Let your array be called CTscan, and be a 1024x1024 matrix.
If you know how many columns you want to delete (let's say 21), you can do this to delete the first 21 columns:
Then, you will have a 1024x1003 matrix.
You can also do the same for rows. To delete the first 23 rows of CTscan:
Then, you will have a 1001x1003 matrix.
To delete the last 21 columns:
CTscan(:, end-20:end) = ;