This is a single center prospective study evaluating 124I-evuzumitide in patients with systemic amyloidosis. The purpose of this study is to 1) Establish the diagnostic accuracy of 124I-evuzumitide in cardiac amyloidosis 2) Evaluate extracardiac uptake 3) identify and characterize the distribution and uptake of 124I-evuzumitide in patients with transthyretin amyloid cardiomyopathy (ATTR-CM) and 4) Correlate the uptake with the structure and function of different organs, including the heart. To achieve these goals, eligible patients will undergo primarily hybrid positron emission tomography and magnetic resonance imaging (PET/MRI). In a subgroup of patients who are unable to undergo PET/MR, computed tomography will be used instead of MRI (i.e. PET/CT). In a subgroup of patients, repeat imaging with the same modality will be done at a interval of 6-12 months. Clinically available data (demographics, phenotype, imaging, laboratory) will also be collected to characterize the disease in each patient.

Inclusion Criteria:

1. Subjects will at least have one of the following conditions: systemic amyloidosis with known organ involvement, carrier of a known pathogenic mutation in the transthyretin gene, multiple myeloma, and monoclonal gammopathy of undetermined significance
2. Patient willing to consent for the study and undergo the study procedures.

Exclusion Criteria:

1. Has severe claustrophobia or any medical condition that would prevent completion of the imaging protocol
2. Has a known allergy to potassium iodide treatment or to gadolinium.
3. Patients on dialysis or those with eGFR <30 cc/min/1.73 m2 will be excluded from undergoing gadolinium-enhanced cardiac MRI.
4. Has received heparin or heparin analogs (e.g., enoxaparin, dalteparin, fondaparinux) within 7 days prior to 124I-Evuzumitide administration