X-RAY  EXPOSURE

Just as ounces, gallons, and liters measure liquids, and grams and pounds measure mass/weight, we also have units to measure amounts of exposure for ionizing radiation.  We have three terms which are of interest to us in this regard.  We will briefly consider their technical definitions and then progress to generalities in a more practical vein to enhance understanding.  These three terms will be our building blocks for purposes of further discussion:

1.  Roentgen

          Roentgen (abbreviated with a capital R) has a specific definition:

the amount of radiation that produces a specific amount of ionization in 1 cc of air at standard conditions.

          It is not necessary, however, to remember this definition; it is enough to know that the Roentgen (R) is a measuring unit of radiation exposure.  The amount of radiation which exits from the tube at certain technique factor settings is measured in Roentgens (R).

2.  rad

          Rad (abbreviated with a lower case r) stands for radiation-absorbed-dose.  The rad is a unit of absorbed energy (if anyone cares, 100 ergs per gram of tissue).  This is the most functional term for our purposes because it represents the actual dose, or how much radiation is actually absorbed by the part being radiographed.  Another way of thinking of this would be to consider it the difference between the amount of radiation which strikes the part being measured and the amount of radiation which strikes the bucky surface, the difference being the amount of radiation which is absorbed by the intervening body tissues. 

          Regarding the rad (r), it may have occurred to you that the amount of radiation which is absorbed by tissue depends upon where the measurement is made.  For example: if, for some unusual reason, the entire body were exposed to a certain amount of radiation all at one time, the dose absorbed by different body parts would vary markedly.  As you can imagine, the amount of radiation absorbed by the skin would be greater than the amount absorbed by the thyroid, or the bone marrow, or the gonads.  For this reason, a measurement in rads must be accompanied by a specification of the  body part in question, where the dose is being measured, for it to have meaning and in order for it to be used as a comparison to other dose measurements. 

3.       rem

          Rem is not abbreviated.  It stands for Roentgen-equivalent-man, and it is the unit of exposure which takes into account the relative biologic effects of varying types of ionizing radiation.  The rem provides a way to measure and compare other forms of ionizing radiation (such as alpha, beta, and gamma rays) which are also capable of producing ionization in tissue.  If we wanted to compare the effect of cosmic radiation with diagnostic x-ray exposure, we could say that the amount of cosmic radiation which could produce the same effect in tissue as 1 rad of x-radiation would be 1 rem. 

One more point must be reviewed, and that refers to a term you came to understand in your study of x-ray technique factors.  As you recall, the prefix “milli” (abbreviated with a lower case m) indicates 1/1000^th of a unit.  mA thus became the shorthand method of indicating a thousandth of an ampere.  We can use this same prefix to indicated thousandths of Roentgens (mR), rads (mr), or rems (mrem). 

Now with all of the hard stuff behind us, we can proceed to explore some representative values to better see this whole subject in perspective. 

To begin with, it is necessary to understand the vast difference between whole body exposure and regional exposure.  Heavy exposure of the whole body to ionizing radiation is especially damaging because it leaves no unaffected tissue to carry on body function.  Following are the effects created by varying WHOLE BODY exposures:

In sharp contrast to the above figures, REGIONAL exposures are far less detrimental because they create effects only in the areas irradiated and leave the rest of the body tissues able to carry on their normal functions.  Indeed, it is possible to deliver 5000-6000 rads of therapeutic radiation over a relatively small body area over a five or six week period, with only  moderate or negligible systemic effects.  Regional doses for diagnostic radiology are available in tables published in various sources.  These figures, given in rads (r) or millirads (mr), can only be considered as representative examples because actual figures vary depending upon the equipment used, the varying screen/film/grid factors utilized, patient size, etc.  Also important for accurate determination of regional exposure is a reference to the particular body part which is being studied; that is, how many mr (considering all of the above variables) would be absorbed by the male gonads…or female gonads…or the bone marrow. 

