Friday, December 8, 2023

Linear no-threshold model in radiation safety

The most conservative theory of radiation is the "linear no-threshold" (LNT) model. According to this model, there is no safe threshold for exposure to ionizing radiation, and any amount of radiation, no matter how small, has the potential to cause harm. The LNT model assumes a linear relationship between radiation dose and the risk of adverse health effects, extending this linear relationship down to zero dose.

In other words, the LNT model suggests that the risk of radiation-induced health effects, such as cancer, increases linearly with increasing radiation dose, and there is no level of radiation exposure considered completely without risk.

Deterministic vs Stochastic effects

Deterministic effects (or tissue reactions) of ionizing radiation are related directly to the absorbed radiation dose and the severity of the effect increases as the dose increases. 

Example: Cataracts

Mnemonic: DDD Deterministic severity Determined by Dose

Stochastic effects of ionizing radiation are chance events, with the probability of the effect increasing with dose, but the severity of the effect is independent of the dose received. Stochastic effects are assumed to have no threshold. 

Example: Cancer

Stochastic Severity No
Probability So

Stannous ions in equilibrium radionuclide angiocardiography

A small amount of stannous chloride (SnCl2) is added to a vial containing a radiopharmaceutical precursor, such as pertechnetate (TcO4^-).

The stannous ions act as a reducing agent, converting TcO4^- to a reduced form of technetium (Tc-99m).

The radiotracer, now in the form of Tc-99m, can be easily incorporated into red blood cells or other carriers.

Technetium gamma decay is an example of isomeric transition

Technetium-99m (Tc-99m) undergoes a type of nuclear decay known as an isomeric transition when it emits gamma radiation. Specifically, Tc-99m undergoes an isomeric transition to its more stable state, technetium-99 (Tc-99), through the emission of gamma rays.

The isomeric transition involves a change in the nuclear energy state of the atom without a change in its chemical properties. In the case of Tc-99m, the metastable state (m) refers to a higher-energy state with a relatively short half-life, and the isomeric transition involves the release of a gamma-ray photon as the nucleus transitions to a lower-energy state.

The gamma rays emitted during the isomeric transition of Tc-99m are of diagnostic interest in nuclear medicine. Tc-99m is widely used as a radiopharmaceutical in various medical imaging procedures, including single-photon emission computed tomography (SPECT). The emitted gamma rays are detected by gamma cameras to create images that provide valuable diagnostic information about the structure and function of organs and tissues in the body.

Photolectric effect and Compton scatter

In the Compton effect, a photon interacts with an outer shell electron, resulting in the ejection of the electron and a scattered photon with reduced energy.

Compton scatter is a significant contributor to image noise. It can result in scattered photons reaching the detector and degrading image contrast.

Compton scatter is more likely to occur with higher-energy photons, such as those around 140 keV.

The photoelectric effect is more pronounced at lower energies and is characterized by the complete absorption of a photon by an inner-shell electron, leading to the ejection of the electron.

The Photoelectric Effect is particularly important in medical imaging applications using low-energy X-rays, where it contributes to contrast in radiographic images. 

Mnemonic: Compton is with COmpletely More energy PhoTONs  (Compton scatter is more likely to occur with higher-energy photon)

photoeLEctric absorbs aLL Lower Energies