In (single photon) emission computed tomography one has to compute the activity distribution f(x) and the distribution of the attenuation coefficient μ (x) from the integral equation $$\mathop \smallint \limits_{L(s,\omega )} f(x){e^{ - (M\mu )(x,\omega )}}dx = g(s,w)$$ where ω is a unit vector, L(s, ω) is the straight line perpendicular to a ω with (signed) distance s from the origin, and $$(M\mu )(x,\omega ) = \mathop \smallint \limits_o^\infty \mu (x + t{\omega ^ - })dt$$ is the fan beam transform. We show that for finitely many sources, Mμ (x,ω) can be determined up to an additive constant for all sources x by the consistency conditions in the range of the attenuated Radon transform.