Diagnostic Reference Levels (DRLs) in CT

The optimisation of patient protection in CT requires the application of examination-specific scan protocols tailored to patient age or size, region of imaging and clinical indication in order to ensure that the dose to each patient is as low as reasonably achievable for the clinical purpose of the CT examination. Diagnostic reference levels (DRLs) are a practical tool to promote the assessment of existing protocols and appropriate development of new and improved protocols at each CT centre by facilitating the comparison of doses from present practice. DRLs were first successfully implemented in relation to conventional X rays in the 1980s and subsequently developed for application to CT in the 1990s.

1. Why do we need DRLs in CT?

Surveys of dose estimates from CT highlight the substantial variations in practice between some CT centres for similar types of examination and similar patient group (adults or children of different sizes). Such observations indicate the need for improvement through implementation of measures to keep all doses within acceptable ranges for the clinical purpose of each examination. Examination-specific DRLs for various patient groups can provide the stimulus for monitoring practice to promote improvements in patient protection. Such DRLs can be set not only at a national level (as investigation levels for unusually high typical doses), but also locally by each CT centre (as characterising its present practice).

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2. What is the purpose of national DRLs (NDRLs)?

National DRLs provide an initial broad check in the optimisation process. They are set on the basis of wide scale surveys of the mean doses representing typical practice for a patient group (e.g. adults or children of different sizes) at a range of representative CT centres for a specific type of CT examination. NDRLs are commonly set at the third quartiles of these national distributions [IPEM, 2004]. As such, NDRLs are not optimum doses, but nevertheless they are helpful in identifying potentially unusual practice (the highest 25% of typical doses). NDRLs are intended to promote awareness, dose audit and comparison as the basis for improving patient radiation protection, with an implied maintenance of diagnostic quality.  For continuing effectiveness, NDRLs should be periodically reviewed and revised on the basis of updated survey data reflecting changes in national practice and technology.

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3.Which dose quantities are used for setting DRLs for CT?

DRLs should be set in terms of the practical dose quantities used to monitor CT practice: volume weighted CT dose index (CTDIvol, expressed in mGy) and dose-length product (DLP in mGy•cm), as commonly displayed by CT scanners. These quantities are not patient doses (directly reflecting risk to individuals), but dose indicators characterising radiation exposure in CT for the purposes of comparison of practice. CTDIvol and DLP are specified in relation to dose measurements in the standard CT dosimetry phantoms, which are acrylic cylinders with diameters of 16cm (commonly referenced for head protocols) and 32 cm (commonly referenced for body protocols on adults). Under similar conditions of CT exposure, displayed values of CTDIvol or DLP referring to the smaller dosimetry phantom are about twice those referring to the larger phantom. In order to allow meaningful comparison of values of CTDIvol and DLP as part of the optimisation of patient protection, it is imperative to know the reference dosimetry phantom for each displayed dose. There is no merit in setting DRLs in terms of other dose quantities, such as effective dose, that are derived from the well-defined monitoring quantities by coefficients that could vary depending on the particular dose model adopted.

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4. How are DRLs used at the local level by a CT centre?

Each CT centre should determine its typical levels of dose (CTDIvol and DLP) for each type of examination (and associated clinical indication) as the mean values observed for representative samples for each patient group (adults and children of different sizes) [IPEM, 2004]. These mean doses should be compared with the relevant NDRL. Mean values above the NDRL should be investigated and either justified as being clinical necessary or reduced through appropriate changes in practice to improve patient protection. Locally determined mean doses for each CT unit can be set as local DRLs (LDRLs) for subsequent comparison with practice at other CT centres in pursuit of improved patient protection. LDRLs should be reviewed annually and revised as necessary following periodic local dose audit to monitor trends (conducted, for example, every 3 years or on significant change of equipment or technique [IPEM, 2005]).

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5. Do DRLs apply to individual patients?

No, DRLs relate to typical practice for a specific CT examination (e.g., brain in relation to acute stroke) and patient group (e.g., by age or gender), as summarised by mean doses observed for a sample of patients. CTDIvol or DLP values for specific examinations on individual patients can be expected to vary somewhat according to patient physique and clinical needs, and so these individual doses should not be compared directly with national or local DRLs, whose purpose is to promote general improvements in overall practice for the examination. However, an investigation should be triggered when doses for individuals within a patient group are consistently exceeding a LDRL, with a view to reviewing and as necessary revising CT technique for optimised patient protection [IPEM, 2004].

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6. What can be done for individual patients?

Whereas CTDIvol and DLP provide a general characterisation of the conditions of exposure in CT, AAPM [BOONE et. al., 2011] has suggested a complementary approach for monitoring doses to individual patients utilising the concept of size-specific dose estimates (SSDE). Values of SSDE are derived from displayed values of CTDIvol, though application of tabulated correction factors specific to patient age or effective diameter (determined from AP and lateral dimensions), in order to provide an estimate of the typical level of dose for an individual patient as a simple summary of their particular level of radiation exposure from a CT examination.

