Several series of NMR spectroscopy experiments were performed to determine the relaxivity of Gd(DTPA)2− in solutions of skim-milk powder at the concentrations of 0%–40%. As discussed in several reports in literature (13 (link),33 (link)), these concentrations cover the approximate range of the macromolecular contents in most biological tissues (the solid content in cartilage is about 20%–35% (34 (link),35 (link))). R-values of 4.2 (mM sec)−1 was found in our experiment for saline at 7T and 25°C, which agrees with the relaxivity values found in literature (14 (link),29 (link),30 (link),33 (link)).
The μMRI T1-GAG imaging followed the well-documented dGEMRIC procedure in the literature (10 (link),11 (link),29 (link)–31 (link)). Briefly summarized, each cartilage block was T1-imaged before (T1before) and after (T1after) a 10-hour immersion in 1 mM solution of a commercially available Gd(DTPA)2− contrast agent (Magnevist, Berlex, NJ) at the room temperature ([H11015]25°C). The T1 im ages were converted to the Gd(DTPA)2− concentration image of cartilage and subsequently to the GAG concentration image of cartilage (30 (link)).
The μMRI experiments were performed using a Bruker AVANCE II MRI console interfaced to a 7T/89 mm superconducting magnet, commercial microimaging accessory, and a home-built 4-mm solenoid coil. The tissue block was placed in the magnet at the magic angle, which minimizes the influence of the dipolar interactions (32 (link)). The echo time (TE) of the imaging sequence was 8.6 msec and the repetition time (TR) of the imaging experiment was 1.5 and 0.5 seconds for the before- and after-soaking experiments, respectively. The 1-mm-thick imaging slice was transversely located in the middle of the 10-mm-long specimen. The 2D in-plane pixel size was 13 μm. The measurement of 2D T1 images used the inversion-recovery pulse sequence with five inversion points (for the T1before, they were 0, 0.4, 1.1, 2.2, 4.0 sec; for the T1after, they were 0, 0.1, 0.3, 0.5, 1 sec), which allowed the calculation of T1 relaxation in the tissue through a single exponential equation on a pixel-by-pixel basis. (To save time, the repetition time TR in μMRI was less than 5T1. In this case, we used a modified T1 fitting function, Y = A(1 – B exp (-t/T1), where B is less than 2 when TR [H11021] 5T1.)
The μMRI T1-GAG imaging followed the well-documented dGEMRIC procedure in the literature (10 (link),11 (link),29 (link)–31 (link)). Briefly summarized, each cartilage block was T1-imaged before (T1before) and after (T1after) a 10-hour immersion in 1 mM solution of a commercially available Gd(DTPA)2− contrast agent (Magnevist, Berlex, NJ) at the room temperature ([H11015]25°C). The T1 im ages were converted to the Gd(DTPA)2− concentration image of cartilage and subsequently to the GAG concentration image of cartilage (30 (link)).
The μMRI experiments were performed using a Bruker AVANCE II MRI console interfaced to a 7T/89 mm superconducting magnet, commercial microimaging accessory, and a home-built 4-mm solenoid coil. The tissue block was placed in the magnet at the magic angle, which minimizes the influence of the dipolar interactions (32 (link)). The echo time (TE) of the imaging sequence was 8.6 msec and the repetition time (TR) of the imaging experiment was 1.5 and 0.5 seconds for the before- and after-soaking experiments, respectively. The 1-mm-thick imaging slice was transversely located in the middle of the 10-mm-long specimen. The 2D in-plane pixel size was 13 μm. The measurement of 2D T1 images used the inversion-recovery pulse sequence with five inversion points (for the T1before, they were 0, 0.4, 1.1, 2.2, 4.0 sec; for the T1after, they were 0, 0.1, 0.3, 0.5, 1 sec), which allowed the calculation of T1 relaxation in the tissue through a single exponential equation on a pixel-by-pixel basis. (To save time, the repetition time TR in μMRI was less than 5T1. In this case, we used a modified T1 fitting function, Y = A(1 – B exp (-t/T1), where B is less than 2 when TR [H11021] 5T1.)
