Learning objectives
To understand how radiation dose is measured in cardiac imaging and the potential
risks of radiation exposure.
To understand the key principles of radiation protection and the concept of ‘As Low
As Reasonably Practicable’.
To understand how radiation dose can be minimised in different imaging techniques,
including the general principles of patient-tailored imaging, good operator technique
and improvements in hardware and software.
Introduction
Exposure to ionising radiation is an important healthcare concern, primarily due to
the potential increased lifetime risk of malignancy. This is important for patients
and staff who are exposed to ionising radiation during diagnostic imaging or interventional
procedures. Ionising radiation refers to radiation in the electromagnetic spectrum,
which has enough energy to remove electrons from an atom. For the purposes of diagnostic
imaging, this includes X-rays and gamma rays.
The UK average background radiation dose is 2.7 mSv per year, and about 0.4 mSv (16%)
of this is from diagnostic medical examinations.1 2 In the UK, the number of CT scans
performed increased fivefold between 1996/1997 and 2012/2013.3 Similar trends are
seen in other countries and with other imaging modalities. The USA has seen a threefold
increase in the annual number of nuclear medicine procedures, and CT procedures have
increased 20-fold between 1985 and 2005.4 Cardiac imaging and interventional procedures
are responsible for approximately 40% of the US cumulative effective dose due to medical
imaging.5
Estimating patient radiation dose
The term ‘radiation dose’ can refer to one of several measures (table 1). Dose area
product (DAP) is used in X-ray and fluoroscopic imaging (eg, invasive coronary angiography
[ICA]). Radiation dose in CT is calculated from dose indices measured in standardised
phantoms. Volume CT dose index (CTDIVOL) can be used to compare protocols, and dose
length product (DLP) can be used to compare doses between patients for the same CT
examination. In nuclear medicine, radiopharmaceuticals emit radiation in the form
of gamma rays or particles, and administered radioactivity is measured as nuclear
decays per second (becquerels [Bq]).
Table 1
Radiation dose parameters
Modality
Parameter
Definition
Units
General
Absorbed dose.
Amount of energy deposited in a material per unit mass.
Gray (Gy) 1 Gy=1 joules per kilogram.
Equivalent dose.
Absorbed dose multiplied by weighting factor based on the type of radiation (weighting
factor of 1 for X-rays and gamma rays).
Sievert (Sv).
Effective dose.
Whole body quantity based on absorbed organ doses weighted based on their radiation
sensitivity and type of radiation; weighted sum of the organ equivalent dose.
Sieverts (Sv).
Fluoroscopy
Kerma (kinetic energy released per unit mass).
Energy transferred per unit mass of irradiated material.
Gray (Gy).
Air kerma.
Energy transferred per unit mass of air measured with an ionisation chamber.
Gray (Gy).
Dose area product.
Product of the air kerma and X-ray beam area.
Gy cm2.
Peak skin dose
Accumulated absorbed dose to the most irradiated area of skin.
Gray (Gy).
Fluoroscopy exposure time
Cumulative time fluoroscopy is used.
Seconds/minutes.
CT
CT dose index (CTDI).
Average absorbed dose from one axial CT scan measured with an ionisation chamber
Gray (Gy).
Weighted CTDI (CTDIw).
CTDI weighted across the field of view with 1/3 for the centre and 2/3 for the edge.
Gray (Gy).
Volume CTDI (CTDIVOL).
CTDIw divided by pitch.*
Gray (Gy).
Dose length product.
CTDIVOL multiplied by total scan length.
mGy cm.
Radioisotopes
Radioactivity.
Rate of nuclear decay events (decays per second).
Becquerel (Bq).
*Pitch=table movement per rotation/slice thickness.
