PET and CT

Introduction

CT is composed of an X-ray generator on one side of the scanner and of an X-ray detector on the other, it produces cross sectional images of the body and gives a 360 degree image of the brain. An X-ray tube rotates around the patient, at different angles, beams of Xrays are emitted and detected. The detected signals will have different attenuations due to the density and composition of different tissues. Dense structures like bones will appear whiter, as they absorb more X-rays. The opposite is true for tissues like fat and air, which will appear darker. 

Applications and limits 

Compared to other techniques, it is very fast as can be done in less than 1 minute and has high spatial resolution. This makes it useful in emergency situations. It is mostly used in clinical settings, for: bleeding, injury and strokes. To increase the contrast, an injection of dye can be applied. 

This technique is fast and inexpensive. However, it has a low-level radiation exposure (equivalent to 0.5-1 yrs of natural background radiation) and a poor image resolution compared to MRI (it is less effective for visualising soft tissues compared to MRI). 

Radioligands

If the metabolic activity in an area of the brain changes, there will also be an alteration of the glucose level in that area. Thanks to the radioligands it will be measured by the scanner. 

Markers can target inflammation, neurotransmitters etc. The radioligand is injected in the blood stream and the PET scan measures the strength of the radiation signal at every point in the brain, mapping its distribution. 

• In clinics, one mostly uses FDG (fluorodeoxyglucose), this shows over and under activity of the brain cells (for ex. used to detect cancer).

• In Research, there are many more targets for radioligands.

1. Positron Emission and Annihilation:
   The isotopes decay by emitting positrons (e+), which are the antimatter counterparts of electrons. Once emitted, positrons travel a short distance (up to ~1 mm) before colliding with electrons in the body. This collision causes annihilation, resulting in the release of two gamma photons traveling in opposite directions (180° apart). The gamma photons have an energy of 511 keV each, calculated using Einstein's equation (E = m_e * c^2) based on the mass of the electron.

The key reaction is:
e+ (positron) + e− (electron) → 2 gamma photons

2. Limitations in Spatial Resolution:

   • The photons are not always emitted exactly 180° apart, leading to slight angular deviations. Positrons travel a small distance before annihilation, causing a minor spatial uncertainty.
     These factors make PET better suited for monitoring time-dependent metabolic processes rather than achieving high spatial resolution.

3. Positronium Formation:
   Positrons can briefly combine with electrons to form an exotic atom called positronium, but its half-life is extremely short (~0.125 nanoseconds) before annihilation occurs.