Radiation

his course will briefly address the (ionizing) radiation used in X-ray & CT examinations.
Most people know that (ionizing) radiation may be harmful to the human body. But why is this exactly? 
Important questions such as ‘how do we measure radiation load?’, ‘how dangerous is radiation?’ and ‘how high is the risk of developing cancer when undergoing a CT examination?’ will be answered in this course.

Ionizing radiation

Ionizing radiation is a collective term for multiple types of radiation (including X-rays, alpha radiation, beta radiation and gamma radiation). These various types have one property in common, which is that they can ‘ionize’ matter. Explanation: radiation contains so much energy that it can release a negatively charged electron from the outer shell of an atom. The atom then looses its neutral charge and gets a positive charge - an ion is formed.
When ionizing radiation comes in contact with living tissue, it can cause biologic effects, leading to tissue damage.
Humans are exposed to ionizing radiation from natural sources and artificial sources (e.g. X-ray). This is explained further in the Artificial radiation sources & natural background radiation section.  

For your convenience, in the remainder of this course the word ‘radiation’ is understood to be ionizing radiation. 

Measuring radiation

There are several ways to describe the radiation load. In the literature, the terms CTDIvol, DLP and effective dose are used.

• Volume CT dose index (CTDIvol)
  Description of the mean local patient dose of a CT scan. This is expressed in mGy (milligray, 1 mGy = 1 joule/kg absorbed radiation energy).
  Benefit: easy and quick comparison of the radiation dose between various CT scanners (Note: not all CT scanners are the same). The CTDIvol can be read directly on the CT scanner display.
  Drawback: no exact dose measurement for individual patients.
   
• Dose-length-product (DLP)
  Measurement of the total patient dose and accounting for scan length. So this is the radiation dose of the entire scan. The DLP is expressed in mGy/cm.
  The DLP can be calculated as follows:  DLP = CTDIvol x scan length (cm).
   
• Effective dose (E)
  Effective dose includes the radiation dose of the entire body and describes the risk of developing cancer. The effective dose is expressed in mSv (millisievert).
  The effective dose can be used to compare the relative risks of the various X-ray examinations. However, this measurement is unsuitable for determining individual patient risks. Gender, build, age and organ sensitivity to X-rays must be included in individual risk calculations (see also the Children section).Not all tissues/organs are equally susceptible to ionizing radiation. The International Commission for Radiological Protection (ICRP) has developed various so-called tissue weighting factors. The tissue weighting factors can be used to estimate individual tissue/organ risks. Through the years and based on research, ICRP have adjusted the tissue weighting factors (see table 1). The figures are virtually unchanged. Remarkably though, the mammary gland is more susceptible to radiation-induced cancer than was thought originally in 1991 (tissue weighting factor from 0.05 to 0.12). Conversely, it has now been shown that the gonads are less susceptible (original tissue factor 0.25, now 0.08).