A method of performing physiological gating in a medical procedure includes acquiring a sequence of images having at least a first image and a second image of a target region, determining a first composite image based on the first and second images, and gating a medical procedure based on the composite image. The barrel includes a plurality of layers. Accordingly, the yield of a process used to generate die cast structures may be greatly increased. As specifically exemplified herein, the polymer backbone is a PEO of about 270 units with about 10-30 core-forming PLAA subunits, and advantageously about 14-24.