Building an NMR machine - Page 1
Forum section: electronics / projects

author: mriPaul
author_rank: Contributor
timestamp: December 19, 2016, 09:25:46 pm
content: I saw a previous string about MRI systems. I want to try something a little more simple. I want to build an NMR system to detect trace amounts of ferromagnetic materials in tissue and plastics. My arsenal includes a few ESR magnets and a high inductance non cooled magnetizer that looks like an ESR magnet we used to magnetize neodymium magnet material. Also my equipment includes o-scopes, wave generators and boxes of electronic components. I think I can get about a 1-2 tesla field. Has anyone tried any of the diy systems on the net with success? If so, what sensitivity should I expect?

author: mmagin
author_rank: Frequent Contributor
timestamp: December 19, 2016, 11:33:18 pm
content: Are you sure this isn't what you want instead?https://en.wikipedia.org/wiki/Ferromagnetic_resonance

author: mriPaul
author_rank: Contributor
timestamp: December 20, 2016, 12:25:46 am
content: Any form of simple detection would be good. Something easy to build would be better. I was hoping I could apply a field and pulse the field with an SCR and have a ringing circuit as a detector. I saw an MRI and metallic particles caused white spots on the image. The signal from the ferro-metal looked very large.

author: mriPaul
author_rank: Contributor
timestamp: December 20, 2016, 02:38:42 pm
content: Thank you evb149. I have one method that has worked in some situations. I call it a force induction magnetometer. The subject (~1 cubic inch)is suspended by a string. A capacitor bank and an SCR dump current into a coil. I look for minute deflection/rotation of the subject as a pass/fail system. Another similar method is a NdFeB hockey puck sized magnet spinning on an axis parallel to the field vector of the magnet. A thin polycarbonate sheet is used to block wind currents. Both of these methods have missed metal particles. We also have hand sized plastic plates that are too massive for this method if a particle is near center.Another method I am considering is the is the pulse induction metal detector. I could compare the driving pulse to the eddy output with a timing circuit. I think my Tektronix o-scope will output a delta time between the two channels. I am considering a Helmholtz pair as the field is more contained.The goal here is optimization and ease of use. I am looking for help because I do not want to build something and later find out that something else is much better. Here is what we know... I think. The plastic parts are washed and any metal particles are embedded in the plastic. Consider this, a Helmholtz pair with a volume of about 25mm x25nn x 25mm= 15625 mm cubed. A small metal particle (.1mm) with its field deformation in an alternating field would be about 1 cubic mm. Because it is mostly steel, give it a permeability of 100 over air. I am now at about 1 part per 160? (A common metal detector is 1000 times larger in volume or detecting a 1mm particle.) I could design and fabricate a Helmholtz coil with pick-up and CAP to get about a 50ms drop to zero and pulse at 100 Hz, I could spin the plate and raster like a record over a clean plastic turn table. If something like this or better has been built and is open sourced, please let me know.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 20, 2016, 04:29:34 pm
content: If the particles are ferromagnetic one could use a kind af magnetometer. An easy one is by inductive pic-up. So a little like the two coil metal detectors. The principle is a little like an LVDT, just with the sample moving instead of the core and a more sensitive detection. If in doubt, do a few more cycles.No need for Helmholz coils to get a very homogeneous field. Usually it is like one large outer coil to produce an AC field and than two smaller, identical ones inside connected in series but reversed direction. You than compare with the sample in the either one of the detecting coils.25 mm dimensions for the samples is not that big.

author: mriPaul
author_rank: Contributor
timestamp: December 20, 2016, 05:11:54 pm
content: Thank you. Let me see if I get this. I have three concentric coils. The driver in the middle at z = 0 and two pick-ups at say z = +/- a few mm. The pick-ups wound in series in opposite direction to produce no signal or a low signal. When one of the coils "sees" iron or steel, the pairs are un-matched and a signal is seen. Do think they are sensitive enough? My particle size could be <0.1mm. I want to keep the size small for sensitivity and most of the items are <25mm.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 20, 2016, 07:08:18 pm
content: The inductive system is quite sensitive. Normally the method / instrument is known as AC magnetic susceptibility.The system is rather strain forward (AC bridge in the kHz range) and not at all fancy technology. The only more fancy instrument used is an lock-in amplifier / synchronous rectifier.Here a sample link, for what such a system might look like:https://nationalmaglab.org/user-facilities/high-b-t-facility/high-b-t-measurement-techniques/ac-magnetic-susceptibility-in-high-bt

author: mriPaul
author_rank: Contributor
timestamp: December 20, 2016, 07:38:41 pm
content: Thank you very much. I see where the guys at the NHMFL are using a second transformer to boost the signal. I just ordered a simple amplifier kit good for 100KHz. My Arduino is here on my desk. I did some testing at the NHMFL while in graduate school.