APPENDIX 1 SUMMARY OF RECOMMENDATIONS
The core recommendations and statements in this
document are the “Basic Principles”, described in section 3.3. Below
are listed the other specific guidelines, taken from the relevant sections,
with their evidence grading:
Introduction and
Guideline development
1.1: These
Guidelines should be reviewed and renewed using an evidence-based methodology
after a period no greater than five years after publication.
GP
Justification and referral
criteria
4.1: All CBCT examinations must be justified on
an individual basis by demonstrating that the potential benefits to the
patients outweigh the potential risks. CBCT examinations should potentially add
new information to aid the patient‟s management. A record of the Justification
process must be maintained for each patient.
ED BP
4.2: CBCT should not be selected unless a
history and clinical examination have been performed. “Routine” or “screening”
imaging is unacceptable practice.
ED BP
4.3: When referring a patient for a CBCT
examination, the referring dentist must supply sufficient clinical information
(results of a history and examination) to allow the CBCT Practitioner to
perform the Justification process
ED BP
4.4: For the localised assessment of an
impacted tooth (including consideration of resorption of an adjacent tooth)
where the current imaging method of choice is MSCT, CBCT may be preferred
because of reduced radiation dose.
GP
4.5: CBCT may be indicated for the localised
assessment of an impacted tooth (including consideration of resorption of an
adjacent tooth) where the current imaging method of choice is conventional
dental radiography and when the information cannot be obtained adequately by
lower dose conventional (traditional) radiography.
C
4.6: For the localised assessment of an
impacted tooth (including consideration of resorption of an adjacent tooth),
the smallest volume size compatible with the situation should be selected
because of reduced radiation dose. The use of CBCT units offering only large
volumes (craniofacial CBCT) requires very careful justification and is
generally discouraged.
GP BP
4.7: Where the current imaging method of
choice for the assessment of cleft palate is MSCT, CBCT may be preferred if
radiation dose is lower. The smallest volume size compatible with the situation
should be selected because of reduced radiation dose.
GP
4.8: CBCT is not normally indicated for
planning the placement of temporary anchorage devices in orthodontics.
GP
4.9: Large volume CBCT should not be used
routinely for orthodontic diagnosis.
D
4.10: For complex cases of skeletal
abnormality, particularly those requiring combined orthodontic/surgical
management, large volume CBCT may be justified in planning the definitive
procedure, particularly where MSCT is the current imaging method of choice.
GP
4.11: Research is needed to define robust
guidance on clinical selection for large volume CBCT in orthodontics, based
upon quantification of benefit to patient outcome.
GP
4.12: CBCT is not indicated as a method of
caries detection and diagnosis.
B
4.13: CBCT is not indicated as a routine
method of imaging periodontal bone support.
C
4.14: Limited volume, high resolution CBCT
may be indicated in selected cases of infra-bony defects and furcation lesions,
where clinical and conventional radiographic examinations do not provide the
information needed for management.
C
4.15: Where CBCT images include the teeth,
care should be taken to check for periodontal bone levels when performing a
clinical evaluation (report).
GP
4.16: CBCT is not indicated as a standard
method for identification of periapical pathosis.
GP
4.17: Limited volume, high resolution CBCT
may be indicated for periapical assessment, in selected cases, when
conventional radiographs give a negative finding when there are contradictory
positive clinical signs and symptoms.
GP
4.18: Where CBCT images include the teeth,
care should be taken to check for periapical disease when performing a clinical
evaluation (report).
GP
4.19: CBCT is not indicated as a standard
method for demonstration of root canal anatomy.
GP
4.20: Limited volume, high resolution CBCT
may be indicated, for selected cases where conventional intraoral radiographs
provide information on root canal anatomy which is equivocal or inadequate for
planning treatment, most probably in multi-rooted teeth.
GP
4.21: Limited volume, high resolution CBCT
may be indicated for selected cases when planning surgical endodontic
procedures. The decision should be based upon potential complicating factors,
such as the proximity of important anatomical structures.
GP
4.22: Limited volume, high resolution CBCT
may be indicated in selected cases of suspected, or established, inflammatory
root resorption or internal resorption, where three-dimensional information is
likely to alter the management or prognosis of the tooth.
D
4.33: Limited volume, high resolution CBCT
may be justifiable for selected cases, where endodontic treatment is
complicated by concurrent factors, such as resorption lesions, combined periodontal/endodontic
lesions, perforations and atypical pulp anatomy.
C
4.34: Limited volume, high resolution CBCT is
indicated in the assessment of dental trauma (suspected root fracture) in
selected cases, where conventional intraoral radiographs provide inadequate
information for treatment planning.
B
4.35: Where conventional radiographs suggest
a direct inter-relationship between a mandibular third molar and the mandibular
canal, and when a decision to perform surgical removal has been made, CBCT may
be indicated.
C
4.36: CBCT may be indicated for pre-surgical
assessment of an unerupted tooth in selected cases where conventional
radiographs fail to provide the information required.
GP
4.37: CBCT is indicated for cross-sectional
imaging prior to implant placement as an alternative to existing
cross-sectional techniques where the radiation dose of CBCT is shown to be
lower.
D
4.38: For cross-sectional imaging prior to
implant placement, the advantage of CBCT with adjustable fields of view,
compared with MSCT, becomes greater where the region of interest is a localised
part of the jaws, as a similar sized field of view can be used.
GP
4.39: Where it is likely that evaluation of
soft tissues will be required as part of the patient‟s radiological assessment,
the appropriate initial imaging should be MSCT or MR, rather than CBCT.
