For mere light signals to become images that can provide information about diseases, mathematics is needed. "If you send lasers into a tissue, as with optical coherence tomography or photoacoustic tomography, you receive different signals. But in many cases, these alone do not allow you to draw any conclusions about the condition of the tissue", explains mathematician Otmar Scherzer from the Johann Radon Institute for Computational and Applied Mathematics of the Austrian Academy of Sciences (OeAW).
For several years, Scherzer and his colleagues have been working to combine imaging techniques such as photoacoustic tomography (PAT) and optical coherence tomography (OCT) into a new analysis tool for medicine. While the latter is commonly used in ophthalmology to diagnose diseases such as glaucoma or diabetic retinopathy, PAT allows a three-dimensional representation of blood vessels and other tissue components.
Combining to see more
"These two methods complement each other very well, as both work with a short and intense laser pulse", explains Scherzer. OCT shows how light is scattered and reflected in the tissue, while PAT measures how much of the light beam is absorbed in the tissue. "To stay with the example of blood vessels, the blood absorbs the laser light. If there is a lot of blood, the PAT image becomes very dark", explains the mathematician, who is responsible for combining the different light signals from the two procedures into one image. This information could ultimately be used for cancer diagnoses. For example, skin cancer can be recognized by the fact that more blood vessels are formed in the skin. "All tumors need an increased blood supply, and this could be seen using the new tool", Scherzer says.
Better clues for tumor detection
For the combined technique, Scherzer and his colleagues already have a specific application in mind: elastography. A plate pressed on the skin sends a short laser pulse into the uppermost tissue layers. The second time, the laser is delivered under the skin without pressure. An elasticity equation can calculate how elastic and stiff the material in the skin is and convert this information into a picture. "The advantage is that you can see how stiff the tissue is, rather than simply whether or not it is stiff." While this does not say anything about whether the tissue is malignant or benign, it would provide a clue as to whether or not you need to examine further, Scherzer continues.
In addition, the combined tomography could help doctors work more accurately in tumor surgery, explains Scherzer. "You could, for example, analyze tissue samples in real-time during surgery to know if you are still in the altered material that needs to be removed, or if you have already reached healthy tissue."
How algorithms and medicine come together
There is still a long way to go before getting to that point. It is true that the computer models and the measuring setups are becoming increasingly precise and better coordinated with each other, but there are still some hurdles. "You can compare it to an old TV, where the colors sometimes shifted slightly and did not lie exactly on top of each other. We have a similar problem when it comes to superimposing the two images precisely enough to get a clear picture." To overcome this problem, intensive collaboration is required: while the mathematicians ensure that the reconstruction algorithms are optimally adjusted, the medical physicists work to arrange the measuring setups more and more accurately.
Another challenge is the samples used to test the procedure. "We are still in the development phase. In other words, we need material for which we know the individual components exactly and which resembles human tissue." So far, the mathematicians have been cooking the special material themselves. It is comparable to aspic, Scherzer says. However, they only have a short time to carry out their measurements, as the material ages within a day and becomes unusable.
"To be honest, I underestimated at the beginning how difficult it would be. However, I am confident that one day we will bring the technology to the point where it can be used in skin and tissue studies in the way that we envisage."