Spatially resolved polymer mobilization revisited - Three-dimensional, UltraShort Echo Time (3D UTE) magnetic resonance imaging of sodium alginate matrix tablets.

Hypothesis: Three-dimensional H UltraShort Echo Time magnetic resonance imaging (H 3D UTE MRI) of the matrix tablet made of hydrophilic polymer hydrated in heavy water (DO) will allow investigation of the hydration-induced spatiotemporal evolution of the material originally included in the matrix tablet during manufacturing (i.e., polymer chains and bound water).

Experiments: The oblong-shaped sodium alginate matrix tablets were used to verify the hypothesis. The matrix was measured before and during hydration in DO for up to 2 h using the H 3D UTE MRI. Five echo times (first at 20 μs) were used, resulting in five three-dimensional images (one image for each echo time). In chosen cross-sections, two parametric images, i.e., amplitude and T* relaxation time maps, were calculated using "pixel-by-pixel" mono-exponential fitting.

Findings: The regions of the alginate matrix with T* shorter than 600 μs were analyzed before (air-dry matrix) and during hydration (parametric, spatiotemporal analysis). During the study, only hydrogen nuclei (protons) pre-existing in the air-dry sample (polymer and bound water) were monitored because the hydration medium (DO) was not visible. As a result, it was found that morphological changes in regions having T* shorter than 300 μs were the effect of fast initial water ingress into the core of the matrix and subsequent polymer mobilization (early hydration providing additional 5% w/w hydration medium content relating to air-dry matrix). In particular, evolving layers in T* maps were detected, and a fracture network was formed shortly after the matrix immersion in DO. The current study presented a coherent picture of polymer mobilization accompanied by local polymer density decrease. We concluded, that the T* mapping using 3D UTE MRI can effectively be applied as a polymer mobilization marker.