  This paper proposes a robust longitudinal registration method for Contrast
Enhanced Spectral Mammography in monitoring neoadjuvant chemotherapy. Because
breast texture intensity changes with the treatment, a non-rigid registration
procedure with local intensity compensations is developed. The approach allows
registering the low energy images of the exams acquired before and after the
chemotherapy. The measured motion is then applied to the corresponding
recombined images. The difference of registered images, called residual, makes
vanishing the breast texture that did not changed between the two exams.
Consequently, this registered residual allows identifying local density and
iodine changes, especially in the lesion area. The method is validated with a
synthetic NAC case where ground truths are available. Then the procedure is
applied to 51 patients with 208 CESM image pairs acquired before and after the
chemotherapy treatment. The proposed registration converged in all 208 cases.
The intensity-compensated registration approach is evaluated with different
mathematical metrics and through the repositioning of clinical landmarks (RMSE:
5.9 mm) and outperforms state-of-the-art registration techniques.
