Journal
ULTRASOUND IN MEDICINE AND BIOLOGY
Volume 38, Issue 5, Pages 846-863Publisher
ELSEVIER SCIENCE INC
DOI: 10.1016/j.ultrasmedbio.2012.01.011
Keywords
Ultrasound contrast agents; Microbubbles; High-frequency ultrasound; Subharmonic; Phospholipid coating; Nonlinear; Individual bubbles
Funding
- Ontario Institute for Cancer Research
- National Sciences and Engineering Research Council
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There are a range of contrast ultrasound applications above 10 MHz, a frequency regime in which nonlinear microbubble behavior is poorly understood. Lipid-encapsulated microbubbles have considerable potential for use at higher frequencies because they have been shown to exhibit pronounced nonlinear activity at frequencies up to 40 MHz. The objective of this work was to investigate the influence of agent formulation on the subharmonic response of lipid-encapsulated microbubbles at high frequencies with a view to providing information relevant to improving contrast agent design and imaging performance. An optical-acoustical setup was used to measure the subharmonic emissions from small (d < 3 mu m) individual lipid-encapsulated microbubbles as a function of transmit pressure, size and composition. In this study, five agent formulations (Definity (TM), MicroMarker (TM) and three in-house agents manipulated to exhibit different levels of shell microstructure heterogeneity) were insonified at 25 MHz over a peak negative pressure (P-n) range of 0.02-1.2 MPa. All agents exhibited distinctly different subharmonic behavior, both in terms of amplitude and active sizes. MicroMarker (TM) exhibited the strongest, broadest and most consistent subharmonic response, 22% greater in power than that of Definity (TM) and as much as 50% greater than the in-house formulations. No clear relation between in-house agents' shell microstructure and nonlinear response was found, other than the variability in the nonlinear response itself. An analysis of the response of MicroMarker (TM) bubbles suggests that these bubbles exhibit expansion-dominated'' oscillations, in contrast to compression-only'' oscillations observed for similar bubbles at lower frequencies (f < 11 MHz). (E-mail: brandon.helfield@sri.utoronto.ca) (C) 2012 World Federation for Ultrasound in Medicine & Biology.
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