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
What is the role of the operator?
responsible for the practical aspects of the examination, which means that once the justification has been made, the operator is the person charged with ensuring that the examination produces the desired outcome, whilst ensuring that the radiation dose to the patient is as low as reasonably practicable (ALARP).
Thus the operator is responsible for ensuring that the optimisation process is carried out on the correct patient and for maintaining the balance between patient dose and image quality.
Each of the following people is acting as an operator.
A. A radiologist reporting a mammogram
B. An interventional radiologist inserting a PICC line
C. A GP who requests an open access skull x-ray
D. A staff member responsible for film processor quality control
E. A medical physicist setting up an AEC device
A. True.
B. True.
C. False.
D. True.
E. True.
The reporting radiologist and the referring GP do not do anything to physically affect the exposure to the patient, whereas the others do. However, the regulators regard the clinical evaluation (reporting) of the outcome of an exposure to ionising radiation as an operator function.
A. An AP radiograph of the abdomen of an obese patient requires more dose than that of a slim patient
B. A typical grid factor is 20:1
C. Patient entrance surface dose (ESD) will decrease with increasing kV when AEC is employed
D. Copper filtration will decrease patient ESD
E. A typical grid ratio is 12:1
F. A 200 speed film/screen combination will result in a lower dose to the patient than a 400 speed combination
G. Ignoring backscatter, field size changes effective dose without altering the ESD
A. True. The patient will be thicker and will therefore attenuate the x-rays to a greater degree. Thus in order to achieve the same exit dose, the entrance dose will need to be greater.
B. False. The grid or Bucky factor which describes the exposure increase required when using a grid will typically be in the region of 4 to 8 and will decrease as the kV increases.
C. True. As kV increases a larger proportion of the radiation penetrates the patient and thus for constant exit dose, less entrance dose is required.
D. True. The copper filter will remove the lower energies in the x-ray spectrum thus increasing the average energy of the beam. See above.
E. True. The session in Module 8a Physics/Anti-Scatter Grids Screens in Radiography (300-0777) has more information on this.
F. False. The speed is the reciprocal of the exposure required to achieve an optical density of 1. So the higher the speed, the lower the dose.
G. True. The session in Module 8a Physics/Patient Dosimetry (300-0692) has more information on this.
How does a smaller field size impact dose?
Strict limitation of the field size to the area being examined should be a matter of routine. This will both reduce the patient dose and result in a better image because of the reduction in scatter.
How does kV impact dose?
Appropriate choice of kV is important. In general, a higher kV will result in a lower patient skin and effective dose for the same exit dose as the mA is often automatically adjusted. If mA kept the same then increased kV will increase dose
How does projection impact dose?
The projection used can influence the radiation dose to the patient. For example a PA chest x-ray will result in a lower effective dose than an AP chest x-ray, a left lateral lumbar spine projection will result in a lower effective dose than a right lateral.
If a tissue, sensitive to radiation, lies close to the surface of the patient where the x-ray beam enters then that tissue would receive a relatively large dose. If the radiograph is taken using the opposite projection, say AP instead of PA, the dose to that tissue will be reduced.
How does focus skin distance effect dose?
Increasing the tube focus to skin distance will reduce the entrance surface dose
why should the fastest speed class consistent with the required image quality be used in all exposures?
Fast image receptor speed reduces patient dose.
However, in conventional radiography, increasing speed by increasing the screen intensification factor results in either increased noise (if it is the conversion efficiency that is increased) or poorer spatial resolution (if it is the screen thickness that is increased). These effects adversely affect image quality.
What are typical frame rates for digital angiography (DA) and digital subtraction angiography (DSA) acquisitions and why?
Frame rates can vary from 1 to 6 fps, depending on the anatomical site under consideration. Greater temporal resolution isn’t required and the increased dose cannot be justified.
The important fact is the rate of blood flow in the area under consideration. For example, from the aortic bifurcation down to the calf a fluoroscopy imaging rate of 2 fps would probably be suitable whilst from the calf downwards 1 fps would suffice. Similarly, hepatic angiography is usually performed at 2 fps whilst in the case of cerebral angiography the initial phase at least may be performed at 4 to 6 fps.
