as ounces, gallons, and liters measure liquids, and grams and pounds
mass/weight, we also have units to measure amounts of exposure for
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
(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
Roentgen (R) is a measuring unit of radiation exposure. The amount of
radiation which exits from the tube at certain technique factor
measured in Roentgens (R).
(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
or how much radiation is actually absorbed by the part
Another way of thinking of this would be to consider
it the difference
between the amount of radiation which strikes the part being measured
amount of radiation which strikes the bucky surface, the difference
amount of radiation which is absorbed by the intervening body
rad (r), it may have occurred to you that the amount of radiation which
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
body parts would vary markedly. As
you can imagine, the amount of radiation absorbed by the skin would be
than the amount absorbed by the thyroid, or the bone marrow, or the
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
for it to be used as a comparison to other dose measurements.
Rem is not
abbreviated. It stands for
Roentgen-equivalent-man, and it is the unit of exposure which takes
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)
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
produce the same effect in tissue as 1 rad of x-radiation would be 1
LET'S MAKE THIS SIMPLER
||unit of x-ray
||unit of x-ray
absorbed by tissue (must state where)
equivalent (other forms of ionizing radiation)
more point must be reviewed, and that refers to a term you came to
your study of x-ray technique factors. As
you recall, the prefix “milli” (abbreviated with a lower case m)
1/1000th of a unit. mA
thus became the shorthand method of indicating a thousandth of an
We can use this same prefix to indicated thousandths
of Roentgens (mR),
rads (mr), or rems (mrem).
with all of the hard stuff behind us, we can proceed to explore some
representative values to better see this whole subject in
begin with, it is necessary to understand the vast difference between whole
body exposure and regional
of the whole body to ionizing radiation is especially damaging because
no unaffected tissue to carry on body function.
Following are the effects created by varying WHOLE
detectable by chromosome analysis
readily detectable in an individual
likely to produce vomiting in about 10% of people exposed
likely to produce transient disability and obvious blood changes in
majority of people so exposed. Also may produce skin redness
(fatal) dose for a short exposure
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
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
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
examples because actual figures vary depending upon the equipment
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,
mr (considering all of the above variables) would be absorbed by the
gonads…or female gonads…or the bone marrow.
all of these enormous variables into account, we find a few general
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
study with fluoroscopy
study (3-5 v)
|Chest study (2v)
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
entrance dosage, were it not for the intervening air space causing a
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!
a comparative figure, we routinely encounter what is known as
radiation.” We all receive
approximately 0.04 rems of whole body radiation dose from cosmic rays
year. In addition we receive
another approximately 0.06 rems from terrestrial sources; that is,
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,
where each of us lives), simply because we exist in this world. That
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.
that the term “rem” is used in the above examples, in order to cover
other forms of ionizing radiation which we routinely encounter, which
x-ray, but which can have biologic effect on living tissue.)
you become too worried about all of this, consider that there is no
knowledge linking routine G-I studies to any transient or long-term
measurable functional defect, and this dosage is approximately three to
times greater than that required even for one of our biggest plain film
a 5-view lumbar study. This places
our routine diagnostic skeletal studies into a very safe range.
effect of ionizing radiation depends on the number of individual atoms
altered by the dosage. In the final
analysis, there is theoretically no dosage of ionizing radiation which
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
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
cells that are damaged or live their full life span.
Just for your general amazement:
300 million cells in our bodies die every minute
and are immediately replaced so that the number remains relatively
constant throughout adulthood.
28 billion skin cells are lost every day; ½ million every 30 seconds!
balancing act is what the doctor considers when he/she orders a
study. We know that a well-produced
diagnostic radiologic study, utilizing optimum radiation protection
is a negligible health risk, at most - more theoretical than real - and
is well justified in the proper investigation of health complaints
potential benefits far outweigh the minimal negative effects on the
once in awhile doctors and technologists want to know how much dosage
was administered to a certain patient. As
previously stated, there are tables available which provide averages
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
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
any given case.
is possible for an individual x-ray facility to be checked so that you
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
inspection, but requires an extended period of time on the part of the
control examiner. It is not really
necessary for practical purposes to do this simply for the purpose of
tube output, but it may be of interest in determining the adequacy of
shielding in the walls and operator’s booth.
tube output is measured by the radiation control examiner, the results
in units of mR/mAs. This indicates
how many milliroentgens (mR) of exposure are actually produced for
selected at the control unit. For
example, if you set your machine utilizing the 200 mA station and the
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!
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
(heaven forbid!), it would result in 720,000
mAs. In an average
booth that has lead shielding constructed according to regulations,
mythical 720,000 mAs exposure would result in an exposure of only about
milliroentgens for the operator standing in the booth - and certainly
never any diagnostic x-ray technique coming anywhere close to a 1 hour,
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.
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
second (40mAs) at 85 kV, the combined mAs would be 60.
Whereas, in the outrageous example in the previous
operator stood behind his leaded booth for a mythical 1 hour exposure
mAs, and received only 4mR doses to himself, the mere 60 mAs exposure
from this routine thoracic spine study would provide a protected
exposure of 1/12,000 that amount, or 0.00033 mR (thirty-three ten
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.
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
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
at an approximate 80-fold increase in one facility!)
leads us to the use of personnel dosimetry badges.
The fact is that probably very few x-ray technicians
private offices fall into the category in which dosimetry service is
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.”
maximum permissible dose (MPD) is 5 rems per year, which, divided into
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.
a private office setting, if a radiographer consistently stands behind
well-leaded operator’s booth, he/she cannot possibly come close to
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,
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
dosimetry service, so the service is superfluous; however, there are
doctors and technologists who like to continue to utilize the dosimetry
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
and limit exposure.
continue to perform her x-ray duties?
The answer is a qualified “yes.” This
the facility is well leaded and
has been inspected for safety
the operator’s booth is fully
leaded and permits only infinitesimal passage of x-ray.
the pregnant technologist
consistently wears a full leaded apron while she is
performing her radiographic duties
the pregnant technologist does
not spend other time in a room adjacent to the radiographic facility,
which is not protected by an intervening lead barrier.
all of these safeguards, it would be virtually impossible for any
create a measurable skin dosage, let alone a significant dosage to the
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
week, distance from other personnel work areas, the types of use
surrounding areas, and the construction materials of the office.
This evaluation would have to be made by a qualified
specialist because the doctor or technologist would likely have no way
knowing the precise but varying rules for each given situation or
whether or not there was any transmission of scatter radiation.
further data on radiation dose.
Dose comparison to other risks