Analysis of Quantitative MRI in Predicting the Biomechanical and Biochemical Properties of Porcine and Cadaveric Intervertebral Disc

• Main Authors: Li-Yu Lin, 林俐妤
• Other Authors: Jaw-Lin Wang
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/63864204277535089864

碩士 === 國立臺灣大學 === 醫學工程學研究所 === 103 === Purpose: To investigate the relationship between rheological properties, dynamic properties, biochemical properties, and T2 relaxation times (T2 values) of porcine and human cadaveric disc. Introduction: The degeneration of composition and structure of intervertebral disc often leads to the instability of spinal segment. The T2 mapping, a magnetic resonance imaging (MRI) quantitative technique, provides a non-invasive method to evaluate changes in the composition and structural integrity within the disc. A recent study reported that T2 mapping changes after diurnal loading, which indicates that the T2 mapping may also be affected by tissue’s structural change due to external loading. A quantitative analysis of relationship between T2 mapping and the static and dynamic properties of disc is less reported in literature. In this study, we hypothesized that T2 values correlated with the biomechanical properties of disc. Material and method: 55 porcine discs and 24 human discs were used in this experiment. The axial T2 weighted MRI of porcine and human discs were first scanned to quantify the morphology of nucleus pulposus (NP) and anulus fibrosus (AF), while the sagittal one were scanned to classify the degree of degeneration of human discs using Pfirrmann’s grading system. The sequence of the T2 mapping are; TR: 3650 ms; 12TE from 13.2 to 158.4ms (every 13.2ms). For the human discs, grade 5 degenerated discs were excluded for analysis. After the MR scanning, the biomechanical tests, including the creep test and dynamic mechanical analysis (DMA) test, and the biochemical content, including the water and glycosaminoglycan (GAG), of the disc were performed and analyzed. The T2 value and biomechanical properties between human and porcine discs were compared using Student’s t-test. The variation of T2 value and biomechanical properties of human discs among different degrees of degeneration were also compared using Student’s t-test. Correlations among T2 values, biomechanical properties and biochemical properties were evaluated using Pearson’s correlation. A p-value of <0.05 was considered to be statistically significant. Results: Comparison of porcine and human discs: The aggregate modulus and permeability of porcine discs are lower than the ones of human discs, while the storage modulus and loss modulus of porcine discs are higher than the one of human discs. Comparison of human discs at different degeneration: The permeability increased, but the aggregated modulus decreased as the human disc is degenerated. The phase angle of human disc increased with disc degeneration. Correlation between biomechanical properties and T2 values: For the creep test, the permeability of porcine discs positively correlated with NP T2 value, while the aggregate modulus of porcine discs is not correlated with any T2 value. The permeability of human discs negatively correlated with NP T2 value, while the aggregate modulus of human ones negatively correlated with AF T2 value. For the DMA test, the phase angle of porcine discs positively correlated with AF T2 value. The storage modulus and loss modulus of porcine disc negatively correlated with NP T2 value, but positively correlated with AF T2 value. The phase angle of human discs negatively correlated with NP T2 value, but positively correlated with the AF T2 value at lower frequencies. Correlation between biochemical properties and T2 values: The GAG and water content of AF in porcine discs is correlated with T2 value. The GAG content of NP in human discs is correlated with T2 value. Discussion: It is hard, if not possible, to find healthy human disc specimens. The biomechanical integrity and biomechanical content of porcine discs can be analogue to the one of healthy human discs. The results of two groups used in this study may reflect the consequence of healthy and degenerated discs in human. For the degenerated human discs, the increased permeability and phase angle of degenerated discs may be due to the micro-structural defect, while the decreased aggregated modulus may be due to the cross-linked collagen fibers. Quantitative T2 MRI was affected by the water content, orientation of collagen network and tissue anisotropy. The correlations of rheological and dynamic biomechanical properties respective to the T2 values are different between the healthy porcine and degenerated human disc due to the degenerative changes of tissue properties in poroelasticity and viscoelasity. For the creep test, the permeability positively correlated with the T2 value of NP in porcine disc. The higher T2 value usually implies higher water content, which may indicate that the higher interstitial fluid flows within the NP of porcine discs. In the degenerated human discs, the NP T2 values decreased but the permeability increased. The lower NP T2 value is reported to connect with the lower GAG content. The insufficient GAG content may hinder the water holding capacity of discs, hence increased the permeability of the human disc. For the DMA test, the phase angle is the most significant property that positively correlated with AF T2 value in porcine and human discs. The higher AF T2 value indicated the less organized fiber orientation and structural integrity, which may hinder the temporal response to the external loading, indicating the higher values of phase angle. In this study, we did not find the correlation between water content and T2 value, but find the correlation between GAG content and T2 value in human discs. It is speculated that T2 signal is affected by the capability of interaction with water molecule, but not the hydration level alone. Conclusion: The biomechanical properties and T2 values changed with the disc degeneration. The T2 values were shown to correlate with the rheological and dynamic mechanical properties of disc, indicating that the T2 imaging may be a potential clinical tool for the diagnosis and monitoring of degenerative disc disease.