Prescribing for infants and children.

Abstract

<b>1496</b> <h3><b>Objectives:</b></h3> A room size PET scanner with head tracking will enable investigation of the brain function of active human subjects in a natural setting. The geometric sensitivity of such a scanner at the center of the field of view (FOV) has been previously studied. The purpose of this work is to characterize the geometric sensitivity of a room PET scanner throughout the entire FOV. <h3><b>Methods:</b></h3> An analytic expression for spatial-dependent geometric sensitivity was derived for the case of a rectangular room with PET detectors along each of the walls. The detectors are at the same distance above the floor and their height is the same, thus forming an open box configuration. For a point source at an arbitrary location in the PET FOV the probability of escape of one or both annihilation photons through the top or bottom bounding planes of the PET detector box is modeled. The sensitivity expression has 12 terms and uses formulae for the solid angle of right square pyramids. The derived expression will apply to any open box arrangement of PET detectors. As an application, sensitivity maps were calculated for a square room with dimensions 3 m on a side, and for detectors with heights of 1 and 2 m. Representative linear profiles through the FOV were generated and compared. <h3><b>Results:</b></h3> Geometric sensitivities from the derived equation agree with values from limiting cases, such as for points on the central axis of the PET FOV. In horizontal slices parallel to the floor, sensitivity in central planes is peaked at the center of the FOV. In horizontal slices closer to the top and bottom of the detectors, sensitivity decreases near the center of the FOV and increases toward the walls of the room. In vertical slices parallel to the walls the sensitivity profiles have a triangular shape at the center of the FOV and approximate a triangular shape closer to the walls. This is similar to the triangular sensitivity profile along the central axis of a conventional cylindrical PET scanner. Geometric sensitivity increases with the height of the detectors, as expected, and the sensitivity maps show similar features for different detector heights. <h3><b>Conclusions:</b></h3> An analytic expression was developed for the 3D spatial-dependent geometric sensitivity of a rectangular room PET scanner. The formula will apply to any open box arrangement of PET detectors. Application of the expression provides insight into the sensitivity function of a room PET system and is useful in system design.