A normal variant on TI-201 and Tc-99m MBI whole-body imaging - The superior right atrial wall (Auricle) and superoanterior right ventricular wall areoften seen as mediastinal lesions

Purpose: The authors have often observed on TI-201 and Tc-99m sestamibi (MI BI) scans in patients with thyroid cancer a small focus of increased uptake in the right midparasternal region (focus A) or sometimes in the lower mid chest at the level of the lower sternum (focus B) just inferomedial to foc us A. The objective of this study was to assess the frequency of this findi ng on TI-201 MIBI studies, to assess the incidence of true pathologic lesio ns corresponding to these foci, and to identify their nature. Materials and Methods: One hundred ten whole body TI-201 studies using 4 mC i (148 MBq) and 84 MIBI studies using 20 mCi (740 MBq: first-pass, planar, and SPECT images) were reviewed. The appearance of either focus A or focus B on three orthogonal SPECT images was correlated with an atlas of cross-se ctional anatomy and computed tomography, If focus A was seen on the immedia te static image (obtained at the end of the first-pass acquisition without moving the patient), this image was coregistered with a selected image from the first-pass study showing the superior vena cava and also with another selected image showing the ascending aorta. Results: Focus A was seen in 40% of TI-201 scans and in 49% of MIBI scans, whereas focus B was seen in 20% of TI-201 scans and 39% of MIBI scans. On c orrelation of the SPECT images with a cross-sectional anatomy atlas, focus A and focus B invariably corresponded to the superior portion of the right auricle and basal superoanterior right ventricular wall, respectively. Thes e myocardial regions are prominent and sometimes appear as discrete foci be cause they are considerably thicker than other parts of the right atrial an d right ventricular muscle, respectively, and because they are seen partly end-on in the anterior projection.