Optoacoustic imaging: Revealing the details of tumour patterns A technique that can image the entire tumour volume with high resolution may help oncologists optimize specific treatments for breast cancer. Jiao Li (Tianjin University, China), Vasilis Ntziachristos (Technical University of Munich, Germany), and colleagues have designed a multispectral optoacoustic mesoscope (MSOM) that illuminates millimetre-sized tumours with laser light of various wavelengths, and detects the ultrasound waves generated by internal absorbers, such as haemoglobin, or external nanoparticle probes. By reconstructing the ultrasound signals over multiple frequencies, the team produced 3D images of features that included the vascular network of a tumour with micrometre-scale detail. Experiments with live mice demonstrated that specific tumours could be identified through differences in spatial patterns, such as altered oxygen levels between tumour cores and peripheries. The study highlights the power of MSOM as a tool for preclinical cancer studies.The characteristics of tumour development and metastasis relate not only to genomic heterogeneity but also to spatial heterogeneity, associated with variations in the intratumoural arrangement of cell populations, vascular morphology and oxygen and nutrient supply. While optical (photonic) microscopy is commonly employed to visualize the tumour microenvironment, it assesses only a few hundred cubic microns of tissue. Therefore, it is not suitable for investigating biological processes at the level of the entire tumour, which can be at least four orders of magnitude larger. In this study, we aimed to extend optical visualization and resolve spatial heterogeneity throughout the entire tumour volume. We developed an optoacoustic (photoacoustic) mesoscope adapted to solid tumour imaging and, in a pilot study, offer the first insights into cancer optical contrast heterogeneity in vivo at an unprecedented resolution of <50 mu m throughout the entire tumour mass. Using spectral methods, we resolve unknown patterns of oxygenation, vasculature and perfusion in three types of breast cancer and showcase different levels of structural and functional organization. To our knowledge, these results are the most detailed insights of optical signatures reported throughout entire tumours in vivo, and they position optoacoustic mesoscopy as a unique investigational tool linking microscopic and macroscopic observations.