Diagnostic apparatus (24) includes a sealed case (40), including a biocompatible material and configured for implantation within a body of a human subject (22). At least one antenna (42) is configured to be implanted in the body in proximity to a target tissue (28) and to receive radio frequency (RF) electromagnetic waves propagated through the target tissue and to output a signal in response to the received waves. Processing circuitry (44, 46), which is contained within the case, is coupled to receive and process the signal from the antenna so as to derive and output an indication of a characteristic of the target tissue
