5: CBCT EQUIPMENT FACTORS IN THE REDUCTION OF RADIATION RISK
TO PATIENTS
The
literature review in section 2.5 showed that the effective dose may vary
significantly between different CBCT equipment. In this section, the
significance of selection of appropriate exposure settings in limiting doses
while maintaining the image quality at acceptable clinical levels
(optimisation) is reviewed. Due account was given to any available national
recommendations on CBCT optimisation (Haute Autorité de Santé, 2009; Health
Protection Agency, 2010; Statens strålevern, 2010).
5.1: X-ray tube voltage and mAs
The
kilovoltage (kVp) of an X-ray tube is the potential
difference between anode and cathode during operation. The tube voltage
determines the energy of the X-rays. Lower tube voltages give lower energy
X-rays and thus increase the dose to the skin of the patient (Horner 1994).
Increasing the kVp may result in a
decrease in skin and effective dose (Geijer et al 2009) but an increase in
scatter. Higher kVp, however, reduces the
beam hardening effect (Ludlow 2011). More research is needed to explore the
optimisation of kVp in CBCT. The product of
the tube current measured in milliamperes (mA) and the exposure time measured
in seconds (s) only affects the number of photons emitted by the X-ray tube and
not their energy. Increased mAs increases dose, but the beam penetration and
image contrast remain the same. The kVp and
mA in dental CBCT equipment is either fixed or can be varied depending on the
CBCT unit (Ludlow et al 2006; Lofthag-Hansen et al 2008; Silva et al 2008;
Okano et al 2009; Roberts et al 2009). Fixed kVp and
mA preclude optimisation.
The optimisation of
radiation dose can be achieved by balancing exposure with image quality needs.
Some diagnostic tasks necessitate higher levels of detail to others. The
possibility of using “low dose” MSCT for certain tasks in head and neck
radiology is established (Loubele et al 2005; Loftag-Hansen 2010). There is a
lack of studies that attempt to optimise these two exposure factors for
different CBCT units and clinical protocols. Where “low dose” options are
available through reduction in mAs, large reductions in effective dose have
been reported (Pauwels et al in press); although this study did not
assess image quality, it used manufacturers‟ low dose protocols. Kwong et al
(2008) found that mA and kVp could be reduced for the equipment studied
without a significant loss of image quality. Sur et al 2010 investigated the
effect of tube current reduction on image quality for presurgical implant
planning in CBCT. They reported that substantial reductions in mA could be made
without clinically significant loss of image quality. The work of Lofthag-Hansen
et al (2010) also provides ample evidence for the scope for reductions in mAs
with acceptable image quality, although they emphasised that exposure
parameters should be adjusted to the diagnostic task in question. While these
studies must be viewed as specific to the CBCT equipment in question, the
weight of evidence in X-ray-based imaging in medicine, along with available
national guidelines on optimisation (Haute Autorité de Santé, 2009; Health
Protection Agency, 2010; Statens strålevern, 2010), is sufficiently strong to
be able to make a recommendation:
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
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CBCT
units can be characterised by their Field of View (FOV). The FOV is a
cylindrical or spherical volume and determines the shape and size of the
reconstructed image. FOVs may vary from a few centimetres in height and
diameter to a full head reconstruction. Several CBCT units offer a range of
FOV, whilst a fixed FOV is provided by other units. Some CBCT machines offer
the option to collimate the beam to the minimum size needed to image the area
of interest. The size of the FOV is associated with radiation dose to the
patient and staff (Hirsch et al 2008; Okano et al 2009; Roberts et al 2009;
Lofthag-Hansen et al 2010; Pauwels et al in press).
The study by Pauwels et al (in press), conducted as part of the SEDENTEXCT project, demonstrated well the influence of FOV upon effective dose. As can be seen, while each class of FOV shows a wide range of effective dose, there is a clear trend for smaller FOVs to offer lower doses.
Reducing the size of
the X-ray beam to the minimum size needed to image the object of interest is,
therefore, an obvious means of limiting dose to patients, as well as improving
image quality by scatter reduction. Based on the dosimetry evidence and the
range of potential clinical applications of CBCT, the Panel judged that
equipment with large, fixed FOV was inappropriate for general dental use, where
diagnostic tasks are often localised to one, or a few teeth.
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
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This recommendation applies to “multipurpose”
dental CBCT equipment, used for a variety of clinical applications. In certain
situations (e.g. specialised endodontic practice) it is likely that only small
volume examinations would be required, and a single [small] field of view
option would be appropriate.
