So let?s discuss ionizing radiation a little bit.
Whenever we discuss the field of radiology
it?s important to have at least a basic understanding of what ionizing radiation is
and what the effects of radiation are.
So radiation is the emission of energy in the form of waves.
Electromagnetic radiation is the type that?s used in radiography and CT scans.
And it can actually be harmful when it?s used in excess
so it?s important to remember that when you?re ordering CT scans on a patient
or even when you are the one performing them.
Radiation can go in multiple different directions.
There can be transmitted radiation
which is the amount of radiation that actually goes through the detector
and hits the detector to create an image.
There is absorbed radiation which is the amount of radiation that interacts
with the patient?s tissues and that is measured in units which are called Gray units.
And the scatter radiation, which is the amount of radiation that?s deflected
to a different direction and it?s neither absorbed nor is it transmitted.
So, if you?re a bystander in a patient who is having a CT scan,
you are susceptible to the scatter radiation which is bouncing off of the patient.
There are multiple different sources of radiation; imaging is just one of them.
Imaging constitutes about 50% of radiation these days, somewhat worldwide,
but usually in countries that use CT scans more often.
Other types of radiation include cosmic radiation
which comes from outer space.
Radiation can also come from radioactive material found within the soil
and Radon is also an important source of radiation.
So what are some biological effects of radiation?
It can cause molecular damage and it can create free radicals within the body
and this is one of the reasons why it can be hurtful.
It also results in disruption of normal cellular metabolic function and mitosis.
And because of these it can have multiple different effects on the human body.
So there are deterministic effects and there are what are called stochastic effects.
So deterministic effects are effects that occur at very high doses of radiation.
It results in cell killing including skin erythema, cataracts, and sterility.
And this really only occurs above a certain threshold,
so with deterministic effects, you don?t have any effect at all
until you reach a certain threshold of radiation
and then all of a sudden you have the effects of cell killing.
Stochastic effects on the other hand are dose independent.
They include carcinogenesis and genetic damage
and if the dose increases the probability of a stochastic effect increases,
so there is no threshold the way a deterministic effect has.
So as you increase dose and as you do more and more CT scans, let?s say,
the probability of a stochastic effect will increase.
Most susceptible to the effects of radiation are the bone marrow, colon, lung, and stomach.
With moderate effects on the bladder, breast, liver, esophagus, and thyroid
and the effects that these organs have are really induction of cancer.
Children are obviously the most susceptible,
they have the most stem cells and stem cells are very susceptible to radiation.
So when imaging a child, it?s always very, very, important to be careful
as to the amount of radiation that you provide.
So let?s take a look at fetal risk of radiation.
If a child is very susceptible to radiation, a fetus is actually even more so.
So fetal risk of radiation actually depends on the days after conception
that the fetus encounters the radiation.
So within the first 1 to 10 days, the fetus has the highest risk of radiation
and it really could result in fetal demise.
And this is all when a fetus receives a dose of about 10mGy or more.
About 20 to 40 days after conception, the fetus can have congenital anomalies
which can present after birth.
At about 50 to 70 days, the radiation can result in microcephaly.
Further out, in about 70 to 150 days, it can lead to growth and mental retardation
and again, these are all things that may or may not occur
and you may not know until they?re going to occur until years after the child is born.
Greater than 150 days after conception, it can result in childhood malignancies.
So it?s very important to remember this chart and to know that in a pregnant female
you really don?t wanna be doing any kind of study that results in risk of radiation
unless you absolutely have to.
You really have to weigh the pros and cons
and to see whether or not this patient really needs the imaging study.
So again, risk of radiation really depends on the level of gestation.
So how can we protect against radiation?
There?s something known as ALARA which stands for As Low As Reasonably Achievable.
You wanna minimize the amount of imaging whenever possible
and you wanna minimize imaging doses whenever possible.
So CT scans can be performed in a variety of different ways.
When you?re performing a CT scan on a child,
it?s always very reasonable to lower the dose
so that the child receives less radiation.
You have to keep in mind however that when you lower the dose of an imaging exam,
you?re also lowering the sensitivity of that exam.
You want exposed personnel to be monitored by a film badge
so all radiologist and all technologist that work in the field always wear a film badge
and that shows them how much radiation that person is receiving.
Lead shielding is always used and you wanna increase your distance from the source.
So as we know, scatter radiation is always present
and you wanna be as far away from that source of scatter as possible
especially when you?re someone that works in the field and you?re exposed to this day to day.
Rooms are now designed with the shielding in place to help prevent radiation exposure.
So let?s take a look at the differences in radiography and CT.
In terms of the mechanism of acquisition, both use ionizing radiation.
CTs are actually a lot more expensive than a radiograph
and they take a few seconds longer to perform.
CTs are not portable, the patient has to go into the CT gantry while radiographs are portable.
So in a patient that?s not able to move around,
radiographs are very good with imaging.
Radiographs take just a few seconds,
CTs take a little bit longer than that but really no longer than about a minute or so.
And radiographs are performed without the administration of intravenous contrast.
CT scans may or may not need intravenous contrast,
so in a patient that has a contraindication to contrast this is also something to keep in mind.
It?s important to remember in terms of the radiation
that CTs actually have a lot higher radiation than a radiograph does.