Radioimmunoimaging and immunohistochemical detection of AL amyloid. Three patients with systemic AL amyloidosis (AL 2, AL 11, AL 12) received an intravenous infusion of approximately 2 mCi (1 mg) of ¹²⁴I-labeled m11-1F4. (A) Fused coronal and sagittal PET/CT images acquired 5-days after infusion using the high-resolution Siemens Biograph 16 (patient AL 2) or molecular CT (patients AL 11, AL 12) instruments. (B) Maximum intensity projection PET images. (C) Polarizing and light microscopy. Consecutive tissue sections from each patient (AL 2, lymph node; AL 11, bone marrow, and AL 12, liver) were subjected to histochemical (HC) staining with Congo red (CR, top) or immunohistochemical (IHC) studies using, as primary reagent, mAb 11-1F4 (middle), or, as a negative control, the antibody preincubated with a 22-mer peptide containing the conformational fibril-related epitope recognized by mAb 11-1F4 (bottom). Photomicrographs were acquired with a Leica DM 500 light microscope equipped with cross-polarizing filters. Digital images were obtained using a cooled charged coupled device camera and dedicated SPOT software (Version 3.5.2) at an original magnification × 160.