What is the effect of tube filtration on dose?
the effect of tube filtration is to modify the x-ray spectrum by preferentially absorbing lower energy x-rays. The effect is to reduce the radiation dose to the patient.
How does Al or Cu filtration change the beam?
Aluminium (Al) filtration reduces the dose to the patient with minimal effect on image quality. In addition to the mandatory aluminium filtration, many fluoroscopic units, especially those used in the interventional environment, incorporate extra copper (Cu) filtration (with thicknesses up to 0.9 mm being used). Copper filtration is primarily intended as a means of reducing entrance surface dose. In situations where this filtration is not switched in automatically, consideration should be always given to its use in high dose examinations.
Note that if copper filtration is used, there will be a reduction in x-ray output requiring an increase in exposure (mAs), and a decrease in contrast caused by the change in x-ray beam quality. The increase in exposure required for heavier patients may be so great that it cannot be achieved, thus making the use of Cu filtration impossible.
Both effective and entrance doses will be greater for a large patient than for a thin patient. Why is this?
The x-rays will need to be more penetrating for a large patient and so the kV will need to increase.
In addition, it is quite likely that there will need to be an increase in mA to achieve a suitable contrast to noise ratio. The result can be a considerable increase in dose compared to that for a thin patient. The increase can be by as much as ten times.
Furthermore, because of tube loading effects, it can turn out that copper filtration cannot be used on larger patients resulting in even higher (skin) dose.
What are dose curves on fluoroscopic machines and how can they impact dose?
Many fluoroscopic devices come with a variety of options for controlling the kV and mA. At the very least there will be selectable dose rate options. These will usually work by adjusting the mA. A lower dose rate may result in a noisier image but it may well be suitable for the task at hand and should be used whenever possible.
There may be also be specific task oriented curves controlling how the kV and mA interact. As well as general curves, there may be curves for paediatric radiology, curves for use when high contrast is required and curves for use with contrast media.
A curve that increases kV more quickly thus utilising a more penetrating radiation beam for a given patient attenuation results in lower dose.
What is the rough dose per frame in DSA/DA
Patient entrance doses are of the order of 1 mGy per frame. Care must therefore be taken to avoid cumulative exposures that might result in the onset of deterministic effects.
In a paediatric case, why should you consider not using a grid in fluoro?
Children are more sensitive to radiation. Therefore, paediatric cases are performed at a lower kV and a smaller volume is irradiated, so there will be less scatter.
Air gap to reduce scatter has less increase in dose needed than a grid
Why is collimation important? What is the effect of collimation on patient dose, staff dose and image quality?
The tighter the collimation, the smaller the volume of the patient that is irradiated (Fig 1). As a result, there will be less scattered radiation and image quality will be improved. The effective dose to the patient will also be reduced. In addition, the reduction in scatter means that there will be a concomitant reduction in staff dose.
The ESD will not be reduced by collimating but the effective dose will.
how can some dose be saved in CT?
To try and make dose savings without affecting image quality or diagnostic reliability, ask yourself the following questions:
Is the pre-contrast CT scan necessary?
Are all the image acquisition phases really required?
Are you scanning too long a region of the patient?
Is there another test available that has the same (or better) diagnostic reliability?
What is the single most important factor in reducing CT dose?
mAs is probably the single most important factor.
Where possible, CT scans should be tailored to the individual patient. The mAs should be adjusted to suit patient size and body part – for example, a child should not be scanned at the same mAs as an adult.
Modulation of the mA, where the tube current adjusts according to patient attenuation, should be used where possible - but be aware of the initial setup conditions, i.e. where you set the maximum mAs.
How does slice thickness in CT impact dose?
Thinner sections require a higher mAs to achieve the desired contrast to noise ratio (CNR)
high contrast regions, such as the pelvis and thorax, do not need high mAs settings. In particular, the same mAs is not required for a pelvis exam as an abdominal exam to achieve the required CNR.
In general, kV is standardised at about 120 kV. What will be the result of lower kV settings such as 80 kV or 100 kV?
A. Higher dose for same image noise
B. Lower contrast than at 120 kV
C. Should be considered for paediatric protocols
A. True. Because the photoelectric effect has a greater effect at lower kV, patient attenuation will be greater and the x-ray beam less penetrating. Therefore more photons will be required at the entrance to get the same number at the exit (Fig 1).
