1070 Fig. 4 Time evolution of the dry glassy core, swollen glassy layer, gel area and total area of the cross section of the HPMC based, CR quetiapine fumarate tablet as measured in the central saggital slice. Kulinowski, Dorożyński, Młynarczyk and Węglarz & & Swollen glassy layer reached maximum at 4 h, exactly at the time when the dry glassy layer disappeared. Starting from 4 h, the gel layer thickness increased at the expense of the glassy swollen layer only. DISCUSSION The results described above, obtained using USP Apparatus 4, agree with both the theoretical study presented by Ju et al. (8) and the combined VIS/FTIR study presented by Kazarian et al. (12). All three studies concern dynamic dissolution testing in either solvent flow or agitation states. In their theoretical study, Ju et al. (8) calculated the time evolution of particular regions of the matrix for a dosage form (500 mg) comprised of 35% HPMC (250 kDa and 10 kDa), 2.5% adinazolam mesylate (water soluble drug), 62% lactose (binder) and 0.5% magnesium stearate (binder). The normalized changes in the thickness of the regions vs. time were presented. The model predicted the overall size of the matrix (total swelling of the system) for HPMC 250 kDa would grow monotonically for the 20 h. This slow dissolution rate of the polymeric matrix would result in the domination of solvent uptake over polymer dissolution. In addition, the model predicted that initially the swollen glassy layer and gel layer thickness would increase in an almost linear manner. At approximately 1 h, the expansion rate of both fractions (swollen glassy layer and gel layer) would decrease, and at 5 h, the thickness of the swollen glassy layer would reach its maximum extent, exactly when the dry glassy core disappeared. After this, the model predicted that the thickness of the swollen glassy layer would decrease until it finally disappeared at 16 h. Our results agree with the model developed by Ju et al. (8), as the behavior of the profiles presented in Fig. 4 demonstrate. This agreement between the model and our results is particularly evident for the HPMC 250 kDa formulation. While both studies, theoretical and experimental, differ in details, constitutive similarities between the studies in terms of subjects and conditions include the following: & & & & HPMC polymeric matrix, dissolution medium (distilled water) in agitated/dynamic state, similar size of the dosage form, active substance soluble in water. MRI parameters such as echo and repetition times (TE, TR) were selected to obtain proper contrast within the outer layer. The aim was to observe the gel layer, i.e. strong entanglement regime region, not the diffusion layer, characterized by a weak entanglement, to be in agreement with assumptions made by Ju et al. (8). Taking into account MRI experimental conditions in our study (TE=19 ms, TR= 625 ms), we may expect detection of signal from regions of the hydrating tablet with T2 above ~8 ms. This estimation is based on the assumption that at least 10% of the initial echo signal, decreased due to T2 decay, is detected. A spin-echo sequence with these parameters does not allow detection of very low hydrated regions, which are classified as part of the dry glassy core, leading to some overestimation of its size using MRI. Such a slight overestimation of the dry core area during hydration (Fig. 4), when compared to Ju’s results (8), may be observed. This discrepancy could be minimized using shorter echo time. However, technical limitations, such as gradient strength, of our MRI system did not allow the use of shorter echo times. In spite of this technical limitation, the correlation between our experimental results (Fig. 4) and the theoretical predictions made by Ju et al. (8) is evident. After dry core disappearance at about 5 h (Fig. 2), the voids/cracks appear in the swollen glassy part of the matrix. We hypothesize that they arise due to internal tensions in compressed polymer/drug/fillers matrix being released during hydration of the outer regions. When studying non-compressed matrices, maximum swelling occurred in