Molecular imaging is the process in which a substance that binds to a target molecule, e.g. a receptor, can be used to image and measure physiological functions in the human body. It is generally separated into non-nuclear and nuclear imaging and nuclear imaging is further separated out into in vivo and in vitro imaging.
In vivo: this is when the tracer radioactivity is measured as it leaves the human body. Radionuclide imaging is an example of this in which a radiopharmaceutical is introduced into the patient and then a gamma camera images the radioactivity leaving the patient (e.g. bone scans)
In vitro: this is when a tracer is introduced into the patient and then tissue / fluid samples taken from the patient and the radioactivity measured from these. No images are produced.
This chapter will focus mostly on nuclear, aka radionuclide, imaging. Nuclear imaging involves the introduction of a radioactive source into the patient. This is done with radiopharmaceuticals which consist of a radionuclide part that emits gamma radiation and a pharmaceutical part which is the physical/chemical component to which the radionuclide is attached to. It is the pharmaceutical that largely determines the physiological behaviour of the radiopharmaceutical and, therefore, the nature of the image obtained.