5.3:
Filtration
Aluminium filtration is
an established component of medical X-ray equipment. Some dental CBCT units are
equipped with copper filtration. Filtration removes lower energy X-ray photons
which results in skin dose reduction but also results in contrast loss (Ludlow
et al 2006; Loftag-Hansen et al 2008; Silva et al 2008; Okano et al, 2009;
Roberts et al 2009). Kwong et al (2008) found that addition of a copper filter
did not affect overall image quality on the CBCT equipment studied. Ludlow
(2011) demonstrated that increased copper filtration (in conjunction with a kV
change) resulted in a substantial effective dose reduction. Qu et al (2010)
cite another manufacturer who has chosen to add copper filtration in a move to
optimise dose. Clearly these publications are specific to the equipment
studied, and further work on optimising filtration in terms of material and
thickness should be performed before a general recommendation can be made.
Research studies on optimisation of filtration
for dental CBCT units should be performed
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5.4:
Digital detector
Dental CBCT units are
equipped with digital receptors where the image is captured and formed. Spatial
and contrast resolution are important aspects of CBCT detectors which influence
image quality.
Two types of digital
detectors have been used for dental CBCT units (Hashimoto et al 2003; Ludlow et
al 2003; Araki et al 2004; Pasini et al 2007; Loubele et al 2008; Ludlow &
Ivanovic 2008; Roberts et al 2009). The first type involves conventional image
intensifiers (II). They consist of an input window, input phosphor,
photocathode, vacuum and electron optics, output phosphor and output window.
The input phosphor converts the X-rays to optical photons which then are
converted to electrons within the photocathode. The electrons are accelerated
and focused by a series of electrodes and then strike the output phosphor which
converts the electrons to light photons which are then captured by various
imaging devices. Most modern image intensifiers have cesium iodide for the
input phosphor because it is a very efficient material in absorbing X-rays.
The second type, flat
panel detectors (FPDs), are composed of an X-ray detection layer and an active matrix
array (AMA) of thin film transistors (TFT). The X-ray detector consists of a
phosphor layer such as cesium iodide which converts the X-ray photons to light
photons. The intensity of the light emitted by the phosphor is a measure of the
intensity of the incident X-ray beam. The AMA has a photosensitive element
which produces electrons proportional to the intensity of the incident photons.
This electrical charge is stored in the matrix until it is read out and it is
converted into digital data sent to the image processor. FPDs have greater
sensitivity to X-rays than IIs and therefore have the potential to reduce
patient dose (Kalender & Kyriakou 2007). They have higher spatial and
contrast resolution and fewer artefacts than IIs but, in general, IIs are
cheaper than FPDs.
The detector is an
important element of the imaging chain and optimisation contributes to dose
limitation. The detector is an important element of the imaging chain and
optimisation contributes to dose limitation. Balancing the image quality and
dose involves the assessment and optimisation of the detector‟s parameters with
relation to dose in the context of image quality and would best be performed in
conjunction with a medical physics expert as part of acceptance and
commissioning testing (see Section 6.2.2) and in subsequent routine tests.
Dental CBCT units equipped with either flat panel
detectors or image intensifiers need to be optimised in terms of dose
reduction before use
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5.5:
Voxel size
The volume element
(voxel) represents a three-dimensional (3D) quantity of data and it can be
pictured as a 3D pixel. The reconstructed image area or FOV consists of a
number of voxels which are isotropic. The voxel size in CBCT systems may vary
from less than 0.1 mm to over 0.4 mm (Hashimoto et al 2003; Loubele et al 2008;
Liedke et al 2009). Scanning protocols with smaller voxel size are associated
with better spatial resolution but with a higher radiation dose to the patient.
Voxel size can influence diagnostic performance, with some tasks which require
a high level of detail (see Section 4.3) having been shown to require smaller
voxels to optimise diagnostic accuracy (Liedke et al 2009; Kamboroğlu &
Kursun 2010; Wenzel et al 2009; Hassan et al 2010; Kamboroglu et al 2010; Melo
et al 2010). Qu et al (2010) showed that the “low resolution” option on one
CBCT machine substantially reduced patient dose. Clearly, when possible, a low
resolution option should be preferred where the nature of the diagnostic task
permits. An important consideration in clinical use is that, due to the long
scanning times, it is likely that nominal spatial resolutions may not be
achieved due to the high probability of motion during the scan.
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
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As with section 5.2
(above), this recommendation applies to “multipurpose” dental CBCT equipment.
