Quantitative relaxometry has shown promise for characterizing pathology¹, and monitoring treatment effects². However, the differences in T₁ and T₂ values between healthy and diseased tissues can be very small. In order to promote the clinical acceptance of quantitative relaxometry techniques as a valid biomarker, a fast quantitative method that can measure the relaxation parameters with high repeatability is needed. At the same time, a relaxometry phantom with stable T₁ and T₂ values is also necessary to evaluate the performance of any quantitative method. Magnetic Resonance Fingerprinting (MRF)³ is a promising method in the quantification of T₁ and T₂ simultaneously with high efficiency. The purpose of this study is to evaluate the repeatability of MRF with the standardized relaxometry phantom developed through collaboration between the ISMRM Ad Hoc Committee on Standards for Quantitative MR and the National Institute of Standards and Technology (NIST)⁴. The phantom has SI-traceable components and is monitored by NIST for stability and accuracy.The NIST phantom has 3 layers of sphere arrays that are designed to have the standardized T₁, T₂ and proton density. The T₁ arrays are made of different concentrations of NiCl₂ solutions, and T₂ arrays are made of MnCl₂ solutions. It was scanned with FISP-based MRF method⁵ using Siemens Skyra 3 T scanner (Siemens AG Healthcare, Erlangen, Germany) with a 20-channel head-neck receiver array. The MRF method has the varied flip angles from 5° ~ 75°. The repetition times change from 12 to 15 ms. A total of 3000 frames were acquired for each slice, resulting in a scan time of 40 seconds per slice. Bloch simulations generated a dictionary containing all possible signals covering T₁ values from 20 ms to 3000 ms, and T₂ values from 2 ms to 800 ms. The phantom was placed in the magnet for 30 minutes to stabilize the fluid before the acquisition. Two slices with the in-plane spatial resolution of 1.2 × 1.2 mm², and the slice thickness of 5 mm were scanned over T₁ and T₂ arrays of the NIST phantom. The experiment was repeated over 35 consecutive days to evaluate the repeatability of MRF-FISP method.Figure 1 shows T₁ and Figure 2 shows T₂ values over 35 consecutive days of the measurement. The precision was calculated by the percent change of the standard deviation normalized by the mean T₁ and T₂ values of 35 days. The precision of the estimation of T₁ and T₂ values of MRF-FISP method are plotted in Figure 3. It shows that MRF measures a wide range of T₁ and T₂ values with less than 5 % variations, except for the shortest relaxation times.A less than 5% variations on T₁ and T₂ estimations of MRF was observed in NIST standardized phantoms with a wide range of T₁ and T₂ values over 35 consecutive days. The result from the T₂ array phantom has more variations than the results from T₁ array. The observed variations could relate to small temperature fluctuations day-by-day because it is known that NiCl₂ based T₁ arrays are less sensitive to the temperature changes compared to the T₂ arrays made of MnCl₂. More study would be needed to further analyze the temperature dependence of these phantoms. NIST/ISMRM standardized relaxometry phantom can be the ideal phantom for evaluating the repeatability of the quantitative relaxometry method. The current study shows that MRF has excellent repeatability across 35 days over a wide range of T1 and T2 values.