RTMR 284
CHAPTER 21
 Rad
 The unit of absorbed dose..
 Patient radiation dose is measured in units of rad
 Rem
 The unit of effective dose..
 Dose to the whole body measured in units of rem
 Basis of dose limits
 mRAD
 The millirad is equal to one one-thousandth of a rad
 Gray - Gy
 The SI unit of absorbed dose equal to an energy
deposition of Joule/kg = 10,000 ergs/gm
 (1 Gy = 100 rads).
 Sievert –Sv
 The SI unit of effective dose.
 Classical /Coherent scattering (occurs at < 10keV)
 Compton effect
 Photoelectric effect
 Pair production (occurs at 1.02 MeV)
 Photodisintegration (occurs at >10 MeV)
 Reference Chapter 7 for more details
 Moderate-energy X-rays interact with outer-
shell electrons
 This interaction




scatters the X-ray (up to 180 degrees)
reduces its energy (retains up to 2/3rds energy)
ionizes the atom
Compton effect occurs more above 80 kVp
 Compton effect occurring does not depend on
the atomic # of the atom involved
 Scattered X-rays provide no useful information on
the image
 Compton-scattered X-rays produce fog
 Compton effect is the source of exposure to
radiographers
 Radiation (large amounts) can be scattered from
the patient, especially during fluoroscopy
 Diagnostic range X-rays undergo ionizing
interactions with inner-shell electrons
 The X-ray is absorbed
 Photoelectric effect occurs more with:
 high atomic number material
 low energy X-rays
 Photoelectric effect occurs more below 80 kVp
 Name two materials that would result in more
photoelectric effect occurring.
 Lead
 Iodine contrast
 Barium
 Bone
 Soft tissue/air
 Fat
82
56
56
14
7
6
 Most of the photons (going through the patient) will
interact & be absorbed by the tissue. Only ~5% of the
incident photons will emerge from the patient to form
the image
 ~50 % of the total number of photons reaching the
image is from scattered radiation
 kVp (Kilovoltage peak)
 part thickness
 field size (area exposed, large coning)
 tissue density
 Scatter radiation is greatest with:
◦
◦
◦
◦
high kVp
large fields (open cone) - most important factor
thick body parts
dense tissue
  skin entrance dose to the patient
 shorter exposures ( motion and blurring)
 increased radiographic latitude
 improved control of radiographic contrast
 tube not heated up as much due to better X-ray
production efficiency at higher voltages
 The next slide is for your comprehension only –
will not need to reproduce!
 Higher atomic number (Z)
 PE absorption greater in absorbers with higher Z
 Compton scatter unaffected by Z number
 Increased kVp
 PE absorption decreases sharply
 Compton scatter remains proportionally greater
 Increased mass density
 PE absorption increases
 Compton scatter increases
 As
 Low
 As
 Reasonably
 Achievable
 What can we do as
technologists to reduce
radiation exposure???
 Cardinal Principles:
 Time
 Distance
 Shielding
 How do we use these
principles in our daily
jobs as technologists?
 Limiting Occupational exposure.
 Whole body: 5 rem / 50 mSv
 Quarter: 1.25 rem / 12.5 mSv
 Cumulative Effective Dose Limit E = N x 10 mSv,
where N is the age in years.
 Study Table 21.3 in textbook.
 Type





Pb
Atten.
Apron 0.50 mm 99.9%
Gloves 0.25 mm 99%
Thyroid 0.50 mm 99%
Glasses 0.35 mm 99%
Drape 0.25 mm 99%
 Holding Pt. Comments?
 Techniques
 Repeats
 Shielding
 Flat contact
 Shadow
 Radiation Dose
 ESE – Skin entrance exp.
 Pediatrics Patients
 What exams have a high
ESE???
 Technologist:
 Have a choice to declare her
pregnancy.
 Dose limit 0.5 mSv/mo.
 5 mSv for pregnancy
 Baby Badge
 Wrap around apron
 Stay away from high dose procedures





Patient:
10-day rule for patients.
Ask EVERY TIME
Keep dose low
Talk to Radiologist if you feel the
exam is unnecessary and the
physician is insisting.