Multi-component maps of relaxation times and water exchange rate are potentially useful for many applications, such as monitoring myelin degenerating disorders and diffuse fibrosis in the heart, but they are not routinely acquired. Recently, MR Fingerprinting with Chemical Exchange (MRF-X) was introduced for simultaneously mapping T₁, T₂, exchange rate, and volume fraction in voxels with two exchanging pools.¹ Here, we present MRF-X maps acquired in human brain and skeletal muscle.Three healthy volunteers underwent a brain MRI at 3T (Skyra, Siemens Medical Solutions, Erlangen, Germany) in this IRB approved study. The MRF-X pulse sequence used a FISP-based readout², variable flip angles (0-75 deg), and constant T_R=6.98ms with 3000 RF excitations. An inversion pulse occurred before RF excitations #1 (TI=21ms), #1201 (TI=100ms), and #2401 (TI=250ms). Data were sampled with a variable density spiral having 48 interleaves, matrix size 192x192, and 300mm² FoV. The same flip angle and TR pattern was repeated 48 times in order to fully-sample k-space, with a 10s pause between each repetition to allow for magnetization recovery (scan time 24min). The MRF-X dictionary was simulated using the two-pool Bloch-McConnell equations³. The dictionary had approximately 69 million entries with the following parameter ranges: T₁^A and T₁^B = [50,100:100:2000, 2200:200:3000 3500:500:500]ms, T₂^A and T₂^B [5,10:10:100,120:20:200,250,300,400,500,1000]ms, exchange rate k_AB=[0.01,0.1,0.25,0.5,0.75,1,2,3,5,7,10,15,20,100]s^-1, and volume fraction of the short species ρ_A=[0:0.02:1]. Quantitative maps were computed by finding the entry in the dictionary that was most highly correlated with the acquired signal evolution at each image pixel. The average MRF-X parameter values were computed over twenty ROIs drawn in white and gray matter. In addition, leg skeletal muscle was scanned in another healthy volunteer using the same sequence and reconstruction. The average MRF-X parameter values were determined in ROIs in the soleus, gastrocnemius, and tibilias anterior muscles.MRF-X maps in different axial positions in the brain from two volunteers are shown in Figures 1 and 2. Measurements averaged over white matter and gray matter from each volunteer are summarized in Table 1. Short (species A) and long (species B) T₁ and T₂ components were observed. Averaged over all volunteers, white matter regions had a mean exchange rate of 6.8s^-1 (mean residence time 150ms) with 24% volume fraction for the short species, and gray matter regions had a mean exchange rate of 0.1s^-1 (little to no exchange) with 75% volume fraction of the short species (although this component had longer relaxation values than the short component in white matter). Table 4 summarizes ROI measurements from leg skeletal muscle in the soleus, tibialis anterior, and gastrocnemius.This work presents the first in vivo use of MRF-X for simultaneously mapping compartment relaxation times, volume fractions, and exchange rates. In white matter, the short and long T₂ measurements agree with previously reported values using mcDESPOT.⁴ Although the biological nature of the two compartments observed with MRF-X is only speculated, the volume fraction and mean residence time in white matter agree with literature values for myelin residence time (140ms) and myelin water fraction (17-32%).⁵ Two non-exchanging species were observed in gray matter that might be attributed to cellular components (short T₁ and T₂) and CSF (long T₁ and T₂). It should be noted that the MRF-X measurements may be less precise when the fraction of one species is very small (e.g. <10%), which could account for noisy patches in the maps. In addition, brain tissue may be more accurately modeled by adding a third non-exchanging compartment to model CSF, which could be incorporated in the MRF-X reconstruction.⁶ Previous studies have suggested that skeletal muscle contains three compartments with 8% short T₂<10ms, 82% T₂=45ms, and 10% T₂>200ms.⁷ The two compartments measured with MRF-X may correspond to the short and moderate T₂ species, and they have appropriate volume fractions in soleus and gastrocnemius. The exchange rates reported in one study⁸ are approximately 7.7s^-1, which are similar to the values determined using MRF-X. Finally, the volume fraction for the short T₂ species using MRF-X is 2.8 times higher in the tibialis anterior compared to the gastrocnemius, which is close to the ratio of 3.8 reported by others.⁹In this preliminary in vivo study, it was shown that MRF-X provides multi-compartment relaxation values, volume fractions, and exchange rates that correspond with literature values in the brain and skeletal muscle. Given the key role that these values can play in abnormal tissues, we believe that MRF-X could provide novel insights into disease states.