…And now for something completely different!

I was lucky enough to be thought of when a dear friend of mine, who runs an antique and curio business, came across this old piece of medical equipment. This set off her NERD meter and she messaged me asking if I was interested!

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I immediately replied yes and she snapped it up for me for next to nothing…the reason being is this thing, the way those knobs are laid out and that big beautiful meter, just screams

ANTENNA TUNER!!!

But before I consider butchering it and using it for a project I decided to find out a bit more about it…

…and basically ran into a brick wall…

There is very little info on the Internet about the device and definitely no manual!

I actually had to read a couple of scientific papers to work out what the unit actually did, and how it works. This was the only source of information on the unit I could find.

So what is it? – It’s basically a medical radiation detector. specifically it detects radiation from medical isotopes that are injected/ingested/inhaled into the body, that accumulate in a specific area, like the restriction/blockage of blood flow, lymphatic system issues or uptake from tumors/cancer. The patient is probed locally at the site of interest with the large silver hand held scintillation detector that detects the radiation, which shows as a deflection on the meter. It dates from the early mid 70’s when, I guess, it was the early-ish days of nuclear medicine…

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So how does it work? It is certainly an interesting device. It is battery powered, by a couple of PP9 9V batteries in series, to give an 18V supply. There is a battery check area on the meter, this is activated by turning the range switch to the “B” position, just up from the “OFF” position. The PP9 battery is still available from a few different places, I’ve ordered a pair to see if this thing still works. It will be interesting to see if the unit is capable of detecting background radiation. I hope it does.

The probes are also very interesting. They employ a Scintillation Crystal and a Photomultiplier Tube to detect radiation from the patient. It’s a 2 stage process. The 1st stage involves the Scintillation Crystal. This is located at the site where the probe contacts the patient, and detects the radio active particles that are emitted from the patient who has been administered a Radio Isotope. This crystal then emits a pulse of light (Scintillation), usually in the visible spectrum. This pulse of light is then detected by the photomultiplier tube and “multiplied”  – resulting in a pulse of current for each radiation particle detected, which is then passed to the instrument itself, where the result is processed and displayed.

The Photomultiplier tube is pushed hard up against the Scintillation Crystal via spring pressure. It is encased in an Aluminium tube, most likely for protection.

There are 2 probes with this unit, one marked “Probe type 235N”, which has a moveable shroud with what looks to be a ferrite or powdered iron inner ring touching the probe surface, with a long, curly connecting lead, and another marked “Probe 235”, which has a moveable, plain aluminium shroud. I haven’t been able to establish the difference between the two probes.

OK, so how does it ACTUALLY work? – much simplified, here is how i think it works…

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The 18V battery supply is boosted by what looks to be a Cockcroft-Walton voltage multiplier, to a high voltage, most likely variable up to around 3Kv. This is all at very low current, judging by the size of the transformer drive circuit, probably in the uA range. The range switch looks like it selects a number of set voltages. The voltage multiplier board is located in the rear of the unit, seen here. The transformer and drive circuitry are hidden behind the panel with the 3 connectors on it.

This high voltage is fed to the photomultiplier tube via the front panel BNC connector and any detected radiation from the probe results in a current pulse that is measured across an anode load resistor by the metering circuit. This pulse looks to then be fed to an integrator circuit, whose role it is to change the pulses received from the probe to a proportional, steady voltage, (i.e. more pulses more voltage) which is then displayed on the meter. I imagine the fast/slow push buttons would relate to the speed of integration. As an example, if the probe detects little or no radiation, (low or no meter deflection) from an area on the patients body and then more radiation (meter deflection increases) when moved to another part of the body, then that increase for that area would be a concern. A practical example of this (now old!) technology was to use this device after injecting the patient with radio isotope Iodine i-133, to detect blood clots in the legs of patients suspected of suffering with deep vein thrombosis. A meter deflection on a particular area of the leg indicated the clot site.

My limited knowledge of nuclear medicine hasn’t helped in deciphering the Isotope control. I’ve learnt that differing Isotopes are used for differing areas on the body, for example, the Isotope Xe133 (Xenon Gas) position is used when checking the lung and brain. How this transpires to readings from a probe that only has 2 connections for +ve and Gnd  is a mystery. More research showed that other Isotopes on the dial are used for checking other areas like kidneys and heart. I’m sure there’s a perfectly clear explanation out there somewhere. Maybe the different voltages are used with the different isotopes…i.e. 1000V range for Chromium51? Who knows? If anyone can add any insight as to how these units operate I would be forever greatful.

The unit is beautifully made. It has a number of quality circuit boards and the wiring is super neat, and has been laced up. it has a number of gold plated edge connectors. It’s all analog, it’s full of IC operational amplifiers operating on a split rail +/- 9V supply. There is a very nice precision 50uA meter movement that would be a bit better quality than your average. The case is a work of art, I have a number of ex scientific instrument cases from England and all are wonderful!!! Should I turn it into an Antenna tuner?

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We have certainly come a long way from using this type of instrument for medical diagnostics. The clarity and resolution of todays 3D medical imaging machines is mind-boggling!!!

On that note I’d like to take this opportunity to wish all BRL blog readers a happy,  healthy and prosperous 2019.

73

Andy, VK5LA

One thought on “…And now for something completely different!

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