1066 shows examples of formulations with similar dissolution profiles but with very different biological characteristics (4). Therefore, a parameterization (quantification) of physico- chemical behavior of the dosage form, e.g. swelling, erosion and other matrix changes occurring during dissolution study, was proposed (5). Up to now, only some of the theoretical studies concerned matrix structure and its evolution. For example, Borgquist et al. (6) simulated evolution of PEO matrix systems, Kiil et al. (7) calculated evolution of the three moving fronts of HPMC systems, while Ju et al. (8) presented a comprehensive mathematical model describing swelling/dissolution behavior and drug release from uncross-linked hydrophilic matrices (e.g. HPMC) in dynamic medium conditions. The model by Ju et al. predicts dynamic existence of three different regions: dry core, characterized by un- hydrated regime; swollen glassy layer with very strong polymeric chains entanglement; and gel layer with strong entanglement (8). While the number of theoretical reports on matrix structure is limited, many experimental approaches have been presented in the literature. Several authors (9–13) have presented studies on identification of the boundary regions (moving fronts) in polymeric controlled release formulations (polymer-water-drug systems) using optical methods. In particular, Colombo et al. (9) introduced two sharp fronts: the glass transition boundary and the erosion boundary. At the glass transition boundary, the glassy material is transformed into gel matrix, while at the erosion boundary, the matrix completely disappears due to dissolu- tion or erosion. A third front, the drug diffusion boundary, where the drug dissolves and starts diffusing, was suggested to be placed between these two fronts. Almost concurrently, Gao and Meury (10) described a slightly different model based on changes in the water- polymer properties inside the HPMC matrix. Gao and Meury (10) identified the following fronts within the swelling HPMC matrix structure: true water penetration boundary, phase transition boundary and erosion boundary. At the erosion boundary (dissolution front), the matrix completely disappears by dissolution or erosion. At the phase transition boundary, the glassy material is transformed into a gel matrix and becomes transparent. The true penetration boundary reflects the extent of water penetration through the glassy material. The glassy polymer contains a large number of channels and holes of molecular dimension (pores in the carrier). Solvent penetration through these pores occurs prior to glassy-rubbery transition (10). In 2004, Van der Weerd and Kazarian (11) applied the FTIR-ATR microscopy method with solution flow to study the evolution of the pure HPMC matrix as a function of time. Subsequently, in 2008 they reported (12) the results of Kulinowski, Dorożyński, Młynarczyk and Węglarz combined optical (VIS) and FTIR studies that demonstrated three boundaries inside the HPMC matrix and showed that the time evolution of boundaries was supplemented with evolution of water and drug concentration, consistent with Gao and Meury’s (10) results. In addition to optical and FTIR-ATR methods, Magnetic Resonance Imaging (MRI) is also a useful technique for the study of tablet dissolution, since it allows the real-time, non- invasive recording of the alterations of 3D spatial distribution of water in polymeric matrices. Furthermore, MR images provide information on water density and mobility (14,15). In comparison with other imaging methods, such as CT, image contrast in MRI can be adjusted by using different pulse sequences and altering their parameters. This is because MR image intensity depends on sample proton density, diffusion, proton relaxation times (T2 and T1) and the selection of pulse sequence parameters, such as echo time (TE) and repetition time (TR). For example, using spin-echo-based pulse sequences, it is possible to image highly hydrated polymers (gel) and solutions in which T2 is longer than ~10 ms. Using the same pulse sequence, it is also possible to obtain images of water penetrating the polymer matrix before the glassy- rubbery transition. However, significant loss in signal intensity occurs due to T2 relaxation times shorter than ~10 ms. Spin-echo-based MRI does not allow measurements of solutions in a “dry” glassy polymer matrix with T2 shorter than 100 μs, a very low level of water in polymer matrix can be visualized with MRI using other pulse sequences designed to solid state MR imaging (14,16). In polymer hydration, water molecules behave as both plasticizer and solvent. Hydration leads to the transition of the polymer matrix from the initial glassy state to the final solution state. This process is accompanied by matrix swelling and erosion as well as drug diffusion and release. NMR signal and thus the MR image intensity depend on the dynamic state of the water-polymer matrix system providing the mechanism for direct spatial visualization of regions with specific properties at the molecular level. In this way, MRI allows for the quantification of both the evolution of the region’s sizes as well as the development of fronts (boundaries) between the regions within the HPMC matrix. It should be noted that the use of MRI in the visualization of the multiple regions in a hydrated polymer matrix requires the correct choice of parameters of the measuring method (TE, TR) in order to achieve measurable contrast between different regions (16). In the 1990s, there were several attempts to use MRI for investigation of HPC/HPMC hydration (17–23) in non- pharmacopoeial conditions. However, according to our knowledge, there was only one report (published by Kojima et al. in 1998) (18) on observations of the additional interface layer between the dry core and the swollen gel layer, as well as evolution of all three layers. Recently, Chen