How does it work? - PET-CT

How does it work? - PET-CT

By positron emission tomography (PET), we can obtain visual information on our internal organs without making any surgical incisions.

Technology

Keywords

positron, computer tomography scanner, X-ray, X-ray tube, tumor, fluorine radioisotope, fluorine, isotope, gamma, gamma radiation, photon, glucose

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Scenes

PET-CT room

  • PET-CT device - A cylindrical machine into which a movable bed is slid.
  • movable bed

By using medical imaging techniques, we can obtain visual information on the internal organs of the body without making any surgical incisions. One of these techniques is positron emission tomography (PET). With this technique, not only the anatomical features of the tissues can be analysed, but also their functioning. The PET-image provides three-dimensional information on the blood flow and cellular metabolic processes taking place in a certain tissue.

When a disease occurs in an organ, functional disorders appear first; anatomical changes only appear in a later phase. This is the reason why PET is so helpful in diagnosing diseases at an early stage. PET is used to detect mostly tumours, but also to find neurological changes and to measure blood flow in the heart's coronary artery.

PET is usually accompanied by another diagnostic technique called computed tomography (CT). The images obtained with the PET-CT device can reveal more anatomical detail, increasing the accuracy of the diagnosis.

PET-CT device

  • detector ring - It contains crystals that are sensitive to X-rays and produce electrical signals when interacting with them.
  • X-ray tube - It emits X-rays that get to the detector by passing through the body.
  • scanner drum

Before performing a PET-CT scan, a small amount of radioactive material (radioisotope, radiotracer) is administered into the patient's body. The gamma rays emitted during the radioactive decay of the radiotracer are detected by a detector ring. The detector can detect rays coming from any direction. The CT and PET scans are done at the same time, this way there is no need to change the position of the patient.

FDG

  • 2-deoxy-2-(18F)fluoro-D-glucose (FDG)
  • Fluorine-18 radioisotope - An isotope that has a short half-life and serves as the source of positrons.

Fluorodeoxyglucose (FDG), a fluorine isotope (fluorine-18) attached to a glucose molecule, is often used for PET scanning. FDG accumulates in tissues where the stimulation of glucose is more intensive, in other words, where more significant metabolic processes take place. Fluorine-18, like other PET-isotopes, such as oxygen-15, nitrogen-13 and carbon-11, has a very short half-life (2-110 minutes). The disadvantage of the short half-life is that after administrating the isotope, the PET scan can only be performed within a short period of time. On the other hand, its advantage is that it means less radioactive load for the patient.

Diagnosing tumours with PET-CT

  • Fluorine-18 radioisotope - An isotope that has a short half-life and serves as the source of positrons.
  • PET-CT device - A cylindrical machine into which a movable bed is slid.
  • tumour - Its cells divide without control. The tumorous cells are not evolved enough to fulfill the role of the healthy cells. The growing number of tumorous cells take away the nutrition from the healthy ones.
  • FDG
  • positron - Subatomic particle with +1 elementary charge; the antiparticle of the electron. In the presence of any material, a positron collides with an electron, annihilation occurs, which results in the production of high-energy photons.
  • electron - Subatomic particles with -1 elementary charge. Their size is less than 10⁻¹⁸ m.
  • gamma photon - It arises during the transition of the excited nucleus into a lower energy state.

Following the intravenous administration of the isotope, the PET-CT scan starts in 50-60 minutes. This is the time necessary for the isotope to reach every organ through the circulatory system. After that, the isotope will be present in only those tissues where significant metabolic processes take place. Isotope enrichment can be observed in the brain, the heart, the bladder and the kidneys. If there is a tumorous tissue in the body, the PET-CT is able to detect it due to the cell proliferation in the tumour since this process requires a lot of energy and it implies an elevated level of glucose as well. As the fluorine isotope decays, it emits a positron. The positron interacts with an electron near the isotope producing gamma rays that can be detected by the PET system.

Imaging

The images taken by the PET and CT devices are projected on each other in anatomical sections, perpendicular to the longitudinal axis of the body. By putting together these sections, it is possible to reconstruct the whole body. Thanks to the images, the exact place of the tumour can be determined.

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