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Fast block flow tracking

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Fast block flow tracking of atrial septal defects in 4D echocardiographyIntroductionMethodsBlock matchingOptical flowBlock matching-optical flowBlock flowResultsIn vivo animal resultsError propagationClinical resultsSynthetic dataDiscussionAcknowledgementsReferencesFast block flow tracking of atrial septal defects in 4D echocardiographyMarius George Lingurarua,*, Nikolay V. Vasilyevb, Gerald R. Marxc, Wayne Tworetzkyc,Pedro J. Del Nidob, Robert D. HoweaaDivision of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USAbDepartment of Cardiac Surgery, Children’s Hospital, Harvard Medical School, Boston, MA, USAcDepartment of Cardiology, Children’s Hospital, Harvard Medical School, Boston, MA, USAReceived 6 November 2006; received in revised form 17 December 2007; accepted 21 December 2007Available online 17 January 2008AbstractWe are working to develop beating-heart atrial septal defect (ASD) closure techniques using real-time 3D ultrasound guidance. Themajor image processing challenges are the low-image quality and the processing of information at high-frame rate. This paper presentscomparative results for ASD tracking in time sequences of 3D volumes of cardiac ultrasound. We introduce a block flow technique,which combines the velocity computation from optical flow for an entire block with template matching. Enforcing adapted similarityconstraints to both the previous and first frames ensures optimal and unique solutions. We compare the performance of the proposedalgorithm with that of block matching and region-based optical flow on eight in vivo 4D datasets acquired from porcine beating-heartprocedures. Results show that our technique is more stable and has higher sensitivity than both optical flow and block matching in track-ing ASDs. Computing velocity at the block level, our technique tracks ASD motion at 2 frames/s, much faster than optical flow andcomparable in computation cost to block matching, and shows promise for real-time (30 frames/s). We report consistent results on clin-ical intra-operative images and retrieve the cardiac cycle (in ungated images) from error analysis. Quantitative results are evaluated onsynthetic data with maximum tracking errors of 1 voxel.Ó 2008 Elsevier B.V. All rights reserved.Keywords: Real-time ultrasound; Echocardiography; Atrial septal defect; Tracking; Mutual information; Block matching; Optical flow; Block flow1. IntroductionAtrial septal defects (ASD) are congenital heart malfor-mations consisting of openings in the septum between theatria. This allows blood to shunt from the left atrium intothe right atrium, which decreases the efficiency of heartpumping. Secundum-type ASD has been reported toaccount for up to 15% of congenital heart malformations(Benson and Freedom, 1992). Although the surgical inter-vention for ASD closure is well established and has excellentprognosis, it is performed under cardiopulmonary bypass(CPB), which has widely acknowledged harmful effects.Recent studies have highli ghted the practicability of min-imally invasive image-guided beating-heart ASD closure.The patient rehabilitation is improved by avoiding the useof CPB. A compelling review of image-guided surgicalapplications can be found in Peters (2006). There are twoalternatives for minimally invasive ASD closure proce-dures: one using a catheter-based closure, usually undercontrast-enhanced fluoroscopy (Faella et al., 2003; Papado-poulou et al., 2005); and another using rigid instrumentsthrough the chest wall (Suematsu et al., 2004), demon-strated in animals. Although clinically available, the cathe-ter-based procedure has major disadvantages: it can only beused on a fraction of ASD (Podnar et al., 2001), it excludesprocedures on small children (Patel et al., 2006 ), and it isgenerally performed under a high X-ray dose (Papadopou-lou et al., 2005). ASD closure studies using rigid instru-ments showed the feasibility of the procedures and1361-8415/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.media.2007.12.005*Corresponding author. Present address: National Institutes of Health,Diagnostic Radiology Department, Bethesda, MD 20892, USA. Tel.: +1301 4965283.E-mail address: [email protected] (M.G. Linguraru).www.elsevier.com/locate/mediaAvailable online at www.sciencedirect.comMedical Image Analysis 12 (2008) 397–412highlighted their limitations (Downing et al., 2002; Suema-tsu et al., 2005). Amongst the latter, the difficult visualiza-tion of surgical instruments, limited spatial resolution ofUS imaging, and the large size of the US probe, make theclinical applicability of beating-heart surgery difficult. Thereliable visualization of structures within the heart remainsanother major challenge to successful mini mally-invasivesurgical interventions (Cannon et al., 2003; Suematsuet al., 2004).Recent advances in ultrasound (US) imaging make thisvisualization modality an ideal candidat e for image-guidedinterventions. 4D US is simple, cheap and fast, and allowsthe surgeon to visualize cardiac structures and instrumentsthrough the blood pool. US also has some major disadvan-tages, being extremely noisy with poor shape definition,which makes it confusing and hard to interpret in the oper-ation room. Tracking tissue in 3D US volumes is particu-larly difficult due to the low spatial resolution caused byinterpolation and resampling in image reconstruction (Fen-ster et al., 2001). Therefore, the development of trackingmethods for volumetric data in 4D applications is neces-sary to assist clinical procedures. In ASD closure proce-dures, ASD tracking in US images is desirable to guideeither the rigid instruments or the catheter.A 3D image of an ASD and its position in the heart isshown in Fig. 1. The US probe is placed on the exteriorwall of the right atrium, as exemplified in Fig. 2. Thedynamic nature of ASD is primarily determined by the car-diac cycle with a mean area change of 61% between end-diastolic and end-systolic ( Maeno et al., 2000). The changein size is highly variable and present measurements showsubjectivity (Handke et al., 2001; Maeno et al., 2000; Pod-nar et al., 2001). There is little to no correlation betweenthe dynamic changes of ASD and its size, heart rate orage of patient. The motion of the US transducer and thelow image quality contribute to the challenges of ASDtracking.A number of approaches to estimate the mo tion of car-diac tissue have been tried. They could be separated


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