UCSD Musculoskeletal Radiology

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Introduction

Prior to 2001 the vast majority of CT imaging of children was conducted using the same

or similar techniques used for adult imaging. In 2001, several articles (1-3) received

considerable media attention by pointing out that this approach was not necessary and

resulted in estimated radiation doses to the smallest children as much as three times that

given to an adult. Since then, considerable work has been published in the literature on

protocols to reduce dose to children undergoing CT examinations (4-17). However, many

of these protocols are scanner specific and not transferable to other CT units.

These instructions provide guidance in either developing CT protocols for children or

Technique reduction factors were developed from radiation measurements obtained with

ionization chambers and anatomical pediatric CT phantoms (18). These phantoms range

in size from infant to large adults and consist of tissue equivalent plastic that has a CT

number of “0”. The abdomen phantoms have a tissue equivalent spine; the thorax

phantoms have tissue equivalent spine and lung tissue; and the head phantoms have tissue

equivalent skull bone.

In order to use these reduction factors, the radiologist should first work with the CT

technologist to be familiar with techniques used for both adults and children. You must

then verify that your adult technique factors do not deliver estimated radiation doses

larger than those recommended by the American College of Radiology’s (ACR) CT

Accreditation Program (19,20). No universal CT technique can be used with all vendors’

CT equipment for the adult patient. Differences in CT scanner design (e.g. bow tie filters,

focal spot-to-detector distance, detector efficiency, etc.) make it impossible to estimate

patient radiation dose based on technique factors alone. Consequently, you must have a

Qualified Medical Physicist (21,22) (i.e., one who is board certified in diagnostic

radiological physics) measure the radiation output from your CT scanner in order to

estimate the dose and help you establish appropriate adult abdomen and head techniques.

Any Qualified Medical Physicist who has assisted other facilities in obtaining

accreditation of their CT scanners should be familiar with this test protocol (23).

Your adult abdomen and head techniques will become your baseline techniques. Using

these baselines, Tables I and II allow you to estimate appropriate reductions in mAs for

children based on their age or PA thickness. Ideally, the PA thickness of the pediatric

patient should be measured with calipers. If you cannot obtain the child’s PA thickness,

Tables I and II list an “average” age that corresponds to the thicknesses in the first

column. The resulting techniques should provide a radiation dose that is approximately

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medical physicist should assist with this.)

These instructions assume all technique factors (other than tube current and/or gantry

rotation time) remain fixed as techniques are adjusted for pediatric patients. Although

several authors have advocated changing other parameters (i.e., kVp) in order to reduce

dose (24-28), these instructions will not apply if parameters other than mAs are changed.

You need to work closely with your medical physicist in these situations to insure image

quality is maintained while your desired dose reductions are achieved.

The data in the two tables list reduction factors for the “mAs” provided you are using

manual techniques. If you are using the automatic exposure control (AEC) features of

your CT scanner for imaging, the AEC system should automatically reduce techniques

for children provided the adult baseline is set up properly. In this case, you should follow

part A of the procedure to verify that the dose estimate from your baseline technique does

not exceed recommended adult values. Place your baseline “mAs” values in the provided

spreadsheet to obtain the correct pediatric techniques for your scanner. You can verify

that the AEC system on your CT scanner is properly functioning by comparing the mAs

values listed on your CT images with the appropriate value listed in Tables 1 & II. If the

mAs values listed on your CT images are less than or equal to the corresponding value in

Tables I & II, your pediatric radiation doses are less than or equal to your estimated adult

radiation doses.

Reducing patient dose in CT increases the quantum mottle or background “noise” in your

images. Since increased quantum mottle affects low contrast image quality more than

high contrast image quality, dose reductions for low contrast images may be limited. For

example, soft tissue differentiation (low contrast) requires lower noise in the image than

studies of bony detail or lung parenchyma (high contrast). These instructions should

provide adequate image quality for your pediatric soft tissue studies since the dose will be

similar to adult techniques and the noise level should not change. You may be able to

reduce doses to a greater degree for high contrast studies.

Procedure

A. Establish baseline techniques for an adult head and abdomen CT.

and an adult head phantom using the FDA 32 and 16 cm CTDI PMMA phantoms

(29) respectively.

CT Accreditation Program recommended upper values of 25 and 75 mGy

respectively (20), work with your medical physicist to reduce either the tube

current (mA) or rotation time (sec) to lower the doses.