Effective dose is widely used when discussing radiation as it can be calculated for
all modalities and gives an overall indication of risk from an exposure. However,
it was designed for use in radiation protection within populations and only considers
cancer risk. The International Commission on Radiological Protection (ICRP) has produced
tissue weighting factors to reflect relative sensitivities of tissues to the carcinogenic
effects of radiation (table 2). These are averages for all ages, genders and body
sizes and assume that the risk is the same as if the absorbed dose was distributed
uniformly throughout the body. Effective dose is calculated by multiplying the average
equivalent dose in each exposed tissue by a tissue weighting factor and summing these
values over the whole body. This is normally done using Monte Carlo simulation software.
A more rapid method involves multiplying a displayed dose indicator (eg, DAP or DLP)
by a conversion factor for a given modality and imaged anatomical region.
Table 2
ICRP 103 tissue weighting factors (modified from ref 49)
Tissue
Weighting factor
Gonads
0.8
Bone marrow
0.12
Colon
0.12
Lung
0.12
Stomach
0.12
Breast
0.12
Remainder tissues*
0.12
Bladder
0.04
Oesophagus
0.04
Liver
0.04
Thyroid
0.04
Bone surface
0.01
Brain
0.01
Salivary glands
0.01
Skin
0.01
*The ‘remainder tissues’ refers to the combination of the adrenals, extrathoracic
region, gallbladder, heart, kidneys, oral mucosa, pancreas, prostate, small intestine,
spleen, thymus, uterus, lymph nodes and muscle. All of these tissues together are
assigned a weighting factor of 0.12.
ICRP, International Commission on Radiological Protection.
For CT, the most widely used conversion factor is 0.014 mSv/mGy.cm. However, this
is based on out-of-date tissue weighting factors, was derived using old CT technology
and was designed for chest CT, which includes a different proportion of radiosensitive
tissues compared with CT coronary angiography (CTCA).6 Therefore, this conversion
factor underestimates radiation dose. A study using computer models of patients scanned
with two scanners from one manufacturer showed that 0.028 mSv/Gy.cm is a more appropriate
conversion factor.6 This is convenient as effective dose values calculated with the
previous conversion factor can simply be doubled. A recent phantom study showed that
conversion factors range from 0.020 to 0.043 mSv/Gy.cm depending on CT scanner type,
protocol and tube voltage.7 The average was 0.026 mSv/Gy.cm, and this has been proposed
as a more appropriate conversion factor for CTCA.7
There are similar issues with the calculation of conversion factors for ICA and nuclear
imaging. Ideally, the conversion factor for ICA would consider screening time at different
projection angles. However, it is more convenient to use a single conversion factor
based on the average for commonly used projections. Conversion factors for DAP in
ICA range from 0.18 mSv /Gy.cm2 to 0.24 mSv/Gy.cm2.8 In nuclear imaging, the effective
dose can be calculated by multiplying the activity of administered radiopharmaceutical
by a tracer-specific conversion factor.9 Uncertainties in dose estimation in nuclear
imaging include errors in measuring administered activity and differences in patient
pharmacokinetics and body habitus.
The choice of conversion factor has important implications for quoted effective doses,
and therefore, the factor used must always be quoted. At each step of the calculation,
uncertainties can be introduced and, for an individual patient, uncertainties in effective
dose may be ±40%.10
Typical effective doses for common procedures (table 3) vary widely, due to differences
in equipment, protocols and demographics.11 Further standardisation, audit and quality
improvement is essential to reduce radiation dose and improve consistency. In addition,
it should be remembered that stated radiation doses are estimates rather than precisely
known values.
Table 3
Estimated radiation dose of non-invasive and invasive cardiac imaging for typical
equipment in typical patients24 31 44 45 50–53
Modality
Effective dose (mSv)
X-ray
Chest X-ray51
0.02
CT
Coronary artery calcium score CT31
1–3
Low dose coronary artery calcium score CT*52
0.2–0.4
CTCA31
2–5
TAVI CT assessment (chest, abdomen and pelvis)24
5–50
Fluoroscopy
ICA24
2–20
TAVI, transapical24
12–23
TAVI, transfemoral24
33–100
Diagnostic electrophysiology study24
0.1–3
Radiofrequency arrhythmia ablation24
1–25
SPECT†‡
99mTc-sestamibi
50
Stress only full dose
10
Rest and stress half dose
6
Rest and stress full dose
13
99mTc-tetrofosmin
50
Rest and stress half dose
6
Rest and stress full dose
11
201Thallium
44
Rest and stress half dose
10.4
Rest and stress full dose
21
PET (rest or stress imaging)†‡
13N-ammonia45
2
15O-water45
2
82Rubidium chloride45
3
18F-FDG53
5
18F-Sodium fluoride53
4
*Using iterative reconstruction and tube voltage of 100 kV or below.
