Sherry Steinmetz

Harmonic imaging with levovist

Our purpose was to test the hypothesis that second harmonic imaging preferentially detects backscatter from microbubbles compared with tissue structural components. A prototype second harmonic scanner was used to image a flow channel in a tissue-mimicking rubber phantom (liver density). Video time-intensity curves were calculated from repeated bolus injections of microbubble echocardiographic contrast material under the same fluid dynamic conditions but with three different imaging modes: (1) fundamental imaging at 2.5 MHz (transmit and receive at 2.5 MHz), (2) fun damental imaging at 5.0 MHz (transmit and receive at 5.0 MHz), and (3) second harmonic imaging (transmit at 2.5 MHz and receive at 5.0 MHz). Each video time-intensity curve was calibrated-such that quantitative backscatter intensity was measured relative to the tissue phantom (0 dB). The peak increase in backscatter from the contrast material in the channel relative to the tissue phantom and the intensity in the channel before the contrast effect (the noise floor) was measured along with the area under the calibrated time-intensity curve relative to the phantom. When referenced to the noise floor in the flow channel, all imaging modes produced approximately 25 dB of enhancement. However, when referenced to the tissue phantom, second harmonic imaging produced a 22.3 +/- 1.8 dB peak enhancement, which was greater than either fundamental imaging at 2.5 MHz (15.5 +/- 0.8 dB; p < 0.001) or fundamental imaging at 5.0 MHz (15.3 +/- 1.5 dB; p < 0.001). The area under the time-intensity curves confirmed that harmonic imaging has approximately 7 dB of relative enhancement to the phantom compared with fundamental imaging at either frequency. Second harmonic imaging specifically enhances backscatter from microbubbles compared with a tissue-mimicking phantom. This specificity for microbubbles is due to a decrease in backscatter for the tissue phantom, rather than an increase in backscatter for the microbubbles. These data support the hypothesis that second harmonic imaging may be able to detect microbubbles in the tissue vascular space by preferentially decreasing the backscatter from tissue structural components.

A mathematical model for the assessment of hemodynamic parameters using quantitative contrast echocardiography

A mathematical model for the assessment of hemodynamic parameters using quantitative echocardiography is presented. The method involves the intravenous injection of an ultrasonic echo contrast agent. The relative enhancement of the backscattered ultrasound intensity is measured as a function of time (the time-intensity curve). From this measurement, the volume flow rate (cardiac output) and the mixing volume are calculated. Relevant acoustic properties of the ultrasound contrast agent are discussed. An in vitro experiment is performed to corroborate the theory presented.

Non-invasive flow measurement of a rotary pump ventricular assist device using quantitative contrast echocardiography.

BACKGROUND Many implantable ventricular assist devices (VADs) have no direct measurement of pump output. The aim of this study was to test the hypothesis that quantitative contrast echocardiography can be used to measure VAD output. METHODS Contrast-enhanced Doppler velocity-time integral (VTI) was measured in the VAD inlet and outlet cannulae. Doppler flow (Doppler Q=Doppler VTIxcannula area) was compared with measured flow (Q). A total of 130 flow measurements were made (at 6400 and 12,000 rpm). RESULTS Doppler Q in the outflow and inflow cannulae showed an excellent correlation with measured Q (outlet Doppler Q=1.0052 xQ+0.048, R2=0.9865; inlet Doppler Q=1.5043 xQ+0.003, R2=0.9904), but inlet Doppler Q was 50% higher. Correcting for the flow profile of the conical inlet tube yielded excellent correlation (inlet Doppler Q=1.0029 xQ+0.002, R2=0.9904). CONCLUSION Noninvasive Doppler flow techniques can be used to accurately measure VAD flow, but the flow profile in the cannula needs to be taken into account.

THE EFFECT OF ECHO CONTRAST AGENT ON DOPPLER VELOCITY MEASUREMENTS

The purpose of this investigation was to determine the effect of echo contrast agents on spectral Doppler velocity measurements. SH U 508A was administered by IV injection in 15 patients. The transmitral flow velocity was measured at the E- and A-wave peaks before the start and at the peak of the contrast effect. The Doppler velocity was determined from the Doppler video spectral display and from power spectral analysis of the audio Doppler signal. The Doppler signal intensity was also measured. The Doppler signal intensity increased 17.4 +/- 3.5 dB (p < 0.0001) following echo contrast injection. This was associated with a significant increase in the spectral peak velocity as determined from either the video display or audio analysis. (p < 0.0001). The velocity corresponding to the audio power peak frequency (the modal velocity) did not change significantly (p = NS) and was independent of Doppler signal strength.