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3. Record the final tube voltage (kVp), tube current (mA), rotation time (sec), pitch

and bow tie filter settings in Tables I and II as your baseline techniques for the

adult abdomen and head.

B. Determine the appropriate mAs for a pediatric thorax, abdomen and head CT.

1. Multiply the baseline (abdomen or head) mAs by the indicated Reduction Factor

to determine the appropriate pediatric mAs and write this in the table for all

patient PA thicknesses/ages or use attached Excel spread sheet to automatically

perform these calculations.)

2. The other techniques in your protocol (kVp, pitch, and bow tie filter) must remain

3. If the pitch of your thorax and abdomen scans is different, ask your medical

physicist to calculate the correct Thorax Baseline from your Abdomen Baseline.

Alternatively, the attached Excel spread sheet automatically performs this

correction if you enter the different pitch values in the spreadsheet.

4. When examining pediatric patients, find the mAs Reduction Factor from the

completed tables that corresponds to the applicable PA thickness/age.

either the head or the abdomen. If you elect to use a reduced kVp for pediatric

examinations, the suggested mAs ratios in these tables do not apply.

Table I: mAs Reduction Factors for the Pediatric Abdomen and Thorax

Abdomen

Baseline:

kVp= mA= Time= sec Pitch Abdomen= Pitch Thorax=

PA Abdomen Thorax

Thickness

(cm)

Approx

Age

mAs Reduction

Factor (RF)

Estimated mAs

mAs Reduction

Factor (RF)

Estimated mAs =

9 newborn 0.43 0.42

12 1 yr 0.51 0.49

14 5 yr 0.59 0.57

16 10 yr 0.66 0.64

19 15 yr 0.76 0.73

22 small adult 0.90 0.82

25 med adult Baseline (BL) 0.91

31 large adult 1.27 1.16

Table II: mAs Reduction Factors for the Pediatric Head

Head kVp= mA= Time= sec

Baseline: Pitch=

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PA Head

Thickness

(cm)

Approx

Age

mAs Reduction

Factor (RF)

Estimated mAs =

12 newborn 0.74

16 1 yr 0.86

17 5 yr 0.93

19 med adult Baseline (BL)

Example Calculations

1. An adult thorax is examined at a technique of 120 kVp, 0.5 sec scan time, 200 mA,

pitch = 1, and FOV = 35 cm. What is the appropriate technique for a five year old

thorax at a pitch of 1?

Abdomen

Baseline:

PA Abdomen Thorax

Thickness

(cm)

Approx

Age

mAs Reduction

Factor (RF)

Estimated mAs =

mAs Reduction

Factor (RF)

Estimated mAs =

2. An adult head is examined at a technique of 140 kVp, 0.5 sec scan time, 400 mA,

pitch = 1, and FOV = 25 cm. What is the appropriate technique for a one year old

head?

PA Head

Thickness

(cm)

Approx

Age

mAs Reduction

Factor (RF)

Estimated mAs =

19 med adult Baseline (BL) 200

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Summary

This two-step approach should result in CT radiation dose estimates no greater than the

corresponding adult doses regardless of the patient’s size. But first, you are encouraged to

reduce your current adult techniques and doses if they are high. If you have not

previously reduced your pediatric techniques to the recommended levels of Tables I and

II, these images will be noisier than those you are accustomed to viewing. However, you

are encouraged to take this step.

The Alliance acknowledges that the above reductions in pediatric CT doses are less

aggressive than those that some institutions have currently achieved. If your current

pediatric protocols use mAs that are lower than those derived from Tables I and II, your

pediatric doses are currently less than your adult doses. You are commended for

achieving this reduction and encouraged to continue with your current program.

References

1. Brenner DJ, Elliston CD, Hall EJ, Berdon WE. Estimated risks of radiationinduced

fatal cancer from pediatric CT. AJR 2001;176:289-296.

2. Patterson A, Frush DP, Donnelly LF. Helical CT of the body: are settings

adjusted for pediatric patients? AJR 2001;176:297-302.

3. Donnellly LF, Emery KH, Body AS, et al. Minimizing radiation dose for

pediatric body application of single detector helical CT: strategies at a large

children’s hospital. AJR 2001;176:303-306.

4. Paterson A, Frush DP. Dose reduction in paediatric MDCT: general principles.

Clin Radiol 2007: 62(6):507-517.

