STATIC IMAGE ANALYSIS
The samples measured by static image analysis typically rest on a slide that is
moved by an automated stage. With the PSA300 static image analysis system
a microscope and digital camera collect images of the particles as the slide is
scanned. Samples prepared on slides can include powders, suspensions, or creams.
Aerosol delivery forms such as metered dose inhalers or dry powder inhalers can
be inspected using static image analysis by actuating the device onto a slide for
measurement. In addition, particles in suspension (such as parenterals) can be
collected on a filter for characterization.
The majority of static image analysis measurements are made on powders, typically
used for solid oral dosage forms. Most powders require a sample preparation
step prior to analysis. Powder preparation devices—using either positive pressure
to impact on a hard surface or pulling and releasing a vacuum—break apart
agglomerates and create an even dispersion on the slide. After the sample has
been prepared and the automated stage has presented multiple fields to the
optics and camera for capture, a series of image processing steps occur in the
software. The first step is to separate the particles from the background by setting a
parameter with some threshold value. Setting this threshold can be done manually
or automatically based on phases in the grayscale image or through a contrast
threshold function based on the particle/background contrast.
After the threshold operation is completed several functions may be applied to the
image to improve the edge definition. The basic functions of erosion and dilation
improve edge definition by performing opposite tasks of removing or adding dark
pixels at the particle edge. Advanced functions using combinations of erosion and
dilation steps such as delineation and convex hull improve the edge definition of
particles, leading to accurate area and perimeter determinations that are critical for
shape factor calculations. Other software functions perform the task of separating
touching particles including the crossed fibers in order to quantify fiber length
distributions and aspect ratios.
figure 30
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BASIC IMAGE ANALYSIS FUNCTIONS
Both static and dynamic image analysis
involve these basic steps.
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