BP
4.40: Limited volume, high resolution CBCT
may be indicated for evaluation of bony invasion of the jaws CBCT by oral
carcinoma when the initial imaging modality used for diagnosis and staging (MR
or MSCT) does not provide satisfactory information.
D
4.41: For maxillofacial fracture assessment,
where cross-sectional imaging is judged to be necessary, CBCT may be indicated
as an alternative imaging modality to MSCT where radiation dose is shown to be
lower and soft tissue detail is not required.
D
4.42: CBCT is indicated where bone
information is required, in orthognathic surgery planning, for obtaining
three-dimensional datasets of the craniofacial skeleton.
C
4.43: Where the existing imaging modality for
examination of the TMJ is MSCT, CBCT is indicated as an alternative where
radiation dose is shown to be lower.
B
CBCT equipment factors in the reduction of radiation risk
to patients.
5.1: Kilovoltage and mAs should be adjustable
on CBCT equipment and must be optimised during use according to the clinical
purpose of the examination, ideally by setting protocols with the input of a
medical physics expert.
B
5.2: Multipurpose dental CBCT equipment
should offer a choice of volume sizes and examinations must use the smallest
that is compatible with the clinical situation if this provides less radiation
dose to the patient.
B BP
5.3: Research studies on optimisation of
filtration for dental CBCT units should be performed.
GP
5.4: Dental CBCT units equipped with either
flat panel detectors or image intensifiers need to be optimised in terms of
dose reduction before use.
GP
5.5: Multipurpose dental CBCT equipment
should offer a choice of voxel sizes and examinations should use the largest
voxel size (lowest dose) consistent with acceptable diagnostic accuracy.
C
5.6: Research studies should be performed to
assess further the effect of the number of projections on image quality and
radiation dose.
GP
5.7: Shielding devices could be used to
reduce doses to the thyroid gland where it lies close to the primary beam. Care
is needed in positioning so that repeat exposure is not required. Further
research is needed on effectiveness of such devices in dose reduction.
GP
Quality standards and quality
assurance
6.1: Published equipment performance criteria
should be regularly reviewed and revised as greater experience is acquired in
testing dental CBCT units.
GP
6.2: Testing of dental CBCT should include a
critical examination and detailed acceptance and commissioning tests when
equipment is new and routine tests throughout the life of the equipment.
Testing should follow published recommendations and a medical physics expert
should be involved.
ED BP
6.3: Manufacturers of dental CBCT equipment
should provide a read-out of Dose-Area-Product (DAP) after each exposure.
D
6.4: Until further audit data is published,
the panel recommend the adoption of an achievable Dose Area Product of 250 mGy
cm2 for
CBCT imaging for the placement of an upper first molar implant in a standard
adult patient.
D
6.5: Assessment of the clinical quality of
images should be a part of a quality assurance programme for CBCT.
GP
6.6: Establishments carrying out CBCT
examinations should perform reject analysis, either prospectively or as part of
retrospective clinical audit, at intervals no greater than once every six
months.
GP
6.7: As a minimum target, no greater than 5%
of CBCT examinations should be classified as “unacceptable”. The aim should be
to reduce the proportion of unacceptable examinations by 50% in each successive
audit cycle.
GP
6.8: Image quality criteria should be
developed for dental CBCT, ideally at the European level.
GP
Staff protection
7.1: It is essential that a qualified expert
is consulted over the installation and use of CBCT to ensure that staff dose is
as low as reasonably achievable and that all relevant national requirements are
met.
ED D
7.2: CBCT equipment should be installed in a
protected enclosure and the whole of the enclosure designated a Controlled
Area.
D
7.3: Detailed
information on the dose due to scattered radiation should be obtained to inform
decisions about shielding requirements.
D
7.4: The provision of Personal Monitoring
should be considered.
D
Economic evaluation
8.1:
Economic evaluation of CBCT should be a part of assessment of its clinical
utility.
GP
Training
9.1: All those involved with CBCT must have
received adequate theoretical and practical training for the purpose of
radiological practices and relevant competence in radiation protection.
ED BP
9.2: Continuing education and training after
qualification are required, particularly when new CBCT equipment or facilities
are adopted.
BP
9.3: Dentists and dental specialists
responsible for CBCT facilities who have not previously received “adequate
theoretical and practical training” should undergo a period of additional
theoretical and practical training that has been validated by an academic
institution (University or equivalent). Where national specialist
qualifications in Dental and Maxillofacial Radiology exist, the design and
delivery of CBCT training programmes should involve a Dental and Maxillofacial
Radiologist.
BP
9.4: CBCT applications specialists and agents
of manufacturers and suppliers of CBCT equipment who provide information and
training to clinical staff should obtain relevant training in radiation
protection and optimization.
GP
APPENDIX 2 RECOMMENDATIONS FOR RESEARCH AND
DEVELOPMENT
An intention of the SEDENTEXCT project was
that these guidelines would be used to identify gaps in research. By doing
this, encouragement could be given to the development of subsequent research
projects which will be formative in the update of future evidence-based
guidelines for the use of dental CBCT.
A number of important gaps in the evidence
became evident to the Panel during the review process. These are reflected in
some guideline statements within the document (recommendations 4.11, 5.3, 5.6,
5.7 and 6.8 in Appendix 1). In addition, the review highlighted the need for
more dose audit data to enable the setting of suitable DRLs for CBCT
examinations.
The key priorities identified by the project
team include:
1. Randomised clinical trials of CBCT versus
conventional radiography, looking at the higher levels of diagnostic efficacy,
notably Outcome Efficacy, and incorporating economic evaluation. The highest
priority area is the use of large volume CBCT in orthodontics.
