To build the 11.7-tesla Iseult MRI, many technological breakthroughs were necessary. This was particularly the case for the RF coil, which surrounds the patient's head, transmits RF pulses to excite the proton spins in the patient's brain tissue, and receives the nuclear magnetic resonance signals produced in response.
At such a high magnetic field, spatial inhomogeneities arise in the head both for the main static field (B0) and for the exciting RF field (B1).
For B0 inhomogeneities, the usual compensation techniques (using passive or active shims) are no longer sufficient here. A system dedicated to the human brain was therefore developped in-house and patented. It led to the so-called SCOTCH cylindrical structure. SCOTCH is a shim array of independently controlled small coils, which completes the system of 8 large corrective coils provided by the manufacturer Siemens Healthineers to correct the inhomogeneities of the static magnetic field in the human brain.
"To make the local magnetic shim sheath, we first acquired static magnetic field maps of brains from 100 subjects on NeuroSpin's 3-tesla MRI," explains Alexis Amadon, a researcher at NeuroSpin. "Then, from each of them, we calculated the lines of currents that should ideally circulate on a cylindrical surface around the head to counteract the field disparities. Then we kept the main components, common to all subjects, and spotted the regions that required the most current. That's what helped define the size, shape and location of the coils." Using this data, the team designed a prototype sheath consisting of a 3-cm-thick cylinder about 30 centimeters in diameter and length, and containing 48 coils arranged in 3 layers.
To counteract B1 inhomogeneities, bring spin excitation uniformity, and homogenize the contrast across the entire brain, two novel techniques are combined, leveraging both spatial and temporal degrees of freedom.
- Parallel transmission: our 27-cm-diameter RF coil has 8 independent transmission channels which address different regions of the brain.
- The kT-points® method also developed in-house (and licensed to Siemens) consists in alternating small B0-gradient pulses in-between short RF pulses.
True technological wonders, the RF coil array and its B0-shim sheath have been designed so that the former fits perfectly into the latter.
They were successfully tested in a head-sized spherical "agar phantom" as well as in an ex vivo brain, providing artifact-free images.