Enhancement of left ventricular cavity opacification by harmonic imaging after venous injection of Albunex

Optimizing contrast-enhanced echocardiography by employing a sonographer driven protocol

Background

The use of enhancing agents in echocardiography has been shown to facilitate improved study quality. Despite the known benefits, its use remains limited by institutional policies.

Methods

We aimed to retrospectively evaluate if allowing sonographers to place a peripheral intravenous catheter and administer enhancing agent led to a decrease in time to complete outpatient transthoracic echocardiograms in comparison to using nursing personnel. Three separate protocols were employed. The ‘nurse driven protocol’ utilized nurses to place a peripheral intravenous catheter and inject enhancing agent. In a ‘mixed protocol,’ a nurse placed a peripheral intravenous catheter and the sonographer gave the enhancing agent. The ‘sonographer driven protocol’ involved the sonographer placing the peripheral intravenous catheter and delivering enhancing agent.

Results

A total of 232 echocardiograms were included for analysis. Patient characteristics across the three protocols were not statistically significant. The ‘mixed protocol’ had an average study time that was significantly less than the ‘nurse driven protocol’ (49.4 min ± 11.4 vs 54.6 min ± 12.9; p = 0.024). The ‘sonographer driven protocol’ also showed a significant reduction in study time (50.3 min ± 12.6) when compared to the ‘nurse driven protocol’ (p = 0.017). The additional task for the sonographer to place the peripheral intravenous catheter did not significantly increase the time to complete the study.

Conclusion

Allowing sonographers to administer enhancing agent reduced individual echocardiogram study times by approximately 5 min, supporting that a ‘sonographer driven protocol’ is more efficient with potential downstream economic benefits.

A Voltage-Sensitive Ultrasound Enhancing Agent for Myocardial Perfusion Imaging in a Rat Model

Echocardiographers with specialized expertise sometimes perform myocardial perfusion imaging using U.S. Food and Drug Administration-approved microbubbles in an off-label capacity, correlating microbubble replenishment in the near field with blood flow through the myocardium. This study reports the in vivo clinical feasibility of a voltage-sensitive ultrasound enhancing agent (UEA) for myocardial perfusion imaging. Four UEAs were injected into Sprague-Dawley rats while ultrasound images were collected to quantify brightness in the left ventricular (LV) cavity, septal wall, and posterior wall in systole and diastole. Formulation IV, a phase change agent nested within a negatively charged phospholipid bilayer, increased the tissue-to-cavity ratio in both systole and diastole in the septal wall, 6 dB, and in the posterior wall, 5 dB, while leaving the LV cavity at baseline. This outcome improves the signal of the myocardium relative to the LV cavity and shows promise as a myocardial perfusion UEA.

Bedside myocardial perfusion assessment with contrast echocardiography

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency medicine 2016. Other selected articles can be found online at http://www.biomedcentral.com/collections/annualupdate2016. Further information about the Annual Update in Intensive Care and Emergency Medicine is available from http://www.springer.com/series/8901.

Application of acoustic droplet vaporization in ultrasound therapy

Microbubbles have been used widely both in the ultrasonic diagnosis to enhance the contrast of vasculature and in ultrasound therapy to increase the bioeffects induced by bubble cavitation. However, due to their large size, the lifetime of microbubbles in the circulation system is on the order of minutes, and they cannot penetrate through the endothelial gap to enter the tumor. In an acoustic field, liquefied gas nanoparticles may be able to change the state and become the gas form in a few cycles of exposure without significant heating effects. Such a phenomenon is called as acoustic droplet vaporization (ADV). This review is intended to introduce the emerging application of ADV. The physics and the theoretical model behind it are introduced for further understanding of the mechanisms. Current manufacturing approaches are provided, and their differences are compared. Based on the characteristic of phase shift, a variety of therapeutic applications have been carried out both in vitro and in vivo. The latest progress and interesting results of vessel occlusion, thermal ablation using high-intensity focused ultrasound (HIFU), localized drug delivery to the tumor and cerebral tissue through the blood-brain barrier, localized tissue erosion by histotripsy are summarized. ADV may be able to overcome some limitations of microbubble-mediated ultrasound therapy and provide a novel drug and molecular targeting carrier. More investigation will help progress this technology forward for clinical translation.

Single pulse frequency compounding protocol for superharmonic imaging

Second harmonic imaging is currently accepted as the standard in commercial echographic systems. A new imaging technique, coined as superharmonic imaging (SHI), combines the third till the fifth harmonics, arising during nonlinear sound propagation. It could further enhance the resolution and quality of echographic images. To meet the bandwidth requirement for SHI a dedicated phased array has been developed: a low frequency subarray, intended for transmission, interleaved with a high frequency subarray, used in reception. As the bandwidth of the elements is limited, the spectral gaps in between the harmonics cause multiple (ghost) reflection artifacts. A dual-pulse frequency compounding method aims at suppressing those artifacts at a price of a reduced frame rate. In this study we explore a possibility of performing frequency compounding within a single transmission. The traditional frequency compounding method suppresses the ripples by consecutively emitting two short Gaussian bursts with a slightly different center frequency. In the newly proposed method, the transmit aperture is divided into two parts: the first half is used to send a pulse at the lower center frequency, while the other half simultaneously transmits at a slightly higher center frequency. The suitability of the protocol for medical imaging applications in terms of the steering capabilities was performed in a simulation study with INCS and the hydrophone measurements. Moreover, an experimental study was carried out to find the optimal parameters for the clinical imaging protocol. The latter was subsequently used to obtain the images of a tissue mimicking phantom containing strongly reflecting wires. Additionally, the images of a human heart in the parasternal projection were acquired. The scanning aperture with the developed protocol amounts to approximately 90°, which is sufficient to capture the cardiac structures in the standard anatomical projections. The theoretically estimated and experimentally measured grating lobe levels are equal to -28.3 dB and -35.9 dB, respectively. A considerable improvement in the axial resolution of the SHI component (0.73 mm) at -6 dB in comparison with the third harmonic (2.23 mm) was observed. A similar comparison in terms of the lateral resolution slightly favored the superharmonic component by 0.2 mm. Additionally, the images of the tissue mimicking phantom exhibited the absence of the multiple reflection artifacts. The in-vivo acquisition allows one to clearly observe the dynamic of the mitral valve leaflets. The new method is equally effective in eliminating the ripple artifacts associated with SHI as the dual-pulse technique, while the full frame rate is maintained.

