PET/CT Gamma Ray Protection with Lead Apron

[EBPoncho]

I was informed I might get a better response by posting to this forum, so sorry for the double post.

Hello all,
I am currently attending a service course for a PET/CT system for my employer. We have been covering the physics behind the function of detectors and my instructor said something that I totally disagreed with and the rest of the class looked at me like I was just dumb. It had to do with why PET/CT techs do not wear lead aprons.

You probably already know the process of PET, but it helps me learn if I can explain in detail.

In positron emission tomography a radioactive isotope is combined with a substance that would interact with some biologic process. During the decay of the isotope a positron is emitted and interacts with an electron, resulting in annihilation and the perpendicular emission of gamma rays eventually interacting with the crystals mounted on the pick up tubes, translating into visible light and being measured.

The instructor said that PET techs do not wear lead aprons because the lead apron would slow down the gamma rays and allow the photons to be absorbed by cells of the human body. Essentially the instructor was saying that wearing a lead apron in a PET room is more dangerous than not.

I totally disagreed and he began to try and explain I was wrong.

I disagreed because I was under the impression that photons of any wavelength or frequency never change speed, they can change direction through interactions with atoms or be completely absorbed and expelled, but photons can surely not be slowed down, it may appear they slow down, but it is merely absorption and expulsion of the photon in several directions that make the photon appear to slow down, but they physically are not able to slow down.

Once I conveyed my message clear enough, the instructor said, well the photons impart energy into the lead apron, transferring energy to the atoms, causing the gamma rays to be of less strength, thus being able to be easier absorbed by the human body. This explanation made more sense to me, but I am still skeptical of this explanation, as I believe some lead is better than no lead, but really this explanation only made sense if we were discussing a charged particle rather than an electromagnetic wave. This happens in the generation of x-rays when electrons are attracted to an anode of an xray tube, the electrons interact in the proximity of a nucleus, altering its path and expelling a softer photon that results in the xray generation profile, but I don't see how an electromagnetic wave can be directed without actually exciting an electron or nucleus.

The argument soon became an argument about whether you can slow a photon down, and I ended up arguing against my co-workers and lab partners. I said lead aprons are negligible protection from the powerful gamma rays produced but they didn't cause the radiation to somehow become more dangerous. One student went so far to say that photon speed has been shown scientifically to slow down, but all the explanations I have read state the appearance of a slower photon, but really the path is being altered not the photon speed. The student then said "when you are driving a car and you turn on your headlights, now that light is traveling faster that it normally would". I immediately thought to ask if he was a creationist, because I know Einsteins Theory of Relativity states that light will always travel at C from any relative observation, it's time dilation that occurs at increasing speeds, but that's beside the point of the previous argument but I thought I would share that because it really got my goat after he said Einstein's theory of relativity has been proven wrong!

Are my physics wrong? Can a lead apron cause gamma rays to become more dangerous to a tech wearing it?

 

[Vanadium 50]

It certainly won't help. A thin lead apron won't absorb much radiation - it will take a gamma ray and it will Compton scatter giving you a lower energy gamma ray and an electron, such that the total energy is more or less unchanged.

Will it hurt? Maybe a tiny bit in theory - it's possible that a high energy gamma would go through you, and a lower energy gamma and electron would be absorbed, so you in fact get more dosage. It's also possible that if you were turned away from the source, the apron could produce a backscattered electron, increasing your dose. I don't think these are large effects, and they may not even be measurable.

For dental x-rays, the biggest benefit of the lead apron is that it minimizes patient movement, and thereby reduces the probability than an additional x-ray needs to be taken.

 

[EBPoncho]

It certainly won't help. A thin lead apron won't absorb much radiation - it will take a gamma ray and it will Compton scatter giving you a lower energy gamma ray and an electron, such that the total energy is more or less unchanged.

Will it hurt? Maybe a tiny bit in theory - it's possible that a high energy gamma would go through you, and a lower energy gamma and electron would be absorbed, so you in fact get more dosage. It's also possible that if you were turned away from the source, the apron could produce a backscattered electron, increasing your dose. I don't think these are large effects, and they may not even be measurable.

