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16 • Disorders of the Temporomandibular Joint
263
the posterior teeth on the affected side. A fractured condylar neck is often associated with premature contact
between the posterior teeth on the ipsilateral side and
a posterior open bite on the contralateral side. Take a
radiograph. A panoramic film is a valuable imaging
method for suspected mandibular fractures. This might
be supplemented by other views (e.g. a reverse Towne’s
view) if the panoramic suggests a fracture. An effusion
would probably not give any radiological signs on a panoramic view. If the diagnosis is joint effusion, then reassurance, resting the joint and use of anti-inflammatory
drugs on a short-term basis would probably suffice. If a
fracture is confirmed, then the patient should be referred
to a hospital oral and maxillofacial surgeon. If the occlusion is normal, then management would be conservative, that is, the same as for an effusion. If the occlusion
is abnormal, this would indicate a 2-week period of intermaxillary fixation (IMF) or, on occasion, open reduction
and fixation.
CASE HISTORY 4
1. The symptoms and signs are consistent with myofascial
pain.
2. Reassure the patient and explain that this is a common
condition. Point out that myofascial pain may spontaneously subside but may recur. Advise jaw rest, soft
diet, the use of anti-inflammatory drugs and construct
a soft bite guard. Consider referral for a course of physiotherapy. Review.
Viva Answers
1. ‘Clicking’ of the jaw is a result of an atypical disc position, usually an anterior displacement. Magnetic resonance imaging studies of normal individuals show that
a substantial proportion have an anterior disc position.
Therefore, the presence of a jaw click is not in itself abnormal and there should be no attempt to ‘treat’ it unless there are other symptoms or signs.
2. Bruxism is associated with the following:
n
tooth attrition with unusual wear facets.
n
tooth sensitivity, particularly of anterior teeth.
n
frequent fracture/replacement of restorations.
n
scalloping of the lateral border of the tongue.
n
ridging of the cheek mucosa along the occlusal plane.
3. No. It is, however, important to recognize the significance of social and psychological factors in TMD. Thirty
to forty percent of patients attending for a TMJ disorder
will also have issues relating to their mental health . The
most common disorder is a depressive illness. It is possible that when people become depressed they become
more aware of physical symptoms.
4. No. The majority of patients with TMJ disorders have
myofascial pain or an internal derangement. Neither
have radiological signs. A panoramic radiograph is often
taken as a “check” for other pathology. This is unsupportable as radiographic screening using panoramic
radiographs has no scientific basis. A radiograph (a panoramic may be the best choice) is indicated where the
suspected diagnosis is degenerative joint disease and
after trauma when a fracture is a possible diagnosis.
5. The main indication for arthrography would be an internal disc derangement that has not resolved following
conservative treatment and where either manipulation
under general anaesthesia or surgery is being contemplated. These days magnetic resonance imaging is preferred to arthrography but is occasionally contraindicated,
for example in patients with pacemakers. Contraindications to arthrography include local infection and iodine
allergy. Very anxious patients and needle phobics may not
be appropriate candidates for arthrography.
6. The lateral pterygoid muscle is not accessible for direct
examination. Some authorities suggest palpation behind the maxillary tuberosity as a method of direct examination, but it is of dubious value, particularly as
anyone would find palpation here uncomfortable. The
muscle is best assessed by measuring the response to
opening the jaw against the resistance of the operator’s
hand. Place your hand under the patient’s chin and ask
them to open against it. If there is muscle spasm, there
will be preauricular pain on attempted opening. This
procedure can be repeated with lateral movements.
7. Arthroscopy is a minimally invasive surgical technique in
which an instrument is passed into the upper joint space,
permitting direct imaging via a camera and surgical procedures to be performed. Lavage, biopsy and sectioning of
adhesions can be performed.
8. A soft bite guard is a useful first-line treatment for patients with myofascial pain. Its method of action is not
understood, but it may work as a habit breaker in parafunction or simply as an absorber of occlusal forces. It is
usually worn at night for 6 weeks.
17
Radiation Protection
CHAPTER OUTLINE
Overview‚ 264
17.1 Ionising Radiation and its
Effects‚ 264
Overview
Ionising radiation is used in medicine and dentistry to visualise dense internal structures. In dentistry, the potential
problems are stochastic tumour-inducing effects. This
chapter discusses the doses and risks in different types of
dental radiography and the indications for the use of different views. Methods to protect both the patient and the
dentist are discussed, together with the components of a
quality assurance programme.
17.1 Ionising Radiation and its Effects
LEARNING OBJECTIVES
You should be able to
• explain ionising radiation.
• understand how it interacts with matter.
