Radiology Course page 3 – X-Ray

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The three ways in which radiation interacts with matter

There are three ways x-rays interact with organic matter: Classical Scattering, the Compton Effect, and the Photoelectric Effect.

A photon may come into contact with an atom and interact with an electron. If the photon does not have enough energy to actually displace the electron from the atom it gives its energy to the electron. The electron then produces another photon with the same energy as the other and sends it off in a different direction. This is called Classical Scattering.

Classical Scattering

If the photon has enough energy it will displace the electron from its orbit around the atom. The electron, called a recoil electron, is lost from the atom. The atom absorbs the energy from the photon, but is now missing one electron. This atom will now have a net positive charge and is called an ion. The scattering of electrons is called the Photoelectric Effect.  (image below)

The Photoelectric Effect

If the photon collides with an atom or electron and has enough energy to displace it, but does not transfer all of its own energy to the atom or the electron, it will continue on weaker as scattered radiation. This is called the Compton Effect.

The compton Effect

These three reactions in themselves are not life threatening. However, the molecular interactions of these altered atoms can result in breaking molecules into smaller pieces, disrupting molecular bonds, and forming new bonds within or between molecules. Radiation can also interact with the water or oxygen in cells to disturb their delicate balance and damage DNA molecules.

High doses of radiation to the entire body can cause acute effects. Long term or chronic effects come from repeated exposure to radiation. The body attempts to repair the damage but cannot keep up if the exposures are regular enough or strong enough.

Operators of x-ray devices should monitor the amount or radiation they are exposed to by using a film badge. These badges are worn while at work and then sent in to a company at regular times to be evaluated for radiation exposure. Operators should step behind a lead barrier when exposing films. If no barrier is available, stand at least 6 feet away and between 90 and 135 degrees to the primary beam. Operators should never hold the film for a patient during exposure.

Radiographs should not be taken unless the benefit for the patient outweighs the risk of the radiation exposure. Lead aprons must be used on all patients, and a thyroid collar used while taking intraoral films. A patient would have to have 25 complete mouth series in a short time to significantly increase his or her risk of skin cancer. The benefit of detecting disease that may not be otherwise detected far outweighs the risk of radiation in the small doses used for dental radiography.

Radiation exposure varies according to the technique, the amount of collimation, the film speed, and the kilovoltage. The paralleling technique using a “long cone” provides the least amount of radiation and the best quality radiograph. Rectangular collimation reduces the area of tissue exposed to the x-ray beam by 60 to 70%.

The REAL practical aspects of radiation safety

Some people do not want you to take diagnostic x-rays because they have heard that the radiation is dangerous.  In fact, dental x-rays pose very little danger.  It is important for the dental professional to know the various terms used when speaking about the effects of diagnostic x-rays on the human body.

There are five units used in measuring radiation: The Roentgen, The Gray, the Rad, the Sievert, and the REM.  Their definitions and relationships re explained below.

Exposure: The measure of radiation quantity, the capacity of radiation to ionize air (Roentgen [R]).

Absorbed Dose: The measure of energy imparted by any type of ionizing radiation per unit mass of any type of matter. Its SI (Systeme Internationale) unit is Gray [Gy], where one Gy= 1 joule/kg. Its traditional unit is rad (radiation absorbed dose). 1 Gy= 100 rads. The absorbed dose is generally applied to any type of mass, biological or otherwise.  It has no real value when comparing the “dangerousness” of different forms of radiation exposure to biological organisms.

Equivalent dose is based on an average absorbed dose in a tissue or an organ and weighted by the radiation weighting factor (i.e. the type of radiation).  This term has largely been replaced by effective dose. The units used to define equivalent dose are the REM and the Sievert, both defined below.

Effective dose: The effective dose is the sum of the weighted equivalent doses for all irradiated tissues or organs.  The tissue weighting factor takes into account the relative detriment to each organ and tissue including the different mortality and morbidity risks from cancer, the risk of severe hereditary effects for all generations, and the length of life lost due to these effects. Effective dosing units are used to compare radiation doses on different body parts on an equivalent basis because radiation does not affect different parts in the same way.

Effective dose is a useful term that allows comparisons to be made between sources of radiation exposure which expose only portions of the body, such as radiographic techniques, and whole-body exposures, including those resulting fromnatural or background radiation.

