Relation of contrast echo intensity and flow velocity to the amplification of contrast opacification produced by intermittent ultrasound transmission

Rapid 3-D imaging of contrast flow: application in a perfused kidney phantom

Previous studies indicate imaging of ultrasound contrast in 3-D is potentially superior to 2-D imaging for vascular characterization. A dual-beam, dynamic refill technique, which relies on volumetric contrast clearance and sequential imaging, was used to image a preserved porcine kidney perfused with contrast. A model was developed for the contrast profile across the renal artery to estimate fractional blood volume. This model was used along with refill curve information to measure absolute perfusion within renal cortex for a 100-cm(3) volume. Perfusion measurements from a slice within the volume were also made using a modified interval imaging technique. The measured perfusion using the dual-beam technique was consistent with the perfusion measured using the interval imaging technique (dual-beam values were 1.06 +/- 0.04 x corresponding interval imaging values). These experiments suggest that ultrasound contrast perfusion measurements are independent of the volume of contrast eliminated before refill.

Rapid 3D imaging of contrast flow: demonstration of a dual beam technique

Perfusion imaging in a 3D volume using ultrasound contrast agent may improve vascular characterization compared with 2D imaging. Conventional 3D acquisition requires excessive scan time. A dual transducer technique using conventional systems has been introduced that allows 3D imaging of contrast dynamics with drastically reduced scan times (LeCarpentier et al. 2003). Two transducers are translated across a volume where the leading transducer effects contrast clearance and the following transducer images at desired contrast refill times. With 2D arrays that allow simultaneous clearance and imaging pulses, scan times could be further reduced and the need for two transducers eliminated. The dual transducer technique was demonstrated on a tube phantom, with observed contrast profiles matching those expected. Measured center velocities of (+/- std dev) 1.46 +/- 0.21 and 2.25 +/- 0.5 did not statistically differ from expected values of 1.75 and 2.50 (all mm/s), (p > 0.05). This technique is introduced for rapid acquisition of 3D contrast refill images.

Incremental value of parametric quantitative assessment of myocardial perfusion by triggered Low-Power myocardial contrast echocardiography

OBJECTIVES

The purpose of this study was to compare the assessment of myocardial perfusion by myocardial parametric quantification (MPQ) with technetium-99m sestamibi single-photon emission computed tomographic (SPECT) imaging in humans.

BACKGROUND

Accurate visual interpretation of myocardial contrast echocardiographic (MCE) images is qualitative and requires considerable experience. Current computer-assisted quantitative perfusion protocols are tedious and lack spatial resolution. Myocardial parametric quantification is a novel method that quantifies, color encodes, and displays perfusion data as a set of myocardial parametric images according to the relative degree of perfusion.

METHODS

Forty-six consecutive patients underwent prospective stress/rest technetium-99m sestamibi gated-SPECT imaging and MCE using intravenous Optison or Definity. Apical two- and four-chamber cine loops at rest and after dipyridamole (0.56 mg/kg) stress were acquired. For each patient, the following assessments of myocardial perfusion were performed: 1). visual cine-loop assessment (VIS); 2). MPQ assessment; and 3). combined VIS + MPQ assessment.

RESULTS

The segmental rates of agreement for myocardial perfusion with SPECT were 83%, 89%, and 92% (kappa = 0.46, 0.58, and 0.68) for VIS, MPQ, and VIS + MPQ, respectively. Similar trends were seen for the classification of the presence or absence of a moderate to severe perfusion defect, with the agreement for VIS, MPQ, and VIS + MPQ being 92%, 97%, and 97%, respectively.

CONCLUSIONS

Myocardial parametric quantification demonstrates good agreement with SPECT and incremental agreement with VIS. Analysis strategies that incorporate MPQ demonstrate better agreement with SPECT than visual analysis alone.

