Quantification of the Thyroid Scan (TS) and correlation to Multiparametric Ultrasounds (MPUS): A textbook case of nuclear molecular imaging

For decades, thyroid scintigraphy (TS) has been considered an interesting tool, especially in the field of hyperthyroidism. In recent years, TS has rapidly gained importance since it provides unique molecular information that cannot be obtained by any other modality. In fact, despite a limited 6 mm spatial resolution, it can highlight molecular and histo-functional changes that characterize most thyroid function disorders. However, to become such a powerful molecular image, the TS must be quantified. How much iodine is taken-up characterizes the Uptake (Up), while where iodine distributes characterizes the Spatial Targeting (ST). Methodology, results and limits of the thyroid Uptake are presented, including suppressed tests. Methods to determine the anatomical thyroid volume are revisited with special focus on planar scintigraphy. Recent developments in quantification make the ¹²³I-TS a new molecular imaging procedure. Since ¹²³I targets the sodium iodide symporter (NIS) and tracks the whole organification process, we derived a fundamental linear relationship between the TSH and the precocious (120–240 min) Uptake (p¹²³IUp). This relationship indicates whether the ¹²³I input follows the physiological TSH stimulation or is predictive of a non TSH-suppressible function, whatever the imaging pattern. This allows identification of toxic or compensated (TSH > 0.1 mU/L) Thyroid Functional Autonomy (TFA), even at baseline. Spatial Targeting, measured with the aid of computational algorithms, provides a reproducible Spatial Targeting Index (STI). This allows estimating a functional thyroid volume, that is likely more informative than the anatomical one. Most aspects of TS quantification and the interest to compare the structure (mostly MultiParametric US) and the function (molecular ¹²³I-TS) are presented.