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Research Papers

Internal Three-Dimensional Strains in Human Intervertebral Discs Under Axial Compression Quantified Noninvasively by Magnetic Resonance Imaging and Image Registration

[+] Author and Article Information
Jonathon H. Yoder

Department of Mechanical Engineering and
Applied Mechanics,
University of Pennsylvania,
Philadelphia, PA 19104
e-mail: hyoder@seas.upenn.edu

John M. Peloquin

Department of Bioengineering,
University of Pennsylvania,
Philadelphia, PA 19104
e-mail: peloquin@seas.upenn.edu

Gang Song

Department of Radiology,
University of Pennsylvania,
Philadelphia, PA 19104
e-mail: songgang@seas.upenn.edu

Nick J. Tustison

Department of Radiology and Medical Imaging,
University of Virginia,
Charlottesville, VA 22904
e-mail: ntustison@virginia.edu

Sung M. Moon

Department of Radiology,
University of Pennsylvania,
Philadelphia, PA 19104
MR Systems,
GE Healthcare,
Florence, SC 29501
e-mail: Sung.Moon@ge.com

Alexander C. Wright

Department of Radiology,
University of Pennsylvania,
Philadelphia, PA 19104
e-mail: alexander.wright@uphs.upenn.edu

Edward J. Vresilovic

Penn State Hershey Bone and Joint Institute,
Pennsylvania State University,
Hershey, PA 17033
e-mail: evresilovic@gmail.com

James C. Gee

Department of Radiology,
University of Pennsylvania,
Philadelphia, PA 19104
e-mail: gee@mail.med.upenn.edu

Dawn M. Elliott

Department of Biomedical Engineering,
University of Delaware,
Newark, DE 19716
e-mail: delliott@udel.edu

1Corresponding author.

Manuscript received February 28, 2014; final manuscript received August 2, 2014; accepted manuscript posted August 12, 2014; published online September 17, 2014. Assoc. Editor: James C. Iatridis.

J Biomech Eng 136(11), 111008 (Sep 17, 2014) (9 pages) Paper No: BIO-14-1099; doi: 10.1115/1.4028250 History: Received February 28, 2014; Revised August 02, 2014; Accepted August 12, 2014

Study objectives were to develop, validate, and apply a method to measure three-dimensional (3D) internal strains in intact human discs under axial compression. A custom-built loading device applied compression and permitted load-relaxation outside of the magnet while also maintaining compression and hydration during imaging. Strain was measured through registration of 300 μm isotropic resolution images. Excellent registration accuracy was achieved, with 94% and 65% overlap of disc volume and lamellae compared to manual segmentation, and an average Hausdorff, a measure of distance error, of 0.03 and 0.12 mm for disc volume and lamellae boundaries, respectively. Strain maps enabled qualitative visualization and quantitative regional annulus fibrosus (AF) strain analysis. Axial and circumferential strains were highest in the lateral AF and lowest in the anterior and posterior AF. Radial strains were lowest in the lateral AF, but highly variable. Overall, this study provided new methods that will be valuable in the design and evaluation surgical procedures and therapeutic interventions.

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References

Figures

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Fig. 1

(a) Loading frame interfaced with Instron (red arrow), showing locking mechanism, segment grips, disc, and sliding tank (white arrows). (b) Loading frame integrated with RF coil (green arrows) in MRI. B0 = direction of magnetic field.

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Fig. 2

Images ((a)–(c)) are oriented to show coronal (left), axial (top-right), and sagittal (bottom) planes. (a) Representative MRI data set. (b) The volume used for strain analysis (pink). (c) AF regions of interest defined in the midaxial plane: A = anterior (red), A–L = anterior–lateral (green), L = lateral (blue), P–L = posterior–lateral (yellow), P = posterior (aqua).

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Fig. 3

Pictorial representation of the image registration process, resultant warp field, and displacement map. The reference image is registered to the deformed image defining a warp field that prescribes how structures within the reference image are mapped to the deformed image. The deformation gradient tensor is applied to calculate the Lagrangian strain tensor.

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Fig. 4

(a) Generation of lamellar structure labels using Sobel edge detection (red), shown in three planes. A representative label is shown in green. (b) and (c) Five identified lamellar labels, shown in midaxial view and as 3D projections, respectively. Labels identified by white arrows.

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Fig. 5

Transformation of Cartesian coordinates to local disc coordinates using the disc's outer contour, scaled to intersect each voxel: (a) circumferential basis vectors defined by the contour's tangent and (b) radial basis vectors defined by the contour's normal. Note the complex vector directions imposed by the lamellar curvature.

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Fig. 6

Registration of a representative lamellar label (green), shown in coronal (left), axial (top-right), and sagittal (bottom) views. Difference between original and registered label is small (red), demonstrating good registration. Scale bar = 1 cm.

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Fig. 7

Axial strains for all discs obtained by manual measurement and by image registration, showing good agreement (R2= 0.79, p < 0.05).

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Fig. 8

Strain maps for 10% axial compression in a representative disc: (a) axial strain in coronal and sagittal views (left and right, respectively); (b) circumferential strain in axial view; and (c) radial strain in axial view. Scale bar = 5 cm.

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Fig. 9

Mean and standard deviation of AF regional strain at midaxial height when loaded to 15% compression for (a) axial, (b) circumferential, and (c) radial strain. A = anterior, A–L = anterior–lateral, L = lateral, P–L = posterior–lateral, P = posterior. Region locations are shown in Fig. 2(c). A solid line represents significance at p < 0.05 and a dashed line trend at 0.05 < p < 0.10.

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