Dose management in CT facility
1 Medical Physics Unit, Konstantopoulio Agia Olga Hospital, Athens, Greece
2 International Atomic Energy Agency, Vienna, Austria
Computed Tomography (CT) examinations have rapidly
increased in number over the last few years due to recent advances such as the
spiral, multidetector-row, CT fluoroscopy and Positron Emission Tomography
(PET)-CT technology. This has resulted in a large increase in collective
radiation dose as reported by many international organisations. It is also
stated that frequently, image quality in CT exceeds the level required for confident
diagnosis. This inevitably results in patient radiation doses that are higher
than actually required, as also stressed by the US Food and Drug Administration
(FDA) regarding the CT exposure of paediatric and small adult patients.
However, the wide range in exposure parameters reported, as well as the
different CT applications reveal the difficulty in standardising CT procedures.
The purpose of this paper is to review the basic CT principles, outline the
recent technological advances and their impact in patient radiation dose and
finally suggest methods of radiation dose optimisation. � 2007 Biomedical
Imaging and Intervention Journal. All rights reserved.
Keywords: Patient doses in computed tomography (CT), dose management in CT, dose optimisation in CT
Computed Tomography (CT) has emerged as one of the most
important imaging techniques of modern times. Starting with a bang in early
1970s with a great promise of exploring inner structure of the organs, it faced
challenge from MRI in late 1970s and has emerged not only survivor but rather
its clinical applications continue to increase [1-4]. The recent advances in CT
such as multidetector-row technology, with sub-second acquisition and CT
fluoroscopy have boosted CT applications, even more enabling interventional
radiological (IR) procedures, which were traditionally performed with C-arm
X-ray units. The continual increase in number of slices that can be scanned in
one rotation of the X ray tube has brought multidetector computed tomography
(MDCT) into dynamic imaging.� MDCT is all set for playing an important role in
angiography where it may be indicated as a replacement for conventional
coronary angiography. The development of hybrid systems such as PET/CT,
SPECT/CT and CT simulators in radiotherapy, and its incorporation in CT
planning and dose delivery systems is moving CT from the domain of diagnostic
radiology to other specialities.
Increasing applications mean increasing collective
radiation dose to the population. But that is not bad� as long as individual CT
examination is clinically justified and doses are optimised to be not more than
what is necessary. But experience shows that individual patient doses are
increasing [1, 6-10]. In one of the reports from the United States, it was
estimated that CT scanning accounts for more than 10 % of all radiological
examinations and about two-thirds of the radiation dose to patients .
Regarding MDCT, one of the main problems in the initial systems, which were�
four detector scanners was the width of the X-ray beam in the z-direction.
Since more than one row of detectors has to be exposed, a broader beam should
be used compared to single row scanners so as to expose the outer detectors of
the row, thus increasing the radiation dose. This problem is minimal in 16
detector scanners and above. Large variation� in exposure parameters and
patient doses even for a single CT examination have been reported [12-17]. It
is noted that at specific exposure parameters, the radiation dose to the
patient from various CT models can be totally different due to changing CT
geometry and filtration. There is also growing realisation that very often CT
image quality is much higher than actually required to produce accurate
clinical diagnosis and a number of studies reported large dose reductions using
modified exposure parameters [18-21]. Taking all these into consideration, as
well as the continuous need to balance between the net benefits and the risks
of using such a modality, various international organisations have published
guidelines so as to standardize CT examinations and optimise radiation dose [22-23].
The European guidelines include image quality criteria for the most frequent CT
examination, good imaging techniques and use of Diagnostic Reference Levels
(DRLs) . Since it is not appropriate to set dose limits on medical
exposures, DRL is a useful quantity that facilitates the investigation of dose
levels in various CT procedures and permits comparison of performance between
different scanners and techniques . DRLs provide the means to improve
patient protection, if it is required, identify poor performance and monitor CT
performance in periodic measurements [24-27]. The foregoing discussion reveals
the need for proper management of radiation dose in a CT facility. This paper
aims� to review the situation with regards to patient exposure in CT
examinations, and provide practical advice to manage the radiation dose while
maintaining diagnostic confidence.
