Samira Mchinda1, Gopal Varma2, Robin Draveny1,3, Arnaud Le Troter1, Victor Carvalho1, Valentin H. Prevost1, Maxime Guye1,4, Jean Pelletier1,5, Jean-philippe Ranjeva1, David C. Alsop2, Guillaume Duhamel1, and Olivier M. Girard1
1Aix Marseille Univ, CNRS, CRMBM, Marseille, France, 2BIDMC, Harvard Medical School, Boston, MA, United States, 3Phelma, INPG, Grenoble, France, 4Aix Marseille Univ, APHM, Hôpital de La Timone, Pôle d’Imagerie Médicale, CEMEREM, Marseille, France, 5Aix Marseille Univ, APHM, Hôpital de La Timone, Pôle de Neurosciences Cliniques, Service de Neurologie, APHM, Marseille, France
Synopsis
Inhomogeneous magnetization transfer (ihMT) is a new
MRI modality that provides strong sensitivity to myelinated tissues.
Previously, a 3D whole brain ihMT sequence, based on GRE readouts interleaved
with bursts of MT saturation pulses, was developed and optimized at 1.5T. In
this work we demonstrate that concentrating the MT pulse energy can mitigate
the sensitivity of ihMT to RF inhomogeneities encountered in high field
systems. Overall, this
allows for 1.5mm3 resolution ihMTR maps to be acquired
at 3T, with a reduced B1-induced bias and strong signal within the whole brain,
allowing robust clinical applications of ihMT at 3T.
Purpose
Inhomogeneous
magnetization transfer (ihMT) is a new MRI modality that provides strong
sensitivity to myelinated tissue (1,2). It can be envisioned as a dipolar
relaxation time (T1D)-weighted sequence (3) which can be configured to reveal
selectively myelin associated long-T1D components (4,5). A strong correlation with myelin
specific histology (6) was recently observed, showing promise for ihMT as an in
vivo myelin biomarker. Previously, a 3D whole brain ihMT sequence, based on
steady-state GRE readouts interleaved with bursts of MT saturation pulses, was
developed and optimized at 1.5T (7). Studying this sequence revealed the strong
impact of the temporal distribution of the MT pulse bursts (leading to more or
less concentrated or distributed RF energy, see Fig.1), which modifies ihMT
signal intensity and associated B1 dependency. Whereas a linear B1 dependency was obtained for a distributed RF
deposition, saturation of the ihMT signal with B1 was observed for the most concentrated configuration
(bursts
of high power MT pulses followed by long readouts). The latter configuration should
provide in principle a reduced sensitivity to the RF inhomogeneities that are
encountered at high field. The purpose of this work is to investigate further
this hypothesis and to assess strategies, different than those used at lower
magnetic field, to implement and optimize ihMT at 3T.Methods
Experiments were performed on a 3T system (Verio, Siemens,
Erlangen, Germany) with a 32-channel receive only head coil on healthy
volunteers. The protocol included 3D MPRAGE imaging for automatic segmentation
of brain structures, B1 mapping (VB17 Siemens WIP #658, spin echo variant
comparing spin- and stimulated- echo signals) for measuring transmit B1
inhomogeneities and ihMT-GRE sequences. The latter were run for a constant
B1RMS of 2.7uT with TR varying from 24 to 265ms (experimental parameters given
in Fig.2), leading to progressive concentration of RF energy within the MT pulse bursts. ihMT-GRE scans were repeated with a reference voltage (Vref) set to 80%
of its nominal values to assess the effect of a -20% drop in B1 intensity on
the ihMT signal, as a way to mimic RF inhomogeneities. In a separate experiment, high resolution 1.5mm3
ihMT-GRE scans were run for high and moderate energy concentration (TR of 265ms
and 145ms, respectively) and residual correlation with B1 was performed in the
whole White Matter (WM) area by a linear fit of a smoothed pixelwise ihMTR vs.
B1 representation. The ihMT signal processing and automatic segmentation were
performed as documented in (7).Results & Discussion
Fig.3A shows the ihMT signal averaged over the whole
WM area as a function of TR for both reference voltage settings. Characteristic
rising and falling curves are observed, analogous to previous findings at 1.5T
describing the so-called ihMT boost effect (7).
From these two curves, which tend to merge at long TRs, it appears that the
sensitivity to B1 variations decrease with power concentration (i.e. with TR increase),
as expected from the B1 dependence regimes characterized at 1.5T (7). The relative ihMT signal variation,
associated with a 20% B1 drop, linearly decreases from 31.3% at TR = 24ms to
only 10.1% at TR = 265ms (Fig.3B). In Fig.4 the sensitivity of ihMT with B1 was
estimated using the high-resolution protocol from the smoothed pixel-wise ihMT
vs. B1 representation of WM data. These data reveal a typical ± 20% B1 variation
over
the whole brain WM. A reduced correlation with B1 could be extrapolated for the most concentrated
configuration (i.e. TR=265ms),
in agreement with previous experiments (Fig. 3b). Of interest the data seem to diverge
from a simple linear correlation model, perhaps due to the ihMT signal heterogeneity
within WM. Extrapolating a reliable quantitative index of ihMT signal
variation with B1 from such pixel-wise data remains to be explored. Finally, the
high-resolution protocol is demonstrated in Fig.5, showing a good sensitivity
for WM and exquisite contrast with Gray Matter (Fig.5A). The corresponding
quantitative evaluation measured on several reference brain structures is
reported in Fig 5B.
Conclusion
Instead of optimizing ihMT for maximum signal
intensity, as was done previously at 1.5T (7), high
field systems require to compromise between signal intensity and sensitivity to
RF inhomogeneities. By concentrating the energy deposition, it is possible to
mitigate the sensitivity of ihMT to the latter, while benefiting from an
enhanced signal as compared to the most distributed short TR variant. Overall,
by benefiting from these improvements, 1.5mm3 resolution ihMTR
maps can be acquired at 3T within less than 15min, with a reduced B1 bias (on
the order of ±10%) and strong signal within the whole brain, allowing robust clinical
applications of ihMT at 3T. Acknowledgements
The authors would like
to thank S. Confort-Gouny, V. Gimenez, L. Pini and P. Viout for MRI and volunteer management. This work
was supported by the following funding sources: ARSEP Foundation (Research
Grant 2017), ANR (Grant ANR-17-CE18-0030) and SATT Sud-Est. This work was
performed by a laboratory member of France Life Imaging network (Grant
ANR-11-INBS-0006).
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