A Physically-Realistic Simulator and Cosine-Based Decoder for MERFISH Spatial Transcriptomics

Imaging-based spatial transcriptomics technologies have opened new avenues for studying cellular organization and gene expression within intact tissues. However, the accuracy of downstream analyses depends critically on the decoding step that reconstructs barcodes from fluorescence patterns and maps them to gene identities. Despite a growing number of decoding methods, systematic benchmarking has been limited. Here, we introduce Serval, a modular framework for developing and benchmarking decoding methods across diverse spatial transcriptomics platforms. Serval separates key decoding stages into independently configurable modules, enabling flexible integration of alternative algorithms. Using this framework, we develop the Cosine decoder, a novel method that improves transcript recovery by optimizing cosine similarity to known barcodes. We evaluate Cosine and baseline methods on synthetic and real MERFISH datasets, showing that Cosine achieves higher transcript recovery and superior correlation with expression references compared to existing methods. Furthermore, we demonstrate that the Serval framework generalizes beyond MERFISH. By extending to the DART-FISH platform, we show that Cosine improves transcript recovery, clustering stability and supports more direct annotation of complex biological structures such as the human primary motor cortex. These results establish that modular decoding frameworks facilitate robust, platformagnostic benchmarking, ultimately supporting more accurate spatial transcriptomics analysis across diverse biological samples.