B. False. There will be a greater differential attenuation at lower kV and so image contrast will be greater. With iodine contrast medium, soft tissue contrast will be better with the use of lower kV and so this technique may be advantageous for some work involving angiography in the head.
C. True. Children are less attenuating than adults and so fewer entrance photons will be required in paediatric cases.
In general, kV is standardised at about 120 kV. What will be the result of a higher kV setting such as 140 kV?
A. A slightly lower dose for same noise
B. Poorer contrast
C. Better penetration
A. True. The x-ray beam will be slightly more penetrating and so fewer entrance photons will be required (Fig 1).
B. True. There will be less differential attenuation between tissues at higher kV.
C. True. Tissue attenuation coefficients decreases as mean photon energy increases.
What is the 10 day rule?
During the first 10 days of the menstrual cycle prior to ovulation, it is thought that there is no increase in risk for any foetal radiation effects since the patient is unlikely to be pregnant.
What can you remember of the effects of radiation in utero?
A. The risk of Down’s syndrome is increased by irradiation in the first trimester
B. Risks of mental retardation are greatest during the late stages of pregnancy
C. A foetal dose of 5 mGy will double the natural risk of cancer
D. There is a risk of mental retardation in a foetus if the mother is given an abdominal CT scan between 8 and 15 weeks after conception
E. Risks of induction of childhood cancer are likely to be lower during the earlier stages of pregnancy
A. False. The session in Module 8a Physics/Biological Effects of Radiation Exposure on the Embryo, Foetus and Infant (300-0678) has more detail.
B. False. They are greatest in the period 8th to the 15th week after conception.
C. False. It is considered that a dose of 20 to 30 mGy doubles the natural cancer risk..
D. False. The dose from a CT scan would be 10-20 mGy but the threshold for mental retardation is 200 mGy.
E. True. The session in Module 8a Physics/Biological Effects of Radiation Exposure on the Embryo, Foetus and Infant (300-0678) has more detail.
What is the cut off between low and high foetal dose?
10 mGy
Concerning dose limits:
A. The effective dose limit for employees is 20 mSv per year
B. An employee who is likely to receive more than 6 mSv effective dose per year is designated as a classified person
C. Dose limits for other persons are often referred to as members of the public dose limits
D. The effective dose limit for other persons is 1 mSv per year
E. The equivalent dose limit for hands is 500 mSv per year
All the statements are true.
For members of staff, the effective dose is often termed whole-body dose.
Equivalent dose to hands, forearms, feet and ankles are often termed together as skin dose or extremity dose.
Which of these nuclear medicine procedures could constitute an external radiation risk to staff?
A. Preparation of radiopharmaceuticals
B. Transport of patient radiopharmaceuticals
C. Administration of radiopharmaceutical
D. Acquiring an image
E. Taking a biological sample
F. Removal of radioactive waste
These all constitute an external radiation risk to staff.
Radiopharmaceuticals are continually emitting radiation, therefore external hazards exist from the moment radioactive material arrives on site to the moment it is removed, either as waste or by the patient leaving. The extent of the external hazard depends on a number of factors
How do each of these radionuclides decay?
Carbon-14
Fluorine-18
Technetium-99m
Iodine-131
Carbon-14: beta
Fluorine-18: positron (remember that positrons rapidly interact with electrons to produce 511 keV annihilation photons)
Technetium-99m: gamma
Iodine-131: both gamma and beta
How can external hazard be assesed by a radionuclide?
One way of assessing the external hazard from photon-emitting radionuclides is by using the air kerma rate (AKR) constant (measured in μGy per hour at 1 m from a source of activity 1 MBq).
The higher the AKR constant, the greater the external hazard.
Why do only some B- emitters pose an external hazard?
the maximum range in air increases with beta energy. Only high energy beta emitters (>0.4 MeV) present an external hazard at a distance greater than 1 metre.
Which of these nuclear medicine procedures could constitute an internal radiation risk to staff?
A. Preparation of radiopharmaceuticals
B. Transport of patient dose
C. Administration of radiopharmaceutical
D. Acquiring an image
E. Taking a biological sample
F. Disposal of radioactive waste
Answers A-F are all true.
