Cardinal Peak Processing Snr Filtering

Cardinal peak processing with SNR filtering

Summary

Apply Cardinal's peakProcess function to detect and filter MS imaging peaks using signal-to-noise ratio thresholds, generating a cleaned peak list suitable for downstream annotation and spatial analysis. This skill reduces noise-driven false positives in high-dimensional MS imaging datasets.

When to use

When you have loaded an unprocessed Cardinal object from MS imaging data (e.g., from Zenodo or native formats) containing thousands of m/z features across many spectra, and you need to produce a curated peak list with known expected peak count (e.g., 687 cleaned peaks from PIGII_206). Apply this skill after feature summarization and normalization (TIC) but before spatial segmentation or metabolite annotation, especially when SNR varies across the dataset.

When NOT to use

• Input is already a pre-processed or vendor-supplied peak list (e.g., from mzML, mzXML, or NetCDF with pre-picked peaks).
• SNR threshold and tolerance parameters are not known or cannot be validated against expected output; use peakProcess only when ground truth peak counts or method parameters are documented.
• Dataset contains LC-MS data (not imaging); peakProcess is designed for spatially resolved spectra and may not scale efficiently for non-spatial MS.

Inputs

• Cardinal object (unprocessed, loaded from Zenodo or local file)
• m/z feature matrix (10,200 features × 4,959 spectra, or similar dimensions)
• Raw spectra with intensity values

Outputs

• Cleaned peak table (687 peaks, or dataset-specific count)
• Peak m/z values and aggregated intensities
• Cardinal object with peakProcess results slot

How to apply

Load the unprocessed Cardinal object and apply TIC (total ion count) normalization to account for injection/ionization variability. Call Cardinal's peakProcess() function with SNR threshold=3, sampleSize=0.1 (use 10% of spectra for peak detection), and tolerance=0.5 m/z (merge peaks within ±0.5 m/z to account for mass calibration drift). The SNR=3 threshold filters peaks where signal intensity is at least 3× the local noise estimate. Set sampleSize conservatively to reduce computational burden while maintaining peak detection sensitivity. The function returns a cleaned peak table with m/z values and intensities. Validate that the output peak count matches the expected value (e.g., 687 peaks) and inspect the m/z distribution (e.g., 150–1000 m/z range) to confirm biological plausibility.

Related tools

• Cardinal (Provides peakProcess() function for SNR-based peak detection and filtering in MS imaging data) — https://github.com/Vitek-Lab/Cardinal3-vignettes
• SpaMTP (Integrates Cardinal peak output with pathway and annotation workflows; inherits Cardinal functionality) — https://github.com/GenomicsMachineLearning/SpaMTP
• Seurat (Receives Cardinal objects converted via CardinalToSeurat() for downstream spatial analysis and visualization)
• R (Runtime environment for Cardinal and SpaMTP libraries)

Examples

library(Cardinal); pig206 <- readRDS(url('https://zenodo.org/records/...')); pig206_norm <- normalize(pig206, method='tic'); peaks_cleaned <- peakProcess(pig206_norm, SNR=3, sampleSize=0.1, tolerance=0.5)

Evaluation signals

• Output peak count matches expected value (e.g., 687 cleaned peaks for PIGII_206); if output differs significantly, re-check SNR threshold, sampleSize, or tolerance parameters.
• Cleaned peak m/z values fall within the documented range (e.g., 150–1000 m/z for pig fetus dataset); peaks outside this range suggest parameter drift or data quality issues.
• Peak intensity distribution is non-zero and continuous across spatial locations; isolated or singleton peaks may indicate overfitting with too-low SNR threshold.
• Subsequent spatial segmentation (e.g., SSC with k=8 clusters) produces interpretable spatial domains; poor clustering suggests peaks are still noisy or non-informative.
• Annotated metabolites using RefineLipids() or AnnotateSM() show expected biological relevance (e.g., lipids, amino acids, adenosine triphosphate); spurious annotations suggest peaks are artifactual.

Limitations

• SNR threshold (=3) is data- and instrument-dependent; no universal cutoff is valid across all MS imaging platforms. The threshold may require optimization if expected peak count is not achieved.
• Tolerance (0.5 m/z) assumes m/z calibration error of ±0.5; datasets with higher mass calibration drift may merge distinct peaks or produce fragmentation artifacts.
• sampleSize=0.1 is a heuristic that trades speed for sensitivity; very large or sparse datasets may require adjustment to ensure peaks are detected in minority of spectra.
• peakProcess assumes spectra are pre-normalized (TIC); unnormalized spectra may produce biased SNR estimates and inconsistent peak detection across spatial regions.
• The article indicates 'Pseudo MS/MS-Based Refinement' and 'Refinement with Paired Targeted Metabolic Data' sections are marked 'To come!', suggesting validation workflows are incomplete and peak results may require manual curation.

Evidence

• [other] Apply TIC normalization followed by peakProcess with SNR threshold=3, sampleSize=0.1, tolerance=0.5 mz to generate 687 cleaned peaks.: "Apply TIC normalization followed by peakProcess with SNR threshold=3, sampleSize=0.1, tolerance=0.5 mz to generate 687 cleaned peaks."
• [other] Load the unprocessed pig206 Cardinal object from Zenodo (10,200 m/z features, 4,959 spectra, m/z range 150–1000).: "Load the unprocessed pig206 Cardinal object from Zenodo (10,200 m/z features, 4,959 spectra, m/z range 150–1000)."
• [other] Summarize features using mean aggregation with Cardinal's summarizeFeatures().: "Summarize features using mean aggregation with Cardinal's summarizeFeatures()."
• [readme] SpaMTP inherits functionalities from two well established R packages (Cardinal and Seurat) to present a user-friendly platform for integrative spatial-omics analysis.: "SpaMTP inherits functionalities from two well established R packages (Cardinal and Seurat) to present a user-friendly platform for integrative spatial-omics analysis."