Bio Flow Cytometry Bead Normalization

Version Compatibility

Reference examples tested with: flowCore 2.14+, ggplot2 3.5+

Before using code patterns, verify installed versions match. If versions differ:

• R: packageVersion('<pkg>') then ?function_name to verify parameters

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Bead Normalization

"Normalize my CyTOF data using beads" → Correct instrument signal drift over acquisition time using EQ calibration bead intensities for consistent measurements across runs.

• R: CATALYST::normCytof() for EQ bead normalization

CyTOF EQ Bead Normalization

Goal: Identify EQ normalization bead events in CyTOF data for signal calibration.

Approach: Score events by mean scaled intensity in known bead channels (Ce140, Eu151, Eu153, Ho165, Lu175) and threshold at the 99th percentile.

library(CATALYST)
library(flowCore)

# CyTOF data typically includes EQ normalization beads
# Fluidigm provides normalizer software, but can also do in R

# Load FCS with beads
ff <- read.FCS('cytof_with_beads.fcs')

# EQ beads contain known amounts of: Ce140, Eu151, Eu153, Ho165, Lu175
bead_channels <- c('Ce140Di', 'Eu151Di', 'Eu153Di', 'Ho165Di', 'Lu175Di')

# Identify bead events (high signal in bead channels)
bead_data <- exprs(ff)[, bead_channels]
bead_scores <- rowMeans(scale(bead_data))

# Beads typically have very high intensity
bead_threshold <- quantile(bead_scores, 0.99)
is_bead <- bead_scores > bead_threshold

cat('Identified', sum(is_bead), 'bead events (', round(mean(is_bead) * 100, 2), '%)\n')

Calculate Normalization Factors

Goal: Compute per-channel normalization factors by comparing sample bead intensities to a reference.

Approach: Calculate median bead intensity per channel, then divide reference values by sample values to obtain correction factors.

# For each acquisition, calculate median bead intensity
# Compare to reference to get normalization factor

calculate_norm_factors <- function(ff, bead_channels, bead_idx) {
    bead_intensities <- exprs(ff)[bead_idx, bead_channels]

    # Median intensity per channel
    medians <- apply(bead_intensities, 2, median)

    return(medians)
}

# Reference values (from first file or known standards)
reference_beads <- c(Ce140 = 500, Eu151 = 600, Eu153 = 550, Ho165 = 450, Lu175 = 400)

# Calculate factors
sample_beads <- calculate_norm_factors(ff, bead_channels, is_bead)
norm_factors <- reference_beads / sample_beads

cat('Normalization factors:\n')
print(round(norm_factors, 3))

Apply Normalization

Goal: Correct marker intensities using bead-derived normalization factors and remove bead events.

Approach: Multiply marker channels by the geometric mean of bead factors, then filter out bead events from the flowFrame.

# Apply normalization to all marker channels (not scatter)
marker_channels <- setdiff(colnames(ff), c('Time', 'Event_length', bead_channels))

normalize_cytof <- function(ff, norm_factors, channels) {
    # Get expression matrix
    expr <- exprs(ff)

    # Apply geometric mean of bead factors to all channels
    global_factor <- exp(mean(log(norm_factors)))

    # Or apply per-channel if you have channel-specific factors
    expr[, channels] <- expr[, channels] * global_factor

    exprs(ff) <- expr
    return(ff)
}

ff_normalized <- normalize_cytof(ff, norm_factors, marker_channels)

# Remove bead events
ff_clean <- ff_normalized[!is_bead, ]
cat('Final cell count:', nrow(ff_clean), '\n')

Time-Based Drift Correction

Goal: Remove signal drift that accumulates during long CyTOF acquisitions.

Approach: Bin bead events by acquisition time, fit LOESS to per-bin median intensities, and scale all events to a reference level.

# Correct for signal drift over acquisition time

correct_drift <- function(ff, time_channel = 'Time') {
    expr <- exprs(ff)
    time <- expr[, time_channel]

    # Bin by time
    n_bins <- 20
    time_bins <- cut(time, breaks = n_bins, labels = FALSE)

    # For each marker, fit LOESS to bead signal over time
    corrected <- expr

    marker_cols <- setdiff(colnames(expr), c(time_channel, 'Event_length'))

    for (marker in marker_cols) {
        bin_medians <- tapply(expr[is_bead, marker], time_bins[is_bead], median)

        if (length(unique(time_bins[is_bead])) > 3) {
            # Fit smooth curve to drift
            drift_data <- data.frame(
                time = as.numeric(names(bin_medians)),
                intensity = as.numeric(bin_medians)
            )

            loess_fit <- loess(intensity ~ time, data = drift_data, span = 0.5)

            # Predict correction factor for all events
            correction <- predict(loess_fit, newdata = data.frame(time = time_bins))
            reference <- median(drift_data$intensity)

            corrected[, marker] <- expr[, marker] * (reference / correction)
        }
    }

    exprs(ff) <- corrected
    return(ff)
}

ff_drift_corrected <- correct_drift(ff)

Batch Normalization with CytoNorm

Goal: Harmonize marker distributions across batches using shared reference samples.

