Exposure to an excessive x-ray dose can be harmful to humans. Here, the basics of x-ray interaction with tissue and how it can cause damage are presented. X-ray dose measurement units, safety limits and best practices are also discussed. The following topics are discussed here:
X-ray Interaction with Live Tissue
The key risk in x-ray exposure involves damage to living cell tissue. The body can usually repair damage to cells, but high-level exposures to radiation can be harmful. Intense doses of radiation can cause damage to human cells, as evidenced by skin burns or loss of hair. The main risk, however, is related to the Ionizing nature of x-rays, which are often linked to cancer, chromosome mutations, congenital abnormalities and infant mortality rates.
X-rays are a form of Ionizing radiation, which act by removing electrons from atoms and molecules of materials that include air, water, and living tissue. Ionizing activity can alter molecules within the cells of the human body. That action may cause eventual harm (such as cancer). More information on Ionizing radiation is presented under X-ray Fundamentals.
In addition to x-rays, other forms of ionizing radiation include Gamma radiation, protons, neutrons, positrons. Other forms of non-ionizing radiation, such as Ultraviolet light, can also cause damage to tissue, especially skin.
The effect of ionizing radiation on living tissues is often used in medicine, where the ionizing radiation is applied to selectively kill cancer cells while sparing normal tissue. In Radiation Therapy, ionizing radiation is used to limit or destroy cancer cells’ ability to reproduce by damaging their DNA, so that the cancer cells can no longer divide and grow. Ionizing radiation is most effective at killing cells that are actively dividing, such as cancer cells. This happens since cancer cells divide more rapidly than normal cells, and they also do not repair this damage as effectively as normal cells.
Radiation therapy is performed by implanting a radioactive source near or inside the tumor, or using an external radiation source which delivers a dose focused on the tumor.
Radiation Dose Measurements
The effect of ionizing radiation dose on the human body depends on the amount of energy deposited in the body by the ionizing radiation. As explained in the X-ray Fundamentals, there are primarily two types of processes which govern the interaction of x-ray rays in matter; Photoelectric Absorption and Compton Scatter. Energy is deposited in the human body when either process occurs. X-rays which exit the body with no interactions do not deposit energy in it.
There are three methods used to measure the energy deposited in the human body:
Absorbed dose measures the amount of energy deposited per unit mass of material. Absorbed dose is a generic quantity, defined for any type of radiation or material and is measured in units of Gray (Gy), which is equal to 1 Joule/kg. Another unit for Absorbed Dose is the Rad (R), which is equivalent to 0.01 Gy. That is, 1 Gy = 100 Rad.
Different types of ionizing radiation have different processes which govern their interaction with matter, including living tissue. Hence, the biological effects of a given magnitude of absorbed dose on a human depends on the type of ionizing radiation. As an example, for the same amount of Absorbed Radiation, the impact on human tissue with Alpha radiation is 20 times higher than that with x-ray or Gamma radiation. That is, if an Alpha radiation and an x-ray radiation each deposit 1 Gy of energy into a body, the biological impact of the Alpha radiation is 20 times higher than that for x-rays.
The unit of measurement for Equivalent Dose is Sievert (Sv). For x and Gamma rays, the Equivalent Dose resulting from 1 Gy of energy deposited into a human (Absorbed Radiation) equals 1 Sv. On the other hand, the Equivalent Dose for 1G of Alpha radiation equals 20 Sv.
An older unit of radiation dose is the Roentgen Equivalent in Man (rem). The conversion between Sievert and Rem units is straight forward as 1 Sievert = 100 rem.
A Sievert is a very large unit of dose. Hence, a millisievert (mSv = 1/1,000 Sv) or microsievert (μSv = 1/1,000,000 Sv) are often used. Similarly, a millirem (mrem = 1/1,000 Rem) is often used.
The risk of cancer induction from an equivalent dose depends on the organ receiving the dose as organs have different sensitivities to radiation. For example, the lens of the eye is more sensitive to radiation than hand and feet, as one would expect.
