Modifications of orientational dependence of microscopic magnetic resonance imaging T(2) anisotropy in compressed articular cartilage

Finite deformation elastography of articular cartilage and biomaterials based on imaging and topology optimization

Tissues and engineered biomaterials exhibit exquisite local variation in stiffness that defines their function. Conventional elastography quantifies stiffness in soft (e.g. brain, liver) tissue, but robust quantification in stiff (e.g. musculoskeletal) tissues is challenging due to dissipation of high frequency shear waves. We describe new development of finite deformation elastography that utilizes magnetic resonance imaging of low frequency, physiological-level (large magnitude) displacements, coupled to an iterative topology optimization routine to investigate stiffness heterogeneity, including spatial gradients and inclusions. We reconstruct 2D and 3D stiffness distributions in bilayer agarose hydrogels and silicon materials that exhibit heterogeneous displacement/strain responses. We map stiffness in porcine and sheep articular cartilage deep within the bony articular joint space in situ for the first time. Elevated cartilage stiffness localized to the superficial zone is further related to collagen fiber compaction and loss of water content during cyclic loading, as assessed by independent T2 measurements. We additionally describe technical challenges needed to achieve in vivo elastography measurements. Our results introduce new functional imaging biomarkers, which can be assessed nondestructively, with clinical potential to diagnose and track progression of disease in early stages, including osteoarthritis or tissue degeneration.

Magnetic resonance imaging (MRI) studies of knee joint under mechanical loading: Review

Osteoarthritis (OA) is a very common disease that affects the human knee joint, particularly the articular cartilage and meniscus components which are regularly under compressive mechanical loads. Early-stage OA diagnosis is essential as it allows for timely intervention. The primary non-invasive approaches currently available for OA diagnosis include magnetic resonance imaging (MRI), which provides excellent soft tissue contrast at high spatial resolution. MRI-based knee investigation is usually performed on joints at rest or in a non-weight-bearing condition that does not mimic the actual physiological condition of the joint. This discrepancy may lead to missed detections of early-stage OA or of minor lesions. The mechanical properties of degenerated musculoskeletal (MSK) tissues may vary markedly before any significant morphological or structural changes detectable by MRI. Recognizing distinct deformation characteristics of these tissues under known mechanical loads may reveal crucial joint lesions or mechanical malfunctions which result from early-stage OA. This review article summarizes the large number of MRI-based investigations on knee joints under mechanical loading which have been reported in the literature including the corresponding MRI measures, the MRI-compatible devices employed, and potential challenges due to the limitations of clinical MRI sequences.

Experimental Influences in the Accurate Measurement of Cartilage Thickness in MRI

OBJECTIVE

To study the experimental influences to the measurement of cartilage thickness by magnetic resonance imaging (MRI).

DESIGN

The complete thicknesses of healthy and trypsin-degraded cartilage were measured at high-resolution MRI under different conditions, using two intensity-based imaging sequences (ultra-short echo [UTE] and multislice-multiecho [MSME]) and 3 quantitative relaxation imaging sequences (T₁, T₂, and T₁ρ). Other variables included different orientations in the magnet, 2 soaking solutions (saline and phosphate buffered saline [PBS]), and external loading.

RESULTS

With cartilage soaked in saline, UTE and T₁ methods yielded complete and consistent measurement of cartilage thickness, while the thickness measurement by T₂, T₁ρ, and MSME methods were orientation dependent. The effect of external loading on cartilage thickness is also sequence and orientation dependent. All variations in cartilage thickness in MRI could be eliminated with the use of a 100 mM PBS or imaged by UTE sequence.

CONCLUSIONS

The appearance of articular cartilage and the measurement accuracy of cartilage thickness in MRI can be influenced by a number of experimental factors in ex vivo MRI, from the use of various pulse sequences and soaking solutions to the health of the tissue. T₂-based imaging sequence, both proton-intensity sequence and quantitative relaxation sequence, similarly produced the largest variations. With adequate resolution, the accurate measurement of whole cartilage tissue in clinical MRI could be utilized to detect differences between healthy and osteoarthritic cartilage after compression.

