Why is it that after I import the patient's CT file (in DICOM format), the image is not displayed completely but only shows as a slit?

This is a projection = a 2D image. It is not a 3D volume.

If you have hundreds of projection images like this then it is possible to reconstruct a 3D image from it, but reconstruction is a complex operation and - since you need very accurate information on the imaging geometry - usually only the manufacturer can do it.

You can ask the people who gave you this image to create a 3D reconstruction and send that to you, too.