Melodious Microscope

Researchers at the Helmholtz Zentrum München use laser flashes and ultrasonic signals to look into live tissue. Extremely short laser flashes penetrate the body of a zebra fish centimetre deep. The living organism responds with weak tones that the human ear cannot perceive. A sensitive microphone reliably records the ultrasonic sounds. This sound pattern contains all kinds of information that the team headed by Professor Dr. Vasilis Ntziachristos, Director of the Institute for Biological and Medical Imaging at the Helmholtz Zentrum München can convert into three-dimensional, high resolution images from inside the fish’s body. “Our new method – Multispectral Opto-Acoustic Tomography (MSOT) – enables us to transcend the borders of light microscopy,” says Laboratory Head, Dr. Daniel Razansky. For light waves are already strongly diffused just under the surface of the object, only allowing exterior views, at most.

Ultrasonic signals, on the other hand, are first generated deep in the living body and so allow a completely new insight, even into molecular processes. The advantage of opto-acoustic imaging differentiates it from classical imaging methods. “X-ray and ultrasound can only make structures visible, but not biochemical reactions,” says Razansky. The researchers from Munich have already demonstrated this successfully in their experiments with threadworms, zebra fish and mice. Hence, they have been able to obtain novel 3D insights into the innards of a zebra fish. They did this by transmitting short laser flashes for just a few nanoseconds into a fish whose body contains fluorescent pigment molecules. The laser flashes light up the pigments.

At the same time, however, these molecules become a little warmer and expand a little. “The rapid expansion of the pigments causes a small shock wave,” explains Razansky. A microphone placed just a few centimetres above the fish’s body picks this up as an ultrasonic wave. The three-dimensional image has a resolution of 40 micrometres.

The conversion of ultrasonic signals into image data was a particular challenge for the researchers. To pick the individual pixels out of the sound waves, they developed a complex mathematical algorithm. In countless tests, they learnt how soft and harder structures in the body distort the sound waves. Only then could a specifically designed computer programme interpret the measured sound data of unknown structures, such as those in the zebra fish. The Helmholtz scientists meanwhile have a better understanding of the connection between acoustic signal and body. “The method could achieve a resolution of ten micrometres in a depth of up to five centimetres,” believes Razansky.

Ntziachristos and Razansky see many applications in medicine, in particular since recent years have seen numerous pigments approved for clinical use. In the future, the technology could facilitate the examination of tumours or coronary vessels in humans, for instance, the early recognition of breast cancer. And if the molecular effects of new cancer drugs are pursued over a longer period of time in an animal, this could accelerate the development of new drugs. Perhaps the effectiveness of cancer drugs could even be observed and assessed directly in the patient’s body. Razansky can even imagine the early diagnosis of Alzheimer’s.

Razansky believes initial clinical studies on patients using the MSOT method might even be feasible in around two years. Before then, they will still have to check numerous marker substances in addition to the already tested fluorescent pigments for suitability. These could circulate in the blood system for several days and help track down pathogenic processes. Docked onto cancer cells, they could make biochemical reactions visible during tumour growth via ultrasonic signals.

To ensure that the method can also be used by physicians in the future, the research group has already designed a prototype for a hand-held MSOT probe that focuses the laser flashes on a specific region of the body and can capture the resulting sound waves. Even the strict Review Consortium of the European Research Council (ERC) was impressed by the potential for biomedical imaging and awarded Ntziachristos an ERC grant worth two million euros at the end of 2008. Medical engineering and technology companies are already showing an interest so that MSOT prototypes could quickly lead to a marketable light-sound microscope for research, hospitals or the pharmaceuticals industry.