See the Medicare Benefit Policy Manual, Chapter 15. The ordering test rules are found beginning at Sec.80.6.
On contrast media, see the 'test design" exception when the order does not specify contrast.
https://www.cms.gov/Regulations-and-Guidance/Guidance/Manuals/downloads/bp102c15.pdf
The guidance: The radiologist at the "testing facility determines that an ordered diagnostic radiology test is clinically inappropriate or suboptimal, and that a different diagnostic test should be performed (e.g., an MRI should be performed instead of a CT scan because of the clinical indication), the interpreting physician/testing facility may not perform the unordered test until a new order from the treating physician/practitioner has been received. Similarly, if the result of an ordered diagnostic test is normal and the interpreting physician believes that another diagnostic test should be performed (e.g., a renal sonogram was normal and based on the clinical indication, the interpreting physician believes an MRI will reveal the diagnosis), an order from the treating physician must be received prior to performing the unordered diagnostic test."
"If the testing facility cannot reach the treating physician/practitioner to change the order or obtain a new order and documents this in the medical record, then the testing facility may furnish the additional diagnostic test if all of the following criteria apply:
• The testing center performs the diagnostic test ordered by the treating physician/practitioner;
• The interpreting physician at the testing facility determines and documents that, because of the abnormal result of the diagnostic test performed, an additional diagnostic test is medically necessary;
• Delaying the performance of the additional diagnostic test would have an adverse effect on the care of the beneficiary;
• The result of the test is communicated to and is used by the treating physician/practitioner in the treatment of the beneficiary; and
• The interpreting physician at the testing facility documents in his/her report why additional testing was done.
The radiologist may modify the order (and should document the reason) in the following circumstances:
Test Design
Unless specified in the order, the interpreting physician may determine, without notifying the treating physician/practitioner, the parameters of the diagnostic test (e.g., number of radiographic views obtained, thickness of tomographic sections acquired, use or non-use of contrast media).
Clear Error
The interpreting physician may modify, without notifying the treating physician/practitioner, an order with clear and obvious errors that would be apparent to a reasonable layperson, such as the patient receiving the test (e.g., x-ray of wrong foot ordered).
Patient Condition
The interpreting physician may cancel, without notifying the treating physician/ practitioner, an order because the beneficiary's physical condition at the time of diagnostic testing will not permit performance of the test (e.g., a barium enema cannot be performed because of residual stool in colon on scout KUB; 170.5PA/LAT of the chest cannot be performed because the patient is unable to stand). When an ordered diagnostic test is cancelled, any medically necessary preliminary or scout testing performed is payable".