Walter L. Robinson, M.S.D.A.B.S.N.M., D.A.B.M.P.,

D.A.B.R.

Consultant Certified Medical Radiation Health & Diagnostic Imaging Physicist

CT Radiation Risks and Dose Reduction

1. History2. Basic dosimetry3. Biology of radiation effects4. Unique issues with radiation in children5. Optimization of risk/benefit ratio6. Use appropriate techniques7. Joint efforts with healthcare providers

Medical Radiation and Children

“ The radiologist must beintroduced to the special

techniques required to handle infants and children, and must have experience in their use.”

Side effects were reported within 3 months of discovery

Radiology…tremendous benefits, but also risks or side effects…

2. Basic dosimetry

• Dose units• Measures of dose• Conversions

Factors Affecting Patient Dose From C.T.1. kVp - If mAs is constant, increasing kVp will increase dose2. mAs - mAs is directly and linearly proportional to dose3. Slice Width – dose can be reduced by increasing slice width;

although SNR is improved, axial resolution/detail is reduced4. Matrix Size and FOV – an increase in these results in improved

contrast and spatial resolution, at the expense of dose5. Window and Level – a narrower window enhances contrast resolution6. Radius of Rotation – short diameter (mobile CT) provides higher

patient dose7. Bowtie Absorber – the use of a bowtie absorber with helical CT

reduces dose through filtration of softer X-rays8. Helical Pitch – changing the pitch to a higher number reduces dose9. Beam Width = # detectors x detector thickness affects; the greater

the beam width, the more efficient the collection of X-rays, and lower the dose.

10. Use multi-planar reconstruction. Helical slice thickness can be chosen after acquisition.

Applying this Knowledge

For instance: acquire 5.0 mm slices with no overlap with a helical pitch of 1.2. Reconstruct 5.0 mm slices with 3.0 mm increments to fill the gaps. This improves noise with some resolution loss. With a lowering of mAs to reduce effective dose, this is a helpful tip to improve the quality of the image.

Helical or spiral scanning slice pattern

Many different measures of dose in medical imaging: e.g.

• Exit dose• Dose (or KERMA) area product (DAP) • Entrance skin exposure (R)• Organ dose (RAD or mGy)• Dose equivalent (REM or mSv)• Effective dose (REM or mSv)• Dose computed from a phantom (e.g. CTDI,

MSAD, DLP, etc.)

Radiation Dose: measures for risk assessment

• Absorbed Dose (Gray – Gy)– For an individual tissue or organ– Difficult to measure; not practical

• Effective Dose Equivalent (Sievert – Sv)– Nonuniform exposure to organ or region– Expression of risk equivalent to whole body exposure

Not “Scanner” Dose Units (mGy)– CTDIvol and DLP: phantom determination– Not helpful in assigning risk without conversion!!

CTDI – CT Dose Index• On scanner consoles• Based on phantom (16 or 32 cm diameter)• Only represents the dose to the phantom

based on CT parameters selected• Does not indicate dose to the child in the CT

scanner• Conversions of CTDI to effective dose are

only rough estimations for children– e.g. no age based chest modifications

C.T. Radiation Absorbed Dose Descriptors

CTDI - Computed Tomography Dose Index – the dose to the central axis point in a single sliceCTDI I - (i = # slices) – so the CTDI 100 = single slice dose in the center of the body in the center of 100 mm CTDI w - (w = weighted) = 2/3 surface (1 cm.) x 1/3 centerCTDI v = CTDI w if pitch is 1.0 = 1.0 RAD. If pitch is 0.9 = 1.1 RAD. If pitch is 1.1 = 0.9 RAD.

If the single slice dose is 1.0 RAD, then the center of 14 slices can be 1.5 to 2.0 RAD, from scattered dose from adjacent slice dose contributions.

C.T. Dose Descriptors Cont’d

DLP = Dose Length Product is the bridge from CTDI to Effective Dose. CTDI v x scan length (slice width x # slices) = DLP in mGy-cmEffective Dose Equivalent (EDE) or Effective Dose = the whole body dose equivalent from a dose to a portion of the body

EDE = Sum of the weighting factors for various organ or tissues compartments x maximum dose to a given organ or tissue compartment (as of 2007 there are 15 compartments)

Bone marrow, breast, colon, lung, and stomach are each 12%; gonads are 8%; bladder, thyroid, liver, and esophagus are 4% each; brain, bone surface, salivary glands, and skin are 1% each, with the remainder 12%.

