With continuing advances in medical imaging technologies, there is an increased demand to extract quantitative information from images. This has been particularly vital in the effort to increase the efficacy and accuracy of diagnoses. Quantitative information is readily available in images because the acquisition techniques intrinsically involve physical processes. Quantitative image quality metrics are critical in the evaluation of medical images for diagnostic merit, particularly when used for the characterization and comparison of different systems. When such metrics are based on measurable physical parameters, they can provide valuable information for system optimization. Image quality describes the "goodness" of an image in displaying information for a task. This thesis explored methods of measuring image quality for two scenarios: (1) to characterize 2D flat-panel detector performance and (2) to measure directional spatial resolution for 3D images from breast tomosynthesis.;In the first chapter, two new wireless digital receptors (DRX-1C and DRX-1, Carestream Health, Inc., Rochester, NY) were evaluated and compared to a conventional flat-panel detector (Pixium 4600, Trixell, Moirans, France) on the basis of detective quantum efficiency (DQE). A secondary goal was also to evaluate the filtration to achieve specified beam qualities for the DQE measurements, closely following the methodology of the International Electrotechnical Commission (IEC) for radiation qualities RQA5 and RQA9. All three DR systems demonstrated similar modulation transfer functions (MTFs) at most frequency ranges, while the DRX-1 showed lower values near the cutoff of approximately 3.5 cycles/mm. At each exposure, the Pixium 4600 and DRX-1C demonstrated similar noise power spectrum (NPS) curves that indicated better noise performance than the DRX-1. Zero-frequency DQEs for Pixium 4600, DRX-1C, and DRX-1 were approximately 63%, 74%, and 38% for RQA5 and 42%, 50%, and 28% for RQA9, respectively. In terms of DQE performance, the DRX-1C image receptor was found to be superior to the Pixium 4600 and DRX-1.;In the second chapter, the directional spatial resolution of simulated breast tomosynthesis images was determined using a cone-based technique and a sphere phantom. Projections were simulated for a voxelized breast phantom with 12 mm diameter sphere inserts using a fluence modeled from a 28 kVp beam incident upon an indirect flat-panel detector with 200 microm pixel size. Characteristic noise and blurring for each projection were added using cascaded systems analysis. The projections were reconstructed using a standard filtered backprojection technique, producing a 3D volume with an isotropic voxel size of 200 microm. Regions of interest (ROIs) that completely encompassed single spheres were extracted, and conical regions were prescribed along the three axes extending from the centroid. Voxels within a cone were used to form an edge spread function (ESF), from which the directional MTF was calculated. A bin size of 0.02 mm and a conical range of 30 degrees were found optimal for maximizing accuracy and minimizing noise of the MTF. A method for removing out-of-plane artifacts of the ESFs along in-plane axes was investigated and yielded a modified MTF. The idea of separating the effective resolution and artifacts from the measured ESF are expected to facilitate the interpretation of MTF measurements in breast tomosynthesis. Similar methods may be applied to characterize the spatial resolution of other 3D imaging modalities.