MRI scanners with very high magnetic fields (B0 > 7 Tesla) for humans are becoming increasingly common around the world. The most powerful to date (nominal field of 11.7 T) is at NeuroSpin. Exploiting these high fields for biomedical research requires, among other things,
understanding and quantifying the expected gains in signal-to-noise ratio (SNR)
and contrast-to-noise ratio (CNR)
Calculations and simulations have been developed over the years
to determine what gains can be achieved with increasing B0. However, to date, no experiment has been able to evaluate the contribution of B0 to SNR alone.
In a study carried out in collaboration with the universities of
Maastricht and
Minnesota, the METRIC team of
BAOBAB (NeuroSpin department)
measured the SNR at the centre of a single spherical phantom in different devices, at several field strengths: 3 T, 7 T, 9.4 T, 10.5 T and 11.7 T. The set-ups and experimental conditions were all otherwise almost identical (antennas, electrical properties and MRI sequence protocols). The researchers
compared their data with expected theoretical values (ultimate intrinsic SNR theory).
After eliminating the influence of tilt angle excitation inhomogeneity, the measurements revealed that
the SNR increase is a function of
B01,94±0,16, a value reasonably close to the theoretical value of B02,13.
As the ultimate intrinsic SNR is a measure of the performance of a radio frequency antenna, this result will help to determine the available room for improvement.
Portrait of Caroline Le Ster, new recruit at CEA
Caroline Le Ster was hired in 2022 by the CEA, in the METRIC team of NeuroSpin (UMR BAOBAB) led by Alexandre Vignaud. Caroline is no stranger to NeuroSpin. After her thesis at the Signal and Image Processing Laboratory (LTSI - University of Rennes 1), which she defended in 2017, Caroline arrived at the brain imaging centre for a post-doc. An ideal place for this young researcher eager to work at the interface between medicine, biology and physics, and to take on technological challenges.
Her first assignment was to compare two state-of-the-art functional MRI sequences acquired at 7T with universal pulses. In particular, her work contributed to showing that the sequence classically used by researchers in cognitive sciences (SMS-EPI) could be advantageously replaced at high magnetic field (> 7 T) by the 3D-EPI sequence, which deposits less energy in the tissues and is less noisy.
She then worked on the development of an MRI thermometry methodology to measure brain temperature in vivo. Her work is helping to provide reliable tools for measuring the rise in brain temperature induced by radiofrequency waves during an MRI examination (see news
https://joliot.cea.fr/drf/joliot/en/Pages/news/Science/2020/thermometry-MRI.aspx).
Today, Caroline is working on the Iseult project. Whether by correcting magnetic field fluctuations or optimising parallel transmission, Caroline aims to make high-resolution functional MRI data acquisition reliable at 11.7 T.
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