Development of magnetic resonance tomographs with permanent magnets

It was generally assumed that new technologies in magnetic resonance tomography were mainly advancing by means of perfection of superconductive MRT systems with a high field (1,5-3 Т). Specific of their perfection was the development of new research methods resting on introduction of new software, more powerful computers, and mightier gradient systems without fundamental changes in the device architecture.

Evolution of MRT with permanent magnets went on an entirely different path. First, instead of tunnel type devices with a horizontal magnetic field, so-called open MRT with a vertical magnetic field were developed. It was found experimentally that solenoid coils in a vertical magnetic field are at least 40% more sensitive than tunnel type MRT coils, while the time of imaging is halved. Evolution of open MRTs was also fast: from devices with a C-magnet and one wide support to devices with two small diameter supports, and recently, MRT Aperto with one small diameter support located angularly to the patient's table has been developed. As a result, space open by 320 between MRT poles was obtained, which gives access to the patient from actually any side. This rules out claustrophobia among patients and enables positioning of any anatomic area of a patient in the MRT isocentre, with cannot be done using superconductive MRTs with the field of 1,5 - 3 Т. That is why Aperto produces best quality images of any anatomic area. Simultaneously, new hi-tech research methods were introduced that raised the quality of visualisation of medical images to the level that could not even be dreamt of in the not so remote past. The problem of temperature instability of permanent magnets has also been resolved. In particular, the Aperto device employs an inbuilt system of thermostating that can maintain the required temperature in the treatment room using an ordinary air conditioner, without special cooling and temperature maintenance systems.

Thanks to a powerful gradient system incorporated in the design of an MRT with permanent magnets (Aperto - 22 mT/m and rate of rise of 55 mT/M/msec), those MRTs can employ research methods previously available only to superconductive MRTs with a high field, such as MRCP (magnetic resonance cholangiopancreatography), balanced with SARGE(BASG) - images, weighted by proton density. Aperto can also produce high quality diffusive-weighted images, effectively employ the spectral method of separation of a signal from adipose tissue and water with inhibition of signals from adipose tissue. At that, the time of examination is roughly equal to that of a similar examination by a superconductive MRT 1.5 Т. For instance, while examination with a superconductive MRT 1.5 Т takes 10 minutes, the same examination using Aperto will take not more than 12 minutes. Aperto also enables examination using ultra-fast single-frame sequences with a fairly high quality, which makes it possible to refuse from anaesthesia at examination of children in some cases.

Aperto gives perfect images also because its magnet secures high homogeneity of the magnetic field ( 3 ppm per the diameter of 35 cm), the power of its radio frequency system is 5 kW, and the noise level of the entire system is among the best in the sector, not exceeding 0.3 dB, producing the best signal/noise ratio. Aperto is fitted with a system of automatic magnet shimming. Aperto is supplied with multi-channel receiver coils. The device is fitted with unique elastic body coils tightly fitting to the patient. All coils are provided with automatic shimming systems.

Thanks to all the above features, Aperto can perform full examination of the patient's entire body, including head and spinal examination, examination of extremities, small pelvis, as well as abdominal examinations without any restrictions whatsoever.

Comparison of the features of Aperto MRT with two superconductive MRT magnets with the field of 1.5 Т performed in the clinic of Kumamoto National University in Japan demonstrated that the quality of images obtained by Aperto at examination of the areas described above is quite comparable to the quality of images obtained by mentioned MRTs. At that, the same area was examined in the same patients using both MRT types. Specialists of the clinic admitted that they had to reconsider their opinion of the diagnostic capabilities of open-type MRTs with a permanent magnet after the beginning of practical use of Aperto.

Chuvikov M.N. (Nevskiy Radiological Forum "New Horizons", collected scientific works)

"Imаge processing system", Moscow, Russia.