• distinguish between somatic and genetic effects.
• inform on the doses and risks in dental radiography.
The use of ionising radiation in medicine and dentistry is
governed by the statutory requirements laid down in the
Ionising Radiations Regulations 2017 (IRR17, which deals
with protection of workers and the public) and the Ionising
Radiation Medical Exposure Regulations 2017 (IRMER17,
which specifically addresses patient protection). The second
edition of Guidance notes for Dental Practitioners on the Safe
use of X-ray Equipment, published by Public Health England
and the Faculty of General Dental Practice (UK), describes
the application of the legislation in general dental practice.
This now includes guidance on dental CBCT and handheld
dental equipment.
Ionising radiation includes x-rays, gamma rays and cosmic rays. These are all high-energy, short-wavelength highfrequency electromagnetic radiations. They behave as tiny
packets of energy called photons. While gamma rays are
used in hospital practice in nuclear medicine, x-rays are the
only usual concern of dentists.
X-rays are produced by an electrical process in an x-ray
tube (Fig. 17:1). Electrons, released from a heated tungsten
filament, are accelerated in a vacuum by the application of
17.2 Radiation Protection‚ 266
Self-Assessment: Questions‚ 269
Self-Assessment: Answers‚ 270
a high voltage (typically 50–75 kV [kilovolts]) and strike a
positively charged target. The sudden halt of the electrons
releases energy, mainly as heat but also as x-rays. The
x-rays are a mixture of photons of different energies, but
low-energy (dose-producing) photons predominate.
The x-rays are filtered, usually using aluminium, to
remove the low-energy photons (Fig. 17:2). The photons
are then shaped into an appropriately sized beam by collimation using steel diaphragms or cylinders (Fig. 17.3).
INTERACTION WITH MATTER
Three possible interactions can occur at the atomic level
when x-rays interact with matter:
n
Photoelectric interaction.
n
Compton interaction.
n
Coherent scatter.
The first two result in absorption of all or part of the
x-ray photon energy and ionisation of an atom.
In a living cell, ionisation can have damaging ef fects.
We are particularly concerned if the DNA of a cell is
damaged. This may occur by a direct interaction of x-ray
photons with the DNA or indirectly when a photon disrupts a water molecule into reactive radicals that go on
to damage DNA.
SOMATIC AND GENETIC EFFECTS OF X-RAYS
The irradiation of cells can result in somatic effects (i.e.,
those occurring in the irradiated somatic cells of an individual) or genetic effects (i.e., those occurring in the germ
cells and transmitted to the offspring of the irradiated individual) because of gonadal exposure. In properly conducted
dental radiography, genetic effects are not usually considered because the gonads should not be irradiated.
Somatic effects can be:
n
tissue effects (formerly known as “deterministic” effects;
e.g., cataract formation, loss of fertility, erythema of the
skin, “radiation sickness”)
n
stochastic ef fects: tumour induction (also occurs in
genetic ef fects).
Tissue effects occur above threshold values. These should
never be reached in dental radiography. The potential risk
in dental radiography is of stochastic effects.
264
A
–
+
targetfilament
Vacuum
Copper block
Tungsten Tungsten
17 • Radiation Protection
265
B
Fig. 17.1 An x-ray tube insert from a dental x-ray set (A) and a diagram (B) showing the structure and the direction of electron flow (arrow).
Unfiltered spectrum
Aft
Number of X-ray photons (intensity)
Photon energy (keV)
Fig. 17.2 The effect of filtration on the x-ray beam. Low-energy photons predominate in the unfiltered spectrum. Filtration removes proportionately
more of these “weak x-rays”, resulting in a filtered beam with a higher mean energy at the expense of a loss of intensity.
DOSES AND RISKS IN DENTAL RADIOGRAPHY
“Radiation dose” is a measure of the energy imparted by
x-ray exposure and its biological effect. At a simple level, we
can measure the energy imparted per unit mass (joules/kg)
but using established methods we can calculate “effective
dose”. This is a “whole body equivalent”’ value which can
be directly related to stochastic effects. Doses and risks vary
enormously according to the type of equipment used, and
the sensitivity of the tissue irradiated, so it is hard to give
firm figures, but recent estimations of risk (and radiation
dose) are given in Table 17.1.
266
These risks are calculated for a 30-year-old adult, using the nominal risk
Master Dentistry
Fig. 17.3 An x-ray set with rectangular collimation. This beam restriction method results in a dose reduction to patients of about 65%
compared with conventional 6-cm-diameter round beams.
