Preprocessing Pipelines & Transforms

Preprocessing Pipelines & Transforms

Overview

PathML provides a modular preprocessing architecture based on composable transforms organized into pipelines. Transforms are individual operations that modify images, create masks, or extract features. Pipelines chain transforms together to create reproducible, scalable preprocessing workflows for computational pathology.

Pipeline Architecture

Pipeline Class

The Pipeline class composes a sequence of transforms applied consecutively:

from pathml.preprocessing import Pipeline, Transform1, Transform2

# Create pipeline
pipeline = Pipeline([
    Transform1(param1=value1),
    Transform2(param2=value2),
    # ... more transforms
])

# Run on a single slide
pipeline.run(slide_data)

# Run on a dataset
pipeline.run(dataset, distributed=True, n_workers=8)

Key features:

• Sequential execution of transforms
• Automatic handling of tiles and masks
• Distributed processing support with Dask
• Reproducible workflows with serializable configuration

Transform Base Class

All transforms inherit from the Transform base class and implement:

• apply() - Core transformation logic
• input_type - Expected input (tile, mask, etc.)
• output_type - Produced output

Transform Categories

PathML provides transforms in six major categories:

1. Image Modification - Blur, rescale, histogram equalization
2. Mask Creation - Tissue detection, nucleus detection, thresholding
3. Mask Modification - Morphological operations on masks
4. Stain Processing - H&E stain normalization and separation
5. Quality Control - Artifact detection, white space labeling
6. Specialized - Multiparametric imaging, cell segmentation

Image Modification Transforms

Blur Operations

Apply various blurring kernels for noise reduction:

MedianBlur:

from pathml.preprocessing import MedianBlur

# Apply median filter
transform = MedianBlur(kernel_size=5)

• Effective for salt-and-pepper noise
• Preserves edges better than Gaussian blur

GaussianBlur:

from pathml.preprocessing import GaussianBlur

# Apply Gaussian blur
transform = GaussianBlur(kernel_size=5, sigma=1.0)

• Smooth noise reduction
• Adjustable sigma controls blur strength

BoxBlur:

from pathml.preprocessing import BoxBlur

# Apply box filter
transform = BoxBlur(kernel_size=5)

• Fastest blur operation
• Uniform averaging within kernel

Intensity Adjustments

RescaleIntensity:

from pathml.preprocessing import RescaleIntensity

# Rescale intensity to [0, 255]
transform = RescaleIntensity(
    in_range=(0, 1.0),
    out_range=(0, 255)
)

HistogramEqualization:

from pathml.preprocessing import HistogramEqualization

# Global histogram equalization
transform = HistogramEqualization()

• Enhances global contrast
• Spreads out intensity distribution

AdaptiveHistogramEqualization (CLAHE):

from pathml.preprocessing import AdaptiveHistogramEqualization

# Contrast Limited Adaptive Histogram Equalization
transform = AdaptiveHistogramEqualization(
    clip_limit=0.03,
    tile_grid_size=(8, 8)
)

• Enhances local contrast
• Prevents over-amplification with clip_limit
• Better for images with varying local contrast

Superpixel Processing

SuperpixelInterpolation:

from pathml.preprocessing import SuperpixelInterpolation

# Divide into superpixels using SLIC
transform = SuperpixelInterpolation(
    n_segments=100,
    compactness=10.0
)

• Segments image into perceptually meaningful regions
• Useful for feature extraction and segmentation

Mask Creation Transforms

H&E Tissue and Nucleus Detection

TissueDetectionHE:

from pathml.preprocessing import TissueDetectionHE

# Detect tissue regions in H&E slides
transform = TissueDetectionHE(
    use_saturation=True,  # Use HSV saturation channel
    threshold=10,  # Intensity threshold
    min_region_size=500  # Minimum tissue region size in pixels
)

• Creates binary tissue mask
• Filters small regions and artifacts
• Stores mask in tile.masks['tissue']

NucleusDetectionHE:

from pathml.preprocessing import NucleusDetectionHE

# Detect nuclei in H&E images
transform = NucleusDetectionHE(
    stain='hematoxylin',  # Use hematoxylin channel
    threshold=0.3,
    min_nucleus_size=10
)

• Separates hematoxylin stain
• Thresholds to create nucleus mask
• Stores mask in tile.masks['nucleus']

Binary Thresholding

BinaryThreshold:

from pathml.preprocessing import BinaryThreshold

# Threshold using Otsu's method
transform = BinaryThreshold(
    method='otsu',  # 'otsu' or manual threshold value
    invert=False
)