5. Verdun FR, Schnyder P, Gutierrez D, Gudinchet F. Patient dose optimization in

pediatric computerized tomography. Rev Med Suisse 2006:2(73):1752-1757.

6. Fefferman NR, Bomsztyk E, Yim AM, Rivera R, Amodio JB, Pinkney LP,

Strubel NA, Noz ME, Rusinek H. Appendicitis in children: low-dose CT with a

phantom-based simulation technique—initial observations. Radiology

2005:237(2):641-646.

7. Shah R, Gupta AK, Rehani MN, Pandey AK, Mukhopadhyay S. Effect of

reduction of tube current on reader confidence in paediatric computed

tomography. Clin Radiol 2005:60(2):224-231.

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8. Mayo JR, Kim KI, MacDonald SL, Johkoh T, Kavanagh P, Coxson HO, Vedal S.

Reduced radiation dose helical chest CT: effect on reader evaluation of structures

and lung finings. Radiology 2004:232(3):749-756.

9. Greess H, Lutze J, Nomayr A, Wolf H, Hothorn T, Kalender WA, Bautz W. Dose

reduction in subsecond multislice spiral CT examination of children by online

tube current modulation. Eur Radiol 2004:14(6):995-999.

10. Ratcliffe J, Swanson CE, Hafiz N, Frawley K, Coakley K, Cloake J. Assessment

of image quality of a standard and two dose-reducing protocols in peadiatric

pelvic CT. Pediatr Radiol 2003:33(3):177-182.

11. Frush DP, Slack CC, Hollingsworth CL, Bisset GS, Donnelly LF, Hsieh J, Lavin-

Wensell T, Mayo JR. Computer-simulated radiation dose reduction for

abdominal multidetector CT of pediatric patients. Am J Roentgenol 179(5):1107-

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12. Morgan HT. Dose reduction for CT pediatric imaging. Pediatr Radiol

2002:32(10):724-728.

13. Westerman BR. Radiation dose from Toshiba CT scanners. Pediatr Radiol

2002:32(10):735-737.

14. Fox SH, Toth T. Dose reduction on GE CT scanners. Pediatr Radiol

2002:32(10):718-723.

15. Frush D. Pediatric CT: practical approach to diminish the radiation dose. Pediatr

Radiol 2002:32(10):714-717.

16. Frush DP. Strategies of dose reduction. Pediatr Radiol 2002:32(4):293-297.

17. Wong ET, Yu SK, Lai M, Wong YC, Lau PC. Br J Radiol 2001:74:932-937.

18. Varchena, V. Pediatric phantoms. Pediatr Radiol (2002) 32; 280-284.

19. McCollough C, Branham T, Herlihy V, et al. Radiation Doses from the ACR CT

Accreditation Program: Review of Data Since Program Inception and Proposals

for New Reference Values and Pass/Fail Limits. Presented at the RSNA 92nd

Scientific Assembly and Annual Meeting, 2006.

20. New CT Accreditation Dose Requirements Effective January 1, 2008

http://www.acr.org/accreditation/FeaturedCategories/ArticlesAnnouncements/Ne

wDoseReq.aspx

21. ACR Technical Standard for Diagnostic Medical Physics Performance

Monitoring of Computed Tomography (CT) Equipment

http://www.acr.org/SecondaryMainMenuCategories/quality_safety/guidelines/me

22. AAPM Definition of a Qualified Medical Physicist,

http://www.aapm.org/medical_physicist/fields.asp

23. Instruction Manual for Testing the ACR CT Phantom

http://www.acr.org/accreditation/computed/qc_forms/Phantom_Testing_Instructio

24. Huda W. Dose and image quality in CT. Pediatr Radiol 2002; 32:709-713.

25. Huda W, Chamberlain CC, Rosenbaum AE, et al. Radiation doses to infants and

adults undergoing head CT examinations. Med Phys 28:393-399.

26. Huda W, Scalzetti EM, Levin G. Technique factors and image quality as

functions of patient weight at abdominal CT. Radiol 217:430-435.

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27. Lucaya J, Piqueras J, Garcia-Pena P, et al. Low-dose high resolution CT of the

chest in children and young adults: dose, cooperation, artifact incidence, and

image quality. Am J Roentgenol 175:985-992.

28. Crawley MT, Booth A, Wainwright A. A practical approach to the first iteration

in the optimization of radiation dose and image quality in CT. Br J Radiol

74:607-614.

29. Shope TB, Gagne RM, Johnson GC. A method for describing the doses delivered