麻豆淫院

Can we reveal objects that are hidden in environments completely opaque to the human eye? With conventional imaging techniques, the answer is no: a dense cloud or layer of material blocks light so completely that a simple photograph contains no information about what lies behind it.

However, a research collaboration between the Institut Langevin and TU Wien has now shown that, with the help of innovative mathematical tricks, objects can be detected even in such cases鈥攗sing what is known as the fingerprint matrix.

The team tested the newly developed method on metal objects buried in sand and in applications in the field of medical imaging. A joint publication on this topic has just been in the journal Nature 麻豆淫院ics.

Seeing and hearing means wave scattering

Whether we take a normal photo or use ultrasound to look inside the body鈥攆rom a physical point of view, the same thing always happens when we create an image: A wave is sent to an object, the object reflects part of the wave, and the reflected portion reaches our eye鈥攐r a measuring device. This reflected wave can be used to determine where the object is located.

However, this normally only works if the object's surroundings are sufficiently transparent. "Otherwise, for example in a dense cloud or in murky water, the phenomenon of multiple scattering occurs," explains Prof. Stefan Rotter from the Institute of Theoretical 麻豆淫院ics at TU Wien.

The wave is scattered not only by the object to be imaged, but also by the surrounding environment鈥攐ften many times over, so that only a greatly altered wave can be registered, in which the object being sought can no longer be recognized.

"Instead of the object, all you see is a diffuse fog鈥攖his is a fundamental problem of imaging techniques, from sonar in submarines to imaging techniques in medicine," says Lukas Rachbauer, one of the co-authors of the study.

First the fingerprint, then the image

To overcome this problem, the French-Austrian research team developed a novel method: First, a specific object is examined in an interference-free environment. Each object scatters waves in a very specific, characteristic way. This wave scattering fingerprint of the object can be described mathematically by a matrix鈥攖he so-called scattering matrix.

The object is then hidden in a highly scattering medium鈥攆or example, buried in sand. "When ultrasonic waves are sent into this sand, they are scattered by the sand, but some of the sound penetrates so far into the sand that it is also scattered by the buried object," says Rotter.

"We cannot see the object, but the backscattered ultrasonic wave that hits the microphones of the measuring device still carries information about the fact that it has come into contact with the object we are looking for in the sand."

If, on the one hand, you know the unaltered scattering matrix, the "fingerprint matrix" of the object, and, on the other hand, you measure the wave scattering generated by the hidden object in a multiple-scattering medium, then you can calculate the position of the object using a mathematical method developed by the research team.

"From the correlations between the measured reflected wave and the unaltered fingerprint matrix, it is possible to deduce where the object is most likely to be located, even if the object is buried," explains Rotter.

Steel balls in sand and medical markers

The method was tested with steel balls in sand, but also in medical applications: to monitor the recurrence of breast cancer, so-called lesion markers are used, which are often difficult to image because they are overlaid by scattered signals. With the new method, they were easy to locate.

In addition, fingerprint matrix technology was used to measure muscle fibers鈥攚hich is particularly important for the diagnosis of heart and muscle diseases.

"The concept of the fingerprint matrix is very generally applicable鈥攏ot only for ultrasound, but also for detection with light," says Rotter. "It opens up important new possibilities in all areas of science where a reflection matrix can be measured."

Some objects also change their "scattering fingerprint" when certain physical parameters change鈥攕uch as pressure or temperature. Such variables could be measured from a distance using the new method. This would be particularly exciting, for example, in the measurement of the human brain, where waves have to penetrate the highly scattering skull.