High-contrast RF correlation imaging of defects in food package seals
Previous research work to detect channel defect in package seals using pulse-echo ultrasound inspired the Backscattered Amplitude Integral (BAI) imaging and RF sample (RFS) imaging techniques. The two former image formation techniques, as well as the newly proposed technique, are evaluated from the same acquired pulse-echo RF data set. All images are formed with a 17.3-MHz 6.35-mm-diameter focused ultrasound transducer (f/2, 173-μm-6 dB pulse-echo lateral beamwidth at the focus, λ-86 μm) scanned over a rectangular grid, keeping the package material in the focal region. All techniques are evaluated with the same set of laboratory-made channel defects: plastic and aluminum foil trilaminate film with 6-, 10-, 15-, 38- and 50-μm-diameter channels filled with water or air. The new RF correlation image technique is formed from the correlation coefficient of each RF echo signal relative to a reference signal that does not pass through a channel defect. Prior to processing, the acquired RF echo signals are first windowed to match within the sample the range where the two materials are bonded, that is, the range where channel defects occur. The statistical study on the laboratory-made channel defects shows that RF correlation technique has the highest detection rate relative to BAI-mode and RFS-mode image for 15-, 10- and 6-μm channel defects. It also is the most effective at smoothing the background, leading to the greatest CNR enhancement.