Positron emission tomography

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Positron emission tomography, commonly referred to as PET, is a method of biomedical imaging. It 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.[1]

How it works[edit | edit source]

PET uses radioactive tracers (also called radiotracer or radioligand), which are chemical compounds that are biologically active, meaning that the compound functions/reacts/ has a biological purpose in the body. These compounds have been altered such that their structure includes a positron-emitting radioisotope (a radioactive atom). This means that the radiotracer’s movement and activity throughout the body can be detected with a PET machine. Many biological compounds have been made into a radiotracer, which allows for observation of how that compound acts throughout a region of the body.[citation needed]

FDG[edit | edit source]

For example, fludeoxyglucose (FDG), an analogue of glucose, is a commonly used measure of metabolism; detection of FDG correlates with regional glucose uptake. Glucose metabolism is an important measure because cancer cells increase their metabolism to support their increased rates of proliferation and distribution throughout the body. Increased metabolic activity is usually accomplished through increased glucose-uptake. Because cancerous tumors have higher levels of metabolic activity, tumors can usually be detected with FDG-PET. In fact, around 90% of clinical PET imaging uses FDG to monitor cancer metastasis.[2]

The procedure[edit | edit source]

A small amount of radiotracer is introduced into the subject’s body via injection, and the subject then enters the PET machine. As the radiotracer breaks down, it emits gamma rays which are picked up by the machine, and then translated into a 3-dimensional image of radiotracer concentration throughout the body. The resulting image can be thought of as a heat map, showing areas of high concentration as more brightly lit.[citation needed]

Clinical reasons for getting a PET scan:[edit | edit source]

  • Generally, to evaluate the function of organs such as the heart and brain
    • I.e., measuring 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
  • To evaluate cancer treatment efficacy
  • To locate the specific site for surgery prior to the surgical procedure
  • To evaluate the brain after trauma[3]

Risks[edit | edit source]

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.[4]

PET research in ME[edit | edit source]

PET research in ME has focused on measures of neuroinflammation. This has been done using a TSPO-binding radioligand to measure microglial activation. TSPO (translocator protein) is a protein that is produced when microglia, the resident macrophages of the brain, become activated. Microglial activation is a commonly used measure of neuroinflammation. Further research using high-quality PET/TSPO methodology is needed to better understand the pathophysiology of neuroinflammation in ME.[5]

Nakatomi et al. 2014[edit | edit source]

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 were found to have positive correlations with cognitive impairment scores (related to brain fog), pain scores, and depression scores. The study concludes that neuroinflammation seems to be present in ME patients and is associated with neuropsychological symptoms.[6]

Statistical parametric maps showing areas of significant contrast of PK11195 in brains of ME/CFS patients versus healthy controls.[6]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[edit | edit source]

Because of the recency of the COVID pandemic, there is a limited amount of PET research in long COVID. Similar to ME, the PET research in long COVID is focused on neurological and brain analysis.

Sollini et al. 2021[edit | edit source]

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.[7]

Fig. 4 Brain [18F]FDG PET analysis from Sollini et al. (2021) . Regions of hypometabolism compared to controls in the 13 long COVID patients (a) and subgroups of patients showing persistence of anosmia (b), fatigue (c), or mild-to-moderate vessel [18F]FDG uptake (d). Regions of significant difference are colour-graded in terms of Z values. Talairach coordinates and further details are available in Table 3 of the supplementary materials [7]

Guedj et al. 2021.[edit | edit source]

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.[8]

Fig. 1 from Guedje et al. (2021) Brain 18F-FDG PET hypometabolism in patients with long COVID. In comparison to healthy subjects, the patients exhibit hypometabolism in the bilateral rectal/orbital gyrus, including the olfactory gyrus; the right temporal lobe, including the amygdala and the hippocampus, extending to the right thalamus; the bilateral pons/medulla brainstem; the bilateral cerebellum (p-voxel < 0.001 uncorrected, p-cluster < 0.05 FWE-corrected; SPM8 3D rendering)[8]

