High contrast ultrasound images of defects in food package seals

Previous work to detect defects in food packaging seals using pulse-echo ultrasound inspired the backscattered amplitude integral (BAI) imaging technique, which could reliably identify channels with diameters 38 μm or larger at a center frequency of 17.3 MHz (λ = 86 μm). The current study presents two new processing techniques that more reliably reveal smaller channels (≈ 6 μm in diameter). The RF sampling technique (RFS) displays a single, time-gated, pressure value from the received (not envelope-detected) RF waveform at each transducer position. The RF correlation technique (RFC) calculates the correlation coefficients of the RF signals with a reference signal that does not pass through a channel. The correlation coefficient can be calculated for the entire RF signal (RFCE) or over a short segment of the RF signal (RFCS). The performance of these imaging methods for detecting channel defects is investigated for plastic and aluminum foil trilaminate films with 6, 10, 15, 38, and 50 μm channels filled with water or air. Data are collected with a focused ultrasound transducer (17.3 MHz, 6.35 mm in diameter, f/2, 173 μm -6 dB pulse-echo lateral beamwidth at the focus) scanned over a rectangular grid, keeping the package in the focus. Performance is measured using detection rates, image contrast, and contrast-to-noise ratio (CNR). Both RFS and RFCS have improved detection rates relative to BAI for channels 15 μm or smaller. The RFCS technique is the most effective at smoothing the background, leading to the greatest CNR improvements.