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Noninvasive ambient pressure estimation using ultrasound contrast agents: invoking subharmonics for cardiac and hepatic applications
Dissertation   Open access

Noninvasive ambient pressure estimation using ultrasound contrast agents: invoking subharmonics for cardiac and hepatic applications

Jaydev K. Dave
Doctor of Philosophy (Ph.D.), Drexel University
Jul 2012
DOI:
https://doi.org/10.17918/etd-3970
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Abstract

Ultrasound contrast media Diagnostic ultrasonic imaging Biomedical Engineering
Ultrasound contrast agents (UCAs) are encapsulated microbubbles that provide a source for acoustic impedance mismatch with the blood, due to difference in compressibility between the gas contained within these microbubbles and the blood. When insonified by an ultrasound beam, these UCAs act as nonlinear scatterers and enhance the echoes of the incident pulse, resulting in scattering of the incident ultrasound beam and emission of fundamental (f0), subharmonic (f0/2), harmonic (n*f0; n ) and ultraharmonic (((2n-1)/2)*f0; n & n > 1) components in the echo response. A promising approach to monitor in vivo pressures revolves around the fact that the ultrasound transmit and receive parameters can be selected to induce an ambient pressure amplitude dependent subharmonic signal. This subharmonic signal may be used to estimate ambient pressure amplitude; such technique of estimating ambient pressure amplitude is referred to as subharmonic aided pressure estimation or SHAPE. This project develops and evaluates the feasibility of SHAPE to noninvasively monitor cardiac and hepatic pressures (using commercially available ultrasound scanners and UCAs) because invasive catheter based pressure measurements are used currently for these applications. Invasive catheter based pressure measurements pose risk of introducing infection while the catheter is guided towards the region of interest in the body through a percutaneous incision, pose risk of death due to structural or mechanical failure of the catheter (which has also triggered product recalls by the USA Food and Drug Administration) and may potentially modulate the pressures that are being measured. Also, catheterization procedures require fluoroscopic guidance to advance the catheter to the site of pressure measurements and such catheterization procedures are not performed in all clinical centers. Thus, a noninvasive technique to obtain ambient pressure values without the catheterization process is clinically helpful. While an intravenous injection is required to inject the UCAs into the body, this procedure is considered noninvasive as per the definition provided by the Center for Medicare and Medicaid Services; invasive procedures include surgical procedures as well as catheterization procedures while minor procedures such as drawing blood (which requires a similar approach as injecting UCAs) are considered noninvasive. In vitro results showed that the standard error between catheter pressures and SHAPE results is below 10 mmHg with a correlation coefficient value of above 0.9 - this experimental error of 10 mmHg is less than the errors associated with other techniques utilizing UCAs for ambient pressure estimation. In vivo results proved the feasibility of SHAPE to noninvasively estimate clinically relevant left and right ventricular (LV and RV) pressures. The maximum error in estimating the LV and RV systolic and diastolic pressures was 3.5 mmHg. Thus, the SHAPE technique may be useful for systolic and diastolic pressure estimation given that the standard recommendations require the errors for these pressure measurements to be within 5 mmHg. The ability of SHAPE to identify induced portal hypertension (PH) was also proved. The changes in the SHAPE data correlated significantly (p < 0.05) with the changes in the portal vein (PV) pressures and the absolute amplitudes of the subharmonic signal also correlated with absolute PV pressures. The SHAPE technique provides the ability to noninvasively obtain in vivo pressures. This technique is applicable not only for critically ill patients, but also for screening symptomatic patients and potentially for other clinical pressure monitoring applications, as well.

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