Brachial artery reactivity in patients with severe sepsis: an observational study

IntroductionUltrasound measurements of brachial artery reactivity in response to stagnant ischemia provide estimates of microvascular function and conduit artery endothelial function. We hypothesized that brachial artery reactivity would independently predict severe sepsis and severe sepsis mortality.MethodsThis was a combined case-control and prospective cohort study. We measured brachial artery reactivity in 95 severe sepsis patients admitted to the medical and surgical intensive care units of an academic medical center and in 52 control subjects without acute illness. Measurements were compared in severe sepsis patients versus control subjects and in severe sepsis survivors versus nonsurvivors. Multivariable analyses were also conducted.ResultsHyperemic velocity (centimeters per cardiac cycle) and flow-mediated dilation (percentage) were significantly lower in severe sepsis patients versus control subjects (hyperemic velocity: severe sepsis = 34 (25 to 48) versus controls = 63 (52 to 81), P < 0.001; flow-mediated dilation: severe sepsis = 2.65 (0.81 to 4.79) versus controls = 4.11 (3.06 to 6.78), P < 0.001; values expressed as median (interquartile range)). Hyperemic velocity, but not flow-mediated dilation, was significantly lower in hospital nonsurvivors versus survivors (hyperemic velocity: nonsurvivors = 25 (16 to 28) versus survivors = 39 (30 to 50), P < 0.001; flow-mediated dilation: nonsurvivors = 1.90 (0.68 to 3.41) versus survivors = 2.96 (0.91 to 4.86), P = 0.12). Lower hyperemic velocity was independently associated with hospital mortality in multivariable analysis (odds ratio = 1.11 (95% confidence interval = 1.04 to 1.19) per 1 cm/cardiac cycle decrease in hyperemic velocity; P = 0.003).ConclusionsBrachial artery hyperemic blood velocity is a noninvasive index of microvascular function that independently predicts mortality in severe sepsis. In contrast, brachial artery flow-mediated dilation, reflecting conduit artery endothelial function, was not associated with mortality in our severe sepsis cohort. Brachial artery hyperemic velocity may be a useful measurement to identify patients who could benefit from novel therapies designed to reverse microvascular dysfunction in severe sepsis and to assess the physiologic efficacy of these treatments.

Left ventricular myocardial mass determination by contrast enhanced colour Doppler compared with magnetic resonance imaging

Objective: To assess the feasibility of using contrast enhanced colour Doppler echocardiography to determine left ventricular (LV) mass and to compare its accuracy with LV mass obtained by magnetic resonance imaging (MRI). Methods: Images were acquired in the short axis plane of the heart, derived from coronal and sagittal scout views and double oblique angulation. The LV mass was calculated by two methods: Simpson’s rule and the area–length method. Levovist (Schering AG, Berlin, Germany) 2.5 g was given by slow intravenous bolus or infusion over about 45 seconds for contrast imaging. LV images were captured in the apical two chamber, four chamber, and three chamber views. Each contrast harmonic colour Doppler image was converted to a cavity-only image by simple image mathematics. Results: 27 (77.1%) of the patients (mean (SD) age 66.2 (8.9) years) were men. There was a mean (SD) interval of 6.6 (8.6) days (range 0–27 days) between echocardiography and MRI. The mean (SD) LV mass determined by MRI Simpson’s rule method was 171.0 (52.4) g (range 105.1–318.7 g). The mean LV mass (SD) determined by the echocardiographic Simpson’s rule method was 178.2 (47.0) g (range 112.6–307.6 g). The mean (SD) MRI area–length LV mass was 187.3 (64.5) g (range 109.0–393.6 g). The linear regression correlation between LV mass determined by MRI Simpson’s and echocardiographic Simpson’s methods was excellent (y  =  1.022x, R2  =  0.986) with a mean (SD) difference of 7.20 (20.9) g. The linear regression correlation between the MRI area–length LV mass and MRI Simpson’s LV mass was excellent (y  =  1.101x, R2  =  0.989) with a mean (SD) difference of 16.3 (22.3) g. Conclusions: LV mass may be obtained reliably by contrast enhanced colour Doppler and two dimensional echocardiography. The contrast Doppler method accurately determines LV mass with excellent agreement with the MRI technique.