CT dosimetry methods
Patient exposure is quite different in CT than in
conventional X-ray examinations, with the X-ray tube rotating around the
patient producing thin slices of the irradiated body region. Therefore, dose
calculation in CT is more complicated and requires the introduction of special
dosimetric quantities such as the Computerised Tomography Dose Index (CTDI) and
the weighted CTDI (CTDIw) for a single slice and the Dose Length
Product (DLP) for a complete examination. These quantities are described in
detail in the European Guidelines . With the launch of multidetector-row
scanners, volumetric CTDI (CTDIvol) was introduced in order to
determine the dose in one rotation.
Computerised Tomography Dose Index
CTDI is defined by the following equation:
T is the nominal slice thickness and D(z) is the dose
profile along a line parallel to the Z-axis (tube rotation axis). CTDI
integrates the radiation dose imparted within and beyond a single slice. It is
measured using a specially designed pencil ionisation chamber with an active
length of 100 mm both in free air at the centre of rotation (CTDIair)
and within cylindrical polymethylacrylate (PMMA) phantoms of 16 and 32 cm in
diameter, simulating the head and body of a patient, respectively. CTDIc
and CTDIp are defined respectively as the CTDI values measured with
a pencil chamber dosemeter positioned within the centre and in the periphery of
the PMMA phantom. CTDIp can thus be considered as a good
approximation of the entrance surface dose (ESD).
CTDIw is used for approximating the average
dose over a single slice in order to account for variations in dose values
between the center and the periphery of the slice. It is defined by the
CTDIp is the average of the four CTDIp
values measured in the periphery of the phantom (12, 3, 6 and 9 o� clock).
CTDIvol is introduced to determine the
radiation dose in one tube rotation in multidetector-row scanners and allows
for variations in exposure in the z direction when the pitch (pitch is the
ratio of table feed in one rotation to slice collimation) is not equal to one
(CTDIvol = CTDIw / pitch). In the case of a single slice
spiral system, CTDIvol is equal to CTDIw.
Dose Length Product
DLP is used to calculate the dose for a series of slices
or a complete examination and is defined by the following equation:
i represents each one of the individual N scans of the
examination that covers a length T of patient anatomy. Certain manufacturers
display the DLP value in each patient examination.
The effective dose is a �dose� parameter that reflects the
risk of a non-uniform exposure in terms of a whole body exposure. It is a
concept used to normalise partial body irradiations relative to whole body
irradiations to enable comparisons of risk . The calculation of effective
dose requires knowledge of the dose to specific sensitive organs within the
body, which are typically obtained from Monte Carlo modeling of absorbed organ
doses within mathematical anthropomorphic phantoms , and recently also
voxel phantoms based on real humans. Effective dose is expressed in the units
of milliSieverts (mSv), and can be compared to the effective dose from other
sources of ionising radiation, such as that from background radiation level (e.g.,
radon, cosmic radiation, etc.), which is typically in the range of 1 to 3 mSv
depending upon the location. The International Commission on Radiological
Protection (ICRP) emphasises that effective dose is intended for use as a
protection quantity on the basis of reference values and therefore should not
be used for epidemiological evaluations, nor should it be used for any specific
investigations of human exposure. Rather, absorbed dose should be used with the
most appropriate biokinetic biological effectiveness and the risk factor data.
The use of effective dose for assessing the exposure of patients has severe
limitations. An effective dose can be of some value for comparing doses from
different diagnostic and therapeutic procedures and for comparing the use of
similar technologies and procedures in different hospitals and countries as
well as from use of different technologies for the same medical examinations.
For planning the exposure of patients and risk-benefit assessments, however,
the equivalent dose or the absorbed dose to irradiated tissues is the more relevant
quantity. It must be remembered that an effective dose, however, does not tell
the complete story with regards to the potential effects of ionising radiation.
