Triggered Replenishment Imaging Reduces Variability of Quantitative Myocardial Contrast Echocardiography and Allows Assessment of Myocardial Blood Flow Reserve

Imaging with ultrasound contrast agents: current status and future

Microbubble ultrasound contrast agents (UCAs) were recently approved by the Food and Drug administration for non-cardiac imaging. The physical principles of UCAs, methods of administration, dosage, adverse effects, and imaging techniques both current and future are described. UCAs consist of microbubbles in suspension which strongly interact with the ultrasound beam and are readily detectable by ultrasound imaging systems. They are confined to the blood pool when administered intravenously, unlike iodinated and gadolinium contrast agents. UCAs have a proven safety record based on over two decades of use, during which they have been used in echocardiography in the U.S. and for non-cardiac imaging in the rest of the world. Adverse effects are less common with UCAs than CT/MR contrast agents. Compared to CT and MR, contrast-enhanced ultrasound has the advantages of real-time imaging, portability, and reduced susceptibility to metal and motion artifact. UCAs are not nephrotoxic and can be used in renal failure. High acoustic amplitudes can cause microbubbles to fragment in a manner that can result in short-term increases in capillary permeability or capillary rupture. These bioeffects can be beneficial and have been used to enhance drug delivery under appropriate conditions. Imaging with a mechanical index of < 0.4 preserves the microbubbles and is not typically associated with substantial bioeffects. Molecularly targeted ultrasound contrast agents are created by conjugating the microbubble shell with a peptide, antibody, or other ligand designed to target an endothelial biomarker associated with tumor angiogenesis or inflammation. These microbubbles then accumulate in the microvasculature at target sites where they can be imaged. Ultrasound contrast agents are a valuable addition to the diagnostic imaging toolkit. They will facilitate cross-sectional abdominal imaging in situations where contrast-enhanced CT and MR are contraindicated or impractical.

Reproducible Computer-Assisted Quantification of Myocardial Perfusion with Contrast-Enhanced Ultrasound

Myocardial perfusion can be quantified by myocardial contrast echocardiography (MCE) and is used for the diagnosis of coronary artery disease (CAD). However, existing MCE quantification software is highly operator dependent and has poor reproducibility and ease of usage. The aim of this study was to develop robust and easy-to-use software that can perform MCE quantification accurately, reproducibly and rapidly. The developed software has the following features: (i) semi-automatic segmentation of the myocardium; (ii) automatic rejection of MCE data with poor image quality; (iii) automatic computation of perfusion parameters such as myocardial blood flow (MBF). MCE sequences of 18 individuals (9 normal, 9 with CAD) undergoing vasodilator stress with dipyridamole were analysed quantitatively using the software. When evaluated against coronary angiography, the software achieved a sensitivity of 71% and a specificity of 91% for hyperemic MBF. With the automatic rejection algorithm, the sensitivity and specificity further improved to 77% and 94%, respectively. For MBF reproducibility, the percentage agreement is 85% (κ = 0.65) for inter-observer variability and 88% (κ = 0.72) for intra-observer variability. The intra-class correlation coefficients are 0.94 (inter-observer) and 0.96 (intra-observer). The time taken to analyse one MCE sequence using the software is about 3 min on a PC. The software has exhibited good diagnostic performance and reproducibility for CAD detection and is rapid and user-friendly.

Evaluation cardioprotective effects of atorvastatin in rats by real time myocardial contrast echocardiography

BACKGROUND

The ability to assess myocardial perfusion in small animals is important, especially to investigate models of myocardial ischemia. Myocardial perfusion is usually assessed by postmortem techniques, eliminating the possibility of follow-up in intervention studies. The purpose of the study was to examine the feasibility of real time myocardial contrast echocardiography (MCE) to evaluate cardioprotective effects of atorvastatin in a rat model of acute ischemia-reperfusion injury.

METHODS

The rats (n=15) underwent 20 minutes of mechanical left descending coronary artery (LAD) occlusion followed by 180 minutes of reperfusion. The animals received either atorvastatin (10 mg/kg), atorvastatin and the nitric oxide synthase (NOS)-inhibitor N-Nitro-L-Argininemethylester (L-NAME) (15 mg/kg), or vehicle. MCE was performed to assess the size of the perfusion defect and the myocardial signal intensities (A(max)) at the baseline, during occlusion, and during reperfusion. For comparison, the infarct size, risk area, and regional myocardial blood flow (MBF) were determined by the standard techniques as well.

RESULTS

The dynamics of ischemia-reperfusion injury could be visualized serially by MCE. The infarct size-to-risk area ratio progressively increased during reperfusion and was markedly reduced in the atorvastatin group. Triphenyltetrazolium chloride (TTC) staining confirmed a 23% reduction in the infarct size by atorvastatin. The infarct size by MCE correlated well with the histological methods (r=0.86, P < 0.001). A(max) was reduced in the anterior segments during LAD occlusion (0.08 +/- 0.01 dB) compared to the baseline (2.9 +/- 0.4 dB), approached higher levels post revascularization of LAD (3.22 +/- 0.50 dB), but decreased during 180 minutes of reperfusion (2.32 +/- 0.40 dB). After 180 minutes of reperfusion, A(max) in the risk area was significantly higher in the atorvastain-treated group compared to the vehicle-treated group (2.32 +/- 0.40 dB vs 1.3 +/- 0.4 dB, P <or= 0.05), indicating preserved MBF. The L-NAME-treated group showed no significant difference compared to the vehicle-treated group (A(max) 1.12 +/- 0.60 dB vs 1.3 +/- 0.4 dB). The regional blood flow ratio (ischemic-to-nonischemic wall) measured by the microspheres was significantly higher in the atorvastatin group compared to the control and the L-NAME groups, respectively (0.92 +/- 0.13 vs 0.45 +/- 0.23 vs 0.51 +/- 0.16, P <or= 0.05).

CONCLUSIONS

Atorvastatin has cardioprotective effects in acute reperfusion injury. Contrast echocardiography allows visual and quantitative evaluation of the dynamics of myocardial ischemia-reperfusion injury and can be used to monitor cardioprotective effects during pharmacological interventions even in small animals.

Simplified contrast ultrasound accurately reveals muscle perfusion deficits and reflects collateralization in PAD

BACKGROUND

Simplified contrast-enhanced ultrasound (CEUS) can be used to evaluate muscle perfusion in peripheral arterial disease (PAD). Here, we report its diagnostic accuracy for detecting symptomatic PAD. Additionally, we hypothesize that the extent of collateral formation is reflected by CEUS.

METHODS

Ultrasound contrast agent was injected into an antecubital vein of 58 control subjects and 52 symptomatic PAD patients and its appearance in the calf muscle was evaluated. Interreader variability was tested using 118 raw data films. Arterial collateralization of PAD patients was assessed by angiographic imaging.

RESULTS

PAD patients showed a significantly longer median time to peak intensity (TTP, 36.9s) than control subjects (19.4s, p<0.001) with longer TTPs in advanced PAD stages. The area under the receiver operating characteristic curve was 0.942 and the mean TTP difference between two blinded readers was 0.28s. A TTP cut off at 30.5s was associated with 91% positive predictive value. PAD patients with good collateralization showed a significantly shorter TTP (34.1s) than patients with poor collateralization (44.0 s, p=0.008) but not a higher ankle-brachial index (ABI).

CONCLUSIONS

CEUS accurately displays perfusion deficits of the calf muscle in symptomatic PAD patients. The degree of arterial collateralization is reflected by CEUS and not by ABI.