For dental x-rays, the biggest benefit of the lead apron is that it minimizes patient movement, and thereby reduces the probability than an additional x-ray needs to be taken.

Thank you so much for the response, it makes sense to me now. I got caught up in the photon slowing down argument and lost focus on whether the photon would have a greater probability of interacting with cells. Good to know

 

[mfb]

An effective shield against gamma rays would be too heavy to carry around. The gamma ray to electron conversion can also happen in the human body, I'm not sure if light shielding would be harmful directly, but it would not help either. It would make everything take more time which increases the radiation exposure.

http://webcache.googleusercontent.com/search?q=cache:gGORvNtUuEUJ:medind.nic.in/iaw/t06/i4/iawt06i4p100.pdf+&cd=3&hl=de&ct=clnk&gl=de&client=firefox-b suggesting that technicians receive a dose of up to a few microsievert per patient.

 

[riodice]

I am sure it has changed by now but when I had a PET scan, a few years ago, the technician was using rolling screens they walked behind coated with gold I was told.

 

[geoelectronics]

Positron emission from Flourine-18 , a common PET-CT radioisotope used in USA, yields 2ea 511 keV Annihilation Radiation photons. Regardless of the radioisotope, the 511 is the same or it wouldn't work in a PET based machine.
High energy? Fairly easily shielded. They make lead equivalent underwear and glasses if you don't want to scare the patients.
George Dowell

 

[DEvens]

The dose from such things is not such as to be easily calculated. The effect of a lead apron is likewise not such as to be easily calculated. It might well be a good project for a MSc level student.

The net dose from any given photon depends very strongly on the energy of the photon. This is a complicated relationship, not as simple as more-energy-worse. At different energy a photon will have different probability of various interactions. It can scatter, it can be absorbed, it can kick an electron out of a molecule.

The primary mode of damage to tissue is breaking apart molecules. This usually requires some minimum energy. The buzz phrase is "ionizing radiation." If a molecule is broken apart it won't be doing what it was supposed to be doing, and might start doing something harmful. Either is bad for the cell it happens in. Cells have some ability to recover from such damage, but eventually enough such damage will kill the cell. And there is also the possibility of long term issues such as inducing cancer.

A higher energy photon may have more potential to do harm. But it may have a smaller chance of interacting. This is why higher energy photons tend to have more ability to penetrate matter. Though even that isn't a simple relationship. A lower energy photon may have a much larger chance of interacting, but produce a smaller amount of damage per interaction. Which energy photon is most harmful is a question of measurement. There are tables of such things in health physics standard reports published by various agencies. These have been validated by measurements on living organisms, frequently rats.

Any given photon that scatters can give up some portion of its energy. It goes right back to your high school physics class and collisions. A photon can scatter in such a way as to impart a portion of its energy to the charge it scatters off. It still has the same speed but now it has got a different, lower, energy.

To calculate the net effect of a lead apron would require some careful work with some good radiation software. The one I have the most experience with is MCNP. And even using such a code would have a certain amount of judgement and assumptions involved. Where does the tech stand? What equipment is in there? How long is the exposure? What concentration of what isotope is in the patient? And several other things. And once the calculation was completed there would be a round of validation against careful measurements on some kind of physical model. And even then, there would likely be some engineering judgement involved.

 

[gleem]

Typical PET facilities are designed to protect the personnel with stationary shielding thick enough to reduce levels to less than 10% of their maximum permissible dose. Using proper design and operational procedures the typical personnel dose as determined with thermoluminescent dosimeters is about the level of the background radiation of about 2 mSv/yr. Lead aprons, glasses or thyroid collars typically worn in radiology are equivalent to 0.5mm of lead and would only absorb about 10% of the 511keV gammas. When not behind a stationary shield maintaining as large a distance and with the shortest time from the patient consistent with the task is SOP.

screens they walked behind coated with gold I was told.

Ree-a-lly? Gold is more than 10000 times more expensive than lead and offers little enhancement for the absorption of gamma rays.

