Radiophysics

Radiation protection and patient dose

 

Both ionizing and non-ionizing radiation as well as ultrasound are used in medical imaging methods. Somatic (occurs in own tissues) or genetic (in descendants) damage in patients or personnel are always a risk of examinations which employ ionizing radiation. Photons of non-ionization radiation (radio wave radiation in a strong magnetic field), as well as ultrasound, carry insufficient energy to cause injuries at diagnostic energy levels. Consequently, radiation protection is needed in practice only in X-ray and isotope examinations and in radiotherapy.

There are many factors which influence image quality. By increasing the amount of radiation (and patient dose) image quality can be increased to a certain level, but simultaneously several factors in the imaging chain can diminish quality. The quality control of imaging methods should be arranged such that high image quality with a dose as low as reasonably achievable (ALARA) is maintained.

The purpose of radiation protection is to eliminate the acute toxicity of radiation exposure and diminish the somatic and genetic risks to patients and personnel. It is useful to remember that the natural background radiation in the Nordic Countries varies between 3-6 mSv (300-600 mrem) per year. There is radiation coming from space, soil (radon gas is a very considerable source of radiation) and construction materials, as well as from our own tissues. Background radiation can vary depending on residential area, life style, etc. This value of 3-6 mSv is the same order as the skin dose from an X-ray image of the body.

Quantities and units of radiation dose

Interactions of X-ray and gamma photons always set electrons in motion with sufficient energy to ionize and excite atoms and molecules (see Interactions of radiation with matter). An electron therefore deposits energy in its wake. Around 10-100 ionizations/ m caused by an electron are generated at diagnostic X-ray energies (approximately 33 eV/ion pair). The concept of linear energy transfer, LET (unit keV/ m) can be used to describe this phenomenon together with the concept quality factor Q, explained later. In addition, part of the energy of the electron is absorbed by secondary electrons, so-called delta particles; they in turn have sufficient energy to cause new ionizations.

Exposure

Exposure implies that ions are generated in air as a consequence of the passage of radiation. Ions can be measured with an ionization chamber, which is an air space between two conducting plates coupled to the positive and negative poles of a voltage source. The exposure = the number of ions with negative (or positive) charges divided by the mass of air in the ionization chamber. The SI-unit is C/kg (C = coulomb). The older unit is roentgen R = 2,58 10-4 C/kg.

Absorbed dose

This quantity is the energy per unit mass, which matter has absorbed from radiation. The SI-unit is the gray Gy = J/kg (the old unit was rad = 0.01 Gy). At X-ray and isotope imaging energies (15-500 keV) one R exposure causes approximately 10 mGy (one rad) absorbed dose in all other tissues except in bone, where the absorbed dose at low energies (around 20 keV) reaches up to around 40 mGy.

Kerma

The concept kerma comes from the words Kinetic Energy Released in Matter. It takes into account the dose generated by the aforementioned delta electrons. It is approximately equal to the absorbed dose in air at diagnostic X-ray energies.

Dose equivalent

When energy has been absorbed in tissue the biological effect varies depending on the organ in question, the type of radiation and energy, dose rate, exposure time etc. These are incorporated in the concept quality factor Q, by which the absorbed dose must be multiplied to get the equivalent dose. Its unit is sievert Sv = J/kg (= 100 rem, the old unit).

In X-ray and isotope imaging, Q is approximately 1, because X and gamma radiation deposit relatively small amounts of energy in tissue. Another concept, effective dose, describes the probability of damage to different organs with a weighting coefficient, which is high for radiation sensitive organs such as gonads, bone marrow, lungs, colon, breast etc. and small for other tissues, e.g. muscle. The sum of the weighting factors equals to 1.

From the foregoing it is clear that in diagnostic imaging, the units Gy and Sv, as well as R, rad and rem, have about the same numeric values, although the concepts have different meanings.

Dose rate

One useful concept in dosimetry is the rate, with which a given amount of radiation strikes tissues, for instance kerma rate and exposure rate mR/min, R/h etc. Activity (see the chapter Radioisotopes and radio pharmaceuticals) is also a concept which incorporates the function of time. Whether X-rays from an X-ray device or gamma radiation from radionuclides are discussed, the same concepts can be used to describe radiation phenomena and the biological effects of radiation.

Radiation biology

Ionization and excitation result in fragmentation of molecular bonds with potentially harmful consequences to cell structure, metabolism and organ function. Injuries are divided into genetic and somatic ones. The former can appear in descendants after a long time has elapsed, and the latter may occur quickly (acute consequences) or after a considerable delay. In the peaceful usage of ionizing radiation acute toxicity does not occur.

A distinction is also made between stochastic and non-stochastic effects of radiation. Stochastic implies that even a single "hit" of radiation to one cell or to a small cell group can cause a biological consequence. Damage may be either hereditary (in gonads) or carcinogenic (in tissue). There is no threshold, i.e. the extent of the damage does not depend on absorbed dose (cancer is contracted or not), although the probability of an adverse event increases with dose. This stochastic nature of radiation is therefore the basis of conservative radiation protection.

