bash syntax

[mick crane <mick.crane@gmail.com>]

I nicked this off some guy on the internet.
  1 #!/bin/bash
  2 count_file="scan_count.txt"
  3 ct=`cat $count_file`
  4 ((ct++))
  5 scanimage > /home/mick/DATA/SCANS/scan-$ct.pnm
  6 echo $ct > $count_file

What's the thing with the ct being in double brackets and not having the 
"$" ?
scan_count.txt just is one line with a number.
mick
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