Our approach to imaging the glycosaminoglycan (GAG) component of cartilage
is based on the fact that since GAGs have abundant charged side groups that
confer a negative fixed charge to the cartilage matrix, mobile ions will
distribute in the tissue to reflect the local GAG concentration. In order
to use proton MRI for detection of the ion, we used the MRI contrast agent
Gd-DTPA 2-. The logic for the method is as follows: Gd-DTPA 2- will distribute into areas in cartilage that are depleted of
GAG at a higher concentration than in areas of high GAG. Gd-DTPA 2- has concentration dependent effects on the magnetic resonance
relaxation time T1; therefore T1 images in presence of Gd-DTPA2- reflect the tissue GAG concentration. We call this Gd-DTPA 2- enhanced method dGEMRIC (delayed Gd-DTPA 2-
Enhanced MRI of Cartilage). For more information on the protocol issues in
the application of dGEMRIC for both in vitro and in vivo applications,
please refer to the protocol section of this website.
This figure illustrates the basis of dGEMRIC: The GAG are represented by
the yellow negative signs fixed to the cartilage matrix. The mobile
negative ions, represented by the red negative signs (in this case Gd-DTPA 2-) will distribute in higher concentration in areas depleted of
Therefore, T1 in the tissue, which is dominated by the Gd-DTPA 2- concentration, will be reflective of the GAG
distribution in the tissue.
If the tissue is in equilibrium with the Gd-DTPA 2-, the
absolute GAG concentration can be determined using a modified
of ion distribution in tissue.