Compressed sensing in quantitative determination of GAG concentration in cartilage by microscopic MRI

PURPOSE

To evaluate the potentials of compressed sensing (CS) in MRI quantification of glycosaminoglycan (GAG) concentration in articular cartilage at microscopic resolution.

METHODS

T1 -weighted 2D experiments of cartilage were fully sampled in k-space with five inversion times at 17.6 μm resolution. These fully sampled k-space data were re-processed, by undersampling at various 1D and 2D CS undersampling factors (UFs). The undersampled data were reconstructed individually into 2D images using nonlinear reconstruction, which were used to calculate 2D maps of T1 and GAG concentration. The values of T1 and GAG in cartilage were evaluated at different UFs (up to 16, which used 6.25% of the data). K-space sampling pattern and zonal variations were also investigated.

RESULTS

Using 2D variable density sampling pattern, the T1 images at UFs up to eight preserved major visual information and produced negligible artifacts. The GAG concentration remained accurate for different sub-tissue zones at various UFs. The variation of the mean GAG concentration through the whole tissue depth was 1.20%, compared to the fully sampled results. The maximum variation was 2.24% in the deep zone of cartilage. Using 1D variable density sampling pattern, the quantitative T1 mapping and GAG concentration at UFs up to 4 showed negligible variations.

CONCLUSION

This study demonstrates that CS could be beneficial in microscopic MRI (µMRI) studies of cartilage by acquiring less data, without losing significant accuracy in the quantification of GAG concentration. Magn Reson Med 79:3163-3171, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

T2 MR imaging vs. computational modeling of human articular cartilage tissue functionality

The detection of early stages of cartilage degeneration remains diagnostically challenging. One promising non-invasive approach is to functionally assess the tissue response to loading by serial magnetic resonance (MR) imaging in terms of T2 mapping under simultaneous mechanical loading. As yet, however, it is not clear which cartilage component contributes to the tissue functionality as assessed by quantitative T2 mapping. To this end, quantitative T2 maps of histologically intact cartilage samples (n=8) were generated using a clinical 3.0-T MR imaging system. Using displacement-controlled quasi-static indentation loading, serial T2 mapping was performed at three defined strain levels and loading-induced relative changes were determined in distinct regions-of-interest. Samples underwent conventional biomechanical testing (by unconfined compression) as well as histological assessment (by Mankin scoring) for reference purposes. Moreover, an anisotropic hyperelastic constitutive model of cartilage was implemented into a finite element (FE) code for cross-referencing. In efforts to simulate the evolution of compositional and structural intra-tissue changes under quasi-static loading, the indentation-induced changes in quantitative T2 maps were referenced to underlying changes in cartilage composition and structure. These changes were parameterized as cartilage fluid, proteoglycan and collagen content as well as collagen orientation. On a pixel-wise basis, each individual component correlation with T2 relaxation times was determined by Spearman's ρ_s and significant correlations were found between T2 relaxation times and all four tissue parameters for all indentation strain levels. Thus, the biological changes in functional MR Imaging parameters such as T2 can further be characterized to strengthen the scientific basis of functional MRI techniques with regards to their perspective clinical applications.

A comparison of multi-echo spin-echo and triple-echo steady-state T2 mapping for in vivo evaluation of articular cartilage

OBJECTIVES

To assess the clinical relevance of T2 relaxation times, measured by 3D triple-echo steady-state (3D-TESS), in knee articular cartilage compared to conventional multi-echo spin-echo T2-mapping.

METHODS

Thirteen volunteers and ten patients with focal cartilage lesions were included in this prospective study. All subjects underwent 3-Tesla MRI consisting of a multi-echo multi-slice spin-echo sequence (CPMG) as a reference method for T2 mapping, and 3D TESS with the same geometry settings, but variable acquisition times: standard (TESSs 4:35min) and quick (TESSq 2:05min). T2 values were compared in six different regions in the femoral and tibial cartilage using a Wilcoxon signed ranks test and the Pearson correlation coefficient (r). The local ethics committee approved this study, and all participants gave written informed consent.

RESULTS

The mean quantitative T2 values measured by CPMG (mean: 46±9ms) in volunteers were significantly higher compared to those measured with TESS (mean: 31±5ms) in all regions. Both methods performed similarly in patients, but CPMG provided a slightly higher difference between lesions and native cartilage (CPMG: 90ms→61ms [31%],p=0.0125;TESS 32ms→24ms [24%],p=0.0839).