Analysis of refill curve shape in ultrasound contrast agent studies

Contrast destruction and replenishment by Flash Echo Imaging (FEI) (also referred to as interval or intermittent imaging) has been qualitatively and quantitatively used for tissue blood refill measurements. Many features and capabilities of contrast refill in tissue blood flow and perfusion remain to be elucidated. To aid the development and full reliable utilization of the technique in medical practice, in this paper we undertake physical and mathematical modeling to evaluate different measures derivable from FEI and to provide a basis for the further study of sensitivity and stability of such measures for the detection and measurement of various flow properties and abnormalities. A phantom was developed and used to conduct a dynamic contrast study. Refill curves were investigated as a means of calculating the mean transit time (MTT) and investigating other information that can be determined from their shape. Exponential and error function fits and the area above these curves were used to estimate MTT. The bubble disruption zone was visually measured and theoretically modeled. Computer simulated refill curves based on the flow phantom for different velocity ranges were then computed and compared to the experimental refill curves. The simulated refill curves closely matched the experimental curves in both shape and MTT. The simulated refill curves matched the shape of the experimental results for different velocity ranges. Another simulation examined how a real circulatory system might influence refill. Different refill curve shapes were obtained for different vascular models. Models including the large arteries and veins showed a much faster initial slope than models where the large vessels were not included. Likewise, simulated "shunting" displayed a different slope than models without "shunting" and specific portions of the refill curve that could maximally distinguish shunting. This computer simulation could lead to some experimental hypotheses about differences between normal and cancerous blood flow.

Prediction of functional recovery of hibernating myocardium using harmonic power Doppler imaging and dobutamine stress echocardiography in patients with coronary artery disease

The aim of this study was to compare the accuracy of harmonic power Doppler imaging (HPDI) and dobutamine stress echocardiography (DSE) in predicting recovery of myocardial function after bypass surgery. HPDI using triggering imaging with the administration of Levovist (Shering AG, Berlin, Germany) and DSE were performed in 34 patients (mean age 64 +/- 5 years) with left ventricular dysfunction. A repeat echocardiogram at rest was performed 3 months after revascularization. Of the 408 revascularized dysfunctional segments, 188 (45%) improved on the repeat echocardiogram. HPDI exhibited overall similar sensitivity (88% vs 87%) and accuracy (74% vs 79%) but lower specificity (61% vs 72%, p<0.05) compared with DSE for predicting recovery of myocardial function. Only delayed opacification at the 1:8 triggering point, demonstrated in 62% of viable segments, exhibited higher sensitivity (63%) and positive (58%) and negative (66%) predictive values than early opacification at 1:4 (25%, p<0.001; 35%, p<0.001; and 49%, p<0.001, respectively) in predicting functional recovery. The presence of contrast enhancement within the revascularized area resulted in a significant improvement after revascularization in wall motion score index and ejection fraction compared with areas with residual contrast defect (1.9 +/- 0.3 vs 2.3 +/- 0.3, p<0.01; 36 +/- 6% vs 29 +/- 5%, p<0.01, respectively). Significant correlations were observed between the contrast score index and the follow-up wall motion score index (r = -0.67) and between the contrast score index and the follow-up ejection fraction change (r = 0.65). Triggered HPDI has high sensitivity in detecting hibernating myocardium and can accurately predict the potential for recovery of ischemic left ventricular dysfunction 3 months after revascularization.

Quantitative assessment of coronary stenosis by harmonic power Doppler with a simple pulsing sequence and vasodilator stress in patients

OBJECTIVES

We examined whether myocardial contrast echocardiography (MCE) with harmonic power Doppler (HPD) employing a simple ultrasound pulsing sequence enables estimation of the severity of coronary artery stenosis in patients.

BACKGROUND

Contrast intensity (CI) during MCE with intravenous microbubble infusion is dependent on the myocardial blood flow velocity (MBFV) and pulsing interval (PI).

METHODS

Based on an in vitro experiment, we devised the MBFV index calculated as the reciprocal of the magnitude of CI decay produced by abrupt PI shortening during intermittent imaging. In 68 coronary artery territories from 49 patients, myocardial HPD images were acquired during intravenous infusion of Levovist, while the long PI with 1:10 electrocardiographic gating was shortened to 1:1, both at baseline and during adenosine triphosphate infusion. The MBFV index in each coronary territory and MBFV reserve as the ratio between hyperemia and baseline were compared with the severity of corresponding coronary artery stenosis assessed by quantitative coronary angiography (QCA) or by pressure guide wire as the fractional flow reserve (FFR).

RESULTS

Both the MCE-derived MBFV index during hyperemia and MBFV reserve exhibited significant negative correlations with the QCA-derived stenosis severity (r = -0.56 and r = -0.64, respectively). The MBFV reserve positively correlated with FFR (r = 0.89). By combining the cutoff values of the MBFV index during hyperemia and MBFV reserve, > or =75% of stenoses defined by QCA were determined, with a sensitivity of 77.3%, specificity of 93.4%, and accuracy of 88.3%.

CONCLUSIONS

Shortening of PI during intravenous MCE with intermittent HPD imaging under vasodilator stress enables assessment of coronary artery stenoses in patients.

