Radiopacity of Hollow Coins
The images below were taken of solid coins side by side with
Hollow Spy-Coins.  Each of the Spy-Coins (Micro Half, Micro
Euro, and Micro Nickel) were fitted with the Micro SD Memory
Card.  As seen from the scans, the US half dollar coin was the
most radiolucent of the three, followed by the US nickel, bottom.  
The .50 Euro coin, center, is the most opaque, being the thicker
coin of the three.
Solid Kennedy Half
MicroHalf Hollow Kennedy
Solid Euro €,50
MicroEuro Hollow €,50
Solid US Jefferson Nickel
MicroNickel Hollow Jefferson Nickel
Images taken with 55 KVP at 10 MaS.  The solid
nickel is just showing a bit of transparency at this
setting.