Positron Emission Tomography

Positron emission tomography (PET; tomography – from the ancient Greek: tome: the cut; graphein: to write) is a nuclear medicine imaging technique that enables the visualization of metabolic processes through the use of low-level radioactive substances. This is helpful in the diagnosis of inflammations, tumors and other diseases with increased or decreased metabolic processes. The method, which is used particularly in oncology (science dealing with cancer), cardiology (science dealing with the structure, function and diseases of the heart) and neurology (science dealing with the brain and nervous system and diseases of the brain and nervous system), can determine the biochemical activity in the organism under investigation by using a radiopharmaceutical (tracer; tracer substance: chemical substance that has been labeled with a radiologically active substance). The basis for positron emission tomography, which has been used in diagnostics for 15 years, is the tracking of molecules in the patient’s body by positron emission using a positron emitter. Detection (discovery) of positrons is then based on the collision of a positron with an electron, as the collision of charged particles results in annihilation (generation of gamma quanta), which is sufficient for detection. The American researchers Michel Ter-Pogossion, Michael E. Phelps, E. J. Hoffman and N. A. Mullani succeeded in realizing this idea, which had already existed for decades, only in 1975, when they published their research results in “Radiology“. However, there had been partially successful attempts to image brain tumors by positron-based imaging as early as the 1950s. Moreover, since positron emission tomography requires an enhancement mechanism as a functional principle, German Nobel laureate Otto Heinrich Warburg, who recognized the increased metabolism of tumor cells accompanied by increased glucose consumption as early as 1930, can also be considered one of the fathers of this imaging technique.

Indications (areas of application)

  • CUP syndrome: Cancer of Unknown Primary (Engl. ): cancer with unknown primary tumor (primarius): in approximately 3 to 5 % of all tumor disease, despite extensive diagnostics, no primarius can be detected, only metastasis (formation of daughter tumors). Autopsy studies can detect the primarius in 50 to 85% of cases, this is found in 27% of cases in the lung, in 24% in the pancreas (pancreas), and less frequently in liver / biliary tract, kidney, adrenal gland, colon (colon), genital organs and stomach; histologically (fine tissue) it is mostly adenocarcinomas.
  • Degenerative brain diseases (Alzheimer’s disease/beta-amyloid PET imaging/synapse loss in the hippocampus; Parkinson’s disease; dementia).
  • Brain tumors (e.g., gliomas).
  • Colon carcinoma (colon cancer)
  • Lung tumors (solitary round lung tumors; small cell bronchial carcinoma/lung cancer, SCLC).
  • Malignant lymphomas
  • Mammary carcinoma (breast cancer)
  • Malignant melanoma (black skin cancer)
  • Esophageal carcinoma (cancer of the esophagus)
  • Head and neck tumors
  • Neuroblastomas
  • Sarcomas (Ewing sarcomas, osteo-sarcomas, soft tissue sarcomas, rhabdomyosarcomas).
  • Skeletal diagnostics
  • Thyroid carcinoma (thyroid cancer)
  • Progress monitoring of lysis therapy (drug therapy to dissolve a blood clot) in condition after apoplexy (stroke).
  • Cerebral circulatory disorders – for size representation of the penumbra (as penumbra (lat. : penumbra) is called in a cerebral infarction the area immediately adjacent to the central necrosis zone and still contains viable cells) and to determine the myocardial vitality, for example, after myocardial infarction (heart attack).

The procedure

The principle of positron emission tomography is based on the use of beta radiation, which allows radionuclides (unstable atoms whose nuclei decay radioactively, emitting beta radiation) to emit positrons. Radionuclides suitable for application are those which can emit positrons in the state of decay. As already described, the positrons collide with a nearby electron. The distance at which annihilation occurs is on average 2 mm.Annihilation is a process in which both positrons and electrons are destroyed, creating two photons. These photons are part of the electromagnetic radiation and form the so-called annihilation radiation. This radiation impinges on several points of a detector, so that the source of emission can be localized. Since two detectors face each other, the position can be determined in this way. The following processes are required to generate sectional images:

  • First, a radiopharmaceutical is applied to the patient. These so-called tracers can be labeled by different radioactive substances. Radioactive isotopes of fluorine and carbon are most commonly used. Due to the similarity to the basic molecule, the body is not able to distinguish the radioactive isotopes from the basic element, which results in the isotopes being integrated into both anabolic and catabolic metabolic processes. However, as a result of the short half-life, it is necessary that the production of the isotopes takes place in close proximity to the PET scanner.
  • The detectors already described must be present in a large number to ensure the detection of photons. The method of calculating the collision point of electron and positron is called coincidence method. Each detector represents a combination of scintillation crystal and photomultiplier (special electron tube).
  • From the combination of spatial and temporal events, it is possible to produce a three-dimensional cross-sectional image, which can achieve a higher resolution than a scintigraph.

On the process of positron emission tomography:

  • After intravenous or inhalation intake of the radiopharmaceutical, the distribution of radioactive isotopes in the fasting patient is waited for, and after about an hour, the actual PET procedure is started. The position of the body must be chosen in such a way that the ring of detectors is in close proximity to the part of the body to be examined. Due to this, for whole-body imaging is necessary to take several body positions.
  • The recording time during an examination depends on both the type of device and the radiopharmaceutical used.

Since the PET scanner has poorer spatial resolution compared to computed tomography and this could only be compensated for by higher radiation exposure, a combination of the two methods is needed that is able to employ the advantages of both:

  • The developed method PET/CT is a highly sensitive method, which works with low additional radiation by applying so-called correction maps of the CT.
  • In addition to the higher resolution, the reduced time required can also be seen as an advantage over conventional PET.

As a disadvantage of the PET/CT procedure is the necessary ingestion of an X-ray contrast agent. Further notes