Reverse Engineer An X-Ray Image Sensor

If you think of a medical x-ray, it is likely that you are imagining a photographic plate as its imaging device. Clipped to your tooth by your dentist perhaps, or one of the infamous pictures of the hands of [Thomas Edison]’s assistant [Clarence Madison Dally].

As with the rest of photography, the science of x-ray imaging has benefited from digital technology, and it is now well established that your hospital x-ray is likely to be captured by an electronic imaging device. Indeed these have now been in use for so long that their first generation can even be bought by an experimenter for an affordable sum, and that is what the ever-resourceful [Niklas Fauth] with the assistance of [Jan Henrik], has done. Their Trophy DigiPan digital x-ray image sensor was theirs for around a hundred Euros, and though it’s outdated in medical terms it still has huge potential for the x-ray experimenter.

The write-up is a fascinating journey into the mechanics of an x-ray sensor, with the explanation of how earlier devices such as this one are in fact linear CCD sensors which track across the exposed area behind a scintillator layer in a similar fashion to the optical sensor in a flatbed scanner. The interface is revealed as an RS422 serial port, and the device is discovered to be a standalone unit that does not require any commands to start scanning. On power-up it sends a greyscale image, and a bit of Sigrok examination of the non-standard serial stream was able to reveal it as 12-bit data direct from the sensor. From those beginnings they progressed to an FPGA-based data processor and topped it all off with a very tidy power supply in a laser-cut box.

It’s appreciated that x-rays are a particularly hazardous medium to experiment with, and we note from their videos that they are using some form of shielding. The source is a handheld fluoroscope of the type used in sports medicine that produces a narrow beam. If you remember the discovery of an unexpected GameBoy you will be aware that medical electronics seems to be something of a speciality in those quarters, as do autonomous box carriers.

19 thoughts on “Reverse Engineer An X-Ray Image Sensor

      1. Some oven and microwave bulbs are “hot”, and will briefly emit X-rays when suitably foiled and pulsed with a DC HV from a gas igniter but watch out as the glass isn’t very good at blocking HV and sometimes cracks.
        Turns out that it needs a protection diode in many cases or the accumulated charge will wreck the piezo crystal, anything with about a Vr of >9KV should be enough ie something from a broken photocopier or a bunch of smaller 1N40xx in series heatshrunk.
        If you see a green flash or diffuse glow then you might be lucky here.
        Another method is using a homebuilt multiplier with a multiple reed relay based switch in series to apply pulses rather than continuous current to the tube under test.
        Normal cheap units can tolerate up to maybe 3KV apiece so put enough in your switch assembly to block the HV with about a 50% margin, for a typical 24KV multiplier running off a CCFL driver it would be about 8.
        Pulse driver can be a simple electromagnet and pot the soldered ends in Epoxy to prevent arc-over.

  1. I have access to dental xray equipment here at work and I have to ensure that it works. SO I used my phone one day and left the camera on, covered the sensor with my coat so it was dark. Blasted it with a 1 second pulse from the xray head. Sure enough you see the impulse as a huge spray of random colored pixels that ramp up in intensity then fade away over the pulse duration. This got me thinking that if you modified the CCD you could make an inexpensive detector. Perhaps some software magic, I don’t know. I just know that I could put items in the path and if you played the file back fast enough you could see the outline of shapes etc…

    Just my 2 cents

    1. The CMOS (I can guarantee that your phone is NOT using a CCD sensor :P) sensor in that phone is something like 2x2mm at best, probably even smaller…The x-rays happily ignore the case and lens assembly, so you are only detecting what is going through that tiny 2x2mm spot, no amount of software will fix that :P
      Also – the rest of the electronics in that phone are not radiation hardened, constant exposure to x-rays will significantly reduce their MTBF and make the device less stable.

      Airport x-ray machines and alike use a strip of discrete semiconductor detectors (read: a special and very expensive photodiode) that is the entire with of the conveyor belt (modern ones have well over a 100 individual detectors), these detectors can even do crude energy measurement, so they can detect scattered x-rays, which makes it possible to get more info on the composition of whatever is being scanned.

      1. “Also – the rest of the electronics in that phone are not radiation hardened, constant exposure to x-rays will significantly reduce their MTBF and make the device less stable.”

        As if crossing the border wasn’t pain enough.

      2. Last time I got dental X-rays, I started recording a video with my phone, and held it up on the back side of my head, opposite the source. Even with the audio to capture the “beep” of the machine, I couldn’t see any fuzz on the video when I reviewed it later. I didn’t cover the sensor for darkness, though. I’ll try to remember that next time!

    2. It has been noted on HaD before, that a phone camera can be used as a radiation detector if it has been blocked to prevent light from reaching the sensor. (To clarify “light” I mean IR through UV).

  2. Nice idea but the difference is the sensor chip accumulates charge on each cell for 0.1-10s then reads back.
    If you could (somehow) underclock a cooled CMOS sensor which I’ve done before it does increase its sensitivity.
    All the newer ones are fixed clock unfortunately in order to maintain USB function.
    I did find some IO dental sensors today though which may be repairable.

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