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Nov 02, 2021
The 11.7 T magnet of the Iseult MRI scanner is fully operational!
The 11.7 T magnet of the Iseult MRI scanner is fully operational!

On the left of the door the patient side with the motorised bed; on the right the access to the magnet. F. Rhodes/CEA

Since the main magnet of the Iseult MRI machine took up residence in its Neuropsin arch in 2017, it has taken 4 years to transform it into the most powerful, self-sufficient magnet based on a high-availability system allowing it to operate for 10 years!

In 2018, for its 1st cooling, it took 14 weeks to go from room temperature to its nominal temperature of 1.8K. Since March 2019, the magnet has been at 1.8K without interruption of service. The other record is its current rise to the nominal magnetic field of 11.7 T: it takes only 5 hours compared to several days for hospital MRI magnets.

These records are the fruit of several years' work by the Irfu teams in charge of the magnet, the cryogenic factory and the monitoring of all the equipment required for its operation.

In July 2021, Siemens experts commissioned the MRI and a first signal was obtained proving the functionality of the MRI device (main magnet with its gradient coil). There is still work to be done by the Siemens teams, together with those of Irfu and Joliot, to obtain the expected image quality, but the main magnet at the heart of the MRI apparatus, with spatial (11.72 ± 0.00000293 teslas) and temporal (24 hours a day) homogeneity, is fully operational.

 

History of the project:

In the early 2000s, the CEA decided to create a neuroimaging research institute, Neurospin, on the Saclay site. To equip this unique research centre, a handful of visionaries proposed to build a very high-resolution MRI for humans in order to explore the brain on a scale that was previously inaccessible. An international call for tenders has been launched for a technical feasibility study of the magnet, the central part of the device. The Irfu (Institute for Research on the Fundamental Laws of the Universe) was the only one to respond positively to this call and presented a preliminary design after several months of study. The Iseult project was launched on 30 April 2004 with the signing of a Franco-German agreement by President Chirac and Chancellor Schröder on the development of molecular imaging by very high field MRI.

Status of MRI scanners in 2004:

While medical MRI scanners now reach a magnetic field of 3 Tesla, this was still far from being the case at the time. Only about 100 3-tesla MRI scanners, two 7-tesla research MRI scanners and a single 8-tesla research MRI scanner were in operation worldwide. Iseult, with its 11.7 tesla (223,000 times the earth's magnetic field), is destined to become the world's most powerful whole-body MRI for medical research.

 
The 11.7 T magnet of the Iseult MRI scanner is fully operational!

Cross-section of the 11.7 T Iseult magnet. The superconducting coils are shown in orange, the 1.8 K structure in blue and the cryostat in grey.

The 11.7 T magnet of the Iseult MRI scanner is fully operational!

Left: the magnet, a mastodon and its journey from Belfort to Saclay
Right: the stacking of the 170 coils

The 11.7T magnet:

The Iseult magnet is a 132-tonne behemoth, 5 m long and 5 m in diameter, with a central opening of 90 cm to allow for whole-body studies. To reach its nominal magnetic field, the magnet is powered by a current of 1,500 amperes and permanently cooled from superfluid helium to 1.8 kelvin (or -271.35°C). At this temperature, the magnet conductor, made of a niobium-titanium alloy, can operate in a superconducting state and carry such a large current without overheating. The magnet consists of a set of 170 coils that create the main magnetic field, and two shielding coils that minimise the magnetic field outside the magnet. In total, the magnet comprises 182 km of superconducting wire.

An Alstom/Irfu collaboration for the manufacture of the magnet:

The manufacture of the magnet takes place from 2010 at Alstom (acquired by General Electric in December 2015) in Belfort. It was carried out in close collaboration between the Alstom and CEA technical teams, with weekly visits by members of the Irfu project team, and the presence on the Belfort site of an Irfu agent on a permanent basis, for nearly six years. The magnet was delivered to the Neurospin site in 2017, after an exceptional transport of almost 5 weeks between Belfort and Saclay.

