THE AAPM/RSNA PHYSICS TUTORIAL FOR RESIDENTS

The article explores the fundamentals of emission computed tomography (CT) from a nonmathematical approach. Tomographic images reveal the internal distributions of radioactivity in three-dimensional objects, and thus allows anatomic localization and improves contrast. Tomography requires a stable distribution of radionuclides, uniform detector response, an accurate center of rotation, and a complete set of projections. In emission CT, a large number of measurements, called projections, are collected at various angles about the patient during the examination. This information is organized by the angles of acquisition into a stack, called a sinogram. Each projection is modified by applying a reconstruction filter (eg, ramp or windowed reconstruction filters). These modified projections are backprojected to form the transverse tomographic images. The quality of tomographic images generated from filtered backprojection depends on the underlying assumptions about the projections. Typical. artifacts that result from violations of these assumptions include motion, uniformity, and attenuation artifacts. In addition, an inaccurate center of rotation, insufficient angular sampling, and errors in selection of pixel size can result in poor-quality reconstructed images.