Using pyTFM in Python

pyTFM makes it easy to perform Traction Force Microscopy and Monolayer Stress Microscopy in python. In this tutorial we will calculate strain energy, contractillity, mean normal stress and average line tension of a cell colony. As always we need two images of fluorescent beads: One image before cell removal and one image after cell removal. Additionally, since we are not using the clickpoints addon, we need to provide 3 masks: A mask for the area on which force generation is evaluated (encircling all deformations and forces that originate from the cell colony), a mask for the Finite Elements Analysis (encircling all forces that originate from the cell colony) and a mask of the cell boundaries. Of course, the easies way to generate these masks is to use clickpoints.

In the example data set in the subfolder “python_tutorial” you can find images for a single cell colony as well as suitable masks. I drew the masks in clickpoints and saved them as PNG files so that you can look at them with a standard image display tool. You can use any other format, as long as you can load them as a boolean (True and False) array to python.

The “python_tutorial” folder also contains the complete code of the tutorial in a single python script “”. This script prints all quantities and produces all figures, that we are going to see in this tutorial. It should work right away.

Calculating Deformation Fields

First, let’s import functions to calculate and plot the deformation field:

from pyTFM.TFM_functions import calculate_deformation
from pyTFM.plotting import show_quiver

Calculating the deformation field requires the images of the beads before and after cell removal. You can provide either paths to the files, or arrays ( which must have the data type int32). The deformation field is calculated with Particle Image Velocimetry, using a cross correlation algorithm. You need to find appropriate values for the window_size and overlap that produce a smooth deformation field. For this data you can use:

# paths to the images
im_path1 = r"/home/user/Software/example_data_for_pyTFM/python_tutorial/04after.tif" # change to your location
im_path2 = r"/home/user/Software/example_data_for_pyTFM/python_tutorial/04before.tif"
# calculating the deformation
u, v, mask_val, mask_std = calculate_deformation(im_path1, im_path2, window_size = 100, overlap = 60)
# the unit of window size and overlap is pixels of the image of the beads

The overlap of 60 is a bit to small, especially for the FEM analysis later. If you want to be more accurate use, an overlap of 95. This will however increase the calculation time from a few seconds to roughly 5 minutes.

Let’s plot the deformation field:

# plotting the deformation field
fig1, ax = show_quiver(u, v, cbar_str="deformations\n[pixels]")

show_quiver accepts most of the plotting parameters listed in Overview of Plotting Parameters.


In some cases the images of the beads don’t share exactly the same field of view. You will notice this in the deformation field. pyTFM provides a function to find and extract the common field of view of both images with subpixel accuracy:

from pyTFM.frame_shift_correction import correct_stage_drift
from PIL import Image
import numpy as np

# load your images; dtype must be float32; images must be grayscale (you need a 2 dimensional array)
image1 = np.asarray("/home/user/Software/example_data_for_pyTFM/python_tutorial/04after.tif"))
image2 = np.asarray("/home/user/Software/example_data_for_pyTFM/python_tutorial/04before.tif"))

# cutting out the common field of few of image1 with image2.
# This also normalizes the images and applies a subpixel
# accurate shift. You can provide an additional list of
# images that will be cut to the same field of view.
image1_cor, image2_cor, other_images, drift = correct_stage_drift(image1, image2, additional_images=[])

# saving the output"/home/user/Software/example_data_for_pyTFM/python_tutorial/04after_corr.tif")"/home/user/Software/example_data_for_pyTFM/python_tutorial/04before_corr.tif")

The images in this tutorial are already corrected.

Calculating Traction Fields

Next, we are going to calculate the traction forces, that the cell colony generated. This is done with the “TFM_tractions” function:

from pyTFM.TFM_functions import TFM_tractions
import numpy as np

We have to set the elastic parameters of the substrate (Young’s modulus, Poisson’s ratio and the height of the substrate). You can also set the substrate height to “infinite”. We also need the pixel size of the image of the beads and the pixel size of the deformation field. The later can be calculate if you know the dimensions and pixel size of the image of the beads:

ps1 = 0.201 # pixel size of the image of the beads
im1_shape = (1991, 2033) # dimensions of the image of the beads
ps2 = ps1 * np.mean(np.array(im1_shape) / np.array(u.shape)) # pixel size of of the deformation field
young = 49000 # Young's modulus of the substrate in Pa
sigma = 0.49 # Poisson's ratio of the substrate
h = 300 # height of the substrate in µm, "infinite" is also accepted

