Radiation protection + dosimetry in practice Flashcards
What are entrance and exit doses normally measured in?
Typical entrance doses will be measured in mGy. Exit doses will be measured in μGy
What dose rate can you get if you put your finger in primary beam in fluoro?
50 mGy/min
What are the two types of radiation which will result in a radiation dose to patients and/or members of staff?
Leakage radiation - There will inevitably be some leakage from an x-ray tube housing. this is limited to a maximum of 1 mGy/hr at 1 metre
Scattered radiation - emitted in all directions and is caused by scattering interactions within the patient. Very few scattered photons are produced by elastic scattering and the large majority are the result of the Compton effect.
What does the amount of scatter in the patient depend on?
The amount (fluence) of scattered radiation depends on the field size, volume of patient irradiated and the quality of the primary beam. Both fluence and quality of the scattered radiation have a strong angular dependence
How does the scatter vary from the exit and entrance side of the patient?
Because photons scattered in the forward direction are attenuated in the body, the amount of scatter increases with scattering angle and is less on the exit side of the patient than on the tube side.
Furthermore, there is preferential attenuation of low energy scattered photons and so the scatter is harder (more penetrating) on the exit side
What factors increase scatter?
If you increase the kV, the amount of scatter will increase because there are more photons produced. The number of photons is proportional to the square of the applied potential
If you increase the mA, there will be more scatter because there are more photons produced. The number of photons is proportional to the tube current
If you image for longer, there will be more scatter because there are more photons produced. The number of photons is proportional to the length of time the kV is applied
If you image a larger volume of the patient, there will be more scatter because there will be more atoms of tissue elements with which the photons in the x-ray beam can interact. The tighter the collimation, the smaller is the volume of tissue irradiated, the less is the amount of scatter and the lower is the potential staff dose
T or F: The bigger the patient, the greater the amount of scatter.
This statement is true because:
There are more interactions possible because of the greater patient volume
Bigger patients will require a higher kV and mA to obtain a diagnostic image
In fluoroscopy, larger patients will probably require longer exposures to obtain a diagnosis
What equipment factors effect beam-on time?
Pulsed operation
Last image hold
Virtual collimation - enables collimation to be changed via the LIH facility, so that the beam does not have to be on while this is done.
Why is the ISL not totally obeyed with scatter?
ISL is only approximately obeyed by scattered radiation because the source is a volume of tissue within the patient not a point source
How is the shielding in an xray room designed?
he intention of the design is to ensure that persons outside the x-ray room do not receive a dose that is more than a fraction (normally 0.3) of the annual dose limit. The fraction is called a constraint.
What is the dose limit for a member of the public?
The dose limit for a member of the public is 1 mSv per annum, so most x-ray rooms are designed so that any member of the public will not receive a dose from x-ray procedures being performed in that room of more than 0.3 mSv.
What is the typical shielding thickness for an xray room?
150 mm thick concrete walls or 2.0 mm lead ply strapped to an existing less substantial wall.
Do lead aprons provide protection from primary beam?
No
What lead equivalence should aprons have?
at least 0.25 mm for x-ray tube voltages up to 100 kV and 0.35 mm for voltages up to 150 kV.
It is not uncommon to find 0.5 mm lead equivalent aprons but it is actually more helpful to use a thyroid shield rather than a thicker apron in order to reduce the effective dose.
What is the normal lead equivalence of a thyroid shield?
Thyroid shields typically have a lead equivalence of 0.5 mm.
Is an overcouch or undercouch configuration better for doe reduction?
an overcouach configuration the operator is subject to a greater amount of scattered radiation compared to a undercouch configuration
In an undercouch fluoroscopy configuration, the dose to the staff member will be up to x10 less than in an overcouch (tube over the couch) configuration. This is because the intensity of the scattered radiation varies with angle of scatter (with reference to the primary beam) and at 120° the scatter is 3 or more times that at 60°.
In mobile ward XR how is dose reduced?
Ensuring that the primary beam is directed away from individuals
Use of the inverse square law
Ensuring that appropriate members of staff wear lead aprons
On wards, it is conventional for the radiographer to define a region of radius 2 m around the patient as a controlled area from which other staff members are excluded. The radiographer/operator wears a lead apron and exercises control of access to the controlled area verbally. The x-ray beam is oriented vertically wherever possible.
What is the dose constraint to the pregnant abdomen?
A dose constraint of 1.3 mSv is often applied to the maternal abdomen over a 9-month declared term.
This is a conservative value and will result in a foetal dose less than 1 mSv (the abdomen will attenuate the x-rays).
Who are ‘members of the public’ according to IRR?
In broad terms, a member of the public is anyone who is not a radiation employee or a patient or willing participant in a research study employing ionising radiation.
Members of the public would therefore include diverse staff groups who do not use x-rays in the course of their employment as well as people unconnected with the hospital.
How is a carer different to a member of the public?
subject to a different dose constraint - 5 mSv in any period of 5 consecutive calendar years.
roles and responsibilities in the radiation protection of the patient:
A. The referrer is responsible for optimisation
B. The practitioner is responsible for optimisation
C. The operator is responsible for optimisation
D. The referrer decides if the examination is justified
E. The practitioner decides if the examination is justified
F. The operator decides if the examination is justified
G. A radiographer can be a practitioner
H. A nurse can be a referrer
A. False.
B. False.
C. True.
D. False.
E. True.
F. False.
G. True.
H. True.
What is the role of the practitioner?
It is the role of the practitioner to justify the examination and determine that the prospective benefit from a positive or negative outcome outweighs the risk to the patient which results from the radiation exposure.
This will be done following consideration of:
The clinical information supplied on the request card
How appropriate the request is
The potential benefit of the requested procedure
The availability and utility of other techniques involving the use of less (or no) ionising radiation
A. A skull x-ray is usually justifiable following a bang on the head
B. A CT scan of the abdomen will usually result in an effective dose of greater than 5 mSv
C. A lumbar spine examination will result in an effective dose of less than 0.1 mSv
D. A request for an abdominal x-ray to investigate constipation is justifiable
E. A request for a chest x-ray to investigate pleural effusion is justifiable
A. False. See the Royal College of Radiologists (RCR) guidelines ‘iRefer: Making the best use of clinical radiology’. A link to the appropriate page on the RCR website is available on the Links page, accessible on the Resource icon.
B. True.
C. False. A lumbar spine examination will result in an effective dose of around 1 mSv.
D. False. In general, the important factors are clinical history and clinical findings. Only in exceptional clinical situations should imaging be used.
E. True. This would be a normal investigation in evaluating this type of patient.
Who carries out a radiological examination?
an operator