2. Research to relate image quality to
diagnostic tasks, leading to the development of objective and clinical image
quality criteria for dental CBCT examinations.
3.
Patient dose optimization studies, notably in filtration, exposure factor
reduction (mAs, kV and number of basis images) and the need for thyroid
shielding.
The SEDENTEXCT
Workshop 31st March 2011
Beyond this,
however, an intrinsic objective of the SEDENTEXCT project was to involve
stakeholders as much as possible in guideline setting and in making
recommendations. On March 31st 2011, a SEDENTEXCT Workshop on dental CBCT was held
in Leeds, UK, under the auspices of the British Society of Dental and Maxillofacial
Radiology. Over 100 participants were present from across Europe, including
dental radiologists, medical physicists, national regulatory or advisory bodies
and equipment manufacturers and representatives. As part of the programme, time
was set aside for a “break out” session with the participants divided into ten
working groups, followed by a plenary meeting. The groups were asked to elect a
spokesperson and each group included at least one SEDENTEXCT project scientist.
Each group was asked to consider one of two questions, to summarise their
recommendations and to bring them to the plenary meeting. Half of the working
groups addressed question 1 and half question 2:
1. “What do you think should be the priorities for
research in dental CBCT in the immediate future?”
2. “What developments in the design and
function of CBCT machines would be of most benefit in the next five years?”
Priorities for research in dental CBCT
The following constitute the recommendations for research
recorded at the Workshop:
·
Clinical
trials of patient clinical outcomes when using CBCT compared with conventional
x-rays
·
Clinical
trials of CBCT-based diagnosis/treatment planning versus conventional imaging
·
Research
on the need for CBCT prior to third molar extraction
· Research
on clinical pathways to identify situations where preliminary conventional
radiographs can be omitted
·
Research
on image quality requirements for different clinical applications
·
Research
on identifying minimum equipment performance standards
It is notable that this list accords well with the
research priorities identified by the SEDENTEXCT team.
Developments in CBCT equipment design and function
The following seven items were recorded with a high
frequency:
·
Metal
(dental restoration related) artefact reduction software/ algorithms need to be
developed
·
Need
for variable size of FOVs/ FOVs to fit with diagnostic tasks/ wider choice of
FOVs/ flexibility/ even smaller volume options
· Dose
indicator/ DAP readout on CBCT equipment should be available and standardised
across manufacturers
·
Automatic
exposure control
·
Optimisation/
further dose reduction strategies incorporated
·
Simple
imaging protocols for dentists/presets for specific clinical applications
An
increase resolution without an increase in dose
Other comments were received but with lower frequency,
including:
·
Improvement
in soft tissue contrast
·
Reconstruction
algorithms optimised to the clinical purpose of the examination
·
Flexibility
for different tasks
·
Easier
localisation of small FOV
·
Improved
patient positioning aids
· Better
head support to prevent movement and allow patients with positioning challenges
(e.g spinal deformity)
·
International
standard for design of CBCT equipment
·
User
access to exposure settings to permit optimisation
·
QA
software integrated into equipment
·
Better
training
·
DICOM
compatibility (allowing 3d model production)
·
More
intuitive software
·
Ordinary
dentist/ hospital practitioner systems need to be different
·
Less
variation in machines – too much choice now
Machines
with panoramic option should have field size limitation facilities
We hope that the
feedback from the Workshop on priorities for development in equipment design
and function will be of interest and value to manufacturers in the years ahead.
The SEDENTEXCT team are very grateful to the participants at the Workshop for
their contributions.
APPENDIX 3 GLOSSARY AND ABBREVIATIONS
A (evidence
grade)
|
At least one meta analysis, systematic
review, or RCT rated as 1++, and directly applicable to the target
population; or a systematic review of RCTs or a body of evidence consisting
principally of studies rated as 1+, directly applicable to the target
population, and demonstrating overall consistency of results
|
AMA
|
Active matrix array
|
B (evidence
grade)
|
A body of evidence including studies rated
as 2++, directly applicable to the target population, and demonstrating
overall consistency of results; or extrapolated evidence from studies rated
as 1++ or 1+
|
BP (evidence
grade)
|
Basic Principle. Consensus principle of the
European Academy of Dental and Maxillofacial Radiology (section 3).
|
GP (evidence
grade)
|
Good Practice (based on clinical expertise
of the guideline group and subsequent consensus of stakeholders)
|
C (evidence
grade)
|
A body of evidence including studies rated
as 2+, directly applicable to the target population and demonstrating overall
consistency of results; or extrapolated evidence from studies rated as 2++
|
CBCT
|
Cone Beam Computed Tomography
|
Controlled area
|
An area subject to special rules for the
purpose of protection against ionizing radiation and to which access is
controlled.
|
Craniofacial CBCT
|
Definition based on field of view size.
“Craniofacial” fields of view have a height which is greater than 10cm,
allowing maxillofacial imaging. This is synonymous with “Large volume CBCT” (vide
infra).
|
CTDI
|
Computed tomography dose index
|
D (evidence
grade)
|
Evidence level 3 or 4; or extrapolated
evidence from studies rated as 2+
|
DAP
|
Dose-Area-Product
|
Dento-alveolar CBCT
|
Definition based on field of view size.