†If attenuation correction CT is performed this radiation dose must also be taken
into account (~0.5–2 mSv).45
‡Typical values based on recommended tracer injected activity for standard patients.
18
F-FDG, 18F-fluorodeoxyglucose; CTCA, CT coronary angiography; ICA, invasive coronary
angiography; SPECT, single-photon emission CT; TAVI, transcatheter aortic valve implantation.
Risks of ionising radiation
Ionising radiation has the potential to cause biological harm either by directly damaging
molecules, such as proteins or DNA, or through the secondary effects of free radicals
generated by ionisation. Risks can be divided into stochastic (random) and deterministic
(non-stochastic) effects. These can be somatic, in the individual exposed or hereditary,
affecting germ cells. Inbuilt DNA repair mechanisms can mitigate radiation effects.
Biochemical markers of DNA damage and repair have been identified after CTCA, ICA
and single-photon emission CT (SPECT),12–14 but these revert back to normal background
levels after 1 day.13
Deterministic effects
Deterministic effects, such as skin erythema and hair loss, have a threshold level
below which they will not occur and above which the severity of the effect increases
with increasing dose (table 4). It is rare for these levels to be exceeded during
normal diagnostic imaging. However, for patients undergoing lengthy interventional
procedures with prolonged fluoroscopic imaging in one position, the threshold for
skin damage may be exceeded. Recent epidemiological studies have indicated that the
threshold dose for the formation of lens opacities is lower than previously thought,
at 0.5 Gy for both acute and protracted exposures. The latter is of particular importance
for interventional cardiologists and electrophysiologists and is the reason why lead
eye protection is recommended.
Table 4
Threshold levels for deterministic effects of radiation. (adapted from54)
Deterministic effect
Absorbed dose threshold (Gy)*
Skin erythema
3–6
Skin burns
5–10
Temporary hair loss
4
Sterility
3–6
Cataracts
0.5
For acute exposures: the time to develop these effects after exposure varies from
1 week for skin changes to 20 years for cataract development.
Stochastic effects
Stochastic effects, such as the development of malignancy and germ cell mutations,
do not have a threshold level and may occur at any radiation dose. Their likelihood,
but not severity, increases with increasing absorbed dose. The ‘linear no threshold’
(LNT) model is widely used to relate radiation dose to the risks of these effects.
In the LNT model, risk increases linearly with radiation dose, and there is no threshold
below which they do not occur. Evidence for the stochastic effects of radiation at
doses greater than 100 mSv comes from epidemiological studies (eg, survivors of atomic
weapon explosions and radiation accidents) and animal models. However, limitations
with these data include the different types of radiation, variation in absorbed doses,
acute and protracted exposures and the lack of evidence at radiation doses below 100 mSv.
The inherent latency between a radiation exposure and the development of malignancy
further complicates matters. Solid malignancies typically have a latency of 10–20
years, compared with lymphoma or leukaemia, which have a shorter latency of 2–5 years.15
The background risk of cancer is high, with a lifetime risk of cancer in the UK of
approximately 50%. Therefore, to assess the effect of low-dose radiation would require
very large epidemiological studies.16 The National Research Council Biological Effects
of Ionising Radiation VII committee report has produced models that assess the potential
lifetime attributable risk of cancer from medical radiation exposures.17 The risk
of malignancy is higher for younger patients and women (figure 1). Using these models,
the estimated additional lifetime risk of cancer from CTCA with a dose of 3.7 mSv
would be approximately 1 in 7000 for a 50-year-old man and 1 in 2900 for a 50-year-old
woman.18 As a rough estimation, the risk of fatal malignancy from a radiation exposure
can be estimated as 5% per Sv or 1 in 20 000 per mSv.19 It has been estimated that
0.59% of the cancers in the UK in 2010 were due to diagnostic radiation exposures.20
However, this must be balanced against the health benefits of performing these tests.
