Skoltech researchers and their colleagues have synthesized and tested protein-polymer microbubbles for use in medical ultrasound imaging of internal organs. Administered intravenously, the microbubbles act as a contrast agent, enhancing the quality of the image. Tested on a rat, the new microbubbles proved to remain in the bloodstream longer than currently available analogues, which promises more time for the physician to complete the exam without hurry. The enhanced contrast means that in some cases ultrasound could be used instead of MRI and CT scans, which are more expensive and harmful to the body. The study was published in Biomaterials Advances.

Ultrasound imaging is a fairly inexpensive and harmless technique for medical diagnostics using widely available equipment. The results, however, are not very precise and their quality depends heavily on the skill of the operator. Contrast agents are a good solution: They can noticeably improve the quality of the images of internal organs during ultrasound exams of the heart, liver, brain, etc.

Using other imaging techniques, particularly MRI and CT scans, is not always an option, because of the high cost, lack of advanced equipment, and their adverse effects on the body: The contrast agents those methods rely on are toxic, and computerized tomography additionally exposes the patient to a high dose of radiation. Ultrasound imaging with contrast could be used to diagnose a wide range of cardiovascular, liver, kidney, and female reproductive system diseases.

Ultrasound contrast agents come in the form of microbubbles with shells made of proteins, lipids, or polymers. Proteins are better for visualization quality, while lipids and polymers provide stability and thus win more time for diagnostics. However, neither of these agents are currently available on the Russian market, and besides, there’s still room for improving the performance of the formulations produced in other countries.

Skoltech PhD student Tatiana Estifeeva, Research Scientist Polina Rudakovskaya and Professor Dmitry Gorin from the Photonics Center, along with their colleagues from other institutions, have created the first “hybrid” microbubble formulation by joining a protein called albumin with a copolymer. The combination produced 100 breeds of microbubbles for ultrasound exams, from which the most efficient contrast agent was selected and tested on a live rat by visualizing its beating heart. The initial selection consisted of several stages: assessing microbubble size, concentration, and acoustic response on organ phantoms and then in animals.

Compared with a protein-based agent without the added copolymer, the new hybrid microbubbles achieved an improvement of both key characteristics: image contrast and the time they remain in the bloodstream, which lengthened between one and a half and two times.

To create the new contrast agent, the team paired bovine serum albumin — a protein often used in pharmacology, including in microbubble synthesis — with biocompatible polymers. A total of 100 distinct combinations were investigated, with different polymers used and the proportion of the polymer in the mixture varying from 2% to nearly 50%. The challenge was to find a formulation that provides the optimal balance of microbubble stability, concentration, and ultrasound response.

Initially, those formulations were rejected that did not produce stable microbubbles. The remaining formulations were tested on a blood vessel phantom to determine bubble size, concentration, and acoustic response. This led the team to identify one leading contender, whose performance was then tested against pure albumin microbubbles and control (a saline injection).

The animal test involved the introduction of the agent into the tail vein of a rat, followed by an ultrasound exam of the heart. The result: While the image without a contrast agent lacked most details and in the image with the protein contrast, only the ventricles were visible, the hybrid agent revealed the entire four-chamber heart. And the effect endured for up to 7 minutes, compared with the 3 minutes for standard agents.

The researchers expect that the new agent’s high contrast could make it a useful tool for diagnosing infertility in women and studying blood vessels of the brain. This would require further work, though.

The study reported in this story was supported by Grant No. 21-73-10254 of the Russian Science Foundation.