Integrated PET/MRI in preclinical studies State of the art

Brunotte F, Haas H, Collin B, Oudot A, Bricq S, Lalande A, Tizon X, Vrigneaud JM, Walker PM. Integrated PET/ MRI in preclinical studies. State of the art. The exquisite tissue contrast of magnetic resonance imaging (MRI), the absence of ionising radiation and the opportunity to obtain new molecular and functional data have strengthened the enthusiasm for coupling MRI rather than computed tomography (CT) to positron emission tomography (PET). When reviewing the current literature one might be surprised by the almost unlimited diversity of what is placed under the name of PET/MRI in the articles. The magnetic field is varying from 0.3 Tesla (T) to 9.4 T, the size of the bore varies also from the wide bore of clinical scanners to volumes limited to a few tens of mL. Many preclinical studies are performed using separate PET and MRI scanners. Sometimes PET and the magnet are in line or sequential. More rarely, fully integrated PET/MRI scanners are used. In that case, mutual interference between PET and MRI has required innovative designs. Initially, the conventional photomultipliers had been installed outside the magnet using long optical fibres. They have now been replaced by avalanche photodiodes (APD), and in the near future silicon photomultipliers (SiPM) could provide an alternative. Tumours and neurological and cardiovascular disorders have been the most studied conditions. Many issues remain to be resolved such as image registration, attenuation correction and animal monitoring. Friendly consoles integrating the control of both imaging modalities also need to be developed. Tijdschr Nucl Geneesk 2013; 35(4):1144-1152 Introduction The idea of multimodal imaging techniques is not new (1). Nowadays, the coupling of positron emission tomography (PET) and computed tomography (CT) is the standard in clinical practice (2) and more recently the integration of single photon emission computed tomography (SPECT) and CT is becoming more and more available (3). These imaging techniques have proven to be extremely effective in diagnosing a variety of diseases (4). Tissue characterisation has been improved by combining the specificity of radiopharmaceuticals and the 3D imaging capabilities of modern CT scanners. Although straightforward, coupling of PET and CT has two serious drawbacks: X-ray exposure contributes to an increased patient irradiation dose and CT tissue characterisation is limited. Magnetic resonance imaging (MRI) has demonstrated a much more convincing ability to provide tissue characterisation through measurement of key parameters such as relaxation times T1 and T2, apparent diffusion coefficient (ADC), tissue perfusion and spectroscopy. One very prolific application has been dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) of tumours, which has permitted an accurate monitoring of tumour perfusion in response to anti-angiogenic treatments (5). Garlick et al described under the name of PANDA (PET and NMR dual acquisition) in isolated, perfused rat hearts in 1997 (6). LSO crystals were inserted in a 9.4 T magnet with a bundle of optical fibres towards external photomultipliers. Fifteen years later the integration of MRI and PET has generated considerable enthusiasm in the clinical field (7) since nuclear medicine specialists are in need of several requirements that MRI can offer: (i) reduction of the radiation exposure of the patients, especially in case of repeated examinations and in paediatrics, (ii) improvement of soft tissue characterisation by simple addition of the advantages of both techniques and finally (iii) the access to additional physiological parameters that could be derived only by combining both approaches. All these advantages could make PET/MRI a tool of importance especially when focusing on a given organ with the aim of monitoring the effect of a treatment. In animals many of the rationale retained for clinical studies also apply. The animal irradiation due to CT images can be as high as