Clinical application and laboratory protocols for performing contrast echocardiography

Technically difficult echocardiographic studies with suboptimal images remain a significant challenge in clinical practice despite advances in imaging technologies over the past decades. Use of microbubble ultrasound contrast for left ventricular opacification and enhancement of endocardial border detection during rest or stress echocardiography has become an essential component of the operation of the modern echocardiography laboratory. Contrast echocardiography has been demonstrated to improve diagnostic accuracy and confidence across a range of indications including quantitative assessment of left ventricular systolic function, wall motion analysis, and left ventricular structural abnormalities. Enhancement of Doppler signals and myocardial contrast echocardiography for perfusion remain off-label uses. Implementation of a contrast protocol is feasible for most laboratories and both physicians and sonographers will require training in contrast specific imaging techniques for optimal use. Previous concerns regarding the safety of contrast agents have since been addressed by more recent data supporting its excellent safety profile and overall cost-effectiveness.

Morbid obesity: obscuring the diagnosis of aortic stenosis in a patient with cardiogenic wheezing

We report the case of a morbidly obese 65-year-old female who presented with repeated hypotensive episodes following dialysis. She was misdiagnosed as suffering from asthma, and eventually was found to have severe aortic stenosis. Obesity has become a common and formidable obstacle to gathering important diagnostic information in patients. Modern diagnostic lab tests and imaging modalities such as transthoracic echocardiography (TTE) can provide spurious data in the morbidly obese population, which can ultimately lead to misdiagnosis. In this clinical vignette, we discuss the relationship between the basic pathophysiologic mechanisms underlying aortic stenosis and patient clinical presentation. We also review the relevant literature and discuss the impact of obesity on the diagnosis of this condition.

Advanced echocardiographic techniques

Echocardiography has advanced significantly since its first clinical use. The move towards more accurate imaging and quantification has driven this advancement. In this review, we will briefly focus on three distinct but important recent advances, three‐dimensional (3D) echocardiography, contrast echocardiography and myocardial tissue imaging. The basic principles of these techniques will be discussed as well as current and future clinical applications.

Heart failure with normal left ventricular ejection fraction: role of echocardiography

PURPOSE OF REVIEW

Nearly half of patients presenting with heart failure have a preserved left ventricular ejection fraction (LVEF), previously known as diastolic heart failure. The diagnosis requires fulfillment of three criteria: signs or symptoms of heart failure, presence of a normal LVEF, and evidence of diastolic dysfunction. Two of the criteria can be evaluated by echocardiography. This article reviews the echocardiographic approach to the patient with suspected heart failure with a normal left ventricular ejection fraction (HFNEF).

RECENT FINDINGS

Echocardiography is the primary modality for evaluating left ventricular (LV) systolic and diastolic function in heart failure patients. Measurements of LVEF from two-dimensional echocardiography can have significant variability despite the use of quantitative methods. The use of contrast agents and three-dimensional echocardiography can improve the accuracy. Newer modalities of tissue Doppler imaging and deformation imaging are challenging the concept that systolic function is preserved in HFNEF. Evaluation of diastolic function with echocardiography requires a comprehensive approach using multiple modalities to quantitate transmitral flow, pulmonary venous flow, mitral annular motion, myocardial deformation, and cardiac structure. The clinical applicability of parameters used for evaluating diastolic function and filling pressures is dependent on the LVEF, necessitating a unique approach in patients with suspected HFNEF.

SUMMARY

A comprehensive examination with knowledge of the potential limitations of echocardiography is required to accurately interpret LV systolic and diastolic function in patients with suspected HFNEF.

Myocardial Contrast Echocardiography in the Evaluation of Hypertensive Heart Disease

Myocardial contrast echocardiography (MCE) has an established role in left ventricular assessment by improving the ventricular opacification and endocardial border definition especially in patients with sub-optimal echocardiographic images. With advances in cardiac ultrasound imaging technology and the development of new contrast agents, the clinical utility of this technique has greatly expanded to include assessment of coronary reperfusion in the setting of acute myocardial infarction, determination of myocardial viability within infarct zones as well as assessment of coronary microcirculation and flow reserve in patients with microvascular coronary disease. Improvements in image quality with intravenous contrast agents can facilitate image acquisition and enhance delineation of regional wall motion abnormalities at peak levels of exercise. Numerous studies have confirmed the clinical utility of contrast enhancement during echocardiographic studies, particularly in patients undergoing stress testing. In this paper, we explore the evidence in support of MCE and its potential clinical applications. Our review aims to summarize (1) the basic principles of myocardial contrast echocardiography including recent advances in the ultrasound technology and contrast agents (2) its clinical applications in the diagnosis of cardiovascular diseases and finally, (3) its potential role in risk stratification and assessment of microvascular perfusion in patients with hypertensive heart disease.

Prognostic value of dipyridamole stress myocardial contrast echocardiography: comparison with single photon emission computed tomography

BACKGROUND

Dipyridamole stress myocardial contrast echocardiography (MCE) can be used to detect coronary artery disease (CAD). Because it measures myocardial blood flow velocity in addition to measuring myocardial blood volume, it was hypothesized that it should have greater prognostic utility than single photon-emission computed tomography (SPECT), which measures only myocardial blood volume. Because blood flow mismatch precedes wall thickening (WT) abnormalities during demand ischemia, it was also postulated that perfusion on MCE would be superior to WT abnormalities on echocardiography for this purpose.

METHODS

The incidence of nonfatal myocardial infarction and cardiac death was determined in 261 patients with known or suspected CAD over a mean follow-up period of 14 months who underwent simultaneous dipyridamole stress MCE and 99mTc-sestamibi SPECT. Comparisons of survival curves were conducted with stratified (and unstratified) log-rank tests.

RESULTS

Abnormal results on MCE were found to be the best predictor of an adverse outcome (odds ratio, 23; 95% confidence interval, 6-201; P<.0001) and provided incremental prognostic value over clinical variables (age>60 years, the presence of >or=3 cardiac risk factors, known peripheral vascular disease, prior myocardial infarction, and left ventricular systolic function), inducible WT abnormalities, and SPECT. Prognoses were worst in patients who had both abnormal results on MCE and inducible WT abnormalities and best in those who had neither. Patients with abnormal results on MCE but no inducible WT abnormalities had intermediate outcomes.

