Which radioactive source is most commonly used for calibrating PET systems?

The most commonly used radioactive source for calibrating PET (Positron Emission Tomography) systems is 18F (Fluorine-18). This is primarily due to its suitability for the specific imaging tasks that PET performs. Specifically, 18F is used in numerous radiotracers, such as 18F-FDG, which is widely utilized in clinical practice for cancer imaging, among other applications.

One of the key characteristics of 18F that makes it ideal for PET calibration is its decay properties. It has a half-life of approximately 110 minutes, allowing it to be manufactured in cyclotrons, transported, and used within a practical time frame for clinical applications. Additionally, 18F emits positrons, which directly interact with the detection systems in PET to produce the pairs of gamma photons that are detected, making calibration with this isotope particularly relevant and effective.

In contrast, the other options have characteristics that make them less suitable for PET calibration. 99mTc (Technetium-99m) is primarily used in single-photon emission computed tomography (SPECT), and while it plays a significant role in nuclear medicine, it does not produce the same type of signals that are essential for PET calibration.