The influence of a finite positron annihilation distance represents a fundamental limit to the spatial resolution of PET scanners. It is appreciated that this effect is a minor concern in whole-body F18 imaging, but it does represent an issue when imaging with higher energy isotopes such as N13 or Rb82. This effect is especially relevant for imaging tasks along tissue gradients such as the cardiac/lung boundary and diaphragm/lung boundary. This work presents a method to determine the positron range effect from a CT scan and to model this effect as shift-variant, anisotropic kernels. The positron annihilation distance across boundaries of tissues is represented with a simple model, which can be quickly derived from CT scans and applied in the reconstruction of PET images. The positron range compensation map is applied in a modified OSEM algorithm to simulated and measured data.