“Dento-alveolar” fields of view have a height smaller than 10cm, suitable for
imaging the lower and upper jaws, but are often substantially smaller than
this.
|
DICOM
|
The Digital Imaging and Communications in
Medicine (DICOM) standard
|
DMFR
|
Dento Maxillo Facial Radiology
|
Dose constraint
|
A restriction on the prospective doses to
individuals which may result from a defined source, for use at the planning
stage in radiation protection whenever optimization is involved.
|
DRLs
|
Diagnostic Reference Levels: dose levels in
medical radiodiagnostic practices for typical examinations for groups of
standard-sized patients or standard phantoms for broadly defined types of
equipment. These levels are expected not to be exceeded for standard
procedures when good and normal practice regarding diagnostic and technical
performance is applied.
|
DVT
|
Digital Volumetric Tomography
|
EADMFR
|
European Academy of Dento Maxillo Facial
Radiology
|
EAO
|
European Association for Osseointegration
|
ED (evidence
grade)
|
Derived from the EC Council Directives
96/29/Euratom or 97/43/Euratom.
|
Effective dose
|
The sum of the weighted equivalent doses in
all the tissues and organs of the body specified by ICRP from internal and
external irradiation.
|
FOV
|
Field of view
|
FDI
|
Federation Dentaire Internationale
|
FPD
|
Flat panel detector
|
High resolution CBCT
|
In the context of the current document, the
use of voxel sizes of 0.2mm or smaller.
|
Holder
|
Any natural or legal person who has the
legal responsibility under national law for a given radiological
installation.
|
HU
|
Hounsfield Unit
|
ICRP
|
International Commission on Radiological
Protection
|
II
|
Image intensifier
|
kV
|
kiloVoltage
|
Large volume CBCT
|
CBCT in which the field of view is larger
than the jaws (mandible and maxilla). Typically this refers to fields of view
which encompass the facial bones and base or skull or larger. This is
synonymous with “craniofacial CBCT” (vide supra).
|
Limited volume CBCT
|
CBCT in which the field of view is limited
to a volume smaller than the jaws (mandible and maxilla). Typically this
refers to small fields of view suitable for imaging one, or a few, teeth.
|
Medical physics expert
|
An expert in radiation physics or radiation
technology applied to exposure whose training and competence to act is
recognized by the competent authorities; and who, as appropriate, acts or
gives advice on patient dosimetry, on the development and use of complex
techniques and equipment, on optimization, on quality assurance, including
quality control, and on other matters relating to radiation protection,
concerning exposure within the scope of Council Directive 97/43 Euratom of 30
June 1997.
|
MPE
|
Medical physics expert
|
MSCT
|
Multislice computed tomography. MSCT refers
to “conventional medical CT”
|
Pixel
|
Picture (two-dimensional) element
|
QA; Quality Assurance
|
All those planned and systematic actions
necessary to provide adequate confidence that a structure, system, component
or procedure will perform satisfactorily complying with agreed standards.
|
Quality Control
|
A part of quality assurance. The set of
operations (programming, coordinating, implementing) intended to maintain or
to improve quality. It covers monitoring, evaluation and maintenance at
required levels of all characteristics of performance of equipment that can
be defined, measured, and controlled.
|
QUADAS
|
A tool for the quality assessment of
studies of diagnostic accuracy included in systematic reviews.
|
Qualified expert
|
Person having the knowledge and training
needed to carry out physical, technical or radiochemical tests enabling doses
to be assessed, and to give advice in order to ensure effective protection of
individuals and the correct operation of protective equipment, whose capacity
to act as a qualified expert is recognized by the competent authorities. A
qualified expert may be assigned the technical responsibility for
the tasks of radiation protection of
workers and members of the public
|
SEDENTEXCT
|
Safety and Efficacy of a New and Emerging
Dental X-ray Modality. A project co-funded by the European Atomic Energy
Community‟s Seventh Framework Programme (Euratom FP7, 2007-11 under grant
agreement no. 212246 (SEDENTEXCT).
|
SIGN
|
Scottish Intercollegiate Guidelines Network
|
STARD
|
Standards for the Reporting of Diagnostic
Accuracy Studies
|
Sv
|
The special name of the unit of equivalent
or effective dose. One sievert is equivalent to one joule per kilogram:
1 Sv = 1 J kg–1.
|
TFT
|
Thin film transistor
|
TLD
|
Thermoluminescent dosemeter
|
TMJ
|
Temporomandibular joint
|
Voxel
|
Volume (three-dimensional) element
|
APPENDIX 4 QUALITY CONTROL MANUAL FOR DENTAL CBCT
SYSTEMS
1 Introduction
A Quality Control Programme lays out the
necessary testing to ensure that all parameters during the examination
procedure are in accordance with the standard operating protocol, thus
resulting in images with diagnostic value, without exposing the patient to
unnecessary risk.
A programme of equipment tests for dental
cone beam CT should consider the following aspects:
·
Performance
of the X-ray tube and generator
·
Patient
dose
·
Quantitative
assessment of image quality
·
Display
screen performance
Such a programme is a requirement of the
European Union Medical Exposures Directivei as part
of the optimisation process to ensure patient dose is as low as reasonably
practicable whilst achieving clinically adequate image quality. Any practice
undertaking medical exposure should have access to the advice of a medical
physics expert on such matters. The Medical Exposures Directive is currently
under revisionii and the
role of the medical physics expert is given higher prominence in the most
recent draft.
Testing and patient dose assessment is
carried out when the equipment is first installed as part of the commissioning
process and then throughout the life of the equipmentiii. This
protocol outlines those physical tests and measurements that are considered to
be part of a standard quality control programme for a dental CBCT unit. It does
not cover quality assurance of the clinical image.
A range of tests are appropriate for dental
CBCT looking at different aspects of the equipment and image display. National
guidance exists in some EU countries iv
and the SEDENTEXCT projectv has
developed phantoms to facilitate carrying out a wide range of measurements.