They could all constitute an internal radiation risk, depending on the circumstances.
Radiopharmaceuticals are put inside patients for diagnostic or therapeutic purposes and are considered to be unsealed radioactive sources. There is therefore a risk that the unsealed source ends up inside staff instead.
The extent of the internal hazard depends on the physical form of the radiopharmaceutical, with aerosol particles or a gas (e.g.133Xe) for lung ventilation imaging posing a greater risk than solid material (e.g. 99mTc scrambled egg) for oesophageal transit time measurement.
Radioactive material has entered the body. To determine the risk, you need to establish…?
How does it enter? - Lungs, stomach or lower GI tract? Blood or lymphatic system?
Where does it go? - Does it circulate through the body? Does it accumulate in a specific organ or tissue?
When does it leave? - Is it excreted by the kidneys immediately? How long does it stay in an organ or tissue? Does it decay?
What is being irradiated? - A single organ or tissue? Adjacent organs or tissues? Number of organs or tissues?
What is the difference between biological half life and effective half life?
Biological half-life (T½ biological) describes the excretion rate of radiopharmaceuticals from tissues or organs inside the body. The biological half-life of a radiopharmaceutical depends on its physical and chemical form and in which tissues or organs it accumulates.
Effective half-life (T½ effective) accounts for both the physical (radioactive) and biological decay. The shorter the effective half-life, the lower is the dose.
What is committed effective dose?
The committed effective dose (E(50)) is the dose that will be delivered over 50 years following the intake of radioactive material. It depends on the activity, radiopharmaceutical and internal route. The latter includes mode of intake into the body and subsequent incorporation into and elimination from organs and tissues.
What is annual limit on intake?
The ALI is the activity that, if ingested or inhaled, would result in a committed effective dose of 20 mSv, which is the annual whole-body effective dose limit for a worker. The lower the ALI, the greater the internal hazard.
Should radioactive material get inside the body, wht can be done to reduce the committed effective dose?
Increase the intake of fluids to flush the kidneys
Administer stable pharmaceuticals to block uptake in specific tissue or organs. For example, stable iodine or potassium blocks radioiodine uptake in the thyroid
These actions are a last resort and all preventative measures should be employed to minimise intake.
What is contamination of radioactivity?
Contamination is an uncontrolled release of radioactive material in a place where it is not wanted.
What are electronic personal dosemeters useful for?
Electronic personal dosemeters (EPD) provide an instantaneous measurement and they are useful for:
Assessing new procedures
Real time dose assessment during incidents involving high activities
Monitoring pregnant staff
A radioactive source emits beta particles of energy 1.5 MeV and gamma photons of energy 150 keV. What is the best type of shield for this source?
A. Thin lead
B. Thin lead followed by thick Perspex®
C. Thick Perspex®
D. Thin Perspex®
E. Thick Perspex® followed by thin lead
The single best answer is E.
The source emits high energy beta particles and low energy gamma photons, so:
A thin Perspex® shield would be ineffective for both types of radiation
A thick Perspex® shield would be appropriate for the beta particles but not the gamma photons (because of the low density and low effective atomic number of Perspex®)
A thin lead shield would significantly attenuate both types of radiation but the beta particles would generate bremsstrahlung x-rays (because of the high atomic number of lead)
A thick Perspex® shield following a thin lead shield might not attenuate the bremsstrahlung radiation sufficiently (because of the low density and effective atomic number of Perspex®)
A thick Perspex® shield followed by a thin lead shield would be effective because the beta particles would be stopped by the Perspex®, and thus not reach the lead to generate bremsstrahlung, and the lead would attenuate any gamma radiation that penetrates through the Perspex®
A thick lead shield would also be effective, because bremsstrahlung x-rays would be generated on the source side and attenuated by the remainder of the lead, bearing in mind that the average beta energy is about 500 keV, and that little bremsstrahlung is produced at the full beta energy. However, the shield would be very heavy and might be difficult to handle
The exact thickness of materials would depend on source activity and the desired external dose rate
Which of the three principles of radiation protection apply to patients undergoing nuclear medicine procedures?
A. Justification
B. Optimisation
C. Dose limitation
A. Correct. No procedure involving ionising radiation should be undertaken unless the benefits outweigh the risks involved.
