Comet Labs WN591 Manual de usuario descargar pdf (Pagina 23)

Help! There is a comet in my computer! 20

Let us look at our model comet image (Figure 11). First, we will determine the position of

the intensity centroid of the tail. The centroid is the x-coordinate at which half of the

total intensity of the tail is to the left and half to the right of this coordinate (half of

“grey value weight” to each side of the coordinate). The total intensity of the whole tail is

10920 (Table 1), so we are looking for the x-coordinate at which the total intensity to

either side will be 10920 / 2 = 5460. We start adding intensity weights at the left side of the

tail. We already know that the total intensity of the left most pixel column of the tail (pixel

column number 12) equals 1011 (Table 1, column Background-corrected comet intensity).

Because this is less than 5430, we can add the weight of the next column to the right, and

so on. We calculate 1011 + 974 + 1020 + 914 + 989 = 4909 (see column Cumulative comet

intensity in Table 1). This is lower than the limit 5460. If we now add the intensity weight

of the next column (pixel column number 17), the sum of weights will be 5807 - higher

than the limit 5460. Hence, we can conclude that the intensity centroid of the tail lies

between pixel columns 16 and 17 (Figure 11). Using the same procedure, we can

determine the position of the centroid of the head between pixel columns 5 and 6. The

distance between the intensity centroids of the head and of the tail is 11 pixels (Figure 11).

Figure 11. The intensity centroids. Positions of the intensity centroids are shown as green arrows

on the model comet image with box segmentation. The positions of the centroids can be

determined from data in Table 1. See text for details.

We measure distances in micrometers, hence we have to convert pixels to micrometers.

The scaling factor of the model image is 0.36 µm per pixel, so the distance is

0.36 µm pixel

-1

× 11 pixels = 3.96 µm.

Now we can also calculate OTM as the percentage of DNA in the tail multiplied by the

distance between the intensity centroids:

OTM = 37% × 3.96 µm = 0.37 × 3.96 µm = 1.5 µm.

OTM is an absolute parameter with a measurement unit – micrometer.

Now that we are finished with calculations of the comet parameters, we can turn to some

general topics of image analysis.

head tail

background

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Comet Labs WN591 Manual de usuario Pagina 23