Image Registration for MRI

Summary

To register two images means to align them so that common features overlap and differences-for example, a tumor that has grownare readily apparent. Being able to easily spot differences between two images is obviously very important in applications. This paper is an introduction to image registration as applied to medical imaging. We first define image registration, breaking the problem down into its constituent component. We then discuss various techniques, reflecting different choices that can be made in developing an image registration technique. We conclude with a brief discussion.

1. Introduction

1.1. Background. To register two images means to align them, so that common features overlap and differences, should there be any, between the two are emphasized and- readily visible to the naked eye. We refer to the process of aligning two images as image registration.

There are a host of clinical applications requiring image registration. For example, one would like to compare two Computed Tomography (CT) scans of a patient, taken say six months ago and yesterday, and identify differences between the two, e.g., the growth of a tumor during the intervening six months (Figure 1). One could also want to align Positron Emission Tomography (PET) data to an MR image, so as to help identify the anatomic location of certain mental activation [43]. And one may want to register lung surfaces in chest Computed Tomography (CT) scans for lung cancer screening [7]. While all of these identifications can be done in the radiologist's head, the possibility always exists that small, but critical, features could be missed. Also, beyond identification itself, the extent of alignment required could provide important quantitative information, e.g., how much a tumor's volume has changed.