The core of the system – the new 7T magnet – is a complete new development. Siemens developed its first actively shielded 7T whole body magnet for MAGNETOM Terra, making this the first ultra-high-field scanner with all components designed, built and serviced by a single vendor. The first of these 7 Tesla magnets was installed at the Erlangen University Clinic, Germany in April 2015. The next MAGNETOM Terra units are scheduled for delivery early 2016, and the start of serial production is scheduled for beginning of 2017.
Rotation of hydrogen nuclei in a magnetic field
In simplified terms, an MRI scanner measures the rotation behavior of hydrogen nuclei that are first aligned in strong magnetic field and then excited with radio waves. The system detects the signal created by the excited nuclei. The decay effects of the MR signal provide information about the characteristics of the scanned tissue. The stronger the magnetic field, the better the ratio of the measured MR signal to the surrounding noise.
The ratio at 7 Tesla is about twice as high as at 3 Tesla – providing advantages like a substantially higher spatial resolution and better image contrast. This makes it possible to view finest details in brain structures, which can’t be seen at lower field strengths. Morphological changes in cartilage and muscle tissue can also be recognized more clearly and at an earlier stage. A higher magnetic field also improves magnetic resonance spectroscopy, a technique that makes it possible to detect chemical elements, for example, to track metabolic processes.
Well-established technology optimized for 7 Tesla
Siemens Magnet Technology (SMT) in Oxford, England, has produced 1.5 and 3 Tesla magnets for MRI systems for more than 20 years. For higher field strengths, Siemens was forced to obtain the magnets from other vendors in the past. Last year in 2014, Siemens announced that it would produce its own 7T magnets in-house. This decision offered developers a chance to design an ultra-high field magnet specifically suited for bringing this to a clinical environment. Essentially, the aim was to cut down weight as far as possible to make the structural requirements for ultra-high field MRI scanners more manageable. The Siemens-developed 7 Tesla magnet is 50 percent lighter than previous variants, at 17 metric tons. A MAGNETOM Terra weighs a total of 20 metric tons, which allows for easier transportation and integration into clinical environments.
To achieve such high field strengths, the magnet coils are made of superconducting materials. During operation and in standby mode, they’re cooled with liquid helium, which means they have to be housed in a tank called the cryostat. Even though the cryostat is sealed, a small amount of helium will normally evaporate and has to be topped up from time to time. Siemens previously optimized low-temperature technology for its 3 T and 1.5T magnets, developing what is known as a Zero Helium boil-off technology. This keeps helium from evaporating even while the MRI system is in operation. The technology has now been applied to the 7T magnet.
Spatial resolution 0.2 millimeters
Another innovative feature on MAGNETOM Terra is its eight parallel transmitter channels – an enhancement that was intended to be available only purely for research in the past. Clinical MRI scanners worked with only one transmitter channel. Multiple channels make it possible to excite a scanned anatomical structure more uniformly so as to get an improved image contrast. The multi-channel transmit feature is only available in the research mode on the MAGNETOM Terra. The receiving side in both clinical and research mode has up to 64 antennas, permitting very high spatial resolution of up to 0.2 millimeters in all directions.
In an MRI scan, the spatial encoding is realized by gradients, producing a spatially dependent magnetic field. An improved performance of the gradient system will translate into an improved image quality. The gradients on the MAGNETOM Terra can go up to 80 millitesla per meter, which provides further benefits, especially in the area of diffusion MRI, a technique used to visualize nerve connections in the brain.
Siemens has the largest installed base of 7T MRI systems. About 65% of all 7T installed worldwide are from Siemens. Since ultra-high field MRI will inevitably play an important role in future clinical practice, plans are already being put into action to get regulatory approval in Europe and the USA for MAGNETOM Terra to be used for certain applications in a clinical setting.
¹ MAGNETOM Terra is still under development and not commercially available yet. Its future availability cannot be ensured. Some features of MAGNETOM Terra will remain ongoing research.
Mr. Dr Norbert Aschenbrenner
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