SF Pulse: Monitor Living Cells for Days Without Staining

Updated 6 November 2026 · Standard formula, shown below

This guide explains how to use the SF (Spatial Frequency) pulse method to monitor living cells continuously for days without staining. You will learn the setup, imaging parameters, and analysis steps to track cell behavior label-free, while avoiding common pitfalls and troubleshooting issues.

What You’ll Need

A microscope with a camera capable of capturing phase or brightfield images at regular intervals (e.g., a standard inverted microscope with an incubator chamber).
An incubator or stage-top incubator to maintain temperature, CO2, and humidity.
Cells cultured in a dish or multiwell plate with optical-quality bottom (e.g., glass-bottom dishes).
Software for image acquisition (e.g., microscope manufacturer’s software or open-source like Micro-Manager).
Image processing software (e.g., ImageJ/Fiji) with the SF pulse plugin or custom code (Python/Matlab) to compute spatial frequency spectra.
Optional: a motorized stage for multi-position time-lapse.

Step-by-Step

Prepare your cells and imaging setup. Culture cells in a glass-bottom dish to a confluency that allows clear imaging (typically 50-80% for monolayer cells). Ensure the incubator chamber is pre-warmed to 37°C, CO2 at 5% (if needed), and humidity >90% to prevent evaporation.
Set up the microscope for phase-contrast or brightfield imaging. Choose a low-NA objective (e.g., 10x or 20x) to get a good field of view and sufficient depth of field. Adjust the condenser and focus for sharp, even illumination.
Configure the time-lapse acquisition. In your acquisition software, set the interval (e.g., every 10-30 minutes) and total duration (e.g., 48-72 hours). Use multiple positions if you want to monitor different areas or replicate wells. Keep the light intensity low (e.g., use neutral density filters) to minimize phototoxicity.
Start the acquisition. Begin the time-lapse. Make sure the incubator is stable and the dish is sealed or covered to prevent drift. Monitor the first few frames to confirm focus and stage stability.
After acquisition, process the images. Import the image sequence into ImageJ/Fiji. If needed, correct for background illumination by subtracting a blank image (taken from an empty area) or using a rolling-ball background subtraction.
Compute the spatial frequency spectrum for each frame. For each image, apply a 2D Fast Fourier Transform (FFT) to obtain the frequency domain. In ImageJ, use the “FFT” command. The SF pulse method focuses on the magnitude of the spatial frequencies, often summarized by the power spectrum or a radial average.
Extract the SF pulse signal. Define a region of interest (ROI) in the frequency domain that corresponds to the cell size or subcellular structures (e.g., a ring at a specific radius). The SF pulse is the integrated intensity within that ROI over time. Alternatively, use a plugin that computes the spectral slope or a particular frequency band.
Plot the SF pulse over time. Export the values for each frame and plot them against time. You should see oscillations or trends that correlate with cell activity (e.g., cell division, migration, or metabolic changes).
Interpret the results. Compare the SF pulse changes with known cell behaviors. For example, an increase in high-frequency content may indicate cell rounding during mitosis, while a decrease may suggest cell spreading or death.

Common Mistakes

Using too high light intensity, causing photobleaching or phototoxicity that alters cell behavior.
Not maintaining stable environmental conditions (temperature, pH) leading to focus drift and cell stress.
Imaging at too high magnification, which reduces the number of cells in the field and makes the SF signal noisy.
Forgetting to correct for background or uneven illumination, which adds low-frequency noise to the spectrum.
Choosing a frequency band that does not correspond to the cell features of interest, leading to a weak or non-informative signal.

Tips

Use a low magnification (10x) and binning (2×2) to increase signal-to-noise while reducing file size.
Test the SF pulse on a known event (e.g., cell division) to calibrate your ROI and frequency band.
Keep the same ROI across all frames; if the stage drifts, apply image registration (e.g., using the “TurboReg” or “Descriptor-based registration” plugins) before FFT.
Use a motorized stage with autofocus (e.g., laser-based) to maintain focus over days.
If using multiple positions, ensure each position has a stable focus and that the stage returns accurately.

Troubleshooting

Signal is too noisy. Increase the number of frames averaged (if possible), reduce the magnification, or increase the binning. Also check for vibration or air currents that cause focus drift.
No oscillations in SF pulse. The frequency band may not be sensitive to the cellular process you’re monitoring. Try a broader band or a different radius. Also verify that your cells are actually changing over time (e.g., check a few images visually).
Focus drift over time. Ensure the incubator is stable, the dish is not leaking, and the stage is locked. Use autofocus if available, or refocus manually at intervals.
Cell death or detachment. Check for evaporation (use a lid or mineral oil overlay) and ensure the CO2 and humidity are correct. Reduce light exposure.
Background changes. If the background intensity changes (e.g., due to media color changes), perform flat-field correction using a reference image taken at each time point, or use a rolling-ball background subtraction with a large radius.

Frequently Asked Questions

What is the SF pulse method?

The SF pulse method analyzes the spatial frequency content of label-free images (like phase contrast) over time. The “pulse” refers to the transient changes in the frequency spectrum that correlate with cellular events like division or migration.

Why avoid staining?

Staining can be toxic to cells, alter their behavior, and often requires fixation or permeabilization, preventing long-term live monitoring. Label-free methods like SF pulse allow continuous observation without perturbing the cells.

What cell types can be monitored?

Any adherent cell type that can be imaged with phase contrast or brightfield can be monitored, including primary cells, stem cells, and cancer cell lines. Non-adherent cells are harder because they move out of focus.

How long can I monitor continuously?

As long as the cells remain healthy and the microscope is stable. With proper incubation and low light, you can monitor for several days (e.g., 3-5 days) without significant phototoxicity.

Do I need special software?

No, you can use free tools like ImageJ/Fiji to perform FFT and extract the SF pulse. Some commercial software may have built-in modules, but the method is accessible with basic image processing.

Can I combine SF pulse with other measurements?

Yes, you can simultaneously record other parameters like cell confluence or migration speed from the same images, as long as you don’t stain the cells.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.