CONCLUSION

In patients with known or suspected CAD undergoing dipyridamole stress, MCE provides powerful prognostic information that is superior to clinical variables, electrocardiography, left ventricular systolic function, WT analysis, and SPECT. MCE may therefore serve as a method of choice for myocardial perfusion assessment in patients with known or suspected CAD. Larger studies are needed to confirm these findings.

American Society of Echocardiography Consensus Statement on the Clinical Applications of Ultrasonic Contrast Agents in Echocardiography

ACCREDITATION STATEMENT: The American Society of Echocardiography (ASE) is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians. The ASE designates this educational activity for a maximum of 1 AMA PRA Category 1 Credit.trade mark Physicians should only claim credit commensurate with the extent of their participation in the activity. The American Registry of Diagnostic Medical Sonographers and Cardiovascular Credentialing International recognize the ASE's certificates and have agreed to honor the credit hours toward their registry requirements for sonographers. The ASE is committed to resolving all conflict-of-interest issues, and its mandate is to retain only those speakers with financial interests that can be reconciled with the goals and educational integrity of the educational program. Disclosure of faculty and commercial support sponsor relationships, if any, have been indicated.

TARGET AUDIENCE

This activity is designed for all cardiovascular physicians, cardiac sonographers, and nurses with a primary interest and knowledge base in the field of echocardiography; in addition, residents, researchers, clinicians, sonographers, and other medical professionals having a specific interest in contrast echocardiography may be included.

OBJECTIVES

Upon completing this activity, participants will be able to: 1. Demonstrate an increased knowledge of the applications for contrast echocardiography and their impact on cardiac diagnosis. 2. Differentiate the available ultrasound contrast agents and ultrasound equipment imaging features to optimize their use. 3. Recognize the indications, benefits, and safety of ultrasound contrast agents, acknowledging the recent labeling changes by the US Food and Drug Administration (FDA) regarding contrast agent use and safety information. 4. Identify specific patient populations that represent potential candidates for the use of contrast agents, to enable cost-effective clinical diagnosis. 5. Incorporate effective teamwork strategies for the implementation of contrast agents in the echocardiography laboratory and establish guidelines for contrast use. 6. Use contrast enhancement for endocardial border delineation and left ventricular opacification in rest and stress echocardiography and unique patient care environments in which echocardiographic image acquisition is frequently challenging, including intensive care units (ICUs) and emergency departments. 7. Effectively use contrast echocardiography for the diagnosis of intracardiac and extracardiac abnormalities, including the identification of complications of acute myocardial infarction. 8. Assess the common pitfalls in contrast imaging and use stepwise, guideline-based contrast equipment setup and contrast agent administration techniques to optimize image acquisition.

Contrast-enhanced versus non-enhanced three-dimensional echocardiography of left ventricular volumes

BACKGROUND

In three-dimensional echocardiography (3DE), individual endocardial trabeculae are not clearly visible necessitating left ventricular (LV) volumes to be measured by tracing the innermost endocardial contour. Ultrasound contrast agents aim to improve endocardial definition, but may delineate the outermost endocardial contour by filling up intertrabecular space. Although measurement reproducibility may benefit, there may be a significant influence on absolute LV volume measurements.

METHODS

Twenty patients with a recent myocardial infarction and good ultrasound image quality underwent 3DE using the TomTec Freehand method before and during continuous intravenous contrast infusion. LV volumes were measured offline using TomTec Echo-Scan software.

RESULTS

The use of contrast enhancement increased end-diastolic (110+/-35 vs. 144+/-53 ml; p<0.01) and end-systolic volume measurements (68+/-31 vs. 87+/-45 ml; p<0.01) significantly compared with non-contrast; the ejection fraction remained unchanged (40+/-13 vs. 41+/-14%, p=NS). Measurement reproducibility did not improve significantly, however.

CONCLUSION

Volumes measured by 3DE are significantly larger when ultrasound contrast is used. Possibly, intertrabecular space comprises a substantial part of the LV cavity. In the presence of an adequate apical acoustic window, ultrasound contrast does not improve LV volume measurement reproducibility. (Neth Heart J 2008;16:47-52.).

Contrast echocardiography in Canada: Canadian Cardiovascular Society/Canadian Society of Echocardiography position paper

As an adjunct to transthoracic, transesophageal and stress echocardiography, contrast echocardiography (CE) improves the diagnostic accuracy of technically suboptimal studies when used in conjunction with harmonic imaging. Intravenous ultrasound contrast agents are indicated for left ventricular (LV) opacification and improvement of LV endocardial border delineation in patients with suboptimal acoustic windows. Demonstrated benefits of CE include improvement in the accuracy of LV measurements, regional wall motion assessment, evaluation of noncompaction cardiomyopathy, thrombus detection, Doppler signal enhancement and conjunctive use with stress echocardiography. Studies have shown the value of CE in the assessment and quantification of myocardial perfusion, and recent clinical trials have suggested a role for contrast perfusion imaging in the stratification of patients with suspected coronary artery disease. While it adds some time and cost to the echocardiographic study, CE frequently obviates the need for additional specialized, expensive and less accessible cardiac investigations, and allows for prompt and optimal subsequent patient management. Despite its proven advantages, CE is presently underused in Canada, and this situation will, unfortunately, not improve until several barriers to its use are overcome. Resolving these important hurdles is vital to the future of CE and to its eventual implementation into clinical practice of promising contrast-based diagnostic and therapeutic applications, including the assessment of perfusion by myocardial CE.

Contrast echocardiography

Myocardial contrast echocardiography (MCE) is a noninvasive imaging technique that relies on the ultrasound detection of microbubble contrast agents. These agents are confined to the intravascular space thereby producing signal enhancement from the blood pool. This review encompasses many of the key concepts regarding the clinical application of MCE. The first section focuses on the composition, safety, and biokinetics of ultrasound contrast agents. Then we discuss new ultrasound imaging methodology that has been developed to enhance detection of contrast agent and to assess perfusion at the tissue level. Next, the clinical applications of contrast ultrasound are reviewed. These include enhancement of the cardiac chambers for better assessment of cardiac function and masses, myocardial perfusion imaging for the detection of coronary artery disease, and the assessment of myocardial viability and microvascular reflow. Finally, we discuss some of the future applications for MCE, which include molecular imaging of disease and drug/gene delivery. The overall aim of the review is to update the clinician on state-of-the-art MCE and how it can be applied in patients with cardiovascular disease.