# Or specify manual threshold
transform = BinaryThreshold(threshold=128)

Foreground Detection

ForegroundDetection:

from pathml.preprocessing import ForegroundDetection

# Detect foreground regions
transform = ForegroundDetection(
    threshold=0.5,
    min_region_size=1000,  # Minimum size in pixels
    use_saturation=True
)

Mask Modification Transforms

Apply morphological operations to clean up masks:

MorphOpen:

from pathml.preprocessing import MorphOpen

# Remove small objects and noise
transform = MorphOpen(
    kernel_size=5,
    mask_name='tissue'  # Which mask to modify
)

• Erosion followed by dilation
• Removes small objects and noise

MorphClose:

from pathml.preprocessing import MorphClose

# Fill small holes
transform = MorphClose(
    kernel_size=5,
    mask_name='tissue'
)

• Dilation followed by erosion
• Fills small holes in mask

Stain Normalization

StainNormalizationHE

Normalize H&E staining across slides to account for variations in staining procedure and scanners:

from pathml.preprocessing import StainNormalizationHE

# Normalize to reference slide
transform = StainNormalizationHE(
    target='normalize',  # 'normalize', 'hematoxylin', or 'eosin'
    stain_estimation_method='macenko',  # 'macenko' or 'vahadane'
    tissue_mask_name=None  # Optional tissue mask for better estimation
)

Target modes:

• 'normalize' - Normalize both stains to reference
• 'hematoxylin' - Extract hematoxylin channel only
• 'eosin' - Extract eosin channel only

Stain estimation methods:

• 'macenko' - Macenko et al. 2009 method (faster, more stable)
• 'vahadane' - Vahadane et al. 2016 method (more accurate, slower)

Advanced parameters:

transform = StainNormalizationHE(
    target='normalize',
    stain_estimation_method='macenko',
    target_od=None,  # Optical density matrix for reference (optional)
    target_concentrations=None,  # Target stain concentrations (optional)
    regularizer=0.1,  # Regularization for vahadane method
    background_intensity=240  # Background intensity level
)

Workflow:

1. Convert RGB to optical density (OD)
2. Estimate stain matrix (H&E vectors)
3. Decompose into stain concentrations
4. Normalize to reference stain distribution
5. Reconstruct normalized RGB image

Example with tissue mask:

from pathml.preprocessing import Pipeline, TissueDetectionHE, StainNormalizationHE

pipeline = Pipeline([
    TissueDetectionHE(),  # Create tissue mask first
    StainNormalizationHE(
        target='normalize',
        stain_estimation_method='macenko',
        tissue_mask_name='tissue'  # Use tissue mask for better estimation
    )
])

Quality Control Transforms

Artifact Detection

LabelArtifactTileHE:

from pathml.preprocessing import LabelArtifactTileHE

# Label tiles containing artifacts
transform = LabelArtifactTileHE(
    pen_threshold=0.5,  # Threshold for pen marking detection
    bubble_threshold=0.5  # Threshold for bubble detection
)

• Detects pen markings, bubbles, and other artifacts
• Labels affected tiles for filtering

LabelWhiteSpaceHE:

from pathml.preprocessing import LabelWhiteSpaceHE

# Label tiles with excessive white space
transform = LabelWhiteSpaceHE(
    threshold=0.9,  # Fraction of white pixels
    mask_name='white_space'
)

• Identifies tiles with mostly background
• Useful for filtering uninformative tiles

Multiparametric Imaging Transforms

Cell Segmentation

SegmentMIF:

from pathml.preprocessing import SegmentMIF

# Segment cells using Mesmer deep learning model
transform = SegmentMIF(
    nuclear_channel='DAPI',  # Nuclear marker channel name
    cytoplasm_channel='CD45',  # Cytoplasm marker channel name
    model='mesmer',  # Deep learning segmentation model
    image_resolution=0.5,  # Microns per pixel
    compartment='whole-cell'  # 'nuclear', 'cytoplasm', or 'whole-cell'
)

• Uses DeepCell Mesmer model for cell segmentation
• Requires nuclear and cytoplasm channel specification
• Produces instance segmentation masks

SegmentMIFRemote:

from pathml.preprocessing import SegmentMIFRemote

# Remote inference using DeepCell API
transform = SegmentMIFRemote(
    nuclear_channel='DAPI',
    cytoplasm_channel='CD45',
    model='mesmer',
    api_url='https://deepcell.org/api'
)

• Same functionality as SegmentMIF but uses remote API
• No local GPU required
• Suitable for batch processing

