CT protocol

A CT protocol is a set of parameters that specify a specific exam and contrast delivery requirements. When a CT is requested, it will be vetted by a radiologist or radiographer to determine the study is justified and what the most suitable parameters by which that CT should be performed - this may lead to a different CT examination being performed or an alternative modality recommended. 

Example CT protocols can be found under the Imaging in practice article. 

Generally speaking, the CT protocol can be split down into two areas: the radiological protocol and the technical protocol. 

Radiological protocol

The radiological protocol is the 'type' of CT exam that will best suit the clinical question and patient presentation. It requires detailed knowledge of the radiological appearance of pathology, the parameters of the available CT scanner, and a thorough knowledge of the types of protocols the institution performs. 

For example, if one were to request a "CT head" for a suspected intracranial hemorrhage the radiological protocol would be a "non-contrast CT head" to ensure there is no intravenous contrast masquerading as acute blood.

In a more complex example, if one were to request a "CT abdomen" to rule out a suspected hepatic hemangioma, it may then be protocoled as a "CT quad-phase contrast liver" to inspect dynamic enhancement patterns. 

Technical protocol

Once the protocol has been determined by the qualified party, it will then be discussed exactly how the scan will be performed from a technical aspect. In simple instances, i.e. a CT head, this is quite straightforward. However, in more technically challenging cases that include multiple radiological protocols, for example, a CTPA + triphasic liver, will require expert radiographer guidance to ensure that the scan is performed correctly taking into account contrast delivery timing, scan speed and patient presentation. 

Protocol variation

CT protocols are not, for the most part, a standardized entity in radiology. Whether up the road from a hospital or in a different country, each institution will have a different set of CT protocols.

There are guidelines that help dictate how protocols should be established such as the Liver Imaging Reporting And Data System ¹. These points aside, it is good to get into the habit of asking:

• what is the clinical question?
• what is the scan hoping to achieve? 
• will the patient require contrast? 
  • what contrast phase will best answer the clinical question?
• can the scanner achieve this exam?
  • i.e. a brain perfusion scan is not possible on older, single-source scanners