Clinical utility of contrast-enhanced echocardiography

Over the past three decades, echocardiography has become a major diagnostic tool in the arsenal of clinical cardiology for real-time imaging of cardiac dynamics. More and more, cardiologists' decisions are based on images created from ultrasound wave reflections. From the time ultrasound imaging technology provided the first insight into a human heart, our diagnostic capabilities have increased exponentially as a result of our growing knowledge and developing technologies. One of the most intriguing developments that brought about a decade-long combination of expectations and disappointments was the introduction of echocardiographic contrast agents. Despite repeated waves of controversy regarding the readiness of this technology for clinical use, it has overcome multiple hurdles and currently provides useful clinical information that helps cardiologists to diagnose heart disease accurately. Since the initial reports on the use of ultrasound contrast media such as agitated saline or renografin, the major advances in the field of contrast echocardiography have included (1) the development of stable perfluorocarbon-filled microbubbles, frequently referred to as second-generation contrast agents; and (2) the development of contrast-targeted nonlinear imaging modes, such as harmonic imaging, pulse inversion, and power modulation, which allow consistent real-time visualization of these agents. These contrast agents in conjunction with the new imaging technology constitute powerful tools that improve our ability to evaluate left ventricular function and myocardial perfusion, and allow differential diagnosis of thrombi and intravascular masses. In this manuscript, we briefly review some of the literature that has provided the scientific basis for the use of echocardiographic contrast agents in the context of these important variables.

Comparison between fundamental and second-harmonic imaging echocardiography for calculation of left ventricular mass in children

In adults, calculation of left ventricular mass (LVM) has been shown to give higher values when based on M-mode measurements obtained by the second-harmonic imaging (SHI) technique than with the older fundamental imaging (FI) technique. No information is available in paediatric subjects. This study, therefore, compares LVM calculated from measurements obtained with SHI and FI in 14 children, aged 6.9-13.0 years. M-mode tracings were obtained in accordance with American Society of Echocardiography (ASE) recommendations. Three experienced sonographers performed measurements on each subject with both SHI and FI. The mean value was used in all calculations. LVM was calculated according to ASE convention and indexed by body surface area. LVM mean values were 58.9 +/- 9.7 g m(-2) for SHI and 57.8 +/- 8.2 g m(-2) for FI (P = 0.45). This preliminary study in a small group of paediatric subjects demonstrates no systematic differences between FI and SHI modalities in the calculation of LVM. The likely explanation is that the left ventricular endocardial border is usually well visualized with SHI as well as with FI in children.

Regional left ventricular perfusion and function in patients presenting to the emergency department with chest pain and no ST-segment elevation†

AIMS

We hypothesized that the assessment of left ventricular regional function (RF) and myocardial perfusion (MP) will provide incremental value over routine evaluation in patients who present to the emergency department (ED) with chest pain (CP) and no ST-segment elevation.

METHODS AND RESULTS

In addition to routine clinical evaluation, patients with suspected cardiac CP and no ST-segment elevation were evaluated in the ED for RF and MP using contrast echocardiography (CE). Cardiac-related death, acute myocardial infarction, unstable angina pectoris, congestive heart failure (CHF), and revascularization were considered as events within 48 h (early). Of the 1017 patients studied, 166 (16.3%) had early events. Adding RF increased the prognostic information of clinical and EKG variables significantly (Bonferroni corrected P<0.0001) for predicting these events. When MP was added, significant additional prognostic information was obtained (Bonferroni corrected P=0.0002). All patients were followed for a median of 7.7 months (25th-75th percentiles: 2.7-12.5) Of these, 292 (28.7%) had events. Adding RF increased the prognostic information of clinical and EKG variables for determining the risk of events significantly (Bonferroni corrected P<0.0001), which was further increased by adding MP (Bonferroni corrected P<0.0001).

CONCLUSION

Early assessment of RF on CE adds significant diagnostic and prognostic value to routine evaluation in patients presenting to the ED with suspected cardiac CP and no ST-segment elevation. MP provides additional significant value. CE could be a valuable tool in the early triage and management of CP patients presenting to the ED.

Safety of contrast dobutamine stress echocardiography: A single center experience

BACKGROUND

Contrast is increasingly being used during dobutamine stress echocardiography. However, there are few data regarding the safety of this combination.

METHODS

We retrospectively analyzed 751 consecutive stress echocardiograms, 332 without contrast and 419 with contrast (299 with Sonouve, 120 with Optison). Reported side effects and physiologic data were then compared.

RESULTS

There were no fatalities. The incidence of side effects was similar in the 3 groups. The Optison group had a lower diastolic blood pressure compared with the noncontrast group ( P < .05) at rest, and the Sonovue group had a higher peak heart rate compared with the noncontrast group ( P < .001). Patients receiving Optison had more premature atrial contractions ( P < .05) but there was no difference in the incidence of ventricular tachycardia, supraventricular tachycardia, or vagally mediated episodes.

CONCLUSION

The use of contrast during dobutamine stress echocardiography was not associated with an increased risk of side effects.

Harmonic imaging improves estimation of left ventricular mass

OBJECTIVES

To assess the effect of tissue harmonic imaging (THI) on assessment of left ventricular mass index (LVMI) measurements by M-mode trans-thoracic echocardiography, when compared with magnetic resonance imaging (MRI).

METHODS

20 hypertensive male subjects were studied. LVMI was measured in all subjects by both gradient-echo MRI (Lscelsint Prestige 1.9 T) and by transthoracic echocardiography (ATL HDI 5000). M-mode echocardiography recordings were taken for each patient, two with fundamental imaging (FI) and two using THI in a randomised order and the images unlabelled. Recordings were analysed off-line, by a blinded observer. LVMI by MRI was calculated using Simpson's rule on serial short axis slices of 8 mm thickness. Data are expressed as mean +/- SD.

RESULTS

There was a difference in LVMI measurements between FI and THI (LVMI) (79 +/- 20 vs. 93 +/- 25 g2; p < 0.001). A lower mean difference was obtained by THI, compared to FI, when compared with MRI (2 +/- 15 vs. -32 +/- 22 g2; p < 0.001) suggesting that FI underestimates LVMI. Inter-observer variability was similar between THI and FI (4.5 +/- 15 vs. 6.4 +/- 15 g2; p = 0.46).

