In order for the images produced by the future MRI to be free of distortions or artifacts, the magnetic field generated by the Iseult magnet must be homogeneous to 0.5 PPM (parts per million) around the patient's brain. To meet this challenging specification, it was necessary to provision means of "shimming" the field, i.e. of correcting all the small defects that would inevitably arise from the manufacturing process. 5904 pieces of shim (small iron platelets) were screwed onto rails and installed inside the magnet tunnel. This first configuration was tested on Thursday, July 9, 2020 by mapping its effect on the magnetic field of Iseult at 3 T. The results are very encouraging as this first shimming iteration allowed to increase the homogeneity of the field in the useful zone from 138.8 to 3.2 PPM (value extrapolated to 11.72 T from magnetic measurements at 3 T).
Iseult is a one-of-a-kind MRI magnet, firstly because of its record magnetic field: 11.72 T; and then because of all what this entails; such as its size (5 m long, 5 m in diameter), mass (over 130 tons), stored energy (338 MJ), etc. Despite this, Iseult remains in the same boat as all the MRI magnets in the world when it comes to the very strict specifications on the homogeneity of the magnetic field it has to generate.
In order to prevent the images produced by the future MRI from being distorted or riddled with artifacts, the magnetic field generated by Iseult must be homogeneous to within 0.5 PPM (Parts Per Million) throughout the entire useful area; a sphere 220 mm in diameter encompassing the patient's brain. For a center field of 11.72 T, this means a maximum difference between the extreme values of the magnetic field of 5.86 µT in the sphere!
Even though Iseult was designed with the field homogeneity in mind from the outset, it was also essential to provision ways of “fine-tuning” the field to correct all the small defects that would inevitably arise during manufacturing. These means of adjustment are called "shims" and the process of adjusting the MRI field homogeneity is what is called "shimming".
On Iseult, to maximize the reliability of the system, which is designed to operate continuously for years, the shimming is completely passive. It consists of a myriad of small iron tablets a few centimetres in size (5904 pieces!), installed inside the magnet tunnel, all around the patient. The iron inside the tunnel magnetizes itself and locally distorts the field lines. So, by choosing the right quantity and positioning of the shims, it is therefore possible to correct the field defects.
In order to determine this "suitable" iron distribution, a program was developed which processes magnetic field maps of the magnet as an input and outputs assembly drawings for the shims from them.
Each rectangle on the above figure corresponds to an individual shim slot; the blackened ones being filled with iron shims (active because ferromagnetic) and the white ones being the neutral shims (in non-magnetic aluminium, there to plug the holes...).
In the end, the 5904 pieces of shim were screwed on their supporting rails in 3 days with the help of many people from NeuroSpin who came to give a helping hand at this ant work. The result can be seen on the pictures: a kind of black and white mosaic with flowing patterns.
This first configuration was tested on Thursday, July 9, 2020 by mapping its effect on the Iseult magnetic field at 3 T. The results are very encouraging because this first iteration has allowed to increase the homogeneity of the field in the useful zone from 138.8 PPM to 3.2 PPM (value extrapolated to 11.72 T from magnetic measurements at 3T). Thus, after a few corrections on the shimming configuration, the targeted homogeneity of 0.5 PPM should be achieved. This, together with the insertion of the gradient coils into the magnet, shows that Iseult is in the final stages of its conversion to true MRI.