15 General introduction and outline of this thesis In patients with tarsal coalitions, metabolic changes in bone metabolism and consequently subtle structural abnormalities of relatively small and anatomically challenging structures may define treatment and outcome. Therefore, the optimal modality should sensitively detect changed bone metabolism and precisely depict anatomical bone changes to inform the orthopedic surgeon on active local disease. In the follow-up of post-operative tarsal coalitions, especially talocalcaneal coalitions, plain radiographs lack sensitivity to discern active pathology. Current scientific literature on optimal use and limitations of bone-SPECT/CT in patients with tarsal coalitions is scarce, especially in the post-operative setting. This thesis analyses a small structured series of patients with tarsal coalition under consideration for surgery and in follow-up after surgery, and subsequently describes scenarios for the use of bone-SPECT/CT following plain radiographs. These scenarios aim to provide an evidence-based imaging strategy in patients with tarsal coalition and insufficiently explained symptoms. FD/MAS, current challenges for patients and their physicians FD/MAS is a rare benign disease caused by a somatic post-zygotic mutation in the G-nucleotide binding protein alpha sub unit (GNAS1) on chromosome 20q13.32 and usually presents around adolescence, with a wide age range [10, 11]. In this metabolic bone disease, normal bone is being replaced by fibrous tissue, which has a structure and quality inferior to healthy bone. Clinical presentation varies from an accidental finding of a solitary small extra-articular lesion with limited to no complaints, to extensive bone disease causing considerable impairment and/or fractures. Any bone may be affected: in monostotic fibrous dysplasia, one single bone is affected, comprising the majority of FD/MAS-patients. In the polyostotic form, multiple bones are affected, as shown in Figure 1, panel A. If FD is accompanied with hyperpigmented skin lesions (café-au-lait macules), and endocrinopathies such as precocious puberty, it is calledMcCune-Albright syndrome. Typical skin presentation is shown in Figure 1, panel B. In severe FD/MAS, the personal impact can be enormous. Not only may it instigate physical problems such as deformity, fractures, pain and hamperedmobility, but also due to psychological effects, such as stigma, depression, and anxiety [13]. 1