[en] Background: Myocardial perfusion imaging (MPI) is an established method for assessing the severity of coronary artery disease, which requires the administration of two doses of 99mTc-Tetrofosmin for diagnosis protocol. Optimization of the activity should be performed as a function of several acquisition parameters for adequate image quality with the lowest radiation exposure. The currently recommended activities have remained unchanged for many years (370/740 MBq rest/stress). A new protocol reducing the activity to be administered has been evaluated with regard to its potential for radiation dose reduction while preserving diagnostic image quality. Methodology: In our protocol for MPI with 99mTc-tetrofosmin®, an activity of 296/629 MBq at rest/stress was applied to 99 patients (36 female, 63 male). A second set of 49 patients (14 female, 35 male) with 259/555 MBq at rest/stress was included in the study. The SPECT scans were acquired with M.GE® dual-head with the standard protocol for MPI. The biodistribution of 99mTc-Tetrofosmin reported by Higley was used in an internal dose calculation program called OLINDA/EXM-1.0® to estimate absorbed radiation doses (mGy) and effective radiation doses (mSv), in our total of patients (female 50 – 100kg / male 43 – 105 kg body mass). Two nuclear medicine physicians have evaluated with visual and quantitative analysis the acceptability of the image quality for clinical interpretation without increasing imaging time. Results: In total there were 148 patients (50 f/ 98 m, age 32 – 88 y). The mean effective dose in females estimated by internal dosimetry was 3.25 and 5.56 mSv at rest/stress respectively. In males the mean effective dose estimated by dosimetry was 2.65 and 4.51 mGy at rest/stress. For male and female patients of the same weight the dosimetry showed that the women’s effective doses are about 17% higher than for men as result of less body muscle mass in women. According to physician’s evaluation, 100 % of the total images were deemed acceptable for interpretation. Conclusion: The international community has been considering the need of decreasing the effective radiation dose as far as possible in patients who require medical imaging involving radiation in order to reduce the associated risks. Recent papers demonstrate progress made in reducing the radiation dose. In this new protocol for reducing the activity to be administered, our results show a radiation dose reduction of 20% at rest and 15% at stress. This study presents new information about the internal dose assessment, concluding that the women’s mean absorbed dose in organs and effective dose is higher than in men. The difference in the effective dose is about 20%. The results of the image quality evaluation suggest that interpretable MPI images can be obtained in adults with a reduction in the administered activity of 20% at rest and 15% at stress. Protocols with high activity of 99mTc-Tetrofosmin should be re-evaluated in favour of those with lower activity, such as the one evaluated here, in order to reduce patients’ absorbed and effective dose. (author)