Compared to traditional T₁ and T₂ contrasts, MR-derived quantitative water content, electrical conductivity and magnetic susceptibility more directly reflect physiological tissue properties (water, ion and ferritin content) and physical tissue properties describing its interaction with electromagnetic fields. Moreover, electrical conductivity can provide additional valuable information for SAR estimation in ultra high field MRI and for patient specific non-ionizing EM hyperthermia therapy¹. This study was based on a single multi-echo GRE sequence with long TR and total acquisition time of 7 ½ minutes² and thus completely compatible with clinical protocols. To the best of our knowledge this is the first report of simultaneous acquisition of water content, conductivity and susceptibility in tumour patients featuring moreover tumour characterisation by dynamic PET with O-(2[18F]-fluoroethy1)-L-tyrosine tracer (FET-PET). Water content information was mainly derived from the magnitude of GRE images while conductivity and susceptibility maps were retrieved from the multi-echo phase profiles³.Two meningioma patients (62-year-old male and 64-year-old Female) were investigated in a hybrid MR-PET 3T scanner as part of the pre-surgery planning. The prototype 3T MR-PET scanner comprised a commercially available 3T Siemens Trio MR system and a custom-built MR-compatible Brain PET scanner⁴. Two dedicated head coils for MR including an outer birdcage coil for transmit and an inner 8 channels coil for receive were placed in the PET detector. The MR Brain PET delivers PET images with an optimal resolution of 3mm. For quantitative mapping, a 2D multi-echo gradient echo sequence was employed with TR=10s and nominal flip angle of 90°. FOV=200×162mm²; slice thickness=1.5mm; TE1=3.87ms;echo separation ΔTE=4.08ms; 12 echoes; acceleration factor=2. To calculate the water content and its complementary proton density, T₁, T₂* and transmit-receive field corrections usually need to be applied. However, in our long TR protocol T₁ saturation can be neglected for brain tissue, which at 3T has T₁<2s. It is still present for cerebral spinal fluid (CSF) and can be calculated using its known T₁ of 4.3s⁵. T₂* mapping is performed by a mono-exponential fitting to the signal decay of GRE. The combined effect of transmit and receive RF field inhomogeneity can be corrected using e.g. the bias field correction algorithm implemented in SPM². The water content map is obtained by calibrating the T₂* and bias field-corrected tissue values to the saturation-corrected CSF signal intensity. Reconstruction of the electrical conductivity is based on electrical properties tomography: σ=(ΔΦ)/(2μω), where Δ represents the Laplacian operator, μ the magnetic permeability, and ω the Larmor frequency. The transceiver phase was calculated from the 12 multi-echo GRE phase profiles by interpolating them at the TE=0 after unwrapping. A local quadratic fitting of the transceiver phase was performed before applying the second-order derivative⁶. For susceptibility reconstruction, the B0 field map was estimated by unwrapping the phase of all GRE echoes and applying linear regression, the effect of background field shifts was eliminated using the in-house software MUBAFIRE⁷. Susceptibility estimation was performed using a minimisation strategy with Tikhonov- and gradient-regularisation⁸. In addition, pre- and post-contrast-enhanced T₁ weighted (MPRAGE) and T₂ weighted (FLAIR) images were obtained and co-registered to the electrical conductivity map. For further clinical analysis and comparison, tumour regions were identified and segmented based on the PET data. Fig1a)- c) show the reconstructed water content, electrical conductivity and susceptibility mapping in one patient. The water content values, defined as mean and SD of the distribution over all voxels assigned to a given tissue class, were 70.3±3.5 % in white matter and 83.5±6.3 % in grey matter. The conductivity values were 0.41±0.15 S/m in the white matter, 0.83±0.32 in Grey matter and 2.11±0.43 S/m in CSF. The reconstructed PET map (average tracer uptake over 30 minutes) of one patient is shown in Fig1d), Fig1e)-f) represents the enhanced T₁weighted MPGRAGE map and T₂ weighted FLAIR. Water content in tumour region defined by PET is 85.54± 0.91%, while conductivity is 0.91±0.18 S/m. Compared to PET and contrast enhanced T₁ weighted images, both the water content and conductivity maps are able to differentiate the tumour region from surrounding tissue.In both cases, the values are higher in the tumour than in surrounding tissue. Fig 2 plots the water content versus conductivity in four region of interests including water matter, grey matter, CSF, and tumour. Susceptibility also provides useful information for defining the edge of the tumour region and thus provides complementary information to conductivity.Simultaneous mapping of water content, conductivity and susceptibility based on multi-echo GRE is feasible. This method makes full use of the magnitude and phase profiles of a standard sequence and can be applied to investigate and understand pathological tissues.