Magnetic resonance imaging of articular cartilage: ex vivo study on normal cartilage correlated with magnetic resonance microscopy

Knee Cartilage Imaging

Articular cartilage injury and degeneration represent common causes of knee pain, which can be evaluated accurately and noninvasively using MRI. This review describes the structure of cartilage focusing on its histologic appearance to emphasize that structure will dictate patterns of tissue failure as well as MR appearance. In addition to identifying cartilage loss, MRI can demonstrate signal changes that correspond to intrinsic structural abnormalities which place the cartilage at risk for subsequent more serious injury or premature degeneration, allowing for earlier intervention and treatment of important causes of pain and morbidity.

Regional variations in MR relaxation of hip joint cartilage in subjects with and without femoralacetabular impingement

The objective of this study was to analyze regional variations of magnetic resonance (MR) relaxation times (T1ρ and T2) in hip joint cartilage of healthy volunteers and subjects with femoral acetabular impingement (FAI). Morphological and quantitative images of the hip joints of 12 healthy volunteers and 9 FAI patients were obtained using a 3T MR scanner. Both femoral and acetabular cartilage layers in each joint were semi-automatically segmented on sagittal 3D high-resolution spoiled gradient echo (SPGR) images. These segmented regions of interest (ROIs) were automatically divided radially into twelve equal sub-regions (30(0) intervals) based on the fitted center of the femur head. The mean value of T1ρ/T2 was calculated in each sub-region after superimposing the divided cartilage contours on the MR relaxation (T1ρ/T2) maps to quantify the relaxation times. T1ρ and T2 relaxation times of the femoral cartilage were significantly higher in FAI subjects compared to healthy controls (39.9±3.3 msec in FAI vs. 35.4±2.3msec in controls for T1ρ (P=0.0020); 33.9±3.1 msec in FAI vs. 31.1±1.7 msec in controls for T2 (P=0.0160)). Sub-regional analysis showed significantly different T1ρ and T2 relaxation times in the anterior-superior region (R9) of the hip joint cartilage between subjects with FAI and healthy subjects, suggesting possible regional differences in cartilage matrix composition between these two groups. Receiver operating characteristic (ROC) analysis showed that sub-regional analysis in femoral cartilage was more sensitive in discriminating FAI joint cartilage from that of healthy joints than global analysis of the whole region (T1ρ: area under the curve (AUC)=0.981, P=0.0001 for R9 sub-region; AUC=0.901, P=0.002 for whole region; T2: AUC=0.976, P=0.0005 for R9 sub-region; AUC=0.808, P=0.0124 for whole region). The results of this study demonstrated regional variations in hip cartilage composition using MR relaxation times (T1ρ and T2) and suggested that analysis based on local regions was more sensitive than global measures in subjects with and without FAI.

Magnetization transfer analysis of cartilage repair tissue: a preliminary study

PURPOSE

To evaluate the magnetization transfer ratio (MTR) after two different cartilage repair procedures, and to compare these data with the MTR of normal cartilage.

DESIGN AND PATIENTS

Twenty-seven patients with a proven cartilage defect were recruited: 13 were treated with autologous chondrocyte implantation (ACI) and 14 were treated with the microfracture technique (MFR). All patients underwent MRI examinations with MT-sequences before the surgical treatment, after 12 months (26 patients) and after 24 months (11 patients). Eleven patients received a complete follow-up study at all three time points (five of the ACI group and six of the MFR group). All images were transferred to a workstation to calculate MTR images. For every MT image set, different ROIs were delineated by two radiologists. Means were calculated per ROI type in the different time frames and in both groups of cartilage repair. The data were analyzed with unpaired t- and ANOVA tests, and by calculating Pearson's correlation coefficient.

RESULTS

No significant differences were found in the MTR of fatty bone marrow, muscle and normal cartilage in the different time frames. There was a significant but small difference between the MTR of normal cartilage and the cartilage repair area after 12 months for both procedures. After 24 months, the MTR of ACI repaired cartilage (0.31+/-0.07) was not significantly different from normal cartilage MTR (0.34+/-0.05). The MTR of MFR repaired cartilage (0.28+/-0.02), still showed a significant difference from normal cartilage.

