Variability in CT Protocol Nomenclature: Understanding the Problem.   Computed tomography protocols define how a scanner is used to acquire a specific type and quality of image.  CT scanner protocols have no standard nomenclature, precluding easy comparisons between institutions.[1] In order to fully understand the variation in radiation dose that may exist between institutions, the protocol used must be clearly defined–this includes an understanding of the clinical indication for which the protocol was designed, and the equipment type and any dose reduction software capabilities of the equipment. For instance, in many institutions the names of specific protocols may come from the equipment itself (protocols installed by equipment manufacturers), or the names may be developed by the clinical staff responsible for carrying out the examinations. Clinical staff change, new protocols are developed, new equipment is acquired, software is updated, but rarely will the protocol names reflect these changes making it impossible to accurately link like protocols. An abdomen pelvis single phase routine CT may be named variably at one institution with 4 different scanners as:

  1. Abdomen-Pelvis routine
  2. Abdomen-Pelvis non-contrast
  3. Abd-Pel routine
  4. Plain abdomen pelvis
Linking protocols by indication

Linking protocols by indication

Calculating the radiation dose for all routine abdomen pelvis examinations carried out at this institution could be done easily if you know that these are all the same protocol. However, if the “routine” at one institution is an examination with and without contrast (dual phase) then the dose estimates might be quite different. This is a problem for radiation dose registries. If these protocols are used for the same indication then the side by side comparison of these protocols might be justified. However, if the dual phase protocol is used for more complex indications then the comparison should not be made. It is also not appropriate to aggregate all CT examinations according to body part scanned; the patient mix at one institution may be quite different than at another. For instance aggregating  dose for “head” exams at a quarternary neurological facility and comparing them to “head” examinations at a community hospital is not a good method for understanding radiation dose variability because the examinations at the former will be highly complex.

At St. Michael’s Centre of Excellence in Medical Imaging, we are developing a protocol dictionary for our Canadian Registry that is based on the Radiology Lexicon and the International Release (RF2) of the Systematized Nomenclature of Medicine Clinical Terms (SNOMED CT).[2] The Radiology Lexicon (RadLex) is an initiative that attempts to standardize radiology protocol nomenclature based on many of the variables that will affect radiation dose.[3] We are building on RadLex using SNOMED CT clinical indications and linking these protocols and indications to the Canadian Association of Radiologists (CAR) Referral Guidelines [4] and the American College of Radiology (ACR) Appropriateness Criteria [5] and recommendations from Choosing Wisely Canada [6] and Image Wisely [7] and Image Gently [8]; recommendations for choosing appropriate imaging tests for adult and pediatric patients. Our dictionary will be tested and validated in the early phase of the Registry development and then released for public use.

[1] Mahesh M. Variability in CT protocols. J Am Coll Radiol. 2013 Oct;10(10):805-6. [2] [3] [4] [5] [6] [7] [8]