Specific organs and tissues are known to be more radiosensitive than others.
While this is reflected in effective dose, the absolute doses to specific
organs or tissues is also important to consider.
CTDI can be measured using pencil ionisation chamber. If
measurements are not possible, the Imaging Performance and Assessment of CT
(ImPACT) Patient Dosimetry Program produced by the Medicines and Healthcare
products Regulatory Agency (MHRA) can also be used. The program is freely
available� on the Internet and continuously updated to include data on the
recently developed CT scanners . It provides CTDI and DLP values for a wide
range of CT scanners and exposure parameters used to perform a CT examination.
Furthermore, the Electrotechnical Commission in 1999 recommended the display of
CTDI value on the CT console . Many manufacturers currently display CTDIw
and CTDIvol values on operator console. In this way, radiation dose
adjustments during modification of exposure parameters can be viewed on the
console before irradiation. The dosimetric quantities chosen to determine DRLs
are CTDIw (CTDIvol in the case of multidetector-row
scanners) and DLP. DRL values are proposed by the European Commission  and
the National Radiological Protection Board (NRPB) . These values should not
be used individually. They should be the tool so as to identify situations in
which dose optimisation should be applied.
Exposure parameters and CT dose
Choosing exposure parameters is a complex task and depends
to a large extent on the anatomical region to be scanned, the size and the
pathology of the patient. The chosen parameters should result in sufficient
image quality so as to aid clinical diagnosis. The main problem in determining
exposure parameters is image noise and its effect on image quality. Some
parameters that are in control of operators are discussed below:
kVp: Most CT systems do not provide users with
flexibility to adjust kilo voltage (kV) or kilo voltage peak (kVp) in a
continuous manner but there are few discreet settings possible. Tube kVp
determines the quality and quantity of radiation. The intensity of X ray beam
is typically proportional to square of kVp applied to the tube. Thus even minor
modifications in the tube potential value can result in significant changes in
image noise and considerable change in radiation dose . According to Kopp
, most of the abdominal CT examinations can be done using 120 kVp and earn
a 20% to 40% reduction in radiation dose compared to a value of 140 kVp.
Furthermore, paediatric CT examinations can be successfully performed using 80
kVp resulting in sufficient image quality [33-35].
mAs: Another important parameter which greatly
affects image quality and dose is the product of tube current and rotation time
(mAs). Radiation dose, at fixed kVp and filteration, is linearly related to
mAs, meaning that by reducing the mAs by half, the dose is also reduced by
half. On the other hand, noise is inversely related to mAs. Therefore, the
reduction by half of mAs will result in a 50 % increase in image noise. A lot
of studies have investigated the modification of mAs as a means of reducing the
radiation dose and results showed that it is an easy and straightforward way of
CT dose optimisation [16-18]. Certain studies have presented results on
individual modification of mAs with respect to patient weight and showed substantial
reduction in radiation dose [16, 18, 36]. It should be noted, however, that mAs
modification should be done carefully in certain examinations such as the
abdominal CT. The reason is that the increase in image noise can greatly
influence image quality, which is very important in organs like the liver and
Pitch: Pitch is another important parameter for
spiral and MDCT. By definition, pitch depends on collimation and table feed.
Therefore, if the patient�s table moves faster this will increase pitch and
consequently decrease the duration of patient exposure and reduce radiation
dose. However, a faster moving table results in certain artefacts, which have
great impact on image quality. According to Kalra , no marked difference in
abdominal image quality was noted between scans obtained with pitch 1.5 and
those with pitch 0.75 resulting in 50% reduction of radiation dose. As far as
collimation is concerned, small values inevitably result in higher mAs and
consequently higher dose so as to maintain image quality. Specifically for
multidetector-row scanners one should be careful as there are two definitions
available for pitch depending on whether single section collimation (pitch: p
which is independent of the number of detector rows) or the total collimation
of the detector array (volume pitch: p* which increases as the number of
detector row increases) is chosen as reference. In scanners using the volume
pitch, values are usually higher (in the order of 6) than in scanners using
conventional definition of pitch (p).