CONCLUSION

In hypertensive males, M-mode echo derived from FI underestimated LVMI. These results imply that widely accepted reference ranges for LVMI using FI are not applicable when THI is used.

Instantaneous quantitative video intensity heterogeneity: evaluation with low mechanical index contrast echocardiography

BACKGROUND

Instantaneous video intensity of myocardium has been poorly characterized. Myocardial video intensity is usually displayed in the fitted curve from the exponential equation, y = a(1 - e (-bt)). However, information from the fitted curve will be as accurate as the original video intensity data from the perfusion image. Therefore, we sought to characterize the intramyocardial instantaneous video intensity from low mechanical index (MI) contrast echo imaging for variation.

METHOD

Low-MI imaging using a nonlinear cancellation technique was performed on 10 subjects with normal myocardium. Quantitative video intensity was analyzed in five segments in the epicardium and subendocardium, as well as in systole and diastole.

RESULTS

Video intensity varied between the epicardium and endocardium in each of the region that was analyzed, with the greatest variation in the inferior region (P < 0.0001). Diastolic and systolic differences were also present.

CONCLUSION

Instantaneous video intensity is heterogeneous within the myocardium. Differences can result from attenuation, myocardial fiber structure, and even isotropic effects of the contrast agent, and should be taken into account when data are fitted into an exponential function.

Semiautomatic detection of left ventricular contours in contrast-enhanced echocardiographic images: Comparison with magnetic resonance imaging

The aim of this study was to evaluate the accuracy of a semiautomatic contour detection method for left ventricular (LV) volume calculation in contrast-enhanced echocardiographic images. In 26 patients, LV volumes were automatically measured by magnetic resonance imaging and second harmonic echocardiography after intravenous Levovist administration. LV cavity edges were manually drawn and semiautomatically outlined using the active contour algorithm, improved by a nonlinear anisotropic filter; LV volumes were calculated by the modified Simpson's rule. Manual and semiautomatic analysis of echocardiographic images lasted 45 +/- 6 and 20 +/- 8 seconds, respectively. Contrast echocardiography volumes were smaller than those by magnetic resonance imaging (mean difference: 16 mL for manual and 18 mL for automatic analysis). LV volumes by echocardiography closely related with those by magnetic resonance imaging using both manual (r = 0.955) and semiautomatic (r = 0.945) analysis; the correlation was closer for end-systolic than for end-diastolic volumes. In conclusion, this method provides a fast measure of LV volumes in contrast-enhanced images while reducing operator dependency.

Advances in myocardial contrast echocardiography and the role of adenosine stress

Advances in contrast echocardiography hold promise for the routine assessment of myocardial perfusion. Continued progress may ultimately position myocardial contrast echocardiography (MCE) as an imaging modality that can provide comprehensive cardiac assessment-anatomic, physiologic, and pathophysiologic. Vasodilator stress with adenosine can play an important role in conjunction with MCE, particularly as it relates to the noninvasive evaluation of myocardial perfusion and coronary blood flow reserve. Adenosine pharmacologic stress testing may provide improved test performance through perfusion detection when compared with traditional use of dobutamine assessments of regional wall motion abnormalities.

Incremental value of cardiac imaging in patients presenting to the emergency department with chest pain and without ST-segment elevation: a multicenter study

BACKGROUND

We hypothesized that imaging of regional myocardial function (RF) and perfusion (PER) will add incremental value for both diagnosis and short-term prognosis to routine demographic, clinical, and electrocardiographic findings in patients presenting to the emergency department (ED) with chest pain and without ST-segment elevation on the electrocardiogram.

METHODS

We compared contrast echocardiography (CE) with gated single-photon emission computed tomography (SPECT) for this purpose. Both CE and SPECT readings included separate and composite assessments of both RF and PER. Adverse events in the first 48 hours after ED presentation included acute myocardial infarction, emergent revascularization, and cardiac-related death.

RESULTS

Concordance between CE and SPECT was 77% (73% to 82%) for all territories, with a higher concordance for the anterior wall of 84% (78% to 89%). Of the 203 patients recruited for the study, 38 (19%) had a cardiac event within 48 hours of ED presentation: 21 had acute myocardial infarction, 16 underwent an urgent revascularization procedure, and 1 died. In multivariate logistic regression models, the number of abnormal segments on CE and SPECT were significant predictors (P <.05) of cardiac events. The composite scores on CE provided 17% incremental information (P =.009, n = 203) and gated SPECT provided 23.5% additional information (P =.020, n = 163) for predicting cardiac events compared with routine demographic, clinical, and electrocardiographic variables. RF and composite evaluation was superior on SPECT compared with CE, whereas PER alone was not.

CONCLUSIONS

Cardiac imaging of RF and PER at the time of ED presentation offers substantially greater diagnostic and prognostic information for early cardiac events in patients presenting to the ED with chest pain and no ST elevation than does the routine demographic, clinical, and electrocardiographic assessment.

Assessment of left ventricular function with contrast echocardiography

Echocardiography continues to be the primary noninvasive imaging modality for the assessment of cardiac structure and function because of significant advances in ultrasound imaging technology over the last two decades, yet quantification of the LV function has remained an elusive goal. The introduction of contrast agents has represented a major advance in clinicians' ability to visualize the endocardium and to assess LV function accurately, particularly in patients who have poor acoustic windows. Ongoing and future refinements in imaging technology used with contrast enhancement have placed automated quantification of LV function on the brink of widespread clinical use.

Prognostic value of contrast stress echocardiography in patients with image quality too limited for traditional noncontrast harmonic echocardiography

Clinical data and contrast stress echocardiography (CSE) results were analyzed in 283 patients to establish the prognostic value of CSE for patients with limited echocardiogram image quality at baseline. The mean follow-up period was 736 +/- 337 days. Only 7 patients (2.5%) had nondiagnostic image quality with contrast enhancement. During follow-up, 24 cardiac events (8.5%) occurred (5 cardiac-related deaths, 2 nonfatal myocardial infarction, 17 coronary revascularizations). Overall sensitivity, specificity, and positive and negative predictive values were 60.9%, 76.8%, 19.7%, and 95.5%, respectively. Kaplan-Meier event-free survival was higher for patients with a negative CSE result as compared with those with a positive CSE finding (P <.0001). In a multivariate Cox proportional hazards model, positive CSE was the strongest predictor of cardiac events (risk ratio 3.7; 95% confidence interval 1.6-8.7). CSE can successfully predict cardiac events for patients with limited noncontrast echocardiographic image quality. A negative CSE result conferred a good prognosis.

