Evaluation of microbubbles as contrast agents for ultrasonography and magnetic resonance imaging

Abstract

Background: Microbubbles (MBs) can serve as an ultrasound contrast agent, and has the potential for magnetic resonance imaging (MRI). Due to the relatively low effect of MBs on MRI, it is necessary to develop new MBs that are more suitable for MRI. In this study, we evaluate the properties of SonoVue® and custom-made Fe(3)O(4)-nanoparticle-embedded microbubbles (Fe(3)O(4)-MBs) in terms of contrast agents for ultrsonography (US) and MRI.

Methodology/principal findings: A total of 20 HepG2 subcutaneous-tumor-bearing nude mice were randomly assigned to 2 groups (i.e., n = 10 mice each group), one for US test and the other for MRI test. Within each group, two tests were performed for each mouse. The contrast agent for the first test is SonoVue®, and the second is Fe(3)O(4)-MBs. US was performed using a Technos(MPX) US system (Esaote, Italy) with a contrast-tuned imaging (CnTI™) mode. MRI was performed using a 7.0T Micro-MRI (PharmaScan, Bruker Biospin GmbH, Germany) with an EPI-T(2)* sequence. The data of signal-to-noise ratio (SNR) from the region-of-interest of each US and MR image was calculated by ImageJ (National Institute of Health, USA). In group 1, enhancement of SonoVue® was significantly higher than Fe(3)O(4)-MBs on US (P<0.001). In group 2, negative enhancement of Fe(3)O(4)-MBs was significantly higher than SonoVue® on MRI (P<0.001). The time to peak showed no significant differences between US and MRI, both of which used the same MBs (P>0.05). The SNR analysis of the enhancement process reveals a strong negative correlation in both cases (i.e., SonoVue® r = -0.733, Fe(3)O(4)-MBs r = -0.903, with P<0.05).

Conclusions: It might be important to change the Fe(3)O(4)-MBs' shell structure and/or the imagining strategy of US to improve the imaging quality of Fe(3)O(4)-MBs on US. As an intriguing prospect that can be detected by US and MRI, MBs are worthy of further study.

Figure 1. In vitro US experiments.

Images of SonoVue® and Fe₃O₄-MBs under the mode of CnTI™ and Flash: (A) In the first half of the tube, in CnTI® mode, SonoVue® showed high echo; in the second half, in Flash mode, the SonoVue® microbubbles broke and the enhanced signal generated by the microbubbles changed to anecho (white arrow). (B) Under the same imaging strategy change, the signal of Fe₃O₄-MBs tube had little change (white arrow).

Figure 2. In vitro MRI experiments.

The in vitro MR images of different volume fraction of SonoVue® and Fe₃O₄-MBs: (A). 1: sodium chloride solution (0.9% w/v), 2–4: SonoVue® with different volume fraction (1.75%, 3.5%, 7.0%). 5–7: Fe₃O₄-MBs with different volume fraction (1.75%, 3.5%, 7.0%). (B) an approximately linear relationship was observed in SonoVue® (r = −0.982, P<0.05) between SNR and microbubble volume fraction. (C) an approximately linear relationship was observed in Fe₃O₄-MBs (r = −0.929, P<0.05) between SNR and microbubble volume fraction.

Figure 3. Representative US findings with SonoVue® and Fe₃O₄-MBs.

Images from the same mouse: (A) gray-scale image of the tumor with a size 0.79 mm×0.34 mm, (B) under CnTI™ mode just before microbubble injection, signals from tumor was suppressed (white arrow), (C) maximum contrast enhancement US tumor image after SonoVue® injection (white arrow), and (D) maximum contrast enhancement US tumor image after Fe₃O₄-MBs injection (white arrow).

Figure 4. SNR time courses of the same mouse tumor in US by using SonoVue® and Fe₃O₄-MBs.

Figure A shows the SNR time course in ROI during SonoVue® injection and Figure B shows the SNR time course when using Fe₃O₄-MBs injection.

Figure 5. Representative MRI findings with SonoVue® and Fe₃O₄-MBs.

Images from the same mouse during microbubble injection: (A) anatomical image showing the superficial tumor (white arrow), (B) pre-injection EPI-T₂* MRI tumor image (white arrow), (C) maximum contrast enhancement of the tumor after SonoVue® injection (white arrow), and (D) maximum contrast enhancement of the tumor after Fe₃O₄-MBs injection (white arrow).

Figure 6. SNR time courses of MRI from SonoVue® and Fe₃O₄-MBs.

Figure A shows the SNR time course in ROI during SonoVue® injection and Figure B shows the SNR time course when using Fe₃O₄-MBs.

Figure 7. Correlation between the US and MRI.

Figure A shows the signal strength correlation between US and MRI in 600 seconds of enhancement when using SonoVue® (r = −0.733, P<0.05). Figure B shows the signal strength correlation between US and MRI in 600 seconds of enhancement when using Fe₃O₄-MBs (r = −0.903, P<0.05).