Multi-Center, Multi-Vendor Reproducibility and Calibration of MRI-Based R2* for Liver Iron Quantification
Authors: Hernando Diego , Zhao Ruiyang , Yuan Qing , Aliyari Ghasabeh Mounes , Ruschke Stefan , Miao Xinran , Karampinos Dimitrios C. , Mao Lu , Harris David T. , Mattison Ryan J. , Jeng Michael R. , Pedrosa Ivan , Kamel Ihab R. , Vasanawala Shreyas , Yokoo Takeshi , Reeder Scott B. 
Published: November 2021

Abstract
Introduction: Excessive accumulation of iron is caused by a variety of conditions, including hereditary hemochromatosis and transfusional hemosiderosis. If untreated, iron overload can lead to damage in those organs where iron accumulates. Therefore, accurate and reproducible evaluation of body iron stores is needed to guide diagnosis, grading, and treatment monitoring of iron overload. While serum ferritin is the simplest means to assess body iron, it is also an acute phase reactant and therefore is not a reliable biomarker of body iron. Liver iron concentration (LIC) is directly and linearly related to total body iron stores. As such, LIC is widely recognized as a useful surrogate biomarker for the evaluation of iron overload. Liver biopsy is limited by its invasive nature and is contraindicated in many patients (eg. thrombocytopenia) due to bleeding risk. Magnetic resonance imaging (MRI) is a standard of care tool to measure LIC. Arguably the most practical method is R2* MRI due to its speed and ease of use, but the cross-vendor reproducibility of R2*-based LIC estimation remains unknown. Therefore, we evaluated the reproducibility and calibration of R2*-based LIC measurement via a single-breath-hold, confounder-corrected R2*-MRI at both 1.5T and 3T, through a multi-center, multi-vendor study.
Methods: Four centers (University of Wisconsin-Madison, University of Texas-Southwestern, Johns Hopkins University, and Stanford University) using MRI scanners of different vendors (GE, Philips, and Siemens) participated in this HIPAA-compliant IRB-approved prospective cross-sectional study. This study recruited subjects with known or suspected iron overload from a variety of etiologies, including hereditary hemochromatosis, transfusional hemosiderosis (due to non-malignant or malignant conditions), and chronic liver disease. Subjects with were recruited for same day multiecho gradient-echo MRI for R2* mapping at both 1.5T and 3T (UW, UTSW, Stanford: 3.0T; JHU: 2.89T). R2* maps were reconstructed from the raw multiecho images and analyzed at a single center. Spin-echo MRI were also performed at 1.5T according to a standardized protocol (FerriScan, Resonance Health, Australia) and processed by a commercial algorithm to obtain FDA-approved reference standard LIC estimates. R2*-vs.-LIC calibrations were generated across centers and field strengths using linear regression and compared using F-tests. A predicted 2.89T calibration was interpolated from the 1.5T and 3.0T calibrations, and compared to the measured (JHU) calibration. Receiver operating characteristic (ROC) curve analysis was performed to determine the diagnostic accuracy of R2* MRI for detection of clinically relevant LIC thresholds.
Results: A total of 200 subjects were recruited and successfully scanned for this study. We confirmed a linear relationship between R2* and LIC. All calibrations within the same field strength (see Figure 1) were highly reproducible showing no statistically significant center-specific differences (F &gt; 3.0461). Pooled calibrations for 1.5T, 2.89T, and 3.0T were generated. At either field strength and for each of the LIC thresholds under consideration (1.8, 3.2, 7.0, 15.0 mg/g), estimated areas under the ROC curve (AUCs) of 0.98 or higher were observed.
Discussion and Conclusions: In conclusion, confounder-corrected R2* MRI enables accurate and reproducible quantification of liver iron overload, over clinically relevant ranges of LIC. The data generated in this study provide the necessary calibrations for broad dissemination of R2*-based LIC quantification.
Figure 1 Figure 1.

Disclosures
Hernando: Calimetrix: Current holder of individual stocks in a privately-held company. Pedrosa: Merck: Honoraria; Bayer Healthcare: Honoraria; Health Tech International: Current holder of stock options in a privately-held company. Vasanawala: HeartVista: Current holder of individual stocks in a privately-held company; InkSpace: Current holder of individual stocks in a privately-held company; Arterys: Current holder of individual stocks in a privately-held company. Reeder: Bayer: Research Funding; Pfizer: Research Funding; Calimetrix, LLC: Current holder of individual stocks in a privately-held company; Reveal Pharmaceuticals: Current holder of individual stocks in a privately-held company; Elucent Medical: Current holder of individual stocks in a privately-held company; Cellectar Biosciences: Current holder of individual stocks in a privately-held company; HeartVista: Current holder of individual stocks in a privately-held company.