Multimodal image registration using IECC as the similarity measure

Purpose: The registration of images from positron emission tomography(PET) to those from magnetic resonance imaging(MRI) using mutual information is usually effective, but fails occasionally because of small region of overlap, low-activity defects in the PETimage, difference in spatial resolution, etc. In this article, the authors propose the pixel-based individual entropy correlation coefficient (IECC) as a new, more accurate and more robust registration criterion. Methods: The authors compare it to the current criteria: Mutual information (MI), normalized mutual information (NMI), and the entropy correlation coefficient (ECC). The anatomical region to be registered was the head. A rigid-body registration was used; no deformation was employed. The authors established the effectiveness of IECC by both simulated data and clinical studies using brain fluorodeoxyglucose (FDG) PET and MRI. Both a normal-activity FDG model and a FDG model with a perfusion defect were used for the PETimage.Reconstruction by both filtered backprojection and by ordered subset-expectation maximization was investigated. Results: The mean errors and SDs of IECC were 1.17 ± 0.85 mm for translation and 1.04 ± 1.28 ° for rotation in clinical PET. Those of MI, NMI, and ECC were 1.86 ± 1.22 , 1.86 ± 0.96 , and 1.68 ± 1.05 mm for translations and 1.52 ± 1.84 ° , 1.74 ± 1.68 ° , and 1.70 ± 1.90 ° for rotations. The mean errors and SDs of IECC were 1.56 ± 0.58 mm for translation and 1.46 ± 1.66 ° for rotation in clinical PETmodel with a 30% perfusion defect. Those of MI, NMI, and ECC were 2.81 ± 1.41 , 2.98 ± 1.80 , and 3.29 ± 2.08 mm for translations and 3.34 ± 3.80 ° , 2.87 ± 3.25 ° , and 3.04 ± 3.44 ° for rotations. The mean errors and SDs of IECC were 1.79 ± 1.04 mm for translation and 1.64 ± 1.62 ° for rotation in clinical PETmodel with a 50% perfusion defect. Those of MI, NMI, and ECC were 3.49 ± 1.92 , 3.57 ± 2.22 , and 3.49 ± 1.89 mm for translations and 4.12 ± 4.24 ° , 3.62 ± 4.87 ° , and 3.44 ± 3.80 ° for rotations. The significant differences between IECC and each of the other three criteria were found for displacement misregistration in almost all parameters ( p 0.01 ) . Conclusions: Accuracy of the IECC criterion was higher than that of the other criteria, usually in a statistically significant way.