Initial comparison of registration and fusion of SPECT-CmT mammotomography images

Abstract

A hybrid, dual modality single photon emission computed tomography (SPECT) and x-ray computed mammotomography (CmT) scanner for dedicated breast and axillary imaging is under development. CmT imaging provides high resolution anatomical images, whereas SPECT provides functional images albeit with coarser resolution. As is being seen clinically in whole body imaging, integration of the images is expected to enhance (visually) and improve (with attenuation correction of SPECT) information provided by either modality for the detection, characterization and potentially staging of breast cancer. The registration of these images considers variations in object positions between the different modalities and imaging parameters (pixel size, conditions of acquisition, scan limitations). Automatic methods can be used which find the geometric transformations of the different imaging modalities involved. Here we demonstrate the initial stages of iterative 2-dimensional registration and fusion of SPECT with parallel beam geometry and CmT with offset cone-beam acquisition geometry for mammotomography with images acquired and reconstructed independently on each system. Two registration algorithms are considered: the first is an intrinsic correlation, Mutual Information (MI) method based on intrinsic image content; the second is a rigid body transform method, Iterative Closest Point (ICP) method based on identification of fiducial markers visible to both emission (SPECT) and transmission (CmT) imaging modalities. Experiments include use of a geometric resolution/frequency phantom imaged under different conditions, and two different anthropomorphic breast phantom sizes (325 and 935mL). Initial results with the geometric phantom demonstrate that MI can be misled by highly symmetric features, and ICP using control points is more accurate to within fractions of a voxel. Initial breast phantom studies indicate that object size and SPECT resolution limitations may contribute to registration errors.