Multimodal Metabolic Imaging with Hybrid Positron Emission Tomography and Magnetic Resonance Imaging (PET/MRI) Systems

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Modern imaging technologies such as magnetic resonance imaging (MRI) and positron emission tomography (PET) allow us to image, non-invasively, the inner workings of the body. PET creates three-dimensional images of radio-labelled, biologically active molecules that have been injected in a patient. The most commonly used PET tracer is a glucose analogue labelled with radioactive fluorine (FDG), which can be used to locate metabolically active regions, such as cancer cells or inflammation. FDG-PET is limited, however, to certain patients and studies due to limited resolution and additional radiation dose. An alternative method for imaging metabolism has been proposed using MRI. It relies on the chemical exchange between protons in metabolites such as glucose and the protons in water that comprise most of the MRI signal. Using MRI methods such as chemical exchange saturation transfer (CEST) and chemical exchange-sensitive spin lock (CESL) imaging, we can measure the change of signal due to chemical exchange and relate it to the concentration of different metabolites, such as glucose, creatine, and glutamate. If comparable to FDG-PET, this would allow the MRI to acquire metabolic images in patients that normally can't be exposed to radioactive tracers, such as pregnant women or young children. MRI can also acquire images with much higher resolution than PET, potentially probing the heterogeneity of metabolic signals.