B. Correct. The radiation dose to the patient must be kept as low as reasonably practicable consistent with the intended purpose.
C. Incorrect. Dose limitation does not apply to medical exposures.
What sort of lisence do you need to administer nuclear substances?
ARSAC licences
What are the main factors that determine the dose received by nuclear medicine patients?
The most important factor in optimising patient dose is therefore activity. Decisions must be made regarding decreasing or increasing activity.
otherwise:
Procedure
Radiopharmaceutical (radionuclide and pharmaceutical)
Administration route
Patient (size, pathology + gender)
What measures can be taken to reduce patient dose from the same procedure and activity?
Avoid maladministration
Avoid misadministration
Encourage excretion
Block critical organs
Which piece of UK legislation requires the use of diagnostic reference levels in nuclear med?
IR(ME)R 2017 require an employer to have written procedures for the use and review of DRLs.
National DRLs for nuclear medicine procedures are taken from ARSAC Notes for Guidance and are expressed as administered activity in MBq. Employers set their own DRLs, which should be lower than or equal to the national DRL.
Administered activities above the DRL must be justified and reasons recorded for individual patients. Should an increase above DRL be deemed appropriate for all patients undergoing a particular exposure, an application to ARSAC must be made.
What conception advise is given to men regarding internal uptake?
No precautions for routine diagnostic examinations
4 month delay following iodine-131 (131I), phosphorus-32 and strontium-89 therapy as Genetic mutations in stem cells can lead to hereditary disease in offspring.
What conception advise is given to women regarding internal uptake?
No precautions for the majority of routine diagnostic examinations
Any administered activity of 131I greater than 30 MBq should be considered as a ‘therapy’ administration for radiation protection purposes – see below
Avoid pregnancy following therapies for period stated in ARSAC Notes for Guidance
What radionuclides are favoured in paeds?
Technetium-99m and iodine-123 are favoured in paediatric examinations due to the relatively small dose coefficients compared to other radionuclides.
What extra considerations are there with paediatric nuclear imaging?
Paediatric procedures
Minimising movement - Diagnostic imaging quality will be reduced if the patient is moving during their examination; this is more likely for paediatric patients than adults for the reasons stated.
Contamination hazards - For babies and infants, nappies will contain radioactive excreta that, for adults, would go to the drainage system. This represents an increased contamination hazard and precautions may be necessary depending on the nuclide and activity administered.
What are dose limits for contacts of the patient?
Carers and comforters1 5 mSv n/a
Other members of household 2 1 mSv Limit 5 mSv in 5 years
General members of the public 2 0.3 mSv Limit 5 mSv in 5 years
From which piece of legislation do the concepts of Controlled and Supervised Areas arise?
IRR 2017 - This means that each employer decides which areas are designated as controlled or supervised through prior radiation risk assessment.
WHat are the criteria for designated an area as controlled?
6 mSv effective dose or 3/10th of any equivalent dose limit for radiation workers
External dose rate exceeds 7.5 μSv/hour (hr)
Significant risk of contamination being spread
Area can be accessed by non-radiation workers
WHat are the criteria for designated an area as supervised?
1 mSv effective dose or 1/10th of any equivalent dose limit for radiation workers
What must local rules contain?
The content of Local Rules will vary between different employers but, in accordance with IRR 2017, must contain the following:
Dose investigation levels
Contingency arrangements for accidents
Details of the RPS(s)
Description of the physical area to which it relates
Summary of working instructions
Who is the RPS?
In general, RPS will be an employee working within the nuclear medicine department, usually a senior technologist or physicist. They should have practical knowledge of the work being undertaken and be suitably trained in radiation protection. A number of RPS may be appointed in large departments. Very often, the RPS will take on other radiation protection responsibilities, such as waste management and IRMER implementation.
Which detectors are specifically used for contamination monitoring?
Geiger-Muller tubes and scintillators
What are typical background readings for detectors?
typical background reading (1 cps) on a Geiger type contamination monitor
typical background reading of 10 cps on a scintillation-type contamination monitor
What may be included in contamination monitoring of personnel?