Marker Quantification

QuantifyMIF:

from pathml.preprocessing import QuantifyMIF

# Quantify marker expression per cell
transform = QuantifyMIF(
    segmentation_mask_name='cell_segmentation',
    markers=['CD3', 'CD4', 'CD8', 'CD20', 'CD45'],
    output_format='anndata'  # or 'dataframe'
)

• Extracts mean marker intensity per segmented cell
• Computes morphological features (area, perimeter, etc.)
• Outputs AnnData object for downstream single-cell analysis

CODEX/Vectra Specific

CollapseRunsCODEX:

from pathml.preprocessing import CollapseRunsCODEX

# Consolidate multi-run CODEX data
transform = CollapseRunsCODEX(
    z_slice=2,  # Select specific z-slice
    run_order=[0, 1, 2]  # Order of acquisition runs
)

• Merges channels from multiple CODEX acquisition runs
• Selects focal plane from z-stacks

CollapseRunsVectra:

from pathml.preprocessing import CollapseRunsVectra

# Process Vectra multiplex IF data
transform = CollapseRunsVectra(
    wavelengths=[520, 570, 620, 670, 780]  # Emission wavelengths
)

Building Comprehensive Pipelines

Basic H&E Preprocessing Pipeline

from pathml.preprocessing import (
    Pipeline,
    TissueDetectionHE,
    StainNormalizationHE,
    NucleusDetectionHE,
    MedianBlur,
    LabelWhiteSpaceHE
)

pipeline = Pipeline([
    # 1. Quality control
    LabelWhiteSpaceHE(threshold=0.9),

    # 2. Noise reduction
    MedianBlur(kernel_size=3),

    # 3. Tissue detection
    TissueDetectionHE(min_region_size=500),

    # 4. Stain normalization
    StainNormalizationHE(
        target='normalize',
        stain_estimation_method='macenko',
        tissue_mask_name='tissue'
    ),

    # 5. Nucleus detection
    NucleusDetectionHE(threshold=0.3)
])

CODEX Multiparametric Pipeline

from pathml.preprocessing import (
    Pipeline,
    CollapseRunsCODEX,
    SegmentMIF,
    QuantifyMIF
)

codex_pipeline = Pipeline([
    # 1. Consolidate multi-run data
    CollapseRunsCODEX(z_slice=2),

    # 2. Cell segmentation
    SegmentMIF(
        nuclear_channel='DAPI',
        cytoplasm_channel='CD45',
        model='mesmer',
        image_resolution=0.377
    ),

    # 3. Quantify markers
    QuantifyMIF(
        segmentation_mask_name='cell_segmentation',
        markers=['CD3', 'CD4', 'CD8', 'CD20', 'PD1', 'PDL1'],
        output_format='anndata'
    )
])

Advanced Pipeline with Quality Control

from pathml.preprocessing import (
    Pipeline,
    LabelWhiteSpaceHE,
    LabelArtifactTileHE,
    TissueDetectionHE,
    MorphOpen,
    MorphClose,
    StainNormalizationHE,
    AdaptiveHistogramEqualization
)

advanced_pipeline = Pipeline([
    # Stage 1: Quality control
    LabelWhiteSpaceHE(threshold=0.85),
    LabelArtifactTileHE(pen_threshold=0.5, bubble_threshold=0.5),

    # Stage 2: Tissue detection
    TissueDetectionHE(threshold=10, min_region_size=1000),
    MorphOpen(kernel_size=5, mask_name='tissue'),
    MorphClose(kernel_size=7, mask_name='tissue'),

    # Stage 3: Stain normalization
    StainNormalizationHE(
        target='normalize',
        stain_estimation_method='vahadane',
        tissue_mask_name='tissue'
    ),

    # Stage 4: Contrast enhancement
    AdaptiveHistogramEqualization(clip_limit=0.03, tile_grid_size=(8, 8))
])

Running Pipelines

Single Slide Processing

from pathml.core import SlideData

# Load slide
wsi = SlideData.from_slide("slide.svs")

# Generate tiles
wsi.generate_tiles(level=1, tile_size=256, stride=256)

# Run pipeline
pipeline.run(wsi)

# Access processed data
for tile in wsi.tiles:
    normalized_image = tile.image
    tissue_mask = tile.masks.get('tissue')
    nucleus_mask = tile.masks.get('nucleus')

Batch Processing with Distributed Execution

from pathml.core import SlideDataset
from dask.distributed import Client
import glob

# Start Dask client
client = Client(n_workers=8, threads_per_worker=2, memory_limit='4GB')

# Create dataset
slide_paths = glob.glob("data/*.svs")
dataset = SlideDataset(
    slide_paths,
    tile_size=512,
    stride=512,
    level=1
)