Scan length: The extent of body length covered in
scanning does not affect the CTDI value but certainly affects DLP. The scanning
length for a particular type of CT examination can vary due to the pathology of
the patient, the size of the patient, the experience of the user, or even the
demographics of a country (height of the population). With the evolution of CT
scanners (non helical machines are almost extinct in developed countries), and
especially with the introduction of multidetector-row scanners and the dramatic
reduction of rotation times to subsecond values, users are tempted to extend
the region of interest beyond the one actually required. For all these reasons,
CT protocols need to be established so as to limit irradiation only to the
particular body region in investigation.
Interventional CT procedures
The evolution of CT technology has facilitated the wider
use of CT in Interventional Radiology (IR) procedures. They are performed
either by using conventional CT equipment in which the catheter needle and the
lesion are observed during consecutive CT slices (blind technique) or by using
CT systems that are combined with a fluoroscopy unit. CT fluoroscopy works in
low tube currents in the level of 50 mA. Images can be acquired with frame
rates that can reach 12 images/second. A special footswitch is used to control
the patient table movement. Usually a monitor� would be placed inside the CT
scanner room. An important advantage of conventional CT is that it does not
involve exposure to the medical personnel as CT fluoroscopy does. On the other
hand, repetition of one or more slices in the region of interest is inevitable
resulting in increased levels of radiation dose to the patient. While data on
patient doses from interventional procedures carried out in angiographic units
are widely available, only two studies investigated the levels of patient
radiation dose in conventional CT intervention reporting maximum skin doses in
the range of 500 mGy to 1000 mGy [37, 38]. A larger number of studies exist for
CT fluoroscopy [39-42].
With the recent achievement of combining PET with CT,
corresponding examinations can be done without moving the patient but just
moving the patient table to reach the body region to be examined. The CT images
are used for producing the attenuation correction maps for PET images. On the
other hand, the PET-CT patient undergoes a CT examination regardless of having
a similar examination in a conventional system, adding to the total patient
dose. To overcome this problem, low dose CT is performed using thicker
collimation and lower mAs. The low dose CT acquisition protocol does not
significantly affect attenuation correction and anatomic delineation in PET
Practical ways if optimising radiation dose in CT
It is one of the ALARA (As Low As Reasonably Achievable)
principles and it is the first rule of optimisation in any radiology
department. Due to the fact that CT procedure is classified as a high radiation
dose procedure , it is essential that it is requested by properly trained
practitioners in close collaboration with the CT radiologist. International
Basic Safety Standards (BSS) require that an examination should be carried out
only in the case of a justifiable clinical indication . In certain clinical
situations, non-ionising techniques such as ultrasound or magnetic resonance
imaging (MRI) could probably provide similar information without irradiating
the patient. The establishment of standard protocols for the most frequent
examinations will limit radiation dose only to the level really required.
Furthermore, the repetition of a non-enhanced CT procedure using contrast
material should be reconsidered. In most cases, a single contrast enhanced
examination is sufficient and the non-contrast procedure could be eliminated
such as in abdominal CT for the evaluation of the liver. It should be noted,
that contrast material and multiphase enhancement studies are a common practice
over the last few years irradiating the same part of the body even up to� four
times . Due to all these reasons, the Royal College of Radiologists in United Kingdom has produced clinical guidelines, which are really helpful in justifying a
CT examination . As far as repetition of a CT scan is concerned, it must be
stressed that a second CT in the same body region is in most cases not
Shielding of organs
Shielding should be used in sensitive groups such as
children and young patients. Shielding of organs such as the thyroid, eye lens
and breast, when they are not in the primary beam can result in 40% to 80%
radiation dose reduction . A reduction of 95% in radiation dose can be
achieved by shielding the testes in abdominal procedures .
