MRI-Based Nonrigid Motion Correction in Simultaneous PET/MRI Se Young Chun 1,3 , Timothy G. Reese 2,3 , Jinsong Ouyang 1,3 , Bastien Guerin 1,3 , Ciprian Catana 2,3 , Xuping Zhu 1,3 , Nathaniel M. Alpert 1,3 , and Georges El Fakhri 1,3 1 Center for Advanced Radiological Sciences, Nuclear Medicine and Molecular Imaging, Radiology Department, Massachusetts General Hospital, Boston, Massachusetts; 2 A.A. Martinos Center, Radiology Department, Massachusetts General Hospital, Boston, Massachusetts; and 3 Harvard Medical School, Boston, Massachusetts Respiratory and cardiac motion is the most serious limitation to whole-body PET, resulting in spatial resolution close to 1 cm. Furthermore, motion-induced inconsistencies in the attenuation measurements often lead to significant artifacts in the recon- structed images. Gating can remove motion artifacts at the cost of increased noise. This paper presents an approach to respiratory motion correction using simultaneous PET/MRI to demonstrate initial results in phantoms, rabbits, and nonhuman primates and discusses the prospects for clinical application. Methods: Studies with a deformable phantom, a free-breathing primate, and rabbits implanted with radioactive beads were performed with simultaneous PET/MRI. Motion fields were estimated from concurrently acquired tagged MR images using 2 B-spline nonrigid image registration methods and incorpo- rated into a PET list-mode ordered-subsets expectation maximization algorithm. Using the measured motion fields to transform both the emission data and the attenuation data, we could use all the coincidence data to reconstruct any phase of the respiratory cycle. We compared the resulting SNR and the channelized Hotelling observer (CHO) detection signal- to-noise ratio (SNR) in the motion-corrected reconstruction with the results obtained from standard gating and uncorrected studies. Results: Motion correction virtually eliminated motion blur without reducing SNR, yielding images with SNR compa- rable to those obtained by gating with 5–8 times longer acquisi- tions in all studies. The CHO study in dynamic phantoms demonstrated a significant improvement (166%–276%) in lesion detection SNR with MRI-based motion correction as compared with gating (P , 0.001). This improvement was 43%–92% for large motion compared with lesion detection without motion correction (P , 0.001). CHO SNR in the rabbit studies confirmed these results. Conclusion: Tagged MRI mo- tion correction in simultaneous PET/MRI significantly improves lesion detection compared with respiratory gating and no mo- tion correction while reducing radiation dose. In vivo primate and rabbit studies confirmed the improvement in PET image quality and provide the rationale for evaluation in simultaneous whole-body PET/MRI clinical studies. Key Words: lesion detection; motion correction; PET/MRI J Nucl Med 2012; 53:1284–1291 DOI: 10.2967/jnumed.111.092353 The intrinsic spatial resolution of modern whole-body PET scanners is in the range of 4–7 mm in full width at half maximum for stationary objects (1,2). However, because of the inevitable respiratory and cardiac motion, this resolu- tion cannot be achieved in clinical imaging of the chest or abdomen, where the effective resolution becomes close to 11 mm (3). Therefore, many whole-body clinical PET stud- ies may not benefit from state-of-the-art PET if motion blurring is not corrected. Organs change location, shape, or local tissue density as they move, and complex nonrigid movement of heart muscle, lung, or abdominal organs results in image blurring (4). In such cases, coincidence data are also inconsistent with the attenuation measurement. Even if the CT-based attenuation data have negligible degradation due to move- ment, the attenuation correction is valid only for emission data acquired under the same stationary state as CT. Such inconsistencies between emission and attenuation data can lead to confusing artifacts (e.g., liver “banana artifacts”) in the reconstructed images (5). The resulting image deg- radation decreases detection of lesions (6). Even when lesions are detected, their radioactivity concentration will be reduced, and apparent standardized uptake value is well below the true standardized uptake value because of motion blurring (6,7). There has been considerable research to reduce motion artifacts due to the movement of internal organs. Inves- tigators have tried to freeze motion by exploiting the near- periodic heart and lung motion using physiologic gating. In this context, a physiologic event such as end-inspiration (respiratory) or the QRS complex of the electrocardiogram (cardiac) is used to mark the beginning of each cycle. The respiratory cycle is typically divided into equal time bins, and data are acquired over many cycles. We will refer to these bins as timing bins to emphasize that their endpoints Received Apr. 26, 2011; revision accepted Apr. 12, 2012. For correspondence or reprints contact: Georges El Fakhri, Center for Advanced Radiological Sciences, Radiology/Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital, 55 Fruit St., Boston, MA 02114. E-mail: elfakhri@pet.mgh.harvard.edu Published online Jun. 28, 2012. COPYRIGHT ª 2012 by the Society of Nuclear Medicine and Molecular Imaging, Inc. 1284 THE JOURNAL OF NUCLEAR MEDICINE • Vol. 53 • No. 8 • August 2012 by on May 20, 2020. For personal use only. jnm.snmjournals.org Downloaded from