Microscopic image analysis techniques for the morphological characterization of pharmaceutical particles: Influence of process variables

This study investigated the effects of various process variables on some of the shape factors most widely used in the morphological characterization of pharmaceutical particles (including circularity, aspect ratio, er, and the recently proposed Vr and Vp). After optimization of the illumination system and greyscale thresholds for discrimination of particle background, we evaluated the effects of process variables within the image capture and analysis system, including the image capture device (video or digital camera), image color information (24-bits-per-pixel RGB or 8-bits-per-pixel black and white), file type (JPG or TIFF), and JPG file compression ratio. A key aspect was evaluation of the effect of scaling factor (µm/pixel), dependent on real pixel size and optical magnification, on shape factor estimates. The results obtained indicate that accurate estimation of shape factors requires use of a scaling factor below a certain maximum; however, use of very low scaling factors will mean that the field of view is very small, so that it will contain very few particles, implying a time-consuming increase in the number of images that must be analyzed. Finally, we use statistical procedures to estimate the minimum number of particles that must be analyzed in order to provide accurate estimates of shape factors. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association