A compact 0.5 T MR elastography device and its application for studying viscoelasticity changes in biological tissues during progressive formalin fixation

Abstract

Purpose: To develop a method of compact tabletop magnetic resonance elastography (MRE) for rheological tests of tissue samples and to measure changes in viscoelastic powerlaw constants of liver and brain tissue during progressive fixation. Methods: A 10-mm bore, 0.5-T permanent-magnet-based MRI system was equipped with a gradient-amplifier-controlled piezo-actuator and motion-sensitive spin echo sequence for inducing and measuring harmonic shear vibrations in cylindrical samples. Shear modulus dispersion functions were acquired at 200-5700 Hz in animal tissues at different states of formalin fixation and fitted by the springpot powerlaw model to obtain shear modulus l and powerlaw exponent a. Results: In a frequency range of 300-1500 Hz, unfixed liver tissue was softer and less dispersive than brain tissue with l ¼ 1.68 6 0.17 kPa and a ¼ 0.51 6 0.06 versus l ¼ 2.60 6 0.68 kPa and a ¼ 0.68 6 0.03. Twenty-eight hours of formalin fixation yielded a 400-fold increase in liver l, 25-fold increase in brain l, and two-fold reduction in a of both tissues.

Fast tomoelastography of the mouse brain by multifrequency single‐shot MR elastography

“…Shown are the real parts of the complex‐valued wave images that are simulated or acquired in a frequency range of 800‐1200 Hz. The 7T EPI MRE sequence was based on the sequence shown in Figure B, whereas 0.5T tabletop MRE refers to a compact MRE device with Bessel fit–based shear wave speed calculation for investigations of cylindrical tissue samples…”

“…B, Signal‐to‐noise‐ratio analysis for AHI and k ‐MDEV using numerical waves. The 2D scalar wave fields in cylindrical coordinates were simulated based on a Bessel function of the first kind to reproduce waves measured in phantoms by tabletop MRE (soft material: SWS = 0.97 m/s as shown in Figure ; stiff material: SWS = 3.45 m/s as found by in vivo MRE). The ground truth is 1 due to normalization of c / c 0 .…”

“…One sample with a diameter of 13.3 mm was investigated in a 7T small‐animal MRI scanner (Bruker BioSpec 70/16, Ettlingen, Germany) using the newly developed imaging method described subsequently. A second sample with 7.5‐mm diameter was investigated in a compact 0.5T tabletop MRE device (Pure Devices, Würzburg, Germany), which we used as a reference standard …”

“…Setup and imaging parameters for phantom experiments at 7T were similar using a cylindrical sample container (inner diameter = 13.3 mm; length = 20 mm), which was inserted into the head cradle. This setup allowed us to induce well‐defined cylindrical waves for Bessel fit–based SWS analysis . Vibration frequencies were 800 Hz to 1200 Hz (100‐Hz increments)—the same as the vibration frequency range used in 0.5T tabletop MRE for reference …”

“…The following 3 SWS reconstruction methods were used: Fitting of cylindrical waves in phantoms based on Bessel functions . This method was applied to 7T and 0.5T data and was considered the reference standard.…”