Final answer:
A CT scan is the imaging study an ER doctor would order when in need of a high-quality, three-dimensional image in a short amount of time.
Step-by-step explanation:
The ER doctor would order a CT scan when in need of a high-quality, three-dimensional image in a short amount of time. Computed tomography, or CT scan, uses computers to analyze multiple cross-sectional X-rays and create detailed 2D views or slices of the scanned area. These slices can then be combined to produce a three-dimensional image.