In certain situations (e.g. specialised endodontic practice) it is likely that
only high resolution, small volume, examinations would be required.
5.6:
Number of projections
The rotation of the
X-ray tube and the detector around the patient‟s head produces multiple
projection images. The total number of acquired projections depends on the
rotation time, frame rate (number of projections acquired per second) and on
the completeness of the trajectory arc. A high number of projections is
associated with increased radiation dose to the patient, higher spatial
resolution and greater contrast resolution. Brown et al (2009) have shown that
increasing the number of projections does not influence the linear accuracy of
CBCT. Reducing the number of projections, while maintaining a clinically
acceptable image quality, results in patient dose reduction through a reduction
in exposure (mAs).
Some models of CBCT
equipment offer the opportunity to perform partial rotations (e.g. 180º instead
of the standard 360º), resulting in approximately 50% dose reductions to the
patient. Some studies suggest that, for certain clinical applications on
specific CBCT equipment, partial rotations can be used while maintaining
acceptable diagnostic accuracy and image quality (Lofthag-Hansen et al 2010;
Durack et al. 2011). Further research studies should look into the effect of
the number of acquired images on the relationship between radiation dose and
image quality.
Research studies should be performed to assess
further the effect of the number of projections on image quality and radiation
dose
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5.7:
Shielding devices
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
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5.8:
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Kamburoğlu K, Murat S,
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Kwong JC, Palomo JM, Landers
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Okano T. Effects of tube current on cone-beam computerized tomography image
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6: QUALITY STANDARDS
AND
QUALITY ASSUTANCE
QUALITY ASSUTANCE
6.1 Quality assurance programme
The purpose of Quality
Assurance (QA) in dental radiology is to ensure consistently adequate
diagnostic information, while radiation doses are controlled to be as low as
reasonably achievable.
A well-designed QA
programme should be comprehensive but inexpensive to operate and maintain for
the dentist and staff. It should cover all aspects of the imaging process,
including objective measures of the imaging equipment performance, patient dose
audit and an assessment of clinical image quality. Such a programme will
include the following:
• Performance of the X-ray tube and
generator
• Quantitative assessment of image
quality
• Display screen performance
• Patient dose assessment
• Clinical image quality assessment
•
Clinical audit
Those aspects of the
programme that deal primarily with equipment performance and patient dose are
commonly referred to as quality control (QC). A QC programme will include
surveys and checks that are performed according to a regular timetable. A
written record of this programme should be maintained by staff to ensure
adherence to the programme and to raise its importance among staff. A specific
person should be named as leader for the QC programme.
In addition, assessment
of the clinical images and other clinical audit should be undertaken on a
regular basis to confirm that the equipment is being used correctly to produce
clinically useful images.
In preparing this
Section, due account was given to relevant sections of published national
guidelines on dental CBCT in Belgium (Advies van de Hoge Gezondheidsraad,
2011), Denmark (Sundhedsstyrelsen, 2009), France (Haute Autorité de Santé,
2009), Germany (Leitlinie der DGZMK, 2009), Norway (Statens strålevern, 2010)
and the United Kingdom (Health Protection Agency, 2010a and 201b).
6.2:
Equipment performance
A programme of testing
X-ray equipment performance is a requirement of the European Union Medical
Exposures Directive (Council Directive 97/43/Euratom, 1997) as part of the
optimisation process to ensure patient dose is as low as reasonably achievable
whilst achieving clinically adequate image quality. The rationale for
maintenance and testing of a dental CBCT system is similar to that of other
dental systems (European Commission 2004) and for X-ray equipment in general
(IPEM91 2005) and will consist of a critical examination and acceptance and
commissioning testing when first installed, followed by routine testing
throughout the life of the equipment. As both patient and operator dose are
potentially higher than for traditional dental X-ray equipment, greater care is
required for dental CBCT in all aspects of an equipment QC programme.
Preliminary guidance on
testing dental CBCT is now available, both within these Guidelines (Appendix 4)
and from the UK (HPA 2010a) outlining the basic tests to be undertaken, both
when the equipment is first installed and then on a regular basis throughout
the life of the equipment. The QC protocol developed by the SEDENTEXCT project
is given in Appendix 4 to these guidelines.
Suggested performance
guidelines are also provided so that users can assess whether their unit is
operating consistently and in line with expectation for these types of units.
However, it should be remembered that this technology is still relatively new
and is developing rapidly. The tests and performance guidelines should be kept
under critical review and may well be subject to change as experience is gained
in testing such units.
