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Diagnostic Reference Levels

Cynthia H. McCollough, PhD, Mayo Clinic, Rochester, MN
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Introduction

Diagnostic reference levels were first mentioned by the International Commission on Radiological Protection (ICRP) in 19901 and subsequently recommended in greater detail in 19962. From the 1996 report:

Diagnostic reference levels are supplements to professional judgment and do not provide a dividing line between good and bad medicine. It is inappropriate to use them for regulatory or commercial purposes. Diagnostic reference levels apply to medical exposure, not to occupational and public exposure. Thus, they have no link to dose limits or constraints. Ideally, they should be the result of a generic optimization of protection. In practice, this is unrealistically difficult and it is simpler to choose the initial values as a percentile point on the observed distribution of doses to patients. The values should be selected by professional medical bodies and reviewed at intervals that represent a compromise between the necessary stability and the long-term changes in the observed dose distributions. The selected values will be specific to a country or region.

Diagnostic reference levels are not the suggested or ideal dose for a particular procedure or an absolute upper limit for dose. Rather, they represent the dose level at which an investigation of the appropriateness of the dose should be initiated. In conjunction with an image quality assessment, a qualified medical physicist should work with the radiologist and technologist to determine whether or not the required level of image quality could be attained at lower dose levels. Thus, reference levels act as “trigger levels” to initiate quality improvement. Their primary value is to identify dose levels that may be unnecessarily high – that is, to identify those situations where it may be possible to reduce dose without compromising the required level of image quality.

Use of Diagnostic Reference Levels to Reduce Patient Dose

The use of diagnostic reference levels as an important dose optimization tool is endorsed by many professional and regulatory organizations, including the ICRP, American College of Radiology (ACR), American Association of Physicists in Medicine (AAPM), United Kingdom (U.K.) Health Protection Agency, International Atomic Energy Agency (IAEA), and European Commission (EC). Reference levels are typically set at the 75th percentile of the dose distribution from a survey conducted across a broad user base (i.e., large and small facilities, public and private, hospital and out-patient) using a specified dose measurement protocol and phantom. They are established both regionally and nationally, and considerable variations have been seen across both regions and countries3. Dose surveys should be repeated periodically to establish new reference levels, which can demonstrate changes in both the mean and standard deviation of the dose distribution.

The use of diagnostic reference levels has been shown to reduce the overall dose and the range of doses observed in clinical practice. For example, U.K. national dose surveys demonstrated a 30% decrease in typical radiographic doses from 1984 to 1995 and an average drop of about 50% between 1985 and 20004,5. While improvements in equipment dose efficiency may be reflected in these dose reductions, investigations triggered when a reference dose is exceeded can often determine dose reduction strategies that do not negatively impact the overall quality of the specific diagnostic exam. Thus, data points above the 75th percentile are, over time, moved below the 75th percentile – with the net effect of a narrower dose distribution and a lower mean dose.

CT Diagnostic Reference Levels From Other Countries

Diagnostic reference levels must be defined in terms of an easily and reproducibly measured dose metric using technique parameters that reflect those used in a site’s clinical practice. In radiographic and fluoroscopic imaging, typically measured quantities are entrance skin dose for radiography and dose area product for fluoroscopy. Dose can be measured directly with TLD or derived from exposure measurements. Some authors survey typical technique factors and model the dose metric of interest.

In CT, published diagnostic reference levels use CTDI-based metrics such as CTDIw, CTDIvol, and DLP. Normalized CTDI values (CTDI per mAs) can be used by multiplying them by typical technique factors, or CTDI values can be measured at the typical clinical technique factors. Tables 1 and 2 below provide a summary of CT reference levels from a variety of national dose surveys.

CT Diagnostic Reference Levels From the ACR CT Accreditation Program

Beginning in 2002, the ACR CT Accreditation Program has required sites undergoing the accreditation process to measure and report CTDIw and CTDIvol for the head and body CTDI phantoms. The typical acquisition parameters for a site's adult head (head), pediatric abdomen (ped), and adult abdomen (body) examinations were used to calculate CTDIw and CTDIvol. For the pediatric exam, sites were instructed to assume the size and weight of a typical 5-year-old child, and doses were measured using the 16-cm phantom. The average and standard deviation of these doses were calculated by year. Summary data for CTDIvol are shown in Table 3 below.

In every case except adult abdomen exams in 2003, both the average dose and the standard deviation fell for each consecutive year. Thus, the establishment of CT reference levels in the United States appears to have helped reduce both the mean dose and the range of doses for these common CT examinations.

Although dose reduction was observed for adult head CT examinations, feedback from sites undergoing accreditation indicated that sites were systematically reducing dose to below the 60 mGy level, even though complaints with regard to head image quality at this dose level were common. The purpose of reference levels is to decrease dose levels only when doing so does not compromise image quality or patient care. Changes in technology (multi-detector-row CT) and practice (3-5 mm image widths) have occurred since the U.K. dose survey that gave rise to the 60 mGy level for the adult head.

As can be seen in Tables 1 and 2, these changes have resulted in an increase in the diagnostic reference level for head CT (U.K. 2003 data now specifies CTDIvol reference levels of 65 mGy for the cerebrum and 100 mGy for the posterior fossa). Thus, the ACR CT Accreditation Program used survey data from the inception of the program to establish the most current U.S. reference levels for head CT (i.e., 2002 data were used to avoid including dose values that were thought to yield inadequate image quality). Beginning January 1, 2008, the ACR CT reference levels were changed to a CTDIvol of 75 mGy (adult head), 25 mGy (adult abdomen) and 20 mGy (pediatric abdomen)15. These values will be reassessed periodically.