Table 17.1 Estimates of Dose and Risk in Dental Radiography
Technique
Intra-oral (bitewing,
periapical)
Panoramic 3–24 0.1–1.3
Lateral cephalogram ,6 ,0.2
CBCT (‘dento-alveolar’) 11–674
CBCT (‘craniofacial’) 30–1073
Computed Tomography
(as used for dental
implant planning)
coefficient for cancer of 5.5 x 10
times greater, while for older patients risk falls until, at 80 years, they are
virtually negligible.
Effective Dose
(Microsieverts)
,2 ,0.1
(median 5 61)
(median 5 87)
280–1410 15–77
22
/Sv. Risks for children are two to three
Risk of Cancer
(per Million)
0.6–37
(median 5 3.4)
1.6–59
(median 5 4.7)
17.2 Radiation Protection
LEARNING OBJECTIVES
You should be able to:
• judge when imaging is justified and choose the best
techniques to use.
• explain how to limit dosage to patients and staff.
• optimize the use of equipment to ensure useable radio-
logical images are produced.
• understand the administrative requirements needed
from the dental team.
The aim of radiation protection is to ensure all exposures
are kept as low as reasonably practicable (ALARP principle).
PROTECTION OF PATIENTS
In dental radiography, protection of patients is achieved by
three main means:
n
Justification.
n
Dose limitation.
n
Quality assurance.
Justification
There are legal and ethical requirements that no radiological
examination should be used unless there is likely to be a benefit in terms of improved prognosis or management of the patient. This implies that no x-ray examination is ever ‘routine’
and that radiographic ‘screening’ is unacceptable. Instead,
radiographs should be prescribed according to the clinical
needs of the patient and following a clinical examination of
the patient. Justification for exposure of comforters and carers,
who may be exposed whilst assisting patients, is also now
required by law.
Selection of Bitewing Radiographs
The nearest we come to “routine” radiography in dentistry is
with the bitewing radiograph. For dentate patients who are
new to the practice (and partially dentate patients where
films can be supported in the mouth), most authorities agree
that a posterior bitewing examination is justified. Thereafter,
the intervals between bitewing examinations should be determined by assessment of caries risk. Current UK guidelines
are shown in Table 17.2, although other evidence-based
guidelines with slightly different intervals are also available.
The bitewing frequency for a patient (child or adult) may
change if the individual changes caries risk category.
If the dentist feels that a radiographic examination is of
help in assessment of bone loss in periodontal disease, then
bitewing radiographs will provide the necessary information
in the premolar and molar regions, providing geometrically
accurate images. Where periodontal probing depths exceed
5 mm, then vertical bitewing radiographs are appropriate.
Selection of Periapical Radiographs
Periapical radiographs are indicated in the following situations:
1. When dictated by localised symptoms/signs (pain, swell-
ing and tenderness of a tooth).
2. Prior to the extraction of erupted third molars, retained
roots, lone-standing upper molars or where there is reasonable clinical suspicion that problems may arise. The
fashion of routine pre-extraction radiographs has arisen
in the absence of any scientific evidence of benefit.
3. Prior to preparation of a tooth for a crown or bridge
retainer.
Table 17.2 Intervals Between Bitewing Examinations
Category
Interval (months) by Caries Risk Category
Low Moderate High
Child 12–18 12 6
Adult 24 or greater 12 6
17 • Radiation Protection
267
4. In endodontics, where basic guidelines suggest radiograph(s) at the following stages:
a. Preoperative
b. Working length estimation
c. Master cone position (pre-condensation)
d. post-condensation
e. At 1 year after treatment completion.
5. Dental trauma.
This list is not exhaustive. Where there is any localised
dental or alveolar problem, a periapical radiograph may be
appropriate.
Selection of Panoramic Radiographs
In terms of image quality, panoramic radiography is inferior to good intra-oral radiographs. Consequently, for most
dental diagnostic uses it is a “second best” imaging technique. Possible situations where it may be useful include:
n
where a bony lesion or unerupted tooth is of a size or
position that precludes its complete demonstration on
intra-oral radiographs
n
in orthodontic assessment when clinically indicated (no
“screening”)
n
preoperative assessment of third molars, unless other
adequate radiographs are available
n
when mandibular fracture is suspected
n
as part of implant dentistry planning, unless other adequate radiographs are available.
Routine “screening” of all new patients is never justifiable;
research has shown that the majority of patients who receive
a “screening” panoramic radiograph receive no diagnostic
benefit from the examination.
Selection of Cone Beam CT Examinations
Cone beam CT is a relatively new imaging technology, providing 3D imaging at a lower dose than medical CT. Its use is
being rapidly taken up by dental specialists, and evidencebased guidelines of its efficacy are still in development. Despite
this, one general principle can be stated: that CBCT should
only be used when the question for which imaging is required
cannot be answered adequately by lower dose conventional
(traditional) radiography.