CONCLUSION

The differences between damaged and repaired cartilage MTR are too small to enable MT-imaging to be a useful tool for postoperative follow-up of cartilage repair procedures. There is, however, an evolution towards normal MTR-values in the cartilage repair tissue (especially after ACI repair).

Multinuclear NMR and microscopic MRI studies of the articular cartilage nanostructure

Studies of the structure of articular cartilage by a number of NMR spectroscopic and imaging techniques are reviewed. Advantage is taken of the fact that the NMR investigations can be done non-invasively on the intact tissue and do not require sectioning, slicing and decalcification as in the case of electron microscopy. The different contributions to 1H T2 relaxation are described and it is pointed out that ignoring the biexponential behavior of the transverse relaxation can lead to serious errors in the proton density measurements and the T2 characterization of the articular cartilage. A way to slow the transverse relaxation and to minimize its angular dependence by the use of dipolar echo is described. 2H double quantum filtered spectroscopic MRI is a powerful technique to follow the orientation and density of the collagen fibers in articular cartilage. Using this technique, it was found that attachment of the cartilage to the bone has a stabilizing effect on the collagen matrix and that the hydroxyapatite in the calcified zone resides near the collagen fibers but does not contribute to their order. In response to mechanical pressure, it was shown that the collagen fibers flatten near the surface and become crimped near the bone. A number of NMR techniques have been described for the measurement of 23Na residual quadrupolar interaction. It was found that this can serve as a very sensitive measure of the depletion of proteoglycans. Finally, a combination of the above techniques was used to study a maturation of articular cartilage in pigs. The increased order and density of the collagen fibers from newborn to adult pigs revealed itself as a shortening of T2 and significant increase of the residual quadrupolar interaction of both 2H and 23Na nuclei.

MR Imaging of the Normal Hip

This article reviews the complex normal anatomy of the hip joint and its surrounding structures on MR imaging, including MR arthrography.Thorough knowledge of the normal appearance of the marrow and osseous and articular anatomy as well as the ligaments, tendons, and surrounding muscles of the hip is essential for imaging diagnosis.

The effect of detachment of the articular cartilage from its calcified zone on the cartilage microstructure, assessed by 2H-spectroscopic double quantum filtered MRI

Most studies on articular cartilage properties have been conducted after detachment of the cartilage from the bone. In the present work we investigated the effect of detachment on collagen fiber architecture. We used one-dimensional (2)H double quantum filtered MRI on cartilage bone plugs equilibrated in deuterated saline. The quadrupolar splittings observed in the different zones were related to the degree of order and the density of the collagen fibers. The method is non-destructive, allowing for measurements on the same plug without the need for fixation, dehydration, sectioning and decalcification. Detachment of the radial from the calcified zone resulted in swelling of the cartilage plug in physiological saline and a concomitant decrease in the quadrupolar splitting. The effect of mechanical pressure on the (2)H quadrupolar splittings for the detached cartilage and for the calcified zone-bone plugs were compared with those of the same zones in the intact cartilage-bone plug. The splitting in the radial zone of the detached cartilage collapsed at much smaller loads compared to the intact cartilage-bone plug. The effect of the load on the size of the cartilage was also greater for the detached plug. These results indicate that anchoring of the cartilage to the bone through the calcified zone plays an important role in retaining the order of the collagen fibers. The water (2)H quadrupolar splitting in intact and proteoglycan-depleted cartilage was the same, indicating that the proteoglycans do not contribute to the ordering of the collagen fibers.

Ultrastructural MR imaging techniques of the knee articular cartilage: problems for routine clinical application

The high incidence of cartilage lesions together with new surgical treatment techniques have necessitated the development of noninvasive cartilage evaluation techniques. Although arthroscopy has been the standard for cartilage evaluation, MR imaging has emerged as the imaging method of choice, allowing morphological evaluation of cartilage and cartilage repair tissue, as well as evaluation of its biochemical content. This article deals with current ultrastructural MR imaging techniques for cartilage evaluation, indicating the advantages as well as the drawbacks for routine clinical application.

