See the invisible!
56 yo F presents with abdominal pain. Routine CT shows inflammation about the gallbladder, but no stone. On spectral analysis, a gallstone is clearly visible!
The reason for this is simple, the density of the gallstone and bile are nearly identical at the Kev used for conventional CT (about 70 keV, which is the typical output of a 120 kVp X-ray tube), but significantly different at higher and lower energies, as demonstrated on spectral curves. The atomic number of bile and gallstone are also clearly different.
Ultrasound confirmed the gallstone lodged in the neck of the gallbladder. patient was taken to surgery where acute cholecystitis was confirmed.