For Instance

RADS EDEPA Chest…………...... 0.02 ………………………0.02 REM

C.T. Head………………7.5 ……………………….0.2 REM

C.T. Abd………………..2.5 ……………………….1.0 REM

C.T. Ped. Abd………….2.0 ……………………….3.0 REM

Annual Background Radiation……………………0.3 REM

Radiation Risk for Children is 3 times that of the Adult

Effective Dose

• It is a radiation dose quantity • It is a computation based on:

Organ dose and radiosensitivityWeighting factors

Biological Effects of Radiation Learned from the Past:

• Deterministic effects• Stochastic effects

There are Two Types of Bio Effects

Dose dependent: –severity depends on dose–there is a threshold–burns, hair loss

This is a deterministic effect

Deterministic Effects

There are Two Types of Bio Effects

Non dose dependent: – severity is independent of dose– risk of event occurring is dependent

on dose– there is “no threshold”– cancer, genetic mutations

This is a stochastic effect

Biological effects of radiation damage to DNA

• Reactions are rapid • Induction of cancer takes many years• The damage to DNA may lead to

genomic instability

4. Unique issues with radiation in children• Plain film history

– Scoliosis• Therapy

– Tinea capitis– Thymus

• Low dose effect and cancer– Atomic bomb survivors– Brenner

… 96 minutes of x rays

Typical Radiation Doses(mSv)

• Average annual technician dose 3.2• Natural background 3.5• Dental x-rays .09• BE (marrow) 8.75• CXR (marrow) .01• Mammogram (breast) .5 - 7.0• Airline passenger .03• Flight crew / attendants 1.6• CT < 1.0 – 30 mSv

Typical Medical Radiation Doses: 5 year-old

(mSv*)• 3-view ankle .0015 • 2-view chest .02 • Tc-99m radionuclide gastric emptying .06 • Tc-99m radionuclide cystogram .18 • Tc-99m radionuclide bone scan up to 6.2• FDG PET 15.3• Fluoroscopic cystogram <.33• Chest CT up to 3 • Abdomen CT up to 5

CXREquivalents

1/14th

139

31076516150250

* This is effective dose; organ doses (in mGy) will differ

One PET CT in a 5 yr old…

• 23.3 mSv• 1165 chest x rays, or…..• 7.5 years of background

radiation

AJR Feb 2001

AJR February 2001

Fatal Cancer Risk

• Estimated • Debated• May be zero• May be, in children, 1 in 500 - 1,000

risk* from a single CT

* Risk is of fatal cancer!!

• NCRP• ICRP• BEIR• NCI• FDA• ACR, AAPM, etc.

Is Low-level Ionizing Radiation Harmful? Support:

Brenner et al, 2003*“Above doses of 50-100 mSv

(protracted exposure) or 10-50 mSv (acute exposure), direct

epidemiologic evidence from human populations demonstrate the exposure to ionizing radiation

increases the risk of some cancer.”

www.pnas.org/cgi/doi/10.1073/pnas.2235592100

Conclusions from BEIR VII (2005) include:

“…the risk of cancer proceeds in a linear fashion at lower doses

without a threshold and … the smallest dose has the potential to

cause a small increase risk to humans.”

“It should be noted, however, that the inability to detect increased [cancer] risks at very low doses

does not mean that those increases do not exist.”

UNSCEAR 2000

Pierce, Preston, Rad Res 151 pg 178-186: 2000

Brenner Pediatric Radiology Apr 2002 pg 230

Sensitivity of children to radiation

Digital uncoupling of final product and dose

Radiation sensitivity inversely with age

Adult risk is 5%/Sv; children is 16%/Sv, or three times higher.

• Tissues are more radiosensitive• Longer lifetime to manifest

radiation-induced injury (cancer, cataracts)

• Each exam (therefore dose) is cumulative – depending upon where the dose is delivered

Radiation Risks in Children:No Debate

Effective Dose Equivalent (EDE)

Equal exposure:Child EDE > adult EDE

Huda et al Radiology 203: 1997 pg 421

Risk vs. Benefits

Let us never forget, that a properly prescribed diagnostic test utilizing C.T. for children has its benefits. Benefits that most of the time far exceed the risk. The risk, after all is to the increased possibility of a cancer in 10-30 years vs. the effective management of the patient’s current condition.

