Download - Radiation
Radiation
Justin McWilliams, MDAssistant ProfessorInterventional Radiology
Quiz
You need a lateral view. Is it better to rotate the image intensifier toward you or away from you?
Why is fluoro time a poor indicator of radiation exposure ?
How many Grays of radiation puts the patient at risk for skin injury?
A 5-second DSA run uses how much more radiation than 5 seconds of fluoro?
A typical embolization procedure exposes the patient to how many CXRs worth of radiation? What is the increased cancer risk of such a procedure? What is the cancer risk to the operator if he does 1 embo procedure
per working day for 30 years?
Radiation
Radiation exposure
Radiation effects
Minimizing radiation to the patient
Minimizing radiation to you
Radiation exposure
X-ray production
X-rays are produced by accelerating electrons through high voltage (50-150 kVp) applied to a tungsten target in an X-ray tube
Amount of X-rays produced are determined by tube current (mA) and the tube voltage (kVp)
Dose is hard to measure in IR! Dose is not administered uniformly throughout
the patient’s body Radiation field is moved, angled, collimated
Both fluoro and DSA are used
Four metrics are used to estimate patient radiation dose Fluoro time Peak skin dose (not yet measured by equipment) Reference dose (air kerma) Dose-area product (DAP)
Reference dose
Also called “cumulative dose”
The Air Kerma for the entire procedure, measured in Gy at a fixed reference point near the isocenter of the tube
Does not account that the radiation field is moved to different areas of the patient during the procedure
Conservative, generally overstates risk
Measurement is likely accurate to within +/- 50%
Dose-area product
Measure of total X-ray energy absorbed by the patient
Basically the air kerma (dose) multiplied by the area of body exposed (area)
Fluoro time
Fluoro time is only a very rough indicator of radiation dose, affected by: Patient size Beam location Beam angle Normal vs. high dose rate Distance of tube from the patient
These can all add up to 10-fold difference in dose for the same fluoro time!
How to measure patient dose?DOSE-AREA PRODUCT (DAP)
Product of the air kerma and the exposed area (in cm2)
Good measure of stochastic risk (cancer risk) because it estimates total radiation energy delivered to a patient
Poor estimator of skin dose and deterministic effects large dose over small area or small
dose over large area?
Unit of measurement (Gy-cm2) does not translate into standard units of dose (hard to use)
CUMULATIVE AIR KERMA Air kerma = Kinetic Energy
Released per unit Mass of Air; basically, how much radiation dose is being delivered at a specific point (about where the patient’s skin is)
Also known as reference dose or cumulative dose
Easy to measure, expressed in Gy
Absorbed dose in tissue will be about equal to the air kerma at that point
Notification threshold = 3 Gy
DAP vs. Air Kerma
Absorbed dose
Patients and staff are exposed to radiation, but only a portion is absorbed into the body
Absorbed dose is measured in Gray or rads 1 Gray = 100 rads
Approximate radiation doses: Fluoro = 2-10 rads/min CXR = 0.02 rads CT abdomen = about 2-10 rads Natural background radiation = 0.3 rads/year
Dose equivalent
Different forms of radiation (X-rays, alpha particles, etc) produce different biologic effects for same absorbed dose
Dose equivalent (rem or Sievert) is used to measure biologic “harmfulness” of a radiation dose
For diagnostic X-rays, 1 rem = 1 rad and 1 Gy = 1 Sv
Effective dose
Effective dose is the dose equivalent to the whole body caused by irradiating just a localized area This is calculated by multiplying the dose to each
irradiated organ by a weighting factor based on the radiosensitivity of that organ
Example effective doses: CXR = ~0.1 mSv PTA = 10-20 msV Biliary drainage = 40 mSv Transcatheter embolization or TIPS = 50-100
mSv
Additional cancer risk = ~5%/Sv
So, a long embolization procedure in a 30 year old increases risk of developing a fatal cancer by about 0.5%
Radiation production in fluoro Fluoro machines operate in automatic
brightness control
When brightness of picture is inadequate, the ABC automatically increases mA or kVp (or both) to increase X-ray penetration Large patients = more dose than small patients
(up to 4-10x higher!) Abdominal fluoro = more dose than chest fluoro Oblique fluoro = more dose than AP fluoro
Direct radiation exposure
Direct exposure rate refers to entrance skin exposure where the X-ray beam enters the patient 2-10 rads/min for fluoro ~50 rads/min for DSA 30 mins of fluoro = 60-300 rads = 0.6-3
Gy
Indirect radiation exposure Indirect exposure rate refers to exposure to
the staff from scattered radiation from the patient
~1/1000 of the skin entrance exposure rate at a distance of 1 meter Large patients increase scatter radiation Larger field (not collimated) increases scatter Scatter much higher on the X-ray tube side of
the patient▪ For lateral view, stand next to II, not next to tube!
