Acoustic Techniques for Assessing the Optison Destruction Threshold

Abstract

Objective-The purpose of this study was to identify the pressure threshold for the destruction of Optison (octafluoropropane contrast agent; Amersham Health, Princeton, NJ) using a laboratoryassembled 3.5-MHz pulsed ultrasound system and a clinical diagnostic ultrasound scanner.Methods-A 3.5-MHz focused transducer and a linear array with a center frequency of 6.9 MHz were positioned confocally and at 90° to each other in a tank of deionized water. Suspensions of Optison (5-8 × 10 4 microbubbles/mL) were insonated with 2-cycle pulses from the 3.5-MHz transducer (peak rarefactional pressure, or P r , from 0.0, or inactive, to 0.6 MPa) while being interrogated with fundamental B-mode imaging pulses (mechanical index, or MI, = 0.04). Scattering received by the 3.5-MHz transducer or the linear array was quantified as mean backscattered intensity or mean digital intensity, respectively, and fit with exponential decay functions (Ae −kt + N, where A + N was the amplitude at time 0; N, background echogenicity; and k, decay constant). By analyzing the decay constants statistically, a pressure threshold for Optison destruction due to acoustically driven diffusion was identified.Results-The decay constants determined from quantified 3.5-MHz radio frequency data and Bmode images were in good agreement. The peak rarefactional pressure threshold for Optison destruction due to acoustically driven diffusion at 3.5 MHz was 0.15 MPa (MI = 0.08). Furthermore, the rate of Optison destruction increased with increasing 3.5-MHz exposure pressure output.Conclusions-Optison destruction was quantified with a laboratory-assembled 3.5-MHz ultrasound system and a clinical diagnostic ultrasound scanner. The pressure threshold for acoustically driven diffusion was identified, and 3 distinct mechanisms of ultrasound contrast agent destruction were observed with acoustic techniques. Keywordsacoustically driven diffusion; Optison; rapid fragmentation; static diffusion; ultrasound contrast agent destruction Diagnostic ultrasound, combined with the injection of ultrasound contrast agents (UCAs) into the vasculature, provides the practitioner with an estimate of blood velocity and fractional blood volume in the myocardium, 1 the kidney, 2 and the brain. 3 Ultrasound imaging has shown promise for non-invasive quantification of tissue blood perfusion. The technique, known as perfusion imaging, takes advantage of the response of UCAs to low-and high-output acoustic pulses. The accuracy of this technique depends on complete and rapid loss of the contrast enhancement on high-power insonation and negligible microbubble destruction during the imaging phase. Rapid destruction of a UCA is possible through a process known as fragmentation, and pressure thresholds for fragmentation of several UCAs have been identified. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript 4,5 A substantial loss of echogenicity is also possible at acoustic pressures lower than the fragmentation threshold. This type of destruction is caused by acoust...

Effects of Acoustic Insonation Parameters on Ultrasound Contrast Agent Destruction

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