To develop and implement an efficient pulse sequence for high-resolution 3D imaging of phosphorus containing metabolites in the human tissue.Phosphorus (³¹P) magnetic resonance spectroscopy (MRS) and imaging (MRI) techniques offer unique opportunities for evaluating tissue metabolism in health and disease.^1,2 They allow direct detection and quantification of high-energy phosphates such as phosphocreatine (PCr) and adenosine triphosphate (ATP),³ and the ATP synthesis hydrolysis cycle (ATPase).^{4 31}P-MR data typically suffer from low signal-to-noise ratio (SNR) due to the low concentration of ³¹P containing metabolites and the low MR sensitivity associated with the ³¹P nucleus. As a result, ³¹P-MR signal in the human tissue is approximately four orders of magnitude lower than ¹H signal. Therefore, undesirable compromises in spatial resolution and coverage have been made in order to perform ³¹P-MR within reasonable acquisition times. Multiple voxel ³¹P-MR data are typically acquired through chemical shift imaging approaches, which are inherently slow. In this work, we developed and implemented a fast ³¹P-MRI approach based on the 3D non-Cartesian FLORET pulse sequence.⁵ Using this method, we imaged the human brain with 1.4 cm nominal isotropic resolution. We also imaged the exercising lower-leg muscles with a 6 s temporal resolution.We performed all imaging experiments on a whole-body 7 Tesla scanner (MAGNETOM, Siemens Medical Solutions, Erlangen, Germany). Our FLORET sequence used a narrowband Gaussian shaped pulse (duration: 6 s; bandwidth: 250 Hz) in order to excite a single metabolite of the ³¹P spectrum. We imaged the brain using an in-house developed dual-tuned (³¹P/¹H) array.⁶ We acquired images of single metabolites (i.e. PCr and γ-ATP) with 1.4 cm nominal isotropic resolution with the following parameters: TR = 5 s for PCr and 2 s for γ-ATP, FA = 75^o, three hubs at 45^o, with 10 interleaves per hub, and a total acquisition time of 15 min per metabolite. To image PCr levels of the exercising lower-leg muscles, we used a commercial dual-tuned quadrature volume coil (Rapid MRI, Ohio). Subjects performed a 2 min plantar flexion exercise using an in-house developed MR compatible ergometer. We collected images serially, before (baseline), during, and after completion of the exercise. The total time of image acquisition was 10 minutes. Acquisition parameters of the FLORET sequence were: TR = 0.4 s, FA = 25^o, 1.6 cm isotropic resolution, acquisition time per image: 6 s, one hub at 90^o, and 15 interleaves.Single metabolite images in the brain acquired using our spectrally selective 3D-FLORET sequence are shown in Figure 1. The pseudo-SNR of the PCr image was 20.1 ± 4.5, which was calculated as the mean signal divided by the standard deviation of the noise background. The ratio of PCr-to-γ-ATP was 1.5 ± 0.1, which was in good agreement with previously reported values.⁷ PCr images of the lower-leg muscles at rest, at the end of the plantar flexion and after recovery are shown in Figure 2, together with the signal evolution of PCr in a region-of-interest drawn in the gastrocnemius muscle. We developed and implemented a spectrally selective 3D-FLORET sequence for imaging ³¹P containing metabolites in human tissue. This highly efficient pulse sequence allowed us to map PCr and γ-ΑTP at 1.4 cm isotropic nominal resolution. To the best of our knowledge, such resolution has only been reported previously at 9.4 Tesla using a mono-tuned volume coil.⁷ While testing the 3D-FLORET sequence, we also attempted to image PCr and γ-ATP in the brain using a spectrally selective turbo spin echo (TSE) sequence. While this sequence has been very effective for imaging skeletal muscle,⁸ it only yielded very poor signal in the brain. This could be attributed to the short transverse relaxation times in the brain compared to the skeletal muscle.^9,10 In the skeletal muscle, we were able to map PCr during exercise and recovery with 6 s temporal resolution. Previous imaging approaches, based on Cartesian k-space sampling, allowed imaging of PCr at 10 – 12 s temporal resolution.^11,12 High temporal resolution through 3D-FLORET imaging could allow more precise estimation of PCr resynthesis rate, a marker of oxidative capacity of the muscle.Spectrally selective 3D-FLORET is an efficient pulse sequence for imaging ³¹P-containing metabolites in the human tissue when high spatiotemporal resolution is needed.