DOI:10.2214/AJR.07.2556
AJR 2008; 190:335-343
© American Roentgen Ray Society
Radiation Dose Reduction in Chest CT: A Review
Takeshi Kubo1,2,
Pei-Jan Paul Lin1,
Wolfram Stiller3,
Masaya Takahashi1,
Hans-Ulrich Kauczor4,
Yoshiharu Ohno5 and
Hiroto Hatabu6
1 Department of Radiology, Beth Israel Deaconess Medical Center, Boston,
MA.
2 Present address: Department of Diagnostic Imaging and Nuclear Medicine, Kyoto
University, Kyoto, Japan.
3 Department of Medical Physics in Radiology, German Cancer Research Center
(DKFZ), Heidelberg, Germany.
4 Department of Radiology, German Cancer Research Center (DKFZ), Heidelberg,
Germany.
5 Department of Radiology, Kobe University Graduate School of Medicine, Kobe,
Japan.
6 Department of Radiology, Brigham and Women's Hospital, 75 Francis St., Boston,
MA 02115.
Received May 13, 2007;
accepted after revision August 15, 2007.
Address correspondence to H. Hatabu
(hhatabu{at}partners.org).
Abstract
OBJECTIVE. This article aims to summarize the available data on
reducing radiation dose exposure in routine chest CT protocols. First, the
general aspects of radiation dose in CT and radiation risk are discussed,
followed by the effect of changing parameters on image quality. Finally, the
results of previous radiation dose reduction studies are reviewed, and
important information contributing to radiation dose reduction will be
shared.
CONCLUSION. A variety of methods and techniques for radiation dose
reduction should be used to ensure that radiation exposure is kept as low as
is reasonably achievable.
Keywords: cancer risk • chest CT • CT • radiation dose • radiation safety
Introduction
CT is a powerful tool for the examination of chest disease because it can
depict the disease process far more clearly than chest radiographs. Technical
developments in CT scanners have enabled larger volume coverage with higher
resolution and lower noise, but this has led to increased radiation exposure
[1–4].
Studies in the United States, United Kingdom, Germany, and Japan have shown
approximately twofold increases in the number of CT examinations performed
between the late 1980s and the early 2000s
[5–8].
The contribution of CT examinations to the collective dose from diagnostic
radiation exposure is estimated to be 67% in the United States and 47% in the
United Kingdom [5,
9]. Currently, the issue of
radiation dose reduction draws wide attention. However, application of
reduced-dose CT techniques in clinical practice varies among institutions
[10,
11], which illustrates the
lack of a standard protocol for effectively reducing radiation dose to
patients in clinical settings.
The objective of this article is to present the available data on reducing
radiation dose exposure in routine chest CT protocols. First, important
aspects of radiation dose in CT will be discussed, followed by a review of
previous studies of CT radiation dose reduction. Finally, important techniques
and factors that contribute to radiation dose reduction in CT will be
outlined.
Radiation Dose Issues in CT
Several points must be considered regarding CT radiation dose. First, there
is an argument that radiation exposure in medical imaging has a significant
impact on cancer risk related to radiation exposure. It is reported that
exposure to ionizing radiation during diagnostic imaging may be responsible
for 0.6–3.2% of malignant tumors in 15 developed countries
[12], and CT examinations are
responsible for most of the collective patient dose. Note, however, that the
increased risk of malignancy related to radiation exposure is based on
estimates derived from mathematic models and not on true epidemiologic proof
that diagnostic radiation caused malignancy. Some argue that the method used
by Berrington de Gonzalez and Darby
[12] overestimates the
mortality risk related to diagnostic radiology
[13].
Second, a large variation in CT scanning parameters and radiation dose
exists in chest CT
[14–17].
According to Diederich and Lenzen
[17], tube voltage for typical
standard chest CT protocols ranged from 120 to 140 kVp and current–time
product (mAs) ranged from 100 to 533 mAs. These facts mean that there is large
variation in scanning parameters and consequently in the patient dose
delivered in chest CT.