Some of the tests are straightforward and can be readily performed by the
clinical staff using the CBCT equipment. Other tests are more complex and the
input of a medical physicist is required.
Routine quality
control tests primarily involve comparison of results with those determined
during commissioning. Significant variation, as indicated by pre-determined
action levels, should be investigated, either with the help of a medical
physics expert or the equipment service engineer.
Not all possible methods of assessment are
considered essential. It is important to perform enough tests to confirm that
the equipment is operating as intended. More complex tests do add extra
information that is helpful in the optimisation process and they are detailed
here for completeness. However, whether the more detailed tests are undertaken
will depend on the availability of expert support and the necessary resources.
The tests are summarised in the table at the
end of the manual. The recommendations of priority, level of expertise,
frequency and action levels are based on published guidancev and the
experience of the SEDENTEXCT team in validating the use of the SEDENTEXCT QC
test phantom. This represents an initial assessment of what is sensible and
achievable but it must be borne in mind that, as experience of testing these
units is obtained over a period of years, these recommendations should be
critically reviewed as new evidence becomes available.
Some manufacturers of dental CBCT systems
provide a quality assurance phantom with their system, which should come with
recommendations on the tests that should be performed, the best way to perform
them, how often they should be performed and how the results should be
interpreted. Some of these quality assurance phantoms are also provided with
software that automatically performs analysis of the acquired image.
2 X-ray tube and
generator
The correct and
reliable performance of the X-ray tube and generator is crucial to the
production of consistent images. Both radiation output and tube kilovoltage
should be regularly monitored whilst tube filtration and leakage should be
performed as part of the equipment commissioning and should be repeated if
major repair work is carried out on the tube head.
2.1 Radiation output
This is assessed by measuring the absorbed
dose in air at a fixed point in the X-ray beam, e.g. by using a small thimble
ionisation chamber placed at the isocentre. It should be noted that the
ionisation chamber should have isotropic sensitivity.
2.1.1 Radiation Output Repeatability
This test monitors the consistency of the
radiation output for a series of radiation exposures using constant exposure
parameters.
2.1.2 Radiation Output Reproducibility
This test monitors the effect of the exposure parameters
(tube voltage and mAs) on the radiation output. Comparison should be made with
the baseline values established at commissioning.
Example: Measure at a range of tube voltages e.g. 70, 80,
90kVp at a
range of typical clinical mAs settings.
2.2 Tube potential
The voltage applied to the X-ray tube determines the
energy of the X-ray photons and is a major factor in determining the contrast
in the image.
Assessment of the tube potential ensures that the
delivered kVp is
close to that set on the unit by the operator. Poor agreement between the two
would affect clinical image quality, equipment radiation output and patient
dose.
2.2.1 kV accuracy
The kVp should be measured directly using a kV
divider device at intervals of 10kVp across the full range the unit is capable of
producing.
2.2.2 kV repeatability
The consistency of the tube potential should be monitored
by repeating five measurements at at least two clinically relevant kVp values,
where possible.
2.2.3 kV reproducibility
The reproducibility of the tube potential
over time should be monitored by comparing the measured results for kVps at
intervals of 10kV across the full range the unit can produce with those
established as baseline values at commissioning.
2.3 Filtration
The filtration of an X-ray tube absorbs the low energy
photons that do not contribute to the image formation but do contribute to
patient skin dose. Having adequate filtration is essential to ensure that
patient dose is controlled. The total filtration should be marked on the X-ray
tube housing.
Total filtration can be estimated by
measuring the Half-Value Layer (HVL). The HVL is the thickness of the absorber
required to reduce the intensity of the incident X-ray beam by half. The HVL is
an estimate of the penetrating power of the X-ray beam which means that the
higher the HVL the more penetrating the X-ray beam is.
2.3.1 How to measure HVL
A dosimeter such as a thimble ionisation
chamber should be positioned at the isocentre of the X-ray beam or at the
surface of the detector. If possible, the scanner should be set to operate in
„service mode‟ so that the X-ray tube is stationary. If this is not possible,
then alternatives should be considered, such as the possible use of the „scout‟
mode. Alternatively the scanner can be operated under normal conditions with
care taken in setting up the dosemeter and the filters. A typical protocol for
measuring HVL should be followedvi, in which the transmission through known
thicknesses of high purity aluminium is assessed. Using this HVL measurement
and knowledge of the X-ray tube design, the total filtration can be estimated
from look-up tablesvii. HVL is measured directly on several modern
dose/kV meters as an alternative to this method.
2.4 Radiation Field of View
The field of view (FOV) of a dental CBCT
scanner is usually defined at the isocentre. The scanner should be set to
operate in „service mode‟ and a film or a CR cassette can be placed at the
isocentre and exposed to different field sizes. The size of the film or the CR
cassette should be chosen so as to extend over the nominal dimensions of the
FOV. The dimensions of the imaged field can be measured and compared to the
nominal FOV, as quoted by the manufacturers, and the dimensions of the FOV
measured at baseline. If the manufacturers state that it is necessary to
irradiate beyond the nominal FOV for the purposes of image reconstruction this
should be taken into account.
If the scanner
cannot be operated at the „service mode‟, then the film or the CR cassette
could be placed on the detector and exposed to the maximum and different FOVs.
If the distance of the focal spot to the detector is known, then the dimensions
of the nominal FOV on the detector can be calculated and compared to the imaged
FOV. Alternatively, two sets of thermoluminescent dosimeters (TLDs) could be
placed using holders at the isocentre with the first set placed vertical and
the second set placed parallel to the z-axis and exposed to one FOV at a time.