Learn more:[edit | edit source]

See also[edit | edit source]

References[edit | edit source]

  1. "ScienceDirect". www.sciencedirect.com. Retrieved February 27, 2019.
  2. C, Annibaldi A and Widmann. "Glucose metabolism in cancer cells. - PubMed - NCBI". www.ncbi.nlm.nih.gov. Retrieved February 27, 2019.
  3. "How Does a PET Scan Work?". www.hopkinsmedicine.org. Retrieved February 27, 2019.
  4. "How Does a PET Scan Work?". www.hopkinsmedicine.org. Retrieved February 27, 2019.
  5. Lara Mejia, Paula S.; Brumfield, Sydney A.; VanElzakker, Michael B. (2019). "Neuroinflammation and Cytokines in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS): A Critical Review of Research Methods". Frontiers in Neurology. 9. doi:10.3389/fneur.2018.01033. ISSN 1664-2295.
  6. 6.06.1 Nakatomi, Yasuhito (June 1, 2014). Kazuhiro Takahashi; Yosky Kataoka; Susuma Shiomi; Kouzi Yamaguti, Masaaki Inaba; Hirohiko Kuratsune; Yasuyoshi Watanabe. "Neuroinflammation in Patients with Chronic Fatigue Syndrome/Myalgic Encephalomyelitis: An 11C-(R)-PK11195 PET Study". The Journal of Nuclear Medicine. 555 (6): 945–950 – via SNM Journals.
  7. 7.07.1 Sollini, Martina; Morbelli, Silvia; Ciccarelli, Michele; Cecconi, Maurizio; Aghemo, Alessio; Morelli, Paola; Chiola, Silvia; Gelardi, Fabrizia; Chiti, Arturo (March 7, 2021). "Long COVID hallmarks on [18F]FDG-PET/CT: a case-control study". European Journal of Nuclear Medicine and Molecular Imaging. doi:10.1007/s00259-021-05294-3. ISSN 1619-7070. PMC 7937050. PMID 33677642.
  8. 8.08.1 Guedj, E.; Campion, J. Y.; Dudouet, P.; Kaphan, E.; Bregeon, F.; Tissot-Dupont, H.; Guis, S.; Barthelemy, F.; Habert, P. (January 26, 2021). "18F-FDG brain PET hypometabolism in patients with long COVID". European Journal of Nuclear Medicine and Molecular Imaging. doi:10.1007/s00259-021-05215-4. ISSN 1619-7070. PMC 7837643. PMID 33501506.

myalgic encephalomyelitis (ME) - A disease often marked by neurological symptoms, but fatigue is sometimes a symptom as well. Some diagnostic criteria distinguish it from chronic fatigue syndrome, while other diagnostic criteria consider it to be a synonym for chronic fatigue syndrome. A defining characteristic of ME is post-exertional malaise (PEM), or post-exertional neuroimmune exhaustion (PENE), which is a notable exacerbation of symptoms brought on by small exertions. PEM can last for days or weeks. Symptoms can include cognitive impairments, muscle pain (myalgia), trouble remaining upright (orthostatic intolerance), sleep abnormalities, and gastro-intestinal impairments, among others. An estimated 25% of those suffering from ME are housebound or bedbound. The World Health Organization (WHO) classifies ME as a neurological disease.

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.

amygdala Part of the brain, within the temporal lobe. Related to memory and emotional behavior.

brainstem Region of the midbrain in adults, includes midbrain, pons, and medulla oblongata and develops.

cerebellum A part of the brain at the back of the skull in vertebrates, beneath the occipital lobe of the cerebrum. Its name reflects the fact that it looks like a smaller version of the cerebrum. Its main known functions are the coordination of unconscious muscle movements and the maintenance of body positional equilibrium.

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From MEpedia, a crowd-sourced encyclopedia of ME and CFS science and history.