Modification of exposure parameters
The most easy and straightforward way of reducing the dose
in CT is to lower the mAs. This can have a significant effect in image quality
but in some CT procedures such as chest and the pelvic exam, this degradation
does not usually have an impact in clinical diagnosis [51, 52]. In abdominal
procedures, however, large mAs reduction is not usually possible. In these
situations, modification of mAs according to patient weight can provide an
alternative to dose optimisation . Aldrich found that image noise is highly
correlated with patient weight and that an acceptable image quality is
associated with a noise level of 4.5. He then developed a simple mAs prediction
equation to optimise radiation dose for all patient weight categories. The
International Atomic Energy Agency (IAEA), through a coordinated research
project (CRP) that involved six countries and nine new technology CT scanners
across the world investigated the potential for patient dose reduction while
maintaining diagnostic confidence in routine chest and abdomen CT examinations
in adult populations. The main objective of the project was to develop a simple
and clear-cut methodology whereby users could determine exposure factors that
could be applied to patients of different body weight, rather than depend on
the current approach of using default values based upon standard sized patient.
The results showed that patient weight can be an excellent predictor of the
required dose for routine chest and abdomen CT and that a noise level of 10
provided acceptable image quality, but the value could be increased for larger
patients. The project also developed recommendations of how to implement the
results to any CT department. Special exposure factors should be used for
children. Reduction of mAs during the IR technique using the conventional CT
machine will greatly reduce patient dose especially in the region of the body,
which is scanned repeatedly in the attempt to position the guidance needle.
Another straightforward way to optimise dose is to
increase the pitch of the exam either by increasing table speed or decreasing
collimation. The choice of pitch will depend on the clinical situation and
pathology of the patient such as in the case of pulmonary nodules in which
increase of pitch is not encouraged due to the resulting� reduction in their
Limitation of scan length
In order to limit the region of the patient being
irradiated, only radiologists properly trained in CT as well as radiation
protection issues related to the CT technique should perform such procedures.
Consideration should be given to program the examination protocol according to
pathology. The large range in DLP values reported in the literature reveal the
differences in technique followed in each CT department . For example, some
operators examine the upper abdomen in cases of hepatic and pancreatic disease,
whereas others examine the whole abdomen, which also includes the pelvic
region. According to Hidajat et al, many clinical studies have to be
performed so as to gain consensus for the optimal length of examination .
Use of anatomy-adapted tube current modulation
Tube current modulation is based on the idea that pixel
noise on the image results from quantum noise in the different projections
taken as the tube rotates around the patient . The value of mAs is
therefore changed during one rotation according to the patient anatomy in each
projection. The idea is similar to the automatic exposure control system in the
X-ray radiography equipment. In the projection with less attenuation from the
patient, such as the posterior-anterior chest projection, less mAs can be used.
In lateral projections in which attenuation from the patient can be high, the
mAs can be increased accordingly.
X-ray filters are used in radiology for cutting off the
X-rays that have lower energy and do not contribute to the image but only to
the patient dose. There are studies in the literature that have investigated
the use of various filters and their effect on dose reduction [53, 55].
According to these studies, bow-tie or beam shaping filters reduce radiation
dose by 50% compared with conventional flat filters. Software noise reduction
filters is an alternative, especially in high contrast examinations such as
chest CT. Kalra used such filters for post-processing reduced radiation dose
chest CT images and found improved levels of noise in the lung, mediastinum and
chest walls with some small compromise in image sharpness and contrast .
Diagnostic Reference Levels
CTDI and DLP measurements should be part of the dose
optimisation programme in a CT department. Determination of local DRLs should
be done using a sample of 10 standard-sized (70 kg) patients in each type of
procedure and mean values of the results should be compared to DRLs set by
professional bodies [22, 31]. In the case of local values being higher than
internationally set DRLs, corrective action should be applied after detailed
investigation and thorough revision so as to reduce patient doses if deemed
necessary. The procedure should be repeated in certain time intervals as part
of the established quality assurance program, or when new techniques or new
equipment are introduced in the department.
