MRI versus competition

drgmscn

We are so fond of magnetic resonance that we sometimes forget there are other imaging modalities - some much older than MRI and some that have not yet reached maturity but are lurking behind the corner. I was a bit surprised when Jimmy Atkinson of Diagnostic Medical Sonography Degree informed me that my blog was selected among the top 50 on their list. They are an agency for online sonography imaging-technician degrees, but they evidently keep an eye on MRI. In any case, it is a honor. Thanks, Jimmy.

MR is of course part of a much broader class of imaging physics. I was thinking since some time about establishing a separate list of books, for example, to cover those aspects of imaging physics, technology and informatics which can not be specifically labeled as MR. On my MRI books list I have so far listed such monographs in the related works category, but that is very incomplete. I will start within this month.

I was also thinking these days about novel imaging techniques which are on the horizon and which might develop much faster than expected. They fall in two categories, both presently associated with security imaging rather than high-resolution medical imaging. However, the line between these two areas is very thin and flexible. Give the 'new' technologies two decades of development and they might yet beat MRI. With the present worries about security at mass-transit locations, they will certainly not suffer from lack of funding!

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The first category is that of plain radio scanning based just on bulk electric susceptibility and magnetic permeability variations. What got me thinking was a recent article on RF Globalnet Newsletter about an application of RTI (Radio Tomographic Imaging). I was often thinking about this possibility, though in a different layout: place a person/object into a sphere made of radio-transparent material, dislocate around it M transmitters and N receivers, and then send coded signals from combinations of the transmitters and detect them by all the receivers (the coding would help to eliminate random noise). In nearly linear-response systems, one can empirically pre-calibrate the response functions for every transmitter-receiver pair and for every voxel of the FOV and thus be able to carry out the image reconstruction of any object (it is essentially just a large matrix inversion). With no need of any theory - just an empirical calibration! Anyway, if something like this works today with a resolution of a couple of feet in a room-size area - and using just 1 transmitter and 28 receivers - imagine what it might do in a 2 m sphere covered by a thousand RF transceivers. At current prices of the chip-size devices, the cost would be peanuts, and one would need no magnet!

In the second category are the body scanners they start nowadays deploying at the airports. They are based on sub-millimeter microwaves emitted spontaneously by [warm] human bodies. Such waves are already close to the far infrared region (remember that infrared radiation is already used for imaging, both in outdoors IR surveillance and in medical thermography). Electromagnetic waves of this kind (the terahertz region) penetrate biological tissues to much greater depth then IR and have enough distinctive spectral features to allow considerable degree of chemical discrimination. Terahertz technology was until very recently considered as prohibitively difficult, but that has changed dramatically in recent years - it is now exploding and becoming a real buzzword in communications. So, once again: give this 20 years and where will it be? Probably peeking into our stomachs (if not minds) and producing detailed medical reports even while we leisurely stroll through a clearance room!

This is one potential advantage which the above modalities have over MRI: the responses are extremely fast - of the order of a micro-second at most, compared to nearly one second in magnetic resonance (dictated by the relaxation times). Consequently, such techniques might be potentially capable of taking "instantaneous" shots of a moving object, void of motional artifacts. Or, alternatively, use the time MRI would take to repeat the scan a hundred thousand times for better quality.

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What should MRI do in view of the growing competition? Evidently, just claiming forever that it is "the best" might backfire. It would be nicer to have all the techniques collaborate. For example, we should have a software capable of combining all available images of a patient into a single 3D image of his/her innards in which each technique would contribute with whatever it is good at and, at the same time, help to remove other technique's artifacts. Then, adding one more imaging technique would amount to just plugging-in another data source. The task is staggering, of course (think of the 3D alignment problems). But when a physician is comparing three different types of scans, is he not doing just that?