The units of effective dose are the same ones used for equivalent dose.   They are the sievert (Sv) and the REM.For diagnostic x-ray purposes, 1 Sv = 1 Gy. The unit traditionally used in the United states was the rem (radiation equivalent man). 1Sv = 100 REM.  A millisievert (mSv) is one thousandth of a sievert, and one miliREM (mREM) is one thousandth of a REM.

The REM and the SIEVERT are large units, so exposure to medical radiation is generally measured in milliREMs (mREM) and milliSIEVERTs (mSV).   

1 mSV = 100 mREM or 1 mREM=.o1 mSV. 

The average dental x-ray taken with film delivers between about .0075 and .0095 mSV effective dose per exposure depending on angulation.  This estimate is based on an average exposure because different speed films (D, E, & F) require different exposure settings on the machine, and because dental x-rays taken at one angle may expose different parts of the body to radiation than another taken at a different angle.  A dental x-ray taken digitally delivers between a third and half of this value (.003 to .005 mSV).

Based on this estimate, a full mouth series of  dental x rays (18 intraoral films) taken with x-ray film delivers about .09 mSV.  A panorex film delivers about .01 mREM.

By comparison, the average person in the US is exposed to about 3 mSV per year just from naturally occurring background sources.  By this measure, it would take approximately 33 full series of dental radiographs or 300 panoramic x-rays to equal the background radiation that the average citizen is exposed to on a yearly basis.  Note that most dentists take a new full series and/or a new panoramic every three to five years on average.

Background radiation comes from outer space, the earth, natural materials (including natural foods), and even other people.   For example, flying cross country exposes a person to about 5 mREM over and above the normal radiation he receives from outer space while simply walking outdoors for the same length of time.   Cooking with natural gas exposes us to about an additional 10 mREM per year because of the naturally occurring radon gas the cooking gas contains.  Living in a brick building adds an additional 10 mREM per year over and above the radiation you would receive from living in a wooden structure.  Simply sleeping next to another person exposes each bed partner to an extra 2 mREM per year.

The American Nuclear Society has an excellent web page that allows you to calculate your own yearly exposure to ionizing radiation.

The Washington State Department of Health has set the maximum safe occupational whole body radiation exposure to 50 mSV per year.  This is the dose considered safe for people who work with ionizing radiation in their professional lives, including x-ray and nuclear technicians.  By this reckoning, it would take over 550 full mouth series of dental x-rays to equal one years maximum safe radiation level for an person employed in a radiation intensive occupation.  It would take 5000 panorex films to get to this limit, or over 10,000 individual x-ray films.

The use of digital radiography further reduces the exposure to about one half of the values in the chart below.  This would mean that it would take about 50 full series of x-rays (taken with a digital sensor) to equal the amount of radiation the average citizen picks up from naturally occurring background sources each year—that means 360 intraoral films:

The table below is adapted and updated from the website of the American Dental Association. A table of equivalent doses is quite helpful in comparing the amount of radiation received from dental x-rays to other medical and natural sources.


What about danger to the x-ray technician?

The x-radiation figures mentioned above pertain to the patient who is in the direct line of fire from the x-ray tube.  The radiation received by the person taking the x-ray comes exclusively from scatter, which is most easily understood by thinking about a flashlight aimed at a wall in a completely darkened room.  The spot on the wall where the flashlight is aimed is the brightest because it is in the direct line of fire, however, the rest of the room is also dimly illuminated by the light that scatters off the wall.  This scatter is what concerns us since nothing but the patient’s face and jaws is directly in the line of fire of the beam.  The flashlight analogy is inexact since x-ray beams are better collimated (they form a tighter beam), and much less x-radiation is scattered from the target than light from the wall because of the nature of the x-radiation itself.  But the analogy still helps you to understand the concept of scatter versus direct illumination.  Furthermore, the strength of the radiation (or light) hitting any unit area falls off geometrically depending on the distance from the source of scatter.  Think of  the flashlight analogy again.  In a very large, dark room the area of the wall two feet from the bright spot is much brighter than an area 20 feet away.  The “brightness” of the scatter illumination falls off as the square of the distance.  A person standing 6 feet away from the target receives one ninth (1/9) as much scatter radiation as a person standing two feet away from the target (6 feet is 3 times further away than 2 feet, and 3 squared is 9).  A person standing 10 feet away (5 times further away) from the target receives one twenty-fifth (1/25).

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