The non-stochastic effect of radiation has a definite threshold (normally different for every tissue and organ). These have been found from past experience, e.g. in cancer treatment with radiotherapy during this century. Diagnostic radiation examinations (where skin dose varies between 0.1 mSv and 0.1 Sv / examination) expose the patient to very small doses so the consequences of non-stochastic effects do not evolve. One clear exception is the dose to a fetus, particularly during the sensitive period of organogenesis. Therefore, the indications for pediatric examinations involving ionizing radiation must be examined particularly closely.

It is estimated that if 200 000-2 000 000 people get a dose of l mSv (the same as the background dose per year without radon) it is probable that one person will develop cancer. It is, however, impossible to separate so few cases from cancers caused by other factors, such as environmental toxins and unknown reasons etc.

Many other factors such as the type of radiation and energy, LET value, dose rate, time between exposures or fractionation of dose, different sensitivity of tissues for radiation, biological variations etc. have a significant effect on the likelihood of injury.

Radiation protection

Because injuries from small doses can partly be stochastic the starting point of radiation protection is to avoid and reduce somatic and genetic doses to as low a level as possible (ALARA, As Low As Reasonably Achievable). The consequences of small doses given over long periods of time are partly unknown, and as the time for a carcinoma to appear can be decades, damages caused by low level radiation are often impossible to separate from diseases caused by other factors. On the other hand it is important to use sufficient radiation to achieve good quality images. These examinations, which are clinically indicated, must be performed with sufficient radiation to achieve an image of diagnostic value.

Patient

The dose can be measured or estimated at different depths in the patient, or in different parts of the environment. Terms like skin dose (or surface or entrance dose), depth dose, dose in patient's centre, exit dose (approximately the same as dose to the screen without a grid) and organ dose are fairly self-evident. Dose diminishes as the depth at which it is measured increases. In the diagnostic examination of the body only a 1/100-1/1000

Figure 7. Patient thickness very strongly influences the entrance dose needed for an image. Measurement conditions are also shown.

part of the initial dose penetrates through. Dose decreases also without matter, even in air. Radiation intensity (as well as light intensity) decreases in inverse proportion to the square of the distance from the focus.

Fig. 7 shows how skin dose and exit dose are changed with patient thickness when the exposure of film to a constant blackness (optical density) is made with an automatic exposure meter. In this case exit dose does not depend on thickness, because a screen-film combination always requires a certain amount of radiation.

Many features of X-ray devices and properties of patient tissues influence the dose needed for good image quality. In Table 2 the most important factors are mentioned.

Table 2. Factors influencing patient dose

 

- radiography (mAs value x number of images) or fluoroscopy (mA value x examination time) - high voltage (kVp) and its stability - filtration - distance from focus - field size - thickness of object and absorption in tissues - lead grid - sensitivity of screen, image intensifier and detectors - usage of image memory (for instance in surgical operations)

There are big differences in the properties of different imaging methods and in radiation detectors. Screen-film-combinations are always used in practise instead of film alone. Screen sensitivities vary from speed value 20 to 1600 (that of the reference screen-film-combination being 100), when the speed of the film alone has a value of about l. Consequently corresponding alterations can be found in patient doses.

Personnel

The first rule in the radiation protection of personnel is to go outside the X-ray laboratory when a patient exposure is made. In fluoroscopic examinations one must work l) quickly, 2) with sufficient protective clothing, and 3) at an appropriate distance from radiation sources. These three measures are of primary importance in both X-ray and isotope work. The staff who are most likely to be exposed to radiation are those who work

Figure 8. Diagnostic x-ray device: generator, X-ray tube and console (control board). Exposure is ended when the ionization chamber(s) in the automatic exposure system has (have) collected enough radiation (ionization) to blacken the film adequately (after development). The positions of the three chambers are shown in the radiation field, as well as spectra (number of photons as a function of energy) in different phases of the X-ray chain.

with fluoroscopic devices (radiologists, surgeons etc.), nurses who hold small children or non-cooperative patients, as well as staff working with the nuclear medicine imaging of patients.

National and international radiation legislation and recommendations are universally in use. According to these regulations, for instance, examination rooms, devices and working conditions must be adapted so that doses are diminished to as low a level as possible and that the quality of images and examinations attains the highest possible level. The most recent ICRP recommendation (publication 60, 1991) puts the maximum dose level of 20 mSv per one year to the whole body of personnel. This value is 40% of the earlier maximum limit, which shows the increasingly conservative attitude in radiation protection.

One should remember that the dose to personnel from scattered radiation is 100-1000 times smaller than the dose in the entrance field on the patient's skin. Therefore it is essential for the radiation worker to avoid putting his hands in the primary radiation field (use lead gloves). The patient's body can also serve as good protection, if one can place oneself in such a position that one does not directly see the entrance field of radiation.

 

Aaro Kiuru