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

磁共振成像(MRI)和正电子发射断层扫描(PET)等现代成像技术使我们能够非侵入性地对人体内部工作进行成像。PET创建了注射在患者体内的放射性标记的生物活性分子的三维图像。最常用的PET示踪剂是一种用放射性氟(FDG)标记的葡萄糖类似物，可用于定位代谢活跃的区域，如癌细胞或炎症。然而，由于分辨率和额外的辐射剂量有限，FDG-PET仅限于某些患者和研究。已经提出了一种使用核磁共振成像新陈代谢的替代方法。它依赖于代谢产物中的质子之间的化学交换，如葡萄糖和水中的质子之间的化学交换，这些质子构成了磁共振信号的大部分。利用化学交换饱和转移(CEST)和化学交换敏感自旋锁定(CESL)成像等磁共振方法，我们可以测量由于化学交换引起的信号变化，并将其与葡萄糖、肌酸、谷氨酸等不同代谢物的浓度联系起来。如果可以与FDG-PET相媲美，这将使MRI能够获得通常不能暴露于放射性示踪剂的患者的代谢图像，如孕妇或幼儿。磁共振成像还可以获得比正电子发射计算机断层扫描更高分辨率的图像，潜在地探测代谢信号的异质性。

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.