Nuclear Medicine

PET-CT

A PET scan (Positron Emission Tomography) is a relatively frequently performed examination in nuclear medicine and plays a major role in oncological staging and detection of infection/inflammation.
The PET camera consists of a ring with detectors, which contain a crystal just like the gamma cameras and a system to amplify the photon signal (photomultiplier). 
The technology continues to improve, and the newer PET scanners are fitted with digital (solid state) detectors.  
As described above, two photons are released when certain radio-isotopes decay (ones emitting positrons), which move in opposite directions. The detector around the patient registers these two photons and then calculates where the signal came from (see Radiation section).

In general, PET cameras have an in-built CT (PET-CT camera). The addition of CT provides additional information about the anatomy. There are currently also hybrid systems that combine PET with MRI. However, they are rarely used in the clinical setting and are reserved for specialised centres.

Just like conventional scintigraphy, PET-CT scanning involves the use of a radiopharmaceutical. There are different kinds of radiopharmaceuticals available; each one provides different information about the biological processes in the body. 
The most well-known one is 18F-FDG (18F-Fluorodeoxyglucose). This is a glucose analogue that is taken up by cells with a high glucose metabolism (for example, brain cells, brown fat, as well as tumour cells and inflammatory cells), just like natural glucose. However, the glucose analogue cannot be processed further in the citric acid cycle and thus accumulates in the cell. Tumour cells employ anaerobic metabolism much more than healthy tissue does, which transforms glucose into lactate, without the presence of oxygen. This is called the Warburg effect. This leads to a high metabolism (high glucose consumption in these cells), and thus the accumulation of 18F-FDG glucose on a PET scan is many times higher than in the surroundings. This property can be used to chart any tumours and metastases. FDG is also taken up by macrophages, revealing infectious/inflammatory conditions (fig. 19).