^99mTc MDP SPECT to evaluate mandibular growth in children: Development of normal standards

Abstract

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Objectives: To evaluate ^99mTc MDP uptake in normal mandibular condyles with SPECT. Asymmetric mandibular growth can lead to facial disfigurement in children. Standards in normal patients are needed to diagnose asymmetry with SPECT. Methods: Thirty-two patients (10 male, 22 female; age 8-25, mean 14 years) referred for a ^99mTc MDP scan not of the face received an additional SPECT of the head. Subjects were injected with ^99mTc MDP (7.4 MBq/kg) and imaged at 3-4 hours. An external reference (vial of known activity) was used. ROIs were drawn on all transaxial slices containing the right and left condyles, the clivus and the external source. Condyle counts were normalized by the clivus counts or a calibration factor (activity divided by ROI counts of the external source). Right and left condyle values were compared by the paired t-test. Age-based normal ranges for the condyle activity normalized by the internal and external standards were determined. Symmetry parameters were calculated using the difference between the right and left condyles normalized by the mean or the ratio of the right and left condyle. Results: There was no significant difference between the activity of the right and left condyles (p=0.23). Using the internal standard yielded a mean of 0.32 (95% CI of 0.07-0.57) and its correlation with age was not significantly different than 0 (r=-0.18). Using the external standard, there was a significant correlation with age (r=-0.53) and age-based normal ranges were determined using regression. The mean difference between the right and left condyles, normalized by the mean, was -0.02 and not significantly different than 0 (95% CI of -0.19 to 0.21). The left-to-right ratio had a mean of 0.99 not significantly different from 1.0 (95% CI of 0.78 to 1.19). Conclusions: ^99mTc MDP SPECT is an effective method of assessing mandibular symmetry. Internal and external standards can be used to evaluate the activity in the condyle. The right-to-left difference normalized by the mean or the right-to-left ratio can be used as parameters of mandibular symmetry. Age-based normal ranges of the ^99mTc MDP condyle activity and symmetry parameters have been determined.