The present invention, in some embodiments thereof, relates to systems and/or methods for image registration and, more particularly, but not exclusively, to systems and/or methods for registration of ultrasound and Computed Tomography CT and/or Magnetic Resonance (MR) images.
Physicians may use several imaging modalities when examining the heart of a patient. Each imaging modality may provide better imaging detail of some tissue types and/or tissue functionalities, but poor imaging of other tissue types and/or functionalities. Therefore, several different modalities may be used to gain an overall clinical picture.
Ultrasound may be used to provide dynamic and/or functional information of the beating heart. Ultrasound may also provide images of the function of the cardiac valves. Computed tomography may provide detailed examination of the heart structure, for example, chambers, blood vessels, connective tissue and/or muscle tissues. However, due to limitations in radiation exposure of the patient, fewer samples may be acquired at different stages of the cardiac cycle, yielding high quality images at some time phases and low quality images at other time phases.
US and CT imaging modalities may be different in terms of, for example, image artifacts, intensity levels and/or spatial shape differences. The differences may be due to physical processes related to image formation of each modality. Ultrasound images are obtained by echo (i.e., reflection) of acoustic waves. CT images are obtained from X-rays, optionally accompanied by injected contrast agents. The differences in the image modalities raise difficulties in registering the US and CT images.
Various attempts have been made to align US and CT images, in order to fuse the information of both modalities and enable better diagnosis, follow up and visualization during treatment procedures. However, the differences in the modalities, for example, image artifacts, intensity levels, and spatial shape differences due to physics related to image formation in each modality, have made image registration challenging.
Huang et al., “Rapid dynamic image registration of the beating heart for diagnosis and surgical navigation.” IEEE Trans Med Imaging. 2009 November; 28(11):1802-14. discloses “ . . . a rapid two-step method for registering RT3D US to high-quality dynamic 3-D MR/CT images of the beating heart. This technique overcomes some major limitations of image registration (such as the correct registration result not necessarily occurring at the maximum of the mutual information (MI) metric) using the MI metric.”