Contrast Echocardiography: Clinical Utility for the Evaluation of Left Ventricular Systolic Function

Despite continued improvements in imaging technology, transthoracic echocardiography does not reliably provide images adequate for interpretation in all patients. In these patients, the administration of ultrasound contrast agents can markedly enhance the diagnostic utility of the test. Contrast echocardiography relies on the ultrasound detection of contrast agents composed of encapsulated microbubbles that are generally smaller than red blood cells. Intravenous administration of microbubble contrast agents results in left ventricular opacification and facilitates delineation of the endocardial border. This procedure has been shown to consistently increase the number of myocardial segments that can be interpreted, to improve accuracy of assessing regional and global left ventricular function, to decrease interinterpreter variability, to increase interpreter confidence, and to be a cost-effective strategy. Accordingly, patient selection for contrast echocardiography should be based not only on adequacy of the baseline images, but also on the clinical question being asked.

Use of Contrast Enhancement for the Assessment of Left Ventricular Function

Endocardial visualization is essential for accurate interpretation of regional wall thickening abnormalities, which constitute the diagnostic hallmark of coronary artery disease, and for reproducible assessment of left ventricular (LV) ejection fraction. Unfortunately, in many cardiac patients, poor image quality does not allow consistent visualization, let alone automated detection, of the endocardial boundary in all segments throughout the cardiac cycle. Continuing efforts directed towards improving endocardial visualization made by both echocardiographic imaging equipment manufacturers and researchers have recently produced new objective, quantitative approaches for the assessment of global and regional LV function. Although these developments have not been integrated into routine clinical practice, they may have a positive impact on endocardial visualization and, thus, eventually improve the ability to accurately and objectively evaluate LV performance. In this review, we discuss some of the latest developments in the use of contrast for the evaluation of LV function.

Which is the better method in detecting significant left anterior descending coronary artery stenosis during contrast-enhanced dobutamine stress echocardiography: coronary flow velocity reserve or wall-motion assessment?

The diagnostic accuracy of dobutamine stress echocardiography (DSE) depends on wall-motion assessment. Coronary flow velocity reserve (CFVR) during DSE can be measured by transthoracic Doppler echocardiography. To investigate comparative diagnostic accuracy between wall-motion and CFVR assessment in the detection of significant left anterior descending coronary artery (LAD) stenosis, 274 patients underwent both contrast-enhanced DSE and coronary angiography. Intravenous contrast agent, Levovist, was injected to enhance left ventricular endocardial border delineation and coronary flow velocity in the LAD. Wall motion was assessed by standard technique, and CFVR was calculated as coronary flow velocity at peak dobutamine stress divided by baseline coronary flow velocity by transthoracic Doppler echocardiography. CFVR could be successfully obtained in 232 patients (feasibility, 85%). After excluding 14 patients with isolated diagonal stenosis, CFVR was significantly lower in 65 patients with significant LAD stenosis than it was in 153 patients without stenosis (1.62 +/- 0.56 vs 2.72 +/- 0.94, P <.001). CFVR < or = 2.0 had a 75% sensitivity, a 81% specificity, and a 79% diagnostic accuracy for detecting significant LAD stenosis, and these values were comparable with those by wall-motion analysis (sensitivity, 78%; specificity, 89%; and diagnostic accuracy, 86%). The measurement of CFVR in the LAD during DSE was feasible and the diagnostic accuracy of CFVR was equivalent to wall-motion assessment in the detection of LAD stenosis.

The role of contrast enhancement in echocardiographic assessment of left ventricular function

With or without contrast, echocardiographic evaluation of left ventricular (LV) function is qualitative, subjective, and experience dependent, because it is mostly based on visual interpretation of gray-scale 2-dimensional images or, at best, relies on tedious and time-consuming manual tracing of the endocardial boundaries. Adequate endocardial visualization is essential for accurate interpretation of regional wall thickening abnormalities, which constitute the diagnostic hallmark of coronary artery disease, and for reproducible assessment of LV ejection fraction. Studies performed in the last decade have estimated the number of patients with suboptimal endocardial delineation by fundamental imaging at 30%. These studies have highlighted the problem of variable image quality as a major limitation of transthoracic echocardiography because failure to adequately visualize a segment, either at rest or during stress, may compromise the ability of this technique to identify areas of dysfunctional myocardium. Several remedies based on different technologic and methodologic approaches have been tested over the years with variable success. In this review we discuss the most recent techniques that have had a positive impact on endocardial visualization, as well as the potential benefits of these approaches.

Left ventricular opacification at rest and during stress

Although echocardiography is the most widely used cardiac imaging modality in the world, it is often limited by poor endocardial border definition. The development of contrast agents that opacify the cardiac chambers after intravenous injection now makes it possible to acquire high-quality images, even in technically difficult cases. Several studies have now shown that contrast echocardiography improves assessment of global and regional wall motion, enhances observer agreement, and salvages technically difficult studies. In addition, contrast echocardiography is valuable in specific settings, such as the intensive care unit or emergency department, where high-quality images are often most difficult to acquire. Finally, obstacles to the penetration of contrast echocardiography into routine clinical practice (such as cost/reimbursement, logistics, and education) are discussed.

Head-to-head comparison of fundamental, tissue harmonic and contrast harmonic imaging with or without an air-filled contrast agent, levovist, for endocardial border delineation in patients with poor quality images

Recent developments in tissue harmonic imaging and intravenous contrast agents have enhanced left ventricular endocardial border delineation (EBD). In a total of 48 patients with poor quality images, apical 4- and 2-chamber views were obtained with fundamental, tissue harmonic and contrast harmonic imaging with or without intravenous Levovist, an air-filled contrast agent. The left ventricle (LV) was divided into 12 segments, and the EBD of each segment was scored: (1) not visible, (2) barely visible, (3) well delineated. The EBD index (EBDI), defined as the sum of the endocardial scores divided by 12 was obtained for each patient. Of a total of 576 LV segments, 231 were scored as 1 by fundamental imaging and that number decreased to 125 segments by tissue harmonic imaging and 116 segments by fundamental imaging with Levovist. The number of segments scored as 1 decreased to 38 segments by tissue harmonic imaging with Levovist, and to 29 segments by contrast harmonic imaging with Levovist. The EBDI by fundamental imaging was 1.85+/-0.29, which improved significantly with the addition of Levovist (2.10+/-0.36, p<0.001) and was nearly identical to that by tissue harmonic imaging (2.15+/-0.32, p=NS). Tissue and contrast harmonic imaging with Levovist further enhanced the EBDI (2.43+/-0.26, 2.51+/-0.27, respectively). Levovist enhances EBD, even in the fundamental mode, to the level obtained with tissue harmonic imaging. Tissue harmonic and contrast harmonic imaging are the best modalities for enhancing EBD after Levovist injection.

