Radiation Safety

1. Radiation Safety Chapter 9 Bushong

2. Patient Radiation Dose • Generally, patient radiation dose is higher during computed tomography (CT) than during radiography or fluoroscopy • Patient radiation dose during CT is approximately 5000 mrad per examination • Patient radiation dose during radiography is approximately 4000 mrad/min with doses ranging from 1000 – 10,000 mrad per examination

3. Patient Radiation Dose • Patient radiation dose during spiral CT is inversely proportional to pitch • Patient radiation dose during CT is nearly uniform throughout the body, while that from radiography or fluoroscopy is maximum at the entrance skin • For a given collimation during spiral CT, higher pitch results in lower patient radiation dose

4. Patient Radiation Dose • Patient radiation dose is often less in spiral CT because overlapping images can be reconstructed without overlapping scans • Patient radiation dose during CT is higher with thinner slices or overlapping slices • As with any x-ray imaging, patient dose can be reduced at the expense of image noise

5. Patient Radiation Dose • When patient dose is specified for a CT examination, it is usually an average value of a dose distribution • The dose profile is most helpful in identifying patient dose • Patient dose in CT is measured with a pencil ionization chamber • Patient dose in CT is described by the CT dose index (CTDI)

6. Radiation Dose • The CTDI is equal to the multiple scan average dose (MSAD) if the slice thickness (ST) is equal to the couch incrementation (CT) • If the ST does not equal CI, MSAD is equal to the CTDI multiplied by CI

7. Personnel Radiation Exposure • Area radiation exposure is figure-eight shaped • Lowest area radiation exposures are in the plane of the gantry and outside the patient aperature • Highest are radiation exposure is near the patient and is due to scatter radiation produced in the patient

8. Personnel Radiation Exposure • Area radiation exposure is approximately 1 mR/scan at 1 m from the scan plane • It is permissible for a CT technologist to remain in the room during examination but protective apparel must be worn • When a technologist is in the room during examination, the radiation monitor should be positioned at collar level above the protective apron

9. Personnel Radiation Exposure • As Low As Reasonably Achievable (ALARA) • Cardinal principles • Time • Distance • Shielding

10. Contrast Media • The number of particles of contrast media (solute) per kg of water (solvent) is osmolity • ***I haven’t exactly heard of osmolity, it appears in several journal articles but is a rather difficult word for which to find an exact definition.**** • Osmolality is defined as the number of particles in solution, or the number of milliosmoles per kilogram, or the concentration of molecules per weight of water • Osmolarity is defined as the number of milliosmoles per liter of solution, or the concentration of molecules per volume of solution • *** Since Bushong seems to be referring to particles per kg I will use the term osmolality rather than osmolity***

11. Contrast Media • High osmolar contrast media (HOCM) is the conventional “ionic” contrast media • Blood has an osmolality of approximately 300 mOsm/kg water • HOCM has 4 to 8 times the osmolality of blood (1200 to 2500 mOsm/kg) • HOCM is considered to be more toxic than low osmolar contrast media (LOCM)

12. Contrast Media • LOCM is “non-ionic” contrast media • LOCM contrast media has 2 to 3 times the osmolality of blood (600 – 800 mOsm/kg water) • Contrast media has a biologic half-life of 10 to 90 minutes • Ninety percent of contrast media is excreted within 24 hours

13. Contrast Media • Peak urine concentration occurs approximately 2 hours following administration • Contrast media can be excreted through liver but is primarily excreted through kidneys • Patients with poor renal function have increased excretion of contrast media through gallbladder and small intestine

14. Contrast Media • Contrast media does not change when excreted in mother’s milk • Contrast media enhance contrast resolution by increased photoelectric effect • Atomic number for iodine is 53 • Degree of contrast enhancement is directly related to iodine concentration • Following intravenous injection, peak iodine blood concentration occurs within 2 min

15. Contrast Media • Following IV injection, the vascular compartment biologic half-life for iodinated contrast is approximately 20 minutes • Iodinated contrast media is transferred from the vascular to the extra vascular compartment in about 10 minutes for equilibrium followed by an exponential decrease in both

16. Contrast Media • Renal accumulation occurs in approximately 1 minute with maximum contrast occurring in 5 to 15 minutes • Severely impaired renal function results in prolonged plasma levels and poor contrast resolution • Contrast media should be warmed to body temperature before administration

17. Contrast Media • Contrast media may be administered by bolus injection or rapid infusion • Adult dosage is 150ml to 250ml • ***At my institution the adult dosage is between 100ml to 150ml but never above 150ml Omnipaque*** • Child dosage is 1 ml/kg to 3 ml/kg • ***my institution uses 1 ml/lbs***

18. Contrast Media • Less contrast is required for spiral CT • An over dosage may be life threatening by compromising the cardiovascular or pulmonary systems • Contrast media is dialyzable since it does not bind to plasma or serum protein • Reaction to contrast media is unpredictable. Personnel must be trained to recognize contrast media reaction and respond appropriately

19. Contrast Media • Fatal contrast reactions reportedly occur from 6 to 100/M