Microscopic structural alterations of liver tissue induced by freeze-thaw cycles give rise to palpable property changes. However, the underlying damage to tissue architecture is difficult to quantify histologically, and published data on macroscopic changes in biophysical properties are sparse. To better understand the influence of hepatic cells and stroma on global biophysical parameters, we studied rat liver specimens freshly taken (within 30 min after death) and treated by freeze-thaw cycles overnight at either -20 °C or -80 °C using diffusion-weighted imaging (DWI) and multifrequency magnetic resonance elastography (MRE) performed at 0.5 T in a tabletop MRE scanner. Tissue structure was analyzed histologically and rheologic data were analyzed using fractional order derivatives conceptualized by a called spring-pot component that interpolates between pure elastic and viscous responses. Overnight freezing and thawing induced membrane disruptions and cell detachment in the space of Disse, resulting in a markedly lower shear modulus μ and apparent diffusion coefficient (ADC) (μ[-20 °C] = 1.23 ± 0.73 kPa, μ[-80 °C] = 0.66 ± 0.75 kPa; ADC[-20 °C] = 0.649 ± 0.028 μm2/s, ADC[-80 °C] = 0.626 ± 0.025 μm2/s) compared to normal tissue (μ = 9.92 ± 3.30 kPa, ADC = 0.770 ± 0.023 μm2/s, all p < 0.001). Furthermore, we analyzed the springpot-powerlaw coefficient and observed a reduction in -20 °C specimens (0.22 ± 0.14) compared to native tissue (0.40 ± 0.10, p = 0.033) and -80 °C specimens (0.54 ± 0.22, p = 0.002), that correlated with histological observations of sinusoidal dilation and collagen distortion within the space of Disse. Overall, the results suggest that shear modulus and water diffusion in liver tissue markedly decrease due to cell membrane degradation and cell detachment while viscosity-related properties appear to be more sensitive to distorted stromal and microvascular architecture.
Keywords: Diffusion-weighted imaging; Freeze-thaw cycles; Liver microarchitecture; Magnetic resonance elastography; Tabletop MRE; Viscoelasticity; Water diffusion.
Copyright © 2019. Published by Elsevier Ltd.