What kind of scanner can detect a hidden chip in someone's body?

Question

So it's the lead-up to what might be World War III and the enemy is kidnapping and implanting mind-control chips into our people, one by one, to turn them into sleeper agents, to all attack en masse when the signal is given. The victims who have been chipped remember the whole ordeal and try and reach out for help, but nobody will believe their crazy-sounding conspiracy babbling. After a while though, when enough people start turning up with the same plight, their claims start to be taken seriously and an investigation begins. The problem is, they can't quite find the chip in anyone!

The obvious/dumb solution, a metal detector, wouldn't work because, this setting being based on the real-world, there's no iron to be detected in the silicon chip wafers or from the precious metal traces on a PCB. And plus, transistors these days are TIIINY! The chip itself may be so small that it would be difficult, if not impossible, for any external scan to detect it, simply due to lack of resolution. The chip can receive commands though, therefore it can be assumed that it it can also reply too, broadcasting detectable (though potentially very weak, possibly intermittent, and on an unknown frequency) electromagnetic waves.

The victims make themselves available to undergo any kind of non-invasive test or process to help locate the chip in their bodies (so no exploratory brain surgery to find it). What kind of scans or other kind of test could have a chance at locating this within one's body, given the size, material composition, and other real-world constraints? CAT scan? MRI? X-ray? Something outside the box? And for the scans that wouldn't work (like the metal detector), bonus points for explaining why.

Answer 1

The hard-science tag is kinda overkill for this one. Please read it's wiki.

1. X-Rays have been successfully used to scan a single atom. There is no chip that can be manufactured today that can't be detected by X-Ray... if you have the right X-ray. But, to be frank, a chip that could do anything more than binary math would be trivially detected by hospital-grade X-rays.

2. An MRI is a metal detector. That's why you can't have any metal in your body or on your person when one is performed. A simple metal detector is using an inductive coil to detect fluctuations in a predictable magnetic field to identify the presence of metals. A Magnetic Resonance Imager is a metal detector on world-class steroids. It would rip ferrous metal out of your body. But it would also trivially detect the presence of non-ferrous metal and, to make a point, anything that doesn't have the expected density of the area being scanned.

3. A computerized tomography scan, better known as a CT or CaT scan, is nothing more than a series of X-ray images and some high quality software. In other words, from a practical perspective, a CT scan is an X-ray.

4. Just to make the list reasonably complete, a sonogram would have all the same consequences and results. It's just using sound waves rather than EM waves.

What all these scanning technologies do (and I'm simplifying), is test for density changes in the body. We know what the "average" or "expected" densities are so we can program software to point at an unexpected change in density and tell the operator, "huh, what's this?" The reason we have skilled analysts is that when all you have to work with is density it's beyond possible to have false positives.

And so what you might want to do is ask yourself, "what materials can I use to create my mind control chip that have densities similar to the area of the body where it's implanted?" What's cool about that question is that you could take advantage of contour changes in the brain where recorded densities are a bit flexible due to the inherent limitations in the technology, like the angles used to take the pictures (and that's one reason why MRIs use a rotating emitter).

In other words, I believe that with some creativity, a believable chip could be expressed in your world that would be difficult, but not impossible to detect, thereby allowing you to maintain an air of mystery to meet the needs of narrative necessity.

BTW: there are two reasons why a simple metal detector (like those used in airports) wouldn't detect the chip:

1. There must be enough metal.

2. Compared to an MRI they're weak. Really weak. A strong enough metal detector would easily detect a mere grain of iron. But it would also be difficult to constrain to the limits of the locale of the detector. In other words, a detector that strong would pull that dude at the end of the conveyor belt who's putting his favorite iron-infused necklace on right back to the detector — probably choking him to death.

Answer 2

Here's what I would do:

I would place several individuals in completely EM shielded areas - nothing in, nothing out. There would be multiple sensors for multiple bands of the EM spectrum and then just wait for an EM signal.

Once the signal is detected, a copy is made Now we have something we know to look for, from there, we make a very sensitive bit of equipment to look for that thing, and then through multiple repetitions, find it.

And by that I mean this: We can make the assumptions that the chip would be located in approximately the same location for all victims. So, we have a group of volunteers who think they've been chipped, we start scanning them, once we get a hit in a specific region, we start scanning that region, once we get a hit in that region of the body, we start scanning an area, once we have a confirmation on an area - then we book the surgery.

So for example - we get a hit off the left arm - that gives us a region. Then we scan the left arm intensely, we get a hit just below the elbow - that gives us an area - then we go in and have a look.

Repeat the process as neccessary N number of times to get closer and closer. Also - once the first one is discovered and we can see what it is/interfaces with, we can then make educated guesses about where it would be on someone else.