Combined positron emission tomography (PET) and magnetic resonance imaging (MRI) is a new tool to study functional processes in the brain. Here we study brain function in response to a barrel-field stimulus simultaneously using PET, which traces changes in glucose metabolism on a slow time scale, and functional MRI (fMRI), which assesses fast vascular and oxygenation changes during activation. We found spatial and quantitative discrepancies between the PET and the fMRI activation data. The functional connectivity of the rat brain was assessed by both modalities: the fMRI approach determined a total of nine known neural networks, whereas the PET method identified seven glucose metabolism-related networks. These results demonstrate the feasibility of combined PET-MRI for the simultaneous study of the brain at activation and rest, revealing comprehensive and complementary information to further decode brain function and brain networks.

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Deep learning (DL) has been reemerging recently in many fields, including computer vision and speechrecognition, because of big data and groundbreaking GPU performance (LeCun et al 2015, Sze et al 2017). Sophisticated deep neural network (DNN) models were proposed in the competition of ILSVRC (ImageNet Large-Scale Visual Recognition Challenge), such as AlexNet (Krizhevsky et al 2012), VGG Net (Simonyan and Zisserman 2014), Microsoft ResNet (He et al 2015), and GoogLeNet (Szegedy et al 2015). DL is adopted quickly in medical imaging applications for lesion detection (Esteva et al 2017), image segmentation (Ronneberger et al 2015) and registration, and automated diagnosis (Dolz et al 2016). DL has been also used in end-to-end trainings to enhance image quality, such as noise and artifacts reduction, across many medical imaging modalities (Han

Simultaneous PET/MR imaging is an emerging hybrid modality for clinical and preclinical imaging. The static magnetic field of the MR imaging device affects the trajectory of the positrons emitted by the PET radioisotopes. This effect translates into an improvement of the spatial resolution in transaxial images. However, because of the elongation of the positron range distribution along the magnetic field, the axial resolution worsens and shine-through artifacts may appear. These artifacts can lead to misinterpretation and overstaging. The aim of this work was to study the relevance of this effect. Methods: Measurements were performed in a 3-tesla PET/ MR scanner. A 1-cm 2 piece of paper, soaked with a radioisotope and placed in air, was scanned, and the magnitude of the shinethrough was quantified from the PET images for various radioisotopes. Additionally, PET/MR and PET/CT images of the lungs and the larynx with trachea of a deceased swine were obtained after injecting a mixture of NiSO 4 and 68 Ga to simulate hot tumor lesions. Results: For the radioactive paper, shine-through artifacts appeared in the location of the acrylic glass backplane, located 3 cm from the source in the axial direction. The ratio between the activity of the shine-through and the activity reconstructed in the original location ranged from 0.9 ( 18 F) to 5.7 ( 68 Ga). For the larynx-withtrachea images, the magnitude of the artifacts depended on the organ orientation with respect to the magnetic field. The shine-through activity could reach 46% of the reconstructed activity (larynx lesion). The lesion within the trachea produced 2 artifacts, symmetrically aligned with the magnetic field and characterized by artifact-to-lesion volumeof-interest ratios ranging from 21% to 30%. Conclusion: In simultaneous PET/MR imaging, the effect of the magnetic field on positrons may cause severe artifacts in the PET image when the lesions are close to air cavities and high-energy radioisotopes are used. For accurate staging and interpretation, this effect needs to be recognized and adequate compensation techniques should be developed. PET/ MR imaging is a powerful technology that is now expanding worldwide (1); integrated PET/MR imaging systems are already commercially available for clinical applications (2,3). Special attention is thus being paid to the ability of simultaneous PET/MR imaging to provide quantitative information and artifact-free images. A particular phenomenon of this integrated technology is the effect of the static magnetic field on positrons. Because of the Lorentz force, positrons follow helical paths along the magnetic field lines. Consequently, the positron range distribution narrows in the plane perpendicular to the magnetic field, thus improving the transaxial resolution (4-9). On the other hand, little attention has been devoted to the behavior of positrons in the axial direction. The positron range distribution elongates along the magnetic field, and the axial resolution subsequently degrades (9-11). The positron range can rea...