Basic principles at SAMI
Positron emission tomography (PET)
The most common route to introduce a radioactive tracer in vivo is through an intravenous (i.v.) injection. The radioligand accumulates with time in the target tissue and through decay it emits a positron (e+) which travels only a short distance (few millimetres, depending on the type of radionuclide) before losing its energy to the surrounding electron rich tissue. Through annihilation, the mass of an electron and the positron is converted into electromagnetic energy, which is released as two high-energy photons (511 keV) in a random direction 180º±0.25 º (non-collinearity) apart. These high energy photons have a high chance to leave the body and therefore can be detected during PET measurements (see figure).
The facts that photons travel in a line, creating a so called line of response, and that each photons detection time can be individually recorded, make it possible to separate random detection events from the hundreds of thousands coincidence events per second. It is the reconstruction algorithm’s task to translate this enormous dataset of coincidence records into meaningful three-dimensional images (x, y, z spatial dimensions, 3D).
The inherent limitation of spatial resolution in PET data collected from a PET measurement could lack the information for an accurate regional identification and quantification. To circumvent these issues, PET imaging is usually coupled with some type of anatomical imaging.
X-ray Computed Tomography (CT)
During a CT measurement, an X-ray source is rotated around the subject while the X-ray sensor is placed opposite from the source to collect information from the body. When X-rays penetrate through tissue there is a certain likelihood that it will get absorbed based on electron density. CT produces a topographic data volume similar to PET which can be used to show different structures of the body differentiated based on their ability to block the projected X-ray. CT imaging is mostly used in bone and lung investigations, but several other diagnostic methods are used as well, where rapid imaging coupled with high resolution is required, like cardiac CT.
Magnetic resonance imaging (MRI)
During MRI imaging, each proton in the subject positioned within a magnetic field is excited by a radio frequency pulse, while this energy is lost the tissue produces a nuclear magnetic resonance signal (NMR) which can be detected. Gradient coils around the subject make it possible to modulate the main magnetic field and therefore to make the NMR signal encode positional information thus providing the precondition for 3D volume generation.
While the drawback of MRI imaging compared to CT is longer acquisition time, it is preferred over CT in cases where increased soft tissue contrast is important. The different properties of MRI imaging provide numerous options to emphasize the different characteristics of the tissue in our interest.
MRI does not use the higher energy parts of the electromagnetic spectrum (gamma-rays, X-rays, etc.), but applies non-ionizing low energy radio waves coupled with a magnetic field to create images of the subject. In CT, the required radiation dose for good image quality can be significant, thus MRI can have an advantage over CT due to decreased radiation load.