Accurate T1 relaxation time measurement is critical for many quantitative MRI applications. An MRI system phantom (Figure 1 A) was developed through collaboration between the ISMRM Ad Hoc Committee on Standards for Quantitative MR and the National Institute of Standards and Technology (NIST).¹ The system phantom has SI-traceable components and is being monitored by NIST for stability and accuracy. We used the system phantom to assess variations of T1 measurements across MRI systems at 1.5 T and 3 T. The purpose of this study was to determine the accuracy, precision, repeatability and reproducibility of the T1 measurements.Two system phantoms were imaged at multiple sites for three vendors across two field strengths using head coils with 8 to 32 channels (Table 1 in Figure 1 C). For each vendor, one location performed repeatability analysis (n=3 measurements) at 3 T; repeatability was assessed with the coefficient of variation.² The study included T1 measurement by inversion recovery (IR) using 2D FSE-IR and variable flip angle using 3D FSPGR. The T1 data was fit using custom software (Phantom Viewer, developed at NIST). Details regarding the phantom and data acquisition protocol are available online: https://collaborate.nist.gov/mriphantoms/bin/view/MriPhantoms/RecommendedImagingProtocols. NiCl₂ was used to dope deionized water and create a range of T1 values from approximately 20 to 2000 ms. Across all measurements, reported temperature ranged from 17.1 °C to 23.3 °C; temperature was reported either of the MRI room or of the bulk water in the phantom. The deviations from the reference NMR T1 measurements at 20 °C for each of the vendors and both methods are shown for 1.5 T in Figure 2 and 3 T in Figure 3. The repeatability study coefficients of variation are presented in Figure 4. The CV for Vendor B, VFA is three times the CV of the other vendors and methods; Vendor B, VFA also has the greatest deviation from the NMR-measured values (Figure 3 B). T1 VFA deviation from reference values for representative data sets vary by position within the head coil (Figure 5).Systematic differences in T1 relaxation time measurement can be identified as a function of measurement method (IR or VFA), sample position and scanner type. As measured in our lab, the temperature variation of the NiCl₂ T1 relaxation time is non-linear and increases by 7.2% over the range of bore temperatures (17.1 °C to 23.3 °C). Thus, the deviation from reference T1 cannot be attributed only to temperature variation. The deviation from NMR-measured value is greater at 3 T than at 1.5 T. The VFA data has a larger variation than IR; B1 inhomogeneity could contribute to the larger systematic error in VFA measurements. Unlike 3 T, the 1.5 T data do not exhibit an obvious pattern of deviation from the NMR-measured values, with the exception of Vendor B, which consistently underestimates T1 with VFA. However, additional data is needed for vendors A and B. This study demonstrates that T1 variations from NMR-measured value are correlated site-to-site within a vendor and by position within the head coil. The ISMRM/NIST system phantom is an excellent tool for evaluating multi-site MRI acquisition protocols.