A Brave New World: The Story of Canon’s New 3T Magnet Development and Manufacture
Hajime Tanabe
In-house developed 3T magnet
Recently we have developed a new superconducting magnet in-house, which has been refined to focus on image quality. During development, the engineering team determined two important points to be improved.
The first point was high magnetic field homogeneity, which is one of the most important factors in obtaining high image quality. The new magnet has better homogeneity than the conventional one. As a result, the maximum field of view (FOV) has expanded, so that wider and more stable images can be acquired.
The second point was to achieve a low quench rate through high quench resistance performance. A quench is the loss of superconducting state, which means the loss of magnetic field homogeneity and when it occurs the MRI system cannot be used. It takes time and cost to recover the magnetic field. The new magnet has improved quench resistance compared to conventional magnets and it is much less likely that a quench occurs, increasing confidence in continuous use of the MRI.
The first point was high magnetic field homogeneity, which is one of the most important factors in obtaining high image quality. The new magnet has better homogeneity than the conventional one. As a result, the maximum field of view (FOV) has expanded, so that wider and more stable images can be acquired.
The second point was to achieve a low quench rate through high quench resistance performance. A quench is the loss of superconducting state, which means the loss of magnetic field homogeneity and when it occurs the MRI system cannot be used. It takes time and cost to recover the magnetic field. The new magnet has improved quench resistance compared to conventional magnets and it is much less likely that a quench occurs, increasing confidence in continuous use of the MRI.
High magnetic field homogeneity and expansion of maximum FOV
There are various indices of magnetic field homogeneity, but as one example, the new magnet achieves 0.05 ppm at 30 cm DSV. It allows you to obtain a large FOV of 55 × 55 × 50 cm. To realise this, a high-level of improvement and process management at both design and manufacturing stage is required.
During design, coil position is optimised at micron-level precision by utilising a dedicated optimisation tool and specific know-how in implementation. At this time, it is necessary to optimise not only magnetic field homogeneity but also to satisfy many other requirements. This is a really important process in the upstream design stage. In addition, robust production is also important to minimise errors for stable mass production. This new magnet achieves high magnetic field homogeneity and is robust against production errors.
During manufacturing, the accuracy of the superconducting coil position has been improved by more than double compared to a conventional magnet. For example, the length of the superconducting coil is from 3 m to 5 m per circumference, but it is wound with a tolerance of less than ±1 mm to the design value. Moreover, this tolerance is maintained over the entire length of nearly 100 km. It has been made possible through high-level winding skills and sophisticated position control even though this is a very complicated technique.
In order to achieve the high magnetic field homogeneity, it is necessary to achieve and maintain a good balance between design and manufacturing. While it is challenging, the results achieved are very rewarding.
During design, coil position is optimised at micron-level precision by utilising a dedicated optimisation tool and specific know-how in implementation. At this time, it is necessary to optimise not only magnetic field homogeneity but also to satisfy many other requirements. This is a really important process in the upstream design stage. In addition, robust production is also important to minimise errors for stable mass production. This new magnet achieves high magnetic field homogeneity and is robust against production errors.
During manufacturing, the accuracy of the superconducting coil position has been improved by more than double compared to a conventional magnet. For example, the length of the superconducting coil is from 3 m to 5 m per circumference, but it is wound with a tolerance of less than ±1 mm to the design value. Moreover, this tolerance is maintained over the entire length of nearly 100 km. It has been made possible through high-level winding skills and sophisticated position control even though this is a very complicated technique.
In order to achieve the high magnetic field homogeneity, it is necessary to achieve and maintain a good balance between design and manufacturing. While it is challenging, the results achieved are very rewarding.
Low quench rate through high quench resistance performance
It is not easy to achieve high quench resistance in a superconducting magnet. It is not a matter of calculation, but the result of years of accumulated experience and know-how.
In the past our engineers experienced quenches, however each time this happened, they investigated the causes and took measures to improve the situation. Our new magnet has achieved a low quench rate through high quench resistance performance, taking maximum advantage of this experience and efforts to consider the solutions in design.
To realise these points, high-level improvement and process management at both design and manufacturing stages were required.
