The study aims to find new tests to check if transplanted bone marrow cells are growing well after Hematopoietic Stem Cell Transplantation (HSCT). Bone marrow is a spongy tissue inside bones that makes blood cells. HSCT is a procedure where unhealthy bone marrow is replaced with healthy cells. To do this, doctors will use a special imaging test called FLT (fluorothymidine) imaging and take blood samples. They want to find out if these tests can show when bone marrow is not growing as expected, a problem called nonengraftment, or if it's just growing slowly, called delayed engraftment. The study will last for 7 years, take place in four centers, and involve up to 56 patients. Participants will have imaging and blood tests several times within the first month after their transplant and will stay in the study for one year. This is a pilot study, meaning it's a small, early-stage study to see if the tests work.

• Study duration: 1 year for each participant, but the full study lasts 7 years.
• FLT imaging is done before and after HSCT to check bone marrow growth.
• Participants need to be able to undergo imaging without sedation and be 4-80 years old.