In this study, we will study 129Xe-MRI and LCI as tools for diagnosis of BOS in pediatric patients who have received Hematopoietic Stem Cell Transplantation (HSCT) and have been identified as eligible for this study. Participants will be required to have vital signs collected, complete breathing tests and complete an MRI. The MRI will require participants to perform breath holds in the MRI scanner with xenon gas while being coached by a research assistant.

Inclusion Criteria:

• Male and female participants aged 6 -17.9 years old.
• Received HSCT treatment at least 6 months prior to enrollment in study.
• Participants should have an FEV1%pred value greater than 40%.
• Participant understands the study procedures and is willing to participate in the study as indicated by signature on the informed consent or assent.
• Participant must be able to perform a breath hold for 16s.
• Participant meets MRI screening criteria

Exclusion Criteria:

• Participant has had a cold or respiratory infection in the last four weeks.
• Participant requires supplemental oxygen or has a daytime room air oxygen saturation ≤ 95%.
• Participant is unable to perform spirometry or plethysmography maneuvers.
• Participant is pregnant or lactating.

Two groups of participants will be recruited. Group 1 will be participants who meet the screening criteria for the development of BOS according to the NIH criteria. Group 2 will be participants who may or may not have abnormal spirometry; however they do not fulfill the NIH criteria for the development of BOS.