Approach: Train spline-based CytoNorm models on reference samples run in all batches, then apply the learned transformations to normalize new samples.

# CytoNorm for cross-batch normalization using reference samples

library(CytoNorm)

# Requires: training samples run on all batches (e.g., same PBMC reference)
# Creates spline-based transformation

# Prepare training data
train_files <- list.files('batch1_reference/', pattern = '\\.fcs$', full.names = TRUE)
train_data <- lapply(train_files, read.FCS)

# Define model
model <- CytoNorm.train(
    files = train_files,
    labels = rep('Reference', length(train_files)),
    channels = marker_channels,
    transformList = NULL,  # If already transformed
    nQ = 100,  # Number of quantiles
    seed = 42
)

# Apply to new batch
test_files <- list.files('batch2/', pattern = '\\.fcs$', full.names = TRUE)
normalized_files <- CytoNorm.normalize(
    model = model,
    files = test_files,
    labels = rep('Test', length(test_files)),
    outputDir = 'batch2_normalized/'
)

Quantile Normalization

Goal: Align marker distributions across samples by mapping to a common reference distribution.

Approach: Rank-order values per channel per sample and replace with interpolated reference quantiles computed from all samples.

# Simple quantile normalization across samples

quantile_normalize <- function(fs, channels) {
    # Extract expression matrices
    expr_list <- lapply(fs, function(ff) exprs(ff)[, channels])

    # Get reference distribution (mean of all samples)
    all_values <- do.call(rbind, expr_list)
    reference_quantiles <- apply(all_values, 2, function(x) sort(x))
    reference <- colMeans(reference_quantiles)

    # Normalize each sample
    normalized_fs <- fs
    for (i in 1:length(fs)) {
        expr <- exprs(fs[[i]])
        for (ch in channels) {
            ranks <- rank(expr[, ch], ties.method = 'average')
            normalized_values <- approx(1:length(reference), sort(reference),
                                        xout = ranks)$y
            expr[, ch] <- normalized_values
        }
        exprs(normalized_fs[[i]]) <- expr
    }

    return(normalized_fs)
}

CATALYST-Based Normalization

Goal: Normalize CyTOF data using CATALYST's built-in bead handling and time-drift correction.

Approach: Use prepData with by_time=TRUE to automatically correct time-dependent drift during SCE construction.

library(CATALYST)

# CATALYST provides bead-based normalization for CyTOF

# Load data with prepData (handles bead removal)
sce <- prepData(fs, panel, md,
                transform = TRUE,
                cofactor = 5,
                by_time = TRUE)  # Correct time-dependent drift

# Or manual bead gating in CATALYST
# sce <- prepData(fs, panel, md, FACS = FALSE)
# sce <- filterSCE(sce, !sce$is_bead)

Visualization

Goal: Visualize bead signal drift and assess normalization effects.

Approach: Plot bead channel intensity over acquisition time with LOESS trend, and compare marker distributions before and after normalization.

library(ggplot2)

# Plot bead signal over time
bead_plot_data <- data.frame(
    Time = exprs(ff)[is_bead, 'Time'],
    Ce140 = exprs(ff)[is_bead, 'Ce140Di'],
    Eu151 = exprs(ff)[is_bead, 'Eu151Di']
)

ggplot(bead_plot_data, aes(x = Time, y = Ce140)) +
    geom_point(alpha = 0.1, size = 0.5) +
    geom_smooth(method = 'loess', color = 'red') +
    theme_bw() +
    labs(title = 'Bead Signal Over Time (Ce140)', x = 'Time', y = 'Intensity')
ggsave('bead_drift.png', width = 10, height = 4)

# Before/after normalization
compare_df <- data.frame(
    Value = c(exprs(ff)[, 'CD45'], exprs(ff_normalized)[, 'CD45']),
    Status = rep(c('Before', 'After'), each = nrow(ff))
)

ggplot(compare_df, aes(x = Value, fill = Status)) +
    geom_histogram(bins = 100, alpha = 0.5, position = 'identity') +
    theme_bw() +
    labs(title = 'Normalization Effect on CD45')

Export Normalized Data

Goal: Save normalized and bead-free data for downstream analysis.

Approach: Write the cleaned flowFrame to a new FCS file using write.FCS.

# Save normalized FCS files
write.FCS(ff_clean, 'normalized_sample.fcs')

# For CATALYST object
# saveRDS(sce, 'normalized_sce.rds')

Related Skills

Workflow order: cytometry-qc → doublet-detection → bead-normalization → clustering

• cytometry-qc - Run first: identify drift and quality issues
• doublet-detection - Run before: remove doublets prior to normalization
• compensation-transformation - Initial data preprocessing
• clustering-phenotyping - Analysis after normalization
• differential-analysis - Batch-aware statistical testing