Figure 1
Estimated relative risk of attributable cancer from CT coronary angiography (CTCA)
at different ages (compared with an 80-year-old man undergoing CTCA). Values obtained
from ref 55.
Principles of radiation protection and regulations
The central principles of radiation protection as applied to medical exposures are
justification, optimisation and limitation. Justification refers to the consideration
that the benefits of exposure to ionising radiation should sufficiently outweigh the
risks. Appropriateness criteria and national guidelines can aid determination of the
suitability of imaging or procedures for individual patients. Individual patient factors
must also be considered. Optimisation refers to using the minimum amount of radiation
to achieve an adequate diagnosis or procedure. This has been described as keeping
radiation dose ‘As Low As Reasonably Practicable’ (ALARP). Limitation means that the
effective dose to an individual should not exceed the recommended dose limits.
In the UK, legislation concerning the use of ionising radiation for medical exposures
includes the Ionising Radiation Regulations 2017 and the Ionising Radiation (Medical
Exposure) Regulations 2017.
Ionising Radiation Regulations (IRR)
The aim of the IRR is to ensure that the dose to staff and members of the public due
to the use of radiation in the workplace is kept as low as reasonably practicable.
This is the duty of the employer. Doses are minimised by the use of engineering controls,
systems of work and personal protective equipment. IRR prescribes annual dose limits
for individuals, which must not be exceeded. Following recommendations from the ICRP,
the annual dose limit to the lens of the eye has been reduced from 150 mSv to 20 mSv
per year in the most recent update to the regulations.
Ionising Radiation (Medical Exposure) Regulations (IR(ME)R)
IR(ME)R covers the justification of medical exposures involving ionising radiation
and the optimisation of radiation dose. Key roles are identified with respect this:
the employer, referrer, practitioner and operator. The referrer requests the investigation
or procedure and must provide enough information to enable this. The practitioner
assesses whether the examination is justified based on this information and, if appropriate,
authorises it. The operator carries out the practical aspects of the exposure. The
employer determines the entitlement to carry out these roles, within the constraints
established by IR(ME)R. IR(ME)R also includes requirements for training, quality assurance
of procedures and equipment, optimisation of exposures and notification requirements.
IR(ME)R established the requirement for diagnostic references levels (DRLs). DRLs
are dose levels for typical examinations based on standard-sized patients, within
a restricted weight range of 50–90 kg. In the UK, DRLs are calculated as the rounded
third quartile value of the distribution of dosimetric or activity values for the
examination being considered. National DRLs are available for a range of commonly
performed procedures (table 5). DRLs are only available for some common examinations,
and there is not currently a national DRL for CTCA. DRLs do not represent an ‘average’
radiation dose and are not a dose limit. Instead they can be used as an indication
of good practice in order to benchmark local practices. Under normal circumstances,
individual exposures are not expected to exceed DRLs consistently, but this must be
considered in conjunction with other aspects of justification, including clinical
factors such as patient body habitus, procedural complexity or net benefit to the
patient. Examples where DRLs may be exceeded would include ICA with complex intervention
for chronic total occlusion or when the local population body mass index is higher
than the restricted weight range used to calculate the DRL.
Table 5
Diagnostic reference levels23
DAP per exam (Gy cm2)
Fluoroscopy time per exam (min)
Coronary angiography
31
4.3
Coronary graft angiography
47
13
Percutaneous transluminal coronary angioplasty (single stent)
40
11.3
Pacemaker (permanent)
7
6
DAP, dose area product.
