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Purpose: Simultaneous PET‐MRI improves inflammatory cardiac disease diagnosis. However, challenges persist in respiratory motion and mis‐registration between free‐breathing 3D PET and 2D breath‐held MR images. We propose a free‐breathing non‐rigid motion‐compensated 3D T2‐mapping sequence enabling whole‐heart myocardial tissue characterization in a hybrid 3T PET‐MR system and provides non‐rigid respiratory motion fields to correct also simultaneously acquired PET data. Methods: Free‐breathing 3D whole‐heart T2‐mapping was implemented on a hybrid 3T PET‐MRI system. Three datasets were acquired with different T2‐preparation modules (0, 28, 55 ms) using 3‐fold undersampled variable‐density Cartesian trajectory. Respiratory motion was estimated via virtual 3D image navigators, enabling multi‐contrast non‐rigid motion‐corrected MR reconstruction. T2‐maps were computed using dictionary‐matching. Approach was tested in phantom, 8 healthy subjects, 14 MR only and 2 PET‐MR patients with suspected cardiac disease and compared with spin echo reference (phantom) and clinical 2D T2‐mapping (in‐vivo). Results: Phantom results show a high correlation (R2 = 0.996) between proposed approach and gold standard 2D T2 mapping. In‐vivo 3D T2‐mapping average values in healthy subjects (39.0 ± 1.4 ms) and patients (healthy tissue) (39.1 ± 1.4 ms) agree with conventional 2D T2‐mapping (healthy = 38.6 ± 1.2 ms, patients = 40.3 ± 1.7 ms). Bland–Altman analysis reveals bias of 1.8 ms and 95% limits of agreement (LOA) of −2.4‐6 ms for healthy subjects, and bias of 1.3 ms and 95% LOA of −1.9 to 4.6 ms for patients. Conclusion: Validated efficient 3D whole‐heart T2‐mapping at hybrid 3T PET‐MRI provides myocardial inflammation characterization and non‐rigid respiratory motion fields for simultaneous PET data correction. Comparable T2 values were achieved with both 3D and 2D methods. Improved image quality was observed in the PET images after MR‐based motion correction. [ABSTRACT FROM AUTHOR]

Purpose: Simultaneous PET-MRI improves inflammatory cardiac disease diagnosis. However, challenges persist in respiratory motion and mis-registration between free-breathing 3D PET and 2D breath-held MR images. We propose a free-breathing non-rigid motion-compensated 3D T 2 -mapping sequence enabling whole-heart myocardial tissue characterization in a hybrid 3T PET-MR system and provides non-rigid respiratory motion fields to correct also simultaneously acquired PET data., Methods: Free-breathing 3D whole-heart T 2 -mapping was implemented on a hybrid 3T PET-MRI system. Three datasets were acquired with different T 2 -preparation modules (0, 28, 55 ms) using 3-fold undersampled variable-density Cartesian trajectory. Respiratory motion was estimated via virtual 3D image navigators, enabling multi-contrast non-rigid motion-corrected MR reconstruction. T 2 -maps were computed using dictionary-matching. Approach was tested in phantom, 8 healthy subjects, 14 MR only and 2 PET-MR patients with suspected cardiac disease and compared with spin echo reference (phantom) and clinical 2D T 2 -mapping (in-vivo)., Results: Phantom results show a high correlation (R 2  = 0.996) between proposed approach and gold standard 2D T 2 mapping. In-vivo 3D T 2 -mapping average values in healthy subjects (39.0 ± 1.4 ms) and patients (healthy tissue) (39.1 ± 1.4 ms) agree with conventional 2D T 2 -mapping (healthy = 38.6 ± 1.2 ms, patients = 40.3 ± 1.7 ms). Bland-Altman analysis reveals bias of 1.8 ms and 95% limits of agreement (LOA) of -2.4-6 ms for healthy subjects, and bias of 1.3 ms and 95% LOA of -1.9 to 4.6 ms for patients., Conclusion: Validated efficient 3D whole-heart T 2 -mapping at hybrid 3T PET-MRI provides myocardial inflammation characterization and non-rigid respiratory motion fields for simultaneous PET data correction. Comparable T 2 values were achieved with both 3D and 2D methods. Improved image quality was observed in the PET images after MR-based motion correction., (© 2024 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)