Finally, the traction field can be calculated by:

tx, ty = TFM_tractions(u, v, pixelsize1=ps1, pixelsize2=ps2, h=h, young=young, sigma=sigma)

We can plot it in the same way as we plotted the deformation field:

fig2, ax = show_quiver(tx, ty, cbar_str="tractions\n[Pa]")

Quantifying the Force Generation

In order to quantify the force generation of the cell colony, we have to select the area where deformations and tractions that are generated by the colony are located. This selection requires a mask, a boolean array, that has the value True in the area that we want to use and False elsewhere. I produced the appropriate mask in clickpoints and saved it as a grayscale image as “force_measurement.png”. After loading the mask, there are two more things we need to do: First, we need to fill all holes in the mask in order to produce a continuous area. Second, we need to resize the mask to the dimensions of the deformation and traction fields:

import matplotlib.pyplot as plt
from scipy.ndimage.morphology import binary_fill_holes
from pyTFM.grid_setup_solids_py import interpolation # a simple function to resize the mask

# loading a mask that defines the area used for measuring the force generation
mask = plt.imread(r"/home/user/Software/example_data_for_pyTFM/python_tutorial/Tractions.png").astype(bool)
mask = binary_fill_holes(mask) # the mask should be a single patch without holes
# changing the masks dimensions to fit to the deformation and traction fields
mask = interpolation(mask, dims=u.shape)

This mask can now be used to calculate the contractillity and the strain energy:

from pyTFM.TFM_functions import strain_energy_points, contractillity

# strain energy:
# first we calculate a map of strain energy
energy_points = strain_energy_points(u, v, tx, ty, ps1, ps2) # J/pixel
# then we sum all energy points in the area defined by mask
strain_energy = np.sum(energy_points[mask]) # 2.14*10**-13 J

# contractillity
contractile_force, proj_x, proj_y, center = contractillity(tx, ty, ps2, mask) # 2.03*10**-6 N

Measuring Stresses in Cell Colonies

Stresses are calculated with the Finite Elements Methods, modeling the colony as a 2 dimensional sheet and applying force opposite to the traction forces to it. The FEM algorithm largely uses the solidspy package. This package is also very instructive if you want to get into FEM in general.

We will use the previous area outlined in “Tractions.png” to model the cell colony. This area covers all cell generated tractions and is larger then the actual cell colony, due to inaccuracies in the calculation of tractions. All measures for stress are evaluated on the actual area of the cell colony. This area is generated from the cell boundaries.

# first mask: The area used for Finite Elements Methods
# it should encircle all forces generated by the cell colony
mask_FEM = plt.imread(r"/home/user/Software/example_data_for_pyTFM/python_tutorial/Tractions.png").astype(bool)
mask_FEM = binary_fill_holes(mask_FEM) # the mask should be a single patch without holes
# changing the masks dimensions to fit to the deformation and traction field:
mask_FEM = interpolation(mask_FEM, dims=tx.shape)

# second mask: The area of the cells. Average stresses and other values are calculated only
# on the actual area of the cell, represented by this mask.
mask_cells = plt.imread(r"/home/user/Software/example_data_for_pyTFM/python_tutorial/cell_borders.png").astype(bool)
mask_cells = binary_fill_holes(mask_cells)
mask_cells = interpolation(mask_cells, dims=tx.shape)

The traction forces in the FEM area are typically slightly unbalanced, leading to a net force and torque acting on the cell colony. We need to correct this:

from pyTFM.grid_setup_solids_py import prepare_forces

# converting tractions (forces per surface area) to actual forces
# and correcting imbalanced forces and torques
# tx->traction forces in x direction, ty->traction forces in y direction
# ps2->pixel size of the traction field, mask_FEM-> mask for FEM
fx, fy = prepare_forces(tx, ty, ps2, mask_FEM)

