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