Finally, we must understand correctly that CT examinations involve much
more radiation exposure than radiographic examinations. Unfortunately, both
patients and physicians often do not have correct knowledge about the
radiation exposure caused by CT examinations
[18,
19]. Lee et al.
[19] showed that all patients
and more than 70% of physicians in the emergency department underestimated the
radiation dose in one abdominal CT examination (equivalent of 100–250
chest radiographs).
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Fig. 1A —62-year-old man with pleural effusion. Standard-dose
(A, 150 mA) and reduced-dose (B, 50 mA) CT images at same level.
In 50-mA image, increased mottles and black streaks over mediastinum and soft
tissues on right lung are noticeable. SD of CT attenuation in region of
interest measured on ascending aorta (20 x 20 pixels) was 8.3 H for
150-mA image and 13.9 H for 50-mAs image.
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Fig. 1B —62-year-old man with pleural effusion. Standard-dose
(A, 150 mA) and reduced-dose (B, 50 mA) CT images at same level.
In 50-mA image, increased mottles and black streaks over mediastinum and soft
tissues on right lung are noticeable. SD of CT attenuation in region of
interest measured on ascending aorta (20 x 20 pixels) was 8.3 H for
150-mA image and 13.9 H for 50-mAs image.
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General Strategy for Radiation Dose Reduction in CT Examinations
Because radiation dose is determined by many factors
[20,
21], there are various ways to
reduce the radiation dose in chest CT. CT scanning parameters that can be
changed in clinical practice are summarized in Appendix 1. Lowering tube
current or tube voltage is the most direct way of achieving radiation dose
reduction. Because tube current is easier to modify and the result is more
predictable, modification of tube current is most widely used. Tube voltage
reduction is particularly useful when patient body diameter is small or when
the contrast of target objects is high. Higher helical pitch can contribute to
radiation dose reduction by reducing exposure time, although the effect on
image quality depends on the type of scanner used. In axial scans, radiation
dose is closely related to section thickness and section spacing.
Individualization of scanning parameters can also reduce radiation dose
through optimization of dose because patient size varies greatly. This is most
important to avoid overexposure in the examinations of children and small
adults [22].
The advent of automatic exposure control for CT scanners has eased the task
of scanning parameter individualization and enabled dynamic modulation of tube
current [23]. Some image
processing parameters that have a primary effect on image quality can also
have an influence on radiation dose. The use of reconstruction algorithms and
image filters can change the radiation dose needed to obtain clinically
acceptable CT images.
Needless to say, radiation dose reduction should be attained without
deterioration in the quality of the examination. In general, image noise is
inversely proportional to the square root of the radiation dose (Fig.
1A,
1B). Consequently, reduced-dose
CT images have a higher noise level than standard-dose CT images, and care
must be exercised to ensure that the former remain suitable for diagnosis.
Direct comparison between reduced-dose and standard-dose CT images of the same
patient may address this issue. Comparisons of image quality or diagnostic
results between standard-dose and reduced-dose CT images have been performed
to investigate the feasibility of reduced-dose CT protocols.
Reduction of Tube Current
Modification of tube current is the simplest method of radiation dose
reduction and has been a mainstay of radiation dose-reduction methods.
Assessment of image quality in reduced-dose CT has been reported in several
articles
[24–28]
(Table 1). Overall impression
of image quality, visualization of structures in the lung, level of noise
(grainy or mottled), and severity of artifacts are assessed with scores and
compared between reduced-dose and standard-dose CT images. The results of the
studies indicate that current–time product can be reduced (from the
typical 200 mAs) to 110–140 mAs without significant degradation of image
quality
[25–27].
In one article, standard-dose CT images obtained at 400 mAs were compared with
reduced-dose CT images obtained at 200, 140, 80, and 20 mAs
[25]. The images obtained at
200 and 140 mAs showed no significant difference in quality compared with
standard-dose CT images. In another study, comparison of images obtained with
40–280 mAs was performed in terms of perceived level of mottle
[26]. The authors concluded
that the dose could be reduced to 120 mAs without compromising image
quality.
