PET MRI physics review

1039 Learning Objectives 1. Understand the challenges and possible solutions that are being considered in the construction of a clinical PET MRI. PET/CT has already shown great value in clinical applications, especially by providing quantitative functional information underlayed with anatomy. Limitations include a lack of adequate soft-tissue contrast, CT radiation dose, and impossibility to perform simultaneously with PET. MRI advantages are excellent soft-tissue contrast in brain and abdomen and no additional ionizing radiation dose for the patient. However, a series of technical challenges must be overcome. These are briefly listed hereafter and discussed in the poster presentation. MRI static magnetic and gradient fields, and radiofrequency signals affect the normal operation of PET PMTs, and cause interference in front-end electronics of PET detectors. MRI, in turn, is affected by PET causing inhomogeneities in the magnetic field and leading to artifacts. Currently, there is no consensus on the best configuration for clinical PET/MRI. Current prototypes include PET/CT tandem, removable PET insert, and integration of a PET detector ring within MR. The latter is the most challenging but most promising approach for simultaneous acquisition. Technical challenges derive from the use of PMTs since they are sensitive to magnetic fields. For this reason, optical fibers can be used and PMTs can be replaced by Avalanche Photodiodes. For MR compatibility issues, magnetic susceptibility should be close to that of human tissue. PET scintillator materials such as NaI(Tl), CsI(Tl), BGO, LSO(Ce) are the ones that share this property. Shielding should be selected by considering magnetic and conductivity-related compatibility. Several scintillator materials and heavy metals have been tested; purified forms of lead would make a good choice. Eddy currents should be taken into account to prevent heat and image artifacts. MR has no facility to measure tissue attenuation. So there are several studied methods for attenuation correction including template-based, segmentation-based, atlas-based methods, and use of ultrashort Echo time sequences