The fundamental goal of X-ray imaging is to produce a diagnostic image while keeping the patient’s exposure to radiation as low as possible. This creates a unique challenge because adjustments that appear to reduce the radiation beam’s “strength” can paradoxically lead to a higher overall dose absorbed by the patient. Specifically, lowering the kilovoltage peak (kVp) often requires compensatory adjustments that ultimately increase the radiation absorbed dose.
Defining the X-ray Beam Controls
The X-ray beam is primarily controlled by two adjustable factors: kilovoltage peak (kVp) and milliampere-seconds (mAs). These controls govern the characteristics of the X-ray photons produced, influencing both image quality and patient dose. The kVp setting determines the maximum electrical potential applied across the X-ray tube, controlling the energy and penetrating power (quality) of the resulting X-ray beam.
The mAs setting is a product of the tube current (mA) and the exposure time (s). This factor controls the total number of X-ray photons created at the source, known as the beam’s quantity or intensity. The total radiation dose delivered to the patient is directly proportional to the mAs setting; doubling the mAs will double the patient’s radiation dose.
How Beam Energy Affects Tissue Interaction
The mechanism for the increased absorbed dose at lower kVp is rooted in how X-ray photons interact with tissues. When the X-ray beam enters the body, it is attenuated through two main processes: the Photoelectric Effect (PE) and Compton Scattering. In the Photoelectric Effect, the X-ray photon is completely absorbed by an inner-shell electron, depositing all of its energy within the patient. This total absorption of energy primarily contributes to the patient’s absorbed dose.
The likelihood of a Photoelectric Effect interaction is highly dependent on the energy of the X-ray photon, decreasing rapidly as the kVp increases. When the kVp is lowered, the beam’s average energy is reduced, significantly increasing the probability of the Photoelectric Effect. This means a larger proportion of photons are completely absorbed rather than passing through or scattering. This leads to higher energy deposition and a greater absorbed dose in the tissue, while Compton Scattering is less dependent on initial photon energy in the diagnostic range.
The Mathematics of Dose Compensation
To produce an image with adequate brightness on the detector after lowering the kVp, the technologist must increase the mAs setting. This compensation is necessary because the lower-energy beam is less penetrating and more of it is absorbed by the patient’s body due to the Photoelectric Effect. The relationship between kVp and the required mAs adjustment is non-linear, making the compensation disproportionately large.
The intensity of the X-ray beam reaching the detector is approximately proportional to the fifth power of the kVp, meaning a small change in kVp has a massive effect on the detector signal. The “15% rule” illustrates this non-linear relationship: a 15% decrease in kVp requires the mAs to be approximately doubled to maintain the same image density. Since patient radiation dose is directly proportional to mAs, this necessary doubling of mAs to compensate for the kVp reduction is the direct cause of the overall higher absorbed dose. For example, if a procedure requires a shift from 80 kVp to 68 kVp, the mAs must be doubled.
Image Contrast and Patient Safety Trade-offs
Despite the increase in patient dose, technologists sometimes choose a lower kVp technique to achieve a specific diagnostic goal. The benefit of using a lower kVp is a dramatic increase in image contrast. Because the Photoelectric Effect is highly sensitive to the atomic number of the tissue and the beam energy, a lower kVp maximizes the differential absorption between tissues, such as bone and soft tissue. This results in an image with a greater difference between the darkest and lightest shades, making subtle structures easier to see.
This decision represents a trade-off between image quality and patient safety. A lower kVp yields a high-contrast image, which can be diagnostically superior for certain exams, but it comes at the expense of a higher absorbed dose due to the required compensatory increase in mAs. Clinical practice involves finding the optimal balance, often selecting the highest kVp that still provides adequate contrast to allow for the lowest possible mAs and patient dose.