These results suggest that a higher tube current–time product beyond
the 110- to 140-mAs range will not translate into significant improvement of
perceived image quality, although image noise will decrease further as
radiation dose increases. Therefore, current–time product above the 110-
to 140-mAs range may be unnecessary for routine chest CT. Most studies of
reduced-dose CT below this mAs level have shown that the perceived image
quality of reduced-dose CT is less than that of standard-dose CT
[25,
28–31],
although some studies cited much lower acceptable mAs values
[24,
32,
33]. These discrepant results
most likely originate from differences in scoring criteria, which is the major
limitation of studies using qualitative image analysis.
Assessment of Diagnostic Quality
Comparison of diagnostic results between standard-dose CT images and
reduced-dose CT images has also been used to evaluate the diagnostic quality
of reduced-dose chest CT images. Clinically speaking, diagnostic quality is
more important than image quality in itself; as long as we can reach the same
diagnosis with both standard-dose and reduced-dose CT images—even if the
quality of reduced-dose CT image appears inferior.
Evaluation of the diagnostic quality of reduced-dose CT in general has been
attempted [25,
28,
31]. Two studies showed no
statistically significant difference in the detection of abnormalities between
standard-dose and reduced-dose CT
[25,
28]. On the other hand, one
article suggested decreased diagnostic quality of reduced-dose CT
[31]. In this study, 150
series of images obtained with a standard protocol of 200–320 mAs at 120
kVp were compared with simulated low-dose images (40 and 100 mAs)
[31]. The authors concluded
that both 40- and 100-mAs images had significantly lower detectability of
lesions than standard-dose CT. Clearly, no consensus exists regarding minimal
clinically acceptable scanning parameters for chest CT in general, which
complicates clinical implementation of radiation dose reduction in chest CT
examinations.
Evaluation of Diagnostic Quality for Pulmonary Nodule Detection
There is an alternative approach to evaluating diagnostic quality of
reduced-dose CT images. Assessments of diagnostic quality of reduced-dose CT
images for specific clinical indications have been undertaken by many groups,
particularly detection of nodules in low-dose CT
[34–42]
(Table 2). These studies
suggest that a current–time product of 50–20 mAs was sufficient
for the detection of pulmonary nodules
[34–38].
Most lung cancer screening programs using CT use a tube current–time
product in this range
[43–49].
More aggressive reduction of radiation dose down to 5 and 6 mAs was also
investigated with favorable results
[39,
40]. However, very low mAs
settings should be used cautiously because there have been reports of
decreased nodule detectability in images obtained with less than 20 mAs of
current–time product
[41,
42].
Radiation Dose Reduction in the Evaluation of Pulmonary Embolism
Pulmonary embolism is a major health problem that is estimated to affect
575,000 patients in the United States each year
[50]. CT pulmonary angiography
(CTPA) has a high negative predictive value for pulmonary embolism and is
replacing pulmonary angiography
[51,
52]. The number of CTPA
examinations has increased because of its noninvasiveness and ready
availability, but the rate of positive CTPA examinations has reportedly
diminished to 5.7% in 2003
[53]. In other words,
detection of one pulmonary embolism takes approximately 17 times more CTPA
examinations with negative findings, thereby inflating the cost of radiation
exposure for the diagnosis of pulmonary embolism. Radiation dose reduction in
examinations for possible pulmonary embolism cases is therefore important.
Tack et al. [54] reported that
images produced with 17.5 effective mAs were sufficient for the detection of
pulmonary emboli. They compared image quality and radiation dose between a
standard-dose protocol (140 kVp and 175 mAs) and a low-dose protocol (100 kVp
and 125 mAs). They showed that a radiation dose reduction of 67% was achieved
and depiction of peripheral pulmonary artery was improved
[55]. From these results,
there seems to be room for radiation dose reduction in CTPA, although it
remains to be seen whether the low tube voltage protocol is applicable to
large patients.
