This is an open-labelled two-arm pilot comparative prospective study. All the subjects will undergo image assessment in the two stages of exchange arthroplasty surgery. At the first stage, each enrolled subject will receive both FDG and Ga68 citrate PET/CT scans before the first operation for periprosthetic Joint Infection (PJI). Thus, the test results of FDG and Ga68 for each individual can be obtained. After the first operation, the surgery/biopsy proof can be obtained as the gold standard. The subjects those with PJI negative will complete the process at the first stage.

And the second stage of this study will be based on the subjects with positive PJI from the first operation. They will receive both FDG and Ga68 citrate PET/MR scans after antibiotic bone cement was implanted. The sensitivity/accuracy of the two tracers for PET/MR can be calculated and compared. This stage is aimed to answer whether PET/MR scan is a feasible imaging tool to provide diagnostic information of infection control status after the resection arthroplasty of hip/knee PJIs, especially with the implantation of antibiotic loaded bone cement.

In the second stage, the investigators shift the imaging modality to PET/MR based on the following reasons: (1) MRI itself has no radiation burden; (2) MRI provides more accurate tissue contrast information and therefore better anatomic delineation; and (3) currently there was no study indicating the existence of ABLC may hamper the interpretation of images.

The study duration is expected to be completed in a period of 3 year. It plans to enrol a total of 40 evaluable subjects with suspicious. And we expect the PJI prevalence will be around 75%, i.e. the anticipated number of subjects of true PJI is around 30. The sample size and the prevanence is given based on the clinical availability and consideration.

The recruited subject subject with periprosthetic joint infection will undergo total four PET scans at two different stages. At first stage, subsequent FDG PET/CT and Ga68 citrate PET/CT scans on different two days will be arranged before infective prosthesis is removed. Eight to twelve weeks after the 1st stage operation, patient will receive two subsequent PET/MR scans using Ga-68 Citrate and FDG on different two days, respectively. The PET images will be visually examined by a nuclear medicine physician and will be reported if any abnormal uptake higher than background by their location and standard uptake value (SUV). The CT/MR images will be reported by a radiologist using standard reading procedures, respectively. The lesions on the PET scan will be correlated to the computed tomography and magnetic resonance images, and the final sensitivity, specificity, and accuracy will be calculated according to the histopathology results or composite clinical and laboratory data.

On the scan day using Ga68-Citrate, baseline electrocardiogram, complete blood count (CBC), and biochemistry profiles including serum alanine transaminase (ALT) and creatinine (Cre) level will be done prior to injection of the radio-pharmaceutical (Ga68-Citrate). Vital signs will be measured before the scan beginning. After completing exam, the electrocardiogram and vital signs will be measure again, and the subject will be released if there is no discomfort. The subject will return to the clinic within 1 week after Ga68-citrate scan. CBC, biochemistry profiles will be checked again for safety monitoring.

Inclusion Criteria:

1. Patients with clinically-proved or -suspicious periprosthetic joint infection and treated with two-stage exchange arthroplasty
2. Age equals or more than 20 years old
3. Willing to sign the informed consent

Exclusion Criteria:

1. Unable to tolerate PET/MR or PET/CT scan, such as those with magnetic implants (e.g. those received intracranial aneurysm surgery, cardiac pacemaker, artificial valves replacement, artificial ears), renal impairment (glomerular filtration rate lower than 30 ml/min/1.73 m2), claustrophobia, unable to lie still, allergy to medium contrast.
2. Unable to give informed consent
3. Patient who is pregnant or lactating
4. Unwilling to use contraceptives during nuclide medicine examinations.
5. Allergy history to FDG or Ga68-citrate, significant abnormal lab data, and high risk to conduct examination after evaluations of PI.