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Radionuclides

Radionuclides

In nuclear medicine, a given radionuclide is administered to the patient, with the aim of investigating a specific physiological phenomenon by means of a special detector, usually a gamma camera, located outside the body. The injected radionuclide is selectively deposited in certain organs (thyroid, kidney, etc.). The size, shape and functioning of these organs can be seen from the gamma camera. Most of these procedures are diagnostic, although in some cases radionuclides are administered for therapeutic purposes.

The radionuclides useful in nuclear medicine are the following:

  • "In life" diagnosis: gamma emitters of short half-life (technetium-99 metastable, indium-111, iodine-131, xenon-133 and thallium-201) and ultra-short half-life positron emitters (carbon-11, oxygen-15 fluorine-18 and rubidium-82).
  • "In vitro" diagnosis: gamma emitters (iodine-125, chromium-51 and cobalt-57) and beta emitters (tritium and sodium-24).
  • Therapy: beta emitters (iodine-131, yttrium-90 and estrogen-90).

Nuclear medicine "in life": Use of radiopharmaceuticals

Radiopharmaceuticals are substances that can be administered to the living organism for diagnostic or therapeutic purposes, investigating the functioning of an organ. Currently, 100 to 300 radiopharmaceuticals are used for diagnostic purposes.

The isotopes used have a short half-life of minutes, hours or days and are prepared in radiopharmacy laboratories, thus guaranteeing their properties and purity.

Radiopharmaceuticals are usually administered as part of simple molecules or linked to more complex molecules to be distributed in the organs that are to be explored.

Positron-emitting radionuclides are used in the technique called positron emission tomography (PET). The positrons emitted by these radionuclides are annihilated with the atomic electrons, giving rise to two gamma rays that propagate in opposite directions and are detected with a gamma camera that has detectors located on both sides of the patient. This method is used to evaluate, among others, the functioning of the heart and the brain.

The quality of the images obtained with these equipment is superior to that of conventional equipment. Currently, due to its high cost and high technology, there are only equipment marketed in countries with a high level of medical technology. The high cost and the high technology that is required are due to the fact that to produce these isotopes it is necessary to have a cyclotron.

Another important technique is scintigraphy, which detects the gamma radiation emitted by the radiopharmaceutical attached to the organ to be studied, in a device called gamma camera, whose detector is placed on the organ, receiving photons from the radiopharmaceutical.

These signals are transformed into electrical impulses that will be amplified and processed by means of a computer. This transformation allows the spatial representation on a screen or plate of X-rays, on paper or the visualization of successive images of the organ for its later study.

Currently, gamma cameras allow three-dimensional cuts of the organ, improving the quality of the studies and the diagnostic sensitivity.

The thyroid scan consists of obtaining the image of the thyroid gland, administering to the patient an isotope, such as iodine-131 and technetium-99, which binds to the cells of this gland. It is used to diagnose the presence of alterations of the form, volume or thyroid function, such as goiter, hyperthyroidism, thyroid cancers, etc.

The adrenal scan allows obtaining information about the form and function of the adrenal glands, whose dysfunctions can cause the appearance of diseases such as Addison's disease, Cushing's syndrome, etc.

With different isotopes and administration forms, cardiovascular diseases (chest angina and myocardial infarcts), digestive diseases (from cysts or tumors to digestive disorders or intestinal absorption) and lung diseases (tumorous involvement of the lungs) can be studied.

The bone scintigraphy allows to diagnose infections and tumors in the bones, by detecting the accumulation of the radiopharmaceutical injected to the patient in the affected areas.

The studies of the central nervous system (CNS) with these scintigraphy techniques are very useful to evaluate the different types of dementias, epilepsies and vascular or tumor diseases, which can not be detected by nuclear magnetic resonance or by computerized axial tomography (CT).

Nuclear medicine "in vitro"

The analytical technique called radioimmunoassay, allows to detect and quantify existing substances in blood and urine, and which are difficult to detect by conventional techniques. It is carried out through the combination of the antibody-antigen binding with the one marked with an isotope, generally iodine-125, of one of these two components, usually the antigen.

To perform this type of analysis, the patient does not come into contact with radioactivity, since the analyzes are carried out on the blood drawn from the patient. For this reason, this specialty of nuclear medicine is called "in vitro".

It is a technique of great sensitivity, specificity and accuracy, which applies to various fields:

  • Endocrinology: determinations of thyroid, adrenal, gonadal and pancreatic hormones with dynamic stimulation and braking tests.
  • Hematology: determinations of vitamin B12, folic acid, etc.
  • Oncology: determinations of tumor markers for the diagnosis and monitoring of tumors.
  • Virology: determinations of hepatitis B and C markers
  • Pharmacology and toxicology: determinations of drugs in blood, detecting possible sensitizations of the organism to allergies.

 

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References

Last review: October 2, 2015