Molecular imaging makes it possible to visualize cell function and to observe molecular processes in vivo, meaning in the living organism, without affecting them. How in vivo molecular imaging works is by using molecular probes called biomarkers to help image certain biochemical pathways or cellular targets.
Scientists are using miniature versions of positron emission tomography (PET) scanners and magnetic resonance scanners to view molecular processes in whole animals like rats and mice. These techniques are being used to study tumor growth and infectious diseases. The number of disciplines where this technology is being applied is steadily increasing.
PET scanning is a way of creating a three-dimensional image of processes taking place in the body. A molecule of interest is labelled with a positron emitting isotope. These positrons collide with electrons to emit two photons, or particles of light. A scanner capable of detecting photons and determining the density of positron collisions, and hence the concentration of the target molecule, in a particular area. There are eleven commonly used isotopes, including those of Carbon, Oxygen, Nitrogen, Fluorine (the most common isotope in clinical use), Copper (two types of isotope), Iodine, Bromine, Rubidium and Gallium.
PET technology enables the detection of minute amounts of substances. Different concentrations of a molecule are revealed to the observer as different colors. It is, unfortunately, extremely costly. This is because most of the probes need to be produced in an on-site cyclotron, a type of particle accelerator.
Briefly, Magnetic Resonance Imaging (MRI) uses the application of a strong magnetic field to alter the magnetization of atoms in the body. Radio frequency fields are used to change the alignment of the magnetization, which causes the atomic nuclei to create a rotating magnetic field that is picked up by the scanner. This technology is capable of producing high resolution images of soft tissues like those in the brain, muscles, heart and tumors. Another advantage of MRI over PET is that it does not rely on radioisotopes. One disadvantage is that it is less sensitive by several orders of magnitude.
Single photon emission computed tomography (SPECT) makes it possible to measure blood flow in the brain, or regional cerebral blood flow (rCBF). It uses gamma rays. By moving the gamma ray camera around the subject’s head, it is possible to create a three-dimensional image. Because the radioisotopes used in SPECT have longer half-lives than those used in PET, this imaging modality is considered safer than PET.
Optical imaging is another method used for in vivo molecular imaging. There are various methods which use absorption, reflectance, bioluminescence or fluorescence to create contrast. This method has the advantage of fewer safety concerns compared to other techniques.
Further advances in the field how in vivo molecular imaging works may lead to the development of better, more personalized drugs. This may have implications for neuroscience research, especially as regards to Alzheimer’s Disease. As the Baby Boomer generation are living longer and healthier lives, this could be a potentially serious social and medical challenge in the years to come.