Final answer:
A HIDA scan is used when ultrasound results are equivocal for cholecystitis, allowing for measurement of gallbladder function and detection of bile duct blockage.
Step-by-step explanation:
When an ultrasound is equivocal for diagnosing cholecystitis, a HIDA scan (also known as cholescintigraphy) is typically the next diagnostic modality used. A HIDA scan involves the injection of a radioactive material, hydroxy iminodiacetic acid (HIDA), which is taken up by the gallbladder to measure its function and can detect blockage in the bile ducts. This is different from the other listed modalities such as a CT scan, MRI, and ERCP, which may be ued for evaluating other aspects of the biliary system or complications related to cholecystitis but are not the primary choice when an ultrasound does not provide a clear diagnosis.