Science Questions

How many medical x-rays are safe?

Sun, 29th Nov 2009

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Steve, Little Walden asked:

How much radiation are you exposed to during a medical x-ray? How does that compare to the dosage levels radiation workers are allowed to receive?


We put this to Phil Clarke and Stuart Yates:

Phil -   Hi, I’m Phil Clark from the Particle Physics group in Edinburgh University.  And first of all, when you’re discussing radiation dosage, it often gets quite complex due to the different ways to measure radiation and there’s often an abundance of different units like, rems, grays, sieverts, Röntgens, Becquerels, Curies and so on so that can confuse things so much.  But the important unit of measurement is what known as the Gray (Gy) and that’s the unit of absorbed dose.

RoentgenIt corresponds to one joule of energy absorbed by a kilogram of material.  Now the different types of radiation like alpha, beta and gamma decays, result in different biological effects.  So what you do is you have to take the grey number and multiply it by what’s often called the Q factor and an example would be for x-rays and electrons, the Q value would be one.  So, if you multiply those two together, you get what’s known as the dose equivalent and the scientific measurement for that is a sievert.

And one sievert is actually quite a large value so you typically measure in millisieverts, so thousands of sieverts.  Now a typical standard chest x-ray produces about 0.1 millisievert and the dosage that are recommended for people working at CERN or the maximum dose is about 6 millisieverts.  And if you’re a radiation worker it goes up to about 20, or if you’re an airline staff member, the usual measurement is 5 millisieverts.  So the amount of radiation you get from an x-ray is actually quite small.

Diana -   That’s the physics of x-ray doses but what about the different types of x-ray scans?

Stuart -   My name’s Stuart Yates and I’m a radiation protection advisor working at Addenbrookes Hospital.  Well, you get a very wide range of different x-rays giving different amounts of radiation dose, but taking a typical example chest x-ray, its’ very common lots of people might be referred to by the GP or hospital doctor.

And a typical x-ray gives you about the same amount of radiation dosage you’d get in three or four days from natural sources of radiation in the environment and also natural radioactivity in food that we eat, for example, Brazil nuts contain radium and so they’re slightly radioactive.  And so typically, a chest x-ray is about the same as eating three or four bags of Brazil nuts in terms of radiation dose.

The CT scans I think can give you more radiation dose or your equivalent perhaps to a few years of natural radiation but then the benefit is also that much greater because the doctors will get that much more information and so one of the key things in all x-rays is that, you will only get that x-ray if the benefit outweighs the risk.

Because radiation comes naturally from cosmic rays from outer space we’re actually quite well-protected at ground level from that radiation because of absorption in the atmosphere.  But when we fly, we’re less protected because we’re higher up in the atmosphere and so typically you’d get the same amount of radiation dose from a chest x-ray as you would from say, a return flight to Southern Europe.

Diana -   So, a simple chest x-ray will give you 0.1 millisieverts.  That’s the 60th of the dose limit for someone at CERN.  However, a CT scan can give you up to 20 millisieverts of radiation which is four years worth of background radiation and that’s unless you live in some parts of Cornwall where it’s only two years worth because the rocks there emit lots of lovely radioactive radon.


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I think a chest x ray (CXR) is the equivalent radiation dose to 4.5 days of environmental solar abdominal CT, on the other hand, is the equivalent of about 4.5 years, so a significant exposure.

Chris chris, Thu, 26th Nov 2009

I guess you also need to take into account the reason why the person is having the X-ray - if life is that dangerous to you then the risk from an x-ray is probably minor in comparison!

Chris chris, Fri, 27th Nov 2009

Every doctor,chemist and biologist and all owe Physics a lot.
physics and math helping all of fields. ScientificBoyZClub, Fri, 27th Nov 2009

I'm sure the dosage-level and risks for a medical X-ray varies considerably depending on the type of X-ray, and in general a CT-scan (computed tomography) scan will be much higher than for a conventional 'shadow' X-ray.

It's a different situation really though, comparing occupational exposure for health workers against patients, as the patient hopefully only receives a few dozen X-rays in their life - whereas the health worker or other scientific staff who get occupational exposure probably see a much smaller exposure, but on a day-in day-out basis.

A professor from my University who used to work with radioactivity and was continually monitored with a film-badge dosimeter says the only time it came back black (recorded significant dosage) was after he'd been on an international flight. I never quite ascertained whether it had gone through the hand-baggage X-ray scanner, or whether it was indeed blackened by in-flight cosmic-rays...

Here's a pic of a film-badge dosimeter:
Here's another pic showing the insides:
(the white tab is a lightproof wrapping for the photographic film contained inside)  The organisation the person works for will usually send the films off to a lab for processing and assessment every few weeks.

And another pic:
I wore a film-badge just like this when doing my final-year undergrad project and as a PhD student where I did various experiments involving radioactivity. techmind, Sat, 28th Nov 2009

X-ray usage has not always been so tightly controlled ...

]. RD, Sun, 29th Nov 2009

This is a relevant news story to your thread I feel

It seems that, almost universally amongst people that I meet, the impact of physics on medicine is greatly appreciated.

Shibs, Sun, 29th Nov 2009

Quote from the reference provided by Shibs above:

"The public has voted the X-ray machine as the best invention, ahead of the Apollo 10 space capsule and Stephenson's Rocket.

Out of nearly 50,000 votes cast, one in five people named it for having made the greatest impact on the past, present and future.

Ten of the most significant objects in science, engineering, technology and medicine were selected for the vote."

I was actually very surprised by this result because, despite the obvious benefits to modern health provision that x-rays have brought, the actual number of lives saved, relative to other interventions like antimicrobials - or even just plain old hand-washing, is tiny.

I think people are swayed by the very visual nature of the x-ray and the fact that because you can see inside someone that must be good. But the reality is that most people die of things that are far too small to see with an x-ray - bacterial and viral infections and arterial obstructions. Cancers, I grant you, you can see on x-ray, but by the time they are sufficiently big to be visible they are usually incurable.

So sorry radiologists, my vote therefore was for penicillin... chris, Mon, 30th Nov 2009

Ok, so your vote was for penicillum, hum? Fine, when you get abdominal pain and think you have appendicitis, I think you should skip the cat scan and ask for penicillen and go home. See what happens. When you need a CT, it is the BEST inventin of the world, we do not miss appendicitis anymore and the lawyers are mad. They are hoping to scare the docs with radiation worries, WHO CARES about radiaition if yo need to know about appendicitis? Just skip your flight to hawaii for the next three years and turn off your TV for 500 hours. Yes, so remember, you get penicillin, and no cat scan. Fine. Put that in your last will and testament. Real guy, Thu, 7th Jan 2010

I have always wondered why the radiation goes up as you fly. Could it be that there is an accumulation of radiation in the atmosphere because of atomic testing in Nevada , Muroara , and all those other places ? Also I always wondered why there is a slightly higher radiation count in the Rockies ? Could it be that there was open air boxcars with yellow cake going to a nuclear processing plant ? lumos, Thu, 18th Nov 2010

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