A phantom placed alongside a patient, i.e., “pocket phantom,” can enable quality assurance of proton density fat fraction (PDFF) measurements. To determine the optimal positioning, the spatial dependence of PDFF measurements must be understood. To address this unmet need, we filled the bore of a 3T MRI with pocket phantoms. Though PDFF bias was low, it increased with distance from isocenter. Absolute bias was <2.5% within 18cm (in-plane) and within 10cm (z-direction). We conclude that the optimal position for a PDFF pocket phantom is on the table, centered below the patient at the level of the examined organ.