Original Research Extended Harmonic Phase Tracking of Myocardial Motion: Improved Coverage of Myocardium and Its Effect on Strain Results Sandra R.R. Tecela ˜o, MSc, 1,2 * Jaco J.M. Zwanenburg, PhD, 2 Joost P.A. Kuijer, PhD, 2 and J. Tim Marcus, PhD 2 Purpose: To extend the harmonic phase (HARP) tracking method in order to track the myocardial tissue that ap- pears near the epicardial contour during systole and reap- pears near the endocardial contour during diastole, due to the longitudinal motion and conical shape of the heart. Materials and Methods: A mathematical model of myocar- dial deformation was used to quantify the accuracy of the extended HARP tracking and of the strain computation. For six healthy volunteers, the number of tracked points and the two-dimensional strain components were computed with the extended and with the original HARP tracking version. Results: High accuracy was obtained for the circumferential strain (maximum error is 0.5% relative to analytical strain). The extended version tracked 22  7%, 51  19%, and 67  20% more points than the original version on the basal, mid, and apical slices, respectively (P  0.001 for each slice), and yielded a decreased circumferential shortening (relative de- crease: 2  4%, 9  4%, and 12  5% for the three slices; P  0.005 for mid and apex), at end systole. These differences in circumferential strain were related to the more complete cov- erage of the myocardial wall with tracked points. Conclusion: The extended HARP tracking also provides strain values from myocardial regions that were not covered by the original HARP tracking. Key Words: cardiac imaging; myocardial tagging; harmonic phase; Hilbert transform; myocardial strain J. Magn. Reson. Imaging 2006;23:682– 690. © 2006 Wiley-Liss, Inc. SINCE THE FIRST articles on MRI and myocardial tag- ging (1,2) were published, myocardial strain analysis with MR has become a more common procedure. The postprocessing performed on the tagged images can go from simple visual inspection of the tag pattern defor- mation to the exhaustive computation of strain maps. Important efforts have been made during the past few years in the latter field. The published methods include active contour models (3), optical flow methods (4,5), template matching methods (6), and harmonic phase (HARP) MR (7). Due to its automatic nature, HARP tracking is a promising tool for clinical application. HARP tracking is a phase-sensitive method to deter- mine the tag line displacement by tracking its angle value over the cardiac cycle, normally starting at the time frame with an undeformed tag pattern. Due to the combination of the conical shape of the heart and its longitudinal motion, new tag lines appear during the systolic phase near the epicardial contour and other tag lines disappear near the endocardial con- tour. Since these new tag lines are not present during the time frame with the undeformed tag pattern, their angle values are not available and, consequently, these lines are not tracked. However, disregarding these tag lines can lead to systematic errors in the strain results, due to the transmural gradient in the strain (8). In this work, an extended HARP tracking method is proposed, which tracks the new tag lines that enter into the image plane during the systolic phase and recovers the tag lines that reappear at the endocardial contour during the diastolic phase. With this extended method, all parts of the myocardium are completely tracked dur- ing all phases of the cardiac cycle. In this study, the accuracy of the extended HARP tracking method is reported and the effect of this method on the strain results of human volunteers is quantified. MATERIALS AND METHODS Original HARP Tracking HARP tracking is an automatic phase-sensitive method to track the tag pattern in the myocardium. The spatial modulation of magnetization (SPAMM) technique (2) is 1 Institute of Biophysics and Biomedical Engineering, University of Lis- bon, Lisbon, Portugal. 2 Department of Physics and Medical Technology, VU University Medical Center, Amsterdam, The Netherlands. Contract grant sponsor: Calouste Gulbenkian Foundation; Contract grant sponsor: Portuguese Foundation for Science and Technology; Contract grant number: SFRH/BD/3005/2000; Contract grant sponsor: Nether- lands Heart Foundation; Contract grant number: 2000B220. *Address reprint requests to: S.R.R.T., Institute of Biophysics and Bio- medical Engineering, Faculdade de Cie ˆncias da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal. E-mail: stecelao@fc.ul.pt Received March 9, 2005; Accepted January 26, 2006. DOI 10.1002/jmri.20571 Published online 28 March 2006 in Wiley InterScience (www.interscience. wiley.com). JOURNAL OF MAGNETIC RESONANCE IMAGING 23:682– 690 (2006) © 2006 Wiley-Liss, Inc. 682