Mapping the fiber orientation in articular cartilage at rest and under pressure studied by 2H double quantum filtered MRI

The one-dimensional (2)H double quantum filtered (DQF) spectroscopic imaging technique was used to study the orientation of collagen fibers in articular cartilage. The method detects only water molecules in anisotropic environments, which in cartilage is caused by their interaction with the collagen fibers. A large quadrupolar splitting was observed in the calcified zone and a smaller splitting in the radial zone. In the transitional zone the splitting was not resolved and a small splitting was again detected in the superficial zone. From measurements performed at two orientations of the plug relative to the magnetic field it was deduced that in the calcified and radial zones the fibers are oriented perpendicular to the bone, bending at the transitional zone and flattening at the superficial zone. The effect of load applied to the cartilage-bone plug was monitored by the same technique. At low loads there is a small decrease in the quadrupolar splitting in the calcified zone, a marked decrease in the radial zone, and an increase of the splitting accompanied by a thickening of the superficial zone. Under high loads, while the thickening and the splitting of the superficial zone further increase, the splitting in the radial and calcified zones completely collapse. Pressure-induced changes in the thickness of the surface zone indicate flattening of the collagen fibers near the surface. The marked collapse of the splitting near the bone at high pressures may result from crimping of the collagen fibers.

Induction of a neoarthrosis by precisely controlled motion in an experimental mid-femoral defect

Bone regeneration during fracture healing has been demonstrated repeatedly, yet the regeneration of articular cartilage and joints has not yet been achieved. It has been recognized however that the mechanical environment during fracture healing can be correlated to the contributions of either the endochondral or intramembranous processes of bone formation, and to resultant tissue architecture. Using this information, the goal of this study was to test the hypothesis that induced motion can directly regulate osteogenic and chondrogenic tissue formation in a rat mid-femoral bone defect and thereby influence the anatomical result. Sixteen male Sprague Dawley rats (400 +/- 20 g) underwent production of a mid-diaphyseal, non-critical sized 3.0 mm segmental femoral defect with rigid external fixation using a custom designed four pin fixator. One group of eight animals represented the controls and underwent surgery and constant rigid fixation. In the treatment group the custom external fixator was used to introduce daily interfragmentary bending strain in the eight treatment animals (12 degree angular excursion), with a hypothetical symmetrical bending load centered within the gap. The eight animals in the treatment group received motion at 1.0 Hz, for 10 min a day, with a 3 days on, one day off loading protocol for the first two weeks, and 2 days on, one day off for the remaining three weeks. Data collection included histological and immunohistological identification of tissue types, and mean collagen fiber angles and angular conformity between individual fibers in superficial, intermediate, and deep zones within the cartilage. These parameters were compared between the treatment group, rat knee articular cartilage, and the control group as a structural outcome assessment. After 35 days the control animals demonstrated varying degrees of osseous union of the defect with some animals showing partial union. In every individual within the mechanical treatment group the defect completely failed to unite. Bony arcades developed in the experimental group, capping the termini of the bone segments on both sides of the defect in four out of six animals completing the study. These new structures were typically covered with cartilage, as identified by specific histological staining for Type II collagen and proteoglycans. The distribution of collagen within analogous superficial, intermediate, and deep zones of the newly formed cartilage tissue demonstrated preferred fiber angles consistent with those seen in articular cartilage. Although not resulting in complete joint development, these neoarthroses show that the induced motion selectively controlled the formation of cartilage and bone during fracture repair, and that it can be specifically directed. They further demonstrate that the spatial organization of molecular components within the newly formed tissue, at both microanatomical and gross levels, are influenced by their local mechanical environment, confirming previous theoretical models.

Structural evaluation of articular cartilage: potential contribution of magnetic resonance techniques used in clinical practice

OBJECTIVE

To determine whether routine magnetic resonance imaging (MRI) techniques can detect age-related structural modifications of bovine articular cartilage.

