Radiation Safety Chapter 9 Bushong

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

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

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

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)

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

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

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

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

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***

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)

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

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

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

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

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

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***

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

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