Comparison of two ultrasound contrast agents for left ventricular opacification

doi:10.21203/rs.3.rs-3207197/v1

Background

Several ultrasound contrast agents have been approved and used for measurement of left ventricular (LV) ejection fraction (EF). However, there has been no direct comparison between these agents.

Purpose

To compare left ventricular imaging parameters between two widely used ultrasound contrast agents for LV opacification when imaged with a low power contrast specific imaging modality.

Methods

In this retrospective study of patients undergoing echocardiography during anti-cancer treatment were included who underwent follow-up echocardiograms (3 months) with two different ultrasound contrast agents - Definity® and SonoVue® - if there were only minor changes in the LV ejection fraction (EF) between the studies (< 5%). Qualitative and quantitative analysis of the contrast effect was performed in apical four- and two chamber views (4CV,2CV). For quantitative analysis of the LV opacification, square regions of interest (ROI) were placed in the apical and basal ventricular lumen and the arithmetic mean and standard deviation of pixel intensity within each ROI was recorded throughout the entire loop. Qualitative visual assessment was performed using a visual score to assess the endocardial border delineation, basal attenuation and apical swirling.

Results

41 female patients with a mean EF of 61 ± 9% (mean ± standard deviation, SD) fulfilled the inclusion criteria. Both contrast agents provided intensive LV opacification in the basal and apical cavities. The video intensities (mean + standard deviation) in the apical and basal cavities were comparable across both contrast agents in the 2CV apical ROI (219.96 ± 21.42 vs 224.69 ± 20.23), the 2CV basal ROI (114.55 ± 42.02 vs 112.79 ± 42.57), the 4CV apical ROI (228.02 ± 17.80 vs 225.51 ± 20.37), and the 4CV basal ROI (100.05, 41.11 vs 92.84, 42.54). Visual assessment revealed no statistical difference between the contrast agents in terms endocardial delineation.

Conclusion

In patients with normal LV function both ultrasound contrast agents, no clinically relevant differences were found regarding LV opacification and endocardial border delineation.

Echocardiography

Contrast agent

Left ventricle

Ejection fraction

Ultrasound enhancing agents are now routinely used in most tertiary cardiac centres due to their ability to improve the quality of echocardiographic images. One major benefit of these agents is that they can enhance endocardial border definition, allowing for improved assessment of chamber size and systolic function. In addition, they can aid in the characterization of intra-cardiac masses, such as tumors or thrombi, by improving the visualization of these structures. Other advantages of ultrasound enhancing agents is that they can enhance Doppler signal, providing more reliable estimation of gradients across chambers, and can assist in the assessment of myocardial perfusion [1–4].

Several ultrasound contrast agents have been approved for use in the measurement of left ventricular ejection fraction (EF). These agents differ in terms of the concentration of microbubbles, type and content of gas, and composition of the microbubble shell [5]. However, no direct comparison has been performed between these agents.

In our study, we will be comparing two commonly used ultrasound contrast agents, Definity® and SonoVue®, using both quantitative and qualitative methods. It should be noted, that another agent has been approved for LV opacification - Optison® (GE Healthcare Inc.), which is a perflutren protein-type A microsphere injectable suspension that was first approved for use in 1997.

The retrospective analysis was performed using data from a cardio-oncology echocardiography registry at the Mazankowski Alberta Heart Institute in Edmonton, Canada. The following inclusion criteria were applied: 1. Patients with early-stage breast cancer referred for echocardiographic measurement of LV ejection fraction using UEA. during cardiotoxic cancer therapy. 2. Consecutive echocardiograms performed using different UEAs - SonoVue® and Definity®. The choice of the UEA was at the discretion of the cardiologist. 3. Less than 5% difference in LV ejection fraction between the two consecutive echocardiograms. 4. Informed consent for using anonymized imaging data for research (Ethics ID: HREBA.CC-16-0309_REN7).

