• D Papp, S J Breda, OeiE H GEHGDepartment of Radiology & Nuclear Medicine, Erasmus MC University Medical Center, Doctor Molewaterplein 40, 3015 GD Rotterdam, The Netherlands., PootD H JDHJDepartment of Radiology & Nuclear Medicine, Erasmus MC University Medical Center, Doctor Molewaterplein 40, 3015 GD Rotterdam, The Netherlands., G Kotek, and J A Hernandez-Tamames.
• Department of Radiology & Nuclear Medicine, Erasmus MC University Medical Center, Doctor Molewaterplein 40, 3015 GD Rotterdam, The Netherlands. Electronic address: d.papp@erasmusmc.nl.
• Magn Reson Imaging. 2020 Jul 1; 70: 91-97.

PurposeQuantification of the T2∗ relaxation time constant is relevant in various magnetic resonance imaging applications. Mono- or bi-exponential models are typically used to determine these parameters. However, in case of complex, heterogeneous tissues these models could lead to inaccurate results. We compared a model, provided by the fractional-order extension of the Bloch equation with the conventional models.MethodsAxial 3D ultra-short echo time (UTE) scans were acquired using a 3.0 T MRI and a 16-channel surface coil. After image registration, voxel-wise T2∗ was quantified with mono-exponential, bi-exponential and fractional-order fitting. We evaluated all three models repeatability and the bias of their derived parameters by fitting at various noise levels. To investigate the effect of the SNR for the different models, a Monte-Carlo experiment with 1000 repeats was performed for different noise levels for one subject. For a cross-sectional investigation, we used the mean fitted values of the ROIs in five volunteers.ResultsComparing the mono-exponential and the fractional order T2∗ maps, the fractional order fitting method yielded enhanced contrast and an improved delineation of the different tissues. In the case of the bi-exponential method, the long T2∗ component map demonstrated the anatomy clearly with high contrast. Simulations showed a nonzero bias of the parameters for all three mathematical models. ROI based fitting showed that the T2∗ values were different depending on the applied method, and they differed most for the patellar tendon in all subjects.ConclusionsIn high SNR cases, the fractional order and bi-exponential models are both performing well with low bias. However, in all observed cases, one of the bi-exponential components has high standard deviation in T2∗. The bi-exponential model is suitable for T2∗ mapping, but we recommend using the fractional order model for cases of low SNR.Copyright © 2020 Elsevier Inc. All rights reserved.

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