Fourier transform second-harmonic generation (SHG) microscopy has been applied to quantitatively compare the information content between SHG images obtained from the forward and backward direction for three tissue types: porcine tendon, sclera, and ear cartilage. Both signal types yield consistent information on the preferred orientation of collagen fibers. For all specimens, the Fourier transform of the forward and backward SHG images produces several overlapping peaks in the magnitude spectrum at various depths into the tissues, indicating that some information present in the forward SHG images can be extracted from the backward SHG images. This study highlights the potential of backward SHG microscopy for medical diagnostics.

Ultrasound contrast agents (UCAs) have been explored as a means to enhance therapeutic techniques. Because the effectiveness of these techniques relies on the UCA concentration at a target site, it would be beneficial to estimate UCA concentration noninvasively. In this study, a noninvasive method for estimating UCA concentration was developed in vitro. Backscatter coefficients (BSCs) estimated from measurements of Definity V R UCAs were fitted to a theoretical scattering model in the 15-25 MHz range using a Levenberg-Marquardt regression technique. The model was defined by the UCA size distribution and concentration, and therefore concentration estimates were extracted directly from the fit. Calculation of the BSC was accomplished using planar reference measurements from the back wall of a Plexiglas V R chamber and an average of 500 snapshots of ultrasonic backscatter from UCAs flowing through the chamber. In order to verify the ultrasonically derived UCA concentration estimates, a sample of the UCAs was extracted from the flow path and the concentration was estimated with a hemacytometer. UCA concentrations of 1, 2, and 5 times the dose recommended by the manufacturer were used in experiments. All BSC-based estimates were within one standard deviation of hemacytometer based estimates for peak rarefactional pressures of 100-400 kPa.

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Ultrasound contrast agents (UCAs) are used clinically to enhance the contrast of ultrasound images. Recently, microbubbles have been explored as a means to enhance therapeutic techniques. Because the effectiveness of these techniques relies on the UCA concentration at a target site, it would be beneficial to acquire real-time estimates non-invasively. A method for measuring concentration, based upon backscatter coefficients (BSCs) at frequencies above resonance, was developed. Calculation of the BSC was accomplished using plane reference measurements from the back wall of a Plexiglas chamber. For each trial, an average of 500 snapshots of ultrasonic backscatter from Definity microbubbles flowing through the chamber was acquired. Immediately following this procedure, a sample of the UCAs was extracted from the flow path in order to optically verify the concentration estimates. Using measurements of attenuation coefficient through the bubble cloud, BSC was calculated for the 15–25 MHz range. UCA concentration was estimated by using a Levenberg–Marquardt fitting algorithm to match the experimental BSC to a linear scattering model. All BSC-based estimates were within one standard deviation of optically derived estimates. These results indicate that the BSC can be used to measure UCA concentration. [This work was supported by NIH Grant No. R37EB002641.]