Self-made RF receiver coil is hard to connect and match to clinical MR scanner due to many reasons.
Can I do the workaround? I use manufacturer`s coil,e.g. small disc-shape receiver coil and connect my self-made coil to it via inductive coupling, say by conducting loop (connected with my coil) pressed to the manufacturer coil windinds? How many losses (mismatch, inductive losses, etc) do I get? Are the similar articles, attemps? Thank you for nuts and bolts!
Some time ago I focused on the home built NMR probe which I designed and connected to an NMR Varian spectrometer... Since the entire RF power has to be delivered in both directions, i.e. spectrometer --> probe, and probe --> spectrometer, there was necessity for building a probe (i.e. an electrical resonance circuit) while tuning it to a desired frequency and matching it to the impedance standard (generally 50 Ohm). Thus, its design was about minimizing energy looses. From the point of view of electronics, there are certain parameters, the so called S-parameters or scattering matrix parameters, which describe how significant the mismatch between RF power emitter and receiver really is. Both the MR spectrometer and MR probe should be concerned as emitter and receiver interchangeably, as the RF power has to delivered in both directions - i.e. for sample irradiation and acquiring the signal.
Summarizing the short introduction above, any new home built coil, if replaced with a "standard" coil, should be equipped with a dedicated electric circuit (also due to its individual electrical frequency characteristic). Moreover, any additional wires change the S-parameters of the circuit, thus changing resonance curcuit conditions and leading to very probable noticeable increase in reflected power and energy looses.
Of course it can funnily enough happen, that the coil you are going to replace with those being primarily used, has a similar or just the same electrical characteristic in the frequency range, which you consider desirable for your experiments. Naturally, in such case your RF spectrometer would be working properly and being ready to use, but in my opinion it seems rarely and unlikely to happen.
If you are interested in the S-parameters formalism and the home built MR probe, which I built and tested, have a look please at my paper: "Efficient design of multi-tuned transmission line NMR probes: The electrical engineering approach" (J. Frydel et. al., SSNMR Feb. 2011)
Article Efficient design of multituned transmission line NMR probes:...
1. you do not answer the question.
2. Suppose separation of transmit (body) and my receiver RF coil. Remember clinical MRI. The receiver coil parallel circuit characteristics are quite simple - near frequency (MR scanner tunes it fine) and and non so bag quality factor (human sample gives many losses). So I do not see any jokes.
1. My answer included a suggestion for an alternative solution, which in my opinion should be much more efficient in general case. I just contributed the knowledge to nuts and bolts, but if you feel that your idea is far away from my suggestion, just ignore my alternative solution.
2. I think we are looking at the "characteristic" itself from two different points of view... An experimental frequency characteristic of a coil measured by a vector analyzer appears "quite surprising" (in the contrary to the theoretical approach for ideal coils), because of negative imaginary part of its impedance at a few frequencies. The reason for that could be explained (among others) by the presence of electrical capacity which occurs between particular windings of the coil. Therefore, in my description I meant by frequency characteristic the values of S-parameters. The circuit characteristic could be understood also in a different way - that is true, but for me the values of S-parameters are just reliable and very helpful at designing electrical circuits. Besides, I am in fact much more experienced in designing of the instrumentation for NMR spectroscopy rather than for clinical MRI. I do not know the exact scanner model which you are using, too, thus can not guess technical aspects of your scanner. However, an additional circuit working in the way you described should lead in my opinion to additional energy losses and signal distortion, even when applied to MRI. Having said that I do not advise you against the idea - just wishing you good luck with your attempts!
What are losses in usual transformer? I think to make a usual transformer from company and my coil (or the part). How could I minimize losses? Magnetic core is possible (I will fix it firmly and it will be remote from the imaged organ)?
According to my knowledge, commercially available transformers work at 50-60 Hz and 250 V with efficiency of 60-95%. I have no idea whether they could function at frequencies in the range of MHz and much higher voltage - it probably depends on a model, nevertheless it will be hard to find a proper one. Transformer efficiency in such case is not easy to estimate due to many factors - additional energy losses will originate from skin effect, eddy currents, home made electric connections, etc., which for sure are going to play a significant role at higher frequencies - their influence should be initially calculated or simulated yet before the first usage. Besides, I am not sure about flashover - it has to be considered due to an electric shock hazard, too - it is a crucial factor when a human body is placed into the MR scanner. Therefore, without careful calculations I personally would not test it at all, and definitely not on humans. In order to minimize the losses I would tend towards designing a dedicated circuit terminated with a coil, while tuned to the desired frequency and matched to (the most probably) 50 Ohm standard, capable to get directly connected with the MR scanner. It is however not a trivial task.
Yes this is quite doable. You make a secondary coil that is inductively coupled. The mutual inductive splitting can be handled in a couple of ways. The best is to use one lobe as your resonant detector. Most of this was worked out a number of years ago but has not been developed in the last 10 years. See the attached paper from Zhou et al in 1994.
https://www.ncbi.nlm.nih.gov/pubmed/8057814
Good luck with your project.
Doug Morris
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