Magnetic Resonance Elastography of Rodent Brain

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.

Keywords: MRI; brain; magnetic resonance elastography; neurodegenerative diseases; rodent.

Figure 1

Illustration of different brain MRE set-ups reported in the litterature. (1) Picture of a piezoelectric actuator linked to a bite-bar by a plastic rod, Clayton 2011. (2) schema of a modal actuator driving a vertical piston hitting the back of the rodent head, Chatelin 2016. (3) Electromagnetic actuator driving a vertical piston glued on the mouse skull, Atay 2008. (4) Schema of the external transducer transmitting excitation to a nose cone maintained on the mouse head by rubber bands and picture of the nose cone and plots maintaining the mouse head, Patz 2017. Advantages (+) and drawbacks (–) of the actuation transmitters and actuators are listed. Actuation movements are symbolized by orange arrows. Pictures extracted from (14) © Institute of Physics and Engineering in Medicine. Reproduced by permission of IOP Publishing. All rights reserved; and (20).

Figure 2

Summary of the rodent brain areas studied by MRE, labeled as follows: -(first line) name of brain area, with relevant publications listed below; -(second line) frequency ranges used to probe the area; -(third line) when available, the minimum value of |G^*| - the average of |G^*| values reported at 900 and 1,000 Hz – maximum reported |G^*| value. |G^*| was chosen as it is the most fequently reported parameter and 900-1,000 Hz studies the most frequent in the literature. 1—Atay 2008; 2—Bertalan 2017; 3—Boulet 2011; 4—Châtelin 2016; 5—Clayton 2012; 6—Diguet 2009; 7—Freimann 2013; 8—Hain 2016; 9—Jamin 2015; 10—Jugé 2016; 11—Klein 2014; 12—Majumdar 2017; 13—Millward 2015; 14—Munder 2017; 15—Murphy 2012; 16—Patz 2017; 17 – Salameh 2011; 18—Schregel 2012; 19—Vappou 2008; 20—Yin 2017.