, wherein one R is hydrogen and the other is an aryl group substituted with a radionuclide chelating moiety; wherein a radionuclide chelating moiety is selected from diethylenetriamine pentaacetic acid (DTPA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), triethylenetetramine (TETA) and 1,4,7-triazacyclononane-triacetic acid (NOTA); wherein hydrocarbylene refers to alkylene, alkenylene, alkynylene, arylene, cycloalkylene, divalent acyl, or a combination thereof; wherein hydrocarbyl refers to alkyl, alkenyl, aryl, cycloalkyl, acyl or any combination thereof; and substituents in substituted hydrocarbyl groups include halogen (e.g., F, Cl, Br, and I); hydroxyl groups, alkoxy groups, aryloxy groups, aralkyloxy groups, oxo(carbonyl) groups, carboxyl groups including carboxylic acids, carboxylates, and carboxylate esters; thiol groups,.alkyl and aryl sulfide groups, sulfoxide groups, sulfone groups, sulfonyl groups, and sulfonamide groups; amines, hydroxylamines, nitriles, nitro groups, N-oxides, hydrazides, azides, and enamines.