Localization of parathyroid adenomas – SPECT subtraction technique using statistical parametric mapping (SPM) registration method

Objectives To develop a 3D subtraction technique using two SPECT images (^99mTc-MIBI and ^99mTc-Tc0₄^-) of (para)thyroid gland which should represent a complement to dual-phase MIBI imaging or 3D MIBI - ¹²³Iodine subtraction. Dual-phase method is unable to detect pathological parathyroid glands with fast wash-out; using of ¹²³I causes interference between energy windows.

Methods The subtraction of Tc0₄^- from MIBI image should determine a parathyroid gland which size might be lower than the spatial resolution of a gamma scanner. The images are subtracted after registration and normalization. Required precise and objective registration of both images is performed by the SPM package. Normalization is performed to the mean activity deposited in the para+thyroid system. Normalization volume is determined as an intersection of activity and geometry masks. A suspicious finding consists of a small group of voxels which is visible in the contrast image as a remainder of MIBI and records a difference in a order of tens or hundreds percent.

Results A group of 59 subjects has been analyzed using this method. All subjects had preceding conclusions made by visual analysis of dual-phase MIBI washout using 3D-visualized tomo images. We added 15 possible positive results from a group of 30 a priori negative results. Up to now, 2 possible positive cases have been surgically confirmed. In 3 positive cases, we discovered another suspicious lesions which had been missed by dual-phase study due to presence of a primary finding.

Conclusions It is a promising method, which must be still verified and evaluated against a suitable gold standard. It may be able to improve the sensitivity at least regarding the dual-phase study. Data for specificity issues are still lacking.