MDCT DRLs in more detail

Diagnostic reference levels (DRLs)

A diagnostic reference level (DRL) is an indicative dose that is not expected to be exceeded under normal imaging conditions for a given diagnostic task.

A DRL is not a regulatory limit, it is a benchmark that when exceeded triggers a review. Conducting a local dose audit and comparing the results to a DRL provides an imaging facility with a simple method of identifying situations where they are delivering an unusually high patient dose.

More information on DRLs, how they are used, and the regulatory requirements for Australian imaging facilities, is provided on the NDRLS in more detail page.

Scanners covered by the MDCT DRLs

The MDCT DRLs are for standard diagnostic CT scanners. The DRLs do not apply to:

  • Dental CT
  • Cone beam CT units
  • Radiotherapy planning CT
  • Attenuation correction/localisation CT for multimodality imaging procedures (e.g. PET/CT or SPECT/CT). There are DRLs for these scans in the nuclear medicine section.

Setting the MDCT adult DRLs

Data submitted to the NDRLS MDCT survey were used to set the current MDCT Adult DRLs. The facility reference level (FRL) from each submitted survey was calculated and the 75th percentile of the distribution of FRLs for each protocol type was used as the basis of the DRL. More information about the data submitted to the survey and how these DRLs were calculated can be found on the MDCT statistics page.

Setting the MDCT paediatric DRLs

The paediatric MDCT DRLs were set using data submitted to a survey conducted by the Royal Australian and New Zealand College of Radiologists (RANZCR). More information about the data submitted to the survey and how these DRLs were calculated can be found on the MDCT statistics page.

Comparing my facility’s CT doses to the DRLs

You can compare your facility’s CT doses using two methods:

  1. Using ARPANSA’s online service

ARPANSA provides a free service for you to compare the doses your facility delivers to the national DRLs for common CT protocols. Data submitted to the service is used to monitor and occasionally update the DRLs.

  1. Using your own tools

Using ARPANSA’s service is not mandatory; you can still fulfil the regulatory requirements using a different service, or by conducting the comparison yourself.

You should compare your facility reference level (FRL) - which is the median dose delivered to patients undergoing a given protocol at your facility - to the DRLs. The median dose is simply the value in the middle of the distribution of delivered doses, i.e. the same number of scans will deliver a dose above the median as deliver a dose below the median.

To calculate your FRL:

  • Record the CT dose index (CTDIvol) and dose length product (DLP) for a minimum of 10, but preferably 20 or more, scans performed using the same protocol on different patients. The sample should be representative of your normal patient cohort (with regards to gender, weight and age). In general, a larger number of patients will provide a better estimate of your FRL:
    • 10 patients may provide reasonable precision for head scans
    • 20 or more patients should be used for torso scans
  • Excel and other spreadsheet based applications have built in functions to determine the median, but if need be you can calculate it manually:
    • Sort the scans in terms of increasing CTDIvol.
    • If there are an odd number of patients, the median CTDIvol is the value in the middle of the list.
    • If there is an even number of patients, then the median is simply the average of the middle two values.
    • Repeat the process for DLP. Note that the median CTDIvol may not be associated with the same patient(s) as the median DLP, so the list may have to be sorted a second time to determine the median DLP.

In order to prove that you have complied with RPS 14, Section 3.1.8 a), you should document and retain your results. This documentation should include your FRL and the corresponding DRL.

Reporting CTDIvol and DLP when conducting a multi-series scan

The only time you should submit data from a multi-series scan to the MDCT survey should be for chest-abdomen-pelvis scans. In those cases:

  • Sum the DLP
  • Average the CTDIvol

Definition of phase and series

As per DICOM, a series is a collection of images generated during a single radiation exposure.

Phase relates to the time between the injection of contrast material and the acquisition. If you have conducted a multiple phase study, you will almost certainly have acquired multiple series.

Collaboration in development of the MDCT DRLs

The most recent MDCT DRLs were developed by ARPANSA in conjunction with a liaison panel composed of representatives from: