Reward processing is essential to human brain function, with dopamine signalling in the nucleus accumbens (NAcc) as a key element. The monetary incentive delay task is widely studied with functional magnetic resonance imaging (fMRI), measuring indirect hemodynamic changes. Functional positron emission tomography (fPET) with 6-[¹⁸F]FDOPA directly quantifies dopamine synthesis, enabling dynamic assessment during task performance within a single scan. We investigated the reliability of 6-[¹⁸F]FDOPA fPET and blood oxygenation level dependent (BOLD) fMRI during a modified monetary incentive delay task in 25 (10 female; age 24.6 ± 6.1 years) healthy participants across two PET/MRI sessions. Intraclass correlation coefficients and coefficients of variance were computed for BOLD beta estimates and striatal dopamine synthesis (Ki) at 30 s and 2 s resolutions in the caudate, putamen and NAcc. 6-[¹⁸F]FDOPA Ki estimates showed fair to good reliability in the NAcc and putamen at rest (ICC = 0.47–0.66), fair reliability during the win condition in the caudate and putamen (ICC = 0.46–0.57), but poor reliability across the loss condition in all regions (ICC = 0.11–0.39). Conversely, fMRI beta showed good reliability in the NAcc during feedback and in the caudate during feedback loss (ICC = 0.61–0.67), but fair reliability elsewhere (ICC = 0.4–0.46) except for poor reliability in the caudate during cue loss (ICC = 0.26). These findings indicate that both methods achieve comparable reliability but in different target areas, with the molecular specificity of fPET offering dynamic assessment of dopaminergic function. Clinicaltrials.gov Identifier: NCT06675851. EudraCT Number: 2019-004880-33.