Incremental value of single photon emission tomography/computed tomography in 3-phase bone scintigraphy of an accessory navicular bone

A 10-year-old male child with bilateral club foot presented to the hospital with pain in his right foot. Plain X-ray did not reveal any abnormality. Magnetic resonance imaging (MRI) showed type II accessory navicular bone in right foot with synchondrosis and surrounding edema. Then patient was referred to our department for three phase bone scan. Blood flow, blood pool and delayed static images of ^99mTc-methylene diphosphonate (MDP) bone scan did not reveal any definite abnormal concentration of radio tracer and was equivocal. However, interestingly, hybrid single photon emission tomography/computed tomography (SPECT/CT) showed a small bony structure medial to right navicular bone with sclerosis and an increased radio tracer uptake suggestive of an accessory navicular bone [Figure 1]. The CT acquisition parameters were 110/130 Kv, 100 mAs, pitch-1, 512 × 512 matrix using standard filters in a 6 slice multidetector SPECT/CT system. The CT images were reconstructed with reconstruction kernel B60s and 1 mm slice thickness in all three planes.

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Figure 1 Three phase ^99mTc-methylene diphosphonate (MDP) bone scan images. Blood flow (a), blood pool (b) and delayed static (c) images did not reveal any definite abnormal concentration of radio tracer in right foot. Single photon emission tomography (SPECT) (d), computed tomography (CT) (e) and hybrid SPECT/CT (f) images revealing a small bony structure medial to right navicular bone with sclerosis and increased MDP uptake (arrows) which was suggestive of an accessory navicular bone

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Accessory navicular bone (Os tibialis externum or navicular secundrium) is one of the several supernumerary ossicles of foot. Its incidence is 6-12%. It is mainly found on the medial side of the proximal navicular bone and in continuity with tibialis posterior tendon. Those patients having flat foot are more prone to develop accessory navicular bone. Three distinct types of accessory naviculars are known in the literature.[1] Type 1 accessory navicular is a sesamoid bone in posterior tibial tendon. It accounts for 30% of cases. Type 2 is an accessory ossification center in the tubercle of the navicular bone which accounts for 70% of cases. Cornuate navicular is an anomaly related to the presence of an osseous bridge connecting the navicular bone and the accessory navicular. Type 2 and cornuate navicular are mostly symptomatic and associated with clinical manifestations particularly pain. The indication of three-phase bone scan is to see if the accessory navicular bone is the cause of symptoms.[2] Radiograph is non-diagnostic except for showing its presence in symptomatic cases. The mechanism of pain in accessory navicular has been attributed to traumatic or degenerative changes at the synchondrosis or to soft-tissue inflammation when fused. In radiographic evaluation the 45° eversion oblique view of the foot is the most important view for identifying this condition.[3] High-resolution ultrasonography is also found to be useful in evaluation of accessory naviculars bone.[4] MRI findings of painful accessory navicular bone usually include persistent edema pattern in the accessory navicular bone and within the synchondrosis.[5] Surgical treatment of painful accessory navicular consists of excision along with its synchondrosis. It has been shown that all symptomatic accessory naviculars bones show positive hyperemia in the initial blood pool phase and positive tracer uptake in delayed skeletal phase.[6] If the uptake in symptomatic accessory navicular is equivalent to adjacent tarsal bones and if there is no hyperemia in the initial phase (as in our case), SPECT/CT should be performed. SPECT/CT in our study showed increased uptake corresponding to the accessory navicular bone. Although the role of ^99mTc-MDP bone scan has been documented,[6] the role of SPECT/CT in accessory navicular bone has not been documented. Our case demonstrated that SPECT/CT provides an incremental value when there is an equivocal three-phase bone scan in symptomatic accessory navicular bone.