The Effective Dose takes this sensitivity into account by multiplying the equivalent dose received by each organ by a weighting factor reflecting that organ’s sensitivity to radiation. When setting safe radiation dose limits, the most sensitive organs, such as the eyes, determine such limits. Just like Equivalent Dose, the unit for measuring the effective dose is also the Sievert (Sv).
Safe Dose Limits
The American National Standards Institute (ANSI) develops and publishes standards for various industries and applications. The ANSI standard which defines the allowed x-ray dose to humans when screened by x-ray systems is the ANSI N43.17-2009, titled: "Radiation Safety for Personnel Security Screening Systems Using X-rays.“ In addition to dose to people, the ANSI N43.17-2009 also requirements on safety interlock interlocks, operational procedures, information to provide to subjects, training for operators as well as other information.
The ANSI N43.17-2009 sets the dose limits for the general public as follows:
0.25 μSv (25 μrem) Effective Dose per screening
250 μSv (25 mrem) in one year
The limits above are for the general body, including the eyes and other body parts. Further, the limits above are set for the general public, including children and pregnant women. The dose limit for occupational workers is higher than those set above. Same is true for body extremities (arms and legs) which have higher dose limits.
The dose limits set by the ANSI N43.17-2009 standard are very low when compared to other sources of radiation we get from other sources, as presented in the figure below. For example, the effective dose from a typical chest x-ray 100 μSv, which is 400 times higher than the limit set by the ANSI N43.17-2009 (0.25 μSv). The daily dose from natural sources at sea level is 40 times higher the limit set by the standard. The atmosphere blocks much of the radiation coming from the sun and other sources in space. Hence, higher elevation increases the effective dose and a round-trip flight from NY to London results in an effective dose of 50 μSv, which is 200 times higher than the daily limit set by the standard.
Effective x-ray dose from multiple sources compared the the NanoDose technology developed by Seethru AI.
Factors Affecting Effective X-ray Dose
When dealing with an x-ray scanner, there are several factors which affect the effective dose delivered to people or material in general. Understanding these factors helps reducing unnecessary x-ray exposure. In general, it is always advised to limit x-ray exposure to As Low As Reasonably Achievable, also known as the ALARA principle.
There main factors affecting x-ray the effective x-ray dose presented below, in no particular order:
Current of the x-ray source
Peak voltage of the x-ray source
Distance from x-ray source
An x-ray source converts electrical energy into x-rays and heat (more information presented here). The electrical power is measured by current and voltage and increasing either or both increases the x-ray dose. For most practical x-ray sources, electrical current is typically measured in milli-Amperes (mA) and is linearly proportional to effective dose. For example, doubling the mA of an x-ray source would double the effective x-ray dose. Exposure time increases effective dose linearly, similar to mA. Hence, doubling the exposure time will double the effective dose.
The efficiency of converting electrical energy into x-rays increases with higher voltage. Peak voltage is typically measured in kilo-Volt peak (kVp) and dose is exponential with kVp. That is, effective x-ray dose of an x-ray source is proportional to kVp raised to power 2, or higher. Hence, doubling the kVp increases the effective x-ray dose 4 times.
Increasing the distance from the x-ray source reduces the dose. The amount of decrease in x-ray dose a function of distance depends on the shape of the x-ray beam. For a cone-beam, effective dose is inversely proportional to squared distance, while for a pencil beam the dose is inversely proportional to distance.
Adding attenuating (shielding) material which absorbs some of the x-ray beam reduces the effective dose delivered to people or material. The reduction of x-ray dose depends on the shielding material, where denser material, such as Lead, are more efficient at attenuating x-rays and hence reduce the dose. Further, dose is exponentially reduced as a function shielding material thickness. More information on x-ray absorption in material is presented here.
For a given application, a combination of the dose factors presented above are typically implemented to meet ANSI N43.17-2009 dose requirements.