  This paper presents a novel algorithm named Direct Simultaneous Registration
(DSR) that registers a collection of 3D images in a simultaneous fashion
without specifying any reference image, feature extraction and matching, or
information loss or reuse. The algorithm optimizes the global poses of local
image frames by maximizing the similarity between a predefined panoramic image
and local images. Although we formulate the problem as a Direct Bundle
Adjustment (DBA) that jointly optimizes the poses of local frames and the
intensities of the panoramic image, by investigating the independence of pose
estimation from the panoramic image in the solving process, DSR is proposed to
solve the poses only and proved to be able to obtain the same optimal poses as
DBA. The proposed method is particularly suitable for the scenarios where
distinct features are not available, such as Transesophageal Echocardiography
(TEE) images. DSR is evaluated by comparing it with four widely used methods
via simulated and in-vivo 3D TEE images. It is shown that the proposed method
outperforms these four methods in terms of accuracy and requires much fewer
computational resources than the state-of-the-art accumulated pairwise
estimates (APE).