The number of TLDs should be chosen so as to extend over the nominal dimensions
of the FOV. The TLDs are read out and the dimensions of the irradiated FOV are
compared with the dimensions of the nominal FOV.
In addition, it
should be confirmed that the X-ray beam is contained within the detector. A
film or a CR cassette should be placed on the surface of the detector and the
edges of the active area of the detector should be marked on the film or CR
cassette and then exposed to radiation. The radiation field should not extend
beyond the marked edges on the film or the CR cassette.
2.5 X-ray beam alignment
This test is to assess the coincidence of the
centre of the radiation and imaged FOV with the isocentre as defined by the
alignment lasers or the scout view.
Any radiopaque
object positioned at the isocentre allows for a measurement of the distance
between the imaged object and the centre of the imaged FOV using the measuring
tool of the scanner‟s software. Note that the accuracy of this measurement is
reliant on the correct calibration of the measurement software (see section
4.6) and the voxel size of the reconstructed image.
2.6 Leakage
Radiation is emitted from all directions from
the focal spot, not just in the direction of the primary X-ray beam. The tube
housing is designed to attenuate the radiation outside the main beam so that
patient and staff are not significantly exposed. This source of secondary
radiation is known as leakage.
On standard X-ray equipment, leakage is
measured during commissioning, usually by a medical physics expert, to confirm
that the tube head design and construction is adequate. It should also be
measured if physical damage to the tube head has occurred or the tube head has
been dismantled during repair.
The measurement of leakage on a dental CBCT
is problematic and can only reliably be achieved if the movement of the tube
head can be stopped (likely to be available in „service mode‟ only) and the
primary beam can be blocked either by the use of collimators or a lead block at
least 1mm thick placed as close to the tube window as possible. If this can be
achieved, standard methods for leakage measurement can be appliedviii, involving the identification of areas of leakage and the measurement of
dose rate at these areas. When interpreting the results, due regard should be
made to the effectiveness of the attenuation applied at the tube window.
If the movement
of the tube head cannot be stopped, securely fixing a lead block as close to
the tube window as possible should still allow meaningful measurements of
secondary radiation to be made at accessible points adjacent to the unit. These
results will give an indication of whether the leakage from part of the tube
housing is higher than expected. The use of film or computed radiography plates
around the tube housing can also be useful in detecting small areas in which
there is less shielding, or where the shielding is absent altogether. If
detected, measurements of secondary radiation can be focussed in these areas.
3 Patient dose
Knowledge of
patient dose is essential for clinicians who are making the decision regarding
the justification of the exposure. It is also important to ensure that doses
are optimised and in line with any national and international guidelines. The
dose quantity „effective dose‟ gives an indication of radiation risk and can be
compared to doses from other radiation sources. However, effective dose cannot
readily be measured and must be inferred from more easily measurable dose
quantities.
3.1 Dose measurement
A variety of dose indices are used to
characterise patient dose.
3.1.1 CTDI
For CT
scanners the CT dose index (CTDI) is usually used. This is a measurement of the
dose integrated across the dose profile along the patient‟s length. It is
measured using a pencil detector either in air or in a perspex phantomix. Such
a dose index has drawbacks for use in dental CBCT units due to the greater beam
size and asymmetry of the dose distribution. However, if a CTDI is quoted by
the manufacturers, it is suggested that this be measured by the medical physics
expert at commissioning for comparison with the specification.
3.1.2 CBCT dose index
The SEDENTEXCT project has investigated the use of a dose
index obtained from measurements using a small volume dosemeter in a Perspex
phantom. This is measured at points across the X-Y plane in the centre of the Z
axis.
Such indices can be used to monitor the
reproducibility of the dose distribution over time, to relate to manufacturer‟s
specification and national or international diagnostic reference levels if set
using a dose index.
3.1.3 Dose area product (DAP)
The product of the dose in the beam multiplied by the
area of the beam at that point is known as the dose area product (DAP) and is a
dose index routinely used in panoramic and cephalometric radiography, as well
as in general radiography and fluoroscopy.
DAP can readily be measured by the medical
physics expert using either a calibrated ionisation chamber that integrates the
dose across the primary beam (DAP meter) or by measuring dose and beam size at
a fixed point. Care should be taken on units where the beam size changes during
the scan and a suitable DAP meter must be used for these units.
If a DAP reading is provided on the equipment
readout, the medical physics expert should confirm the accuracy of such a
readout. The readout may then be used by the dentist to audit and monitor dose
and compare to any national or international audit levels (see diagnostic
reference levels).
3.2 Diagnostic reference levels
The European
Medical Exposures Directive requires that diagnostic reference levels are set
and used as part of the optimisation process. Exactly how this requirement is
applied varies from country to country depending on how it has been implemented
into national legislation. However, the overall aim is that patient dose is
audited and the dose for a typical patient is compared to past levels and any
national and international levels. This will give the dentist confidence that
doses in their practice are not unnecessarily drifting upwards and that they
are in line with accepted levels.
Diagnostic reference levels may be set using
a variety of dose indices. The UK Health Protection Agency has recommended the
use of dose area product (DAP) and has proposed setting reference levels for
the UK for both adult and child procedures. The adult level is for the clinical
protocol for the placement of an upper first molar implant in a standard male
patient and the child level is for the clinical protocol used to image a single
impacted maxillary canine of a 12 year old male. Based on current national
audit data an initial achievable level of 250 mGy cm2 is
proposed and further data is requested so that national reference levels for
both adult and child can be set.