CONCLUSIONS

3D-TESS provides results similar to those of a conventional multi-echo spin-echo sequence with many benefits, such as shortening of total acquisition time and insensitivity to B1 and B0 changes.

KEY POINTS

• 3D-TESS T 2 mapping provides clinically comparable results to CPMG in shorter scan-time. • Clinical and investigational studies may benefit from high temporal resolution of 3D-TESS. • 3D-TESS T 2 values are able to differentiate between healthy and damaged cartilage.

MRI properties of a unique hypo-intense layer in degraded articular cartilage

To investigate the characteristics of a hypo-intense laminar appearance in articular cartilage under external loading, microscopic magnetic resonance imaging (μMRI) T1, T2 and T1ρ experiments of a total of 15 specimens of healthy and trypsin-degraded cartilage were performed at different soaking solutions (saline and 100 mM phosphate buffered saline (PBS)). T2 and T1ρ images of the healthy tissue in saline showed no load-induced laminar appearance, while a hypo-intense layer was clearly visible in the deep part of the degraded tissue at the magic angle. A significant difference was found between T2 values at 0° and 55° (from 16.5  ±  2.8 ms to 20.2  ±  2.7 ms, p  =  0.0005), and at 0° and 90° (16.5  ±  2.8 ms to 21.3  ±  2.6 ms, p  <  0.0001) in saline solution. In contrast, this hypo-intense laminar appearance largely disappeared when tissue was soaked in PBS. The visualization of this hypo-intensity appearance in different soaking mediums calls for caution in interpreting the data of relaxation times, chemical exchange and collagen fiber deformation.

Comparison of load responsiveness of cartilage T1rho and T2 in porcine knee joints: an experimental loading MRI study

OBJECTIVE

To compare changes in T1rho and T2 values of the femoral cartilage in porcine knee joints under staged loading and unloading conditions.

DESIGN

Sixteen porcine knee joints with intact capsules and surrounding muscle were imaged using a custom-made pressure device and 3.0 T magnetic resonance imaging. Sagittal T1rho and T2 images were obtained for the lateral and medial condyles under the following compression loads: none (Load 0), 140 N (Load 140), 300 N (Load 300), and no compression after decompression (Post-load). The percentage changes of cartilage T1rho and T2 values under each loading condition from those at Load 0 were calculated for weight-bearing overall and eight subdivided regions of interest (ROIs) in both femoral condyles. The actual contact pressure under Load 140 and Load 300 was measured using pressure-sensitive film.

RESULTS

For the overall ROI, the mean decreases of T1rho and T2 values were 4.4% and 5.1% under Load 140% and 10.9% and 10.6% under Load 300 in the medial condyle and were 5.2% and 4.0% under Load 140% and 10.6% and 6.0% under Load 300 in the lateral condyle. In the medial condyle, the actual contact pressure correlated highly with percentage changes in T1rho (r = -0.84, P < 0.01) and T2 (r = -0.79, P < 0.01), but those correlations were relatively low in the lateral condyle.

CONCLUSION

Although there were side-dependent variations in the correlations with actual pressure, cartilage T1rho and T2 showed similarly sensitive responses to applied load.

Loading-induced changes on topographical distributions of the zonal properties of osteoarthritic tibial cartilage--A study by magnetic resonance imaging at microscopic resolution

The topographical distributions of the zonal properties of articular cartilage over the medial tibia from an experimental osteoarthritis (OA) model were evaluated as a function of external loading by microscopic Magnetic Resonance Imaging (µMRI). T2 relaxation times and cartilage thicknesses were measured at 17.6 µm resolution from 118 specimens, which came from thirteen dogs (six 8-week and seven 12-week after surgery), with and without mechanical loading. In addition, bulk mechanical modulus was measured topographically from each tibia surface. The total thickness decreased significantly under the external loading, in which the relative thickness of the superficial zone (SZ) and the transitional zone (TZ) increased whereas the radial zones (RZs) decreased. In the bulk data, T2(55°) decreased significantly (p<0.001) at all OA-time-points, but T2(0°) decreased without significance (p>0.05) at 8-week. Complex relationships were found in the zonal tissue properties as a function of external loading with the progress of OA. T2 in the superficial zone changed more profoundly than the same properties in the radial zone as a function of external loading at all OA time-points. This study confirms that OA affects the load-induced changes in the molecular distribution and structure of cartilage, which are both depth-dependent and topographically distributed. Such detailed knowledge of mechanobiological changes in specific tibial cartilage zones and locations with OA progress could improve the early detection of the subtle softening of cartilage that accompanies pre-clinical stages of OA.

