The images acquired as part of a diffusion protocol suffer from geometric distortions. Some of these distortions are common to all images in the set, and lead to loss of anatomic fidelity and systematic bias in tractography. Other distortions are unique to that particular diffusion gradient and will lead to decreased precision of estimates of magnitude and main direction of diffusion. This chapter discusses the mechanisms that cause the distortions and ways to correct them. It reviews both methods that attempt to measure the off-resonance fields that cause the distortions and methods that...

The images acquired as part of a diffusion protocol suffer from geometric distortions. Some of these distortions are common to all images in the set, and lead to loss of anatomic fidelity and systematic bias in tractography. Other distortions are unique to that particular diffusion gradient and will lead to decreased precision of estimates of magnitude and main direction of diffusion. This chapter discusses the mechanisms that cause the distortions and ways to correct them. It reviews both methods that attempt to measure the off-resonance fields that cause the distortions and methods that attempt to calculate them from the distorted images using image registration techniques.