Recruiting18F-FAPI PET/MRI Imaging in Myelofibrosis: a Prospective Observational Study.This study intends to use a prospective, observational, self-controlled, multi-center study design to evaluate the feasibility and diagnostic efficacy of 18F-FDG PET/CT and 18F-FAPI PET/MRI imaging for the evaluation of systemic fibrosis in MF patients based on the results of bone marrow pathological biopsy, and to analyze the relationship between imaging results and clinical prognosis.