Effects of cryopreservation on the depth-dependent elastic modulus in articular cartilage and implications for osteochondral grafting

Cryopreservation of articular cartilage is often used in storage of experimental samples and osteochondral grafts, but the depth-dependence and concentration of glycosaminoglycan (GAG) are significantly altered when cryogenically stored without a cryoprotectant, which will reduce cartilage stiffness and affect osteochondral graft function and long-term viability. This study investigates our ability to detect changes due to cryopreservation in the depth-dependent elastic modulus of osteochondral samples. Using a direct-visualization method requiring minimal histological alterations, unconfined stepwise stress relaxation tests were performed on four fresh (never frozen) and three cryopreserved (-20 °C) canine humeral head osteochondral slices 125 ± 5 μm thick. Applied force was measured and tissue images were taken at the end of each relaxation phase using a 4× objective. Intratissue displacements were calculated by tracking chondrocytes through consecutive images for various intratissue depths. The depth-dependent elastic modulus was compared between fresh and cryopreserved tissue for same-depth ranges using analysis of variance (ANOVA) with Tukey post-test with a 95% confidence interval. Cryopreservation was found to significantly alter the force-displacement profile and reduce the depth-dependent modulus of articular cartilage. Excessive collagen fiber folding occurred at 40-60% relative depth, producing a "black line" in cryopreserved tissue. Force-displacement curves exhibited elongated toe-region in cryopreserved tissue while fresh tissue had nonmeasurable toe-region. Statistical analysis showed significant reduction in the elastic modulus and GAG concentration throughout the tissue between same-depth ranges. This method of cryopreservation significantly reduces the depth-dependent modulus of canine humeral osteochondral samples.

New insights into the thermal behaviour of organic ionic plastic crystals: magnetic resonance imaging of polycrystalline morphology alterations induced by solid–solid phase transitions

Morphology alterations induced by solid–solid phase transitions in Organic Ionic Plastic Crystals (OIPC) elucidate molecular dynamics, micro-structural behaviour and conductive properties of OIPCs.

Topographical variations of the strain-dependent zonal properties of tibial articular cartilage by microscopic MRI

The topographical variations of the zonal properties of canine articular cartilage over the medial tibia were evaluated as the function of external loading by microscopic magnetic resonance imaging (µMRI). T2 and T1 relaxation maps and GAG (glycosaminoglycan) images from a total of 70 specimens were obtained with and without the mechanical loading at 17.6 µm depth resolution. In addition, mechanical modulus and water content were measured from the tissue. For the bulk without loading, the means of T2 at magic angle (43.6 ± 8.1 ms), absolute thickness (907.6 ± 187.9 µm) and water content (63.3 ± 9.3%) on the meniscus-covered area were significantly lower than the means of T2 at magic angle (51.1 ± 8.5 ms), absolute thickness (1251.6 ± 218.4 µm) and water content (73.2 ± 5.6%) on the meniscus-uncovered area. However GAG (86.0 ± 15.3 mg/ml) on the covered area was significantly higher than GAG (70.0 ± 8.8 mg/ml) on the uncovered area. Complex relationships were found in the tissue properties as the function of external loading. The tissue parameters in the superficial zone changed more profoundly than the same properties in the radial zone. The tissue parameters in the meniscus-covered areas changed differently when comparing with the same parameters in the uncovered areas. This project confirms that the load-induced changes in the molecular distribution and structure of cartilage are both depth-dependent and topographically distributed. Such detailed knowledge of the tibial layer could improve the early detection of the subtle softening of the cartilage that will eventually lead to the clinical diseases such as osteoarthritis.