Radiation effects
Deterministic effects
Radiation effects with a threshold dose; effect is not observed unless threshold is exceeded
Skin dose thresholds
Early erythema – 3 Gy – 1-2 days – sunburn
Epilation – 3-7 Gy – 3 weeks – hair loss
Main erythema – 10 Gy - onset 1-4 weeks – burning, itching If >14 Gy, progresses to dry
desquamation 1 week later If >18 Gy, progresses to moist
desquamation (blistering, sloughing) 1 week later
Ulceration – 24 Gy – 2-12 months
Stochastic effects
No threshold
Any dose increases the chance of the effect, with higher doses increasing the chances
Radiation-induced cancer
Radiation-induced cancer Approximate additional risk of fatal
cancer for an adult for an examination: Extremity X-ray: <1/1,000,000 CXR: 1/100,000 to 1/1,000,000 Chest CT: 1/10,000 to 1/1,000 Multiphase abdominal CT: 1/1,000 to 1/500
These risks are additive to the ~25% background risk of dying of cancer
Minimizing radiation exposure to the patient
Patient factors
Very small (<10 kg) or very large (>135 kg) patients
Age (3x risk for newborns, 1x risk at age 25, 0.2x risk for patients in 60s)
Pregnant patients
Prior radiation exposure within last 2 months
Diabetes, autoimmune diseases, connective tissue diseases increase risk of skin effects
Procedure factors
Ultrasound instead of fluoro when possible (biliary, arterial access)
Patient should be as far from tube, and as close to II, as possible (good to be tall!)
Don’t step on the pedal
Pulse fluoro mode (7.5 or 15 frames/sec instead of 30/sec)
View and save images with “last image hold”
Exclude bone from the image
Procedure factors (cont)
Collimate to smallest field of view possible Avoid exposure to eyes, thyroid and gonads
Position and collimate without fluoro 5-8% of radiation exposure is delivered during preparation for
imaging, positioning the table and adjusting collimators
Avoid magnification ABC uses more radiation to brighten and sharpen the image in mag
view
Avoid high-dose or detail modes
Use higher kVp (but can reduce contrast)
Minimize overlap of fields and repeated acquisitions
3-6-9 rule
Minimizing radiation exposure to staff
Reduce radiation time
Less time on the pedal
Use last image hold
Pulsed fluoro
Low dose fluoro
Increase distance from radiation
Inverse square law Double distance from patient = ¼ the
radiation dose from scatter radiation Nonessential personnel should be
outside a 6-foot radius from the X-ray source
Step out of room for DSA runs
Shielding
Lead apron (0.5 mm Pb equivalent) blocks about 95% of scatter radiation
Thyroid shield, leaded glasses are essential Most radiosensitive organs
Lead drapes and clear leaded glass barriers
Follow up
Post procedure
Record dose in the medical record
If dose exceeded deterministic thresholds Discuss possible effects and
management with patient Have patient or family member notify IR
if deterministic effects occur Institute a clinical follow-up plan for the
patient
Follow up plan
Necessary when large radiation dose was used
Telephone call at 2 weeks or so Redness? Blistering? Hair loss? Location of radiation field
May need follow up for >1 year
Quiz
Quiz
You need a lateral view. Is it better to rotate the image intensifier toward you or away from you?
Quiz
You need a lateral view. Is it better to rotate the image intensifier toward you or away from you?
Toward you! Keep the beam away from you, because most of the scatter occurs at the point the beam enters the patient
Quiz
Why is fluoro time a poor indicator of radiation exposure ?
Quiz
Why is fluoro time a poor indicator of radiation exposure?
Does not include DSA runs Dose varies greatly for the same
fluoro time Thin or obese patient AP or oblique views Magnification Distance from X-ray source
Quiz
How many Grays of radiation puts the patient at risk for skin injury?
Quiz
How many Grays of radiation puts the patient at risk for skin injury?
3 Grays!
Quiz
A 5-second DSA run uses how much more radiation than 5 seconds of fluoro?
Quiz
A 5-second DSA run uses how much more radiation than 5 seconds of fluoro?
About 10x more radiation for DSA!
Quiz
A typical embolization procedure exposes the patient to how many CXRs worth of radiation? What is the increased cancer risk of such
a procedure? What is the cancer risk to the operator if
he does 1 embo procedure per working day for 30 years?
Quiz
A typical embolization procedure exposes the patient to how many CXRs worth of radiation? About 1000! What is the increased cancer risk of such a
procedure? About 0.5% for a 30 year old! What is the cancer risk to the operator if he does
1 embo procedure per working day for 25 years?100 mSv (patient equivalent dose) x 1/250 (scatter fraction at 18 inches) x 1/20 (fraction of radiation that gets through the lead) x 5000 (# of procedures) = 100 mSv
A career in IR is probably equivalent to having an embolization
procedure done on yourself (0.5% additional cancer risk)
References
Mitchell E and Furey P. Prevention of radiation injury from medical imaging. J Vasc Surg 2011; 53:22S-27S.
Miller D, et al. Clinical radiation management for fluoroscopically guided interventional procedures. Radiology 2010;257:321-332.
Cousins C and Sharp C. Medical interventional procedures – reducing the radiation risks. Clin Radiol 2004;59:468-473.
Wagner L. Angiography radiation dose – limiting dose to the patient while maintaining effective image quality. http://www.uth.tmc.edu/radiology/RSNA/2008/RSNA_wagner_2008.pdf