# Run pipeline in parallel
dataset.run(
    pipeline,
    distributed=True,
    client=client
)

# Save results
dataset.to_hdf5("processed_dataset.h5")

client.close()

Conditional Pipeline Execution

Execute transforms only on tiles meeting specific criteria:

# Filter tiles before processing
wsi.generate_tiles(level=1, tile_size=256)

# Run pipeline only on tissue tiles
for tile in wsi.tiles:
    if tile.masks.get('tissue') is not None:
        pipeline.run(tile)

Performance Optimization

Memory Management

# Process large datasets in batches
batch_size = 100
for i in range(0, len(slide_paths), batch_size):
    batch_paths = slide_paths[i:i+batch_size]
    batch_dataset = SlideDataset(batch_paths)
    batch_dataset.run(pipeline, distributed=True)
    batch_dataset.to_hdf5(f"batch_{i}.h5")

GPU Acceleration

Certain transforms leverage GPU acceleration when available:

import torch

# Check GPU availability
print(f"CUDA available: {torch.cuda.is_available()}")

# Transforms that benefit from GPU:
# - SegmentMIF (Mesmer deep learning model)
# - StainNormalizationHE (matrix operations)

Parallel Workers Configuration

from dask.distributed import Client

# CPU-bound tasks (image processing)
client = Client(
    n_workers=8,
    threads_per_worker=1,  # Use processes, not threads
    memory_limit='8GB'
)

# GPU tasks (deep learning inference)
client = Client(
    n_workers=2,  # Fewer workers for GPU
    threads_per_worker=4,
    processes=True
)

Custom Transforms

Create custom preprocessing operations by subclassing Transform:

from pathml.preprocessing.transforms import Transform
import numpy as np

class CustomTransform(Transform):
    def __init__(self, param1, param2):
        self.param1 = param1
        self.param2 = param2

    def apply(self, tile):
        # Access tile image
        image = tile.image

        # Apply custom operation
        processed = self.custom_operation(image, self.param1, self.param2)

        # Update tile
        tile.image = processed

        return tile

    def custom_operation(self, image, param1, param2):
        # Implement custom logic
        return processed_image

# Use in pipeline
pipeline = Pipeline([
    CustomTransform(param1=10, param2=0.5),
    # ... other transforms
])

Best Practices

1. Order transforms appropriately:

   • Quality control first (LabelWhiteSpace, LabelArtifact)
   • Noise reduction early (Blur)
   • Tissue detection before stain normalization
   • Stain normalization before color-dependent operations

2. Use tissue masks for stain normalization:

   • Improves accuracy by excluding background
   • TissueDetectionHE() then StainNormalizationHE(tissue_mask_name='tissue')

3. Apply morphological operations to clean masks:

   • MorphOpen to remove small false positives
   • MorphClose to fill small gaps

4. Leverage distributed processing for large datasets:

   • Use Dask for parallel execution
   • Configure workers based on available resources

5. Save intermediate results:

   • Store processed data to HDF5 for reuse
   • Avoid reprocessing computationally expensive transforms

6. Validate preprocessing on sample images:

   • Visualize intermediate steps
   • Tune parameters on representative samples before batch processing

7. Handle edge cases:

   • Check for empty masks before downstream operations
   • Validate tile quality before expensive computations

Common Issues and Solutions

Issue: Stain normalization produces artifacts

• Use tissue mask to exclude background
• Try different stain estimation method (macenko vs. vahadane)
• Verify optical density parameters match your images

Issue: Out of memory during pipeline execution

• Reduce number of Dask workers
• Decrease tile size
• Process images at lower pyramid level
• Enable memory_limit parameter in Dask client

Issue: Tissue detection misses tissue regions

• Adjust threshold parameter
• Use saturation channel: use_saturation=True
• Reduce min_region_size to capture smaller tissue fragments

Issue: Nucleus detection is inaccurate

• Verify stain separation quality (visualize hematoxylin channel)
• Adjust threshold parameter
• Apply stain normalization before nucleus detection

Additional Resources

• PathML Preprocessing API: https://pathml.readthedocs.io/en/latest/api_preprocessing_reference.html
• Stain Normalization Methods:
  • Macenko et al. 2009: "A method for normalizing histology slides for quantitative analysis"
  • Vahadane et al. 2016: "Structure-Preserving Color Normalization and Sparse Stain Separation"
• DeepCell Mesmer: https://www.deepcell.org/ (cell segmentation model)