Radiation

1,126 views

Published on

0 Comments
1 Like
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
1,126
On SlideShare
0
From Embeds
0
Number of Embeds
60
Actions
Shares
0
Downloads
67
Comments
0
Likes
1
Embeds 0
No embeds

No notes for slide

Radiation

  1. 1. Justin McWilliams, MDAssistant ProfessorInterventional Radiology
  2. 2.  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?
  3. 3.  Radiation exposure Radiation effects Minimizing radiation to the patient Minimizing radiation to you
  4. 4.  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)
  5. 5.  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)
  6. 6.  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%
  7. 7.  Measure of total X-ray energy absorbed by the patient Basically the air kerma (dose) multiplied by the area of body exposed (area)
  8. 8.  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!
  9. 9. DOSE-AREA PRODUCT (DAP) CUMULATIVE AIR KERMA Product of the air kerma and the  Air kerma = Kinetic Energy exposed area (in cm2) Released per unit Mass of Air; basically, how much radiation dose Good measure of stochastic risk is being delivered at a specific (cancer risk) because it estimates point (about where the patient’s total radiation energy delivered to skin is) a patient  Also known as reference dose or Poor estimator of skin dose and cumulative dose deterministic effects  large dose over small area or small  Easy to measure, expressed in Gy dose over large area?  Absorbed dose in tissue will be Unit of measurement (Gy-cm2) about equal to the air kerma at does not translate into standard that point units of dose (hard to use)  Notification threshold = 3 Gy
  10. 10.  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
  11. 11.  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
  12. 12.  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%
  13. 13.  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
  14. 14.  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
  15. 15.  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!
  16. 16.  Radiation effects with a threshold dose; effect is not observed unless threshold is exceeded
  17. 17.  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
  18. 18.  No threshold Any dose increases the chance of the effect, with higher doses increasing the chances Radiation-induced cancer
  19. 19.  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
  20. 20.  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
  21. 21.  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
  22. 22.  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
  23. 23.  Less time on the pedal Use last image hold Pulsed fluoro Low dose fluoro
  24. 24.  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
  25. 25.  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
  26. 26.  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
  27. 27.  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
  28. 28.  You need a lateral view. Is it better to rotate the image intensifier toward you or away from you?
  29. 29.  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
  30. 30.  Why is fluoro time a poor indicator of radiation exposure ?
  31. 31.  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
  32. 32.  How many Grays of radiation puts the patient at risk for skin injury?
  33. 33.  How many Grays of radiation puts the patient at risk for skin injury? 3 Grays!
  34. 34.  A 5-second DSA run uses how much more radiation than 5 seconds of fluoro?
  35. 35.  A 5-second DSA run uses how much more radiation than 5 seconds of fluoro? About 10x more radiation for DSA!
  36. 36.  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?
  37. 37.  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)
  38. 38.  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_wa gner_2008.pdf

×