Diagnostic Reference Levels

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 1999 6

60

1050

35

900

-

-

35

780

ACR 20027

60

-

35

-

-

-

-

-

UK 2003 8

-

930

20

470

-

-

20

560

Germany 2003 9

60

1050

25

770

-

-

24

1500

Switzerland 2004 10

60

800

20

710

30

540

-

-

Taiwan 2007 11

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 2002 12

75

1200

25

-

-

-

-

-

UK 2003 8

65 - 100

930

14

470

-

-

14

560

Netherlands 2008 13

-

-

-

-

-

-

15

700

EC 2004 14

60

-

25

-

-

-

15

700

ACR 2008 15

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

 

2002

2003

2004

2002

2003

2004

2002

2003

2004

Adult Head

Adult Abdomen

Pediatric Abdomen

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 %tile

76.8

63.9

60.0

22.6

23.4

21.1

20.6

20.5

18.4

90 %tile

99.0

82.2

74.0

29.5

30.6

25.8

26.6

25.6

23.4