Spatial Resolution

The physical size of individual measurement zones, ranging from subcellular (50-100 nm) in MERFISH to multi-cellular (55-100 µm) in Visium arrays. Smaller spots reduce transcriptional signal mixing from multiple cells, enabling finer cell-type discrimination. The capture area directly determines the minimum resolvable tissue feature and the degree of spatial deconvolution required downstream.

The distance between the centers of adjacent measurement spots, which governs sampling density independently of spot size. Technologies with spacing smaller than spot diameter achieve overlapping coverage, while sparse spacing leaves interstitial tissue gaps unmeasured. Dense spacing is critical for resolving fine spatial gradients and cell-cell interaction boundaries.

A fundamental physical constraint defined by the Abbe diffraction limit (~200-250 nm for visible light), which restricts the minimum resolvable distance between two distinct point sources. Super-resolution microscopy techniques like STED and STORM bypass this limit through stimulated emission depletion or stochastic fluorophore switching, achieving resolutions down to 20-30 nm for single-molecule RNA detection.

The ability to distinguish signals along the optical axis perpendicular to the tissue plane, measured in micrometers. High axial resolution enables 3D spatial reconstruction of tissue architecture and prevents out-of-focus signal contamination. Confocal microscopy achieves ~500-800 nm axial resolution via pinhole rejection, while light-sheet microscopy provides ~1-5 µm resolution with reduced phototoxicity for thick specimens.

The proportion of target mRNA molecules actually detected at a given location, which directly impacts effective resolution. Low efficiency (<10%) creates spatial dropout artifacts where true expression is missed, requiring spatial imputation algorithms. High efficiency (>50%) preserves rare transcript detection and enables reliable single-cell spatial analysis without excessive zero-inflation.

The number of distinct RNA species that can be simultaneously resolved at each spatial location, ranging from whole-transcriptome (>20,000 genes) in array-based methods to targeted panels (100-1,000 genes) in imaging-based approaches. Higher multiplexing enables unbiased discovery of spatial patterns, while targeted panels provide superior sensitivity and faster imaging cycles for pre-selected gene sets.

A foundational technique that uses spatially indexed oligonucleotide arrays to capture mRNA from tissue sections. The physical size and spacing of these barcoded spots directly determine the assay's spatial resolution, ranging from 55-micrometer spots in early Visium platforms to sub-micrometer features in newer technologies. Resolution is fundamentally limited by the barcode feature density on the capture surface.

A method for directly reading out the sequence of RNA transcripts within preserved tissue by performing rolling circle amplification and sequencing-by-ligation on-site. ISS achieves subcellular resolution by localizing individual mRNA molecules to their exact coordinates within a cell. This technique bypasses the spot-based limitations of capture arrays, offering the highest possible spatial resolution for transcriptomic mapping.

A statistical measure of the degree to which a variable's values at nearby locations are more similar than expected by random chance. Metrics like Moran's I quantify this dependence. The concept is critical for spatial resolution because it defines the effective resolution limit—the distance at which biological signals become spatially independent. Strong autocorrelation at fine scales demands high-resolution technologies to resolve meaningful patterns.