Pulse inversion harmonic imaging improves endocardial border visualization in two-dimensional images: comparison with harmonic imaging

Pulse inversion harmonic imaging (PIHI) is a new modality that increases the detection of harmonic echoes and myocardial contrast by cancelling linearly transmitted signals. We tested whether PIHI improved the detection of endocardial borders in noncontrast 2-dimensional echocardiography. We compared PIHI with tissue harmonic imaging (THI), which decreases linearly transmitted signals using filters. Fundamental mode (FM) was compared with THI and PIHI in 50 consecutive patients. The global and segmental endocardial visualization scores measured with FM were significantly improved by using either THI or PIHI. The improvement of the global score compared with FM was slightly higher using PIHI than THI, because of an improved visualization of the base and the anterior wall with the PIHI technique compared with THI. The ratio of myocardial-to-cavity signal was similarly increased from FM with THI and PIHI. PIHI, a new modality for detection of myocardial contrast, can also be used for endocardial border visualization. It provides an improvement relative to THI for specific regions of the endocardium.

Contrast echocardiography improves the accuracy and reproducibility of left ventricular remodeling measurements: a prospective, randomly assigned, blinded study

OBJECTIVES

We sought to assess the impact of contrast injection and harmonic imaging, on the measure by echocardiography of left ventricular (LV) remodeling.

BACKGROUND

Left ventricular remodeling is a precursor of LV dysfunction, but the impact of contrast injection and harmonic imaging on the accuracy or reproducibility of echocardiography is unclear.

METHODS

We prospectively collected LV images by using simultaneous methods. Then, LV volumes were measured off-line, in blinded manner and in random order. The accuracy of echocardiography was determined in comparison to electron beam computed tomography (EBCT) in 26 patients. The reproducibility of echocardiography was assessed by three blinded observers with different training levels in 32 patients.

RESULTS

End-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV) and ejection fraction (EF), as measured by EBCT (195 +/- 55, 58 +/- 24 and 137 +/- 35 ml and 71 +/- 5%, respectively) and echocardiography with harmonic imaging and contrast injection (194 +/- 51, 55 +/- 20 and 140 +/- 35 ml and 72 +/- 4%, respectively), showed no differences (all p > 0.15) and excellent correlations (all r > 0.87). In contrast, echocardiography using harmonic imaging without contrast injection underestimated the EBCT results (all p < 0.01). Reproducibility was superior with rather than without contrast injection for intraobserver and interobserver variabilities (all p < 0.001). Values measured by different observers were different without contrast injection, but were similar with contrast injection (all p > 0.18). Consequently, intrinsic patient differences represented a larger and almost exclusive proportion of global variability with contrast injection for EDV (94 vs. 79%), ESV (93 vs. 82%), SV (87 vs. 53%) and EF (84 vs. 41%), as compared with harmonic imaging without contrast injection (all p < 0.005).

CONCLUSIONS

For assessment of LV remodeling, echocardiography with harmonic imaging and contrast injection improved the accuracy and reproducibility, as compared with imaging without contrast injection. With contrast injection, variability was almost exclusively due to intrinsic patient differences. Therefore, when evaluation of LV remodeling is deemed important, assessment after contrast injection should be the preferred echocardiographic approach.

Qualitative and quantitative effects of harmonic echocardiographic imaging on endocardial edge definition and side-lobe artifacts

Harmonic imaging is a new ultrasonographic technique that is designed to improve image quality by exploiting the spontaneous generation of higher frequencies as ultrasound propagates through tissue. We studied 51 difficult-to-image patients with blinded side-by-side cineloop evaluation of endocardial border definition by harmonic versus fundamental imaging. In addition, quantitative intensities from cavity versus wall were compared for harmonic versus fundamental imaging. Harmonic imaging improved left ventricular endocardial border delineation over fundamental imaging (superior: harmonic = 71.1%, fundamental = 18.7%; similar: 10.2%; P <.001). Quantitative analysis of 100 wall/cavity combinations demonstrated brighter wall segments and more strikingly darker cavities during harmonic imaging (cavity intensity on a 0 to 255 scale: fundamental = 15.6 +/- 8.6; harmonic = 6.0 +/- 5.3; P <.0001), which led to enhanced contrast between the wall and cavity (1.89 versus 1.19, P <.0001). Harmonic imaging reduces side-lobe artifacts, resulting in a darker cavity and brighter walls, thereby improving image contrast and endocardial delineation.

[Usefulness of contrast echocardiography with harmonic imaging in a patient with hypertrophic cardiomyopathy and midventricular obstruction]

Echocardiography is routinely used for the evaluation of cardiac function. Definition of the endocardial border is essential for the assessment of global and regional left ventricular contractility. This is sometimes difficult due to an inadequate acoustic window. New echocardiographic techniques may be useful to accurate and noninvasively diagnose certain conditions which may otherwise remain undiagnosed with traditional techniques. We present a case of a patient diagnosed with segmental wall motion abnormalities (lateral and apical hypokinesis) by conventional echocardiography. The use of harmonic imaging with contrast changed the initial diagnosis and the patient was diagnosed with severe hypertrophic cardiomyopathy with midventricular obstruction, without segmental wall motion abnormalities.

Contrast echocardiography is superior to tissue harmonics for assessment of left ventricular function in mechanically ventilated patients

BACKGROUND

Assessment of left ventricular function by echocardiography is frequently challenging in mechanically ventilated patients. We evaluated the potential value of contrast-enhanced imaging and tissue harmonic imaging over standard fundamental imaging for endocardial border detection (EBD) in these patients.

