Colocalization by Cross Correlation
Colocalization by Cross Correlation
This plugin attempts to determine: the average distance between non-randomly spatially associated particles, the standard deviation of that distance (which should also reflect the width of the PSF in the image for diffraction limited images), and a statistical measure of confidence of the association. It does this by performing a cross-correlation function (CCF) between the two images, operating in a similar manner to Van Steensel’s CCF except this plugin performs the CCF in all directions and provides additional information, such as a confidence level. It currently works on 2D/3D single-channel images, and requires a mask of all possible localizations for the signal in one of the images.
Installing the Plugin:
Available on the list of ImageJ updates sites. Requires Fiji.
How to use Colocalization by Cross Correlation:
Randomization mask: An example of an appropriate mask for analyzing cross-correlation of cytoplasmic proteins(right), generated from an actin stain (left).
Prepare a mask for the Costes randomization: To get the best possible results, you will want to create and save a segmented mask for one of your images. This mask should contain all possible localizations for that stain/dye, or it should be a mask of localization for your null hypothesis (i.e if you hypothesize a protein is localized to the mitochondria, you would want your mask to encompass the entire cell). As an example, say you are studying correlation between two nuclear proteins, then you would want your mask to cover the nucles, which could be created easily using a DAPI or Hoechst stain (the mask itself does not need to be generated from either image your are trying to correlate). If you were studying cytoplasmic proteins, you would want your mask to cover the entire cytoplasm. The mask is very important and not using it could easily lead to undesired correlations. This is because without a mask this plugin will find correlations at any distance, and, if say you are studying nuclear proteins, can easily correlate one nuclei to the nuclei of a neighboring cell (cells are often highly repetitive and spaced relatively evenly). However, when an appropriate mask is used, these cell to cell correlations will be subtracted out during the analysis.
Prepare your data images: The two images should undergo appropriate background subtraction to lower the background pixel values to zero or near zero. Even low level background (pixel values of 10 to 20 on a 16 bit image) can lower the confidence level and result in false negatives. Additionally, the images (particularly 3D) need to be correctly scaled (Image > Properties), otherwise all axes will be assumed to have the same scale and your mean correlation distance will be in pixels.
Determine your point-spread function (PSF) dimensions: During a Costes randomization images are randomized by PSF-sized chunks, rather than individual pixels. To facilitate this, the plugin needs to know the approximate size of the image PSF in pixels (rounded to the nearest whole unit). The PSF size can be calculated using the Abbe diffraction formula for fluorescence microscopy (lateral: 0.5λ/NA, axial: 2λ/NA^2), and then simply needs to be divided by the corresponding scaled pixel size (in Image > Properties).
The plugin can be found in the Analyze > Colocalization menu after it has been installed. At the dialog menu, select the two images and the randomization mask to be used, and enter the PSF width and height in pixel units. Image 1 is the image that will be randomized and requires an appropriate mask. If the possible localizations for your dye/stain encompasses the entire image, you can check the no mask box. For the Costes’ randomization cycles count, you can typically input a low number (5-8), however if you have sparse signal within a large mask volume you will probably want to do more cycles. If the “Show intermediate images” box is checked, the plugin will open images showing the correlations (before and after subtraction of random associations), and a sample randomization image. This can be useful for understanding the function of the plugin, or for visualizing the direction of the correlation if your sample has a polarized axis. More details on the intermediate images can be found below.
Interpreting the results:
To help describe the results, we will use the simple, idealized example shown below. This example analyzes the cross-correlation of two 2D images composed of equally sized dots (shown as a composite image). Most of these dots are paired to dots of the other channel at a set distance, however some are not at this distance or are not paired. We can see from the results that the strongly correlated dots are approximately 44 pixels apart, with a standard deviation of ~5.5 pixels (for actual microscope data, this distance would be in scaled units instead of pixels). The contribution image highlights the dots that are around this range of each other. We used correlation at a distance for this example to highlight one of the strengths of this technique, but this can be done for traditional “colocalized” images that have a mean cross-correlation distance near zero.
