Pilot Imaging Study of Leukemia

Inclusion Criteria: 1. Aged 4 to 80 years 2. Evidence of high-risk hematopoietic malignancy with relapsed/refractory disease: acute lymphocytic leukemia, Acute myeloid leukemia, Ambiguous lineage leukemia, myeloma 3. Karnofsky/Lansky score of ≥ 50 4. Agree to use contraceptive measures during study protocol participation (when age appropriate) 5. Patient or parent/guardian capable of providing informed consent. 6. Ability to undergo 18F FLT imaging without sedation 7. Bilirubin \< 2.5 mg/dL, AST/ALT \<5x upper limit of normal, Serum creatinine \< 1.0 or 2x the upper limit of normal (whichever is higher) 8. Pulse oximetry of \> 90% on room air 9. Ability to undergo 18F FLT imaging without sedation 10. Anticipated immunotherapy (Arm A to include patients who received immune therapy with co-enrollment on a separate protocol or other immunotherapy) and Arm B, those who received other non-immune therapies to treat their cancers (excludes HSCT but includes chemotherapy or non-HSCT radiotherapy). Exclusion Criteria: 1. Patients with uncontrolled infections 2. Pregnancy or lactating 3. History of prior fluorothymidine allergy or intolerance.