IR(ME)R now includes a duty for the employer to have a programme of radiation equipment
quality assurance and to ensure that prior to an exposure information is provided
on the benefits and risks to the individual. Additionally, the administration of radioactive
substances is now governed under IR(ME)R, with applications for licences currently
managed by the Department of Health and Social Care.
Safe use of radiation in specific modalities
Safe use of fluoroscopy
Fluoroscopy is used in ICA, percutaneous coronary intervention, device implantation
and electrophysiology. Optimisation of radiation exposure during fluoroscopy minimises
the radiation dose to both patients and staff. Radiation dose can be minimised with
hardware improvements, software improvements and good operator technique. Technological
advances contributing to dose reduction include automated exposure control, spectral
beam shaping filters, pulsed fluoroscopy, flat panel detectors, ‘last image hold’,
fluoro-loops and fluoro-save features.21 Good operator technique includes appropriate
collimation (reduction in size) of the primary beam, minimising exposure time and
keeping the detector close to the patient.22 Viewing saved fluoro-loops rather than
repeated screening reduces radiation dose. Oblique fluoroscopic angles increase the
radiation dose.23 Real-time feedback to staff can help improve understanding of radiation
dose, particularly for trainees.
Staff radiation exposure during fluoroscopy comes from the primary beam and scattered
radiation. The transradial approach has a slightly increased operator radiation exposure
compared with the transfemoral approach due to the higher patient radiation exposure,
proximity of the operator to the X-ray tube and the less effective position of ceiling
mounted shielding.22 24 25 Subclavian access means an even closer operator position
and thus higher radiation dose.24 The simplest method to reduce radiation dose to
staff is to use the ‘inverse squared law’, that is, doubling the distance between
the operator and radiation source reduces dose by a factor of four. Further dose reduction
is enabled by the use of appropriate personal protective equipment such as aprons,
thyroid shields, glasses, hats, gloves and shin covers made using lead-equivalent
material. At 80 kV, an 0.35 mm lead equivalent apron will transmit 3.1% of the radiation
exposure. It is important that aprons are well fitting and close at the sides, as
operators are often angled towards the X-ray tube. Lead aprons and eye protection
should be checked annually for defects and carefully stored. Shielding to reduce staff
exposure to scattered radiation includes ceiling mounted shields, lead table skirts
and patient drapes. Shielding in the walls and doors of the fluoroscopy room aim to
reduce the radiation dose to those outside the controlled area to well below the public
dose limit. Particular care must be taken during complex interventions where prolonged
screening and the presence of extra staff increases occupational exposures. Careful
planning can reduce exposure times, such assessing implantation angles on CT prior
to transcatheter aortic valve implantation (TAVI) or electroanatomical mapping for
ablations. Dose monitoring for staff can be performed with passive or active personal
dosimetry monitors. Various types of these are available to monitor whole body, collar,
lens or extremity dose.
The average occupational whole body exposure for UK cardiologists in 2009/2010 was
0.12 mSv.2 Occupational whole body exposure for interventional cardiologists and cardiac
electrophysiologists can be two to three times higher than diagnostic radiologists.26
However, the effect on subsequent morbidity and mortality is uncertain. A study of
43 763 radiologists and 64 990 psychiatrists showed an increase in melanoma, non-Hodgkin’s
lymphoma and cerebrovascular disease among radiologists practising before 1940 but
no excess mortality in those who started practising after 1940.27 Another study identified
increased leukaemia mortality among men performing fluoroscopy guided interventions
who had graduated before 1940 but no overall increase in mortality compared with psychiatrists.28
A study of 90 957 technologists performing fluoroscopy between 1994 and 2008 found
an increase in brain cancer, breast cancer and melanoma.29 These effects may be due
to radiation, but confounding factors may also be implicated.
