Tony Puffer, SMT: "Dozens of NMR measurements for every magnet we produce!"


pufferSiemens Magnet Technology, one of the world’s leading manufacturer of MRI magnets, produces more than 1,000 superconducting magnets each year. A cutting-edge industrial process, punctuated by an impressive array of tests and measurements from prototype to product and from factory to on-site installation. Tony Puffer, SMT Business Excellence Manager, answers our questions for Metrolab.


With global demand for MRI scanners increasing constantly, SMT has equipped nearly 40% of the scanners already in use for all the major manufacturers including Siemens, Toshiba, Philips and Hitachi. What is your product range policy?
Tony Puffer: In 2007, 1.5 T magnets represent more than 80% of our output, and our range is based around four standard products which differ in terms of their performance and dimensions. For  example,  one of our magnets, with its larger inner bore and short length has been designed to improve patient comfort and as a result has significantly challenged the existing Open market in the USA. In addition to our 1.5T range, we have also just introduced a new 3.0T magnet that will with its excellent performance challenge existing products in this growing market. We are constantly fine-tuning our range: we have introduced four new models over the past four years and we will continue to improve our product portfolio every year.


More than 10% of the 570 employees at your Oxford site work in the R&D department. What major research areas are you focusing on?
T. P.: There is increasing demand for "low-cost" MRI scanners from existing markets who are under cost pressure and from emerging countries, such as China. To optimise the cost of our magnets, we have a team of talented Engineers looking at using new materials and optimising the way we use existing materials. Another area of development is with "open" magnets: the shorter they are, the more comfortable they are for the patients, but the more they cost! Here the challenge is to strike the best compromise between specification and cost. Lastly, we are focusing our efforts on new 3.0T magnets, which represent a growing share of the market.


For each new product you build several prototypes… How do you test them?
T. P.: We have to check that the magnet complies with every specification requirement. In practice, the test protocols are very thorough and rigorous. Of course, we use NMR measurements under a strict protocol to check field intensity and homogeneity, temperature behaviour, and field drift – which must be less than 0.1 ppm per hour… At this stage, Metrolab's mapping or single-point measurement instruments are invaluable! There is also a series of safety tests, one of which is to check that the helium recovery system is working properly: we have to provoke the magnet to quench to test the quench circuit. And on the mobile systems we even go as far as simulating their transport by road using a vibrating table!


Do these tests at the R&D stage mean there is no need to check the magnets again as they leave the production line?
T. P.: Not in the slightest! Once it has been assembled, each magnet is sent to our magnet test department, where a whole series of tests are performed.  When the magnet is energised and we check the stability and homogeneity of the field generated, still using NMR probes. For each magnet this array of tests takes several days, and represents dozens of NMR measurements!


Once they have been installed on the customer’s premises, MRI scanners must still be shimmed… what does this entail?
T. P.: The electromagnetic environment of the room disrupts the field generated by the magnet. Yet, as we were just saying, the magnet must be perfectly homogeneous in order to function correctly. Shims are small metal parts that our installation engineers place inside the magnet bore to correct the field. They proceed by iterations, using a Metrolab 24-position Magnetic Field Camera, which moves around the longitudinal axis of the magnet. For each new mapping operation a calculation programme indicates the remaining corrections to be made… and where the corresponding shims must be placed.


All in all, how long does the operation take?
T. P.: Our teams can shim a magnet in three or four iterations. If it takes an hour for each mapping operation, and another hour to do the calculations and position the shims, that makes six to eight hours. That’s no mean feat compared with the four to six days it took when we had to do the magnetic map point by point, with the inherent risks involved. Metrolab’s development of the MFC has really revolutionised shimming!


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