4 Quantitative
image quality performance
A range of image quality indicators can be
measured using phantoms designed for such measurements. A variety of different
phantoms are available.
Phantoms, such as the Catphan, designed for
use on CT scanners can be used for dental CBCT units but are difficult to
position and tend to use soft tissue-equivalent materials for the more accurate
evaluation of grey scale accuracy.
Dental imaging has a few specific
requirements (e.g. hard tissue visualisation and sub-millimetre spatial
resolution) which are not assessed by phantoms not specifically designed for
the purpose. Some manufacturers provide phantoms with their scanners and the
SEDENTEXCT project has designed a phantom specifically with dental CBCT units
in mind.
In addition, software tools are required to
analyse the images of the phantom. These may be available as part of the image
viewing software or may be separately provided with the phantom. The SEDENTEXCT
phantom is provided with standard software for image analysis.
Acquisition of such a phantom and software
tools is essential if the image quality measurements are to be performed. MPEs
should normally have access to such phantoms and software and will be able to
carry out these measurements.
4.1 Image density values
A clinically useful image relies on the
system‟s ability to distinguish between and clearly display the different
materials in an image. The accuracy with which a system can continue to do this
over time can be determined quantitatively.
4.1.1 Setting a baseline
·
Acquire
an image of the image density value section of the phantom. This should be an
area in which there are many different materials clearly distinguished from one
another
·
Draw a
region of interest in each of the different materials and record the mean pixel
value and standard deviation in each
4.1.2 Routine measurements
·
In
future visits, expose the same test object using the same protocol, draw a
region of interest in each of the different materials and record the mean pixel
value and standard deviation in each
·
Compare
the mean pixel value for each material with that measured on the first visit
4.2 Contrast detail assessment
Assessing a system‟s ability to display
details of known varying contrast can give important information as to the deterioration
of image quality over time. A phantom containing objects with a range of
different sizes and/or contrasts is required.
4.2.1 Setting a baseline
·
Acquire
an image of the contrast detail section of the phantom. This should be an area
in which there are various details of the same material that vary in diameter
and depth, or various details of different materials
·
The
simplest check of contrast detail is counting the number of details that can be
clearly resolved on a reporting monitor
o It may be useful to derive a single value
for contrast detail assessment, for example the threshold detection index, the
image quality factor or the contrast to noise ratioxii.
Action levels will depend on the test object and scoring methodology used
o Some phantoms may provide software that
calculates contrast detail values after analysing images. In these cases,
follow the instructions that come with the phantom
4.2.2 Routine measurements
·
Acquire
an image of the contrast detail section of the same phantom using the same
exposure protocol as at baseline
·
Count
the number of details on the image using the same monitor as at baseline where
possible
o If a threshold detection index, image
quality factor or contrast to noise ratio is being used, compare with the
baseline results
o If
automated scoring with phantom software is being used, results should be
compared with baselines
Scoring test objects by eye is very subjective.
It should be ensured that where there are different personnel scoring the
details, they use a similar methodology.
4.3 Uniformity and artefacts
It is important that the entire detector is
capable of producing a useful image, and so it must be ensured that there are
no significant areas of damage or problems with detector calibration that could
lead to artefacts in acquired images. Similarly it must be confirmed that
damaged or dead pixels are appropriately corrected for in the final image.
4.3.1 Where a QC phantom is available:
·
Acquire
an image of the uniformity section of the phantom. This should be a large
homogeneous area so that it can be assured that any deviations on the image are
the result of the imaging system and not the phantom itself
·
A
visual check of the uniformity of the image will reveal any significant
uniformity problems
·
Where
quantitative tools are available, draw a region of interest in the centre of
the test object and then four evenly spaced regions around the periphery and
measure the mean pixel value in each. Assess the image uniformity using the
results
4.3.2 Where no QC phantom is available:
·
Acquire
an image with nothing in the beam. Be aware that this could give odd images on
some scanners if the reconstruction relies on a head or equivalent phantom
being present. In these cases consider the use of a scout view
·
A
visual check of the uniformity of the image will reveal any significant
uniformity problems. In this case, some windowing of the image may be necessary
to better assess uniformity
·
Where
quantitative tools are available, draw a region of interest in the centre of
the test object and then four evenly spaced regions around the periphery and
measure the mean pixel value in each. Assess the image uniformity using the
results
4.4 Noise
There are many processes that could affect
the quality of a clinical image, including tube output, detector efficiency and
image processing. A quantitative assessment of the noise in an image can
identify any deterioration in image quality with time and help determine the
cause of the deterioration.
4.4.1 Setting a baseline
·
Acquire
an image of the uniformity section of the phantom. This should be a large
homogeneous area so that it can be assured that any deviations on the image are
the result of the imaging system and not the phantom itself
·
Draw a
region of interest in the centre of the test object, with diameter no greater
than one fifth the diameter of the test object. Record the standard deviation
·
Repeat
for five consecutive axial slices and calculate the average standard deviation.
4.4.2 Routine measurements
·
Acquire
an image of the uniformity section of the same phantom using the same protocol
as at baseline
·
Draw a
region of interest in the centre of the test object, as close in size and
position to that at baseline as possible, and record the average standard
deviation across five consecutive axial slices
Further analysis:
Consideration should be given to the
calculation of a signal to noise ratio in addition to the noise measurements
described above. The information provided by signal to noise ratios can be
useful in investigating potential problems with the system where they are
suggested by noise measurements alone.
4.5 Spatial Resolution
Spatial resolution is a measure of the
ability of the system to detect small high contrast detail.
