  We derive explicit formulas for the reconstruction of a function from its
integrals over a family of spheres, or for the inversion of the spherical mean
Radon transform. Such formulas are important for problems of thermo- and photo-
acoustic tomography. A closed-form inversion formula of a
filtration-backprojection type is found for the case when the centers of the
integration spheres lie on a sphere in Rn surrounding the support of the
unknown function. An explicit series solution is presented for the case when
the centers of the integration spheres lie on a general closed surface.
