Image resolution
The image resolution is determined during presetting the scan
parameters in
the scanner software. During the scan the 2D gel is resolved into a
mosaic of
equally sized squares (pixels). Smaller pixels mean higher image resolution,
i.e. more
details are visible on the image.
Resolution is measured in several units. Typical units used by
scanners are
micron (1 micron corresponds to a pixel size of 1 micrometer), ppi
/
dpi (pixels per inch / dots per inch) or in the metric system pixels
per
millimeter. For image analysis of protein spots, the smallest spots
should
have about 5 to 10 pixels in diameter. As an example, take a very small
spot on a
two-dimensional electrophoresis gel with 1 mm in diameter. With a
resolution of
200 dpi this spot will have a diameter of about 8 pixels.
Higher resolutions
mean
that you have more data available for the analysis. However, the
increase in
precision that comes with higher resolution is marginal once you have a
certain
minimum resolution. On the other hand, every doubling of the resolution
results
in a fourfold increase of the image file size. Scanning takes more time
in
higher resolutions, files take up more space on the disk, and image
analysis
requires more time and memory.
The following table shows the relation between resolution and spot
sizes for
some typical resolutions.
Typical image resolutions and corresponding
spot and file sizes
| image resolution [dots per inch] |
pixel width [inch] |
image resolution [micron] |
diameter [pixel] of a 1mm-diameter-spot |
pixels in a circular 1mm-diameter-spot |
diameter [pixel] of a 5mm-diameter-spot |
pixels in a circular 5mm-diameter-spot |
width of a 20cm gel image [pixels] |
file size [MB] 8 bit graylevels |
file size [MB] 16 bit graylevels |
| 100 |
0.0100 |
254 |
4 |
12 |
20 |
304 |
787 |
0.6 |
1.2 |
| 150 |
0.0067 |
169 |
6 |
27 |
30 |
685 |
1181 |
1.3 |
2.7 |
| 200 |
0.0050 |
127 |
8 |
49 |
39 |
1217 |
1575 |
2.4 |
4.7 |
| 300 |
0.0033 |
85 |
12 |
110 |
59 |
2739 |
2362 |
5.3 |
10.6 |
| 400 |
0.0025 |
64 |
16 |
195 |
79 |
4869 |
3150 |
9.5 |
18.9 |
Increasing the resolution after the scan brings no additional
information to
the image. Reducing the image resolution after the scan results in loss
of
data. However, this can be acceptable provided that the spot diameter
for small
spots does not drop below 5 pixels.
|