Artefact, arising from most metals (usually dental restorations), in the scan degrade the image quality significantly
throughout the axial plane, producing radiating dark bands.
This is one reason why CBCT should not be used as a method
of caries detection, as the artefacts can mimic radiolucency.
Similarly, metal posts in roots may produce the same effect.
Detailed, evidence-based selection criteria for CBCT have been
developed recently for Europe (European Commission, 2012.
Radiation Protection 172. Evidence based guidelines on cone
beam CT for dental and maxillofacial radiology).
Dose Limitation
Patient doses in dental radiography can be minimised by
considering:
n
operating potential (kilovoltage): for intra-oral radiography a minimum of 50 kV is set and 65–70 kV is recommended. This is often fixed on dental intra-oral x-ray sets
but is usually used to control exposures on panoramic
x-ray equipment
n
tube current-exposure time product (mAs). The current
(mA) is often fixed on dental intra-oral x-ray sets, while
the exposure time (s) is often fixed for panoramic and
CBCT machines
n
AC/DC generation of x-rays: ‘DC’ (constant potential)
generators lead to fewer low-energy (dose-producing)
x-ray photons
n
filtration: aluminium filters absorb low-energy x-ray
photons
n
collimation: on intra-oral x-ray sets, the beam can be
restricted to a rectangle of 4 cm by 3 cm, leading to a
substantial dose reduction over the conventional 6-cmdiameter round beam; all new equipment should be
fitted with rectangular collimation and it should be
retro-fitted on older equipment; on panoramic machines,
selective field size collimation facilities may be available.
For CBCT equipment, there should be a choice of fields of
view, and examinations must use the smallest that is
compatible with the clinical situation if this provides less
radiation dose to the patient.
n
image receptor speed: for intraoral radiography, digital
systems may offer some reduction in mAs (and hence
dose) compared with film. Where film is used, E- or
F-speed films should be used. For digital panoramic equipment, this factor is out of the control of the operator, but
for film-based panoramic radiography, a rare-earth
screen/film combination should be used (a combination
of ISO speed 400 or better is used). For CBCT equipment,
the choice of image receptor is out of the control of the
operator, although this will influence radiation dose.
n
lead shielding of patients: the only requirement to use a
lead apron in dentistry is for comforters and carers in the
primary x-ray beam with a patient
n
Diagnostic Reference Levels (DRL): the entrance dose
generated by dental x-ray sets (including CBCT) is
measured and audited by a radiation protection advisor or medical physics expert and compared against
national or local levels (Table 17.3). Those operating
above need to investigate and instigate measures to
reduce doses to below local DRL. This is a legal requirement in the UK.
Quality Assurance
A poor-quality image means that the patient receives reduced, or no, benefit from the risk of the x-ray examination.
Even in the best hands, radiographs may be produced that
are “rejects”. A quality standard of no greater than 10% of
plain film radiographs (5% for digital images) being nondiagnostic has been set for general dental practice. Good
quality of radiographs can be addressed by attention to all
of the criteria listed in Table 17.3.
A quality assurance programme of regular checks,
cleaning and servicing should be established to maintain
high standards and to fulfil the legal requirement.
PROTECTION OF STAFF
While practitioners rightly consider the well-being of patients
first, the needs of dental staff and the public (who are not patients) should not be ignored. Dentists and ancillary staff may
be exposed many times each day to x-rays if staff protection is
not ensured. The following are the important considerations.
268
Master Dentistry
Table 17.3 Methods of Assuring Good-Quality Radiographs
Area Improving Methods
Radiographic technique Use of film-holding/beam-aiming devices for intra-oral radiography
X-ray set Regular maintenance and servicing, as recommended by the manufacturer
Film, cassettes and digital sensors Use film before expiry date; store in cool dry conditions, handle with care
Darkroom Must be light tight and have correct safelights
Manual processing Use a thermometer and timer and use time/temperature processing
Automatic processing Clean and service regularly
Viewing images Film: use an illuminated viewing box and keep the surface clean.
“After-care” Mount, name and date film radiographs
Careful positioning for panoramic radiography
Careful selection and instruction of patients
Triennial survey of radiation safety by appropriately trained person
Ensure cassettes are light tight and that intensifying screens are cleaned
Handle digital sensors with care.
Clean work surfaces
Fix and wash films adequately
Change chemicals as advised by manufacturer
Change chemicals as advised by manufacturer
Use a magnification aid for intra-oral radiographs.
