Mutual information-based multimodality registration of cardiac ultrasound and SPECT images: a preliminary investigation

Background: Ultrasound (US) and single photon emission computed tomography (SPECT) are the two most commonly prescribed procedures for diagnosing coronary artery disease (CAD). We have demonstrated the feasibility of multimodality registration of two-dimensional (2D) and three-dimensional (3D) cardiac US images with cardiac SPECT images with an aim to simultaneously present the complementary anatomical and perfusion information from the two modalities. We have also tested the clinicians' assessment of the clinical adequacy of the registered images. Methods and Results: We have demonstrated temporal and spatial registration for nine sets of cardiac US and SPECT cine loops covering the entire cardiac cycle. Temporal alignment was performed by interpolation of existing SPECT images at cardiac phases corresponding to available US images. Spatial registration was performed in 3D image space using a mutual information-based approach. Experts from echocardiography and nuclear medicine determined the clinical utility of the registration by rating each registration on a scale of 1 to 5, a rating of 3 or above indicating clinical utility. 2DUS-SPECT registration (five cases) received an average rating of 4.2, whereas 3DUS-SPECT registration (four cases) received an average rating of 2.85. By one-sample t test, the overall evaluations (mean 3.58) were greater than the pre-specified clinical cut-off of 3 with p < 0.05, indicating likelihood of clinical utility. Conclusion: Our method demonstrates the feasibility of registering cardiac US and SPECT images in their present as well as possible future forms. Such registration has the potential to provide a more accurate and powerful tool for diagnosing CAD.