We study both analytically and numerically the gravitational fields of stars in f (R) gravity theories. We derive the generalized Tolman-Oppenheimer-Volkov equations for these theories and show that in metric f (R) models the Parameterized Post-Newtonian parameter γPPN = 1/2 is a robust outcome for a large class of boundary conditions set at the center of the star. This result is also unchanged by introduction of dark matter in the Solar System. We find also a class of solutions with γPPN ≈ 1 in the metric f (R) = R − µ 4 /R model, but these solutions turn out to be unstable and decay in time. On the other hand, the Palatini version of the theory is found to satisfy the Solar System constraints. We also consider compact stars in the Palatini formalism, and show that these models are not inconsistent with polytropic equations of state. Finally, we comment on the equivalence between f (R) gravity and scalar-tensor theories and show that many interesting Palatini f (R) gravity models can not be understood as a limiting case of a Jordan-Brans-Dicke theory with ω → −3/2.

We study the interior spacetimes of stars in the Palatini formalism of f (R) gravity and derive a generalized Tolman-Oppenheimer-Volkoff and mass equation for a static, spherically symmetric star. We show that matching the interior solution with the exterior Schwarzschild-De Sitter solution in general gives a relation between the gravitational mass and the density profile of a star, which is different from the one in General Relativity. These modifications become neglible in models for which δf (R) ≡ f (R) − R is a decreasing function of R however. As a result, both Solar System constraints and stellar dynamics are perfectly consistent with f (R) = R − µ 4 /R.PACS numbers: 98.80. -k, 95.35.+d, 04.50.+h

Regardless of patient size, most patients in this study were positioned too low, which negatively affected both patient dose and image noise. Miscentering was more pronounced in smaller pediatric patients.