Molecular imaging includes the field of nuclear medicine along with various other fields that together offer different ways of producing images. Nuclear medicine uses radiolabelled molecules (radiotracers) that produce signals by means of radioactive decay. However, other molecules can be used to produce signals to image via means of sound (ultrasound), magnetism (magnetic resonance imaging- MRI), or light (optical techniques of bioluminescence and fluorescence).
In contrast with conventional anatomical or physiological imaging, in vivo molecular imaging characterises and monitors biological processes at the cellular and molecular levels. Molecular imaging techniques include PET, single photon emission computed tomography (SPECT), optical fluorescence, optical bioluminescence, magnetic resonance imaging (MRI) and ultrasound.