Hands: Should be monitored after handling radioactive materials and on leaving a radioactive area. Applies for both bare hands and gloves
Skin: (arms, face, legs), hair: these areas are not normally included in routine monitoring unless there is reason to suspect they have become contaminated. For example, if contamination was found on the hands, staff may then monitor their face and hair to ensure no contamination has been transferred inadvertently. If there were a high risk of skin or hair contamination through normal working practices, risk assessment would have identified the need for protective equipment to be worn
Clothing: (e.g. white coats, overalls, gowns, tunics, trousers, etc.): the need for routine monitoring of clothing is determined from the radiation risk assessment. Very often, procedures that carry a high risk of contamination will require the use of protective clothing and these items will be monitored and removed before leaving the designated area
Shoes: Same considerations as for clothing. Disposable overshoes, where required, will be monitored and removed before leaving the designated area
What are the general principles after a radiactive spill?
Priority is personnel: Whether it be an employee, patient or member of the public.
Restrict movement: Persons should move away from the contaminated area to reduce further contamination and minimise external exposure but not so far as to spread contamination
Decontaminate: Wash hands, remove clothing and shoes, wash other areas
Monitor: Check for residual contamination
Repeat: If contamination persists, repeat decontamination and re-monitor
What are the most significant radiation risks associated with IV injection?
Tissue extravasation: causing a high localised tissue dose to patient, the need for a second administration and a delay to the procedure
Contamination: of the patient, of personnel or of surfaces
Staff finger doses: handling the syringe and injecting the patient
Which piece of legislation controls the disposal of radioactive waste?
The environmental permits issued under EPR 2016 for each site state the permitted disposal routes for radioactive waste.
What are the two broad categories of detector?
Charge collection detectors - Electronic charge is released either directly as a result of radiation interactions, or indirectly from the detection of light emitted in the interactions (luminescence). The detector collects and measures this charge to give a real-time indication of radiation dose or dose-rate. Examples include: Gas-filled detectors, Scintillation detectors, Solid-state detectors
Other detectors- Some detectors are based on materials that undergo changes when exposed to radiation. These changes are measured at a later time to show the total radiation dose received. Examples include: Photographic film, Thermoluminescent detectors (TLDs), Optically stimulated luminescent detectors
How do charge collection detectors work?
In charge collection detectors, the electrons released by each interaction are collected and measured to give a real-time signal.
Charge collection detectors operate in one of two modes:
Current mode - the average rate of charge collection (electric current) is measured to indicate the radiation dose-rate.
Pulse mode - charge collected from each interaction is passed through a circuit to produce an individual voltage pulse. In pulse mode, the detector needs to be able to count each individual interaction. If the next interaction occurs before the detector is ready, then it is not counted. The time during which a subsequent interaction will be missed is called the ‘dead-time’.
How can CCDs in pulse mode tell what energy of radiation it detects?
In pulse mode, the amplitude of each voltage pulse is proportional to the charge collected and therefore to the energy deposited by a particle or photon of radiation. This means that charge collection detectors operating in pulse mode are capable of distinguishing between (resolving) radiations of different energies.
How do gas filled CCDs work?
Gas-filled detectors acquire their signal from the interaction of radiation with a gas. The essence of operation is that radiation ionises gas molecules to create ion pairs (electrons and positively charged ions). The electrons and ions flow under the force of an applied voltage to collecting electrodes This produces an electric current or pulse that can be measured.
What are the three types of gas filled ccd?
There are three main types of detectors: ionisation chambers, proportional detectors, and Geiger-Müller (GM) tubes. The main difference in construction between these three types is the charge collection voltage it leads to very different detector properties
What are the advantages and disadvantages of ionisation chambers?
Advantages
Ionisation chambers are flexible (they can do dose, dose-rate and have energy information).
Disadvantages
Ionisation chambers are not so good for low E radiation.
What are the advantages and disadvantages of GM tubes?
relatively low cost, simple construction and ease of use. It is particularly suitable for detecting radioactive contamination. Due to the signal collection method, it takes the GM tube some time to recover from each radiation event and so it cannot be used for accurate measurements of high count-rates.
What are the most common scintillator detectors?
caesium iodide doped with thallium (CsI(Tl)) and sodium iodide doped with thallium (NaI(Tl))
The amount of light emitted by the scintillator is proportional to the energy deposited by the incident radiation. Scintillation detectors are operated in pulse mode and are able to resolve radiations of different energies.
