DETERMINATION OF SMALL OBJECTS FROM PINHOLE SCINTIGRAMS

This study assessed the possibility of measuring the linear dimensions of small structures using pinhole scintigraphy. A number of glass obj ects were made with a spherical, cylindrical or conical shape. Their m aximum dimensions (diameters and heights) were 3.5-22.5 mm. These glas s objects were filled with I-131, placed inside a plastic neck phantom and imaged using a gamma camera equipped with a pinhole collimator. T he source-to-collimator distance was varied from 2 to 12 cm. An algori thm for image seg mentation (threshold selection) was used to divide t he image into object and background. On the segmented image, the numbe r of non-zero pixels in the direction of the principal axes was multip lied by the appropriate calibration factor to obtain the linear dimens ions of the object. Spatial resolution of the pinhole collimator, expr essed as the full-width at half-maximum (FWHM), varied from 8 to 10 mm for the range of source-to-collimator distances examined. We found th at, for dimensions up to 1.5 x FWHM, finite spatial resolution affects the accuracy of measurement. Non-linear correlation between true and calculated dimensions was used to take the latter into account. Our re sults are now being used to improve quantitation of remnant thyroid ti ssue masses for the calculation of radioiodine ablation doses. ((C) 19 98 Lippincott-Raven Publishers).