Image Gently, CT Imaging and Radiation Protection of ...courses.washington.edu/radxphys/course_contents/ct/ct_pediatric_imaging/class_2021...– The pediatric nuclear medicine community

Kalpana M. Kanal, PhD, DABRDirector, Diagnostic Physics Section and Resident Education

University of Washington Medical Center, Department of RadiologyDiagnostic Physics Section

A copy of this lecture is available at: http://courses.washington.edu/radxphys/r3.html

Image Gently, CT Imaging and Radiation Protection of Pediatric Patients

http://courses.washington.edu/radxphys/r3.html

cf: Coy, D, et al. Body CT: The Essentials

Size Specific Dose Estimate(SSDE, mGy)

©UW and Kalpana Kanal, PhD 3

• Little body fat compared to adults • Variations in normal growth and development • Small physical size present challenges• Lack of ability to breath hold, and potential need for sedation

or other immobilization must be considered• Radiation doses - important due to the increased vulnerability

(sensitivity) of tissues• Children have a longer lifetime in which to manifest stochastic

(i.e. cancer) risks• Children incur a higher dose when identical CT settings are

used• CTDIvol and DLP metrics need to be understood well

Challenges of Pediatric Imaging


• The Alliance for Radiation Safety in Pediatric Imaging, the sponsors of the “Image Gently” campaign was formed in 2007

• Fundamental principles of rad protection during ped imaging:– Justification – ensuring examination is indicated– Utilization – ensuring correct imaging modality is selected and– Optimization – providing for appropriate balance between dose and

image quality

• www.imagegently.org

Alliance for Radiation SafetyImage Gently

http://www.imagegently.org/


• Interventional Fluoroscopy: Step Lightly– Treat kids with care: step lightly on the fluoroscopy pedal.

Stop and child size the technique. Consider ultrasound or when applicable, MR guidance.

• General Fluoroscopy: Pause and Pulse– Stop and child-size the technique. Use lowest pulse rate

possible. Consider ultrasound or MRI when applicable.

Alliance for Radiation Safety Fluoroscopy


• NM– The pediatric nuclear medicine community has developed the

Pediatric Radiopharmaceutical Administered Doses: 2010 North American Consensus Guidelines to directly address the need to lower radiation dose and standardize the radiopharmaceutical dose for children throughout the United States

• CR/DR– Promote standardization of exposure index and awareness of the need

to optimize radiation exposure in routine radiographs in children– The group is working with digital radiography manufacturers to

develop educational materials to promote appropriate use of their equipment in children

– Recently, a digital radiography safety checklist was created for technologists to remind them of the critical steps needed when performing computed radiography or direct radiography in children

Alliance for Radiation Safety NM and CR/DR


• The Image Wisely Campaign, the adult counterpart to the Image Gently Campaign, is based on the success of the Alliance for Radiation Safety in Pediatric Imaging.

• The constituent founding organizations are AAPM, ACR, ASRT, and the Radiological Society of North America (RSNA), with individuals from the Alliance providing consultation.

Image Wisely


• Is CT the right test in this clinical situation? Is ultrasound or MR imaging more appropriate?

• If CT is indicated, have appropriate adjustments based on size, scan indication, and region scanned been made?

• Have general pediatric specific protocols been developed and uploaded to the scanner for common examinations?

• Are additional modifications to the protocol warranted, i.e. follow-up examinations?

• If multiphase scanning is indicated, the highest exposure should be for the phase that provides the most critical information. Consider lower doses for phases such as pre-contrast scanning if necessary or delayed images, such as with CT urography

• Has appropriate use of sedation, and immobilization been implemented?

Checklist


• Has the environment been made as child friendly as possible?• Is shielding being used when appropriate? • Have equipment and protocols been developed and reviewed

periodically with qualified medical imaging physicists?• Has the pitch (in helical scanning) been adjusted to lower dose?

(Between 1 and 1.5)• Has the appropriate scan field of view (bow tie filter) been

selected?• Has patient positioning been optimized so that the patient is in the

center of the gantry, the scan technique and length of the projection scout views have been minimized and only those phases necessary are being performed?

• Have appropriate dose reduction technology such as tube current modulation been activated? Understand these technologies

Checklist


CT Dose IndicesCTDIvol

• CTDIvol = [2/3 * surface dose + 1/3 * central dose] / Pitch

• The original application of CTDIvol was to provide a standardized method to estimate and compare the radiation output of different CT scanners.

CTDIvol was defined to be a dose index of CT scanners, not a patient dose index.