Observation of the ischemic cascade in humans using contrast echocardiography during dobutamine stress

Experimental studies have postulated the ischemic cascade and the present study was designed to elucidate whether it can be observed in the clinical setting. Fifty-three patients suspected of having coronary artery disease were studied. Myocardial perfusion abnormalities (MPA) and wall motion abnormalities (WMA) were assessed simultaneously by infusion of Levovist during dobutamine stress echocardiography. Time - intensity data of myocardial opacification were fitted for Y=A (1-e(-)(beta) (t)) from which the rate of increase (beta) of intensity were derived both at rest and during stress. Wall motion was also given a score. Bright opacification was observed in 50 patients: 25 showed significant stenosis (>50%) in the left anterior descending artery (group II) on coronary angiography and 25 did not (group I). Significant differences were found in the beta ratio (stress/rest) between the 2 groups at a low-dose (2.0+/-0.3 vs 1.5+/-0.5, p<0.05) and at a high-dose of dobutamine (2.7+/-1.0 vs 1.1+/-0.5, p<0.001), whereas the wall motion score differed only at a high-dose. Of the 25 patients in group II, MPA preceded WMA in 12, both occurred at the same stage in 12, and neither MPA nor WMA was seen in 1. These data prove the ischemic cascade clinically, using contrast echocardiography, by demonstrating that MPA precede WMA during dobutamine stress in patients with coronary stenosis.

Effect of microbubble fragility on transit rate measurement by contrast echography

We sought to propose a simplified method to measure flow velocity based on ultrasonic microbubble destruction, and investigated the effect of microbubble shell fragility on such measurement. Acoustic density (AD) from the second harmonic short axis image of flow was obtained at variable velocities (2 to 73 mm/s) in an in vitro model during long (1000 ms) and short (33 ms) interval ultrasound (US) pulsing, allowing complete and partial microbubble replenishment between pulses, respectively. Microbubbles with shell elastic modulus of 0.4 MPa and 16 MPa were tested. By shortening pulsing interval, AD diminished gradually, rather than abruptly, to a plateau level for both microbubbles. The extent of AD decay was greater for the fragile than the strong microbubbles. A linear relationship existed between the magnitude of AD decay and flow velocity only in the higher and lower velocity range for the fragile and the strong microbubbles, respectively. Thus, difference in contrast intensities during long and short pulsing intervals, respectively, allowing complete and partial replenishment may provide for velocity measurement, in which choice of optimal microbubble fragility for the range of velocity to measure may increase the accuracy.

Assessment of myocardial viability in prior myocardial infarction by intravenous bolus microbubble injection: a new time domain index to estimate regional relative myocardial blood volume

We tested whether the duration of myocardial opacification by harmonic power Doppler imaging after intravenous bolus microbubble injection (with a definition of "the end of opacification") would reflect the remaining vascular bed in infarcted segments. In 28 patients with previous myocardial infarction and 20 control patients, we performed harmonic power Doppler imaging after intravenous bolus injection of 1.5 g of Levovist. Using multiframe trigger mode in which 4 consecutive frames were imaged at every sixth end systole, which formed 1 "burst," we recorded anterior/septal and inferior/posterior walls separately on the center of each apical view with individual boluses. The duration of segmental opacification was measured as the number of "bursts" in which color signals persisted until the fourth frame. The duration was similar between the anterior/septal and inferior/posterior walls (13 +/- 3 vs 13 +/- 3 bursts, not significant) in the control group. In myocardial infarction patients, the duration was significantly shorter in the infarcted than in the control segments (6 +/- 6 vs 14 +/- 3 bursts, P < .001) and their ratio and difference exhibited significant correlations (r = 0.82, P < .001 and r = 0.91, P < .001, respectively) with the activity ratio on thallium Tl 201 single-photon emission computed tomography at rest. Thus, the duration of opacification by harmonic power Doppler imaging after intravenous bolus microbubble injection, the measurement of which was standardized by using multiframe trigger mode, may be useful in assessing regional myocardial viability in patients with previous myocardial infarction.

