3. Table of Content
Introduction to radiation
protection
Review of CT
Radiation protection in CT
Equipment & layout of room
Principles & guidelines
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5. Radiation risk
Riskis proportional to absorbed dose
Risk is quantified by determining the
“effective dose” and is expressed as
millisievert (mSv)
Atomic bomb survivors who experienced
doses that were slightly higherthan doses
encountered in CT demonstrated
increased cancers.
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8. ALARA policy
The goal of the radiation safety program is to ensure
that radiation dose to workers, members of the public,
and to the environment is as low as reasonablyas low as reasonably
achievableachievable (ALARA) below the limits established by
regulatory agencies.
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9. Factors in radiation protection
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12. CT examination
"Computed tomography (CT)" means the production of a tomogram
by the acquisition and computer processing of x-ray transmission data.
Computed tomography includes the capability of producing axial
tomograms.
CT examination is a “high dose” procedure and Justification in CT is of
particular importance for RP.
The absorbed dose to tissues from CT can often approach or exceed the
levels known to increase the probability of cancer as shown in
epidemiological studies.
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13. Effective dose estimation
CTDI is equivalent to dose distribution integrated over the
z (longitudinal) patient axis divided by the slice thickness
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15. Justification in CT
A series of clinical factors play a special part in
justification:
Adequate clinical information, including the records of
previous imaging investigations, must be available
In certain applications prior investigation of the patient
by alternative imaging techniques might be required
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16. Optimization in CT
Optimal use of ionizing radiation involves the
interplay of the imaging process:
the diagnostic quality of the CT image
the radiation dose to the patient
the choice of radiological technique
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17. Patient dose in CT
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These
parameters
affect on
patient
absorbed dose :
18. Patient dose in CT
mAs ( directly proportional to radiation dose)
KVp ( inversely but not linearly proportional to dose)
Pitch (inversely related to dosage)
Slice thickness (requires an increase in mAs)
Number of scans (doubling the radiation dose)
Scan time (faster scans lead to an increased scan area)
Scanning mode (step-based vs.spiral technique, single-slice vs.
multi-slice)
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20. Stationary CT rooms
Computed
tomography rooms
typically have high
workloads and high
kilo voltage
technique settings.
gantry
coach
operator
Isodosecurves12/17/17 Isfahan University of Medical Sciences 20
21. Room layout
The official UK guidance on the design of
radiological facilities suggests that CT x-ray
rooms should be designed to a minimum
dimension of 38 m².
at least 1/16-Inch lead shielding or equivalent is
required for the walls, doors, floors, ceilings,
and operator's barrier. The concrete equivalence
would be about 4 to 6 inches of standard-density
concrete , depending on workload and distance
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23. Equipment & facilities
Protective apron, glasses, gloves, collar & gonad
shields and of course film badge & other dosimeter
should be used.
All of devices should be certificated and
calibrated.
Radiation safety committee and specially medical
physicist have important role in performing
radiation protection programs.
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24. Medical physicist’s tasks
Facilities shall be established and implemented under
the supervision of the medical physicist (MP)
MP must perform an initial or acceptance test of
each CT system prior to use on patients
MP must perform an annual evaluation of the CT
system and quality control program
Facility conducts a continuous quality control
program designed/overseen by the MP
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26. ICRP recommendations
to reduce personnel dose
Personal dosimeter shall be monitored (worn
properly, read at an appropriate frequency)
Personal protective equipment to all staff should be
available
Protective clothing should be worn by staff
Duration of exposures, dose-rate and X-ray field
size should be minimized
Positioning of personnel during examinations
should be assessed
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27. ICRP recommendations
to reduce patient dose
Actions for 3 groups are defined:
Operator
Manufacture
Physicians
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28. What can operators do?
Limit the scanned volume
Reduce mAs values
Use automatic exposure control
Shielding of superficial organs
Use of spiral CT with pitch factor>1
Separate factors for children
Use of partial rotation
Adequate selection of image reconstruction
parameters
Record of dose, exposure factors12/17/17 Isfahan University of Medical Sciences 28
29. Actions for
manufacturers
Introduce automatic exposure control
Be conscious of high doses in CT
Include safety features to avoid unnecessary dose
Display of dose
Convenience in using low dose protocols
Draw attention of users to selecting separate
protocols for paediatric patients
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30. Actions for physician
Ensure that patients are not irradiated unjustifiably
Clinical guidelines advising which examinations are
appropriate and acceptable should be available
Consider whether the required information be obtained by
MRI, ultrasonography
Consider value of contrast medium enhancement
Clinician has the responsibility to communicate to the
radiologist about previous CT examination
CT examination of chest in young girls and young females
needs to be justified in view of high breast dose
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31. General rules in x-ray
departments
Using film badge & regular periodic dosimetry
Using standard devices , testing them periodically,
keeping according to manufacture order.
Applying protective equipment for all of patient
Training technologists in order to proper usage of
imaging machines & protective equipments
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32. Special rules in CT departments
Use of low dose scanning protocol
Warm up , calibration & check up of CT machines result
in patient dose reduction
CT technologist should assess patient information and
then choose best technique according to indication, size
and location of lesions
Accurate selection of slice tilt specially in brain scan
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33. AEOI recommendations
Atomic Energy Organization of Iran (AEOI)
publishes guidelines and reports periodically
latest guideline consists of 6 chapter about rules for
radiation activities in diagnostic departments
objectives, applications, key words definitions, and
responsibilities are mentioned in introduction of
named report.
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34. AEOI report INRARP6CP02
( Chapter 3 )
Choose technical factors to acquire acceptable
quality images and low dose to patient
Use minimum slice number and minimum mAs
according to clinical goals
Don’t perform pre contrast scan routinely
Take care of lenses radiation by tilting the gantry
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35. AEOI report INRARP6CP02
( Chapter 3 ) cont.
Mount danger sign and warning light
Pregnant women should informed technician
Occupational exposure should not exceed over
permitted dose limit
Fixed , portable and adjustable protective devices
should be available
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The effective dose in chest CT is in the order of 8 mSv (around 400 times more than chest radiograph dose) and in some CT examinations like that of pelvic region, it may be around 20 mSv
Protection needs to extend from the floor to a height of not less than 2 m and be
continuous.