Beta particles, which are high-energy electrons or positrons emitted during certain types of radioactive decay, have limited penetration power in matter. It is true that they can typically travel only a short distance, usually a few millimeters to a few centimeters, in dense materials like human tissue or lead.
However, in the case of a PET (Positron Emission Tomography) scanner, the limited penetration of beta particles is not a hindrance. In fact, it is precisely the characteristic of beta particle interactions that allows PET scanning to work.
PET scanning is a medical imaging technique that relies on the detection of positrons, the positively charged counterparts of electrons. In PET imaging, a radioactive tracer is injected into the patient, which emits positrons through the process of radioactive decay. These positrons quickly interact with nearby electrons in the tissue, resulting in a process called annihilation.
During annihilation, a positron collides with an electron, and both particles are annihilated, converting their masses into energy in the form of two gamma rays (high-energy photons). These gamma rays, being highly energetic, are capable of traveling several centimeters within the body without significant attenuation.
PET scanners are equipped with detectors surrounding the patient's body. When the gamma rays from positron annihilation reach the detectors, they are detected and recorded. By analyzing the detected gamma rays and their locations, computer algorithms can reconstruct three-dimensional images of the distribution of the radioactive tracer within the body.
The key to PET scanning is the precise detection of the gamma rays resulting from positron annihilation. The limited range of beta particles is actually advantageous because it restricts the annihilation events to a small localized region near the tracer's concentration. This allows for accurate localization of the emission source and the reconstruction of detailed images.
In summary, although beta particles themselves have limited penetration depth, the annihilation process they undergo generates high-energy gamma rays that can travel far enough to be detected by the PET scanner's detectors, enabling the imaging of metabolic processes and the distribution of the injected tracer in the body.