Now we are ready to perform a Finite Elements Analysis. This is split into two steps: First, the FEM grid is setup. The grid is build up of nodes. For each node the connectivity to other nodes (stored in “elements”), constraints on the displacements (stored in “nodes”) and forces acting (stored in “loads”) as well as elastic properties (Youngs’s modulus and Poisson’s ratio stored in mats ) on the node are defined. Note that the Young’s modulus of the material has no influence whatsoever on the resulting stresses and that the Poisson’s ratio has only a small influence. Consequently, both Young’s modulus and Poisson’s ratio can be left at their default value (1 Pa and 0.5 respectively). The applied forces are simply obtained from the underlying tractions with a negative sign. Next, the FEM system is solved by calculating the deformations, followed by the strain and, based on the strain-stress relation of a linearly elastic 2-dimensional material.

from pyTFM.grid_setup_solids_py import grid_setup, FEM_simulation

# constructing the FEM grid
nodes, elements, loads, mats = grid_setup(mask_FEM, -fx, -fy, sigma=0.5)
# performing the FEM analysis
# verbose prints the progress of numerically solving the FEM system of equations.
UG_sol, stress_tensor = FEM_simulation(nodes, elements, loads, mats, mask_FEM, verbose=True)
# UG_sol is a list of deformations for each node. We don't need it here.

The stress tensor completely defines the forces inside the cell colony. We can for example extract the average mean normal stress and the coefficient of variation of the mean normal stress (quantifying how much the stress varies in the colony) from the stress tensor. We will use the mask “mask_cells” which marks the actual area of the cell colony for these measurements.

# mean normal stress
ms_map = ((stress_tensor[:, :, 0, 0] + stress_tensor[:, :, 1, 1]) / 2) / (ps2 * 10**-6)
# average on the area of the cell colony.
ms = np.mean(ms_map[mask_cells]) # 0.0043 N/m

# coefficient of variation
cv = np.nanstd(ms_map[mask_cells]) / np.abs(np.nanmean(ms_map[mask_cells])) # 0.41 no unit

Calculating the Line Tension

A particularly interesting feature are forces taht are transmitted across cell-cell boundaries. This is quantified by the line tension. First, we need to load a mask marking all cell borders. Note that this is the same mask that we used to get the area of the cell colony, only this time we are not going to fill any holes or resize the mask.

# loading a mask of the cell borders
mask_borders = plt.imread(r"/home/user/Software/example_data_for_pyTFM/python_tutorial/cell_borders.png").astype(bool)

The cell-cell borders are stored in an object “borders”, which among other things contains a spline interpolation of each border, assigns each border to a cell and contains a list of borders located at the edge of the cell colony:

from pyTFM.grid_setup_solids_py import find_borders

# identifying borders, counting cells, performing spline interpolation to smooth the borders
borders = find_borders(mask_borders, tx.shape)
# we can for example get the number of cells from the "borders" object
n_cells = borders.n_cells # 8

We can use the cell-cell borders together with the stress tensor to calculate the line tension. The line tension is a force vector acting on a small slice of a cell border. We are going to calculate the average length of this vector (“avg_line_tension”) and the average normal component of the line tension (“avg_normal_line_tension”):

from pyTFM.stress_functions import lineTension

# calculating the line tension along the cell borders
lt, min_v, max_v = lineTension(borders.lines_splines, borders.line_lengths, stress_tensor, pixel_length=ps2)
# lt is a nested dictionary. The first key is the id of a cell border.
# For each cell border the line tension vectors ("t_vecs"), the normal
# and shear component of the line tension ("t_shear") and the normal
# vectors of the cell border ("n_vecs") are calculated at a large number of points.

# average norm of the line tension. Only borders not at colony edge are used
lt_vecs = np.concatenate([lt[l_id]["t_vecs"] for l_id in lt.keys() if l_id not in borders.edge_lines])
avg_line_tension = np.mean(np.linalg.norm(lt_vecs, axis=1)) # 0.00569 N/m

# average normal component of the line tension
lt_normal = np.concatenate([lt[l_id]["t_normal"] for l_id in lt.keys() if l_id not in borders.edge_lines])
avg_normal_line_tension = np.mean(np.abs(lt_normal)) # 0.00566 N/m,
# here you can see that almost the line tensions act almost exclusively perpendicular to the cell borders.

Finally let’s produce a plot of the line tension:

from pyTFM.plotting import plot_continuous_boundary_stresses

# plotting the line tension
fig3, ax = plot_continuous_boundary_stresses([borders.inter_shape, borders.edge_lines, lt, min_v, max_v], cbar_style="outside")