Myocardial blood flow measurements in rats with simple pulsing contrast echocardiography

Relationship between contrast intensity and ultrasound (US) pulsing interval has been utilized to quantify myocardial blood flow (MBF) during myocardial contrast echocardiography (MCE). We tested if an MCE method employing a simple pulsing sequence during intravenous contrast infusion has the ability to quantify MBF in rats. We performed MCE in 17 rats using a 5- to 12-MHz broadband transducer during microbubble infusion via the femoral vein. Acoustic density (AD) from the anterior wall of the left ventricle imaged in the short axis plane was plotted against the frame number after shortening the pulsing interval (PI) from 1:20 to 1:1 end-systolic ECG gating. The relation between AD and frame number was fitted to a decay function. The rate of the AD decay was decreased during dipyridamole infusion, but was increased by causing coronary stenosis. The AD during long PI imaging remained unchanged during the interventions. Estimated MBF by MCE after correction by heart rate exhibited a close correlation (r = 0.83) with the present "gold standard" of colored microsphere-derived MBF. Thus, the decay rate of the contrast intensity obtained with the high-frequency transducer after abrupt shortening of PI during intravenous microbubble infusion may provide for noninvasive measurement of MBF in rats.

Accuracy and reproducibility of coronary flow rate assessment by real-time contrast echocardiography: in vitro and in vivo studies

Real-time myocardial contrast echo (MCE) provides the potential to assess myocardial blood flow from time-intensity refilling curves after high-energy bubble destruction. This study validated the accuracy of this approach and the effect of specific examination variables and instrument settings on results. The effects of examination depth and angle as well as dynamic range, pulse repetition frequency, and line density were assessed with the use of in vitro incremental flow rates produced in an in vitro tissue phantom. In vivo recordings of real-time imaging with an infusion of a contrast agent (Optison) were obtained in 7 open-chest dogs with graded left anterior descending artery stenosis at baseline and during adenosine hyperemia, and were compared with flow probe measurements. After bubble destruction, time-intensity data were fitted to an exponential function, and the rate of intensity increase (b) and peak plateau intensity (A) were derived from refilling curves. In vivo real-time values for b, but not A, correlated closely with flow probe measures (r = 0.93). A similar correlation for b was observed in vitro (r = 0.98). The correlation between flow rate and b was influenced by several examination variables, including depth, angle, and instrument settings. Real-time MCE provides accurate quantification of coronary flow by assessing the rate of microbubble refilling. However, this parameter may be affected by several examination and instrument variables. Therefore, real-time MCE refilling measures are best applied by comparing baseline values with those of stress studies.

Comparison of contrast-enhanced fundamental imaging, second-harmonic imaging, and pulse-inversion harmonic imaging

RATIONALE AND OBJECTIVES

To investigate the feasibility of recent contrast-specific ultrasound techniques in depicting vascular flow and the effects of changing the output power of the transducer and insonation mode on contrast enhancement, the authors performed an experimental study with a flow phantom.

METHODS

While changing the mechanical index and the sound insonation mode (continuous and intermittent), images were obtained with three contrast-enhanced ultrasound techniques: fundamental, second-harmonic, and pulse-inversion harmonic imaging (PIHI) after a bolus injection of microbubble contrast agent. The images were compared on a time-intensity curve.

RESULTS

In assessing fixed flow (10 cm/s), PIHI showed the best depiction of flow signal. In intermittent scanning, increases in the mechanical index caused stronger flow signals and longer enhancement duration in all techniques. However, continuous scanning revealed poor depiction of flow signal regardless of the technique or changes in the mechanical index because of significant bubble destruction.

CONCLUSIONS

Microbubble-enhanced PIHI with intermittent scanning at a high mechanical index can depict vascular flow highly effectively without shortening the duration of enhancement.

Comparative value of dobutamine and adenosine stress in the detection of coronary stenosis with myocardial contrast echocardiography

BACKGROUND

Controversy continues as to whether adenosine or dobutamine is the superior pharmacological stress agent for myocardial contrast echocardiography (MCE).

METHODS AND RESULTS

We compared real-time MCE refilling curves and wall thickening during adenosine and dobutamine stress in 14 open-chest dogs with left anterior descending and left circumflex coronary artery stenoses that reduced hyperemia by 40% to 60% and 70% to 90% (mild and severe non-flow-limiting stenosis, NFLS) and resting flow by 10% to 30% and 35% to 50% (mild and severe flow-limiting stenosis, FLS). MCE was performed with low-energy imaging during Optison infusion. After high-energy bubble destruction, time-intensity data from risk beds were fitted for an exponential function as y=A(1-e(-)(bt)), from which the rate of intensity increase (b) and maximal plateau intensity (A) were derived. Although severe NFLS and greater stenoses decreased b with both dobutamine and adenosine, with mild NFLS it was reduced in 58% of animals with dobutamine versus 8% with adenosine. The absolute decrease in b, however, was greater for adenosine than dobutamine with FLS. The A parameter was decreased with both adenosine and dobutamine only with the most severe FLS. Wall thickening was decreased with dobutamine in 33% of animals with severe NFLS and in all animals with any FLS; with adenosine, in all with severe FLS.

