The feasibility of in vivo ³⁹K magnetic resonance imaging in humans was proven recently^1,2. In muscle tissue the resonance of this spin-2/3 nucleus is split due to non-vanishing quadrupole interaction with electrical field gradients (EFG)³. In case of axial symmetry, theory predicts that the frequency shift Δf_Q of the two satellite resonances depends on the angle $\beta$ between the privileged direction of the EFG and the static magnetic field according to $$\Delta f_{Q} = \frac{e^2 Q q}{4 \hbar} ( 3 \cos^2{\beta} -1 ),$$ where $\hbar$ is the Planck constant, $eQ$ is the electric quadrupole moment (5.85 fm²)⁴ and $eq$ is the EFG. The dipole-dipole interaction of two coupled spin-1/2 nuclei follows the same angular dependency. In proton spectroscopy of muscle tissue this was observed for the total creatine (tCr) resonance^{5, 6}. In this work, the angular dependence of splitting of the ³⁹K resonance is compared in vivo to the one of the tCr resonance in ¹H spectroscopy.All measurements were performed on a 7 T whole body system (MAGNETOM 7 T, Siemens AG, Germany) with an accessible patient bore size of 60 cm. ³⁹K MR signal was acquired with a custom-built surface coil (d = 10 cm, 2 windings) whereas for ¹H MR spectroscopy a commercially available surface coil was used (¹H Loop Coil 7 T; SIEMENS AG, Germany). The surface coils were placed in two sessions between the calves of a healthy volunteer (28y, f) positioned right laterally (fig. 1). The angle between the static magnetic field B₀ and the front edge of the tibia was varied from -10° up to +100° in increments of 10° by bending the knees. For each position a ³⁹K spectrum was acquired with the FID sequence (TR = 290 ms, BW = 2000 Hz, bas. res. = 512, $\alpha$ = 90°, nex. = 1024, TRO = 256 ms, TAQ = 5 min, oversampling 2, fig. 2). The occurring resonances were plotted with the AMARES algorithm^{7, 8}. By prior knowledge the intensity and line width of the two satellite resonances as well as their absolute distance to the central resonance were assumed to be equal. The angular dependency of the satellites’ frequency shift was plotted and equation $$\Delta f_{satellites} = A ( 3 \cos^2{(\beta-\beta_0)} -1 )$$ was fitted to the data. After performing B₀ shimming and water suppression by a water excitation technique (WET), a ¹H spectrum with a PRESS sequence (TE = 20 ms, TR = 2000 ms, BW = 1200 Hz, bas. res. = 2048, $\alpha$ = 90, nex. = 92, TRO = 1706 ms, TAQ = 3:04 min, voxel size 2 $\times$ 2 $\times$ 2 cm³ placed in musculus gastrocnemius, fig. 2) was acquired for different angular positions of the calves. The distance of the two peaks are plotted in fig. 3 versus the angle between the tibia and B₀.The frequency shift of the satellite resonances ($\Delta f_{satellites}$) in the ³⁹K spectrum changes with the angle between the tibia and B₀. However, the splitting never reaches zero with the chosen fit options. The fitting results in $A = (96 \pm 12) 1/s$ and $\beta_0 = (-1 \pm 6)°$, representing the unknown angle between the tibia and the muscle fibers. The splitting of the tCr resonance ($\Delta f_{dipole-dipole\ splitting\ tCr}$) in the ¹H spectrum is only distinguishable for angles between -10° and 30°. In comparison to the splitting of the ³⁹K resonance caused by non-vanishing quadrupolar interaction the splitting of the tCr resonance in the ¹H spectrum due to dipole-dipole interaction is one order of magnitude lower. Nevertheless, their behavior under angle variation is similar for the data acquired.³⁹K spectroscopy was performed unlocalized. Therefore signals are likely from different muscle types. Three resonances were assumed for all spectra to avoid the subjective decision on when to change the model, even though the resonances are not clearly visible especially of high angles. Nevertheless, the fitted model for the angular dependence assuming axial symmetry describes the data properly. ¹H spectroscopy was compromised by the B₀ shim, especially at large bending angles when the coil could not be positioned exactly in the isocenter. Therefore the splitting of the tCr resonance was only observable for angles smaller than 30°.In conclusion, the similar behavior of quadrupole splitting of the ³⁹K resonance and the dipole-dipole splitting of the tCr resonance in the ¹H spectrum under angle variation was shown in vivo at human calf muscle. The potassium ions and the creatine seem to be located in an equivalent electromagnetic environment.