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Laboratory | Medical imaging
Clinical MRI methodological team
Team Leader: Alexandre Vignaud
NeuroSpin is equipped with a 8 channel
transmission system. Up to now, our efforts were mainly focused on the none-selective
RF excitation to homogenize the flip angle (small or large angle regimes) or
the true rotation matrix on the human brain at 7T. Motivated by the success and
simplicity of the Spoke method to settle uniform 2D excitation, our team has
extended the concept to 3D though the kT-points method. The
potential of the approach has been demonstrated first on density weighted
imaging using spoiled gradient recalled echo. The RF pulse tailored for one
specific subject played with 5 kT-points has proven to improve
drastically flip angle spatial homogeneity at 7T on the human brain for a
duration under the millisecond with a low energy deposition. The pulse
parameterization has been combined with optimal control technique to tackle problem
imposed by MP-RAGE inversion. RF inhomogeneity mitigation in this sequence is
displayed on Figure 3 (CP defines
standard transmission mode using Circular Polarisation) where white –gray
matter (WM-GM) contrast is restored.
numerous applications, initial state is an unknown, which forbid to use small
flip angle approximation classical approach to optimize kT-points. In
this case, rotation angle and axis are specified. In this context, we have
introduced GRadient Ascent Pulse Engineering (GRAPE) to synthetize desired rotation
matrix for refocusing angles needed in 3D SPACE, Turbo Spin Echo at variable
flip angles with a phase of rotation axis unleashed. Several decades of pulse
are then designed using GPUs to implement 3D T2-weighted in vivo
imaging. Results are presented on Figure 4.
Nevertheless, parallel transmission exploitation at UHF will reach its maximum potential only if the parameters which have a role on security are under control. Thus, our group has developed fast algorithms to design RF pulse, under SAR explicit constraints using Virtual Observation Points compression model, which allow to design 3D pulses in very competitive time (< 10 s for both small and large flip angles). In the context, the temperature would be a more relevant parameter than SAR. It has been demonstrated that the efficiency and the pulse duration could be improved if the temperature was taken into account instead of SAR. Unfortunately, it would require a good understanding of the SAR-temperature dependency in the biological tissues. The recent efforts from few groups to concentrate on this challenge consists to measure temperature induced in vivo by a radiofrequency energy source with PRF (Proton Resonance Frequency shift) an thermometry MR method shown on Figure 5.
CEA is a French government-funded technological research organisation in four main areas: low-carbon energies, defense and security, information technologies and health technologies. A prominent player in the European Research Area, it is involved in setting up collaborative projects with many partners around the world.