Safe use of CT
CT of the heart can take several forms including non-contrast ECG-gated coronary artery
calcium score (CACS), contrast-enhanced ECG-gated CTCA and CT imaging prior to TAVI
(table 3). In the UK, the average patient dose for CTCA is 5.9 mSv (209 mGy.cm, conversion
factor 0.028 mSv/mGy.cm).30 CACS uses standardised acquisition parameters in order
to provide consistent assessment of calcium, and this has a radiation dose of 1–3 mSv.31
Low-dose CACS is possible but may overestimated or underestimated calcium scores.32
33 CT for patients undergoing TAVI involves assessment of the heart and vascular system,
which includes CT of the chest, abdomen and pelvis. The radiation dose for this is
higher than CTCA due to the faster heart rates and larger scan range.
The dose for CTCA depends on patient factors, such as heart rate and body mass index.
The indication for the test affects radiation dose as, for example, assessment of
coronary artery bypass grafts involves a larger craniocaudal scan range than assessment
of native coronary arteries. Radiation dose reduction techniques for CTCA include
prospective ECG-gating, reducing tube voltage and tube current, tube current modulation
and minimising the scan range.25 Improved image reconstruction techniques, such as
iterative or model-based reconstruction, can provide diagnostic quality images at
a lower radiation dose.34 CTCA using state-of-the-art technology in patients with
slow heart rates and low body mass index can achieve average radiation doses as low
as 0.29 mSv.2
Safe use of nuclear imaging
Throughout the world, nuclear techniques are the most common form of cardiac imaging.
In the USA, cardiac nuclear imaging accounted for 26% of overall medical radiation
exposure in 2006.26 Radiation dose in nuclear imaging depends primarily on the choice
of radiotracer.
A variety of radiotracers are available for SPECT imaging, with thallium having the
highest radiation dose (table 3).28 Adjusting the injected radiotracer activity based
on weight or body mass index can reduce radiation dose.28 Radiation dose can be further
reduced by protocol improvements such as stress-only imaging, hardware improvements
such as using newer solid state detectors (cadmium zing telluride and thallium-activated
caesium iodide) and software improvements such as iterative reconstruction, resolution
recovery and noise reduction.29 In addition, SPECT cameras with dedicated geometry
and collimators optimised for cardiac imaging can further reduce radiation dose.35
Longer imaging time with lower injected activity could reduce radiation dose, but
this must be balanced against movement artefacts.36 The CT used for attenuation correction
can also be optimised to reduce radiation dose. Eight best practices have been established
to reduce radiation dose in SPECT. (box 1)32 33 Worldwide, the mean radiation dose
for SPECT myocardial perfusion is 10.9 mSv, with Europe having the lowest dose at
7.9 mSv.27 However, using stress only imaging and the latest technology means that
doses as low as 0.99 mSv are possible.27
Box 1
Eight best practices for SPECT myocardial perfusion imaging (adapted from ref 42).
Eight best practices
Avoid thallium stress.
Avoid dual isotope.
Avoid too much technetium.
Avoid too much thallium.
Perform stress-only imaging.
Use camera based dose-reduction strategies.
Weight-based dosing for technetium.
Avoid inappropriate dosing that can lead to ‘shine through’ artefacts.
Positron emission tomography (PET) can be used to assess myocardial perfusion and
conditions such as sarcoidosis, vasculitis and endocarditis. New tracers and new uses
for established tracers are developing, such as the use of 18F-fluorodeoxyglucose
and 18F-sodium fluoride. PET radiotracers generally have a lower radiation dose than
SPECT tracers (table 3). Techniques to reduce radiation dose in PET include time-of-flight
reconstructions, iterative reconstruction, three-dimensional acquisitions, weight
or body mass index-adjusted radiotracer activity and the use of single attenuation
correction CT images.24 37 Similar to SPECT, the attenuation correction CT must also
be optimised to further reduce radiation dose.38 39
Cumulative radiation dose
Many patients will undergo multiple imaging procedures over their lifetime. The effect
of repeated exposures to low dose ionising radiation is uncertain. It has been suggested
that patients should have a cumulative radiation dose record, but at present, it is
unclear what to do with these records. The process of justification is based on benefit
versus risk for the patient at that time point, rather than on their cumulative radiation
dose.40 However, this sort of record can act as a reminder that all radiation exposures
should be justified and optimised. When applied to the individual, cumulative dose
records can have significant errors, but population-based dose registries may have
benefits for quality improvement and research.41
Congenital cardiac imaging
Patients with congenital heart disease are likely to undergo multiple exposures to
ionising radiation during their lifetime. Therefore, the use of imaging modalities
that do not use ionising radiation should be considered. Children are at an increased
risk of malignancy due to their more rapidly dividing cells and longer life expectancy.