Definity® vials were stored in a fridge and agitated with the manufacturer supplied device (Vial mix). Then from a pre-filled 10 ml syringe containing 0.9% NaCl solution 0.5 ml were removed and 0.5 ml Definity was added. Definity® is removed from the vial with a 1mL syringe and 21-gauge needle after venting the vial. The SonoVue® vials are stored at room temperature preparation and is prepared by injection of 5mL of 0.9% NaCl saline into a vial with the agent specific venting needle. The vial is then shaken for a few seconds and the agent is ready to inject. In our centre, sonographers that are certified for UEA administration start the intravenous cannula and administer the agent after obtaining informed written consent from the patient.

Echocardiograms were performed with a IE33 using a S5 transducer (Philips). For measurement of the LV ejection fraction at least 2 loops including 2 cardiac cycles of the 4 and 2 chamber views (4CV, 2CV) were recorded according to ASE guidelines. Off-line analysis was performed using the Philips IntelliSpace Cardiovascular echocardiography archiving and analysis system and Qlab software (no 10.5). The loops of the 4CV and 2CV on which the reading cardiologist had made the measurements of the end-diastolic and end-systolic volumes were selected for further analysis. Audio Video Interleave (AVI) loops of the echocardiographic recordings were created, anonymized and saved for visual assessment. A square region of interest (ROI) with an area of 23.96mm² was placed in the LV cavity 2 cm below the apical endocardium. An identical ROI was placed immediately above the mitral valve annulus (Fig. 1). The mitral ring of the left ventricle was identified by bright echoes resulting from the fibrous tissues of the mitral ring. After running the loop, the ROI were adjusted to ensure the margins remained in their intended location for the duration of the recording. In some patients, the ROI were adjusted to account for shadows from rib bones, patient movement, and breathing. In 6 patients, the stable position of the ROI was only possible after the activation of motion compensation software within the QLAB software to enable the ROI to adjust alongside the moving ventricle. Echocardiographic sequences with cardiac motion resulting in the ROI movement outside of the ventricles were excluded.

A screenshot of the end-diastolic frame with the placements of the ROI within it along with a graph displaying the mean ultrasound intensity at each ROI over time was recorded to document the correct ROI position (Fig. 1). The intensity within each ROI is measured using the QLAB software which calculates the average brightness of all the pixels within each 23.96mm2 ROI square. The brightness of each pixel is quantified on a brightness scale of 0-256 with 256 indicating pure white, and 0 indicating black.

Each loop was also reviewed for visual assessment to assess the quality of each scan. Scans were graded on a 1–3 scale for each of tracing, swirling and attenuation (Fig. 2). For tracing, the scan a score of 1 indicated that the entire contour of the LV was traceable, a score of 2 if small gaps in the contour > 0.5cm were not clearly defined, and a score of 3 if major gaps in the contour or shadowing were present. For apical swirling, a score of 1 was assigned if there was no swirling, a score of 2 if there was swirling but the apical LV border still can be traced in end-diastole and end-systole, and a score of 3 if severe swirling was present which did not allow to trace the LV border on end-diastolic and end-systolic frames. For attenuation, a 1 was assigned if no attenuation was present, a 2 if there was mild attenuation still allowing good display of the mitral ring and the basal endocardial borders, and a 3 if basal region of the LV was dark. The 3 numbers for each scan could then be summed and compared to other loops.

Statistical analysis

Descriptive statistics were performed to report the mean as central tendency measure and the standard deviation as measure of dispersion of Definity® and SonoVue®.

Paired T test was performed as inferential statistics procedure to compare the means of Definity® and SonoVue®.

From January 2018 to May 2019, the recordings of 62 patients fulfilled the inclusion criteria: Patients on treatment for breast cancer, consecutive echocardiograms at 3 months interval and difference of EF <5%. Of these 62 patients, 41 had LV contrast studies with different UEAs. . In all but one case, the patients were scanned with DefinityÒ prior to the echocardiogram with SonoVueÒ. In 6 patients, part of the loop had to be trimmed due to extreme patient movement distorting results by providing near zero intensity levels.

