1-Norm: Analyzing Changes in Mechanical Wave Motion in Soft Biological Tissue Caused by a Freeze and Thaw Cycle

The mechanical properties of biological tissue are altered as a result of a freezing and thawing process, leading to a decrease in the stiffness, as has been demonstrated in an earlier study. In the current study, we use mechanical wave motion in conjunction with Magnetic Resonance Imaging to study the influence of the freeze-thaw cycle on tissue heterogeneity. To accomplish this, we generate mechanical wave motion inside biological tissue samples using a piezoelectric actuator and Magnetic Resonance Imaging to investigate the propagation and scattering of mechanical wavefronts inside the tissue sample. We use a technique, known as 1-Norm analysis, which uses a Fourier transformation to quantify the scattering of mechanical waves inside biological tissues. The validity and relevance of the 1-Norm has been demonstrated on both anisotropic gelatin-based fiber phantoms and on excised porcine lumbus muscle tissue. In an earlier work, the investigators had demonstrated that higher values of 1-Norm corresponded to a higher degree of mechanical wave scattering, implying that the 1-Norm served as a measure for the extent of tissue inhomogeneity. This finding forms the basis of our current investigation on frozen tissue samples. Three porcine kidney pairs were harvested from three freshly sacrificed porcine subjects at a local slaughterhouse. Samples were prepared into disc shaped specimens of diameter 9.6 mm, and embedded inside glass capillary tubes. A first set of experiments was performed on freshly excised tissue allowed to settle to ambient room temperature (~? hours since excision), and a subsequent set of experiments were performed after freezing the samples at -20^oC for 24 hours and thawing them to ambient room temperature. Mechanical wave motion was introduced into the samples (placed inside glass capillary tubes) using piezoelectric actuators, and the resulting mechanical waves were imaged using Magnetic Resonance Imaging on a 0.5 T table top MRI scanner, at four mechanical frequencies of 500 Hz, 1000 Hz, 1500 Hz and 2000 Hz. MRI data was acquired with a field of view of 9.6 mm x 9.6 mm, a slice thickness of 5 mm and 128 x 128 pixels in the read and phase directions. A 1-Norm analysis on the delineated mechanical wave front (at zero wave displacement), revealed a decrease in the value of 1-Norm for the frozen and thawed tissue sample in comparison to the fresh tissue sample. This was due to decreased scattering of mechanical wavefronts, which indicated a reduction in the tissue heterogeneity as a result of freezing and thawing. This finding complements prior research work led by our group, which used a rheological fitting of shear stiffness parameters to the Maxwell model and demonstrated a reduction in the tissue stiffness as a result of freezing and thawing. Our present study uses a new approach that does not involve fitting data into rheological models, but uses contours of mechanical wave motion to calculate the inhomogeneity of the underlying tissue sample, which is manifested in the form of scattering of mechanical wavefronts. In the long term, we hope to establish the 1-Norm as a quantitative means of assessing inhomogeneity.