Medical apparatus includes a probe, having a distal segment configured for insertion into a body of a patient. The probe includes at least one optical sensing unit, which is disposed along the distal segment and includes first and second radiation sources, configured to emit optical radiation in different, respective, first and second wavelength bands toward tissue in the body in proximity to the distal segment. An optical sensor is configured to receive the optical radiation in the first and second wavelength bands that is scattered from the tissue and to output first and second electrical signals responsively to an intensity of the received optical radiation.