There was no statistical difference in the weight, systolic/diastolic blood pressure, heart rate, LV ejection fraction or GLS (which was done pre contrast administration) between the recordings with DefinityÒ and SonoVueÒ (Table 1). With both agents very similar signal intensities were measured (Figures 2 and 3). In the four-chamber view, the mean intensity of the apical ROI was 228.02 +/- 17.8 for DefinityÒ and 225.51 +/- 20.37 for SonoVueÒ, p = X (Table 2). In the basal ROIs, there was also no significant difference between DefinityÒ and SonoVueÒ (Table 2). The measurements in the two-chamber view were lower compared to the those in the 4 chamber views. In both views and with both contrast agents, the signal intensities were at the base compared to the apex. In the apical 4 chamber view, the difference in mean (2.500) was not statistically significant t(80)= 0.5913, P=0.5560, with 95%CI [-5.914 TO 10.914], as well in the basal 4 chamber view, t(80)= 0.7906 P=0.4315, with 95%CI [-11.075 TO 25.675]. In the 2 chamber view, no statistical difference was found in the apical t(80)= 0.1088 P=0.9136, with 95%CI [-9.646 TO 8.646] as well as the basal views t(80)= 0.1927 P=0.8477, with 95%CI [-16.793 TO 20.393].

As a metric for attenuation, the basal and apical ROI signal intensities were compared. In both the 4 and 2 chamber views, no significant differences were found between the two contrast agents (Figure 2,3). In the 4-chamber view, the mean basal-to-apical ratios were 0.43 for Definity® and 0.41 for Sonovue® respectively (Table 3). In the 2-chamber view, the corresponding basal-to-apical ratios were higher, 0.53 for Definity® and 0.53 for Sonovue®

The mean MI for Definity® was 0.12 with a standard deviation of 0.02, and the mean for SonoVue® was 0.09 with a standard deviation of 0.02. The results of visual analysis are shown in Table 4. There were no major differences in the scoring between the two readers and between the two agents and in the delineation of the endocardial borders or presence of apical swirling between DefinityÒ and SonoVue® (Figure 4)

This is the first clinical study to compare two ultrasound contrast agents for LV opacification. No significant differences in imaging parameters were found between the two agents. The two agents used in this study differ in microbubble size and composition (Supplemental Table).

Given that Definity® has a higher concentration of microbubbles per millilitre, we expected more attenuation compared to SonoVue®. Indeed, very bright signals in the apical LV cavity and no signals from the contrast agent have previously been observed in clinical practice when undiluted Definity® was injected followed by a bolus of saline. Therefore, now diluted injections are preferred. The most recent CEUS cardiac exam protocols of the International Contrast Ultrasound Society (ICUS) recommend diluting one vial in up to 60 ml [1]. For bolus injections, no definite degree of dilution is recommended. In our experience using a prefilled syringe with 0.9% NaCl solution appears to be most practical. The entire content of the vial or parts can be added after disposing the corresponding volume of the saline syringe. By mixing 0.5 ml of Definity with 9.5 ml saline a similar concentration of microbubbles is achieved as for undiluted SonoVue®.

We expected comparable results considering the previous studies using either of the contrast agents [5–6]. However, the degree of agreement was not expected considering the differing characteristics of the UEA and the ultrasound machine which determine the quality of the opacification of the LV. Apart from different gases in the microbubbles, the size and concentration of microbubbles as well the stiffness of the shell of the microbubbles determine whether the LV becomes homogeneously opacified or artifacts like apical swirling or basal attenuation limit accurate tracing of the LV cavity. The results of this study suggest that the concentration and total volume of the injected gas may be a major determinant for good LV opacification. However, comparable LV opacification is only achieved when the transmit energy of the transducer is adjusted to the elastic properties of the microbubbles. The default mechanical index for SonoVue is 0.10 because the shell of the SonoVue microbubbles is softer and the bubbles resonate when exposed to lower energy than the mechanical index used for Definity (0.12) which has a stiffer shell. Apart from the different mechanical indices (MI) both UEAs can be imaged with the same preset for low MI contrast specific imaging.

