Years ago I had a stress test for heart issues where I had to walk on a treadmill until my heart rate was high enough then they injected a radioactive tracer while I was on a scanning table of some sort. The doctor explained they ordered a custom made isotope to inject which had a very short half life. It was scheduled to be at the correct emission rate when injected and within a few hours it was harmless. If I had to delay my appointment they could not use it and had to order another sample.
I had a Nuclear Stress test, where I was injected with this tracer. After the test, I went home and thought “I wonder if my Geiger counter is sensitive enough to pick up the residual radiation?” (I have a tiny Geiger counter I made from a kit, something like this).
I turned it on, and it went berserk!
It was clicking so fast, it sounded like a buzz.
I then walked over and set it on a bench and made an iPhone movie of me walking up to it. It started out click…click…click and as I got closer, click.click.click, until when I bent down and picked it up, click click click click click click click…buzzzzzzzzzzzzzzzz
Even the next day, there was quite a significant count above background.
As an aside: Gammas are the hardest kind of radiation to shield against… but for the same reason, it’s the least-dangerous kind of radiation to have actually inside your body. An alpha will interact with anything and everything it encounters, which means your cells if that’s what it encounters first… or a piece of paper, if that’s what it encounters. Betas are much the same, though not quite to the same degree. But a gamma ray, when it encounters matter, will usually just ignore it and fly right on past. So it’ll probably get past your shielding… but it’ll probably also go right out of your body without doing anything to it, too.
I had such a test though it was a bone scan in December 2017. I was given a form to carry with me ass an explanation in case I went through any detectors in the next week. Three weeks later I took a flight and had the form ready, but the TSA never said anything. (I was a bit disappointed actually.)
As part of the diagnosis of a lacrimal sac disorder I had some guy put technetium-99m in my eyes. When they told me about this I asked if they were making it up as they went along, but no; it turns out to be an accepted procedure.
My review of current literature indicates that thallium is still used more commonly than technetium in stress tests currently. Technetium does give better quality gated images, and when used in conjunction with gadolinium, gives less attenuation artifact. But it can be more expensive and more difficult to procure, hence the use of thallium.
We use technetium (Tc99m) routinely for heart scans. We occasionally use Thallium to look for “hibernating myocardium”. We don’t use any PET agents for the heart at our hospital- those have very short half-lives and are usually done at universities.
“Tech” is the workhorse of nuclear medicine for several reasons, including its practical half-life. It has a half-life of 6 hrs, which is long enough to lots of practical imaging, but not so long that it lingers for days in a patient’s system. It is largely gone by 24 hours (4 half-lives).
Thallium (Tl-201) has a half-life of 72 hours.
One consequence of shorter Tc half-life is that a higher dose can be given without increasing patient exposure, which leads to better images, especially with SPECT (which is like nuclear imaging CT). (However, this advantage is mitigated, when one realizes that you can do Thallium stress one day with redistribution the next day from a single injection, while a similar Technetium scan would require two injections. )