 

The equipment needed to operate the magnet:

For many years, in the shadow of the activities and highlights of Belfort, intense work had been going on at Neurospin to install the cryogenic plant and all the auxiliary equipment needed to operate the magnet. The control systems, PLCs, power supplies, and all cryogenic equipment had already been installed and tested individually. After the installation of the magnet in its arch, the teams worked for almost two years to connect the magnet to all its auxiliary equipment and prepare it for commissioning. At each new stage, a battery of tests on the cooling circuits, electrical circuits and instrumentation had to be carried out.

 
The 11.7 T magnet of the Iseult MRI scanner is fully operational!

Overview of the Iseult magnet and its auxiliary equipment in the NeuroSpin building.

First descent to 1.8K:

Cooling of the magnet starts at the end of 2018, and it will take 14 weeks to go from room temperature to nominal temperature. The cooling process will have required 250 000 litres of liquid nitrogen and 18 500 litres of liquid helium. Electrical and magnetic tests have been carried out continuously to verify the integrity of the magnet and monitor the behaviour of the magnet core.

First current rise:

The field rise then takes place in several stages with numerous electrical and magnetic tests, and the testing of the emergency shutdown procedures. In total, 1300 procedures for detecting the occurrence of potential faults were tested and validated. The magnet will reach its nominal magnetic field for the first time in July 2019. This first ascent to nominal current takes 24 hours, and the magnet remains at 11.7 tesla for about 15 hours, with the project team on site all night to monitor everything and watch for any problems.

 

Dressing the magnet to make it an MRI machine:

A new phase then begins, with the final work to fit out the arch, and the installation of the equipment needed to make the image (in particular the assembly of the gradient coils, another critical piece of equipment for the project supplied by Siemens in the framework of the Iseult collaboration). In parallel, tests of the magnet are continuing to adjust the spatial (11.72 ± 0.00000293 teslas) and temporal homogeneity, two key parameters for image quality.

For the first time, Irfu has developed a high availability system. While the processes are known and mastered in the nuclear sector at the CEA, this is the first time that this type of operation has been implemented for a superconducting MRI magnet. All critical equipment has been doubled up to allow operation in case of problems or maintenance. The magnet is equipped against power outages, computer network failures, and has a cooling autonomy of 7 days even in case of a major problem in the cryogenic plant. The protection of the magnet is ensured by a PLC which relies on a voting system between the different acquisitions to make a decision.

 
The 11.7 T magnet of the Iseult MRI scanner is fully operational!

Left: installation of the shim bars for homogeneity adjustment
Right: installation of the gradient coils

A 24-hour MRI:

For several months, the teams will continue to test the reliability of the cryogenic and electrical installations designed to guarantee the availability of the magnet 24 hours a day. An on-call team is being trained for future operations to intervene in case of an emergency.

On 7 March 2021, the magnet celebrated its second anniversary at 1.8K, without interruption of service, which represents a major achievement for such a complex installation, and the performance was recognised worldwide by several citations in different scientific journals.

 

Interference between the magnet and other systems:

Once all the ancillary equipment has been placed around and in the central hole of the magnet, the operation of the magnet has been tested to study the influence of this other equipment and their interactions, even in the event of an emergency stop or failure. The electrical protection systems, the acquisition and instrumentation systems and the magnet's cooling are being closely scrutinised, in particular to assess the impact of the gradient coils, which manage cryogenic losses and electrical voltages in the core of the windings. The risk is to cause a rapid discharge of the magnet, which makes it possible to protect the magnet by lowering the current in a few minutes without any risk for the patient, but with important consequences on cryogenics and a potential shutdown of several months to put the installation back into service.

The effect of road traffic in the vicinity of the building and the effect of external electromagnetic waves were also studied to validate the effectiveness of the filtering systems developed specifically in the magnet. After 8 runs at 11.7T, and a period of 3 weeks at nominal current 24 hours a day without specific monitoring, the magnet is now fully operational, and it is now possible to run the current up to nominal magnetic field in less than 5 hours, whereas it can take several days for hospital MRI magnets to reach their nominal magnetic field. On 26 July 2021, the baton is passed to Siemens experts to commission the MRI and prepare for the first image.

 
The 11.7 T magnet of the Iseult MRI scanner is fully operational!

The first images of the pumpkin from the 11.7 T MRI of the Iseult project - 400 micron resolution ©CEA

#182 - Last update : 11/02 2021

 

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