METHODS AND RESULTS

Fifty patients underwent standard transthoracic 2D echocardiography and were imaged in fundamental and tissue harmonic modes and subsequently with intravenous contrast (Optison). Two echocardiographers reviewed all studies for ease of visualization of endocardial border segments and scoring of wall motion. EBD for each wall segment was graded from 1 to 4 (1 = excellent EBD). Wall motion was scored by a standard 16-segment model and 1 to 5 scale. Studies were categorized as nondiagnostic if 4 of 6 segments in the apical 4-chamber view were either poorly seen or not seen (EBD score 3 or 4). Quantification of ejection fraction was independently performed offline. Visualization of 68% of all segments improved with contrast echocardiography versus 17% improvement with tissue harmonics compared with fundamental mode. Significant improvement (poor/not seen to good/excellent) occurred in 60% of segments with contrast echocardiography versus 18% with tissue harmonics. A total of 850 segments were deemed poor/not seen, 78% of which improved to good/excellent with contrast echocardiography versus 23% with tissue harmonics. Interobserver agreement on EBD was 64% to 70%. Conversion of nondiagnostic to diagnostic studies occurred in 85% of patients with contrast echocardiography versus 15% of patients with tissue harmonics. Scoring of wall motion with fundamental mode, tissue harmonics, and contrast echocardiography was possible in 61%, 74%, and 95% of individual segments, respectively (P <.001). Wall motion scoring was altered in 17% of segments with contrast echocardiography and in 8% with tissue harmonics. Interobserver agreement on wall motion scoring was 84% to 88%. Contrast echocardiography permitted measurement of ejection fraction 45% (P =.003) more often over fundamental mode versus a 27% (P =.09) increase with tissue harmonics.

CONCLUSIONS

Contrast echocardiography is superior to tissue harmonic imaging for EBD, wall motion scoring, and quantification of ejection fraction in mechanically ventilated patients.

Is the technically limited echocardiographic study an endangered species? endocardial border definition with native tissue harmonic imaging and Optison contrast: a review of 200 cases

OBJECTIVE

Our goal was to determine whether contrast adds diagnostic value to both fundamental and native tissue harmonic imaging (NTHI) for endocardial border definition.

METHODS

Two hundred consecutive patients who underwent stress echocardiography imaging were studied in either fundamental (n = 52) or NTHI mode (n = 148) with an Acuson Sequoia echocardiographic system. Contrast agent (Optison) was administered (0.5 to 1 mL) for enhancement of endocardial borders. Two- and 4-chamber views were analyzed before and after administration of contrast at peak stress for grading of 5 endocardial border segments. Scores from 0 to 5 were assigned to each study for all the images both before and after contrast (0 = 0 segments completely visualized; 5 = 5 segments completely visualized).

RESULTS

The use of Optison contrast significantly enhanced border definition when imaging was performed in either fundamental or NTHI mode. Addition of contrast resulted in better endocardial border definition in fundamental mode (4.1 + or - 1.0 versus 2.3 + or - 1.3, P <.001). However, in NTHI mode, the presence of contrast resulted in enhanced definition of endocardial border compared with its absence (4.8 + or - 0.5 versus 3.3 + or - 1.1, P <.001). The combination of NTHI and contrast resulted in more visualization of endocardium when compared with the combination of fundamental imaging and contrast (4.8 + or - 0.5 versus 4.1 + or - 1.0, P <.001). In addition, interobserver agreement for border detection increased from 83% in fundamental mode without contrast to 95% with the use of NTHI with Optison (P <.001).

CONCLUSION

As defined in 200 cases, the combination of NTHI with Optison contrast results in nearly complete and consistent endocardial border definition.

Measurement of left ventricular volumes and ejection fraction after intravenous contrast agent administration using standard echocardiographic equipment

The enhancement of endocardial border delineation using second harmonic imaging and contrast administration improves the measurement of ventricular volumes. In the majority of existing echocardiographic equipment, however, harmonic imaging is not yet available. The aim of this study was to assess the feasibility of the measurement of left ventricular volumes and ejection fraction after intravenous administration of the contrast agent Levovist using standard echocardiographic equipment and fundamental imaging modality. In 10 patients with good-quality two-dimensional echo imaging, 4 g (400 mg/mL concentration) of Levovist was injected intravenously. Hewlett-Packard Sonos 2000 ultrasound equipment without second harmonic imaging capability was used. To avoid the destruction of microbubbles, the echo machine was set to produce only one end-systolic and one end-diastolic frame in each cardiac cycle (dual triggering). Native and contrast imaging measurements of left ventricular volumes and ejection fractions calculated by modified Simpson's rule were compared in the fundamental mode. Intraobserver and interobserver variability values were assessed. End-diastolic volumes in native continuous and triggered mode and by contrast echo were 126 +/- 48, 121 +/- 46, and 130 +/- 50 mL, respectively (NS), whereas end-systolic volumes were 79 +/- 48, 76 +/- 45, and 79 +/- 46 mL, respectively (NS). Calculated ejection fraction using the three different imaging modalities were 0.41 +/- 0.16, 0.41 +/- 0.16, and 0.42 +/- 0.16 (NS). The intraobserver and interobserver reproducibility values were excellent in triggered mode. Standard echocardiographic equipment with fundamental imaging modality in the triggered mode is suitable for the measurement of left ventricular volumes after intravenous Levovist administration. In clinically difficult patients, contrast echocardiography in triggered mode may be applied even if echocardiographic equipment does not have harmonic imaging possibility.

Improved reliability for echocardiographic measurement of left ventricular volume using harmonic power imaging mode combined with contrast agent

Harmonic power imaging (HPI) is a new echocardiographic modality that enhances the detection of contrast agents in the left ventricle. The endocardium can be delineated by conventional echocardiography using ultrasound contrast agents, although the images tend to be faint. The present study was designed to assess left ventricular volume using HPI after intravenous injection of the contrast agent Levovist (Schering SA, Berlin, Germany) in 25 unselected patients. End-diastolic volume, end-systolic volume, and ejection fraction were determined for each patient with angiography and with 4 different ultrasound modalities: (1) conventional mode without contrast, (2) contrast conventional mode, (3) contrast harmonic intermittent imaging mode, and (4) contrast triggered HPI. The use of HPI improved correlations between the echographic and angiographic measurements for all parameters as well as precision and bias determined by Bland and Altman analysis. The relative errors for interobserver variability were also lower with HPI. This study demonstrates that echocardiographic determination of left ventricular volumes and ejection fraction is more accurate and reproducible using HPI combined with Levovist.