In medicine, it is often possible to determine if an organ is not working properly by examining its structure, as there is a strong relationship between form and function. In cases of traumatic injury, the loss of form is directly linked to the loss of function. However, in many other situations, it is not possible to determine the functionality of a tissue or organ just by looking at its internal structure.

Nuclear Medicine uses molecules with radioactive atoms to track their location in the body, similar to a GPS. Gamma rays, which have higher energy than X-rays, are emitted by these radioactive atoms and can pass through the body without interacting with it. This allows external gamma ray detection cameras to register them.

Positron Emission Tomography (PET) is a Nuclear Medicine image modality that is widely used in oncology, neurology and cardiology studies. The main concept behind PET is that the labeled substance with the radioisotope injected in the patient acts as a tracer for a specific physiological process. As a result, the area of the image where the radioisotope-labeled substance has accumulated appears bright like a light bulb. 

This technology provides valuable insights into the functioning of organs and physiological processes within the body. After injection, the positrons emitted from the tracer will annihilate when it encounters an electron in its surroundings and as a result two photons will be obtained traveling in opposite directions at 511keV (see figure 1). 

Therefore, in order to obtain the radiotracer map inside the body, that is a functional tomographic image after the reconstruction process, the following will be needed:

1. Arrival time of the photons in the detectors to discern what possible annihilation they come from (windows of typically a few nanoseconds). If we also have relative temporal information between the two photons coming from the same annihilation (order of
hundreds of ps allowed by current technology), we can also improve the contrast in the image. This is known as time-of-flight information (TOF).

2. The three-dimensional position of the photons in the detector to correctly assign the arrival trajectory of the photons (x,y and Depth of interaction or DOI).

3. The energy of the incident photons to ensure that they come from an annihilation
process.  Positron Emission Tomography (PET) is an extremely sensitive imaging technique that can detect very small quantities of molecules. This sensitivity is crucial because it allows fo accurate measurement without disrupting the body's normal functioning. Additionally, it helps identify potential toxic substances when present in excess, preventing harmful effects. Most PET cameras use scintillation detectors, which emit light when stimulated by gamma radiation. The emitted light is then collected as pulses for further analysis.