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We evaluated an analytic attenuation correction method for the National Institutes of Health (NIH) Advanced Technology Laboratory Animal Scanner (ATLAS) small animal positron emission tomography (PET) scanner. This method is based on the body outline of emission images and an average empirical μ (linear attenuation coefficient) value. We evaluated this method using a computed tomography (CT)-based attenuation correction (ACs) as the "gold standard." A specially fabricated attenuation calibration phantom with cylindrical inserts that mimic different body tissues was used to derive the relationship to convert CT values to μ for PET. The methods were applied to three test data sets: 1) a uniform cylinder phantom, 2) the attenuation calibration phantom, and 3) a mouse injected with [18F]FDG. CT-based correction achieved activity restorations expected for all three data sets. The analytic method showed similar correction factors for the cylinder phantom and the mouse for which the attenuation medium volume matches the detectable emission source distribution. The correction factor for the attenuation calibration phantom was smaller because the emission image outline underestimated the attenuation medium volume.