Radiation Physics, Cellular Interactions & Biological Risk Models

Ionizing radiation transfers energy to matter

CT uses x-rays that can ionize atoms and produce secondary electrons. Those electrons deposit energy and can damage biological molecules. Understanding the physical quantities prevents confusion between scanner dose indices, tissue energy absorption and radiation-protection risk quantities. A number without its unit, reference object and purpose is not an adequate description of exposure.

Diagnostic x-rays interact through processes including photoelectric absorption and Compton scattering. Photoelectric absorption transfers photon energy to an electron, while Compton scattering transfers part of the energy and changes the photon's direction and energy. Secondary electron tracks account for much of the local energy deposition. Pair production requires at least 1.022 MeV and is not a diagnostic CT interaction at ordinary tube potentials.

Absorbed dose and unit conversion

Absorbed dose is energy imparted per unit mass. The SI unit is the gray (Gy), equal to one joule per kilogram. One Gy equals 100 rad; one rad equals 0.01 Gy or 10 mGy. Therefore 1 mGy equals 0.1 rad or 100 mrad. These conversions concern the same physical quantity expressed in different units.

For a worked conversion, 25 mGy equals 0.025 Gy and 2.5 rad. If an idealized 0.2 kg tissue sample receives 0.004 joule, absorbed dose is 0.004/0.2 = 0.02 Gy, or 20 mGy. This example defines energy and mass explicitly; the scanner's CTDIvol display does not supply those patient's tissue inputs automatically.

Exposure describes ionization in air produced by photons and is a different quantity from absorbed dose in tissue. The historical roentgen is an exposure unit, not interchangeable with Gy or Sv without the relevant conversion conditions. Air kerma likewise describes kinetic energy transferred to charged particles per mass of air. In CT, dose-index measurement may use an ionization chamber, but the physical measurement and the final indexed quantity must be identified.

Equivalent and effective dose

Equivalent dose applies radiation weighting to an organ's absorbed dose. For diagnostic x-rays, the radiation weighting factor is one, so an organ dose of 10 mGy corresponds numerically to 10 mSv equivalent dose for that same organ. This does not make CTDIvol of 10 mGy equal to a patient effective dose of 10 mSv.

Effective dose combines organ equivalent doses using tissue weighting factors for a reference population. Its unit is Sv or mSv. It supports protection and broad comparisons, but is not a precise individual cancer probability or the absorbed dose of one organ. Tissue weighting factors sum to one under the chosen protection system. Do not mix factors from different ICRP recommendations or assign the entire remainder-tissue factor to each remainder organ individually.

For a hypothetical partial calculation, an organ equivalent dose of 10 mSv with a stated tissue weight of 0.12 contributes 1.2 mSv to the sum. Other irradiated organs must also be included before calling the result a complete effective dose. The example tests multiplication and the meaning of a weighted contribution, not a shortcut from one organ to whole-body risk.

Direct and indirect cellular injury

Direct action occurs when energy is deposited in a critical target such as DNA. Indirect action occurs when radiation interacts with other molecules, particularly water, producing reactive species that can damage the target. Diagnostic x-rays are low-linear-energy-transfer radiation, and indirect effects are an important pathway. Do not assign one invariant percentage of total biological injury to each pathway for every tissue and exposure.

Water radiolysis can generate reactive species including hydroxyl radicals. A free radical has an unpaired electron; it is not necessarily an uncharged species. Such reactions can affect DNA, proteins and membranes. Oxygen can modify damage fixation and biological response, but the oxygen effect depends on the experimental and tissue conditions rather than one universal clinical multiplier.

DNA damage includes altered bases and single- and double-strand breaks. Cells may repair damage, misrepair it, undergo death or continue with an altered genome. A double-strand break is generally more difficult to repair accurately than an isolated single-strand break, but neither injury has one inevitable outcome. The biological consequences depend on damage complexity, dose distribution, repair capacity and cellular state.

Relate physical measurements to biology

Dose to a specific organ or tissue is more directly relevant to a tissue reaction than a population-weighted effective dose. Conversely, a local dose measurement alone does not quantify every organ's contribution to whole-body detriment. Keep these roles separate when reviewing a perfusion examination or a long image-guided procedure.

A patient may receive repeated local exposures while the displayed index for each acquisition appears ordinary. The relevant question is what tissue was irradiated, how often and under which settings. Preserve the dose record and seek physicist assistance when a tissue-dose assessment is needed; do not estimate biological consequences solely from an unlabeled sum.

Reference: FDA CT radiation-risk explanation.