Effects of formalin fixation and collagen cross-linking on T2 and magnetization transfer in bovine nasal cartilage

Changes in Proton Dynamics in Articular Cartilage Caused by Phosphate Salts and Fixation Solutions

The objective was to study the effect of phosphate salts and fixation solutions on the proton dynamics in articular cartilage in vitro. Microscopic magnetic resonance imaging (μMRI) T(2) anisotropy and nuclear magnetic resonance (NMR) double quantum-filtered (DQF) spectroscopy were used to study the full-thickness articular cartilage from several canine humeral heads. The in-plane pixel size across the depth of the cartilage tissue was 13 μm. The acid phosphate salt was an effective exchange catalyst for proton exchange in the cartilage with an organized structure of collagen fibrils, while the alkaline phosphate salt was not. For cartilage tissue containing less organized collagen fibrils, both acid and alkaline phosphate salts have no significant effect on the T(2) value at low concentration but decrease the T(2) value at high concentration. The solutions of NaCl, KCl, CaCl(2), and D-PBS were found to have no significant effect on T(2) and DQF in cartilage. This study demonstrates the ability to modify the proton exchange in articular cartilage using the solutions of phosphate salts. The ability to modify the proton exchange in articular cartilage can be used to modulate the laminar appearance of articular cartilage in MRI.

Postmortem MRI of human brain hemispheres: T2 relaxation times during formaldehyde fixation

Unlike in vivo imaging, postmortem MRI allows for invasive examination of the tissue specimen immediately after the MR scan. However, natural tissue decomposition and chemical fixation cause the postmortem tissue's MRI properties to be different from those found in vivo. Moreover, these properties change as postmortem fixation time elapses. The goal of this study was to characterize the T(2) relaxation changes that occur over time in cadaveric human brain hemispheres during fixation. Five hemispheres immersed in formaldehyde solution were scanned on a weekly basis for 3 months postmortem, and once again at 6 months postmortem. The T(2) relaxation times were measured throughout the hemispheres. Over time, T(2) values near the edges of the hemispheres decreased rapidly after death, while T(2) values of deep tissue decreased more slowly. This difference is likely due to the relatively large distance from the hemisphere surface, and other barriers limiting diffusion of formaldehyde molecules to deep tissues. In addition, T(2) values in deep tissue did not continuously decay to a plateau, but instead reached a minimum and then increased to a plateau. This final increase may be due to the effects of prolonged tissue decomposition, a hypothesis that is supported by numerical simulations of the fixation process.

The effect of paraformaldehyde fixation on the delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) measurement

The delayed Gadolinium-Enhanced Magnetic Resonance Imaging of Cartilage (dGEMRIC) method allows for both qualitative and quantitative measurement of the spatial distribution of glycosaminoglycan [GAG] in excised cartilage. The objective of this study was to determine the effect of paraformaldehyde fixation on dGEMRIC measurements. Five bovine and seven human cartilage pieces were punched into 5-mm plugs, fixed for 18 h in 4% paraformaldehyde solution, and washed. The magnetic resonance imaging (MRI) parameter T1 was measured prior and post fixation in cartilage without (T1(0)) and with (T1(Gd)), the ionically charged MRI contrast agent Gd(DTPA)(2-). Images of tissue before and after fixation were qualitatively very similar. The ratios of T1(0), T1(Gd), and calculated [GAG] after fixation, relative to before fixation, were near or slightly higher than 1 for both bovine cartilage (1.01 +/- 0.01, 1.04 +/- 0.02, 1.05 +/- 0.03, respectively) and for human cartilage (0.96 +/- 0.11, 1.03 +/- 0.05, 1.09 +/- 0.13). Thus, these data suggest that dGEMRIC can be used on previously fixed samples to assess the three dimensional spatial distribution of GAG.

Magnetic resonance microscopy of collagen mineralization

A model mineralizing system was subjected to magnetic resonance microscopy to investigate how water proton transverse (T(2)) relaxation times and magnetization transfer ratios can be applied to monitor collagen mineralization. In our model system, a collagen sponge was mineralized with polymer-stabilized amorphous calcium carbonate. The lower hydration and water proton T(2) values of collagen sponges during the initial mineralization phase were attributed to the replacement of the water within the collagen fibrils by amorphous calcium carbonate. The significant reduction in T(2) values by day 6 (p < 0.001) was attributed to the appearance of mineral crystallites, which were also detected by x-ray diffraction and scanning electron microscopy. In the second phase, between days 6 and 13, magnetic resonance microscopy properties appear to plateau as amorphous calcium carbonate droplets began to coalesce within the intrafibrillar space of collagen. In the third phase, after day 15, the amorphous mineral phase crystallized, resulting in a reduction in the absolute intensity of the collagen diffraction pattern. We speculate that magnetization transfer ratio values for collagen sponges, with similar collagen contents, increased from 0.25 +/- 0.02 for control strips to a maximum value of 0.31 +/- 0.04 at day 15 (p = 0.03) because mineral crystals greatly reduce the mobility of the collagen fibrils.