Download Depth-dependent Ion Concentrations in Healthy and Lesioned

Depth-dependent Ion Concentrations in Healthy and Lesioned Articular Cartilage by µCT and µMRI
+1Xia, Y; 2Oravec, D; 1 Mittelstaedt, D; 1Badar, F; 2Yeni, Y; 3 Matyas, J
+1Oakland University, Rochester, MI; 2 Henry Ford Hospital, Detroit, MI; 3University of Calgary, Alberta, Canada.
[email protected]
INTRODUCTION:
The heavily sulfated glycosaminoglycan (GAG) molecules carry a high
concentration of negative charges which are closely packed in articular
cartilage, thereby generating an osmotic pressure that contributes to the
stiffness of the tissue as a load-bearing material. The reduction of GAG
will result in a biochemically and biomechanically weakened cartilage,
eventually leading to clinical diseases such as osteoarthritis (OA). Since
the amount of GAG is proportional to the fixed charge density in the
tissue, the infusion of an anionic contrast agent can in principle be used
to assess the GAG concentration in cartilage. Recently, a gadoliniumbased contrast agent is being used in x-ray micro computed tomography
(µCT) to image the GAG concentration in articular cartilage [1-2]. This
approach is similar to the dGEMRIC procedure in micro magnetic
resonance imaging (µMRI) [3-4], where the GAG concentration can be
determined. In this preliminary project, µCT was used to image the ion
concentration in a number of tibial cartilage blocks from the canines 12weeks after the anterior cruciate ligament transection. A comparison was
made between the ion concentration in µCT and the gadolinium
concentration in µMRI experiments [4].
METHODS:
Samples: Articular cartilage from the medial tibia of four canines 12weeks after the anterior cruciate ligament transection procedure was
harvested into full thickness blocks (~ 2x2x10 mm) using a table saw.
Fresh specimens were stored in saline and imaged within 24 hours. In
addition, the identical specimens were also harvested from one canine
that did not undergo the ACL procedure (the normal-normal).
µCT: Micro-CT imaging of the canine tibial cartilage/bone specimens
was performed by a house-built micro computed tomography scanner
following the procedure by Xie et al [2]. Each specimen, placed in a
vial, was imaged twice using the same protocol, before and after soaking
for 30 min in a solution of 40% Hexabrix (Mallinckrodt, MO). X-ray
tube potential and current were 80 kV and 50 mA, respectively. The
cone beam acquisition geometry yielded a voxel size of 8 microns.
Acquisition was made using 720 views, 16 frames/view, a frame rate of
4/s and full 360 degrees rotation. Acquired projections were
geometrically aligned using third-order B-spline interpolation and
filtered using a SINC function terminated at the Nyquist frequency of
the detector panel. Projections are reconstructed using an adaptation of
the standard FDK cone-beam-reconstruction. The polycarbonate holding
tube was used as a radiographic reference to compare attenuation levels
between images.
µMRI: As a comparison, microscopic MRI experiments [4] were
performed on a Bruker AVANCE II 300MHz micro-imager, using a
quantitative T1 procedure on the humeral cartilage blocks, which were
harvested and prepared identically as the tibial blocks and were soaked
in the 1 mM Gd(DTPA)2- solution (Magnevist, Berlex, NJ). The echo
time (TE) of the imaging sequence was 8.6 msec; and the repetition time
(TR) of the experiment was 1.5 and 0.5 sec for the before- and aftersoaking experiments respectively. The 2D in-plane pixel size was 13
µm, with a 1 mm slice thickness. The acquisition of two T1 images
before- and after-soaking enabled the calculation of the gadolinium and
GAG concentrations in cartilage quantitatively [4].
RESULTS:
Fig 1a is an intensity image from a µCT experiment; and Fig 1b is a
gadolinium concentration image from a µMRI experiment, which was
calculated based on two quantitative T1 images (before- and aftersoaking in Gd(DTPA)2- solution). Both µCT and µMRI images (Fig 1)
have sufficient resolutions to resolve the depth dependency of the ion
concentrations in articular cartilage.
The ion concentrations as measured by the µCT procedure and by the
µMRI procedure were compared in Fig 2. Although the physical
mechanisms of the imaging procedures are quite different in these two
techniques, the distributions of the charged ion particles that were
diffused into the cartilage tissue based on the local concentration of the
GAG molecules show identical trends in these two measurements.
Fig 3 shows the depth dependent profiles of the µCT attenuation ratios
(after-soaking/before-soaking) from several specimens of tibial ACL
cartilage. It is clear that there are large variations among the attenuation
profiles of the 12wk ACL tissues, which could be due to the spatial
distribution of the particular lesions on the tibial surfaces of these
canines - a clear indication of the complex changes of the GAG
concentrations in the lesion tissue. Despite of these complex variations,
the depth dependent trends of these profiles are consistent – decreasing
from the articular surface to the cartilage/bone interface, which indicate
an increasing amount of GAG in tissues.
CONCLUSIONS:
This preliminary study shows that the µCT procedure can be used to
probe the ion concentrations in articular cartilage and that the µCT
profiles have similar trends as the ion concentration profiles in µMRI.
With the appropriate calibrations and calculations in the µCT imaging
procedures, these ion concentration profiles can be translated to the
GAG profiles in the tissue [4]. The ability to image the GAG
concentration non-destructively at high resolution will facilitate further
understanding in the progression of the tissue degradation leading to the
clinical diseases.
Acknowledgement: Y Xia is supported by the R01 grants from the
National Institutes of Health (AR 045172 and AR 052353). The authors
are grateful to Dr C Les and Dr H Sabbah (Henry Ford Hospital, Detroit)
for providing the canine joints, to Dr F Nelson (Henry Ford Hospital,
Detroit), Mr Adam Schroeder (Covidien Imaging Solutions, Michigan),
and Ms J Spann (Michigan Resonance Imaging, Rochester Hills,
Michigan) for helping with the contrast agents.
REFERENCES:
[1] A Aula et al, Osteoarthritis Cartilage 2009; 17: 1538. [2] L Xie et al,
Osteoarthritis Cartilage 2010; 18: 65. [3] A Bashir et al, Magn Reson
Med 1996; 36: 665. [4] Y Xia et al, J Magn Reson Imag 2008; 28: 151.
Poster No. 1609 • ORS 2011 Annual Meeting