Why is skin dose different to absorbed dose in air?

Although in some cases we are interested in skin dose, most radiographic equipment reports the radiation output in air. In this post, we explore why the absorbed dose in air is different to that on the patient’s skin and what factors we must look out for. This question will be explained by breaking it up into five smaller questions.

Does the difference arise due to the mass attenuation coefficient?

The mass attenuation coefficient is a measure of how likely an X-ray beam is to reduce in intensity while traversing through a volume of material. This value is highly dependant on interactions occurring between the incident photons and the electrons in the material. The most common interactions that can happen in the diagnostic energy range are photoelectric absorption and Compton scatter. Therefore, the attenuation coefficient will be the sum of attenuation occurring due to photoelectric absorption and Compton scattering.

A typical X-ray beam of 100 kVp will have an average keV of approximately ⅓ of the kVp, which in this case will be 33 keV. Note: this is a rough estimate and is dependant on added or inherent filtration in the beam and in reality could be different from ⅓. The mass attenuation coefficient of soft tissue at 30 keV is 0.1616 (1) and for air is 0.1537(2). That means there is an increased absorption of approximately 1.05 in soft tissue as compared to air.

Therefore, due to the different mass attenuation coefficient (attenuation properties of the different materials), there will be approximately 5% increase in skin dose as compared to the absorbed dose in the air at the same location. This is typically called the f-factor and falls within the range of 1.04-1.06 in the diagnostic energy range.

What is this increased absorption attributed to? We discuss that in further detail below.

Does the increased absorption arise due to the photoelectric effect?

When discussing absorption or patient dose, the photoelectric effect can play a major role. However, in this case, the photoelectric effect difference between air and soft tissue will not be very significant, at least not in the initial interactions.

As discussed in the post on X-ray Interactions with matter, the rate at which photoelectric absorption is most likely to occur is at photon energies just above the k-shell binding energies. Air is made up of mostly Nitrogen and Oxygen, the k-shell binding energies of these two materials is 0.4099 and 0.5431 keV (3). Human tissue is mostly made up of Carbon, Nitrogen, Hydrogen, and Oxygen with k-shell binding energies of 0.2842, 0.4099, 0.0136 and 0.5431 keV (3).

An X-ray beam having an average photon energy of 33 keV will have a very low likelihood to undergo any photoelectric absorption in both the air and in the skin. This is because the difference in energies between the average photons in the beam and the k-shell binding energies in both materials is too large. Therefore the difference in photoelectric absorption rates at this dose range in the patient’s skin and air is negligible.

In fact, theoretically, due to the Z³ dependence of the effect, there would be a greater likelihood of photoelectric absorption in air than in the patient’s skin. The effective atomic number of Air (7.6) is higher than that of soft tissue (7.4) which would mean the increased likelihood is:

(7.6/7.4)³ = 1.08, or approximately 8% increased likelihood for photoelectric absorption in air as compared to soft tissue.

However, the mass density of air is much less than that of soft tissue and this slightly increased likelihood based purely on the effective atomic number becomes insignificant.

What about Compton scatter, does that explain the difference?

Assuming the dose output (beam quality) remains the same from the tube, then the main contributor to the relationship between Compton interactions and material is the electron density. The ratio of the atomic number to the mass (Z/A) of air is ~0.50 and for soft tissue it is ~0.55 (4). This means the number of electrons in skin tissue will be greater than the number of electrons in the air. This difference is partly due to the absence of hydrogen from air, which has a high electron density.

Therefore, the likelihood of Compton scatter is greater in skin tissue than it is in the air because there are simply more electrons to interact with. These interactions deposit some of the initial energy into the skin and then secondary scattered photons may go on to undergo more interactions (either escape the patient unimpeded, further scatter events or full absorption through photoelectric interactions).

What other factors result in the increased dose?

In the diagnostic energy range, as the radiation is travelling through patients body it is most likely to undergo Compton scattering. The scattered photons can travel backwards towards the skin location and undergo more interactions contributing to skin dose. This phenomenon is simply called backscatter and it increases the skin dose as compared to measurements taken in air without any scattering sources. There have been a number of studies that have published tables of typical backscatter factors (BSF). The BSF is related to the rate of Compton interactions. Thus any factor that will lead to a relative increase in Compton scatters over photoelectric effect will increase the BSF. These factors include beam quality, field size, and tissue thickness. Typical backscatter factors in the diagnostic energy range are between 1.25 and 1.40.

This means that due to the effect of backscatter, skin dose is likely to be 25-40% greater than the absorbed dose to air at the same location.

What about other sources of radiation in the room such as leakage radiation?

Leakage radiation is radiation that escapes from the X-ray tube assembly and is not intended as the primary beam used for imaging. This radiation is characteristic of each tube design, housing and collimation, therefore under the same conditions, the leakage radiation incident on the detector in air or on the patient’s skin will be equal.

Conclusion

In conclusion, there are two main contributors to an increased dose in the patient’s skin as compared to air when measured at the same location. The first is the difference in mass attenuation coefficients. This difference is mainly due to the increased electron density in tissue which increases the rate of X-ray interactions. The increase due to the mass attenuation coefficient difference between air and tissue is approximately 5%. The more major source of the increased dose is the photons that undergo scatter further in the patient’s body and scatter backwards towards the patient’s skin. These scattered photons can undergo additional interactions contributing to skin dose. The increase due to backscatter is approximatley 35%.