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