Robust Spatio-Temporal Registration of 4D Cardiac Ultrasound Sequences

Abstract

Registration of multiple 3D ultrasound sectors in order to provide an extended field of view is important for the appreciation of larger anatomical structures at high spatial and temporal resolution. In this paper, we present a method for fully automatic spatio-temporal registration between two partially overlapping 3D ultrasound sequences. The temporal alignment is solved by aligning the normalized cross correlation-over-time curves of the sequences. For the spatial alignment, corresponding 3D Scale Invariant Feature Transform (SIFT) features are extracted from all frames of both sequences independently of the temporal alignment. A rigid transform is then calculated by least squares minimization in combination with random sample consensus. The method is applied to 16 echocardiographic sequences of the left and right ventricles and evaluated against manually annotated temporal events and spatial anatomical landmarks. The mean distances between manually identified landmarks in the left and right ventricles after automatic registration were (mean ± SD) 4.3 ± 1.2 mm compared to a reference error of 2.8 ± 0.6 mm with manual registration. For the temporal alignment, the absolute errors in valvular event times were 14.4 ± 11.6 ms for Aortic Valve (AV) opening, 18.6 ± 16.0 ms for AV closing, and 34.6 ± 26.4 ms for mitral valve opening, compared to a mean inter-frame time of 29 ms.

Figure 1

The normalized cross-correlation over time for a reference (dashed) and floating sequence before (dotted) and after (solid) temporal alignment (a), resulting temporal registration (b).

Figure 2

Correspondence density of feature matching between all reference and floating frames in one case. The size of each circle is proportional to the number of correspondences for a single frame pair. The black lines are contour lines of the time displacement, in milliseconds, after temporal alignment, illustrating the deformable temporal alignment.

Figure 3

Resulting rigid registration for three sequence pairs across the cardiac cycle. Each row shows three frames from a single case throughout the cardiac cycle.