Ref: RSNA/AAPM module: Imaging Gently: CT Imaging and Radiation Protection of Pediatric Patients


Patient Attenuation Considerations

Mass (kg) PA (cm) # HVL LAT (cm) # HVLPreterm 2 6 1.5 6 1.5Newborn 3 9 2.2 10 2.51 yr 10 12 3 14 3.55 yr 19 16 4 22 5.512 yr 31 18 4.5 27 6.8Adult 68 22 5.5 33 8.3Adult 100+ 35 8.8 48 12



Radiation output of the scanner in all three cases is identical

The entrance radiation dose to 32 cm phantom is 65% greater than the 10 cm phantom – closer to tube

The exit dose of the beam after traveling through 32 cm of plastic is far less than the exit dose after traveling through only 10 cm of plastic

The superficial layers of tissue of the patient’s body shield and reduce the patient’s core tissue dose.

Why does patient dose decrease with increasing patient size?



• The under or over estimate of the patient dose by the displayed CTDIvol as a function of patient size depending on if the 16 or 32 cm CTDI phantom is used

• The displayed CTDIvol underestimates the patient dose and misleads the radiologic technologist or radiologist if they assume CTDIvol represents the patient dose.

Affect of Patient Size on CTDIvol and DLP



Consultation between qualified medical physicist, radiologist and technologist is essential in IQ and dose management

Management of Image Quality and Radiation Dose for Pediatric Patients



A method to establish reduced mAs scan protocols for children that isindependent of the manufacturer, model, and age of the CT scanner may be foundat www.imagegently.org.

When you fill in the baseline values, the table automatically calculates the mAs for thedifferent ages.

Management of Image Quality and Radiation Dose for Pediatric Patients

http://www.imagegently.org/


A lower kVp decreases patient dose and increases quantum mottle in the image while an increase in the kVp has the opposite effect, if mAs is unchanged.

The degree of change in radiation dose and quantum mottle caused by changes in high voltage is often reduced by changing the mAs in the opposite direction of the change in high voltage

Recommended Adjustment of Kilovoltage(kV)



To improve contrast or to perform CT angiography, 100 kVp is reasonable for medium sized pediatric patients.

Neonates to small pediatric patients may be imaged as low as 80 kVp

80 kVp images at the maximum tube current of the CT scanner will not produce an adequate number of x-rays to avoid artifacts and maintain reasonable quantum mottle in the image for larger pediatric patients

Recommended Adjustment of Kilovoltage(kV)



• CTDIvol displayed on console of CT scanner underestimates dose to pediatric patients

• Task Group 204 of AAPM prepared a report that provides scaling factors as a function of patient size or age that can be used to estimate the dose to patient

• CTDIvol displayed = 5.5, 32 cm phantom used. Lateral Diam = 15 cm

• SSDE = CTDIvol * Correction Factor• SSDE = 5.5 * 2.29 mGy• SSDE = 13 mGy

Size Specific Dose (SSDE) for Pediatric Patients

Ref: AAPM Report 204

VS.

Adult Head

VS.

Adult Abdomen

VS.

Pediatric Head

VS.

Pediatric Abdomen


• The CTDIvol numerical value is only half the story, it is always prudent to know/understand the underlying CTDI phantom

• If you’re comparing, benchmark to the same CTDI phantom𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑣𝑣𝑣𝑣𝑣𝑣,16−𝑐𝑐𝑐𝑐 ~ 2.2 𝑥𝑥 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑣𝑣𝑣𝑣𝑣𝑣,32−𝑐𝑐𝑐𝑐

• Remember that CTDIvol is primarily an indicator of output, and is not specific to the patient

• Use SSDE to move in the direction of accounting for patient size, understand its limitations

Lessons for Dose Summaries


Which statement below is correct concerning CTDIvol, with all other parameters constant?

A. CTDIvol increases with a decrease in the tube current.B. CTDIvol is unaffected by the composition and thickness of the

bow-tie filter of the scanner.C. CTDIvol increases as the pitch of the CT scanner increases.D. CTDIvol typically does not represent the absorbed radiation

dose to the pediatric patient.

Review Question


Which statement below is true concerning DLP?

A. DLP cannot be used to estimate changes to patient dose from changes to CT scan parameters.

B. DLP decreases if CTDIvol during the examination increases.C. DLP represents the energy deposited in the patient during

the CT scan acquisition. D. DLP is a less accurate estimation of patient dose than CTDIvol.

Review Question

28

THANK YOU FOR YOUR ATTENTION

Astronomical Clock, Prague

Documents

Image Gently, CT Imaging and Radiation Protection of ...courses.washington.edu/radxphys/course_contents/ct/ct_pediatric_imaging/class_2021...– The pediatric nuclear medicine community