Patrick G. Rafter

Measurement of myocardial blood flow velocity reserve with myocardial contrast echocardiography in patients with suspected coronary artery disease: comparison with quantitative gated technetium 99m sestamibi single photon emission computed tomography

BACKGROUND The ability of high and low mechanical index (MI) imaging methods during myocardial contrast echocardiography (MCE) to assess the physiologic significance of coronary stenoses were compared with technetium 99m sestamibi single photon emission computed tomography (SPECT) in patients. METHODS Intermittent ultraharmonic imaging (high MI) and power modulation angio (low MI) were performed during continuous infusions of the echo-enhancing contrast agent, Optison, at rest and after dipyridamole stress in 39 patients. Technetium 99m sestamibi SPECT was performed simultaneously. Images from the 3 apical windows were divided into 6 walls. Myocardial blood flow (MBF) velocity and MBF velocity reserve were quantified from pulsing interval versus acoustic intensity MCE curves in each wall using postprocessed images. RESULTS Approximately 25% of the myocardial walls could not be analyzed from MCE because of artifacts. MBF velocity and MBF derived from both MCE methods increased significantly after dipyridamole in healthy patients (n = 143 and 129 walls for high and low MI, respectively), compared with those with either reversible (n = 11 and 10 walls for high and low MI, respectively) or fixed defects (n = 18 and 14 walls for high and low MI, respectively) on SPECT. Consequently, MBF velocity and MBF reserve were significantly greater for patients with normal perfusion. Receiver operator characteristic curves obtained for MBF velocity reserve provided a sensitivity and specificity of 82% and 87%, respectively, for high MI; versus 64% and 96%, respectively, for low MI imaging after uninterpretable images were excluded from analysis. CONCLUSIONS Both high and low MI MCE imaging techniques can be used to determine the presence of perfusion defects as identified by technetium 99m sestamibi SPECT. Low MI imaging methods have a number of drawbacks that limit its sensitivity compared with high MI techniques.

Automated ultrasound system for performing imaging studies utilizing ultrasound contrast agents

An ultrasound system that has transmit and receive circuitry that, pursuant to a plurality of image settings, transmits ultrasound signals into a patient, receives echoes from a patient and outputs a signal representative of the echo. Control circuitry is provided that sequentially adjusts the image settings so as to cause the transmit and receive circuitry to have a sequence of imaging configurations during an ultrasound imaging study. A memory may be provided that stores a plurality of state diagrams, each defining a sequence of imaging configurations for a particular imaging study, which are accessible by the control circuitry, wherein the control circuitry accesses a selected state diagram to conduct an imaging study. Such a system is particularly useful for imaging studies that utilize contrast agents.

Wideband phased-array transducer for uniform harmonic imaging, contrast agent detection, and destruction

A system and method for ultrasonic harmonic imaging of both tissue and tissue perfused with a contrast agent are disclosed. The ultrasonic harmonic imaging system comprises a wideband phased-array transducer, a transmitter for transmitting waves into the tissue, a receiver for receiving ultrasonic responses from the tissue, a control system electrically coupled to the transmitter and the receiver for controlling operation of the transmitter and receiver, a video processor, and a monitor. In a preferred embodiment, the transducer insonifies tissue at a fundamental frequency of less than approximately 1.5 MHz. A method for detecting an ultrasonic harmonic response of a contrast agent may be performed by introducing at least one contrast agent into the target object, destroying the contrast agent with an ultrasonic pressure wave and measuring the harmonic response from the destroyed contrast agent. A method for detecting an ultrasonic harmonic tissue response may be performed by insonifying the tissue at a low frequency for increased uniformity in depth and using transmit apodization of a wideband phased-array transducer to improve harmonic generation in the near-field.

Myocardial injury induced by ultrasound-targeted microbubble destruction: evidence for the contribution of myocardial ischemia.

Ultrasound-targeted microbubble destruction (UTMD) can cause left ventricular (LV) dysfunction and tissue alterations in rats when high ultrasound (US) energy and long duration of imaging are used. However, the mechanism underlying these alterations remains unclear. The aim of the present work was to investigate the possible role of ischemia in the pathogenesis of the UTMD-induced LV damages in rats. To address this issue, rat hearts were exposed in situ to perfluorocarbon-enhanced sonicated dextrose albumin (PESDA) and US at peak negative pressures of 0.6, 1.2 or 1.8 MPa for 1, 3, 9, 15 or 30 min. Blood pressure and electrocardiogram were continuously recorded during insonation. LV function was assessed before and immediately after US exposure, as well as at 24 h and 7 d. At each time point, groups of rats were euthanized and their hearts were harvested for morphologic analysis. Rats exposed to either PESDA alone or US alone showed no functional or morphologic abnormalities. By contrast, rats exposed to both PESDA and US exhibited transient LV dysfunction, transient ST-segment elevation, premature ventricular contractions, microvascular ruptures, contraction band necrosis and morphologic tissue damage. These bio-effects were spontaneously and completely reversible by one week, except in the groups exposed to the highest peak negative pressure for the longest duration, in which mild dysfunction persisted and interstitial fibrosis developed. In conclusion, simultaneous exposure of rat hearts to PESDA and US in vivo results in significant bio-effects that are similar to myocardial ischemia, including transient regional LV dysfunction, transient ST-segment elevation and myocyte contraction band necrosis.