METHODS

The cartilage of 3-month-old, 3-year-old, and 13-year-old animals was studied. T1- and T2-weighted MR sequences were performed using a 1.5T clinical imager and a 3-inch surface coil. Histologic slices (5 microm) of cartilage specimens were stained with picrosirius red (for collagen) and toluidine blue (for glycosaminoglycans [GAGs]). A polarized light study was performed to determine the collagen network organization. Except for the 13-year-old animal cartilage, the biochemical content was studied on slices cut parallel to the surface to determine GAG and hydroxyproline (collagen) content. Cartilage profiles were performed to determine the MR pixel intensity and the histologic color intensity.

RESULTS

On T1-weighted images, the cartilage was homogeneous, with pixel intensity profiles presenting low variations. On T2-weighted images, the cartilage was laminar in the 3-month-old animals and became homogeneous thereafter. The pixel intensity varied through the cartilage depth with a profile that depended on the age of the animal. The collagen and GAG staining showed abrupt transitions in the 3-month-old animal, while in older animals the cartilage became more homogeneous with a mild gradient of matrix constituents with depth. These results were confirmed by findings of a biochemical study. In addition to these matrix content variations, the bovine cartilage presented modifications of its collagen network organization with aging.

CONCLUSION

The MR T2-weighted sequences depicted signal variations with age in bovine cartilage concomitant with modifications in its structure. If confirmed in clinics, these observations will reinforce the place of MRI in characterizing cartilage with aging and pathologic processes.

T2 relaxation reveals spatial collagen architecture in articular cartilage: a comparative quantitative MRI and polarized light microscopic study

It has been suggested that orientational changes in the collagen network of articular cartilage account for the depthwise T2 anisotropy of MRI through the magic angle effect. To investigate the relationship between laminar T2 appearance and collagen organization (anisotropy), bovine osteochondral plugs (N = 9) were T2 mapped at 9.4T with cartilage surface normal to the static magnetic field. Collagen fibril arrangement of the same samples was studied with polarized light microscopy, a quantitative technique for probing collagen organization by analyzing its ability to rotate plane polarized light, i.e., birefringence (BF). Depthwise variation of safranin O-stained proteoglycans was monitored with digital densitometry. The spatially varying cartilage T2 followed the architectural arrangement of the collagen fibril network: a linear positive correlation between T2 and the reciprocal of BF was established in each sample, with r = 0.91 +/- 0.02 (mean +/- SEM, N = 9). The current results reveal the close connection between the laminar T2 structure and the collagen architecture in histologic zones.

Quantitative in situ correlation between microscopic MRI and polarized light microscopy studies of articular cartilage

OBJECTIVE

To establish the correlation between the non-invasive imaging by magnetic resonance microscopy (microMRI) and the histological imaging by polarized light microscopy (PLM) accurately, quantitatively, at the highest possible MRI resolution (13.7 microm), and based on the same piece of tissue (articular cartilage from canine shoulder joint).

DESIGN

In microMRI experiments, the laminar appearance (the magic angle effect) of the proton intensity images and the anisotropic characteristics of the T(2)relaxation images were analysed. In PLM experiments, the images of the optical retardation and collagen-fibre orientation in cartilage were constructed in two dimensions.

RESULTS

The T(2)profile has a distinctly asymmetric bell-shaped curve and three featured zones. The retardation profile has a non-zero minimum at the middle of the transitional zone of the tissue. The angle profile has a smooth variation across the transitional zone. These facts suggest that the collagen fibres in the transitional zone are not entirely random but have a residual order. In addition, the peak of the T(2)profile coincides with the minimum of the retardation profile, both represent the most isotropic region of the tissue. A hyperbolic tangent function was found to best describe the transition of the collagen fibres in cartilage. A set of criteria was developed for each technique to define the features in the quantitative measurements.

