Dept. of Computer Science » Pattern Recognition » Our Team » Prümmer, Marcus 

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. 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.

Cooperation Partners:

Department of Radiology, Stanford University.

Siemens AG, Medical Solutions, Forchheim, Germany.

Cardiac C-arm CT is a promising technique that enables 3D cardiac image acquisition and real-time fluoroscopy on the same system. The goal is to bring 3D imaging to the interventional suite for improved therapy planning, guiding, and monitoring. For the reconstruction of 3D cardiac image data, a complete set of projections from a specific heart phase is required. One approach to reduce motion blurring caused by the beating heart is to acquire multiple sweeps using the C-arm and retrospectively select the projections (RECG) that are closest to the desired cardiac phase [1]. In order to further improve the temporal resolution, novel image processing algorithms that utilize retrospective motion correction were investigated in this project. The main focus of this work is to extend the well established FDK algorithm to incorporate motion correction during the back-projection step using a pre-computed motion field. In our experiments we investigated the following two scenarios: (i) Can the image quality from a single sweep be improved given a known motion field? (ii) Can improved image quality be achieved using a lower number of sweeps in combination with motion correction?

Cardiac C-arm CT protocols are, among other things, constrained by the rotation speed and frame rate of the C-arm and detector, the breath hold duration, overall injected dose of contrast agent, scan synchronization with the ECG and X-ray dose. First empirical studies show that protocols of four sweeps are reasonable for images acquired in diastoly, but temporal resolution can still be improved because of heart rate variations during the scan. We have shown that increased temporal resolution can be achieved using a first order motion estimation via 3D-3D non-rigid registration applied on a pure RECG reconstructed time series, that is blurred and has motion artifact, with correction applied in an extended FDK (FDK-4D) algorithm. For scenario (i) we showed that assuming a 4D motion field is given, the FDK-4D algorithm is able to decrease motion blurring significantly using only one single sweep for the reconstruction. For scenario (ii) we showed that even for an insufficient ECG synchronization using three and four sweeps, the reconstructed images of the diastolic and systolic phase are less blurred using FDK-4D compared to pure RECG. In conclusion, increasing temporal resolution using an estimated 4D motion field in the FDK-4D algorithm can decrease motion blurring substantially.

This work was headed by Rebecca Fahrig and supported by Siemens AG, Medical Solutions, NIH grant R01 EB 003524 and by the Lucas Foundation.

In principal, to reconstruct a 3D-image of the beating heart at a particular cardiac phase, a complete set of X-ray projection data representing that phase isrequired. One approximate approach is the retrospectively ECG-gated FDK reconstruction (RG-FDK). From the acquireddata set of Ns multiple C-arm sweeps, those projection images which are acquired closest in time to the desired cardiacphase are retrospectively selected. However, this approach uses only 1/Ns of the obtained data. Our goal is to utilize data fromother cardiac phases as well. 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 weinvestigate the computation of the 4D heart motion based on prior reconstructions of several cardiac phases using RG-FDK. A 4D motion estimation framework is presented using standard fast non-rigid registration. A smooth 4D motion vectorfield (MVF) represents the relative deformation compared to a reference cardiac phase. A 4D deformation regridding byadaptive supersampling allows selecting any reference phase independently of the set of phases used in the RG-FDK fora motion corrected reconstruction. Initial promising results from in vivo experiments are shown. The subjects individual4D cardiac MVF could be computed from only three RG-FDK image volumes. In addition, all acquired projection datawere motion corrected and subsequently used for image reconstruction to improve the signal-to-noise ratio compared to RG-FDK.

Applying the concept of electrocardiogram gating (ECG) during the acquisition of multiple,serial, backward and forward, ECG-triggered rotational acquisitions using a C-arm system allows the 3D+t reconstructionof the heart. The process of retrospective gating is a crucial component of 3-D reconstruction. The gold-standard is still given by the ECG signal. However, an alternative gating method, based on the acquired projection data is required. Our goal is to provide an image-based gating method without ECG.

In this work, we developed a multi-modal non-rigid 2D-3D registration technique. This method allows a non-rigid alignment of a patient pre-operatively computed tomography (CT) to few intra operatively acquired fluoroscopic X-ray images obtained with a C-arm system. This multi-modal approach is especially focused on the 3D alignment of high contrast reconstructed volumes with intra-interventional low contrast X-ray images in order to makeuse of up-to-date information for surgical guidance and other interventions.