Microbubble responses to a similar mechanical index with different real-time perfusion imaging techniques.

The purpose of this study was to determine differences in contrast enhancement and microbubble destruction rates with current commercially available low-mechanical index (MI) real-time perfusion imaging modalities. A tissue-mimicking phantom was developed that had vessels at 3 cm (near field) and 9 cm (far field) from a real-time transducer. Perfluorocarbon-exposed sonicated dextrose albumin microbubbles (PESDA) were injected proximal to a mixing chamber, and then passed through these vessels while the region was insonified with either pulses of alternating polarity with pulse inversion Doppler (PID) or pulses of alternating amplitude by power modulation (PM) at MIs of 0.1, 0.2 and 0.3. Effluent microbubble concentration, contrast intensity and the slope of digital contrast intensity vs. time were measured. Our results demonstrated that microbubble destruction already occurs with PID at an MI of 0.1. Contrast intensity seen with PID was less than with PM. Therefore, differences in contrast enhancement and microbubble destruction rates occur at a similar MI setting when using different real-time pulse sequence schemes.

Detection and display of acoustic window for guiding and training cardiac ultrasound users

Successful ultrasound data collection strongly relies on the skills of the operator. Among different scans, echocardiography is especially challenging as the heart is surrounded by ribs and lung tissue. Less experienced users might acquire compromised images because of suboptimal hand-eye coordination and less awareness of artifacts. Clearly, there is a need for a tool that can guide and train less experienced users to position the probe optimally. We propose to help users with hand-eye coordination by displaying lines overlaid on B-mode images. The lines indicate the edges of blockages (e.g., ribs) and are updated in real time according to movement of the probe relative to the blockages. They provide information about how probe positioning can be improved. To distinguish between blockage and acoustic window, we use coherence, an indicator of channel data similarity after applying focusing delays. Specialized beamforming was developed to estimate coherence. Image processing is applied to coherence maps to detect unblocked beams and the angle of the lines for display. We built a demonstrator based on a Philips iE33 scanner, from which beamsummed RF data and video output are transferred to a workstation for processing. The detected lines are overlaid on B-mode images and fed back to the scanner display to provide users real-time guidance. Using such information in addition to B-mode images, users will be able to quickly find a suitable acoustic window for optimal image quality, and improve their skill.

2C-1 Contrast Resonance Imaging with Embedded Ultrasound Imaging Pulses

A new ultrasound contrast imaging modality called Contrast Resonance Imaging (CRI) is proposed, which is based on our finding that, for a short imaging pulse, the nonlinear scattering signal from resonant contrast microbubbles can be greatly enhanced if the imaging pulse is embedded at the end of a relatively long pulse. The new imaging technique is evaluated with in vitro laboratory A-line measurements, achieving a contrast-to-tissue ratio of approximate 40 dB. For a proof of the concept, the new imaging mode is implemented on an iE33 ultrasound scanner and images of a contrast flow phantom with robust contrast signals are obtained

Fast frame rate 2D cardiac deformation imaging based on RF data: What do we gain?

Fast acquisition sequences enable >200Hz 2D cardiac B-mode imaging with little compromise in image quality. Fast imaging minimizes frame to frame decorrelation, enabling radiofrequency (RF)-based tracking for enhanced signal-to-noise ratio (SNR) in motion and deformation imaging. Our objective was to compare the performance and clinical usefulness of Fast RF-based Tracking (FRFT) to the established modalities Tissue Doppler Imaging (TDI) and image-based speckle tracking (ST).

Contrast resonance imaging with microbubble resonance enhancement and suppression

A new set of multiple-pulse contrast modalities with combinations of power modulation (PM), pulse inversion (PI) and contrast resonance (CR) are proposed in order for contrast imaging sensitivity improvement. For the CR mode, an imaging pulse is ldquoembeddedrdquo in a long ldquoresonancerdquo pulse so that the dynamic ldquoresonantrdquo state of a contrast bubble encountered by the embedded imaging pulse is very different from the static state of the bubble for the imaging pulse alone. Numerical and experimental evaluations of the mode combinations indicate great potential for contrast imaging sensitivity improvement (without resolution degradation) over the standard PM or PI&PM mode.