Reduced-Dose CT for Other Clinical Indications
Applications of reduced-dose CT for specific clinical indications other
than detection of pulmonary nodules have also been reported
(Table 3). Reduced-dose CT was
reported to be useful in follow-up chest CT of oncology patients
[30,
56]. Yamada et al.
[30] noted no difference in
terms of detectability of abnormal findings between standard-dose (140 kVp, 96
mAs) and low-dose (140 kVp, 45 mAs) CT images. Chiu et al.
[56] found almost perfect
concordance in image interpretation between standard-dose (120 kVp, 240 mAs
with contrast enhancement) and reduced-dose (140 kVp, 43 mAs without contrast
enhancement) CT. Dinkel et al.
[29] investigated CT in
follow-up studies of lymphomas and extrapulmonary primary tumors using a
low-dose protocol (15 mAs at 120 kVp) and a standard-dose protocol (150 mAs at
120 kVp). Although disease conspicuity decreased in the lung apex and
mediastinum, detectability of lesions was not affected in the reduced-dose CT
images.
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TABLE 3: Studies of CT Examinations with Reduced Tube Current–Time Product
for Specific Clinical Indications Other Than Pulmonary Nodules
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There have also been reports of successful application of reduced-dose CT
for evaluation of asbestosis
[57], emphysema
[58,
59], bronchiectasis
[60,
61], and pulmonary embolism
[54], citing a
current–time product of 17.5–70 mAs as an adequate parameter for
these indications.
Reduction of Tube Voltage
Tube voltage (peak kilovoltage) reduction is used less frequently than tube
current modification because there are some limitations. Tube voltage changes
are limited; usually, users can select from several preset peak kilovoltages
(typically 80, 90/100, 120, or 135/140 kVp). Delicate adjustment of radiation
dose cannot be achieved through manipulation of peak kilovoltage. Substantial
decrease in radiation dose can occur by selecting a lower tube voltage
setting. For example, 17% peak kilovoltage reduction from 120 to 100 kVp will
result in a greater than 30% dose reduction
[21] because radiation dose is
proportional to peak kilovoltage to the power of more than 2
[62]. Moreover, a large
decrease in radiation dose is inevitably accompanied by a considerable
increase in image noise that can necessitate a compensatory increase in tube
current. Reduction of peak kilovoltage from 120 to 80 kVp requires an almost
fourfold increase in tube current to keep image noise constant
[63]. Therefore, modifications
of two factors (kVp and mAs) are usually involved in tube voltage
reduction.
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Fig. 2 —Effect of helical pitch on radiation dose. A and
B, Schematic presentations of scanning with standard (A) and
high-pitch (x2) (B) protocols. Total scanning time (i.e.,
exposure time) for high-pitch protocol is half that of standard protocol.
Consequently, total radiation dose will be halved in high-pitch protocol and
tube current will be kept constant.
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Despite these limitations, tube voltage reduction may be useful for
selected applications. Decreased beam energy augments contrast created with
the injection of iodine-containing contrast media because of the K-absorption
edge of the iodine [64]. For
contrast-enhanced chest CT, a reduced-dose protocol using a low tube voltage
(80 kVp and 135 or 180 mAs) has been reported to compare favorably with
standard-dose protocol (120 kVp and 90 mAs)
[65]. Radiation dose was
estimated to be reduced 56% and 41% with these two low-tube-voltage protocols.
A recent study showed that pulmonary CT angiography performed with 100 kVp
showed improved visualization of subsegmental arteries while radiation dose to
the patient decreased significantly
[55]. Reduced peak kilovoltage
contrast-enhanced CT has also been applied to abdominal CT examinations
[66,
67].
Low tube-voltage radiography may be useful for the detection of calcified
tissue. One group of investigators reported a 57% dose reduction using 80 kVp
instead of 120 kVp with almost the same accuracy of calcium detection
[68].
Dion et al. [69] applied a
low-tube-voltage protocol (100 kVp, 200 mAs) to examine smaller (< 55 kg)
adult patients and showed image quality comparable to that of standard-voltage
(120 kVp, 200 mAs) images and reduced-tube-current (120 kVp, 100 mAs) images.