Digital: keep monitor surface clean. Regular checks of monitor brightness, contrast and resolution using
test pattern (e.g., SMPTE)
Position
For intra-oral radiography, nobody except the patient (and
occasionally, justified comforter or carer wearing a lead
apron) should be within the controlled area (Fig. 17.4)
unless specific guidance has been received from a medical
physics expert/radiation protection adviser. This controlled area has a strict definition: not within an area of
2-m radius centred on the patient and, for intra-oral
radiography, never in line of the primary beam. Barriers
of suitable material may be used where distance is not
feasible as protection. Because of higher scatter doses
often seen with CBCT equipment, it is more likely that
barriers will be required.
Workload
While it is probably impossible for a member of dental staff
to receive a dose approaching the limits set by law for workers, radiation dose monitoring has been recommended for
anyone taking more than 100 intra-oral or 50 panoramic
radiographs per week.
X-ray set
Primary
beam
Wall
Fig. 17.4 The “controlled area” for an intra-oral x-ray set. The primary beam would be unacceptably intense for many metres, and we rely on walls to
attenuate the x-ray beam to an acceptable level. While this diagram is two-dimensional, remember that the controlled area extends above and below
the patient and x-ray set. Ceiling and floor materials may not provide an adequate barrier to limit the controlled area.
17 • Radiation Protection
269
Local Rules
Every dental practice must have a set of local rules for
radiation safety. By reading these and adhering to them,
radiation safety of staff should be assured.
‘Good Practice’ Guidelines
Guidelines on the safe use of radiation, including the aforementioned Guidance Notes for Dental Practitioners on the
Safe use of X-Ray Equipment, are produced by Public Health
England, but are also available from the European Commission and the International Atomic Energy Agency.
ADMINISTRATION OF RADIATION PROTECTION
The steps involved in an exposure of ionizing radiation
in medicine/dentistry are carried out by suitably trained
personnel allocated to specifically defined IRR and IRMER
“roles”.
Employer (Legal Person)
The employer (e.g., NHS Trust, Health Authority, principal
in general dental practice) has legal responsibility to ensure
that regulations are followed. The employer must ensure
that referrers have written guidance (referral/selection criteria) on referral of patients for x-ray examination.
Registration
The new regulations require that all employers register
their ‘work with a radiation generator’ to the appropriate
regulatory body. In the UK, these are the Health and Safety
Executive (HSE) and in Northern Ireland the Health and
Safety Executive of Northern Ireland (HSENI).
Referrer
The referrer is a registered medical/dental practitioner.
Other registered health care professionals, such as a hygienist or therapist, may also act as referrer if entitled by their
employer’s procedures. The duty of the referrer is to supply
adequate clinical information to allow the practitioner to
justify the examination. In general practice, the referrer is
the dentist, and appropriately trained dental therapists and
dental hygienists, working within their competence if
indemnified to do so.
Practitioner
The practitioner is an individual who is qualified to justify
radiological examinations. In hospitals, this is the radiologist or, depending on local arrangements, the radiographer.
In general dental practice, it is the dentist and appropriately
trained dental therapists and dental hygienists, working
within their competence and if indemnified to do so.
Operator
The operator is the person who carries out the radiological
examination. In hospitals this is the radiographer. In general dental practice, it is the dentist or a suitably qualified
therapist, hygienist, dental nurse and occasionally clinical
dental technicians.
NB: It should be noted that orthodontic therapists and dental
technicians cannot perform any of the above roles, nor do they
have the scope to undertake additional training to do so.
Radiation Protection Supervisor
The Radiation Protection Supervisor is an individual who
takes the role of checking that legal requirements and “good
practice” are being followed. In general dental practice, this
is usually a dentist.
Radiation Protection Adviser (RPA)
All facilities, including general dental practices, must appoint
an RPA. This is a medical physicist who provides expert support in ensuring that the Ionising Radiation Regulations are
followed and good practice is maintained.
Medical Physics Expert (MPE)
Under IRMER17, such an expert should be appointed
to give advice on matters relating to radiation physics
applied to exposure, for example, optimization of patient
dose, such as advice on dental x-ray equipment. Invariably
they may be the same individual or organization acting
as RPA.
