Relaxivity and diffusion of gadolinium agents in cartilage

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Abstract
Prior work indicates that the distribution of Gd(DTPA)2‐ (as measured by T1) is a good surrogate measure of the distribution of gycosaminoglycan (GAG) in cartilage. In addition to the measured T1 in the presence of Gd(DTPA)2‐, the precision of the measurement of Gd(DTPA)2‐ concentration depends on the T1 without Gd(DTPA)2‐ (T), and the relaxivity (r) of Gd(DTPA)2‐ in cartilage, parameters that are influenced by cartilage composition. These parameters were measured in native and GAG‐depleted cartilage in order to estimate the bounds on the values one might expect for cartilage in arbitrary states of degeneration. The range of T was 0.3 sec; the range of r was 0.6 (mM*s)‐1 at 8.5 T and 1.4 (mM*s)‐1 at 2 T. These data suggest that Gd(DTPA)2‐ will be underestimated (and GAG overestimated) if the values for T and r are assumed to be those of native cartilage. (For example, in a severe case a 90% loss of GAG would be underestimated as a 70% loss.) Gd(HPDO3A) was investigated as a nonionic “control agent” and found to have relaxivity and diffusion properties that were comparable to Gd(DTPA)2‐ (rGd(HPDO3A)/rGd(DTPA) ≈ 1; DGd(HPDO3A)/DGd(DTPA) ≈ 0.85). Since Gd(HPDO3A) distributes uniformly through cartilage (independent of GAG), the distribution of T1 with Gd(HPDO3A) can be used as a surrogate measure of variations in T and r, if present. From the perspective of transport, if Gd(HPDO3A) has fully penetrated the cartilage, Gd(DTPA)2‐ would have in the same time frame. Therefore, the data confirm the efficacy of using Gd(HPDO3A) as a “control agent” for dGEMRIC. Magn Reson Med 48:1068–1071, 2002.