Molecular origin of a loading-induced black layer in the deep region of articular cartilage at the magic angle

PURPOSE

To investigate the molecular origin of an unusual low-intensity layer in the deep region of articular cartilage as seen in magnetic resonance imaging (MRI) when the tissue is imaged under compression and oriented at the magic angle.

MATERIALS AND METHODS

Microscopic MRI (μMRI) T2 and T1 ρ experiments were carried out on 18 specimens, both native and degraded (treated with trypsin). The glycosaminoglycan (GAG) concentrations in the specimens were quantified by both sodium ICP-OES and μMRI Gd(DTPA)(2-) -contrast methods. The mechanical modulus of the specimens was also measured.

RESULTS

Native tissue shows no load-induced layer, while the trypsin-degraded tissue shows clearly the low-intensity line at the deep part of tissue. The GAG reductions were confirmed by the sodium ICP-OES (from 81.7 ± 5.4 mg/mL to 9.2 ± 3.4 mg/mL), MRI GAG quantification (from 72.4 ± 6.7 mg/mL to 11.2 ± 2.9 mg/mL). The modulus reduction was confirmed by biomechanics (from 4.3 ± 0.7 MPa to 0.3 ± 0.1 MPa).

CONCLUSION

Both T2 and T1 ρ profiles in native and degraded cartilage show strongly strain-, depth-, and angle-dependence using high-resolution MRI. The GAG reduction is responsible for the visualization of a low-intensity layer in deep cartilage when it is loaded and oriented at 55°.

Hypointense signal lesions of the articular cartilage: a review of current concepts

Discussion of articular cartilage disease detection by MRI usually focuses on the presence of bright signal on T2-weighted sequences, such as in Grade 1 chondromalacia and cartilage fissures containing fluid. Less emphasis has been placed on how cartilage disease may be manifested by dark signal on T2-weighted sequences. The appearance of the recently described "cartilage black line sign" of the femoral trochlea highlights these lesions and further raises the question of their etiology. We illustrate various hypointense signal lesions that are not restricted to the femoral trochlea of the knee joint and discuss the possible etiologies for these lesions.

The effects of mechanical loading and gadolinium concentration on the change of T1 and quantification of glycosaminoglycans in articular cartilage by microscopic MRI

Microscopic MRI (µMRI) T1 experiments were carried out to investigate the strain dependence of the T1 change and glycosaminoglycans (GAG) quantification in articular cartilage at a spatial resolution of 17.6 µm. Both native and trypsin-degraded specimens were immersed in various concentrations of gadolinium (Gd) (up to 1 mM) and imaged at different strains (up to 50% strains). Adjacent specimens were treated identically and analyzed biochemically by an inductively coupled plasma optical emission spectrometer. The T1 profile in the native tissue was found to be both strain-dependent and depth-dependent, while there was no obvious depth-dependence in the degraded tissue. For the native tissue, compression reduced the tissue T1 when Gd in the solution was low (less than 0.4 mM) and increased the tissue T1 when Gd in the solution was high. A set of critical points, where the tissue T1 showed no change at a certain Gd concentration between two different loadings, was observed for the first time in the native tissue. It is concluded that the GAG quantification by MRI was accurate as long as the Gd concentration in the solution reached at least 0.2 mM (tissue not loaded) or 0.4 mM (tissue loaded).

MRI of articular cartilage at microscopic resolution

This review briefly summarises some of the definitive studies of articular cartilage by microscopic MRI (µMRI) that were conducted with the highest spatial resolutions. The article has four major sections. The first section introduces the cartilage tissue, MRI and µMRI, and the concept of image contrast in MRI. The second section describes the characteristic profiles of three relaxation times (T1, T2 and T1ρ) and self-diffusion in healthy articular cartilage. The third section discusses several factors that can influence the visualisation of articular cartilage and the detection of cartilage lesion by MRI and µMRI. These factors include image resolution, image analysis strategies, visualisation of the total tissue, topographical variations of the tissue properties, surface fibril ambiguity, deformation of the articular cartilage, and cartilage lesion. The final section justifies the values of multidisciplinary imaging that correlates MRI with other technical modalities, such as optical imaging. Rather than an exhaustive review to capture all activities in the literature, the studies cited in this review are merely illustrative.