What are the pros and cons of film dosimeters?
Advantages
Film dosemeters:
Are inexpensive
Produce a permanent record of the exposure
Don’t weigh too much
Can measure high occupational doses (multiples of annual limit)
Can be used to measure patient dose
Disadvantages
Film dosemeters:
Are not very radiation sensitive (normal threshold about 0.1-0.2 mGy)
Are sensitive to heat and humidity
Are not tissue equivalent
Require manual intervention to develop
Do not provide an instantaneous readout
What is the most common TLD material?
lithium fluoride (LiF).
What do you think the advantages and disadvantages of TLDs are?
Advantages
Thermoluminescent detectors:
Can be manufactured in many physical forms, making it a very flexible dosemeter (for instance, it can be formed into a very small dosemeter)
Are slightly more sensitive than film
Can be re-used after an annealing cycle, post read-out
With multi-detector dosemeters, can ascertain radiation-type information regarding the exposure
Deal with a wide dose range with a linear response to dose
Disadvantages
Thermoluminescent detectors:
Produce no permanent record: once it is read out, the signal cannot be retrieved (in general)
Are more expensive than film
Are labour-intensive to process, if this is done manually
Do not provide an instantaneous readout
What can ionisation chambers be used in?
In-beam diagnostic x-ray measurements (quality control and dose-area product meter); automatic exposure controllers (to terminate x-ray beam when detector has received the desired amount of radiation); radionuclide calibrator
What can proportional counters be used in?
Contamination and environmental monitoring
What can GM tubes be used in?
Contamination and environmental monitoring
What can scintillators be used in?
Contamination and environmental monitoring
What can Solid-state semiconductor detectors be used in
Personal dosimetry; radiation spectroscopy
What can film dosimeters be used in?
Personal dosimetry; patient skin dose measurement (high dose applications)
What can Thermoluminescent dosemeters be used in?
Personal dosimetry (especially useful for extremity monitoring); patient skin dose measurement
What can Optically stimulated luminescent dosemeters be used in?
personal dosimetry
Concerning radiation detectors:
A. Film is a cheap dosemeter
B. The characteristics of a gas-filled detector are governed by its applied electric field
C. Gas-filled detectors are more efficient at stopping high-energy radiation than solid-state semiconductor detectors
D. Scintillators turn the radiation-induced excitation into a light signal that is captured by a PM tube
A. True. Due to its low cost, film was one of the most common forms of personal dosemeter, although reusable types such as TLDs and OSL dosemeters are increasingly popular.
B. True. Depending on the collecting voltage, gas-filled detectors can be used in ionisation chamber, proportional detector or GM tube modes.
C. False. Solid-state detectors capture a larger proportion of the high-energy radiation than gas-filled detectors due to their higher density.
D. True.
Which has the best energy resolution: SSD, scintillation counter, GM tube?
Solid-state detectors have very good energy resolution.
Scintillation detectors produce a signal that indicates the energy of the radiation, but with poorer resolution than solid-state detectors.
Geiger-Müller tubes are not able to resolve energies because all pulses are the same height.
What is absorbed dose to tissue definition?
Absorbed dose to tissue (DT in Gy) is the energy (in joules) imparted to an organ (T) divided by the mass of the organ (in kilograms).
What is equivalent dose definition?
Equivalent dose (H) (in Sv) is the ‘biologically’ important dose to an organ (T) that takes into account the weighting factors (wR) for each of the types of radiation (R) that the organ is exposed to
What is a tissue weighting factor?
The tissue weighting factor (wT) is a factor that links the probability of a stochastic radiation effect and equivalent dose for a specific organ. The factors vary by organ depending on their radiosensitivity.
What is effective dose?
The effective dose (E) (in Sv) is the sum of the weighted equivalent doses for all tissues/organs of the body
What is entrance surface dose?
Entrance surface dose (ESD) is the dose to a point on the patient’s skin where the central axis of the radiation field first strikes the patient. It includes an element due to backscatter from within the patient.
How can ESD be measured?
directly with a dosimeter (such as a thermoluminescent dosimeter (TLD)) placed appropriately on the patient - however, this approach is losing favour due to the time it takes to accomplish.