Tube voltage reduction may be applied to small adult patients and
children.
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Fig. 3 —Functions of automatic exposure control. A, Exposure
adjustment to overall size of patient's body. B, Modulation of
z-axis: tube current occurs between rotations as patient couch moves.
C, In angular modulation, tube current is modulated during one tube
rotation to account for attenuation inconsistencies at different angles of
gantry rotation.  = location of X-ray tube in gantry measured as angle
from arbitrary place.
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Helical Pitch
Helical pitch (beam pitch) is defined as table increment (table feed) per
gantry rotation divided by the X-ray beam width
[70]. In principle, raising
the pitch shortens the total scanning time and consequently decreases the
radiation dose, which is inversely proportional to pitch
[71]
(Fig. 2). However, the
implication of helical pitch modification is not the same for single-detector
CT and MDCT scanners. Because image noise stays constant, change to a higher
helical pitch in single-detector CT can be used as a method for radiation dose
reduction, especially in pediatric CT examinations in which reduction of total
scanning time is also beneficial
[72,
73]. On the other hand, on
MDCT scanners, higher pitch generally leads to increased noise. In fact, some
CT scanner models compensate automatically for the increase in noise by
increasing current–time product
[74]. Therefore, increasing
pitch is not a practical method of radiation dose reduction, at least not in
MDCT.
Length of Scan
The longitudinal length of the scan should be kept to a minimum because
total radiation dose (CTDIDLP: dose–length product) delivered
to the patient increases linearly as total scan length is increased. Inclusion
of upper abdominal and supraclavicular regions in the chest CT protocol would
be meaningful for patients with malignant disease because these areas may
harbor metastatic lesions. However, in examinations of noncancer patients,
obtaining extra scans outside the lung should be avoided because they will
lead to significant increases in radiation dose without resulting in
clinically relevant information
[75].
Individualization of Scanning Parameters
Image quality depends largely on the size of the patient. Therefore,
application of a single protocol is inherently inefficient. Large patients
will have images of poor quality and small patients will have excessive
exposure to radiation. Therefore, scanning parameters must be adjusted
according to body size. Methods of parameter adjustment have been studied by
several groups
[76–82].
These studies suggest that body weight is the best index of body size for
parameter adjustments. A linear formula balancing tube current and body weight
has been proposed to obtain consistent image quality
[76,
80]. Body circumference has
also been reported to be a useful guide for scanning parameter modification
[63].
Automatic Exposure Control
Automatic exposure control, which has been an essential function in
conventional radiography systems, is now becoming an important function in CT
scanners [23]. The automatic
exposure system in CT scanners essentially provides programmed dynamic
adjustment of the tube current. Tube current is adjusted to achieve consistent
image quality between patients and within a single patient. Because CT images
are degraded by underexposure but not by overexposure, there is a strong
tendency toward overexposure. Therefore, automatic exposure control can
contribute to dose reduction in CT examinations, although it can increase
radiation dose in large patients who would suffer underexposure with a
standard protocol.
Three types of automatic exposure control work on different levels
(Fig. 3): exposure adjustment
to the overall size of the patient's body, exposure adjustment along the
craniocaudal axis of the patient (z-axis modulation), and exposure
adjustment during gantry rotation (angular modulation). Usually, scan
projection radiographs (also variously known as Scanogram [Toshiba], ScoutView
[General Electric], Topogram [Siemens Medical Solutions], Surview [Philips
Medical Systems], and so forth) are obtained in one or two projections to
estimate the attenuation value of the patient, which is used to adjust tube
current. Adjustment to the overall size of the patient is made to diminish the
difference in noise among patients of different body sizes, thus contributing
to individualization of the tube current. Modulation of the z-axis
enables variable tube currents between rotations as the patient couch moves so
that each axial slice has almost constant mean image noise. Angular modulation
adjusts tube current in one tube rotation to account for attenuation
inconsistencies at different angles of gantry rotation. To use automatic
exposure control, users specify parameters such as "noise index,"
"reference mAs," or "reference image" to obtain images
of desired quality, instead of setting actual current–time product.