MR imaging of articular cartilage physiology

The newer magnetic resonance (MR) imaging methods can give insights into the initiation, progression, and eventual treatment of osteoarthritis. Sodium imaging is specific for changes in proteoglycan (PG) content without the need for an exogenous contrast agent. T1ρ imaging is sensitive to early PG depletion. Delayed gadolinium-enhanced MR imaging has high resolution and sensitivity. T2 mapping is straightforward and is sensitive to changes in collagen and water content. Ultrashort echo time MR imaging examines the osteochondral junction. Magnetization transfer provides improved contrast between cartilage and fluid. Diffusion-weighted imaging may be a valuable tool in postoperative imaging.

Strain-dependent T1 relaxation profiles in articular cartilage by MRI at microscopic resolutions

To investigate the dependency of T(1) relaxation on mechanical strain in articular cartilage, quantitative magnetic resonance T(1) imaging experiments were carried out on cartilage before/after the tissue was immersed in gadolinium contrast agent and when the tissue was being compressed (up to ∼ 48% strains). The spatial resolution across the cartilage depth was 17.6 μm. The T(1) profile in native tissue (without the presence of gadolinium ions) was strongly strain-dependent, which is also depth-dependent. At the modest strains (e.g., 14% strain), T(1) reduced by up to 68% in the most surface portion of the tissue. Further compression (e.g., 45% strain) reduced T(1) mostly in the middle and deep portions of the tissue. For the gadolinium-immersed tissue, both modest and heavy compressions (up to 48% strain) increased T(1) slightly but significantly, although the overall shapes of the T(1) profiles remained approximately the same regardless of the amount of strains. The complex relationships between the T(1) profiles and the mechanical strains were a direct consequence of the depth-dependent proteoglycan concentration in the tissue, which determined the tissue's mechanical properties. This finding has potential implications in the use of gadolinium contrast agent in clinical magnetic resonance imaging of cartilage (the dGEMRIC procedure), when the loading or loading history of patients is considered.

Reversed laminar appearance of articular cartilage by T1-weighting in 3D fat-suppressed spoiled gradient recalled echo (SPGR) imaging

PURPOSE

To investigate the reversed intensity pattern in the laminar appearance of articular cartilage by 3D fat-suppressed spoiled gradient recalled echo (FS-SPGR) imaging in magnetic resonance imaging (MRI).

MATERIALS AND METHODS

The 3D SPGR experiments were carried out on canine articular cartilage with an echo time (TE) of 2.12 msec, a repetition time (TR) of 60 msec, and various flip angles (5 degrees to 80 degrees ). In addition, T1, T2, and T2* in cartilage were imaged and used to explain the laminar appearance in SPGR imaging.

RESULTS

The profiles of T2 and T2* in cartilage were similar in shape. However, the T2 values from the multigradient-echo imaging sequence were about 1/3 of those from single spin-echo sequences at a pixel resolution of 26 mum. While the laminar appearance of cartilage in spin-echo imaging is caused mostly by T2-weighting, the laminar appearance of cartilage in fast imaging (ie, short TR) at the magic angle can have a reversed intensity pattern, which is caused mostly by T1-weighting.

CONCLUSION

The laminar appearance of articular cartilage can have opposite intensity patterns in the deep part of the tissue, depending on whether the image is T1-weighted or T2-weighted. The underlying molecular structure and experimental protocols should both be considered when one examines cartilage images in MRI.

Site-specific effects of compression on macromolecular diffusion in articular cartilage

Articular cartilage is the connective tissue that lines joints and provides a smooth surface for joint motion. Because cartilage is avascular, molecular transport occurs primarily via diffusion or convection, and cartilage matrix structure and composition may affect diffusive transport. Because of the inhomogeneous compressive properties of articular cartilage, we hypothesized that compression would decrease macromolecular diffusivity and increase diffusional anisotropy in a site-specific manner that depends on local tissue strain. We used two fluorescence photobleaching methods, scanning microphotolysis and fluorescence imaging of continuous point photobleaching, to measure diffusion coefficients and diffusional anisotropy of 70 kDa dextran in cartilage during compression, and measured local tissue strain using texture correlation. For every 10% increase in normal strain, the fractional change in diffusivity decreased by 0.16 in all zones, and diffusional anisotropy increased 1.1-fold in the surface zone and 1.04-fold in the middle zone, and did not change in the deep zone. These results indicate that inhomogeneity in matrix structure and composition may significantly affect local diffusive transport in cartilage, particularly in response to mechanical loading. Our findings suggest that high strains in the surface zone significantly decrease diffusivity and increase anisotropy, which may decrease transport between cartilage and synovial fluid during compression.

