A Study to Evaluate Organ Level Uptake Repeatability of 124I AT-01 in Subjects With Systemic Amyloidosis

Inclusion Criteria: 1. Understands the study procedures and is capable of giving signed informed consent, as described in Appendix 1, which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol. 2. Male or female ≥18 years of age. 3. For women of childbearing potential: agreement to remain abstinent or use contraceptive methods that result in a failure rate of \<1% per year during the treatment period and for at least 90 days after the last dose of 124I-AT-01. 1. A woman is considered of childbearing potential if she is postmenarchal, has not reached a postmenopausal state, and has not undergone surgical sterilization. 2. Examples of contraceptive methods with a failure rate of \<1% per year include bilateral tubal ligation, male sterilization, established, proper use of hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices, and copper intrauterine devices. 3. Contraception methods that do not result in a failure rate of \<1% per year such as cap, diaphragm, or sponge with spermicide, or male or female condom with or without spermicide, are not acceptable. 4. The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the subject. 4. For men: agreement to remain abstinent or use contraceptive measures and agreement to refrain from donating sperm, as defined below: a) With female partners of childbearing potential, men must remain abstinent or use a condom plus an additional contraceptive method that together result in a failure rate of \<1% per year during the treatment period and for at least 120 days (a spermatogenesis cycle) after the last dose of study intervention. Men must refrain from donating sperm during this same time period. 5. Able to undergo two PET/CT scans as part of the study, including ability to lie supine for up to 1 hour. 6. Has a history of AL or ATTR systemic amyloidosis with at least one organ with clinically demonstrable amyloid involvement defined by: 1. AL systemic amyloidosis: Positive tissue biopsy for AL amyloid, and achieved a hematologic very good partial response or complete response based on their most recent assessment, and at least one of the following: 1) Organ biopsy positive for amyloid, or 2) Natriuretic peptide (NT-proBNP) \>650 pg/mL, or 3) left ventricle septal wall thickness \>12 mm by echocardiogram or cardiac magnetic resonance (CMR), or 4) 24-hour urine protein \>500 mg, or 5) Urine albumin-to-creatinine ratio \>300 mg/g 2. ATTR (wild type or variant) systemic amyloidosis: Positive cardiac biopsy for ATTR amyloid, or at least two of the following: 1) Positive extracardiac tissue biopsy for ATTR amyloid or positive transthyretin gene mutation associated with amyloid, or 2) left ventricle septal wall thickness \>12 mm by echocardiogram or CMR, or 3) pyrophosphate (PYP) scintigraphy with myocardial uptake ≥grade 2. Exclusion Criteria 1. Is pregnant or breast-feeding. 2. Is mentally or legally incapacitated, has significant emotional problems at the time of the study, or has a history of psychosis. 3. Has received in the last 6 months or are currently receiving treatment with anti-amyloid monoclonal antibody therapy or are expected to begin treatment prior to completing this study. 4. Has received heparin or heparin analogs within 7 days of Day 1. 5. Has a significant co-morbidity (e.g., Easter Cooperative Oncology Group (ECOG) score of 3 or greater), New York Heart Association (NYHA) Class IV heart failure, uncontrolled infection, or other ongoing serious illness. 6. Has active thyroid disease. 7. Has a known allergy to potassium iodine treatment. 8. Is receiving hemodialysis or peritoneal dialysis. 9. Has severe claustrophobia that would prevent completion of the PET/CT imaging protocol. 10. Has received an investigational agent within five half-lives of the agent or 30 days, whichever is longer, prior to Screening. 11. Has any illness that, in the opinion of the Investigator, might confound the results of the study or pose additional risk to the subject.