[en] This paper describes a three-dimensional (3D) method for quantifying organ and lesion volume in SPECT. A 3D edge detection technique based on a 3D gradient operator is used to define the boundary of a radionuclide distribution in typical low contrast, high noise SPECT images. The method combines pre- and post-reconstruction filtering of the SPECT images with a 3D adaptation of interpolative background subtraction to provide adequate noise reduction for reliable boundary definition. Simulation studies have shown the method to provide a direct measurement of relatively small volumes of SPECT images with good reproducibility and minimal operator bias. The method is applicable to quantifying volume of both hot and cold lesions