Some of the tests
require specially devised phantoms. Such phantoms are commercially available,
including that developed during the SEDENTEXCT project (Leeds Test Objects
Ltd., Boroughbridge, UK). Some manufacturers of dental CBCT systems also
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, including the SEDENTEXCT
phantom, are also provided with software which automatically performs analysis
of the acquired image.
Published equipment performance criteria should
be regularly reviewed and revised as greater experience is acquired in
testing dental CBCT units
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6.2.1 Critical
examination
6.2.2 Acceptance and
commissioning tests
The
essential content of these tests includes:
·
testing
of equipment performance parameters
·
acquiring
base line values for future routine tests
·
verification
of how the systems are pre-programmed for use in practice
All acceptance and
commissioning testing protocols include tests of the X-ray tube output, voltage
consistency and accuracy, filtration, exposure time and radiation field. These
can be tested in the same way as for other modalities, like general radiology
digital detector systems or MSCT scanners. A dosimeter with wave form display
may be helpful to confirm correct operation of the X-ray tube. Testing of the
correct operation of any automatic exposure control device, if fitted, is also
essential.
Classical tests of
digital detectors (linearity, homogeneity, spatial resolution, low contrast
resolution, (dark) noise, etc.) can be run if unprocessed raw data of the
projective images are available. Reconstruction software can be tested
indirectly via an assessment of image quality, using test objects with specific
inserts. At present, there are no standardized reconstruction software tools
available that would allow comparative studies among modalities. With ever more
sophisticated acquisition schemes (like variable angles, off-axis radiation,
tube output modulation, different FOVs, etc.) it is very unlikely that the
reconstruction software will be standardized in the future.
6.2.3
Routine tests
Both medical physics
experts and local personnel have a role in routine tests. A typical frequency
for medical physics tests is annually (Health Protection Agency 2010a, 2010b;
Statens strålevern, 2010). Local personnel should run a series of routine
consistency tests more frequently in line with current national guidance,
usually monthly (Qualitätssicherungs-Richtlinie, 2004; Health Protection Agency
2010a, 2010b; Statens strålevern, 2010). When introducing a new modality, its
operation should be monitored more frequently, until the system is working
reliably at its optimal point in terms of dose and image quality. Optimisation
studies may be advisable at this stage.
Routine testing may be
helped with automatic procedures built into the system. These can include the
evaluation of test objects against performance levels set by the company or by
national or international protocols, the review of retakes (automatically
stored into the system) and system self checks. Full documentation should be
provided by the installers on these (automated) procedures. Exportable reports
are preferable.
A simple but very
sensitive test for constancy checks in digital imaging is a regular acquisition
of a homogeneous block of material. Local artefacts in the digital detector
induce (usually circular) artefacts in the reconstructed slices. Tube- or
detector-related instabilities would produce variations in signal intensities.
It is important that
the performance of the display equipment and environment is also monitored, as
well as the X-ray equipment and detectors, as these can lead to significant
degradation of the image being used by the clinician.
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
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6.3 Patient dose
An objective of the QA
programme is to ensure doses are kept as low as reasonably achievable. It is,
therefore, necessary to ensure that patient doses are monitored on a regular
basis and compared to agreed standards. Standard dose levels are normally
referred to as Diagnostic Reference Levels (DRLs) as described in the European
Guidelines No 136 (European Commission 2004).
6.3.1
Dose quantities
Dose quantities that
are to be used for the regular assessment of patient dose must be relatively
easy to measure in a clinical situation. Although effective dose is usually
considered to be the best overall descriptor of patient dose, it can not be
readily measured and a simpler quantity is required for routine dose audit.
Entrance surface dose (ESD) and dose area product (DAP) are quantities that are
routinely used in conventional radiology (European Commission 1999). In the
field of CT, the computed tomography dose index (CTDI), and dose length
product, DLP, are routinely used. Ideally, the dose quantity used should give a
good correlation to the effective dose and hence overall patient risk.
In the UK, the Health
Protection Agency has proposed the use of DAP (HPA 2010). This is promising as
it provides one reading per exposure that gives an indication of both the dose
level in the beam and the area irradiated. Some CBCT units already provide this
information after each exposure. If this became universal, as CT scanners now
all provide an indication of DLP, it would greatly facilitate patient dose
audit. The accuracy of such readouts should be checked by the medical physics
expert during routine testing.