CONCLUSIONS

The criteria offer, for the first time, a set of quantitative and objective means to subdivide the tissue thickness into the zones in histology and in MRI. It is shown that the microMRI zones based on the T(2)characteristics are statistically equivalent to the histological zones based on the collagen fibre orientation (t-probabilities of 0.730, 0.973, 0.647, 0.850 for the superficial, transitional, radial zones and the total thickness).

MRI techniques in early stages of cartilage disease

Cartilage degenerative diseases affect millions of people. Our understanding of these diseases and our ability to establish efficacious treatment strategies have been confounded by the difficulty of nondestructively evaluating the state of cartilage. Imaging strategies that allow visualization of cartilage integrity would revolutionize the field by allowing us to visualize early stages of degeneration and thus to evaluate predisposing factors for cartilage disease and changes resulting from interventions (eg, therapies) in culture studies, tissue-engineered systems, animal models, and in vivo in humans. Here we briefly review current state-of-the-art MRI strategies relevant to understanding and following treatment in early cartilage degeneration. We review MRI as applied to the assessment of the whole joint, of cartilage as a whole (as an organ), of cartilage tissue, and of cartilage molecular composition and structure. Each of these levels is amenable to assessment by MRI and offers different information that, in the long run, will serve as an important element of cartilage imaging.

Magic-angle effect in magnetic resonance imaging of articular cartilage: a review

RATIONALE AND OBJECTIVES

The laminar appearance of articular cartilage in magnetic resonance (MR) images has been a source of confusion, especially concerning the number, intensity, thickness, and origin of the layers. The laminar appearance is associated with the magic-angle effect in the MR imaging (MRI) of articular cartilage.

METHODS

This article introduces the topic with background information about cartilage and the magic-angle effect and then reviews the literature about the magic-angle effect. The review concludes with a brief discussion of the future directions of study and the potential clinical relevance of the laminae in MR images of articular cartilage.

CONCLUSIONS

The magic-angle effect is commonly seen in MR images of several tissues. The direct cause of the laminar appearance of articular cartilage is the T2 relaxation anisotropy in the tissue, which is closely linked to the structure of the collagen fibers, their orientation in the magnetic field, and the water-proteoglycan interaction that amplifies the prevailing orientation of the collagen fiber network. The laminar appearance of cartilage has an intrinsic spatial heterogeneity over the two-dimensional joint surface, which leads to inconsistencies in the reported total number of cartilage laminae and the laminar patterns observable in MRI, depending on where the sample was taken. Two additional thin, low-intensity laminae may also be visible at the boundaries of the cartilage with fluid and with bone; whether these boundary laminae are identified and counted with the others may introduce inconsistency in the results reported by various researchers.

Heterogeneity of cartilage laminae in MR imaging

The purpose of this study was to investigate the discrepancy in the number of laminae observed in magnetic resonance (MR) images of articular cartilage (the magic angle effect in MRI of cartilage). Microscopic MR imaging (muMRI) experiments were carried out at 14-micrometer pixel resolution on full-depth cartilage-bone plugs from several locations (central, intermediate, and peripheral) on the humeral heads of two young healthy beagles. When the articular surface of the plug was perpendicular to the direction of the magnetic field, the cartilage appeared to have two layers in the plugs from the central locations of the humeral head, three layers in the plugs from the greater tubercle side of the humeral head, and three or five layers in the plugs from the lesser tubercle side. This heterogeneity of cartilage laminae was observed within a single humeral head and was symmetrical about the median plane of the animal. This result suggests that some structural variations related to cartilage structure in various regions of load bearing may cause some unique laminar patterns seen in MRI of cartilage. This novel and new observation may resolve the controversy about whether cartilage appears as two or three layers in MR images. A comprehensive model for the collagen structure over a curved two-dimensional surface of a joint is suggested as a replacement of the classic three-zone model of fiber orientation in collagen. This heterogeneity of cartilage laminae is speculated to be related to the load-bearing status of the tissue in the joint. The ability to visualize such structural heterogeneity is important because of the direct connection between collagen structure and the mechanical characteristics of cartilage.