Investigation of spatial sampling resolution of the real-time ultrasound pulse-echo BAI-mode imaging technique

Hermetically-sealed flexible food packages require effective seal integrity tests for quality assurance purpose. 10 μm-diameter channel defects can propagate microorganisms into food. The channel diameter limits of human inspection are about 50 μm in transparent food packages. A reliable methodology is thus needed to detect 50 μm-diameter and smaller channels. The pulse-echo Backscattered Amplitude Integral (BAI)-mode imaging technique [UFFC Trans, 45:30, 1998] has been developed and demonstrated by us to 100% reliably detect 38 μm-diameter and larger channels in plastic films; tested using a static stop-and-go transducer scanning pattern. In this study, we examined experimentally the spatial sampling issue of the BAI-mode imaging technique with a new real-time transducer scanning protocol to simulate continuous package production line motion. A focused transducer (17.3 MHz, -6 dB pulse-echo focal beam diameter of 173 μm) acquired RF data in a zigzag raster scanning pattern from plastic film samples bearing point reflectors arranged in a rectangular grid of varying spacings. The contrast-to-noise ratio (CNR) and the average BAI value difference (ΔBAI) between defected and background regions were assessed to quantitatively study image quality versus the varying grid size and the changing spatial scanning step sizes. For any given spatial grid size, the CNR and ΔBAI values degraded as scanning step size in each spatial dimension increased. When the ΔBAI dropped below 5% of the maximum BAI value, the point targets could not be separated in the image