Imagine trying to understand a city by analyzing its power grid. You could look at the entire network at once or zoom in to see how individual homes and businesses are connected. Mass spectrometry imaging (MSI) works similarly, but instead of power lines, it maps the distribution of molecules within a biological sample at an extraordinarily fine spatial resolution.

At its core, mass spectrometry (MS) identifies and quantifies molecules based on their mass-to-charge ratio. MSI takes this a step further by adding spatial context, allowing us to create detailed maps of where specific molecules are located within a tissue, such as a section of the brain.

The process starts with preparing a thin tissue section, which is then placed in the mass spectrometer. A laser or ion beam bombards the tissue, ejecting molecules from the surface. These molecules are ionized and sent through a mass analyzer, where they are separated based on their mass-to-charge ratio. The result is a spectrum that reveals which molecules are present and, crucially, their precise location within the tissue.