Detail

Imaging
29.07.2016

A look beneath the skin: New non-invasive imaging method for showing oxygen in tissue

Learning how to look inside a body without having to cut it open is still an important part of medical research. One of the great challenges in imaging remains the visualization of oxygen in tissue. A team led by Prof. Vasilis Ntziachristos, Director of the Institute of Biological and Medical Imaging (IBMI) at Helmholtz Zentrum München and Chair for Biological Imaging at the Technical University of Munich (TUM), has developed a new approach to this task. Their method, which uses optoacoustic spectral detection and accounts for the distribution of light in tissue, is presented in ‘Nature Communications’.

The image shows 4 fields of tissue imaging measurements ranging from red (100% oxygen) to green (0% oxygen).

New non-invasive imaging method for showing oxygen in tissue, Source: Helmholtz Zentrum München

Imaging of tissue oxygenation is not straightforward; different techniques have been considered but each of them has their shortcomings. In recent years, research in this field has focused on optoacoustic methods. These, especially Multispectral optoacoustic tomography (MSOT), form one of the key areas of Vasilis Ntziachristos' research. Put in simple terms, MSOT turns light into sound and then into visual information: First, a weak pulsed laser beam is directed at tissue. Absorbing molecules and cells respond with a minuscule vibration, which, in turn, creates sound signals. The sound signals are then picked up by sound sensors and translated into images. The way molecules and cells react to the laser beam offers insight into their optical and thus into their biochemical properties.

While MSOT can, in theory, be used to tell how much oxygen can be found in blood, there is one major obstacle: The intensity of light changes with depth, not only because light has been filtered through all the tissue layers that it passed through, but also because different tissue structures may have different properties that affect how light is scattered and absorbed. In the past, there have been several attempts to solve this problem by calculating how the tissue will affect the propagation of light. “However, due to the high optical complexity of tissues, this approach so far could not be flexibly applied in optoacoustic images of tissues of living subjects,” says Stratis Tzoumas, first author of the study.

A new description of light distribution in tissue

Ntziachristos, Tzoumas, and their colleagues came up with a completely different approach. Instead of describing the spatial distribution of light, their imaging method eMSOT - the e stands for "eigenspectra" - avoids simulating the path of light through complex tissue altogether. Instead the new method is based on the discovery that the spectrum of light propagating in tissue can be described by using a small number of basic spectra.  eMSOT uses data from a conventional MSOT-device combined with a new algorithm that is based on this novel way of describing the light spectrum to correct for the effects of light propagation in tissue and obtain accurate images of blood oxygenation in tissue.

Improved accuracy

With eMSOT, the scientists were able to visualize the blood oxygenation level of living tissue up to one centimeter below the skin surface. "Theoretically, the imaging depth can be extended to more than that," says Stratis Tzoumas. "There is, however, a limit at about three because at some point, light cannot penetrate the tissue any further." The scientists observed a vastly improved accuracy in eMSOT over previous optical and optoacoustic approaches. Apart from being non-invasive and radiation-free, eMSOT also delivers comparable or higher resolution both spatially and temporally, than other optical imaging methods. "Information about the amount of oxygen in tissue is important when it comes to various fields in research and treatment - for example tumor growth or in measurements of metabolism" says Vasilis Ntziachristos. "It may be that eMSOT becomes the gold standard method, once it is ready for clinical use.”

Further Information

Original Publication:
Tzoumas, S. et al. (2016): Eigenspectra optoacoustic tomography achieves quantitative blood oxygenation imaging deep in tissues, Nature Communications, DOI: 10.1038/ncomms12121

The Helmholtz Zentrum München, the German Research Center for Environmental Health, pursues the goal of developing personalized medical approaches for the prevention and therapy of major common diseases such as diabetes and lung diseases. To achieve this, it investigates the interaction of genetics, environmental factors and lifestyle. The Helmholtz Zentrum München is headquartered in Neuherberg in the north of Munich and has about 2,300 staff members. It is a member of the Helmholtz Association, a community of 18 scientific-technical and medical-biological research centers with a total of about 37,000 staff members. 

The Institute for Biological and Medical Imaging (IBMI) conducts research into in vivo imaging technologies for the biosciences. It develops systems, theories and methods of imaging and image reconstruction as well as animal models to test new technologies at the biological, preclinical and clinical level. The aim is to provide innovative tools for biomedical laboratories, for diagnosis and for the therapeutic monitoring of human diseases. 

Technical University of Munich (TUM) is one of Europe’s leading research universities, with more than 500 professors, around 10,000 academic and non-academic staff, and 39,000 students. Its focus areas are the engineering sciences, natural sciences, life sciences and medicine, reinforced by schools of management and education. TUM acts as an entrepreneurial university that promotes talents and creates value for society. In that it profits from having strong partners in science and industry. It is represented worldwide with a campus in Singapore as well as offices in Beijing, Brussels, Cairo, Mumbai, San Francisco, and São Paulo. Nobel Prize winners and inventors such as Rudolf Diesel, Carl von Linde, and Rudolf Mößbauer have done research at TUM. In 2006 and 2012 it won recognition as a German "Excellence University." In international rankings, TUM regularly places among the best universities in Germany. 

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