Here’s a detailed description of each of the result windows:
Correlation plot: A radial profile plot will be displayed, it contains the radial profile of the original cross-correlation image (black line), the radial profile of the cross-correlation after subtraction of random associations (red line), and a Gaussian curve fit to the subtracted profile (blue dots). The distance between the red line and the black line is a visual indicator of the confidence value described below. The Y-axis is the average of the cross-correlation function. While not technically arbitrary, it is most easily viewed as a measure of relative correlation. The range of the graph is set automatically to fit the Gaussian curve. If you wish to view all the data select More > Set Range to Fit all…
Gaussian fit analysis: This text window will contain (in scaled units), the mean distance for the discovered correlation, and the standard deviation of that correlation (sigma), as well as an estimation of confidence for this correlation (range 0-100). The confidence is determined by taking the area under the curve (AUC) of the subtracted correlation radial profile (in range of mean ± 3×sigma) divided by the AUC of the original correlation radial profile (in same range). Values closer to 100 indicate a strong likelihood of true correlation. Values near zero indicate low to no correlation between the two images. I currently estimate that values of ~20 or greater indicate some degree of correlation (within the range specified by the Gaussian curve), though this is with very limited data. Confidence values can be affected by many things, including image noise and background, an inappropriate randomization mask, and optical abberations. Performing a simple background subtraction can improve confidence scores signficantly.
Contribution of each image to the gaussian fit: Two new images will be created that display the signal from each analyzed image that contributed to the cross-correlation and gaussian fit result. It is important to note that this image is always generated and the data it contains will always be visible, even if you do NOT have a strong correlation between the two images. The pixel intensity values should NOT be used as an indicator of overall correlation between the images (that’s what the confidence value is for), but the relative brightness within an image can be used as an approximate indicator of how strongly that particular signal contributed to the correlation result. This relative brightness indication can be easily seen in the example data above: In our original data, all the dots are of the same size and intensity, however, in the resulting contribution images, the dots that remain are varied in their brightness based on how much they contributed to the cross-correlation result (you’ll notice that the brightest dots are all oriented in the same direction).
How it works & intermediate images description:
This plugin starts by performing an initial cross-correlation to create a cross-correlation image. Bright regions in the cross-correlation image correspond to a high correlation between the images when the second image is shifted by a vector equal to the distance form the center of the cross-correlation image to the bright region. Thus, if you see a bright spot in the cross-correlation image that is 2 µm to the left of the image center, it means their is a positive correlation between image 1 and image2 when image2 is shited left 2 µm.
To remove the contribution from potential larger repeating structures (such as cells and nuclei), and to correct for correlation that results from high signal density, a second cross-correlation image generated from randomized images is then subtracted from the original correlation image. This occurs through cycles of:
Randomizing Image1 using the Costes randomization method and the mask of all theoretically possible localizations for the signal in Image1 (i.e. a mask of the cells, or the nuclei), which is provided by the user.
Calculating the cross-correlation of the randomized image with Image2, then averaging this over successive cycles.
After the subtraction process, we generate a radial profile of the subtracted data, showing the correlation between the images at different offsets, then fit a gaussian curve to it. We also generate a radial profile for the original correlation data before subtraction, as this is needed to establish a measure of confidence. The confidence is calculated as the area under the curve (AUC) of the subtracted correlation radial profile (in the range of mean ± 3×sigma) divided by the AUC of the original correlation radial profile (in same range) as a percentage. The confidence value, along with the mean and sigma of the gaussian fit are displayed in a log window. Higher values of confidence, closer to 100, indicate that two images likely have a true spatial correlation at the indicated distance.
To generate the contribution images, we further modify the subtracted cross-correlation image, by effectively multiplying it with the gaussian fit in order to create a cross-correlation image that only retains the data within our gaussian curve range. This gaussian-modified cross-correlation image is then used to back-calculate the contribution images. Image1Contribution = (image2 ∗ gaussModifiedCorr) × image1. Image2Contribution = (image1 ★ gaussModifiedCorr) × image2. Key: ∗ -> convolve, ★ -> correlate.