ESTIMATION OF EFFECTIVE DOSE TO PATIENTS UNDERGOING MYOCARDIAL PERFUSION SPECT/CT

Abstract

99mTc-sestamibi SPECT/CT has become widely used for the assessment of cardiovascular diseases. The integrated SPECT/CT results in better quality corrected SPECT imaging with higher patient exposure than SPECT alone. The purpose of this study is to investigate the effective doses to patients undergoing myocardial perfusion SPECT/CT and compare with background equivalent radiation time (BERT). The effective doses had been evaluated in 10 patients at rest and 10 patients at stress study of myocardial perfusion SPECT/CT at King Chulalongkorn Memorial Hospital. The absorbed dose and effective dose from 99mTc-MIBI were calculated by the equations and factors provided by Medical Internal Radiation Dose (MIRD) committee of the Society of Nuclear Medicine with the “S” tables modified with the organ masses of the Asian reference man . The absorbed dose and effective dose from CT scan were determined by using Monte Carlo simulation, ImPACTSCAN program version 1.0.4. The result showed that the organ received the highest absorbed dose was the gall bladder of 35.80±6.00 µGy/MBq for rest study and 34.81±7.33 µGy/MBq for stress study respectively. The organs involved in the excretory pathway of MIBI (small intestine, upper large intestine, lower large intestine and kidneys) received the high absorbed doses. The mean±SD and range of injected activity of 99mTc-MIBI for rest study was 935.00±60.00 (821.00 – 993.00) MBq. For stress study, the mean±SD and range of injected activity was 886.28±108.60 (577.00 – 993.00) MBq. The mean±SD and range of dose coefficients of 99mTc-MIBI was 11.64±0.70 (10.30 – 12.40) µSv/MBq for rest study and 10.18±0.40 (9.30 – 10.70) µSv/MBq for stress study. The mean±SD and range of effective dose from 99mTc-MIBI was 10.85±1.24 (8.45 – 12.29) mSv for rest study and 9.01±1.25 (5.73 – 10.23) mSv for stress study. The mean±SD and range of effective dose from CT was 1.14±0.11 (0.9 – 1.3) mSv for rest study and 1.12±0.13 (0.9 – 1.34) mSv for stress study. The mean±SD and range of total effective dose from rest study of myocardial perfusion SPECT/CT was 11.99±1.2 (9.75 – 13.49) mSv, and its related mean BERT was 3.99±0.4 (3.25 – 4.50) years. The mean±SD and range of total effective dose from stress study of myocardial perfusion SPECT/CT was 10.35±1.2 (6.83 – 11.45) mSv, and its related BERT was 3.5±0.2 (2.28 – 3.82) years. These dose values can be used as reference of Asian population for determination of patient radiation dose undergoing myocardial perfusion SPECT/CT and assist for optimization of myocardial perfusion imaging.