The background ROI used in determining percentage gastroesophageal (GE) reflux is placed over the:

The background region of interest (ROI) in a scan assessing gastroesophageal (GE) reflux is typically placed over the lower left lung. This region serves as a reference point for measuring the normal background activity in the body during a nuclear medicine study.

When evaluating GE reflux, it is essential to differentiate between the activity from the injected radiotracer in the esophagus and any activity from surrounding structures. The lower left lung is chosen because it generally has a consistent and stable background radioactivity level. This allows for a clear comparison and more accurate determination of the percentage of reflux by providing a baseline against which the uptake in the esophagus can be measured.

The other locations mentioned, like the small intestine, right hepatic lobe, and upper esophagus, would not serve effectively as background ROIs for this purpose. The small intestine could show varying radiotracer activity that is not relevant to assessing reflux, while the hepatic lobe and upper esophagus might also exhibit varying or significantly different activity levels influenced by other factors, leading to misinterpretation of the reflux percentage. Thus, placing the background ROI over the lower left lung is considered the optimal choice for accurate assessment in this context.