Self-Assessment Questions
TRUE/ FALSE
1. The following are ionising radiation:
a. X-rays
b. Radiowaves
c. Microwaves
d. Gamma rays
e. Cosmic rays
2. The following are everyday risks to patients in dental
radiography:
a. Tissue (deterministic) effects
b. Somatic stochastic effects
c. Genetic stochastic effects
d. Salivary gland cancer
e. Cataract formation
3. The dose of radiation from a panoramic radiograph is:
a. About the same as 1 to 3 days of background radiation
b. Much less than the dose from a chest radiograph
c. Equivalent to that from a set of posterior bitewing
radiographs
d. Always 3–24 mSv (microsieverts)
e. Associated with a risk of cancer typically higher than
that from a lateral cephalogram
4. A lower radiation dose for a periapical radiograph can be
achieved by:
a. Using a 50 kV x-ray set rather than a 70 kV x-ray
set
b. Using a constant potential (DC) x-ray set rather than
a pulsating potential (AC) x-ray set
c. Using a lead apron
d. Using D-speed film
e. Using a digital radiography system
5. Essential records for each dental exposure include:
a. Name of the IRMER practitioner
b. Name of the referrer
c. Dose estimation
d. Exposure settings
e. Radiographic report
270
Master Dentistry
Single Best Questions
1. Technical faults in panoramic radiography are commonplace. You have to repeat an exposure because there
is low density and contrast throughout the image.
Which of the following faults do you think the most likely?
a. Patient movement
b. Excessive x-ray exposure
c. Insufficient x-ray exposure
d. Radiographic cassette leaks light
e. Old film stock
2. In another panoramic radiograph, the anterior teeth are
narrowed resembling matchsticks, and there is an exaggerated “smile” of the occlusal plane. Which is the most
likely positioning error?
a. Too far forward
b. “Chin up”
c. Too far forward and “chin down”
d. “Chin down”
e. Too far back and “chin up”
3. One of the simplest means of radiation protection of
patients is the use of referral (selection) criteria to select
the appropriate radiological investigation, thus minimising x-ray examinations that do not alter management of
the patient’s problem. Select your first choice of imaging
when a patient presents with an acute pericoronitis on
an erupting wisdom tooth. This is the first occasion it
has occurred.
a. No imaging
b. Horizontal bitewing
c. Panoramic radiograph
d. Periapical radiograph
e. Cone beam CT
4. A 12-year-old patient who has mild crowding, a Class I
malocclusion and a skeletal Class 1 pattern wants and
seems suitable for orthodontic treatment using a simple
upper removable appliance. What is your principal
choice of imaging?
a. No imaging
b. Lateral cephalogram
c. Cone beam CT
d. Panoramic radiograph
e. Bitewing radiographs
5. You perform a basic periodontal examination on a new
patient with gingivitis. Scoring code 3 is the highest associated with the lower left first molar. Which of the
following list of would you choose to further your treatment plan?
a. Panoramic radiograph
b. Horizontal bitewings
c. No imaging
d. Full mouth periapicals
e. Vertical bitewings
6. A patient presents with a large, painless hard bony
swelling in the lower first molar region. Which of the
following is your first choice of imaging?
a. Cone beam CT
b. Magnetic resonance imaging
c. Contrast enhanced CT
d. Conventional CT
e. Sectional panoramic radiograph of the left side
Essays
1. The risk to patients from dental radiography is so low as
to be negligible. Discuss.
2. How would you carry out radiographic quality assurance
in a dental practice still using film?
Viva Questions
1. When should you use a lead apron in dental radiography?
2. Where should the operator stand when exposing a
patient for an intra-oral radiograph?
3. How would you respond to a patient who expressed concern about the x-ray exposure from a dental radiograph?
4. How would you improve the risk/benefit when exposing
a patient for a panoramic radiograph?
5. Why are x-rays considered to be dangerous?
6. How do you assess when a patient should have a set of
bitewing radiographs?
Self-Assessment Answers
TRUE /FALSE
1. a. True. These are produced by bombarding a positively
charged target with electrons.
b. False. Low frequency and, therefore, have insufficient
energy to ionise atoms.
c. False. Low frequency and, therefore, have insufficient
energy to ionise atoms.
d. True. Naturally occurring radiation from radioactive
materials.
e. True. Cosmic rays come from outer space but con-
tribute a substantial part of our natural background
radiation. All of these radiations are electromagnetic
(EM) radiation. EM radiation behave as photons, tiny
packets of energy with a waveform (they have a frequency and a wavelength). The higher the frequency
is, the greater the energy in each photon. X-rays,
gamma rays and cosmic rays are all high-frequency
EM radiation and can ionise atoms.
2. a. False. These effects have threshold doses considerably
higher than that which might be received during
dental radiography. They may, however, occur during
radiotherapy.
b. True. These effects (tumour induction) have no
threshold dose. However, the risk is believed to be directly related to the dose. With low doses associated
with dental radiography, the risk is low.
c. False. It is generally accepted that gonadal doses in
dental radiography are so low as to be negligible. This
is particularly plausible when considering panoramic
radiography (the beam is highly collimated and is
angled slightly upwards) and intra-oral radiography
using film holders (paralleling techniques) and rectangular collimation.
d. True. There is published evidence of an association
between dental radiography and salivary gland (and
brain) tumours. However, this work refers back to a
17 • Radiation Protection
271
time of higher radiation doses and it must be remembered that the risks are small.
e. False. Cataract formation is a tissue (deterministic)
effect that should never occur as a consequence of
dental radiography.
