Ultrasound determination of absolute backscatter from arterial wall structures
Ruben Lara-Montalvo
MS Thesis, Dept. of Electrical and Computer Engineering, Worcester Polytechnic Institute, December 20, 2002.
SUMMARY
This thesis presents an ultrasound technique for measuring the absolute integrated backscatter (IBS) of arterial wall structures through an intervening inhomogeneous soft tissue layer. The aberrating effect of this tissue layer is minimized by normalizing the measured IBS from the wall region of interest with the IBS from an adjacent range cell in blood. The technique may become a tool to differentiate between stable and vulnerable plaques in the carotid artery.
COMPLETE ABSTRACT
This thesis presents an ultrasound technique for measuring the absolute integrated backscatter (IBS) of arterial wall structures through an intervening inhomogeneous soft tissue layer. The aberrating effect of this tissue layer is minimized by normalizing the measured IBS from the wall region of interest with the IBS from an adjacent range cell in blood. The technique may become a tool to differentiate between stable and vulnerable plaques in the carotid artery.
Stroke is a leading cause of serious, long-term disability in the United States accompanied by heavy social and economic burden. In 1997, $3.8 billion ($5,955 per discharge) was paid to Medicare beneficiaries discharged from short-stay hospitals for stroke. Studies performed on carotid arteries suggest that the morphology and composition of atherosclerotic plaque is predictive of stroke risk caused by emboli or thrombi due to plaque rupture or fissuring. There are currently many techniques used in the diagnosis of atherosclerotic plaque such as magnetic resonance imaging (MRI), optical coherence tomography (OCT) and ultrasound, especially intravascular ultrasound (IVUS).
The IBS is a measure of signal strength. IBS calculates the normalized energy from the radio frequency (RF) backscatter signal from a given sample volume. Using IBS we can minimize the random fluctuation in the RF signal amplitude while maintaining adequate spatial resolution. IBS has been used successfully in biomedicine as well as in Non Destructive Evaluation (NDE) applications.
In order to use the IBS from a wall structure echo to differentiate between stable and vulnerable plaques we need to overcome the effect of acquiring the RF signals through an aberrating tissue.
It has been proven that the variance of the IBS estimate of the blood backscatter signal (intrinsic to the stochastic nature of the measurement) can be quantified and reduced to a specified tolerable level thus allowing us to use IBS from blood as a reference. We have shown that that the absolute backscatter of an arterial wall structure s wall,abs can be determined calculating the ratio between the wall IBS and blood IBS and multiplying this ratio by a pre-stored blood backscatter value, as measured with a specified ultrasound transducer.
The experimental setup was done using a commercial HP ultrasound (US) scanner has been interfaced with a personal computer (PC), which controls imaging parameters, data acquisition and analysis. Phantom vessels (silicon rubber tubes) with synthetic lesions are placed in a measurement tank which is connected to a flow system containing blood mimicking fluid (BMF). Between the vessel and the transducer is either water or an inhomogeneous medium (beef or tissue phantom). The PC controls the US scanner through the serial communication port and transfers the acquired data from a Logic Analyzer using an Ethernet connection. Efforts were made to ensure that all measurements were performed keeping the signals in the linear region. We determine the location of the lumen by acquiring a sequence of consecutive RF signals (normally 100) containing the backscatter signal from the vessel region and building a data matrix with each data set as a column. We then proceed to high-pass filter the rows of the matrix thus removing the nearly stationary clutter signal and leaving behind the time-varying stochastic backscatter signal from blood or BMF. This lumen detection method can be advantageous in situations when the lumen width is not constant, such as in stenosis or irregular vessel shapes. The IBS of the vessel wall region of interest is calculated directly using the “wall acquisition” data, while the estimated mean IBS of the BMF is found after clutter removal using the “lumen acquisition” data. These IBS are then normalized to compensate for power/gain settings. In order to account for the possible presence of air bubbles in the hydraulic system, we reject the data sets with abnormal amount of energy.
The system uses the current measured signal amplitude to predict the optimal power/gain setting to do the “lumen acquisition”, reducing number of iterations. The results are in the form of IBS profiles (IBS versus lateral beam position). Specifically, conventional IBS profiles, i.e. profiles not normalized with the IBS of the blood- mimicking fluid, have been measured for phantom vessels containing lesions, with and without an intervening inhomogeneous medium; these results are contrasted with the corresponding normalized IBS profiles. For a given vessel, the normalized and nonnormalized IBS profiles measured through water are similar (apart from a scale factor). With the inhomogeneous tissue present, the non-normalized IBS profile is corrupted by phase aberration and differs dramatically from the profiles obtained through water. Most iv importantly, we have shown that the normalized IBS profile with the inhomogeneous tissue present closely resemble the IBS profiles measured through water.
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