Assessment of Cardiac Function by Acoustic Radiation Force (ARF) Based Methods of Ultrasound Elastography

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Date

2017

Authors

Vejdani Jahromi, Maryam

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Wolf, Patrick D

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Abstract

Heart Failure (HF) is a major cause of morbidity and mortality in the world. This disorder is characterized by compromised systolic and/or diastolic function of the myocardium that reduces the pumping and/or filling efficiency resulting in diminished cardiac output. Cardiovascular researchers have been attempting to develop tools for assessment of cardiac function for decades. Evaluating cardiac function helps clinicians to diagnose and to follow the progress of HF patients.

The gold standard technique for cardiac functional assessment, including systolic and diastolic function, is the pressure-volume (PV) loop measurement; however, this measurement is not typically used clinically due to the invasiveness of the technique. PV loop measurement requires the introduction of a pressure or pressure-volume catheter into the left ventricle. Cardiovascular researchers have been attempting to develop non-invasive tools for assessment of cardiac function primarily by measuring surrogates of ventricular contractility and compliance.

These measures are based on imaging and include Ejection Fraction and Doppler and ultrasound strain imaging. These measurements are indirect measures that rely on cardiac motion or volume changes. The measurements are load dependent and could be affected by the heart rhythm and valvular disorders. Despite research toward this goal, there is no clinically accepted noninvasive technique to provide a direct myocardial measurement of cardiac function.

Acoustic radiation force (ARF) based ultrasound elastography techniques were developed in early 2000s and have been used to measure the static stiffness of tissue. These techniques are being used in the clinic for diagnosis of disorders and malignancies in tissues such as liver. When applied to the heart, it was shown that dynamic changes in the stiffness of the myocardium during the cardiac cycle could be recorded using modified versions of these static techniques.

This had the potential to be a direct measure of the time-varying elastance measured during the cardiac cycle using pressure-volume measurements. The question arose as to whether these ARF based measurements of dynamic stiffness could be used for cardiac functional assessments during systole and diastole; and if so, what the relationship is between these measurements and the gold standard method. The goal of this research was to assess the ability of ARF based measurements of cardiac dynamic stiffness to provide meaningful indices of cardiac function.

In this dissertation both acoustic radiation force impulse (ARFI) and shear wave elasticity imaging (SWEI) ultrasound elastography techniques were studied. These are qualitative and quantitative measures of stiffness, respectively. While the focus of the studies was more on SWEI due to its quantitative nature, ARFI measurements of cardiac function were also investigated and compared to SWEI.

The studies were performed in isolated rabbit hearts in Langendorff or working modes because the preparation has several significant advantages. 1) Parameters including preload, afterload, and coronary perfusion can be accurately controlled. 2) The heart’s left ventricular free wall can be easily imaged from multiple angles. 3) Confounding neurohormonal reflexes of the body can be eliminated.

SWEI measurements of stiffness were used to characterize changes in contractility induced using the Gregg effect. The Gregg effect is the active effect of coronary perfusion on cardiac contractility. It was shown that SWEI measurements of stiffness could detect the changes in contractility induced by this known effect and that the effect was blocked using a Ca channel blocker.

The relationship between ARFI and SWEI measurements was characterized and the possibility of deriving functional indices such as systolic/diastolic ratio and isovolumic relaxation time constant (τ) using either of these techniques was evaluated. It was shown that in the same imaging configuration, the measurements of ARFI and SWEI are linearly related to one another. This could be important, as the ARFI technique will likely be the first cardiac elastography measurement technique to be implemented using transthoracic ultrasound throughout the cardiac cycle.

The Garden Hose effect was used to investigate SWEI’s ability to measure cardiac compliance. SWEI was used to detect the passive effect of coronary perfusion on cardiac compliance and the relationship between perfusion pressure and stiffness was characterized. Finally, SWEI derived measurements of diastolic function were compared to the gold standard PV measurements of cardiac diastolic function including end diastolic stiffness and the relaxation time constant. It was shown that SWEI could detect the changes in cardiac stiffness after induction of global ischemia. These changes were similar to the changes in the PV measure of diastolic stiffness. Furthermore, the results indicated that SWEI could be used to derive the relaxation time constant similar to the relaxation constant derived from intra-ventricular pressure recordings.

In summary, the results of the studies presented in this thesis illustrate the assessments of systolic and diastolic function using ARFI and SWEI ultrasound based elastography. It is concluded that these measurements can be used to derive cardiac functional indices that would have the advantages of an ultrasound based technique; they would be noninvasive, less expensive and could be widely applied outside of the cath lab.

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Vejdani Jahromi, Maryam (2017). Assessment of Cardiac Function by Acoustic Radiation Force (ARF) Based Methods of Ultrasound Elastography. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/14376.

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