Optimisation of PET-CT dose
In most PET-CT examinations, the quality of CT does not
need to be in the level of the diagnostic CT. The reason is that CT images are
just used to produce the attenuation correction maps needed for PET images.
Therefore, lower exposure factors could be used such as lowering the mAs to
70-90 mAs. It should be noted however, that in some situations artefacts are
possible when using low mAs values due to photon starvation effects. CT
optimisation should be done carefully so as not to produce artefacts to the
images. When implants were present, artefacts will most probably be present, so
attention should be drawn to the correct interpretation of PET-CT images.
Another possible cause of artefact could be the use of iodine-based contrast
materials. The reason is that, at PET energies (511 keV), the iodine
attenuation coefficient is close to that of water and this can cause artefacts
in PET images. In this case, inspecting both the CT and PET images could help
overcome this problem [57, 58].
�Training of staff
It was recently found that the concept of CT radiation
dose is not fully understood or appreciated . Therefore, the requesting
physicians must be adequately informed of all associated risks when requesting
a CT scan and must know how to balance the benefits and possible risks.
Furthermore, the radiologists must be familiar with modification techniques
according to the patient�s clinical situation and not simply apply set clinical
New technological improvements such as the
multidetector-row CT and the PET-CT systems opened the field for new and wider
applications. Dose data in the literature indicate that manufacturers are
focussing their efforts towards improving image quality with reduced radiation
dose compared to older generation equipment, recognising the fact that dose
reduction has, in recent years, been an important issue for users. However, the
new technology CT can be operated so easily and quickly compared to previous
years and tempts operators to overuse the modality. It should be also stressed
that CT cancer screening studies are steadily increasing with the introduction
of multidetector-row scanners posing substantial risk for lung cancer
development from yearly screening and lower risk for colon screening due to
shorter screening intervals . In these cases, radiation dose optimisation
is essential because it results in a 5 to 10 reduction factor. It is also
widely recognised that the exposure factors applied are usually higher than
actually required for getting an image with diagnostic confidence. Continuing
development in scanner technology will no doubt further extend the indications
for and scope of CT examinations. Ongoing clinical studies monitoring the
associated patient dose can play a role in achieving excellent imaging at a
reasonable patient dose. Initial steps for dose optimisation could include:
- Clear justification of examination
- Avoid repetition of examination
- Use of tube current modulation
- If the clinical situation and the pathology of the patient permit,
increase the pitch of examination.
- In the case of scanners that do not have tube current modulation,
modification of exposure parameters should be done. The easiest way could be to
modify mAs based on the patient weight.
- Special exposure parameters should be determined for children.
- Proper shielding of organs
- Limitation of scan length
- Use of special filtration
- Measurement of CTDI and DLP in all types of CT examinations. Compare
with proposed DRLs. Repeat dose measurements in certain time intervals or when
new techniques or new equipment are introduced. In the case of dose results
that are higher than DRLs, corrective action should be applied.
- Close and frequent literature research should be done to detect any
- In the case of PET-CT, modification of exposure factors such as the mAs
should be done. The modification should be carefully done so as to avoid the
introduction of artefacts in PET images.
- In the case of conventional CT IR procedures, lower mAs during the
position of the needle if the clinical situation permits (size and position of
lesion) could be used.
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|Received 8 January 2007; accepted 21 January 2007
Correspondence: Medical Physics Unit, Konstantopoulio Agia Olga Hospital, 1 Ifaistou Str., 14569 Anixi, Athens, Greece. Tel.: +30-210-8131052 ; Fax: +30-210-8131052 ; E-mail: email@example.com (Virginia Tsapaki).
Please cite as: Tsapaki V, Rehani M,
Dose management in CT facility, Biomed Imaging Interv J 2007; 3(2):e43
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