In case the sonographer has used a higher MI than 0.10 before injection on SonoVue®, apical swirling (resulting from bubble destruction in the near field) may be noticed. This can be easily corrected by reducing the MI. On the other hand, a lower MI than 0.12 used for imaging with Definity® may result in basal attenuation. Trouble shooting for swirling and attenuation has been in the recent guidelines and textbooks and should be part of sonographers training (4,5).

The results of this study are relevant for daily practice in a busy echocardiography laboratory in which the two different agents are available or there is switch of agents, and physicians have different preferences. That can result in errors of dosing with risk prolonged imaging due imaging artifacts. It is helpful that for both agents do not differ in the volume of the bolus injections. When using the low MI contrast specific imaging setting which is recommended by the EACVI, 1 ml bolus injections provide adequate imaging with both UEAs. Only mechanical index needs to be reduced to 0.10 when studies are performed with SonoVue® compared to Definity® for which 0.12–0.14 is used. For follow up studies of cardio-oncology patients, the two used UEAs are interchangeable in terms of diagnostic performance.

Our results need to be interpreted in the context of some limitations. The study was retrospective and the sample size was not large enough to include a significant number of patients with LV enlargement and/or dysfunction. A prospective study just to compare two approved agents, which have a very small but not negligible risk of side effects would be ethically debatable and hard to receive sponsorship support. The limited number of patients receiving different UEAs on consecutive echocardiograms is another limitation. However, our data is novel and is likely the only practical method for comparison between contrast agents. As there is no difference in the imaging parameters and no evidence of a difference in safety, the choice of contrast agent for follow up echocardiographic measurements of EF mostly depends on contrast availability, cost and ease of use.

Declaration of interest

Harald Becher: Bracco Imaging – research grants, honorarium for presentations at Bracco sponsored meetings. Lantheus Imaging – research grants, consulting

Funding

Dr. Becher has an endowment of the Heart and Stroke Foundation Canada which supported the study. No funding was received from the manufacturers of UEAs.

The dataset supporting the conclusions of this article are available upon request.

All authors of the manuscript have read and agreed to its content and are accountable for all aspects of the accuracy and integrity of the manuscript in accordance with ICMJE criteria.

That the article is original, has not already been published in a journal, and is not currently under consideration by another journal

That each co-author made substantial contribution to the article.

Porter TR, Feinstein SB, Senior R, Mulvagh SL, Nihoyannopoulos P, Strom JB, et al. CEUS cardiac exam protocols International Contrast Ultrasound Society (ICUS) recommendations. Echo Res Pract [Internet]. 2022;9(1):7. Available from: http://dx.doi.org/10.1186/s44156-022-00008-3.
Porter TR, Abdelmoneim S, Belcik JT, McCulloch ML, Mulvagh SL, Olson JJ, et al. Guidelines for the cardiac sonographer in the performance of contrast echocardiography: a focused update from the American Society of Echocardiography. J Am Soc Echocardiogr [Internet]. 2014;27(8):797–810. Available from: https://www.sciencedirect.com/science/article/pii/S089473171400426X
Porter TR, Mulvagh SL, Abdelmoneim SS, Becher H, Belcik JT, Bierig M, et al. Clinical applications of ultrasonic enhancing agents in echocardiography: 2018 American society of echocardiography guidelines update. J Am Soc Echocardiogr [Internet]. 2018 [cited 2023 May 11];31(3):241–74. Available from: https://pubmed.ncbi.nlm.nih.gov/29502588/
Senior R, Becher H, Monaghan M, Agati L, Zamorano J, Vanoverschelde JL, et al. Clinical practice of contrast echocardiography: recommendation by the European Association of Cardiovascular Imaging (EACVI) 2017. Eur Heart J Cardiovasc Imaging [Internet]. 2017 [cited 2023 May 11];18(11):1205–1205af. Available from: https://pubmed.ncbi.nlm.nih.gov/28950366
Becher H, Helfen A. Contrast echocardiography: Compendium for Clinical Practice. Springer/Nature, London; 2019
Suwatanaviroj T, He W, Mirhadi E, Paakanen R, Pituskin E, Paterson I, et al. Variability of left ventricular volume and ejection fraction measurements using contrast echocardiography: The influence of the left ventricular length measurements in a large cohort of patients during monitoring cardiotoxic effects of chemotherapy. Echocardiography [Internet]. 2018 [cited 2023 May 11];35(3):322–8. Available from: https://pubmed.ncbi.nlm.nih.gov/29272561/
Suwatanaviroj T, He W, Pituskin E, Paterson I, Choy J, Becher H. What is the minimum change in left ventricular ejection fraction, which can be measured with contrast echocardiography? Echo Res Pract [Internet]. 2018 [cited 2023 May 11];5(2):71–7. Available from: https://pubmed.ncbi.nlm.nih.gov/29643125/
European Medical Agency Assessment Report SonoVue 2014 https://www.ema.europa.eu/en/documents/variation-report/sonovue-h-c-303-ii-0025-epar-assessment-report-variation_en.pdf

