Hybrid PET/MRI : Advantages, Challenges and Prospects for PET/MRI Integrated Systems

The PET/MRI hybrid system became eligible for staging the diagnosis of malignant tumors, metastasis, and recurrence using 18F-Fluorodeoxyglucose (FDG). Inserting PET in an MRI is technically challenging and requires PET detectors to be compatible with the magnetic field. The key technology that enables this is the semiconductor-based photomultiplier tube. The advantage of the PET/MRI hybrid system is not simply that MRI has replaced computed tomography (CT), but rather that PET and MRI data can be acquired simultaneously. The high-precision fusion images show improved diagnostic accuracy. Positional misalignment due to movements, such as respiratory motion, as seen on PET/CT, can be minimized through simultaneous acquisition, and higher-precision fusion images can be obtained by performing PET and MRI acquisitions under respiratory gating. One of the disadvantages of PET/MRI over PET/CT is that CT cannot be used for gamma-ray attenuation correction, which is necessary for PET image reconstruction. Although a method has been established to generate a pseudo-CT image from MRI for attenuation correction, generating a pseudo-CT image that includes bone components is particularly difficult. In PET/MRI systems approved for clinical use, bone components are not considered for attenuation correction outside the head region. Recently, active research has been conducted utilizing deep learning to generate pseudo-CT images with bone components from MRI, regardless of the anatomical region, or to perform attenuation correction using PET data alone. Recently, new PET tracers have been approved in Japan. As PET/MRI can produce highly accurate fusion images with MRI, these tracers and target lesions are likely to benefit more from PET/MRI than from PET/CT. As such, the scope of PET/MRI applications is expected to continue to expand, and efforts should be made to promote its widespread adoption so that more facilities can utilize this technology.