 

[geoelectronics]

"I disagreed because I was under the impression that photons of any wavelength or frequency never change speed, they can change direction through interactions with atoms or be completely absorbed and expelled, but photons can surely not be slowed down, it may appear they slow down, but it is merely absorption and expulsion of the photon in several directions that make the photon appear to slow down, but they physically are not able to slow down. "

Well, yes and no. Photons travel at c= the speed of light. However is it is traveling in a medium that has a refractive index of more than 1, the speed of light in that medium does slow down. A lot. But what you mean it does not go slower than the peed of light. That's absolutely correct. It does however interact with matter in different ways, and each interaction causes it to "cool off", in that it lowers its EM frequency. Photons are created with the maximum energy they will ever have and can only loose energy. But speed will always = c.

"The instructor said that PET techs do not wear lead aprons because the lead apron would slow down the gamma rays and allow the photons to be absorbed by cells of the human body. Essentially the instructor was saying that wearing a lead apron in a PET room is more dangerous than not. "

Ask him what about the Rays that have already been backscattered off objects and are aready reduced in energy,why not shield against those? The fact is, the more tissue a ray penetrates, the more ionization is done. Rays don't have to be absorbed to ionize. Ask him to get out his half-value and tenth-value shielding charts and show them to you.

"Are my physics wrong? Can a lead apron cause gamma rays to become more dangerous to a tech wearing it? "
To paraphrase your question: are lower energy rays more apt to ionize more than higher energy ones...This fear probably goes back to the early days of unfiltered X-Ray generators and the "burning rays" caused by the unneeded lowest energy X-Rays. The actually did not penetrate far at all, added nothing good to the image, and were indeed absorbed by the skin and did cause burns. Some simple aluminum filters on the generator eliminated those.

This is where physics and medicine diverge. Physics pretty much knows what's going on at the atomic level, while biological interactions are not precise enough tn low levels to measure in a one-off situation.

Get your own PPE.

George Dowell

 

[Tom.G]

Here is some info found at: https://www.sciencedirect.com/topics/engineering/positrons

A quick read indicates that:
Since a Positron is considered a particle, similar to an Electron but opposite charge, it seems somewhat reasonable that it can have a speed. Some treat a Positron equivalently as a wave packet whose energy decreases rather than its speed. In either view, when it annihilates by hitting an Electron, there are two Gamma Rays emitted.

Note this statement from the below quote which supports your instructors comment.
When the energy of the positron is close to zero, the probability of interaction with an electron is highest.

PET, Theory
T.J. Spinks, in Encyclopedia of Spectroscopy and Spectrometry, 1999
Positron annihilation
Positrons are emitted from nuclei of a given isotope with a range of energies up to a characteristic maximum ‘end point’ energy Emax (Table 2), the mean energy being roughly one-third E max. Positrons lose their energy by Coulomb interactions with atomic electrons, following a tortuous path until they are brought to rest within a precisely defined range (dependent on their energy and the effective atomic number of the medium). The ranges for mean and maximum energies in soft tissue are given in Table 2. When the energy of the positron is close to zero, the probability of interaction with an electron is highest. From direct interaction or after the formation of a transient system with an electron known as positronium, two photons, each of energy 511 keV (the rest mass energy of the electron or positron), are emitted in opposite directions with the disappearance (annihilation) of both particles. The ‘back-to-back’ photon emission arises from the conservation of momentum. However, there is only precisely 180° between the photon directions if the net momentum is zero at annihilation. The small residual momentum of the positronium system leads to an angular spread of about ± 0.3°. Positron range and the angular spread determine the physical limits of spatial resolution in PET.
Table 2. Positron ranges in soft tissue for the principal positron emitters

Positron emitter	Positron energy (MeV)	Positron range in soft tissue (mm)	Contribution to resolution (mm FWHM)
Maximum(Emax)	Mean	Maximum	Mean
18F	0.635	0.250	2.6	0.61	0.2
11C	0.970	0.386	4.2	1.23	0.3
13N	1.200	0.491	5.4	1.73	0.4
15O	1.740	0.735	8.4	2.97	1.2

Cheers,
Tom

 

[geoelectronics]

Tom, the positron must be at rest to annihilate in the normal low energy sense. There is a high energy mode too but does not apply here.

The positron never leaves the patient, rather it annihilates inside the patient yielding Annihilation Radiation (like X-Rays) photons. It's the photons (Rays) the OP is talking about.

Hope that clears it up.

George Dowell