The accumulation of potassium in tumors has recently been shown to affect the efficacy of immune cells. Here, the authors demonstrate that photoacoustic imaging can provide measurements of the potassium concentration in vivo inside the tumor microenvironment. Photoacoustic imaging has undergone a tremendous development over the past two decades, owing to its unique ability to provide images of optical absorption at depth in tissue with the spatial resolution of ultrasound: via the photoacoustic effect, the absorption of nanosecond pulsed light generates ultrasound which propagates with very little scattering through soft tissue. The distribution of absorbed light in tissue may be imaged up to centimeter depths with a spatial resolution approximately a hundred micrometers. In order to detect potassium, the authors used a nanosensor whose optical absorption properties depend specifically on potassium concentration. An image of the potassium concentration may then be derived from a set of photoacoustic images obtained at several wavelengths, based on the knowledge of the spectral response of both the nanosensor and blood (a major endogenous source of photoacoustic signal). In mice experiments in vivo, the concentration of potassium in a subcutaneous tumor derived from photoacoustic imaging was in agreement with that measured consecutively by mass spectrometry on the harvested tumor. This study further supports the advent of photoacoustic imaging as a powerful quantitative imaging technique relevant to measure not only endogenous absorption but also any analyte that can affect the optical absorption of dedicated nanosensors.
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