Radionuclide Imaging and Therapy

What are radionuclides?

Radionuclides are radioactive forms of elements. Some are found in nature, while others are created by humans, either intentionally or as a consequence of nuclear processes. Every radionuclide emits radiation at a certain pace, which is quantified in terms of half-life, and each radionuclide emits radiation at a different rate.

Radionuclide Imaging and Therapy: Half-life

When it comes to radioactivity, the half-life is the amount of time it takes for one-half of the atomic nuclei in a radioactive sample to decay. By releasing particles and energy, nuclear species may spontaneously transform into other nuclear species, which is called decay. Alternatively, the half-life of an isotope is the average time it takes for the number of unstable nuclei to halve.

Radionuclide imaging Techniques

There are three techniques of radionuclide imaging used in today's medical physics. They are planar scintigraphy, also known as single-photon emission computed tomography (SPECT), positron emission tomography (PET), and hybrid techniques.

Planar scintigraphy or single-photon emission computed tomography

The detection of gamma-emitting radionuclides by planar imaging or single-photon emission computed tomography (SPECT) requires radiopharmaceuticals containing gamma-emitting radionuclides. Both methods rely on gamma cameras for detection, with collimated detectors registering emitted gamma rays.

A series of collimators direct gamma rays into an array of scintillation crystals, which transforms them into optical photons and detects them with photomultiplier tubes (PMT). A two-dimensional image (scintigram) of radioactivity distribution is created from these data. SPECT has the advantage of three-dimensional imaging (tomography) for better radioactivity distribution detection and physiological and functional data.

Collimators describe the optimum energy as being between 140 and 160 keV. 99mTc, 111In, and 123I are the most frequent gamma-emitting radionuclides utilized in planar scintigraphy and SPECT. (The numbers 99m, 111, and 123 indicate the mass number, also known as the nucleon number, and can be calculated by the addition of protons and neutrons.)

In order to minimize unwanted irradiation, the radionuclide's half-life should be short enough to fade away as soon as possible after imaging.

Positron emission tomography

By detecting the regional concentration of the imaging agent, positron emission tomography (PET) gives a unique opportunity to monitor and quantify in vivo physiological molecular interactions in real time. It is the most precise and non-invasive technique available.

PET necessitates the use of a radiopharmaceutical that contains a positron-emitting radionuclide. In order to attain a lower energy state, the positron-emitting radionuclide requires an extra neutron. The stabilization is accomplished by spontaneous decay, which results in the production of a neutron as well as the emission of a positron and a neutrino.

The positron travels a given distance (positron range), which is determined by the density of the environment and the positron energy. When its kinetic energy drops, it makes contact with an electron, resulting in its annihilation and the creation of two 511 keV photons. After that, PMT registers the photon counts. Single photon events are rejected, allowing for precise measurement of the radioactivity content in the target region. The registered events are rebuilt into pictures that represent the radioactive source's spatial distribution throughout the body.

It is possible to do recurrent examinations during the same day because of an isotope with a short half-life of up to 68 minutes (68Ga).

Radionuclide Imaging and Therapy: Hybrid techniques

The addition and integration of computed tomography (CT) to SPECT and PET for the acquisition of morphologic information while the patient is in the same position resulted in further development and improvement of the nuclear imaging technique. This is critical for pinpointing the exact location of the lesions, particularly in the abdominal region.

Imaging processes are shortened with the use of a CT attenuation map. PET-CT is a hybrid imaging technique that combines the sensitivity of PET with the temporal and spatial resolution of CT. PET and SPECT quantification accuracy is also improved by CT attenuation and scatter correction.

Radiotherapy

Ionizing radiation is extremely sensitive to rapidly proliferating cells, which is utilized in the application of radiation to regulate or destroy fast-dividing cancer cells. External and internal radiotherapy are both possible. The radiation source can be sealed and implanted ( brachytherapy ) or supplied intravenously for in vivo molecular interaction during internal radiotherapy.

The role of radiotherapy is getting bigger every day in nuclear medicine. Targeted radiotherapy of small tumors, micrometastases, and single cancer cells can all benefit from radionuclides that produce Auger electrons in the subcellular range. Radionuclides differ in terms of the type of radiation they emit, as well as their radiobiological efficacy and range of action, allowing for tumor type selection.

Radiotherapy has either found an application or shown potential in solving lymphoma, breast, prostate, colon, thyroid, lung, and brain cancer types, as well as in bone pain palliation.