Figure 1.

Overview of different sources of site effects. The left column lists categories of technical factors that are bound to a specific acquisition site and that may significantly influence the primary image properties. “Scanner effect” is often used as an abbreviation for “image acquisition platform” in a wider sense while it incorporates numerous technical components. In active tasks where researchers or technicians intervene as part of the study protocol (e.g., provide instructions to participants), additional site specific effects of a systemic nature may appear. The right column lists factors that may be variables of interest (such as a disease status) or variables of no interest, depending on the study question. Each of them may be site-specific and thus co-vary with the specific site in a multi-site analysis, imposing the challenge of disentangling the two sources of “site effects”. It should be noted that “variables of no interest” may still be essential for an adequate statistical model of the biological effects (e.g., age).