1. Objective to compare the effect of single energy imaging and mar technology in reducing artifacts of knee and hip replacement, and to explore the appropriate keV range of Mar technology in removing metal artifacts.,2. Objective to compare the effects of different MRI sequences (mavric / semac, stir, ideal, FSE) in removing artifacts of spinal internal fixation, knee and hip replacement.

Conventional spiral CT [4] examination will form black-and-white stripe artifacts around the metal implant, making the display of adjacent important structures unclear. In routine MRI examination [5], metal will interfere with the uniformity of local magnetic field, resulting in signal loss, and the artifacts will seriously affect the observation of bone and its surrounding structures.

The purpose of this study was to comprehensively evaluate the effect of Mar technology on reducing metal artifacts after internal fixation and joint replacement, and then to explore the appropriate keV range of Mar technology to reduce metal artifacts. We also aim to compare the reduction effect of different sequences of MRI on internal fixation artifacts after internal fixation and joint replacement, and to explore the sequence suitable for clinical diagnosis and treatment.

Inclusion Criteria:

• Patients after spinal internal fixation or knee or hip replacement

Exclusion Criteria:

• Patients with poor image quality caused by their own factors such as patients' movement can not meet the diagnostic requirements

From December 2019 to October 2020, patients who were reexamined after spinal internal fixation or knee and hip replacement in Peking University Third Hospital.