Specificity? "High-Contrast and spectroscopic-based specificity of optical imaging."
Not really a EE question...

The full sentence is:


  It is a hybrid modality, combining the high-contrast and
  spectroscopic-based specificity of optical imaging with the high
  spatial resolution of ultrasound imaging.


Specificity means " high-contrast " specific advantage of optical imaging.

Could be reworded as 


  It is a hybrid modality, combining the high-contrast advantage of
  spectroscopic-based  optical imaging with the advantage of high
  spatial resolution of ultrasound imaging.


Basically taking the good specific part of two methods integrated in one system.