Fig. 2. The effects of systemically administered Mn²⁺ as a signal and contrast enhancer in crayfish. (A) Single transverse slices through the head of an adult male crayfish (mass 33.5 g) illustrate the difference in contrast before (top) and 32 min after (bottom) injection of 60 µl of a 120 mmol l^–1 MnCl₂ solution. The brain is undetectable before injection but exhibits the strongest signal and appears below the rostrum (Ro) and between the bases of the bilateral antennae (An) afterwards. Scale bar, 4 mm. (B) Single transverse slices through the thorax of an adult male crayfish (mass 34.0 g), before (top) and 32 min after (bottom) injection of 60 µl of a 120 mmol l^–1 MnCl₂ solution. Boundaries around the air-filled stomach (St) are more distinct and contrast around and within different areas, e.g. hepatopancreas (He) and gill chamber (GC) is clearly enhanced after injection. The walking legs (WL) are pointing forward and are surrounded by water (Wa) in the tube. Scale bar, 6 mm. Imaging parameters: T_R=1.5 s, T_E=17.6 ms; matrix dimensions: 256x128, field of view=5.12 cmx5.12 cm, number of averages=2, slice thickness, 1000 µm; voxel size, 200x400x1000 µm; acquisition time (each), 6.4 min. Inset: Red lines indicate the position of the transverse slices through the head and thorax, as shown in A and B, respectively.