Abstract : Objectives: Respiration motion may impair tumor quantification with micro-SPECT imaging in upper abdomen area. A respiratory signal-based gated reconstruction addresses this problem, but deteriorates the signal to noise ratio and other intensity-based quality measures. The objective is to provide a 3D reconstruction without respiratory artifacts and without degrading the image quality by taking advantage of the distinctive breathing pace of anesthetized mice. Materials and methods: SPECT/CT acquisitions were carried out with a camera eXplore speCZT CT 120, GE Healthcare and on 8 NOD-SCID mice with peritoneal metastases of colic adenocarcinoma (HT29) expressing the sodium iodide symporter (NIS), after injection of 99mTc-pertechnetate. We compared 3 different SPECT reconstruction methods: standard 3D without breathing motion correction (3D), the 10 images issued of respiratory gating (4D), and a new method based on motionless phase detection, simulating breath-hold, at the end of expiration (3D-bh). Results: We measured lesions maximum standardized uptake values (SUV max) and volumes (40% of SUV max) of 72 peritoneal metastases. 3D-bh uses 61% of detected events (vs. 6% for 4D) inducing noise level better than 4D and similar to 3D. 3D-bh allows a more precise measurement of the lesions volumes than 3D when compared to 4D. Conclusions: 3D-bh reconstruction provides an optimized tradeoff between count statistics and accuracy of measures. It leads to lesion size and uptake measurements similar to gated 4D SPECT, while maintaining image count statistics closer to standard 3D SPECT.