Research project

Cardiac C-arm CT is a an imaging technique under development that combines 3D cardiac image acquisition and real-time fluoroscopy on the same system. The combination (hybrid imaging system) combines the advantages of cardiac CT that is already used in 3D/4D imaging of the heart and C-arm systems that are commonly used during interventions because of their real-time projection mode, and high spatial resolution for 2D imaging. For cardiac CT imaging an ECG-gated angiographic X-ray image sequence of a specific heart phase is required to reconstruct a high quality 3D heart image. Retrospective ECG gating is an established technique used in cardiac CT systems to provide the reconstruction algorithm with X-ray images of the heart in the phase corresponding to the phase that is reconstructed. This requires a high temporal resolution and places therefore stringent requirements on the hardware for C-arm CT which are by current hardware implementations not fulfilled. In order to approach the temporal resolution of clinical CT with C-arm CT hardware, we are investigating novel image processing algorithms for non-parametric heart motion modeling to correct the data used in the image reconstruction process to reduce motion blurring.
Efficient Motion Correction for 4D-FBP
To increase temporal resolution we compute the patients’ individual 4D heart deformation based on a sequence of initial retrospectively ECG-gated FDK reconstructions, for several different cardiac phases. A standard non-rigid registration technique is applied to compute the 4D motion vector field (MVF). In this work we developed an approximative FDK-like algorithm (FDK- 4D) to reconstruct dynamic objects. The principle idea of our approach is a temporally dependent spatial warping of the filtered back-projections according to the subjects’ individual MVF. It is an approximate scheme for a FBP along curved rays determined by the subjects’ motion. It is computationally efficient since the only additional cost during the motion corrected back-projection is the spatial warping using e.g. trilinear interpolation of the FBPs according to the MVF.

SNR Enhancement by Retrospectively Motion Corrected FDK-like Reconstructions
Retrospective ECG gating techniques have already been adapted from clinical cardiac CT that allow 3D reconstruction using retrospectively gated projection images of a multi-sweep C-arm CT scan according to the desired cardiac phase. However, it is known that retrospective gating of projection data does not provide an optimal signal-to-noise-ratio (SNR) since the measured projection data is only partially considered during the reconstruction. In this work we introduce a new reconstruction technique for cardiac C-arm CT that provides increased SNR by including additional corrected and resampled filtered back-projections (FBP) from temporal windows outside of the targeted reconstruction phase.
4D Non-Model based Heart Motion Estimation
In order to minimize blurring and motion artifacts, cardiac motion has to be compensated for, which can be achieved using a temporally dependent spatial 3D warping of the filtered-backprojections. In this work we investigate the computation of the 4D heart motion based on prior reconstructions of several cardiac phases using standard retrospectively ECG-gated FDK (RG-FDK) reconstruction. A 4D motion estimation framework is presented using standard fast non-rigid registration. A smooth 4D motion vector field (MVF) represents the relative deformation compared to a reference cardiac phase. A 4D deformation regridding by adaptive supersampling allows selecting any reference phase independently of the set of phases used in the RG-FDK for a motion corrected reconstruction. Initial promising results from in vivo experiments are shown.

4D Heart Motion Modeling and Evaluation Study
In order to approach the temporal resolution of clinical CT with C-arm CT hardware, we are investigating novel image processing algorithms for non-parametric heart motion modeling to correct the data used in the image reconstruction process to reduce motion blurring. 3D ultrasound systems are used for evaluation of the reliability of the different non-parametric heart motion models based on real cardiac data. Such systems allow the quantification of the true heart motion.