three-dimensional image of the distribution of the radiotracer can be obtained by employing filtered back projection or other tomographic techniques. The radioisotopes used in SPECT have relatively long half lives (a few hours to a few days) making them easy to produce and relatively cheap. This represents the major advantage of SPECT as a molecular imaging technique, since it is significantly cheaper than either PET or fMRI. However it lacks good spatial (i.e., where exactly the particle is) or temporal (i.e., did the contrast agent signal happen at this millisecond, or that millisecond) resolution. Additionally, due to the radioactivity of the contrast agent, there are safety aspects concerning the administration of radioisotopes to the subject, especially for serial studies. Positron emission tomography Positron emission tomography (PET) is a nuclear medicine imaging technique which produces a three-dimensional image or picture of functional processes in the body. The theory behind PET is simple enough. First a molecule is tagged with a positron emitting isotope. These positrons annihilate with nearby electrons, emitting two 511 keV photons, directed 180 degrees apart in opposite directions. These photons are then detected by the scanner, which can estimate the density of positron annihilations in a specific area. When enough interactions and annihilations have occurred, the density of the original molecule may be measured in that area. Typical isotopes include , , , , , , , , , and , with being the most clinically utilized. One of the major disadvantages of PET is that most of the probes must be made with a cyclotron. Most of these probes also have a half life measured in hours, forcing the cyclotron to be on site. These factors can make PET prohibitively expensive. PET imaging does have many advantages though. First and foremost is its sensitivity: a typical PET scanner can detect between 10−11 mol/L to 10−12 mol/L concentrations. See also Chemical imaging MICAD Predictive medicine Translational medicine Mouse models of breast cancer metastasis References Medical imaging