Alternative proposals
have been explored by the SEDENTEXCT team and dose indices based on point
measurement within PMMA phantoms have been proposed (see patient dose section
in Appendix 4). Further work is, however, required to establish whether such
indices are appropriate for the setting of DRLs.
Manufacturers of dental CBCT equipment should
provide a read-out of Dose-Area-Product (DAP) after each exposure
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6.3.2
Establishing Diagnostic Reference Levels
The UK‟s Health
Protection Agency have carried out a preliminary audit of DAP across 41 dental
CBCT units and have proposed an achievable dose of 250 mGy cm2
for
CBCT imaging appropriate for the placement of an upper first molar implant in a
standard adult patient. It should be noted that large FOV units in the sample
exceed this and the dose audit data had been normalised to an area
corresponding to a 4cm x 4cm field of view at the isocentre of the equipment.
It is for this reason that they have referred to this dose level as an
“achievable dose” rather than a DRL. It is indicative of the dose that should
be achieved if using a CBCT unit suitable for this clinical use. Some dental
implant systems require “full arch” imaging as part of the needs for
manufacture of imaging guides/stents. Authorities or clinicians performing dose
audit should specify that this requirement should be suspended for the dose
measurement and the smallest field of view compatible with single implant site
assessment used.
They also propose setting a DRL for a
child view based on the clinical protocol used to image a single impacted
maxillary canine in a 12 year old male. As yet, however, insufficient audit
data is available to set this level.
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
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6.3.3 Using DRLs
Dentists should be aware of their
average doses for the different types of examinations they undertake with their
CBCT equipment and how these compare with the European and any national DRLs,
once established.
If a DAP readout is provided on the
equipment, the dentist should undertake audit of DAP readings for standard size
patients, ideally with the help of a medical physics expert. If DAP is not provided
it is expected that the dentist will need to seek help from the medical physics
expert to establish typical patient doses. These assessments should be carried
out on a regular basis, at least every three years or as required by national
legislation.
These measurements can be seen to be a
part of any QA programme adopted by the dental practice. Dose results that
exceed established DRLs, or which significantly differ from previous audits,
should be investigated with the help of a medical physics expert. Any resulting
recommendations should be implemented.
It should be noted that
CBCT units with fixed large FOV are likely to exceed the achievable dose stated
above for the placement of an upper first molar implant in a standard adult
patient. Consideration must be given as to whether the use of such a unit for
this view can be considered to be justified.
A cone beam dental system usually comes
with pre-programmed settings for different types of patients (e.g. children
versus adults) or clinical indications. In the absence of any patient specific
tube output modulation, the pre-programmed protocols can be verified by means
of dose measurements in air, at the level of the detector, or using a DAP
meter. In the ideal case, the dose measurements are performed for all standard
imaging protocols for which a DRL has been defined. When tube output modulation
is used, dedicated phantoms may be required or clinical dose audit based on a
group of standard patients.
It is good practice to
investigate whether the doses have been selected based upon relevant criteria.
In particular, it should be verified that doses for children are significantly
lower than those for adults and that separate programs are available for local
pathologies as well as imaging the complete upper or lower jaw. Other settings
to be tested include the correct pre-programming of lower kV, the use of tube
output modulation, high versus low resolution scanning etc.
Systematic patient dose
surveys are straightforward if DICOM header tags are completely filled in and
if software is available to obtain the dose related information automatically.
The intrinsic dose information has first to be checked against measured data,
has then to be expressed or recalculated into survey related quantities and
then be collected over a period of time. The medical physics expert should
ensure that the practitioner is aware if DRLs are exceeded.
6.4:
Clinical Image Quality Assessment
The consistent
production of adequate diagnostic information from radiological examinations is
central to optimization (EC Directive 97/43 Euratom). There is, however, ample
research evidence showing that radiographic image quality is often less than
ideal in primary dental care (reviewed in European Commission 2004). The higher
radiation doses of CBCT compared with conventional dental radiography mean that
standards must be rigorously maintained.
In addition to the assessment of image
quality by quantitative methods, as described above in the image quality
section, it is important that the quality of clinical images is assessed. This
can be approached in three ways:
1. Comparison with standard reference
images from high quality CBCT examinations.
2. Reject analysis, in which the rate of
unsuccessful CBCT examinations is recorded and the reasons for rejection
analysed.
3. Systematic audit
of CBCT examinations against established clinical image quality criteria.
Assessment of the clinical quality of images
should be a part of a quality assurance programme for CBCT
GP
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6.4.1
Comparison with standard reference images
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