CONCLUSIONS

Both dobutamine and adenosine significantly reduce MCE refilling rates in the setting of severe stenosis and in the absence of contractile abnormalities. Dobutamine decreases refilling rate and wall thickening at a less reduced flow grade than adenosine, but adenosine produces a greater magnitude of change than dobutamine.

Quantitative assessment of myocardial perfusion during graded coronary stenosis by real-time myocardial contrast echo refilling curves

OBJECTIVES

The present study examined the ability of real-time myocardial contrast echocardiography (MCE) to delineate abnormalities produced by graded coronary stenoses and to correlate signal intensity (SI) parameters derived from destruction/refilling curves with regional myocardial blood flow (MBF) and contractile function.

BACKGROUND

Recent technological advances have enabled myocardial opacification by MCE to be achieved during real-time imaging.

METHODS

In eight open-chest dogs, we created LAD occlusion and graded stenoses that were either flow-limiting at rest (FLS) or reduced adenosine hyperemia (non-flow-limiting at rest = NFLS). Myocardial contrast echo used Optison infusion and low-energy real-time power pulse inversion imaging. High-energy FLASH frames destroyed bubbles every 15 cardiac cycles. Myocardial SI-versus-time plots were fitted to a one-exponential function to obtain the rate of SI rise (b) and peak SI in the last frame.

RESULTS

Dyssynergy was not observed during any NFLS, but perfusion abnormalities were. Visual detection of decreased opacification was possible with severe NFLS and FLS. b demonstrated a significant reduction with severe NFLS and near significant with moderate NFLS; peak SI did not. All exponential parameters were significantly decreased with FL stenosis and occlusion. The MBF ratio in LAD/LCx beds (fluorescent microspheres) correlated with b (r = 0.79) and the product of the peak SI and b (r = 0.80).

CONCLUSIONS

In an open-chest dog model, parameters derived from microbubble refilling of the imaging field by real-time MCE correlate well with myocardial blood flow and can identify coronary stenosis.

Assessment of myocardial perfusion with contrast echocardiography at rest and with stress: an emerging technology

Over the past 20 years, there has been considerable progress in the field of myocardial contrast echocardiography (MCE). What began as a modality limited to selected cardiac catherization laboratories may soon become a rapid and accurate bedside tool for assessing myocardial perfusion. Because MCE via intravenous contrast injection can be performed at the bedside and avoids the use of radiation exposure, it offers multiple potential clinical applications, including assessment of reperfusion after fibrinolytic therapy, postinfarction risk area, and myocardial viability. The addition of perfusion data to wall motion may augment the results of stress echocardiography. This report describes the technologic advances in contrast agents and related imaging technologies that enable myocardial perfusion to be assessed by echocardiography. In addition, the latest clinical studies of myocardial perfusion by MCE are presented.

Pulse-inversion contrast harmonic imaging: ultrasonographic assessment of cerebral perfusion

Pulse-inversion contrast harmonic imaging is a new ultrasonographic technique that can assess brain perfusion. In an adult with moyamoya disease and multiple recurrent strokes, this method detected subtle hemispheric differences in temporal-lobe perfusion, presumably due to neovascularisation, which were not shown by xenon-computed tomography or magnetic resonance perfusion imaging.

Direct video-microscopic observation of the dynamic effects of medical ultrasound on ultrasound contrast microspheres

RATIONALE AND OBJECTIVES

Ultrasound can cause destruction of microbubble contrast agents used to enhance medical ultrasound imaging. This study sought to characterize the dynamics of this interaction by direct visual observation of microbubbles during insonification in vitro by a medical ultrasound imaging system.

METHODS

Video microscopy was used to observe air-filled sonicated albumin microspheres adsorbed to a solid support during insonation.

RESULTS

Deflation was not observed at lowest transmit power settings. At higher intensities, gas left the microparticle gradually, apparently dissolving into the surrounding medium. Deflation was slower for higher microsphere surface densities. Intermittent ultrasound imaging (0.5 Hz refresh rate) caused slower deflation than continuous imaging (33 Hz).

CONCLUSIONS

Higher concentrations of microbubbles, lower ultrasound transmit power settings, and intermittent imaging each can reduce the rate of destruction of microspheres resulting from medical ultrasound insonation.