FDG PET Scan

PET stands for positron emission tomography. This is a form of nuclear medicine used purely as a diagnostic modality.

 

As a reminder alpha radiation is made of two protons and two neutrons (helium nucleus), beta radiation is caused by a neutron being converted into a proton and emitting an electron and gamma radiation is purely radiation- an electromagnetic wave.

In conventional nuclear medicine the isotopes are formed from a nuclear reactor where the atom is bombarded with neutrons. This creates a neutron rich nucleus leading to an unstable isotope which can therefore undergo decay.

However the isotopes used for PET are formed in a cyclotron. The cyclotron uses a ionized or charged particle and accelerates it using magnetic fields to propel it at high speeds into another particle to attempt to combine the nuclei. This will typically cause a neutron deficient nucleus. This can cause two possible situations: one where the proton grabs an electron to convert into a neutron or the second where the proton degenerates into a neutron by emitting a positron.

A positron is a particle of antimatter. Antimatter is effectively the reverse of matter. When matter and antimatter combine annihilation occurs with the emission of energy (the concept behind Dan Brown’s “Angels and Demons”. A positron is the antimatter form of an electron: an electron with a positive charge. As soon as the positron is produced it will inevitably combine with an electron with which it will perform mutual annihilation resulting in release of energy in the form of two gamma rays emitted at almost 180 degrees to one another.

It is these gamma rays which are emitted almost in opposite directions which are detected by the PET machine. Considering that antimatter virtually cannot travel in the universe of matter without being annihilated the gamma rays can be used to pinpoint the location of the positron emission extremely accurately.

These days PET machines are combined with a CT scanner which allows superimposition of the images to show where the increased positron emissions exactly.

There are several common isotopes which can be produced reliably with cyclotrons- carbon-11 (half-life:20min), Nitrogen-13 (half-life:10min), Oxygen-15 (half-life:2min) and fluorine (half-life:110min). Evidently the issue here is the short half-life as the cyclotron would need to be onsite rather than being able to transport the isotope from offsite for the majority of the isotopes. The exception would be fluorine but even then the half-life is short compared to conventional nuclear medicine isotopes such as technetium 99m (half-life:6hrs).

The current workhorse of PET is known as FDG – 2-18-Flouro 2-Deoxy-D-Glucose. This uses Flourine 18 and attaches it to one of the branches of the glucose molecule where the 2^nd OH portion is.

The fluorine 18 acts as the tracer while the glucose molecule is taken up by the body. However with the alteration of the glucose molecule while it can be uptaken by the glycolytic cells it cannot be metabolized. As a result it is able to highlight the cells or regions that have a high metabolic activity. The target is usually malignant tissue but it also highlights inflammatory tissue. Normal tissue that would be highlighted are the brain and usually the cardiac muscle. If the person is anxious paraspinal muscles may be active. Also those with high insulin levels such as diabetics or post prandially all the tissue will likely be to have enhanced uptake. There will also be heightened concentration in the renal system due to excretion.

This technique is quite commonly used in the attempt to pick up any malignant tissue such as when screening for metastases.