Use of 99mTc-MDP SPECT for assessment of mandibular growth: development of normal values

Planar scintigraphy using Tc-99m-labeled methylene diphosphonate (Tc-99m-MDP) has been established as a method for evaluating mandibular growth and asymmetry. However, Tc-99m-MDP single photon emission computed tomography (SPECT) provides improved image contrast and quantitative capability potentially allowing the clinician to make a more accurate assessment of the stage of activity of normal and asymmetric mandibular growth. The purpose of this study was to utilize Tc-99m-MDP SPECT in normal subjects to develop a standardized approach to imaging the mandible that could subsequently be used to evaluate patients with potential for mandibular growth abnormalities. Thirty-two patients (22 females, 10 males, 8-25 years of age, mean of 14 years) having Tc-99m-MDP bone scans (7.4 MBq/kg, maximum of 518 MBq) for indications not involving the head were asked to have an additional SPECT that included the mandibular condyles. Internal (clivus) and external (source of known activity included within the field of view) standards were both investigated. Two- and three-dimensional (2-D and 3-D) regions of interest (ROI) were drawn over each condyle and the clivus in a single slice containing the maximum pixel value within the structure. A 3-D ROI was also drawn about the external standard. Maximum and average pixel values within the 2-D and 3-D ROIs were recorded. Correlation analysis indicated that essentially the same information was obtained whether the maximum or average pixel value within a 2-D or 3-D ROI was used. The right to left ratio provided a parameter that can be used to assess the magnitude of mandibular metabolic asymmetry. Use of an external standard provides the potential for quantifying condylar uptake for assessment of the activity of mandibular growth. Normal ranges for different age groups were determined for condylar uptake using both internal and external standards. Tc-99m-MDP SPECT provides a quantitative method for assessing mandibular condylar uptake as a reflection of the activity of mandibular growth. The maximum pixel value of a 2-D ROI using external standardization and the right to left ratio of the condylar uptake provide the potential for quantitative evaluation of patients with mandibular asymmetry. These parameters will be further evaluated in an upcoming clinical investigation.