Taking all of these enormous variables into account, we find a few general figures which can be useful for comparative purposes.  These figures are estimates of typical REGIONAL dose in rads for SKIN ENTRANCE EXPOSURE (typical equipment, 400-speed film/screen imaging):

(Note that “r” is used in these examples to represent the amount of radiation dosage to the skin during the production of the above studies.  To be more specific, one Roentgen of exposure would produce one rad of skin entrance dosage, were it not for the intervening air space causing a slight loss of energy.) 

Dose to specific organs would, of course, be less than the skin entrance dose, because the organs are obviously deep to the skin.  For example, while the skin entrance dose for a 2-view thoracic study is approximately 1/2 rad, the male gonad dose for that same study is only approximately 1/10 of one millirad! 

As a comparative figure, we routinely encounter what is known as “background radiation.”  We all receive approximately 0.04 rems of whole body radiation dose from cosmic rays every year.  In addition we receive another approximately 0.06 rems from terrestrial sources; that is, land, rocks, etc.  Then there is another approximately 0.025 rems from our food, water, and air.  This means that the average natural background radiation is approximately 0.125 rems per year, varying from approximately 0.1 - 0.4 rems, (depending upon where each of us lives), simply because we exist in this world. That compares approximately with having a chest study performed every year.  If one lives in Denver, he receives more background radiation, because of the elevation, than if one lives at sea level in Seattle. 

(Note that the term “rem” is used in the above examples, in order to cover the other forms of ionizing radiation which we routinely encounter, which are not x-ray, but which can have biologic effect on living tissue.) 

Before you become too worried about all of this, consider that there is no body of knowledge linking routine G-I studies to any transient or long-term symptoms or measurable functional defect, and this dosage is approximately three to four times greater than that required even for one of our biggest plain film studies, a 5-view lumbar study.  This places our routine diagnostic skeletal studies into a very safe range. 

The effect of ionizing radiation depends on the number of individual atoms which are altered by the dosage.  In the final analysis, there is theoretically no dosage of ionizing radiation which is low enough so that it causes no damage whatsoever, at the atomic level.  We know, however, that a vast number of atoms can be altered without any effect being discernible by any known testing method.  An astronomical number of whole cells die normally every day, and the body is designed to maintain itself and replace cells that are damaged or live their full life span.

Just for your general amazement:

This balancing act is what the doctor considers when he/she orders a radiologic study.  We know that a well-produced diagnostic radiologic study, utilizing optimum radiation protection techniques, is a negligible health risk, at most - more theoretical than real - and that it is well justified in the proper investigation of health complaints because its potential benefits far outweigh the minimal negative effects on the body. 

Every once in awhile doctors and technologists want to know how much dosage of x-ray was administered to a certain patient.  As previously stated, there are tables available which provide averages for every standard type of diagnostic x-ray study.  This is only an average, however, because individual x-ray tubes emit varying amounts of energy; and patient size, varying screen/film/grid combinations, etc, can bring figures out of the “average” range.  The values derived from tables are perfectly adequate for general purposes, however, and are usually the only information which is available in any given case. 

It is possible for an individual x-ray facility to be checked so that you will know what the output of your specific machine is.  This test is performed by radiation control specialists, and it involves the use of a professional dosimetry unit.  This procedure cannot be accomplished in a brief routine inspection, but requires an extended period of time on the part of the radiation control examiner.  It is not really necessary for practical purposes to do this simply for the purpose of checking tube output, but it may be of interest in determining the adequacy of lead shielding in the walls and operator’s booth. 

When tube output is measured by the radiation control examiner, the results are given in units of mR/mAs.  This indicates how many milliroentgens (mR) of exposure are actually produced for every mAs selected at the control unit.  For example, if you set your machine utilizing the 200 mA station and the 1-second time setting, you know that you would end up with 200 mAs.  The question is, how many mR of exposure does your machine produce for this 200 mAs?  To make matters more complicated, the mR/mAs varies depending on the kV setting!   

