The study will enroll up to 22 participants in two different sets. Up to 16 participants will be enrolled in Set A. Within that total, up to 5 dose levels of TAK-831 may be evaluated, with up to 6 participants per dose level, although typically, there will be 2 to 3 participants per dose level. All participants in Set A will also receive up to 3 doses of [18F]PGM299. Up to 6 participants will be enrolled in Set B. All participants in Set B will be assigned to single treatment group to receive 2 doses of [18F]PGM299.