In general, beam power is optimized to provide only those photon densities necessary for reliable pellet tracking. These comments are not meant to imply that no risk is involved with XRMB procedures. Rather, they indicate that the experimental XRMB radiation exposure risks relative to other diagnostic flood-field techniques are comparable in magnitude and quite low. The entire machine is encased in 2-4 inch lead plates, making its weight approximately 15 tons. It was built by the U.W. Physical Sciences Laboratory .
Pellets are in turn attached to threads for pellet retrieval in case of aspiration.
Instead of irradiating the entire head during pellet tracking, the Microbeam generates a small rastered scan of the field where each pellet is expected to be. From each raster, the position of the pellet is determined and a prediction made as to where it will be at the next sample. The series of x-y positions for a pellet during a trial constitutes that pellet's trajectory. A schematic shows details of the process. After acquisition, acoustic and movement data can be plotted as a time-series.