If all of these measurements seem meaningless, stop and consider this:  if the 200mA station were used at a time setting of 1 second, it would result in 200mAs.  If the 200 mA station were utilized for a setting of 1 minute (which, fortunately, never happens!), it would result in 12,000 mAs.  If the 200 mA station were utilized at a setting of 1 hour (heaven forbid!), it would result in 720,000 mAs.  In an average operator’s booth that has lead shielding constructed according to regulations, this mythical 720,000 mAs exposure would result in an exposure of only about 4 milliroentgens for the operator standing in the booth - and certainly there is never any diagnostic x-ray technique coming anywhere close to a 1 hour, or even a 1 minute exposure!  Most x-ray exposures are measured in fractions of seconds.  A 1-second exposure is a long exposure.  In rare cases, up to 2 seconds may be used for a very large/dense part.  

In a far more practical vein let us consider an example closer to reality.  If an AP thoracic spine radiograph were produced at 200mA at 1/10 second (20mAs) at 85 kV, and a lateral thoracic film were produced at 200mA at 2/10 second (40mAs) at 85 kV, the combined mAs would be 60.  Whereas, in the outrageous example in the previous paragraph, the operator stood behind his leaded booth for a mythical 1 hour exposure at 720,000 mAs, and received only 4mR doses to himself, the mere 60 mAs exposure resulting from this routine thoracic spine study would provide a protected operator an exposure of 1/12,000 that amount, or 0.00033 mR (thirty-three ten thousandths of one one-thousandth of a Roentgen!).  The important point to grasp is that it is not possible to even measure any mr dose to an operator for routine diagnostic exposures, if all the rules are followed. 

There is, however, a rapid increase in radiation dosage that can be measured beyond the edge of the operator’s booth.  This dosage vastly increases the more the collimator leaves are opened.  All of this is designed to make us understand how it is in our best interest to control the ionizing radiation which we are privileged to utilize.  Lead shielding is designed for an important purpose.  Something seemingly simple like forgetting to shut a leaded door to the x-ray room can increase the radiation in the hallway by a huge amount (measured at an approximate 80-fold increase in one facility!) 

This leads us to the use of personnel dosimetry badges.  The fact is that probably very few x-ray technicians in private offices fall into the category in which dosimetry service is legally required.  The rule is this:  dosimetry service is legally required if there is a chance that the operator could receive 1/10 of the “maximum permissible dose.” 

The maximum permissible dose (MPD) is 5 rems per year, which, divided into quarters equals 1250 mrems per quarter.  Dosimetry is legally required if the operator could receive 1/10 of this amount.  That means that an individual must utilize dosimetry service if he/she is likely to receive a 125 mrem exposure per quarter. 

In a private office setting, if a radiographer consistently stands behind a well-leaded operator’s booth, he/she cannot possibly come close to receiving that kind of exposure in the normal occupational setting.  Poorly constructed facilities, built to barely meet the state and federal regulations, coupled with haphazard radiation protection procedures, can very well result in a radiation exposure reading on a badge.  Private office facilities which are well constructed and well operated consistently receive negative reports (zero exposure), when they utilize a dosimetry service, so the service is superfluous; however, there are some doctors and technologists who like to continue to utilize the dosimetry service simply to verify that their readings are consistently negative.  In contrast to the private office setting, however, certain procedures in radiology labs and hospitals do result in exposure to technicians, and dosimetry badges are therefore vital to monitor and limit exposure. 

Can a pregnant technologist continue to perform her x-ray duties?  The answer is a qualified “yes.”  This assumes that:

With all of these safeguards, it would be virtually impossible for any radiation to create a measurable skin dosage, let alone a significant dosage to the uterus. 

It must be remembered, however, that many facilities are not fully leaded.  Often, minimal lead shielding is utilized which meets legal requirements but which does not fully prevent the transmission of scatter radiation.  The amount of legally required shielding depends on the exposure settings of the radiographs which are produced, the radiographic workload during the week, distance from other personnel work areas, the types of use expected from surrounding areas, and the construction materials of the office.  This evaluation would have to be made by a qualified radiation control specialist because the doctor or technologist would likely have no way of knowing the precise but varying rules for each given situation or measuring whether or not there was any transmission of scatter radiation.

See further data on radiation dose. (PDF)

Dose comparison to other risks (PDF)