imported ct resolution is not good

The study contains 3 volumes, each one is low-resolution along a different axis. You cannot get a single high-resolution volume (see this topic for details), but if you just want review 2D slices then you can choose to see the series in each of the orthogonal view that has the highest resolution in that particular orientation (click on the push-pin icon in the top-left corner of a slice view to see the volume selector).