3. a. True. The annual average dose to the UK citizen from
all forms of radiation is 2600 mSv (microsieverts).
The doses from dental radiography can be related to
this. Using the doses given in the text of this chapter,
a panoramic radiograph would be equivalent to approximately this number of days.
b. False. Doses from chest radiography vary but a typi-
cal range is 20–40 mSv, in the same general range as
panoramic radiographs.
c. True. Using the dose ranges given in this chapter,
two bitewing films might be around the same level
of dose as a panoramic radiograph. However, the
ranges involved mean that this may not always be
the case.
d. False. The dose range given assumes “good practice”
and up-to-date equipment. Many older machines
with higher doses are used in dental practices, and
the common practice of overexposing to compensate
for underdevelopment during processing means that
doses may be considerably higher.
e. True. Looking at the figures quoted in the text of this
chapter, you will see that the risk from a panoramic
radiograph is typically an order of magnitude greater
than that of a lateral cephalogram.
4. a. False. Lower voltages give a higher proportion of
weaker x-rays. Weak x-rays are more likely to undergo absorption (photoelectric interactions) in the
patient’s tissues.
b. True. A ‘DC’ x-ray set produces a smaller proportion
of weak x-rays.
c. False. Using standard “good practice” technique (par-
alleling technique and rectangular collimation)
none of the primary beam should be directed towards the trunk of the patient. Scattered radiation
is principally internal and would be unobstructed
by a lead apron.
d. False. This is the slower of the two intra-oral film
speeds usually available.
e. True. Both types of digital intra-oral system (CCD-
based and photostimulable phosphors) can produce
a periapical radiograph using a substantially lower
x-ray exposure.
5. a. True. This is a requirement of IRMER17.
b. False. This is only necessary if patients are referred
between practices for imaging.
c. False. This is only required if there has been a depar-
ture from a standard protocol, or a radiation accident
has occurred (e.g., a timer failure).
d. False. This is only required if a departure from stan-
dard protocols is applied.
e. True. Regulation 12 of IRMER17 requires the em-
ployer to ensure that a clinical evaluation is recorded
of each medical exposure. This needs to demonstrate
that the whole image has been evaluated, and pertinent findings noted.
Single Best Answers
1. c. Low density and contrast means either insufficient
exposure or underdevelopment during processing. If
film is still being used, old film stock will also demonstrate this feature. However, all exposures with this
batch of film would be affected. Old cassettes may get
damaged during years of use and start to leak light.
Light fogs the film, leading to irregular areas of black
at the edge of the radiograph in a position corresponding to the leak. Again, this fault will occur on
every radiograph.
2. c. Narrowing of the teeth occurs when the patient is
positioned too far forward in the panoramic machine
(or focal plane too far back). In this case the Frankfort
plane is also incorrect and the patient is chin down.
The use of bite blocks and positioning lights should
reduce the risk of these faults.
3. a. If this is the first occasion of pericoronitis, then, wisdom tooth removal will not be a consideration. As
such, radiographic examination is unlikely to alter
management.
4. d. The panoramic radiograph is ideal for viewing the
developing dentition and most orthodontists would
say they need one. Interestingly, research shows that
in simple cases, clinical examination supplemented
by study models, without radiography, is often sufficient for treatment planning. For a simple orthodontic treatment such as this, it is very unlikely that a
cephalogram could add anything useful to treatment
planning. You may need bitewing radiographs too—
for caries evaluation.
5. b. Horizontal bitewings showing the crestal bone levels
will suffice in this case and involve the fewest exposures and the least radiation. Vertical bitewings are
used for depths greater than 6 mm. Periapical radiographs are used if perio-endo lesions are suspected.
Percentage bone loss can only be established from
panoramic or periapical radiography. However, there
must be a clear indication that the benefit of grading
bone loss in this way will outweigh the radiation dose
risks.
6. e. From the list provided, this is the first choice of imaging. An additional occlusal view would be useful but
may not be possible in dental practice. If necessary,
one of the other modalities would usually follow.
ESSAY PLANS
1. The essay plan would cover:
Define the risk. Exposure to x-rays carries with it risks.
X-rays are ionising radiation that cause ionisation of
atoms by photoelectric and Compton interactions.
