Gergely Bertalan1, Bettina Müller2, Leif Schröder3, Heiko Tzschätzsch1, Mehrgan Shahryari1, Helge Herthum1, Jing Guo1, Jürgen Braun4, and Ingolf Sack1
1Department of Radiology, Charite Universitätsmedizin Berlin, Berlin, Germany, 2Tierhaltung CCM, Charite Universitätsmedizin Berlin, Berlin, Germany, 3Molecular Imaging, Leibniz Forschungsinstitut für Molekulare Pharmakologie, Berlin, Germany, 4Department of medical Informatics, Charite Universitätsmedizin Berlin, Berlin, Germany
Synopsis
The purpose of this study was the development of multifrequency MR
elastography (MRE) of tissue samples with 40 micrometer pixel edge size for
analyzing the mechanical properties of murine neural tissue. The new technique revealed in specimens of cerebellum and cortical brain areas that white
matter is significantly stiffer than gray matter. Microscopic multifrequency
MRE provides insight into micro
mechanical structures of ex-vivo soft tissues and might be used in the future
to investigate fresh biopsy samples.
Introduction
Viscoelastic constants measured at different length scales provide
insight into the biophysical properties of soft tissues [1]. State-of-the-art
measurement methods for biomechanical parameter quantification in tissue
specimens can be based on scanning force microscopy, tensile tests, shear
rheometry or indentation techniques. Some of these methods can be applied to
very small tissue samples and can resolve parameters in the micrometer range. However,
all these methods are surface based and cannot directly be compared with
in-vivo measurement methods such as MR elastography (MRE). MRE can quantify
mechanical properties in bulky samples of biological soft matter at different
mechanical excitation frequencies and can be applied to in-vivo tissues
non-invasively [2]. Preclinical MRE in small animals can measure mechanical
properties on the millimeter or sub-millimeter scale [3]. Recently, high
resolution multifrequency MRE with tomoelastography post-processing was
introduced for studies of tissue mechanical parameters of the mouse brain [4].
We here present tomoelastography of neural tissue specimens in the range of
tens of micrometer resolution (μ-tomoelastography). The technique provides unprecedented spatial
resolution in MRE of small tissue samples and might thus bridge the gap between
macroscopic MRE measurements and micromechanical test methods.Methods
Unprocessed
neuronal tissues from four C57BL-6 mice were investigated in a 9.5T
horizontal bore NMR spectrometer (Bruker, Avance III Ultrashield 400 plus,
Ettlingen, Germany) equipped with imaging gradient insert, 5mm diameter
quadrature volume coil. Tissue samples were placed in a glass tube with 3.3mm inner
diameter (Wilmad NMR tube, St. Louis, MO).Tissue samples of cerebellum (CB),
spinal cord (SC) and cerebral cortex (CO) with corpus callosum (CC) were investigated
by multifrequency MRE 24 h post mortem. Cylindrical waves of 454, 500, 555,
625Hz were induced by a piezo based driver similar to the one used for tabletop
MRE [3]. Wave images polarized along the main cylinder axis were acquired in a
transversal view using a multi-shot spin-echo MRE sequence [4]. The frequency
of the sinusoidal motion encoding gradients (MEG) was identical to the
vibration frequency. Number of MEG cycles were 8, 8, 10 and 12 for 454, 500, 555
and 625Hz, respectively. Further acquisition parameters were: 1400ms repetition
time (TR), 30ms echo time (TE), 110*110
matrix size, 4.4*4.4mm2 field of view (FoV), 40*40µm²
in-plane resolution and 0.6mm slice thickness. Total acquisition time for 4 frequencies
each sampled with 6 wave dynamics was ca. 1h. Shear wave speed (SWS) maps as a
surrogate of stiffness were generated from the complex valued wave images using
multifrequency wave-number inversion (k-MDEV) [5]. Reference T2 RARE images
were acquired with the same resolution but with longer TR in order to increase
image quality (TR=3s, TE=10ms, rare factor of 6). SWS maps were overlaid on
corresponding T2 RARE images using Elastix [6]. The method was validated in cylindrical
phantoms made of ultrasound gel (Sonogel, Germany) at three different
resolutions (100, 70 and 40µm)
using identical parameters as for ex vivo measurements. Results
Figure 1 shows shear wave images and parameter
maps acquired in phantoms at three different resolutions. Mean SWS was 0.93±0.05m/s with 100µm and 70µm and 0.92±0.06m/s with 40µm pixel edge sizes. Figures 2, 3 and 4 show SWS
maps for CB, CO+CC and SC for all animals. Figure 5 shows group mean SWS values
in different areas of the brain and the spinal cord. Cerebellum and cortical brain
tissue are characterized by abundant white matter which was significantly
stiffer than surrounding tissue containing mainly gray matter. No significant
difference was found between gray and white matter in spinal cord tissue, which
is probably an effect of the small group size.Discussion & conclusion
Our study presents first MRE-measured stiffness
maps of murine neuronal tissues with an in-plane resolution of 40*40µm2. In phantom experiments,
increasing the resolution from 100 to 40µm did not influence SWS values demonstrating the
validity of our μ-tomoelastography
method. By pushing the limits of spatial resolution to an unprecedented small
pixel size, μ-tomoelastography revealed
for the first time mechanical tissue structures and stiffness variation in murine
CB, CO, CC and SC specimens including relatively stiff white matter and
relatively soft gray matter. The visually stiffer appearance of white matter
compared with gray matter in spinal tissue has to be validated by a larger group
size. It should be noted that there are divergent reports in the literature
about relative stiffnesses of gray and white matter which might be resolved by
multifrequency MRE when accounting for different length scales, dynamic ranges,
anisotropy and in-vivo versus ex vivo. μ-tomoelastography provides
insight into micro mechanical structures of ex-vivo soft tissues and might be
used in the future to investigate fresh biopsy samples. Acknowledgements
Support
from the German Research Foundation (Sa901/16, Sa901/17, Ste1450/8-1, GRK2260
BIOQIC, SFB1340 “Matrix in Vision” and Cluster of Excellence Exc 257 NeuroCURE), from the German
Federal Ministry of Education and Research (BMBF; 01EO0801, Center for Stroke Research Berlin) as
well as from the European Union’s Horizon 2020 Program (ID 668039, EU FORCE –
Imaging the Force of Cancer) is gratefully acknowledged.References
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