Microwaves are used to non-destructively assess the uniformity of structural wood beams. The near-field radiation penetration from a surface-mounted cavity-backed slot antenna depends on the conductivity and permittivity of the wood, but it cannot be predicted from the antenna impedance alone. The Specific absorption rate (SAR) is typically used to estimate the power absorbed by living tissues and can be determined by numerical modelling. In this paper, SAR analysis was used to image microwave radiation inside isotropic wood beams with a square cross-section with a density ranging from 60 to 1000 kg/m 3 . The wood beams were imaged with and without defects.