The absorbed radiation dose from internally deposited radionuclide is a major factor in assessing risk and therapeutic utility in nuclear medicine diagnosis or treatment. The quantification of absolute activity in vivo is necessary procedure of estimating the absorbed dose of organ or tissue. To understand accuracy in the determination of organ activity

the experiments on 99Tcm activity quantification were made for a body phantom using dual-head SPECT with the two-view counting technique. Accuracy in the activity quantification is credible and is not affected by depth of source organ in vivo. When diameter of the radiation source is ≤2 cm

the most accurate activity quantification result can be obtained on the basis of establishing the system calibration factor and transmission factor. The use of Buijs’s method is preferable

especially at very low source-to-background activity concentration rations.