The lifetime attributable risk of malignancy from ionising radiation in patients with
congenital heart disease depends on age, sex, surgical complexity and subsequent life
expectancy.42
The use of procedures involving low-dose ionising radiation in patients with congenital
heart disease is increasing.26 Chest X-rays are the most frequent source of ionising
radiation for children under 6 years with surgical procedures for congenital heart
disease.43 However, cardiac catheterisations and CT account for between 81% and 95%
of the cumulative radiation dose.24 The radiation dose reduction techniques discussed
above should all be applied in children, and the protocol should be tailoring to their
age or size.44
Pregnancy
Cardiovascular diseases affect 1% of pregnant women, and during pregnancy, radiation
risks apply to both the mother and the fetus. Imaging modalities that do not use ionising
radiation may be preferable. However, if the benefit outweighs the risk, then an informed
discussion between patient and clinician is essential, with involvement of gynaecology
and medical physics as appropriate.
Radiation risks depend on the radiation dose and gestational age.24 43–46 For most
X-ray imaging, the dose to the fetus is from scattered radiation, as the fetus can
be kept out of the direct X-ray beam. However, for radiotracers with urinary excretion,
the bladder is an important source of foetal exposure. Risks to the fetus are greatest
in the 3rd to 8th week, so careful timing of investigations may reduce risks. Deterministic
effects may occur at a threshold absorbed dose to the fetus above 150 mGy and include
fetal malformation, mental retardation and spontaneous abortion.24 44 The estimated
risk of childhood cancer from a dose of 1 mGy is about 1 in 17 000.44
The dose to the fetus from cardiac imaging is usually low with an estimate of <0.0001 mGy
for a chest X-ray, 1 mGy for a prospectively gated CTCA and 0.074 mGy for an invasive
coronary angiogram.47 More complex procedures and electrophysiology studies are associated
with higher foetal doses, estimated as 0.0023–0.012 mGy per minute.47
The lactating breast has an increased risk of radiation-induced carcinogenesis. After
nuclear imaging, the amount of radiotracer in breast milk will depend on the radiotracer
and injected activity. For 99mTc-tetrofosmin, 0.082% of the injected activity is excreted
in breast milk.48 A recommendation to interrupt breast feeding may be given, depending
on the radiotracer and administered activity, so that the dose to the infant is <1 mSv.
Conclusion
Appropriate use of non-invasive and invasive imaging using ionising radiation is essential
to minimise radiation exposure. The ALARP principle should be applied for both patient
dose and occupational exposure. Methods to reduce radiation dose for all modalities
include protocol optimisation and patient-tailored imaging. Personal protective equipment
is important to minimise occupational exposure. Further technological improvements
will continue to reduce radiation dose in all forms of imaging.
Key messages
Medical imaging is an increasing source of population radiation exposure, with cardiac
imaging and interventional procedures responsible for 40% of the cumulative effective
dose due to medical imaging.
The term ‘radiation dose’ is of little use as it can refer to one of several indices.
Effective dose gives an overall indication of associated cancer risk but has limitations,
in particular uncertainties in the calculation of effective dose for an individual
patient may be ±40%.
Risks from ionising radiation may be stochastic (random) or deterministic (non-stochastic).
Radiation protection involves justification that the risks of the exposure are outweighed
the benefits and that exposures must be optimised to be ‘As Low As Reasonably Practicable’.
Radiation dose reduction techniques for all modalities include patient tailored imaging,
good operator technique and hardware and software improvements.
Personal protective equipment is important to minimise occupational radiation exposure.
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