6. Optimization of benefit/risk ratio

• Appropriate to do exam• Appropriate timing of exam• Appropriate modality• Get clinician/radiologist together• Technologist• CT diagnosis should not be delayed

due to fear of radiation

To improve Benefit/Risk Ratio

1. Prudent patient selection –especially children2. Discussion of non-radiation alternative imaging modalities3. Review of patient medical radiation history – especially

abdomen/pelvis, C.T. fluoro, and conventional fluoro4. Educate and credential referring physicians, E.R.

physicians, and radiologists to the relative risks of medical radiation, especially C.T. and Fluoro

5. Develop CT techniques with medical physicist, CT technologist, and Service representatives to develop low dose techniques while optimizing quality. Enlighten CT techs to newer equipment pediatric techniques-built in

6. Strive for ACR and “Image Gently” recommended published diagnostic radiation levels (DRLs or RRLs)

Prudent Patient Selection

What is considered prudent?

When you feel you can achieve a 95% assureity of a diagnosis that produces the outcome of life over death.

References for Unnecessary High CT Doses

• Pediatric Radiology 2005 35:555-564 (musculoskeletal)

• AJR 2004 183:809-816 (chest)• AJR 2003 181:939-944 (sinus)• AJR 2002 179:461-465 (chest)• AJR 2002 179:1101-1106 (abdomen)• AJR 2002 179:1107-1113 (abdomen)• Pediatric Radiology 1999 29:770-775

(brain)

How Do We Respond?

• Pediatrician or E.D. Physicians’ responsibility:

Be sure the test is necessaryUse the least invasive modality which gives a high certainty of successDiscuss case with radiologist when unsure

Physician’s responsibility:

Understand radiation doses of modalitiesOrder on medical indications not parental/legal pressureDiscuss options with radiologist Consider information for parents

How Do We Respond?

• Radiologists’ responsibilityUnderstand radiation dosesReview requests for higher dose studiesDiscuss with cliniciansUse appropriate technical factors

How Do We Respond?

Clinical RadiologyOctober 2004; 39:928-934

Weight mAAbdomen

Lbs Kg Chest or Pelvis10–19 4.5–8.9 40 6020–39 9.0–17.9 50 7040–59 18.0–26.9 60 8060–79 27.0–35.9 70 10080–99 36.0–45.0 80 120

100–150 45.1–70.0 100–120 140–150>150 >70 >140 >170

Suggested Tube Current (mA) by Weight of Pediatric Patients for Single-Detector Helical CT

Donnelly et al. AJR. 176;303

Radiologist - Parameters• mAs

– Linear to dose (25-60% reduction in pediatric doses is possible for older CT scanners. Newer scanners may have suggested pediatric techniques selectable by the technologist

• kVp– Non linear to dose

20% ↓ kV = 30-40% ↓ dose

• MDCT > radiation

• Large abnormalities, or…• High contrast regions

– Lungs– Bones– CTARemember: A change of pitch from

1.1 to 0.9 can be compensated by a decrease in mAs by ~20%.

“Lower” Dose Pediatric MDCT

15 mAs

8mAs

CT Dose Reduction

• Bone studies: lower mA– Initially 100 mA : 1.3 cGy– Lowered... 40 mA : 0.5 cGy– Currently 20 mA : 0.2 5cGy

A Word about Fetal DosesACR’s appropriateness criteria – for Pelvic Exposures

Try or consider the imaging modalities in the following order: Ultrasound, MRI, Non-pelvic radiographs, non-bone nuclear medicine scans, non-pelvic fluoroscopy or CT, radiography of abdomen/pelvis, PET or bone scans, then pelvic fluoroscopy, CT, CT fluoroscopy.

For Pelvic CT, Fluoroscopy, or CT Fluoroscopy consider a pregnancy test on potentially gravid females before performing these tests. Have medical physicist project fetal dose if exposure to female is absolutely required.

Fetal Dose ContinuedDoses calculated to be less than 5 RAD represent a 40% increase in the risk rate of a cancer to the child. For example if risk is 1/100,000, then after 5 RAD to the fetus, the risk becomes 1.4/100,000.

Doses exceeding 10 RAD may have consequences including mildly diminished mental capacities.

Doses exceeding 15 RAD probably should be recommended for genetic and spiritual counseling, as malformations, Down’s Syndrome, and more significant risks for cancer could be in the future life of the child. It should also be considered that the child may spontaneously abort. This is a situation for genetic and spiritual counselors, in conjunction with some deep parental emotional considerations. Also, to be considered is that the child probably will be borne healthy.

Conclusion

We are part of the way there

• We Need to be Proactive Involve Non-Imagers Control Our DepartmentsEngage Our Community

There is No Ionizing Radiation When You Don’t

Do the C.T. Exam

Remember……..

The ALARA* Concept in

Pediatric CT

*As Low As Reasonably Achievable