Noninvasive Imaging Method Can Penetrate Deeper Into Living Tissue

Massachusetts Institute of Technology, Cambridge, MA

The new technique enables laser light to penetrate deeper into living tissue, which captures sharper images of cells at different layers of a living system. On the left is the initial image, and on the right is the optimized image using the new technique. (Image: Courtesy of the researchers)

Metabolic imaging is a noninvasive method that enables clinicians and scientists to study living cells using laser light, which can help them assess disease progression and treatment responses. But light scatters when it shines into biological tissue, limiting how deeply it can penetrate and hampering the resolution of captured images.

Now, MIT researchers have developed a new technique that more than doubles the usual depth limit of metabolic imaging. Their method also boosts imaging speeds, yielding richer and more detailed images.

This new technique does not require tissue to be preprocessed, such as by cutting it or staining it with dyes. Instead, a specialized laser illuminates deep into the tissue, causing certain intrinsic molecules within the cells and tissues to emit light. This eliminates the need to alter the tissue, providing a more natural and accurate representation of its structure and function.

The researchers achieved this by adaptively customizing the laser light for deep tissues. Using a recently developed fiber shaper — a device they control by bending it — they can tune the color and pulses of light to minimize scattering and maximize the signal as the light travels deeper into the tissue. This allows them to see much further into living tissue and capture clearer images.

Greater penetration depth, faster speeds, and higher resolution make this method particularly well-suited for demanding imaging applications like cancer research, tissue engineering, drug discovery, and the study of immune responses.

“This work shows a significant improvement in terms of depth penetration for label-free metabolic imaging. It opens new avenues for studying and exploring metabolic dynamics deep in living biosystems,” said Sixian You, assistant professor in the Department of Electrical Engineering and Computer Science (EECS), a member of the Research Laboratory for Electronics, and senior author of a paper on this imaging technique.

This new method falls in the category of label-free imaging, which means tissue is not stained beforehand. Staining creates contrast that helps a clinical biologist see cell nuclei and proteins better. But staining typically requires the biologist to section and slice the sample, a process that often kills the tissue and makes it impossible to study dynamic processes in living cells.

In label-free imaging techniques, researchers use lasers to illuminate specific molecules within cells, causing them to emit light of different colors that reveal various molecular contents and cellular structures. However, generating the ideal laser light with certain wavelengths and high-quality pulses for deep-tissue imaging has been challenging.

The researchers developed a new approach to overcome this limitation. They use a multimode fiber, a type of optical fiber that can carry a significant amount of power, and couple it with a compact device called a “fiber shaper.” This shaper allows them to precisely modulate the light propagation by adaptively changing the shape of the fiber. Bending the fiber changes the color and intensity of the laser.

Building on prior work, the researchers adapted the first version of the fiber shaper for deeper multimodal metabolic imaging. “We want to channel all this energy into the colors we need, with the pulse properties we require. This gives us higher generation efficiency and a clearer image, even deep within tissues,” said Honghao Cao, an EECS graduate student.

Once they had built the controllable mechanism, they developed an imaging platform to leverage the powerful laser source to generate longer wavelengths of light, which are crucial for deeper penetration into biological tissues.

When the researchers tested their imaging device, the light was able to penetrate more than 700 micrometers into a biological sample, whereas the best prior techniques could only reach about 200 micrometers.

The deep imaging technique enabled them to see cells at multiple levels within a living system, which could help researchers study metabolic changes that happen at different depths. In addition, the faster imaging speed allows them to gather more detailed information on how a cell’s metabolism affects the speed and direction of its movements.

This new imaging method could offer a boost to the study of organoids, which are engineered cells that can grow to mimic the structure and function of organs.

With these and other biomedical applications in mind, the researchers plan to aim for even higher-resolution images. At the same time, they are working to create low-noise laser sources, which could enable deeper imaging with less light dosage. They are also developing algorithms that react to the images to reconstruct the full 3D structures of biological samples in high resolution.

“Being able to acquire high resolution multi-photon images relying on NAD(P)H autofluorescence contrast faster and deeper into tissues opens the door to the study of a wide range of important problems,” said Irene Georgakoudi, a professor of biomedical engineering at Tufts University who was not involved with this work. “Imaging living tissues as fast as possible whenever you assess metabolic function is a huge advantage in terms of ensuring the physiological relevance of the data, sampling a meaningful tissue volume, or monitoring fast changes. For applications in cancer diagnosis or in neuroscience, imaging deeper and faster enables us to consider a richer set of problems and interactions that haven’t been studied in living tissues before.”

For more information, contact Melanie Grados at This email address is being protected from spambots. You need JavaScript enabled to view it..