Mass spectrometry imaging

Mass spectrometry imaging (MSI) is a spatially resolved analytical technique that maps the distribution of molecules across surfaces — tissue sections, material interfaces, or geological samples — by acquiring mass spectrometry data at discrete spatial coordinates and reconstructing the results as images. Each pixel in an MSI dataset is a complete mass spectrum, making the technique a hybrid of microscopy and molecular analysis.

The dominant methods differ in how they generate ions from a surface. Matrix-assisted laser desorption ionization (MALDI) uses ultraviolet lasers to desorb and ionize molecules from a matrix-coated tissue section, achieving spatial resolutions of 10-50 micrometers. Desorption electrospray ionization (DESI) sprays charged solvent droplets onto the surface, enabling ambient imaging without sample preparation. Secondary ion mass spectrometry (SIMS) uses primary ion beams to ablate surface layers with nanometer-scale resolution, though at the cost of extensive fragmentation.

In cancer pathology, MSI reveals tumor heterogeneity that histology cannot see — metabolic gradients, lipid signatures of necrosis, and drug penetration profiles that predict therapeutic response. The technique treats tissue not as a uniform mass but as a chemical landscape with spatial structure, bridging anatomy and molecular biology in a single dataset.

The epistemic shift is subtle but decisive. Traditional mass spectrometry asks what molecules are present? Mass spectrometry imaging asks where are they, and what spatial relationships do they form? The question changes from composition to organization — from chemistry to spatial systems science.