Q:

Is it possible to compare 3D images?

Is it possible to compare two 3D images?
If possible what are the possible solutions(i mean methods)?
And the purpose i want to compare two 3D images it to identify an object in that source image.
even i have one more question, at present i have been comparing converting 3d images into 2d, but i was barely used to match them by correlating.
How i can detect the object in an image(ROI)?
How to form border lines of the object if detected?
How to remove the other data in the image except leaving the object?

A:

You just asked about all possible questions in 3D image processing. You will not get a neat answer. Please divide, and then conquer. Please find below a few keyword to put on on track!
Is it possible to compare two 3D images? 
-> image similarity metrics, such as Mean Squares, Normalized Correlation, Pattern Intensity, Mutual Information
-> you can only do it if your images are registered. Check rigid registration or non-rigid registration.
Identify an object in that source image.
-> that is the topic of segmentation (in 3D), or computer vision (in 2D)
How i can detect the object in an image(ROI)?
How to form border lines of the object if detected?
-> that is the topic of Edge detection, see Canny Edge for example.
How to remove the other data in the image except leaving the object?
-> trivial, you just subtract once you've it.
Lots of resources in Medical Imaging. See what ITK can do. Some open source software include 3DSlicer, 3DSeg, ...