In addition, IEC has proposed incorporating into CT equipment the display of dose notification and dose alert values for CTDIvol or DLP that can be set by CT centres to flag to the operator levels of potential concern for specific scan settings on the scanner prior to exposure. AAPM, for example, has recommended initial notification values for CTDIvol of 80 mGy (expressed in terms of the 16 cm dosimetry phantom) in relation to scans of the adult head and 50 mGy (32 cm dosimetry phantom) for scans on the adult torso [AAPM, 2011].

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7.What is the difference between NDRLs, LDRLs and Dose Notification Values?

NDRLs for each examination and patient group are set on the basis of distributions of the typical (mean) doses observed in wide scale (national) surveys, commonly by adopting the third quartile values to provide investigation levels for unusual practice (doses in top 25% ). LDRLs represent the typical local practice at a CT centre, as the mean doses determined from samples of patients. Dose notification values can be set locally on each CT scanner in order to prompt confirmation by the operator, prior to scanning the patient, of scan settings that are likely to lead to values of dose indices above established ranges.

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8. Which examinations should have DRLs?

DRLs are intended to promote improvements in patient protection by allowing comparison of current practice. National and local DRLs should (ideally) be set for each examination and each patient group (adults and children of different sizes). In order to allow meaningful comparison of truly similar examinations conducted for similar purpose and requiring similar scan technique, it is crucial to specify detailed descriptions of CT procedures, including a clinical indication (such as CT abdomen in relation to liver metastases), rather than simply broad categories of examination (such as CT abdomen). This usefully allows the comparison of ‘apples with apples’ rather than a mixed bag of fruit.

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9. How should we account for patient size?

The technique factors required for a CT examination and the resulting dose are dependent on patient size and each CT centre should establish specific scan protocols for each patient group as part of optimised practice. Protocols for paediatric CT examinations can, for example, be developed for patients grouped by ranges in age, weight or cross-sectional area, reflecting necessary changes in optimised scan technique. National and local DRLs should also be established with similar regard to patient size. It is important to know the reference CT dosimetry phantom (diameter of 16 cm or 32 cm) for the values of CTDIvol and DLP displayed for each protocol in order to allow meaningful comparison of doses. In particular, paediatric protocols may reference only the 16 cm phantom for all scans, or a mixture of phantoms depending on scan region and protocol (as is commonly the case for adults), depending on the CT manufacturer.

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10. Are DRLs effective in improving patient radiation protection?

DRLs have already proved useful as a tool in support of dose audit and practice review for promoting improvements in patient protection. Their application since 1989 in the UK within a coherent framework for managing patient dose has been instrumental in promoting increased awareness of dose and helping to reduce unnecessary x-ray exposure. UK national DRLs for conventional X ray examinations on adult patients, for example, have typically fallen by a factor of two over the last 20 years owing to improvements in imaging practice. Further application of national and local DRLs for specific CT examinations and patient groups (adults and children of different sizes) should help ensure efficient imaging practice for CT, especially as its utilization continues to increase.

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11. Where should I start in the absence of well-established national and local support for DRLs (as, for example, a small CT facility in a less-resourced country)?

The priority for such a CT centre might be to estimate typical levels of CTDIvol and DLP in relation to present practice for a few common CT examinations on adult patients according to the following steps:

  • Record displayed values of CTDIvol and DLP for samples of 10 or more typical adult patients undergoing the following CT procedures for common clinical indications: CT head (eg in relation to acute stroke), CT chest (eg in relation to lung cancer) and CT abdomen (eg in relation to acute abdominal pain).
  • Calculate for each type of examination the mean values of CTDIvol and DLP; these are your local DRLs (LDRLs).
  • Compare your LDRLs with published values for similar practice in order to provide a broad indication of your relative performance and urgency of need for improvement in your scanning technique. Some sources of established national and international DRLs are given below:
  • If your values are below an NDRL, this does not necessarily indicate satisfactory performance. Scanning techniques should always be reviewed for potential reduction in their levels of dose without compromising the clinical purpose of the CT examination.
  • If your values are above an NDRL, there is a more urgent need to investigate whether simple changes can be made to the scan settings selected for an examination in order to reduce values of CTDIvol and/ or DLP whilst still providing the required clinical information. Scan settings to review (such as tube current, rotation time, collimation, number of axial acquisitions, and pitch and scan length in helical scanning during multi-phase examinations) are discussed here.
  • Levels of dose should be reassessed following revision of CT technique in order to allow further comparisons (see steps above).

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12. What are the potential pitfalls in comparing LDRLs with published values?

Published NDRLs can prove useful in allowing broad comparison of your LDRLs, although potential problems in this process include the following:

  • NDRLs established for other countries (with potentially different CT practice and technology) may not be wholly relevant to your particular circumstances.
  • The types of CT examination specified for the published NDRLs (as being with or without detailed clinical indications) may not be directly relevant to your particular practice.
  • Published dose values (CTDIvol or DLP) may not be expressed in relation to the same standard CT dosimetry phantom (diameter of 16 cm or 32 cm) pertaining to your data, especially for paediatric CT.
  • Advances in technology, such as iterative reconstruction, will need to be taken into account when updating NDRLs or benchmarking LDRLs against doses relating to dissimilar technology.

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