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

目的： 探討豬與人體屍骨椎間盤軸向(Axial plane)量化核磁共振(T2弛豫時間)與椎間盤靜態及動態生物力學與生化性質的關係。 背景介紹： 椎間盤退化影響脊椎之生物力學功能，常導致各類型的神經壓迫，引發疼痛，影響患者生活品質，因此若能對椎間盤退化之力學性質提供一非侵入診斷及預測方法，對早期椎間盤退化給予適當預防措施及治療建議，將可減緩退化或減低相關臨床症狀發生。量化核磁共振技術有別於傳統T2影像，除了能作為退化分期之依據，也能根據椎間盤本身組織特性，提供量化分析方法。雖然文獻已證實生物軟組織之T2弛豫時間(T2值)會受到組織之生化含量及其結構鍵結方式影響，但對於椎間盤力學性質及量化核磁共振技術的關係，仍少有研究討論。 材料與方法： 本實驗分兩部分，第一部分先以55個豬隻椎間盤建立實驗方法，評估健康椎間盤之T2值與力學性質之關係；第二部分使用6副人體腰椎，共計24個椎間盤，評估不同退化程度椎間盤之T2值與力學性質之關係。試樣以3T核磁共振沿著椎間盤進行軸向及矢狀面掃描，軸向掃描利用不同的回音時間(TE)，將所得影像擬合，利用感興趣區域(Region of Interest, ROI)的圈選得到組織各個區域之T2值。接著將椎間盤進行一小時靜態潛變測試，負載大小在豬與人體屍骨分別為0.8MPa及0.3MPa。經過一天的恢復後，再進行動態力學測試(頻率0.031~10Hz，應力範圍0.1-0.8MPa；頻率0.031~3.15Hz，應力範圍0.1-0.6MPa)，並進行椎間盤的生化及微結構分析，生化測試包含水分及醣胺聚醣含量。 本實驗統計方法：豬與人體椎間盤及人體各個退化分期內椎間盤之力學性值差異皆採用學生t檢驗檢測；並採用Pearson相關係數分析，評估椎間盤組織T2值與整體椎間盤靜態、動態力學性質及其組織所對應之生化性質關係，統計皆以p<0.05表示達到顯著差異。 結果： 豬相較於人體椎間盤，有較低的聚合模數及液體滲透度，但有較高的儲存模數及損失模數。人體椎間盤退化等級越高，其液體滲透度增加，聚合模數降低，相位角有增加的現象。椎間盤潛變測試結果與組織T2值的相關性分析中可發現，豬隻椎間盤之液體滲透度與髓核T2值呈正相關，但人體椎間盤液體滲透度與髓核T2值呈負相關；而人體椎間盤聚合模數與椎間環T2值呈負相關。椎間盤動態力學測試結果與組織T2值的相關性分析中可發現，豬隻椎間盤與人體椎間盤相位角皆與椎間環的T2值呈正相關，人體椎間盤相位角同時也與髓核的T2值呈負相關。椎間盤組織生化性值及其所對應之T2值的相關性分析中可發現，豬隻前側椎間環T2值會與其醣胺聚醣及水分含量呈正相關；人體椎間盤髓核T2值會與醣胺聚醣含量呈正相關。 討論： 豬隻椎間盤力學性質及組織特性可類比年輕之健康人體椎間盤，由於年輕人體試樣取得實屬不易，因此本實驗使用豬隻椎間盤及人體椎間盤分別代表年輕及退化過程中椎間盤兩個族群。不同退化分期之力學差異，可能是因為退化造成之組織內微結構的破壞，使退化較嚴重之人體椎間盤有液體滲透度及相位角上升的現象；另外膠原纖維間產生之交聯，可能使退化椎間盤聚合模數有下升之趨勢。 量化T2值已被證實與水分、膠原蛋白組織結構方式及組織異相性相關，椎間盤力學性質受到組織內液體流通特性及黏彈性質所調控，因此健康椎間盤與退化椎間盤組織T2值與靜態及動態相關性分析的結果並不一致。豬隻健康椎間盤髓核與椎間環功能性分工完整，液體滲透度主要由中央髓核調控，髓核T2值較高表示內部水分含量較多，因此與液體滲透度呈正相關；人體髓核T2值下降表髓核水分及醣胺聚醣含量減少，加上膠原纖維增生，椎間盤保水能力差，使液體在內部流動能力增加，因此髓核T2值與液體滲透度負相關。而人體椎間環T2值上升表示交聯增加、水分減少、此退化跡象與椎間盤聚合模數下降有關。 豬與人體椎間盤之相位角皆會與其纖維環T2值呈正相關，纖維環T2值上升表示纖維環排列方式不規則，層與層之間區別性較弱，此結構特性可能會使椎間盤於外力作用下響應時間增長，因而造成相位角增加。 人體椎間盤髓核T2值與醣胺聚醣含量呈正相關，與水分含量則不具有相關性，此結果顯示組織T2值並非因組織內部水分含量影響，而是受到水分子間的交互作用所調控。 結論： 本研究建立豬與人體椎間盤組織T2值與整體靜態及動態力學性質及生化性質之關係，分別代表年輕及退化過程中的椎間盤兩個族群，雖然兩族群椎間盤組織T2值與整體力學相關性結果並不一致，但皆可發現具相關性存在，顯示以椎間盤組織T2值評估整體靜態及動態力學性質為一具潛力之分析方法。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