An indicator element of a percutaneous needle guide for a medical scanning device includes a pointer for pointing to a percutaneous needle entry site on an epidermis of a body, when the guide is attached to the device and the device is positioned over the epidermis for scanning. These code designs combine trellis-coded quantization with irregular repeat accumulate codes. An intermediate structure is disposed between the device substrate and the optical structure. Thereafter a chemical substance is adsorbed onto the plurality of metallic islands so as to produce a chemical substance-metallic islands moiety.