Non‐invasive measurement of brain viscoelasticity using magnetic resonance elastography

Abstract

The purpose of this work was to develop magnetic resonance elastography (MRE) for the fast and reproducible measurement of spatially averaged viscoelastic constants of living human brain. The technique was based on a phase-sensitive echo planar imaging acquisition. Motion encoding was orthogonal to the image plane and synchronized to intracranial shear vibrations at driving frequencies of 25 and 50 Hz induced by a head-rocker actuator. Ten time-resolved phase-difference wave images were recorded within 60 s and analyzed for shear stiffness and shear viscosity. Six healthy volunteers (six men; mean age 34.5 years; age range 25-44 years) underwent 23-39 follow-up MRE studies over a period of 6 months. Interindividual mean +/- SD shear moduli and shear viscosities were found to be 1.17 +/- 0.03 kPa and 3.1 +/- 0.4 Pas for 25 Hz and 1.56 +/- 0.07 kPa and 3.4 +/- 0.2 Pas for 50 Hz, respectively (P < or = 0.01). The intraindividual range of shear modulus data was 1.01-1.31 kPa (25 Hz) and 1.33-1.77 kPa (50 Hz). The observed modulus dispersion indicates a limited applicability of Voigt's model to explain viscoelastic behavior of brain parenchyma within the applied frequency range. The narrow distribution of data within small confidence intervals demonstrates excellent reproducibility of the experimental protocol. The results are necessary as reference data for future comparisons between healthy and pathological human brain viscoelastic data.

Convertible pneumatic actuator for magnetic resonance elastography of the brain

“…An important restriction that has to be considered prior to usage of such an actuator type is its limitation to lowfrequency excitation [8]. Recently published brain elastography studies exploited frequencies in the range of 25-100 Hz [10], [12] and [14]. This is very similar to the frequency range achieved with a pneumatic approach, where a loud speaker is connected to a flexible hose itself connected to an actuator placed near the organ of interest [15], [16] and [17].…”

“…An example of such an MRE setup has been used in Ref. [12], where the subject's head is placed on a head cradle coupled to an external loudspeaker, acting as a source of vibrations, with a flexible carbon fiber rod of variable length. Alternatively, a pneumatic concept where the subject's head is resting on two flexible drum membranes which are positioned in a "V" shape underneath the head and are periodically acoustically excited with a phase difference of 180° has been reported in Ref.…”

Non-identifiability of the Rayleigh damping material model in Magnetic Resonance Elastography

“…Magnetic Resonance Elastography (MRE) can directly visualise and measure tissue elasticity [5][6][7][8], and has been applied to resolve stiffness characteristics of a variety human tissues and organs, such as muscle [9][10][11][12], breast [13][14][15][16][17], liver [18][19][20] and the brain [21][22][23][24][25][26][27][28][29]. MRE acquisition requires application of mechanical waves to tissue within the MRI, phase-contrast MR pulse sequence extended with motion encoding gradient (MEGs), and sophisticated inverse problem methods to identify an elastic modulus map of the tissue.…”

“…To date, reconstruction approaches generally assumed tissue to be linearly elastic [30], although some groups have employed more advanced models, such as viscoelasticity (VE) [23,26,27] and poroelasticity [31]. Rayleigh damping (RD) is an extension of VE, which utilises an additional damping parameter to provide a more complex description of the elastic energy attenuation.…”

“…More specifically, tissue composed from tightly arranged cells displays less damping compared to the highly-saturated porous tissue. Thus, accurate quantification of damping properties might aid in early diagnosis of various degenerative diseases, such as Alzheimer's disease [23], Normal Pressure Hydrocephalus (NPH) [33] and Multiple Sclerosis (MS) [26].…”