Magnetic Resonance Imaging and Image Analysis for Assessment selected from the original image with convenient Lookup Table (LUT) applied. LUT can be an arbitrary color set. Application of LUT transformed grayscale image into a color image for presentation purposes in order to better visualize matrix structure (as in Fig. 3). It does not change actual values of pixel intensities—image analysis as described earlier was performed on original images. Fig. 1B presents histogram of the image in Fig. 1A. The histogram consists of three distinct modes with appropriate assignment of physical properties (see “Discussion”). According to selected thresholds (0.16 and 0.43), original image was segmented yielding three binary images (Fig. 1C) of dry glassy (C1), swollen glassy (C2) and gel (C3) regions. RESULTS A set of MR images of the swelling tablet was collected during 12 h of hydration inside the flow-through cell. In the Fig. 2 examples of the acquired images are presented, an interface region (swollen glassy region) between dry glassy inner part of the table and the outer gel layer is clearly visible. Another view of the changes inside the swelling HPMC matrix during hydration inside the flow-through cell is shown in Fig. 3. Fig. 3 shows the histogram modes appearing and decaying during the dosage form hydration. We hypothesize that the modes shown in Fig. 3 reflect physico-chemical properties of the system as proposed by Ju et al. (8), i.e., dry glassy, swollen glassy and gel regions. It can be observed that the swollen glassy mode is broader than dry glassy core, and the gel mode is substantially broader than the swollen glassy core (approximately 4 times broader). While homogeneity of a particular region is directly related to the mode width, it implies that the more hydrated the polymeric matrix, the more inhomogeneous its structure becomes in terms of solvent (water) concentration and Fig. 3 Evolution of image intensity histogram (0:15–12:00 h) of the quetiapine fumarate tablet during the hydration inside USP Apparatus 4 with temporal resolution of 15 min. 1069 mobility, as well as differences in magnetic susceptibility (solvent/gel-air). The evolution profiles of the regions are shown in Fig. 4. At the beginning, total area of tablet cross-section was equal to the area of dry glassy core. Initially, the area of dry glassy core decreased with the rate of 0.976 mm2/min (R2=0.9429) during the first 30 min. Subsequently, the linear decrease of the dry glassy core with the rate of 0.28 mm2/min (R2= 0.9932) was observed. At 4 h, the dry glassy core disappeared completely. The initial swollen glassy area growth rate was 0.99 mm 2/min (R2 = 0.9129). T his d ecre ased to 0.25 mm2/min (R2 = 0.9899) at 30 min, and the maximum of the swollen glassy area was reached at 4 h. Between 4.25 and 10.25 h, the linear decrease of swollen glassy area with rate 0.16 mm2/min (R2 = 0.9983) was observed. The increase of the gel area covering the tablet surface was observed over 12 h. After the short period of initial gel formation (with rate of 0.98 mm2/min (R2 = 0.9789) during the first 30 min), the linear growth of the gel area with rate of 0.21 mm2/min (R2 = 0.9966) was sustained for the period of 0.5 h to 10.25 h. During the first 30 min, the total glassy (dry glassy + swollen glassy) area remained approximately constant, while all other regions changed rapidly. Then, only a slight decrease with a rate of 0.07 mm2/min (R2 = 0.9725) was observed up to 4 h after the beginning of the study. The summary of the behavior of profiles is listed below: & & & Initial high expansion rate (steeper slope) of the swollen glassy and gel layers within the first 30 min was observed due to the initial water uptake. Almost instantaneous rate change at 30 min resulted in a slower expansion of the swollen glassy and gel layers, the moment when a stable gel layer is formed. Dry glassy core disappeared at approximately 4 h.