How do processes that decrease the radiotracer activity in both kidneys differ from processes that affect a single kidney?
Processes affecting both kidneys are often more systemic in nature. Glomerulonephritis can cause decreased flow in both kidneys. Acute or chronic renal failure also results in decreased flow and function in both kidneys.
There can always be bilateral vascular or obstructive processes, and these should also be considered.
Finally, technical factors, such as injecting the wrong radiotracer or acquiring images at the wrong photopeak, can be considered if the kidneys are not visualized