Table 1. Findings Measurements during SonoVue® and Definity® echocardiograms performed using low MI contrast specific imaging, no statistically significant differences, LMIC low MI contrast specific imaging, MI mechanical index

	Definity®		SonoVue®
	Mean	SD	Mean	SD
MI Weight (kg)	0.12 69.3	0.02 14.23	0.09 69.4	0.2 13.9
Systolic BP	123.4	16.0	122.3	13.2
Diastolic BP	73.3	9.0	72.5	8.5
Heartrate/min	71.4	10.7	74.0	11.3
EF [%]	61.0	8.3	61.1	9.0
GLS [%]	21.4	2.2	21.3	2.0

Table 2. Average brightness of ROIs in apical and basal LV cavity.

	Definity®		SonoVue®
ROI location	mean	SD	mean	SD
4CV apical	228.0	17.8	225.5	20.4
2CV basal	100.1	41.1	92.8	42.5
4CV basal/apical	0.43		0.41
4CV apical	220,0	21.4	224,7	20,2
2 CV basal	114.6	42	112.8	42.6
2CV basal/apical	0.53		0.53

Table 3. Visual assessment score for the categories of tracing, swirling, and attenuation. SD standard deviation

	Definity®				SonoVue®
	2CV		4CV		2CV		4CV
	Mean	SD	Mean	SD	Mean	SD	Mean	SD
Tracing	1.24	0.58	1.15	0.48	1.07	0.35	1.10	0.37
Swirling	1.29	0.46	1.20	0.46	1.15	0.36	1.05	0.22
Attenuation	1.41	0.59	1.54	0.50	1.07	0.35	1.12	0.40

Table 4. Components the UEAs (5,8), *0.5 ml Definity + 9.5 ml 0.9% NaCl

	Definity®	SonoVue®
Gas Shell Microbubble (spheres) diameter Concentration of spheres -undiluted	Perflutren Phospholipids 1.1-3.3 μm 1.2 x 10¹⁰ spheres/mL	Sulphur hexafluoride Phospholipids 1.5-2.5 μm 1.5-5.6 x 10⁸ spheres/mL
Concentration of spheres – diluted*	1.8 x 10⁹ spheres/mL	N/A
Gas volume injected -undiluted Gas volume injected - diluted	150 µl/ml 15 µl/ml	8 µl/ml NA

No competing interests reported.

Comparison of two ultrasound contrast agents for left ventricular opacification

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Abstract

Figures

Introduction

Patients and Methods

Statistical analysis

Results

Discussion

Declarations

References

Tables

Additional Declarations

Status:

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