Positron emission tomography–magnetic resonance imaging
Positron emission tomography–magnetic resonance imaging (PET-MRI) is a hybrid imaging technology that incorporates magnetic resonance imaging (MRI) soft tissue morphological imaging and positron emission tomography (PET) functional imaging.
Presently, the main clinical fields of PET-MRI are oncology, cardiology and neurology. Research studies are actively conducted at the moment to understand benefits of the new PET-MRI diagnostic method. The technology combines the exquisite structural and functional characterisation of tissue provided by MRI with the extreme sensitivity of PET imaging of metabolism and tracking of uniquely labeled cell types or cell receptors. There is discussion and investigation into utilizing PET-MR with Ion Therapy for the purpose of cancer treatment. with MRI's ability to accurately depict the proton density of tissue is a good match for the benefits and technical challenges of treatment planning utilizing Ion Therapy systems.
Currently three companies offer combined PET-MR systems: Philips, Siemens and GE. The first two clinical whole body PET-MRI systems were installed by Philips at Mount Sinai Medical Centre in the United States and at Geneva University Hospital in Europe in 2010.
Currently Siemens and GE are the only companies to offer a fully integrated whole body and simultaneous acquisition PET-MRI system. The Siemens system (Biograph mMR) received a CE mark and FDA approval for customer purchase in 2011. The first fully RoHS compliant system was delivered in 2014. Over forty facilities have since installed this technology.
The GE system (SIGNA PET/MR) received its 510K & CE mark in 2014.
Currently, the combination of positron emission tomography (PET) and magnetic resonance imaging (MRI) as a hybrid imaging modality is receiving great attention not only in its emerging clinical applications but also in the preclinical ﬁeld. Several designs based on several different types of PET detector technology have been developed in recent years, some of which have been used for ﬁrst preclinical studies.  
A preclinical PET-MRI system with sequential acquisition is commercially available from Mediso Medical Imaging Systems since 2011. The first nanoScan PET-MRI system was installed at Karolinska Experimental Research and Imaging Centre of Karolinska Institutet in March 2011. The compact imaging system utilizes magnetically shielded position sensitive photomultiplier tubes and a compact 1 Tesla permanent magnet MRI platform. The integration has no adverse effects on the PET or on the MRI performance due to increased magnetic shielding of the PET component and low fringe field of the MRI component based on the performance evaluation of the system. Bruker Biospin, in collaboration with the University of Tübingen, have produced commercial prototype systems for their preclinical MRI magnets utilising a Siemens PET platform. The first such system comprising a 7T Clinscan MRI with the PET insert was installed at the CAI in Brisbane Australia in 2012.
Hybrid PET MRI systems require special devices that balance tradeoffs between PET attenuation and MRI performance. XLR Imaging Inc and Ceresensa supply PET/MRI coils and As of May 2013, Hybrid MR Inc has formed to provide 3rd party manufacturing of preclinical PET compatible MRI devices.
PET-MRI versus PET-CT
Comparisons have been made between PET-MRI and PET-CT, some sources stating that PET-MRI is simply a X-ray radiation-free version of PET-CT (PET-MRI has as well Radiation from Biomarker). In reality, there are differences beyond X-ray radiation dose.
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