multi - modal imaging capsule for image - guided proton beam therapy , consisting of a biocompatible polymer layer , 18 o - enriched water , and a contrast agent . the biocompatible capsule may be inserted near or inside a tumor under the guidance of x - ray , magnetic resonance , or ultrasonography imaging . upon proton beam irradiation , the capsule emits positrons , allowing the tumor to be imaged and tracked by a pet detector .