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).