BACKGROUND: The DPUK PET-MR Harmonisation Study aims to quantify within-site and across-site variability of brain scans across scanning equipment. Brain images were acquired from 8 PET-MR scanners to optimise clinical trial design and evaluate PET-MR methods. Scans were collected from healthy elderly participants in three groups: Repeated, Intra-manufacturer, and Inter-manufacturer (Table 1). This study examines variability in imaging-derived phenotypes (IDPs) from T1-weighted MRI scans and evaluates harmonisation methods. METHOD: All T1w scans were processed using the UK Biobank (UKB) pipeline to extract volumetric IDPs (total brain, tissue-specific, hippocampus). Within-subject (scanner) variability was quantified using the Coefficient of Variation (CoV) for each subject pair and IDP. In the inter-manufacturer group, CoV and cross-subject variability (biological variability measured as across subjects' standard deviation of within-subject mean IDP) were assessed following three harmonisation approaches: IQM-based: Identify image quality metrics (IQMs) (e.g., noise, contrast) that significantly differ across scanners and exhibit low correlation (-0.5