Imaging physics, beam interactions and useful calculations
Imaging physics begins by identifying the quantity and process being described. X-ray generation, transmission through the patient, detection and display are different stages. Electrons accelerated in an X-ray tube can produce a continuous bremsstrahlung spectrum and characteristic radiation under the appropriate conditions. Tube potential and current-time product affect different aspects of output, and an image's displayed brightness is not a reliable independent measurement of the radiation received by the patient.
Photon interactions help explain attenuation and scatter. Photoelectric absorption and Compton scattering depend differently on photon energy and material. Scatter can reduce image contrast and contributes to occupational exposure around an irradiated patient. Collimation, geometry and appropriate protective arrangements address different parts of this problem. An antiscatter grid may improve contrast but can require an exposure change and can produce cutoff if misused. It should not automatically be added to every small-patient examination.
An attenuation calculation needs its assumptions. In a supplied monoenergetic exponential model, intensity decreases as the exponential of minus the linear attenuation coefficient times thickness. The half-value layer is the natural logarithm of two divided by that coefficient. If the coefficient is explicitly 0.693 per centimetre, the half-value layer is approximately one centimetre. A clinical polyenergetic beam also changes its spectrum during attenuation, so a simple model should not be mistaken for every real shielding calculation.
Likewise, the inverse-square relation describes an appropriate point-source geometry. If a stated model gives four milligray per hour at one metre, it predicts one milligray per hour at two metres. An actual distributed scatter field, intervening material or changed geometry may not obey that simplified example exactly. Occupational monitoring and shielding design therefore require the responsible physical and regulatory assessment rather than a calculation alone.
Absorbed dose measures energy deposited per mass and is expressed in gray. Equivalent and effective dose use weighting concepts and are expressed in sievert. These are not interchangeable with a detector exposure index, an image pixel value or a direct probability of injury in one individual. CT dose indices describe standardized measurement contexts; they are not exact patient organ doses. For a stated CT calculation, dose-length product is volume CT dose index multiplied by scan length. Applying a regional conversion coefficient provides an approximate effective-dose estimate under that model. Preserve the units, assumptions and purpose of each quantity before interpreting or comparing it.