Contrast echocardiography for myocardial perfusion imaging using intravenous agents: progress and promises

Myocardial Contrast Agents – Safety Considerations and Clinical Efficacy in Stress Echocardiography

Transthoracic echocardiographic examination is known to be a safe, non-invasive and reproducible method, used in every day clinical practice to obtain important information about cardiac structure and function. Unfortunately, a significant proportion of studies have highlighted the considerable technically difficultly in producing diagnostic images due to a poor acoustic window and more than 33% of patients undergoing stress echocardiography have suboptimal echocardiographic images. All these limitations have led to the use of contrast agents to improve the quality of standard ultrasound examination to provide a better delineation of left ventricle endocardial borders or to obtain information that cannot be achieved by using standard echocardiography, such as assessing myocardial microcirculation and therefore perfusion. This paper sought to review the clinical efficacy and safety of ultrasound contrast agents focusing on stress echocardiography.

Ultrasound triggered image-guided drug delivery

The integration of therapeutic interventions with diagnostic imaging has been recognized as one of the next technological developments that will have a major impact on medical treatments. Important advances in this field are based on a combination of progress in guiding and monitoring ultrasound energy, novel drug classes becoming available, the development of smart delivery vehicles, and more in depth understanding of the mechanisms of the cellular and molecular basis of diseases. Recent research demonstrates that both pressure sensitive and temperature sensitive delivery systems hold promise for local treatment. The use of ultrasound for the delivery of drugs has been demonstrated in particular the field of cardiology and oncology for a variety of therapeutics ranging from small drug molecules to biologics and nucleic acids.

Absence of clinically significant increase in pulmonary artery pressure after intravenous perflutren injection for myocardial perfusion imaging in pigs

OBJECTIVE

We sought to evaluate the impact of a continuous intravenous infusion of perflutren on systemic pulmonary artery pressures at clinically relevant doses for myocardial perfusion imaging in pigs.

METHODS

Five anesthetized, ventilated, open-chest pigs were administered perflutren intravenously at a rate of 0.0364 mL/kg/min over approximately 5 minutes.

RESULTS

Optimal, sustained myocardial opacification was achieved in all animals. Perflutren produced transient, reversible increases in pulmonary artery pressures versus baseline: 10.6% (3.0 +/- 1.4 mm Hg; 95% confidence interval 1.7-4.2; P < .01) for systolic, 15.2% (2.5 +/- 1.4 mm Hg; 95% confidence interval 1.3-3.7; P < .05) for diastolic, and 11.6% (2.6 +/- 1.1 mm Hg; 95% confidence interval 1.68-3.65; P < .01) for mean pressures. Heart rate and systemic arterial pressures displayed nonsignificant increases during perflutren infusion compared with baseline.

CONCLUSION

A continuous intravenous infusion of perflutren at a rate achieving optimal, sustained myocardial perfusion imaging in pigs induces a mild, transient, not clinically significant increase in pulmonary artery pressures without affecting heart rate or systemic arterial pressures.

Augmentation of cardiac protein delivery using ultrasound targeted microbubble destruction

Gas-filled microbubbles have become an important tool as ultrasonic contrast agents. We have previously shown that ultrasound-targeted microbubble destruction (UTMD) can direct plasmids to the heart. The aim of this study was to evaluate UTMD for protein delivery. Six different groups of rats received 1 microg of luciferase protein with varying protocols: (1) luciferase-loaded microbubbles and ultrasound; (2) luciferase only; (3) luciferase and ultrasound; (4) luciferase-loaded microbubbles; (5) unloaded microbubbles incubated with luciferase and ultrasound; (6) unloaded microbubbles with ultrasound followed by luciferase. Relative luminescence units per mg protein per s were determined in hearts and control organs. The rats that received ultrasound and luciferase-loaded bubbles showed a six-fold higher cardiac luciferase uptake compared with control groups that did not include bubbles. None of the other groups significantly augmented cardiac luciferase activity. We conclude that ultrasound-targeted microbubble destruction can substantially and noninvasively augment organ-specific delivery of proteins.

Assessment of myocardial perfusion in patients with coronary artery disease. Comparison of myocardial contrast echocardiography and 99mTc MIBI single photon emission computed tomography

BACKGROUND

Myocardial perfusion (MP) can be assessed in real time when using a low mechanical index (MI) and harmonic imaging following an intravenous injection of contrast agent. The aim of the study was to determine the feasibility and accuracy of the real-time imaging of contrast echocardiography (MCE) for detecting myocardial perfusion defects at rest and during dobutamine stress echocardiography (DE) compared with 99m Tc MIBI SPECT. The study group consisted of 44 patients (24 men, 20 women, mean age 58.9+/-7.8) with suspected coronary artery disease (CAD). All patients underwent DE. Wall motion (WM) and segmental perfusion were estimated in real time before and at peak stress using a low MI (0.4) after 0.3 ml bolus injections of intravenous Optison. All patients underwent a rest and exercise 99mTc MIBI SPECT study (SPECT). A 16-segment model of the left ventricle was used for the analysis of MP, WM and SPECT by a blinded reviewer. All patients underwent coronary angiography. Significant coronary artery disease was defined as >60% luminal diameter stenosis.

RESULTS

All patients had significant CAD. Twenty-nine patients had single-vessel and 15 patients had double-vessel disease. For all patients, agreement between MCE and SPECT was 89%, between MCE and WM -86%, and between SPECT and WM -82%. The agreement between MCE and SPECT for LAD, RCA and Cx territories was 81, 91 and 73%, respectively. The sensitivity of MCE and SPECT for detecting perfusion defects due to significant CAD (confirmed angiographically) was 97% and 93%, respectively, and the specificity was 93 and 84%, respectively.

CONCLUSION

MCE in real-time imaging with Optison has significant potential for the identification of MP abnormalities. MCE correlates very well with SPECT images.