Molecular and morphological adaptations in compressed articular cartilage by polarized light microscopy and Fourier-transform infrared imaging

Fifteen articular cartilage-bone specimens from one canine humeral joint were compressed in the strain range of 0-50%. The deformation of the extracellular matrices in cartilage was preserved and the same tissue sections were studied using polarized light microscopy (PLM) and Fourier-transform infrared imaging (FTIRI). The PLM results show that the most significant changes in the apparent zone thickness due to 'reorganization' of the collagen fibrils based on the birefringence occur between 0% and 20% strain values, where the increase in the superficial zone and decrease in the radial zone thicknesses are approximately linear with the applied strain. The FTIRI anisotropy results show that the two amide components with bond direction perpendicular to the external compression retain anisotropy (amide II in the superficial zone and amide I in the radial zone). In contrast, the measured anisotropy from the two amide components with bond direction parallel to the external compression changes their anisotropy significantly (amide I in the superficial zone and amide II in the radial zone). Statistical analysis shows that there is an excellent correlation (r=0.98) between the relative depth of the minimum retardance in PLM and the relative depth of the amide II anisotropic cross-over. The changes in amide anisotropies in different histological zones are explained by the strain-dependent tipping angle of the amide bonds. These depth-dependent adaptations to static loading in cartilage's morphological structure and chemical distribution could be useful in the future studies of the early diseased cartilage.

Averaged and depth-dependent anisotropy of articular cartilage by microscopic imaging

OBJECTIVES

To identify the common connections among the averaged and depth-dependent anisotropic properties of articular cartilage by performing a meta-analysis of several published multidisciplinary imaging results. The imaging techniques involved include microscopic magnetic resonance imaging (microMRI), polarized light microscopy (PLM), Fourier-transform infrared imaging (FTIRI), and transmission electron microscopy (TEM).

METHODS

Several physical properties of cartilage are incorporated in this meta-analysis. These tissue properties include T(2) anisotropy from microMRI, angle and retardance from PLM, infrared anisotropy from FTIRI, and image morphology from TEM. Because the specimens in these studies all came from the same type of canine humeral joints, it is possible to correlate these multidisciplinary tissue properties using a common platform.

RESULTS

An ellipse model was used to identify the connections among these tissue properties in terms of the anisotropy of articular cartilage, in each histological zone as well as for the entire noncalcified tissue. It was found that many aspects of these tissue properties can be interpreted beyond their usual meanings as measured, based on 3 features of an ellipse: the concentration, the orientation, and the anisotropy.

CONCLUSIONS

The ellipse model is a useful graphical concept in cartilage imaging since it helps to bring together the measured physical/morphological/chemical quantities in these imaging tools and the anisotropic structure of articular cartilage. Two possible mechanisms for the angular transition of collagen fibrils in cartilage are discussed.

Morphological changes in articular cartilage due to static compression: polarized light microscopy study

We studied the deformation of the extracellular matrices in articular cartilage using a new compression-preservation method in histology. A Hoffman clamp was used to compress the tissue, which remained throughout the paraffin procedure and was removed from the embedded tissue block just before microtoming. Then 14 cartilage-bone blocks from 2 canine humeri were compressed for various strain levels from 5% to 65%. The histological sections were studied using a polarized light microscope, which generated a pair of two-dimensional maps of the fibril orientation (angle) and fibril organization (retardance) for each section. Results were 3-fold. One there was little change in the angle and retardance profiles of the tissue for strain levels 0-15% and a significant change in these profiles for strain levels 15% and above. Two for higher compression, more fibrils became aligned parallel to the articular surface; and three at approximately 30% strain, a second "transitional zone" was formed in the deep part of the tissue. We concluded that this novel compression procedure can be used effectively to study the altered architecture of the collagen matrix in compressed cartilage.