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Investigation of Regional Influence of Magic-Angle Effect on T2 in Human Articular Cartilage with Osteoarthritis at 3 T

Posted Dec 31, 2014 Updated Apr 10, 2023
By Ligong Wang PhD , Ravinder R. Regatte PhD
6 min read

Rationale and Objectives

The objectives of this research study were to determine the magic-angle effect on different subregions of in vivo human femoral cartilage through the quantitative assessment of the effect of static magnetic field orientation ( B 0 ) on transverse (T 2 ) relaxation time at 3.0 T.

Materials and Methods

Healthy volunteers ( n = 5; mean age, 36.4 years) and clinical patients ( n = 5; mean age, 64 years) with early osteoarthritis (OA) were scanned at 3.0-T magnetic resonance using an 8-channel phased-array knee coil (transmit-receive).

Results

The T 2 maps revealed significantly greater values in anterior than in posterior regions. When the cartilage regions were oriented at 55° to B 0 (magic angle), the longest T 2 values were detected in comparison with the neighboring regions oriented 90° and 180° (0°) to B 0 . The subregions oriented 180° (0°) to B 0 showed the lowest T 2 values.

Conclusions

The differences in T 2 values of different subregions suggest that magic-angle effect needs to be considered when interpreting cartilage abnormalities in OA patients.

The primary macromolecules in human cartilage are collagen type II and proteoglycans. Proteoglycan is responsible for much of the compressive stiffness through electrostatic repulsion, whereas collagen provides the tensile and shear strength . The earliest biochemical changes in osteoarthritis (OA) are the modifications at the molecular level of cartilage matrix, which occur without obvious morphologic changes. The loss of glycosaminoglycan and the collagen breakdown are the typical characteristics of early OA .

Human articular cartilage is a highly ordered and depth-dependent ultrastructure and is essentially defined by the organization of the collagen fibrils . Collagen fibers in histology have three predominant organizational zones across the depth of the cartilage tissue. In the superficial (tangential) zone, the collagen fabrils are parallel to the cartilage surface, whereas in the radial (deep) zone, the fabrils are oriented perpendicular to the surface. However, in the transitional (middle/intermediate) zone, the arrangement of collagen fibers is almost random. The characteristic arrangement of collagen fibers results in the “magic-angle effect” and exhibits anisotropic properties when measured at different tissue depths and from different physical orientations in the proton magnetic resonance (MR) images .

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Materials and methods

Human Subjects

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Imaging Hardware

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Imaging Protocol

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MR Images Analysis and Processing

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Results

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Figure 1, (a) The schematic diagram of the arrangement of collagen fibers across different layers of human cartilage. Three magnified details of the local regions of human femoral cartilage display the corresponding subregions oriented 90°, 55° (magic angle), and 180° with respect to the external static magnetic field B 0 , respectively. (b) The plot showing the (3 cos 2 θ −1) factor as a function of angle with respect to B 0 for nuclear dipolar interaction. Two arrows identify the discrete sampling points where (3 cos 2 θ −1), the θ equals approximately 55° and 125°, respectively, and the magic-angle effect may emerge in these two sampling positions. Other arrows show the sampling positions where the (3 cos 2 θ −1) factor has the maximal and minimal values, respectively.

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Figure 2, Two representative T 2 ( top row ) slices obtained from an osteoarthritis (OA) patient were displayed (a and b ). A series of subregions on the femoral cartilage segmented at every 20 ° with respect to B 0 (c and d ). The T 2 profiles (e) of the corresponding subregions segmented in c and d .

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Figure 3, The bar charts of the average anterior and posterior T 2 values in subregions oriented 55°, 90°, and 180° relative to B 0 for healthy controls, respectively.

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Figure 4, The bar charts of the average anterior and posterior T 2 values in subregions oriented 55°, 90°, and 180° relative to B 0 for osteoarthritis subjects, respectively.

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Figure 5, The combined bar charts of the average anterior and posterior T 2 values in subregions oriented 55°, 90°, and 180° relative to B 0 for healthy and osteoarthritis (OA) subjects, respectively.

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Discussion

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Conclusions

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Acknowledgments

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Academic Radiology, Volume 22, Issue 1
Journals General Radiology
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