Automatic exposure control techniques have been shown to be effective in
radiation dose reduction
[83–88].
For chest CT examinations, a 22% radiation dose reduction was reported with
angular modulation [85], and a
26% radiation dose reduction was reported with z-axis modulation
[88], without significant
changes in image quality.
Users of automatic exposure control should remember that target image
quality must be selected by users. If the user-specified image quality setting
is higher than is actually needed, overexposure to the patient ensues as a
result of increased X-ray output by the scanner. Dose reduction with automatic
exposure control is achieved only when the user selects the image quality
appropriate to the clinical purpose of the examination.
Imaging Filters
Imaging filters are software applications designed to improve image quality
by removing noise and artifacts. When filters work effectively, it is possible
to reduce radiation dose without affecting image quality. There are two types
of imaging filters: spatial domain filters, which manipulate data in the
reconstructed images, and raw-data based filters, which modulate the data in
raw-data (projection-data) domain before reconstruction. Kalra et al.
[89] used six spatial
domain–based filters for reduced-dose chest CT images and showed that
for all filters, improved noise level was achieved at the cost of loss of
sharpness. Therefore, these filters will be applicable to images reconstructed
for soft-tissue interpretation when sharpness of images is not critical,
although the role of the filters is limited for lung parenchymal
interpretation because of loss of image sharpness.
Application of raw-data-based filters to reduced-dose chest CT has also
been reported [90,
91]. The advantage of this
type of filter is less degradation of spatial resolution, which is important
in diagnosis of lung parenchyma
[91]. Raw-data-based filtering
seems to contribute to dose reduction by suppressing the streak artifacts that
can be prominent in reduced-dose CT.
Simulation of Reduced-Dose CT Images
Images of reduced-dose CT can be simulated by adding noise to standard-dose
CT images [92,
93]. Simulated images can be
used to investigate the effect of parameter change in CT examinations without
giving additional radiation to the patients. Mayo et al.
[92] tested the feasibility of
computer-based noise simulation technique, which was subsequently used in a
clinical dose reduction study
[31]. Simulation techniques
were also used for the assessment of reduced-dose pulmonary arteriograms for
the evaluation of pulmonary embolism
[54] and emphysema
[59]. Dose simulation
technique is also useful in user education of the CT scanners, making
selection of the parameters easier.
Conclusion and Future Directions
In this article, we covered topics radiologists must know in order to
reduce radiation dose in chest CT. Because this review is not intended to be a
systemic review of the topic, the references cited in this article may not
reflect all issues related to radiation dose of chest CT. Readers are advised
to refer to other articles for comprehensive coverage of the issue of
radiation dose in CT [1,
2,
4].
Because the number of CT examinations is increasing rapidly, radiation dose
reduction is a task that needs urgent attention. Radiation dose adjustment
according to specific indication of the examination and patient size plays a
pivotal role in radiation dose reduction in CT. Appropriate use of
reduced-dose protocols for common clinical indications requires further
investigation. Automatic exposure control facilitates radiation dose
adjustment according to patient size and optimizes radiation dose within a
single patient with dynamic tube current adjustment, enabling further
reduction of radiation dose. Conceivably, benefits of radiation dose reduction
in CT may include the use of saved radiation doseto acquire more information
such as organ perfusion or motion analysis with fast volume coverage using
MDCT scanners. Radiation dose reduction is key in the pursuit of novel
applications of MDCT.
Medical personnel involved in radiologic imaging should be familiar with
the variety of methods and techniques for radiation dose reduction to ensure
that radiation exposure is kept as low as reasonably achievable.
Acknowledgments
The authors thank Sumiaki Matsumoto and Shiva Gautam for manuscript
preparation and Donna Wolfe and Alba Cid for editorial assistance.
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Abstract
Introduction
