Image registration has become essential in several domains where reliable acquisition of aligned images cannot be assured  or turns out to be complex. Needful to mention, the significance of this problem is made most apparent when alignment of a group of images must be guaranteed and the images are rather different in nature although they describe the exact same thing.
Misalignment can be due to movement of the subject or objects of interest, change of view-point, change in general conditions at the scene and even morphic changes or ones due to change in mass and elasticity of organs . Changes in form can be observed over time in the scene or within its constituent parts, e.g. the inevitable and involuntary change in the form of the lungs. In some circumstances, as later discussed, misalignment is due to profound changes in the form of objects (usually subjects and anatomy) being scanned. Alignment is a key step that must be completed before analysis of collections of images is safely embarked on. It allows better understanding of the contents of all images as a group.