Paediatric Sarcoma, Hybrid Imaging & FAPI - Raising the Diagnostic Standard for Paediatric Sarcoma Patients, Combining Novel Radiotracer FAPI and Whole-body PET/MRI

The goal of this clinical trial is to evaluate \[68Ga\]FAPI-46 whole-body PET/MRI as a diagnostic tool to improve tumour lesion examination at primary staging, during treatment control and at relapse in paediatric sarcoma patients. The main questions the trial aim to answer are: * Is \[68Ga\]FAPI-46 whole-body PET/MRI better at detecting malignancy in paediatric sarcoma patients compared to current standard of care imaging modalities? * Is the overall diagnostic performance of \[68Ga\]FAPI-46 whole-body PET/MRI feasible to implement in clinical practice? The intervention of \[68Ga\]FAPI-46 will be compared to current standard of care imaging modalities. All participants will receive their standard of care imaging protocol as rutinely scheduled, which includes: * A localized MRI scan of the tumour region and * An \[18F\]FDG whole-body PET/CT scan In addition to the standard of care imaging protocol, all patients will receive the following experimental scans: * An \[18F\]FDG whole-body PET/MRI * A \[68Ga\]FAPI-46 whole-body PET/MRI * A \[68Ga\]FAPI-46 whole-body PET/CT All patients and legal guardians will fill out questionnaires on their experiences. Patients may undergo the entire trial imaging procedures, including the standard of care imaging protocol and the experimental scans, a maximum of five times.