4.5.1 Limiting resolution
This test measures the smallest high contrast
detail that can be detected, usually by using a phantom in which small lines
get closer and closer together.
Method
Place a suitable object made of a high
contrast material on the detector and expose at clinically relevant exposure
factors. Magnify the reconstructed image of the test object and optimise the
window level. Score the number of resolvable groups of lines and convert to the
corresponding resolution. Be sure to use the same exposure factors as at
baseline year on year.
4.5.2: Modulation Transfer Function (MTF)
Measurement of the limiting resolution will
assess the system‟s ability to transfer the high frequencies (finest details)
but it does not provide any indication on how other frequencies are
transferred. This can be assessed by measuring the modulation transfer function
(MTF) of the system. The MTF can be calculated by measuring the Point Spread
Function (PSF) or the Edge Spread Function (ESF).
The PSF can be
measured directly by imaging a high contrast wire. The wire is embedded in a
suitable medium and placed perpendicular to the scan plane. The PSF is obtained
by plotting the pixel values across the image cross-section of the image of the
wire. Resolution can be measured directly from the PSF by measuring the full
width at half maximum (FWHM).
The ESF is
measured by imaging an edge of a block of material embedded in a suitable
material with the face of the block perpendicular to the scanned plane. The ESF
is obtained by plotting the pixel values across the image.
Differentiating the ESF will give the Line Spread
Function (LSF). The LSF can be used to asses the spatial resolution of the
system similar to the PSF.
The MTF can be calculated as the modulus of the Fourier
transform of the PSF or the LSF. The values quoted are the frequencies at which
the modulation falls to 50% or 10% of its initial value.
A more detailed description of the MTF method
is given in the IPEM Report 32, Part VII.x
4.6 Geometric Accuracy
Where it may be clinically useful to perform
measurements of distance or angle on an image, it must be ensured that
measurements made on a system accurately reflect true distances and angles. A
phantom is required that contains an area with objects at known distances and
angles from one another.
·
Acquire
an image of the geometric accuracy section of the phantom.
· Where
quantitative test tools are available, measure distances and angles across a
variety of the objects within the phantom
· Compare
the measured values with known distances and angles. A more detailed analysis
can be performed by calculating the aspect ratio and pixel pitch if required.
·
Ensure
the aspect ratio is correct by calculating measured x / measured y for
distances of the same intended length. The ratio should be 1±0.04
Ensure
the pixel pitch is as stated by the manufacturer by calculating measured
distance (mm) / number of pixels covering the measured distance. Measure the
pixel pitch for various distances in the x and y axes
5 Display equipment
Regardless of the quality of the x-ray
equipment with which an image is acquired, a clinical image can only be
digitally displayed as well as the monitor on which it is viewed is capable of.
It is essential therefore to ensure that any monitor that is used to report on
clinical images is well set up and subject to regular QC.
The QC programme outlined in the report of
the AAPM task group 18xi, or equivalent, is an appropriate
methodology for MPE tests. Regular in-house checking of the display monitors
should also be performed, as follows:
5.1 General condition
·
A
suitable test pattern, such as an AAPM TG18 or SMPTE image, should be installed
on the computer and viewed on the monitor, which should be clean
·
It
should be ensured that all distinct greyscale levels on the test pattern can be
individually resolved. The small black and white squares within the larger
black and white squares should also be clearly resolved
·
Where two
monitors are used for reporting, it should be ensured that the perceived
contrast of each of the distinct greyscale levels is consistent between the two
5.2 Monitor resolution
It
should be ensured that all of the bars on each of the resolution patterns on
the AAPM TG18 or SMPTE test image can be clearly resolved
Summary
Notes
* Level of expertise: MPE indicates that this test would normally
require the input of a medical physics expert with sophisticated test equipment
whereas in house indicates that the tests can normally be performed by clinic
staff using standard phantoms
** Action level: Results outside these levels should be investigated and
action taken. The advice of a medical physics expert or service engineer may be
required
N.B. This table represents initial guidance
based on current experience of dental CBCT units. It should be kept under
critical review as experience is gained in testing such units.
References
i Council Directive 97/43/Euratom of 30 June 1997 on health protection of
individuals against the dangers of ionizing radiation in relation to medical
exposure Official Journal of the European Communities No L 180/11 1997
ii Draft Euratom Basic Safety Standards Directive – Version
24 February 2010 http://ec.europa.eu/energy/nuclear/radiation_protection/doc/art31/2010_02_24_draft_euratom_basic_safety_standards_directive.pdf
iii IPEM report 91 Recommended standards for the routine
performance testing of diagnostic X-ray imaging systems IPEM 2005
iv HPA-RPD-065 Recommendations for the design of X-ray
facilities and quality assurance of dental Cone Beam CT (Computed tomography)
systems JR Holroyd and A Walker Health Protection Agency 2010
v SEDENTEXCT http://www.sedentexct.eu/
vi IPEM Quality assurance in dental Radiology Report No
671995
vii Cranley K. & Fogarty G.W.A. 1988 The measurement of
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viii IEC (International Electrotechnical Commission) 2008
Medical electrical equipment - Part 1-3: General requirements for basic safety
and essential performance - Collateral Standard: Radiation protection in
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Tomography X-ray scanners 2nd Ed IPEM 2003
x IPEM Report 32 Measurement of the Performance characteristics of
diagnostic X-ray systems used in medicine Part VII Digital imaging system IPEM
2010
xi Samei E, Badano A, Chakraborty D, Compton K, Cornelius
C, Corrigan K, Flynn MJ, Hemminger B, Hangiandreou N, Johnson J, Moxley M,
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