It can also be calculated from a knowledge of the exposure factors used, the distance from the x-ray tube focal spot to the patient’s skin (fsd) and the output at a reference point from the tube in question obtained during quality control (QC) testing.
What is DAP and how can it be measured?
Dose-area product (DAP) is a quantity that integrates the radiation beam area and intensity into a single metric. This is done using a DAP meter, which is a large-area transmission ionisation chamber
Where is the DAP monitor normally sited?
The DAP meter is situated in the tube collimator assembly
What are the advantages and disadvantages of DAP?
Advantage
Dose-area product is useful in radiography and fluoroscopy as the reading is instantaneous. It is especially useful in fluoroscopy where the beam area and beam intensity are changing as the radiologist changes the view. It correlates well with the total energy imparted to the patient.
Disadvantage
However, as it merges two types of information (beam area and intensity) it is not so useful for giving information on maximum skin dose, which is increasingly important in high-dose interventional procedures, especially in neurology and cardiology.
What is monte carlo modelling?
To estimate organ dose, and hence effective dose, a form of mathematical modelling is employed, called Monte Carlo modelling. It uses:
A mathematical ‘reference man’ phantom which has organs of known chemical composition in defined positions as shown in the image and
equations that predict radiation transport through matter
What 2 components affect dosimetry in nuclear?
The radionuclide involved will dictate the energy and type of radiation emitted from within the patient (and hence the pattern of energy deposition) and its half-life determines the physical irradiation time
The route of administration and biokinetics of the radiopharmaceutical will determine the distribution of the activity within the body and how the body’s systems excrete the radiopharmaceutical. The latter determines the biological half-life, which in combination with the physical half-life, determines the effective half-life of the radiopharmaceutical
What factors complicate dosimetry in CT?
multitude of irradiation geometries, as the x-ray tube rotates around the body. This has the effect of massively complicating the Monte Carlo modelling. Also, the irradiating x-ray beam is attenuated by a special CT bow-tie filter, creating a beam whose quality (hardness) changes across its fan shape. Again, this complicates the modelling process.
What does CTDI measure?
the amount of radiation being used within the scan plane
What is DLP?
DLP = CTDIvol × scan length (units of mGy.cm) related to the risk from the examination
What is dosimetry in mammography based on?
Dosimetry is based upon the measurement of ESD to a Perspex phantom. Factors derived from Monte Carlo modelling are used to estimate absorbed dose to the radiation-sensitive glandular breast tissue.
What is a diagnostic reference level?
Diagnostic reference levels are defined in IR(ME)R 2017 as:
Dose levels in medical radiodiagnostic or interventional radiology practices, or, in the case of radiopharmaceuticals, levels of activity, for typical examinations for groups of standard-sized individuals or standard phantoms for broadly defined types of equipment
What is a local DRL?
IR(ME)R 2017 require employers to establish DRLs, having regard to European levels (you will find European levels very similar to UK levels). Therefore, the notion of ‘local DRLs’ has emerged. DRLs should be representative of practice in your hospital; it is not necessary to have a DRL for every examination even if there is a national DRL for that examination. It is acceptable to use a national DRL as a local DRL if no local patient dose information is available. Again, a local DRL is seen as an aid to optimisation, by showing up poor practice when similar local practice is compared to it (the audit cycle).
What is ambient dose equivalent?
The ambient dose equivalent, H*(d), at a point in a radiation field, is the dose equivalent that would be produced in a tissue equivalent sphere at a depth, d.
For strongly penetrating radiations, a depth of 10 mm is recommended and denoted by
H*(10). For weakly penetrating radiation, a depth of 0.07 mm is used for the skin, and for the lens of the eye, a depth of 3 mm is used.
What is Directional dose equivalent?
Directional dose equivalent is analogous to ambient dose equivalent, but with a specified field direction.
What is personal dose equivalent?
The personal dose equivalent, Hp(d), is the dose equivalent in soft tissue, at an appropriate depth, d, below a specified point on the body.
What are the 2 main approaches to monitor internal contamination?
external dose-rate monitoring (assuming the radiation is penetrating enough to escape from the body), or monitoring of samples taken from the individual (blood or urine). This sample can then be analysed to assess the activity within the sample and hence an estimate of body burden can be made.
