A dual tuned ¹³C/¹H radiofrequency (RF) head coil (Rapid Biomedical GmbH, Rimpar, Germany) is evaluated for its potential use in PET/MR hybrid imaging. The RF coil features a birdcage design with 16 rungs that are tuned to the Larmor frequencies of ¹H = 123 MHz and ¹³C = 31 MHz (Fig. 1). When integrated in the setting of PET/MR hybrid imaging, the RF coil is placed in the field-of-view (FOV) of the PET-detector during simultaneous PET and MR data acquisition. This causes attenuation and scattering of the emitted photons and, consequently, leads to an attenuation bias in PET quantification. The rungs of the birdcage were extended by 3 cm compared to an MR-only birdcage in order to have the birdcage rings out of the PET detector ring's area. For attenuation correction of the remaining rigid and stationary hardware components in the PET-FOV, the method of a CT-based 3D template µ-map can be applied that corrects for the quantification bias [1-3]. In this work, the attenuation of a double-tuned RF head coil was quantified in phantom experiments; a CT-based template µ-map was generated and applied in AC. The results were then confirmed in PET/MR measurements of six patients.All measurements were performed on an integrated whole-body PET/MR hybrid system (Biograph mMR, Siemens Healthcare, Erlangen, Germany). For quantitative PET imaging and evaluation of attenuation a cylindrical shaped phantom (volume 9.5 l) was used. A phantom holder enabled the phantom to be positioned a few cm above the patient table with or without the RF head coil in place (Fig. 2). Attenuation was determined by difference measurements. 1) 120 MBq ¹⁸F-radiotracer was injected in the water phantom and the RF coil was placed central in the PET-FOV surrounding the phantom (Fig. 2A). The PET data were acquired for 30 min in listmode. 2) The RF coil was removed, and the PET data were decay time corrected (Fig. 2B). The 3D template AC µ-map of the RF head coil and of the phantom were acquired using a dual-source CT scanner (SOMATOM Definition Flash, Siemens Healthcare) with the parameters: tube voltage 140 keV, tube current 400 mA, matrix size of 512 × 512 pixels, voxel size 0.3 × 0.3 × 0.6 mm³. The CT data attenuation values (140 keV) were adjusted to the PET energy level of 511 keV, by a bilinear function [2]. All PET data reconstructions, using the generated AC map of the RF coil and the phantom, were performed with the reconstruction software (e7 tools, Siemens Molecular Imaging, Knoxville, USA). The software also provides the hardware AC µ-map of the PET/MR systems patient table (Fig. 3). PET/MR hybrid imaging using the ¹H MR-option and ¹⁸F-FDG as PET-radiotracer was performed on six patients (4 male, 2 female; 68 years ± 6 years; 168 cm ± 16 cm; 71 kg ± 11 kg; 285 MBq ± 50 MBq; 3h 16min post injection ± 46min) with the following MR sequences: Dixon-VIBE for the soft tissue µ-map, T2 TIRM transversal dark fluid (FLAIR). To measure the attenuation difference in vivo, PET data acquisition on patients was performed twice: with and without the RF coil in place. Regions-of-interest were placed in a central slice of the PET images to determine the tracer activity concentration values (Bq/ml) with and without RF coil.In the phantom measurements with and without the RF head coil in place, an overall global attenuation of true PET events by 8.7% was determined. Attenuation was homogeneously distributed across the phantom with higher attenuation (up to 25%) along the bottom parts of the coil (Fig. 4A). Applying the CT-based µ-map of the RF coil successfully reduces the mean attenuation bias of 10% to below 1% across the phantom volume (Fig. 4B,C). These quantitative results were confirmed in the six patient PET/MR measurements (Fig. 5). Differences in Bq/ml of 13.5% ± 4.3% were found for PET images without and with head coil in place. These differences were reduced to 1.6% ± 1.1% when AC of the coil was applied; indicating accurate attenuation correction.A dual tuned ¹³C/¹H radiofrequency head coil was evaluated towards its potential use in integrated PET/MR hybrid imaging. Due to its open and symmetric birdcage design the RF coil attenuates the PET-signal rather homogeneous. With rigid geometry and a fixed position on the systems patient table it qualifies for CT-based template AC which was successfully applied. In conclusion, the RF head coil was successfully integrated into PET/MR hybrid imaging and can now be used for multinucleus hybrid imaging in further patient studies.