Abstract
Magnetic resonance elastography (MRE) is a non-invasive imaging technique, using the propagation of mechanical waves as a probe to palpate biological tissues. It consists in three main steps: production of shear waves within the tissue; encoding subsequent tissue displacement in magnetic resonance images; and extraction of mechanical parameters based on dedicated reconstruction methods. These three steps require an acoustic-frequency mechanical actuator, magnetic resonance imaging acquisition, and a post-processing tool for which no turnkey technology is available. The aim of the present review is to outline the state of the art of reported set-ups to investigate rodent brain mechanical properties. The impact of experimental conditions in dimensioning the set-up (wavelength and amplitude of the propagated wave, spatial resolution, and signal-to-noise ratio of the acquisition) on the accuracy and precision of the extracted parameters is discussed, as well as the influence of different imaging sequences, scanners, electromagnetic coils, and reconstruction algorithms. Finally, the performance of MRE in demonstrating viscoelastic differences between structures constituting the physiological rodent brain, and the changes in brain parameters under pathological conditions, are summarized. The recently established link between biomechanical properties of the brain as obtained on MRE and structural factors assessed by histology is also studied. This review intends to give an accessible outline on how to conduct an elastography experiment, and on the potential of the technique in providing valuable information for neuroscientists.
Highlights
Elastography is an imaging technique which derives mechanical property maps from a visualization of shear waves propagating within soft biological tissues
The goal of this review is to present pros and cons of the diverse actuation, imaging, and inversion methods developed so far as well as some important results drawn out in the field of rodent brain Magnetic resonance elastography (MRE), in order to identify the remaining obstacles to pulling this field out of the hands of expert only
MRE is still a recent imaging technique and lacks standardization at small-animal scale, making reliable quantitative data difficult to obtain. It could benefit from concertation of different MRE experts and, as for human MRE, the development of a commercial solution could help to democratize rodent MRE and ascertain preliminary results
Summary
Elastography is an imaging technique which derives mechanical property maps from a visualization of shear waves propagating within soft biological tissues. Several studies in humans reported physiological differences between brain areas, along with alterations in viscoelastic parameters in various diseases (tumor, hydrocephalus and neurodegenerative disease) recently reviewed in [7, 8] These proof-of-concept studies motivated application of MRE in rodents as a crucial step to identify the pathophysiological processes underlying changes in viscoelasticity [9]. As of today, this powerful non-invasive technique has provided first in vivo mechanical characterizations of healthy and pathological rodent brain even though disparity in results can be pointed out. The goal of this review is to present pros and cons of the diverse actuation, imaging, and inversion methods developed so far as well as some important results drawn out in the field of rodent brain MRE, in order to identify the remaining obstacles to pulling this field out of the hands of expert only
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