Positron emission tomography

Positron emission tomography (PET) is a method of biomedical imaging that uses nuclear functional imaging techniques to observe metabolic processes in the body. In clinical settings, it is predominantly used in oncology for tumor metastasis imaging, neurology, and cardiology.

Basics of PET

Radiotracers

PET uses radiotracers (also called radioactive tracers or radioligands), which are biological compounds that have been altered to include a radioactive atom (called a positron-emitting radioisotope). Radiotracers are biologically active, meaning they are designed to interact with your body, often by binding to a particular chemical or receptor.

The type of radiotracer used depends on what metabolic process the doctor wants to look at. For example, fludeoxyglucose (FDG) is a radiotracer that mimics glucose. Places where FDG congregate tend to have increased glucose uptake, a sign that the region has increased metabolic activity. This can be helpful for detecting tumors, which need more energy to spread through the body.

The process

The radiotracer is injected into the subject's body, and the subject enters the PET machine. Over time, the radiotracer will break down, emitting gamma rays. The machine picks up these rays and translates them into a 3-dimensional image showing where the radiotracer is concentrated in the body. The resulting image can be thought of as a heat map, showing areas of high concentration as more brightly lit.

Risks

The risks for the amount of radiotracer injected into the body is small enough that there is usually no need to take precautions against radioactive exposure. If you are pregnant or breastfeeding, you should notify the doctor/researcher to protect against injury to the fetus or contaminating breastmilk.

Clinical reasons for getting a PET scan:

PET is often used to evaluate the function of organs such as the heart and brain. Common uses include:

• To measure perfusion of the heart muscle
• To diagnose neurological conditions such as Alzheimer’s, Huntington’s, Parkinson’s, epilepsy, and stroke
• To detect the spread of cancer or evaluate cancer treatment efficacy
• To locate the specific site for surgery prior to the surgical procedure
• To evaluate the brain after trauma

PET research in ME/CFS

PET research in ME has focused on measures of neuroinflammation. This has been done using a radiotracer that binds to the translocator protein (TSPO). Microglia, the resident macrophages of the brain, produce TSPO when they activate. Microglial activation is a commonly used measure of neuroinflammation.

Nakatomi et al. 2014

Nakatomi et al. (2014) was the first case-control study using PET to measure neuroinflammation through TSPO expression in ME. They used PK11195, a first-generation TSPO-binding radioligand. They found increased PK11195 in cingulate cortex, hippocampus, amygdala, thalamus, midbrain, and pons. PK11195 concentrations in certain regions had positive correlations with cognitive impairment scores (related to brain fog), pain scores, and depression scores. The study concludes that neuroinflammation is associated with neuropsychological symptoms in ME patients.

Statistical parametric maps showing areas of significant contrast of PK11195 in brains of ME/CFS patients versus healthy controls.This image was originally published in JNM. Nakatomi, Yasuhito, et al. Neuroinflammation in Patients with Chronic Fatigue Syndrome/Myalgic Encephalomyelitis: An 11C-(R)-PK11195 PET Study. J Nucl Med. Mar 24, 2014; 55(6):945-950. © SNMMI (non-commercial reuse only)

PET research in long COVID

Sollini et al. 2021

Sollini et al. (2021) compared 13 adult long COVID patients to a group of 26 melanoma patients with a negative PET/CT. COVID patients were matched for sex/age.

• In 4/13 long COVID patients, CT images showed lung abnormalities presenting mild [18F]FDG uptake.
• Long COVID patients also had brain hypometabolism in the right parahippocampal gyrus and thalamus (uncorrected p ≤ 0.001).

This study concluded that [18F}FDG PET/CT can be a tool to analyze the multi-organ nature of long COVID.

Guedj et al. 2021.

Guedj et al. (2021) completed PET scans for 35 long COVID patients which utilized a whole-brain voxel-based analysis. They compared these patients to a local database of 44 health subjects which were controlled by age and sex. Long COVID patients exhibited bilateral hypometabolism compared to health patients. Study supports that PET scans may be valuable for long COVID patients in order to perform a whole-brain voxel-based analysis. Additionally, prevalence of hypometabolism was statistically greater for COVID patients compared to healthy patients.

See also

• Brain
• Computed Tomography Scan
• Long COVID
• Magnetic Resonance Imaging
• Neuroinflammation

See myalgic encephalomyelitis.

See chronic fatigue syndrome.

microglia A type of immune cell, called a macrophage, that lives in the brain. For historical reasons, macrophages have different names based on the part of the body that they normally live in. Macrophages that normally live in the blood are called monocytes. Macrophages that normally live in the skin are called Langerhans cells. Macrophages that normally live in the liver are called Kupffer cells. And macrophages that normally live in the central nervous system are called microglia. Microglia were originally classified as glial cells, under the assumption that the cells had a merely structural function, before it was realized that the cells were in fact immune cells. As the "sentinel cells" of the central nervous system, microglia survey their environment for abnormalities such as infection or tissue damage, and then initiate an immune response to fight the infection or repair the tissue damage.

cognition Thought processes, including attention, reasoning, and memory.