Tissue Masks

Tissue Masks

Overview

Tissue masks are binary representations that identify tissue regions within whole slide images. They are essential for filtering out background, artifacts, and non-tissue areas during tile extraction. Histolab provides several mask classes to accommodate different tissue segmentation needs.

Mask Classes

BinaryMask

Purpose: Generic base class for creating custom binary masks.

from histolab.masks import BinaryMask

class CustomMask(BinaryMask):
    def _mask(self, obj):
        # Implement custom masking logic
        # Return binary numpy array
        pass

Use cases:

• Custom tissue segmentation algorithms
• Region-specific analysis (e.g., excluding annotations)
• Integration with external segmentation models

TissueMask

Purpose: Segments all tissue regions in the slide using automated filters.

from histolab.masks import TissueMask

# Create tissue mask
tissue_mask = TissueMask()

# Apply to slide
mask_array = tissue_mask(slide)

How it works:

1. Converts image to grayscale
2. Applies Otsu thresholding to separate tissue from background
3. Performs binary dilation to connect nearby tissue regions
4. Removes small holes within tissue regions
5. Filters out small objects (artifacts)

Returns: Binary NumPy array where:

• True (or 1): Tissue pixels
• False (or 0): Background pixels

Best for:

• Slides with multiple separate tissue sections
• Comprehensive tissue analysis
• When all tissue regions are important

BiggestTissueBoxMask (Default)

Purpose: Identifies and returns the bounding box of the largest connected tissue region.

from histolab.masks import BiggestTissueBoxMask

# Create mask for largest tissue region
biggest_mask = BiggestTissueBoxMask()

# Apply to slide
mask_array = biggest_mask(slide)

How it works:

1. Applies same filtering pipeline as TissueMask
2. Identifies all connected tissue components
3. Selects the largest connected component
4. Returns bounding box encompassing that region

Best for:

• Slides with a single primary tissue section
• Excluding small artifacts or tissue fragments
• Focusing on main tissue area (default for most tilers)

Customizing Masks with Filters

Masks accept custom filter chains for specialized tissue detection:

from histolab.masks import TissueMask
from histolab.filters.image_filters import RgbToGrayscale, OtsuThreshold
from histolab.filters.morphological_filters import BinaryDilation, RemoveSmallHoles

# Define custom filter composition
custom_mask = TissueMask(
    filters=[
        RgbToGrayscale(),
        OtsuThreshold(),
        BinaryDilation(disk_size=5),
        RemoveSmallHoles(area_threshold=500)
    ]
)

Visualizing Masks

Using locate_mask()

from histolab.slide import Slide
from histolab.masks import TissueMask

slide = Slide("slide.svs", processed_path="output/")
mask = TissueMask()

# Visualize mask boundaries on thumbnail
slide.locate_mask(mask)

This displays the slide thumbnail with mask boundaries overlaid in a contrasting color.

Manual Visualization

import matplotlib.pyplot as plt
from histolab.masks import TissueMask

slide = Slide("slide.svs", processed_path="output/")
tissue_mask = TissueMask()

# Generate mask
mask_array = tissue_mask(slide)

# Plot side by side
fig, axes = plt.subplots(1, 2, figsize=(15, 7))

axes[0].imshow(slide.thumbnail)
axes[0].set_title("Original Slide")
axes[0].axis('off')

axes[1].imshow(mask_array, cmap='gray')
axes[1].set_title("Tissue Mask")
axes[1].axis('off')

plt.show()

Creating Custom Rectangular Masks

Define specific regions of interest:

from histolab.masks import BinaryMask
import numpy as np

class RectangularMask(BinaryMask):
    def __init__(self, x_start, y_start, width, height):
        self.x_start = x_start
        self.y_start = y_start
        self.width = width
        self.height = height

    def _mask(self, obj):
        # Create mask with specified rectangular region
        thumb = obj.thumbnail
        mask = np.zeros(thumb.shape[:2], dtype=bool)
        mask[self.y_start:self.y_start+self.height,
             self.x_start:self.x_start+self.width] = True
        return mask

# Use custom mask
roi_mask = RectangularMask(x_start=1000, y_start=500, width=2000, height=1500)

Excluding Annotations

Pathology slides often contain pen markings or digital annotations. Exclude them using custom masks:

from histolab.masks import TissueMask
from histolab.filters.image_filters import RgbToGrayscale, OtsuThreshold
from histolab.filters.morphological_filters import BinaryDilation

class AnnotationExclusionMask(BinaryMask):
    def _mask(self, obj):
        thumb = obj.thumbnail

        # Convert to HSV to detect pen marks (often blue/green)
        hsv = cv2.cvtColor(np.array(thumb), cv2.COLOR_RGB2HSV)

        # Define color ranges for pen marks
        lower_blue = np.array([100, 50, 50])
        upper_blue = np.array([130, 255, 255])

        # Create mask excluding pen marks
        pen_mask = cv2.inRange(hsv, lower_blue, upper_blue)

        # Apply standard tissue detection
        tissue_mask = TissueMask()(obj)

        # Combine: keep tissue, exclude pen marks
        final_mask = tissue_mask & ~pen_mask.astype(bool)

        return final_mask

Integration with Tile Extraction

Masks integrate seamlessly with tilers through the extraction_mask parameter:

from histolab.tiler import RandomTiler
from histolab.masks import TissueMask, BiggestTissueBoxMask

# Use TissueMask to extract from all tissue
random_tiler = RandomTiler(
    tile_size=(512, 512),
    n_tiles=100,
    level=0,
    extraction_mask=TissueMask()  # Extract from all tissue regions
)

# Or use default BiggestTissueBoxMask
random_tiler = RandomTiler(
    tile_size=(512, 512),
    n_tiles=100,
    level=0,
    extraction_mask=BiggestTissueBoxMask()  # Default behavior
)

Best Practices

1. Preview masks before extraction: Use locate_mask() or manual visualization to verify mask quality
2. Choose appropriate mask type: Use TissueMask for multiple tissue sections, BiggestTissueBoxMask for single main sections
3. Customize for specific stains: Different stains (H&E, IHC) may require adjusted threshold parameters
4. Handle artifacts: Use custom filters or masks to exclude pen marks, bubbles, or folds
5. Test on diverse slides: Validate mask performance across slides with varying quality and artifacts
6. Consider computational cost: TissueMask is more comprehensive but computationally intensive than BiggestTissueBoxMask

Common Issues and Solutions

Issue: Mask includes too much background

Solution: Adjust Otsu threshold or increase small object removal threshold

Issue: Mask excludes valid tissue

Solution: Reduce small object removal threshold or modify dilation parameters

Issue: Multiple tissue sections, but only largest is captured

Solution: Switch from BiggestTissueBoxMask to TissueMask

Issue: Pen annotations included in mask

Solution: Implement custom annotation exclusion mask (see example above)