During design, an appropriate superconducting margin (quench margin) was considered. For example, the value of minimum quench energy (MQE), which is an indicator of superconducting margin, has been decided based on accumulated experience and know-how, while taking cost into account. In addition, the magnet had to satisfy magnetic field homogeneity and many other design and manufacturing requirements. Furthermore, the structure of the superconducting coil was optimised to minimise mechanical stress. The superconducting coil is used at an extremely low temperature of 4.2 K (-269 °C). Thus, the stress when cooling from room temperature to extremely low temperatures is many times higher than that caused by the electromagnetic force of 3T. The superconducting coil is made up of a variety of materials, so the stress analysis of the cooling process has been conducted with multiple patterns of a superconducting coil model. Finally, the process that provides the optimal balance between stress after cooling and strength for each part has been adopted.
During manufacturing, appropriate processing of the superconducting coil components made it possible to double the adhesive strength of the epoxy that solidifies and fixes the superconducting magnet properly compared to conventional methods. The management of the epoxy and solidification process is also based on the accumulated experience and know-how of our engineers.
There is a risk of quench if the temperature rises by just 1 °C from 4.2 K (-269 °C) due to heat generated by cracking of the epoxy or friction caused by movement. Moreover, under an extremely low temperature, the specific heat is lower by two to three orders than at room temperature, which makes it easier for the temperature to increase. The new magnet has quite a high quench resistance even under extreme conditions, so customers can scan with confidence and peace of mind.
In the past our engineers experienced quenches, however each time this happened, they investigated the causes and took measures to improve the situation. Our new magnet has achieved a low quench rate through high quench resistance performance, taking maximum advantage of this experience and efforts to consider the solutions in design.
To realise these points, high-level improvement and process management at both design and manufacturing stages were required.
During design, an appropriate superconducting margin (quench margin) was considered. For example, the value of minimum quench energy (MQE), which is an indicator of superconducting margin, has been decided based on accumulated experience and know-how, while taking cost into account. In addition, the magnet had to satisfy magnetic field homogeneity and many other design and manufacturing requirements. Furthermore, the structure of the superconducting coil was optimised to minimise mechanical stress. The superconducting coil is used at an extremely low temperature of 4.2 K (-269 °C). Thus, the stress when cooling from room temperature to extremely low temperatures is many times higher than that caused by the electromagnetic force of 3T. The superconducting coil is made up of a variety of materials, so the stress analysis of the cooling process has been conducted with multiple patterns of a superconducting coil model. Finally, the process that provides the optimal balance between stress after cooling and strength for each part has been adopted.
During manufacturing, appropriate processing of the superconducting coil components made it possible to double the adhesive strength of the epoxy that solidifies and fixes the superconducting magnet properly compared to conventional methods. The management of the epoxy and solidification process is also based on the accumulated experience and know-how of our engineers.
There is a risk of quench if the temperature rises by just 1 °C from 4.2 K (-269 °C) due to heat generated by cracking of the epoxy or friction caused by movement. Moreover, under an extremely low temperature, the specific heat is lower by two to three orders than at room temperature, which makes it easier for the temperature to increase. The new magnet has quite a high quench resistance even under extreme conditions, so customers can scan with confidence and peace of mind.
Conclusion
When developing Vantage Galan 3T / Supreme Edition, several technical matters have been addressed during development, even though there were no issues with the superconducting magnet itself. Engineers from each department worked closely together and were able to discover unique solutions. This is one of the benefits of Vantage Galan 3T / Supreme Edition being developed in-house, which we like to refer to as All Canon 3T MRI system.
The quest to improve magnet design and performance never ends. We aim to continue to maintain high quality and develop products that will satisfy customer's clinical requirements even further. //
The quest to improve magnet design and performance never ends. We aim to continue to maintain high quality and develop products that will satisfy customer's clinical requirements even further. //
Disclaimer
Some features presented in this article may not be commercially available on all systems shown or may require the purchase of additional options. Due to local regulatory processes, some commercial features included in this publication may not be available in some countries. Please contact your local representative from Canon Medical Systems for details and the most current information.
Hajime Tanabe
Group Manager
Magnet Development Group
MRI Systems Development Department, MRI Systems Division
Canon Medical Systems Corporation
Group Manager
Magnet Development Group
MRI Systems Development Department, MRI Systems Division
Canon Medical Systems Corporation
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