Ionisation can damage important molecules such as
DNA in cells, leading to cell death or mutations. With
dental radiography, the risk is of somatic stochastic
effects (tumour induction). The chance of these effects is directly related to dose; there is no threshold
dose below which they are sure not to occur.
272
Master Dentistry
Quantify the risk. Risks are related to doses. Doses in
dental radiography are variable, depending on many
factors, but typical ranges are ,2 mSv for an intraoral radiograph and 3–24 mSv for extra-oral. These
relate to cancer risks in a 30-year-old adult which
are typically less than 1 per million. Doses are a fraction of annual average exposures to the UK population and risks are less than those of dying from other
causes, such as accidents at work. Risks are higher
in children, who are, therefore, a group of greater
concern.
Is risk negligible? It is important to remember that radiog-
raphy is not a ‘normal’ part of someone’s life. It is an
additional risk. It is something carried out by clinicians ‘to’ a patient and a tangible benefit must be demonstrable. However low the risk, every effort must
be made to maximise the benefit and minimise the
risk through justification, dose limitation and quality
assurance.
2. The essay plan would cover:
Definition. Quality assurance (QA) can be defined as the
organised effort of staff to ensure the consistent production of high-quality radiographs at the lowest possible cost with minimum exposure of patients and
personnel to radiation. QA is an essential component
of radiation protection.
Identifying problems. Begin with a staff meeting to dis-
cuss issues related to radiography and try to instil an
appreciation of the importance of good quality in diagnosis and radiation protection. Try to identify any
existing, known, problems by discussion. Audit film
quality to see whether you reach the quality standard
of no more than 10% unsatisfactory radiographs (or
5% for CBCT). Carry out a ‘film reject analysis’ to
identify the principal problems. For example, if ‘pale’
low-contrast radiographs are a problem then this
could be caused by poor exposure selection, a faulty
x-ray machine or, where film is used, underdevelopment. If the main problem is ‘blurred’ panoramic radiographs, then examine the films to determine
whether this is caused by positioning faults, movement or poor intensifying screen/film contact.
Action. Address first the problems identified by film reject
analysis. For example, if a major problem was underdevelopment of film, make a fresh start with processing by cleaning the processing tanks and using fresh
solutions. Monitor processing times and check developer temperature.
QA programme. Establish a programme of regular checks.
In your essay answer give a possible programme. There
is no ‘correct’ make-up of a QA programme as this
would be tailored to the particular dental practice, but
demonstrate that you understand the principles:
Daily activities
n
Maintain a log of film quality
n
Check developer temperature and process test film
using test object
n
Clean x-ray viewer
n
Clean darkroom work surfaces.
Weekly activities
n
Check film stock
n
Clean intensifying screens.
Biweekly activities
n
Change developer and process reference film.
Monthly activities
n
Check darkroom light-tightness and safelights
(Coin test).
Annual activities
n
Have x-ray sets serviced.
Triannual activities
n
Survey x-ray sets for radiation safety.
Viva Answers
1. When a foetus is in the line of the primary (main) x-ray
beam.
2. Outside the controlled area. This area is defined as in the
line of the primary beam until it is attenuated by distance (well beyond the confines of any dental surgery)
and a space around the patient and x-ray set in all other
directions with a 2-m radius (see Fig. 17.4). Strictly, the
dimensions are set by the Radiation Protection Adviser.
3. First, explain that radiographs are only prescribed when
they are justified (when they will give a clinical benefit).
Second, explain that doses are kept “as low as reasonably achievable” by using well-maintained equipment
and the best materials (this should be the case!). It may
be worth discussing dose levels, in particular relating
the likely x-ray dose to the annual average radiation exposure to the UK population.
4. Reduce the risk by using up-to-date equipment, either a
digital system or an analogue system using a rare-earth
screen/film combination. Carefully select the exposure
and use accurately monitored processing of film. Maximise the benefit by only exposing the patient when it is
clinically justified and by systematically examining the
radiograph to identify all abnormalities of relevance to
treatment.
5. X-rays are high-energy radiation that cause ionisation
of atoms. Ionisation can disrupt important molecules in
the cells of living tissue, in particular DNA. This can lead
to cell death or mutations. Mutation may lead to tumour
formation.
6. No radiographic examination should be performed until
a full history and complete clinical examination have
been performed. Posterior bitewing examination should
be carried out for all new dentate/partially dentate
patients unless approximal surfaces can be directly visualised clinically. Frequency of subsequent bitewing examinations should be based upon caries risk status (see
Table 17.2). Caries risk should be reassessed at each
course of treatment so that (for example) an individual
is not ‘condemned’ to a permanent high-risk category.