A radioisotope-labelled compound of quaternary ammonium salt of a polycyclic aromatic amine with formula I, in which formula I the wavy line indicates a single bond between the non-nodal carbon atom of a polycyclic aromatic system and the R 1 substituent, R 1 is a substituent independently selected from hydrogen, halogens, C 1-4 alkoxy, a nitro group, an amino group optionally having 1 or 2 hydrogen atoms replaced with C 1-6 alkyl or having 2 hydrogen atoms replaced with C 2-5 alkylene to form a heterocyclic ring, a chain C 1-6 carbon group optionally having a halogen, carboxy, formyl or C 1-4 alkanesulfonic substituent, and a phenyl group optionally having 1-5 substituents independently selected from halogens, a chain C 1-6 carbon, a halogenated chain C 1-6 carbon substituent, C 1-4 alkoxy, an amino group optionally having 1-2 atoms of hydrogen replaced with C 1-6 alkyl, R 2 is a chain aliphatic substituent having the -CH 2 - fragment as the terminal member of the chain, to which chain a phenyl group is optionally attached optionally having 1-3 substituents selected from halogens and C 1-6 alkyl, and, if the chain contains at least 2 carbon atoms, in which chain there is optionally a bivalent link between the chain carbon atoms, said link selected from the oxygen atom -O-, sulfur atom -S- and C 3-6 cycloalkylene, wherein the R 2 substituent contains a total of 1-16 carbon atoms, and the hydrogen atom at one of the carbon atoms is replaced with an atom of 18 F fluorine radioisotope, R 3 and R 4 are combined to form a bivalent butadienyl-1,3 substituent whose terminal carbon atoms are linked to the adjacent non-nodal carbon atoms of the B ring to form an aromatic C ring fused with the A and B ring system, having R 1 substituents at non-nodal carbon atoms, n is an integer of 9, X - is a pharmaceutically acceptable counterion, which is an anion of a mono-basic inorganic acid, a mononegative anion of a multi-basic inorganic acid, an anion of an alkanecarboxylic acid, an anion of an aliphatic sulfonic acid, an anion of an aromatic sulfonic acid, an anion of an acidic amino acid, or a hydrate or solvate thereof. the wavy line indicates a single bond between the non-nodal carbon atom of a polycyclic aromatic system and the R 1 substituent, R 1 is a substituent independently selected from hydrogen, halogens, C 1-4 alkoxy, a nitro group, an amino group optionally having 1 or 2 hydrogen atoms replaced with C 1-6 alkyl or having 2 hydrogen atoms replaced with C 2-5 alkylene to form a heterocyclic ring, a chain C 1-6 carbon group optionally having a halogen, carboxy, formyl or C 1-4 alkanesulfonic substituent, and a phenyl group optionally having 1-5 substituents independently selected from halogens, a chain C 1-6 carbon, a halogenated chain C 1-6 carbon substituent, C 1-4 alkoxy, an amino group optionally having 1-2 atoms of hydrogen replaced with C 1-6 alkyl, R 2 is a chain aliphatic substituent having the -CH 2 - fragment as the terminal member of the chain, to which chain a phenyl group is optionally attached optionally having 1-3 substituents selected from halogens and C 1-6 alkyl, and, if the chain contains at least 2 carbon atoms, in which chain there is optionally a bivalent link between the chain carbon atoms, said link selected from the oxygen atom -O-, sulfur atom -S- and C 3-6 cycloalkylene, wherein the R 2 substituent contains a total of 1-16 carbon atoms, and the hydrogen atom at one of the carbon atoms is replaced with an atom of 18 F fluorine radioisotope, R 3 and R 4 are combined to form a bivalent butadienyl-1,3 substituent whose terminal carbon atoms are linked to the adjacent non-nodal carbon atoms of the B ring to form an aromatic C ring fused with the A and B ring system, having R 1 substituents at non-nodal carbon atoms, n is an integer of 9, X - is a pharmaceutically acceptable counterion, which is an anion of a mono-basic inorganic acid, a mononegative anion of a multi-basic inorganic acid, an anion of an alkanecarboxylic acid, an anion of an aliphatic sulfonic acid, an anion of an aromatic sulfonic acid, an anion of an acidic amino acid, or a hydrate or solvate thereof.