CT Diagnostic Reference Levels for Other CT Applications

Because the practice of CT encompasses many more exam types than routine head and body exams, reference levels for many common CT examinations are important for continuing dose optimization efforts in CT. To this end, several national surveys have begun to assess a broader range of exam types. Additionally, the ACR has begun a project to automatically collect CTDIvol data directly from the DICOM header, thus allowing considerably faster accumulation of data sufficient to establish reference levels for additional exam types. This information will extend the value of the diagnostic reference level concept to the majority of CT applications, enabling individual CT users and the community at large to answer the question, "What doses are typical and what doses are too much?"

 

Table 1: Adult Diagnostic Reference Levels for CTDIw (mGy) and DLP (mGy·cm)

  Head  Abdomen  Abdomen & Pelvis 
   Whole Exam  Whole Exam  Pelvis Whole Exam 
   CTDIw  DLP  CTDIw  DLP  CTDIw DLP   CTDIw DLP 
 EC 19996  60  1050  35   900 - -  35  780
 ACR 20027  60 35  - -  -
 UK 20038  -  930 20 470  - 20 560 
 Germany 20039  60 1050  25 770  - 24 1500 
 Switzerland 200410  60 800  20 710  30 540   -
 Taiwan 200711  72 850  31 680  28 520  -
 EC = European Commission; ACR = American College of Radiology; UK = United Kingdom


 

Table 2: Adult Diagnostic Reference Levels for CTDIvol (mGy) and DLP (mGy·cm)

  Head  Abdomen  Abdomen & Pelvis 
   Whole Exam  Whole Exam  Pelvis Whole Exam 
   CTDIvol  DLP  CTDIvol  DLP  CTDIvol DLP   CTDIvol DLP 
 Sweden 200212  75  1200 25   - - -  - -
 UK 20038  65-100  930 14 470 - 14 560
 Netherlands 200813  -  - - - - 15 700 
 EC 200414  60  - 25 - - 15 700 
 ACR 200815  75  - 25 - - -  -
 EC = European Commission; ACR = American College of Radiology; UK = United Kingdom


 

Table 3: CTDIvol (mGy) statistics from the first 3 years of the ACR CT Accreditation Program

  Adult Head Adult Abdomen  Pediatric Abdomen 
   2002 2003  2004   2002 2003  2004   2002 2003  2004 
 Mean 66.7 58.5 55.8 18.7 19.2 17.0  17.2 15.9 14.0
 Std. Dev. 23.5 17.5 15.7 8.0 8.7 7.6 9.7 8.6 7.0
 75 Percentile 76.8 63.9 60.0 22.6 23.4 21.1 20.6 20.5 18.4
 90 Percentile 99.0 82.2 74.0 29.5 30.6 25.8 26.6 25.6 23.4



References

  1. International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection (Report 60). Annals of the ICRP. 1991; 21(1-3).
  2. International Commission on Radiological Protection. Radiological Protection and Safety in Medicine (Report 73). Annals of the ICRP. 1996;26(2):1-31.
  3. Matthews K, Brennan P.C., “The Application of Diagnostic Reference Levels: General Principles and an Irish Perspective,” Radiography, May 2009.
  4. Hart D, Wall B.F., “U.K. Population Dose From Medical X-ray Examinations,” European Journal of Radiology, June 2004.
  5. Shrimpton P.C., Wall B.F., Hart D., “Diagnostic Medical Exposures in the U.K.,” Applied Radiation and Isotopes, January 1999.
  6. European Commission, “European Guidelines for Quality Criteria for Computed Tomography.” EUR 16262 EN. Luxembourg: European Commission; 2000.
  7. American College of Radiology, “ACR Practice Guideline for Diagnostic Reference Levels in Medical X-Ray Imaging,” 2002 (revised 2008).
  8. Shrimpton P., “Assessment of Patient Dose in CT” in Bongart G.Z., Golding S.J., Jurik A.G., et al., European Guidelines for Multislice Computed Tomography, 2004.
  9. Brix G., "Notice of Diagnostic Reference Values for Radiology and Nuclear Medicine Studies," Federal Office for Radiation Protection, ed. Germany; 2003 (available in German only).
  10. Aroua A., Besancon A., Buchillier-Decka I., et al., "Adult Reference Levels in Diagnostic and Interventional Radiology for Temporary Use in Switzerland," Radiation Protection Dosimetry, July 20, 2004.
  11. Tsai H.Y., Tung C.J., Yu C.C., Tyan Y.S., "Survey of Computed Tomography Scanners in Taiwan: Dose Descriptors, Dose Guidance Levels, and Effective Doses," Medical Physics, April 2007.
  12. Swedish Radiation Protection Authority, “Regulations and General Advice on Diagnostic Standard Doses and Reference Levels Within Medical X-ray Diagnostics,” Swedish Radiation Protection Authority, August 7, 2002.
  13. Bouwman R.W., Crompvoets-Jeukens C.R.L., Geertse T.D., et al., “Diagnostic Reference Levels in the Netherlands,” NCS Platform for Radiology and Nuclear Medicine, October 2008. (in Dutch only).
  14. Bongartz G., Golding S.J., Jurik A.G., et al., ”European Guidelines for Multislice Computed Tomography,” Funded by the European Commission. Contract number FIGM-CT2000-20078-CT-TIP, March 2004.

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