CT Radiation Risks and Dose Reduction 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
Medical Radiation and Children 1. History 2. Basic dosimetry 3. Biology of radiation effects 4. Unique issues with radiation in children 5. Optimization of risk/benefit ratio 6. Use appropriate techniques 7. Joint efforts with healthcare providers
The radiologist must be introduced to the special techniques required to handle infants and children, and must have experience in their use.
Radiology tremendous benefits, but also risks or side effects Side effects were reported within 3 months of discovery
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 dose 2. mas - mas is directly and linearly proportional to dose 3. Slice Width dose can be reduced by increasing slice width; although SNR is improved, axial resolution/detail is reduced 4. Matrix Size and FOV an increase in these results in improved contrast and spatial resolution, at the expense of dose 5. Window and Level a narrower window enhances contrast resolution 6. Radius of Rotation short diameter (mobile CT) provides higher patient dose 7. Bowtie Absorber the use of a bowtie absorber with helical CT reduces dose through filtration of softer X-rays 8. Helical Pitch changing the pitch to a higher number reduces dose 9. 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 Exit dose in medical imaging: e.g. 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) CTDI vol 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 slice CTDI 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 center CTDI 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-cm Effective 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 EDE PA 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 radiosensitivity Weighting 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*) CXR Equivalents 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 1/14 th 1 3 9 310 765 16 150 250 * 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!!
Is Low-level Ionizing Radiation Harmful? Support: NCRP ICRP BEIR NCI FDA ACR, AAPM, etc.
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.
UNSCEAR 2000 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.
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.
Radiation Risks in Children: No Debate 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
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 children 2. Discussion of non-radiation alternative imaging modalities 3. Review of patient medical radiation history especially abdomen/pelvis, C.T. fluoro, and conventional fluoro 4. 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 necessary Use the least invasive modality which gives a high certainty of success Discuss case with radiologist when unsure
How Do We Respond? Physician s responsibility: Understand radiation doses of modalities Order on medical indications not parental/legal pressure Discuss options with radiologist Consider information for parents
How Do We Respond? Radiologists responsibility Understand radiation doses Review requests for higher dose studies Discuss with clinicians Use appropriate technical factors
Clinical Radiology October 2004; 39:928-934
Donnelly et al. AJR. 176;303 Weight Suggested Tube Current (ma) by Weight of Pediatric Patients for Single-Detector Helical CT ma Abdomen Lbs Kg Chest or Pelvis 10 19 4.5 8.9 40 60 20 39 9.0 17.9 50 70 40 59 18.0 26.9 60 80 60 79 27.0 35.9 70 100 80 99 36.0 45.0 80 120 100 150 45.1 70.0 100 120 140 150 >150 >70 >140 >170
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
Lower Dose Pediatric MDCT Large abnormalities, or High contrast regions Lungs Bones CTA Remember: A change of pitch from 1.1 to 0.9 can be compensated by a decrease in mas by ~20%.
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 Doses ACR 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 Continued Doses 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 Departments Engage Our Community
Remember.. There is No Ionizing Radiation When You Don t Do the C.T. Exam
The ALARA* Concept in Pediatric CT *As Low As Reasonably Achievable