The present invention provides a method of registering image data for a series of images acquired over a period of time. The images may be acquired from a medical imaging device, such as an MRI scanner. The method allows image-by-image registration to a partially compressed dataset in which motion artifacts are suppressed in a series of target images generated to resemble the original dynamic data. The registration process is refined with each iteration of the process. The method uses Principal Component Analysis which allows accurate registration to be performed without the need to make any assumptions about pharmacokinetic model parameters, which not only avoids the problems of accurate model-fitting but permits the use of much larger target areas of the dataset.