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1 All authors: Department of Radiology, SUNY Upstate Medical University, 750 E. Adams St., Syracuse, NY 13210.
Received October 23, 2000;
accepted after revision January 22, 2001.
Address correspondence to W. Huda.
Abstract
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SUBJECTS AND METHODS. Ten sets of CT images were obtained from patients undergoing CT-guided chest biopsies. For each patient, six images of the same region were obtained at settings between 40 and 280 mAs. CT data were used to reconstruct tomographic sections with a field of view limited to the normal contralateral lung. Images were printed using lung and mediastinal image display settings. Image quality was determined by asking radiologists to assess the perceived level of mottle in CT images. Five chest radiologists ranked the relative image quality of six images. Patient effective doses were computed for chest CT examinations performed at each milliampere-second setting. Radiologists indicated whether any perceived improvement of image quality at the higher radiation exposures was worth the additional radiation dose.
RESULTS. The differences in quality of chest CT images generated at greater than or equal to 160 mAs were negligible. Reducing the radiographic technique factor below 160 mAs resulted in a perceptible reduction in image quality. Differences in CT image quality for radiographic techniques between 120 and 280 mAs were deemed to be insufficient to justify any additional patient exposure. However, the use of 40 mAs results in an inferior image quality that would justify increased patient exposure.
CONCLUSION. Radiographic techniques for unenhanced chest CT examinations can be reduced from 280 to 120 mAs without compromising image quality.
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Radiation doses in CT are well below the threshold doses for the induction of deterministic effects such as erythema and epilation [5]. Patient risks from chest CT examinations are therefore restricted to the stochastic processes of carcinogenesis and the induction of genetic effects. The radiation received by a patient undergoing any type of diagnostic radiologic examination is best quantified by the effective dose [6]. Typical values for the effective dose for patients undergoing chest CT examinations are about 5 mSv [7]. Patient doses may be lower if high-resolution CT is performed using thin sections [8], or higher if both unenhanced and contrast-enhanced images are generated.
For any CT examination performed at a fixed radiographic tube potential, the patient dose is directly proportional to the value for milliampere-seconds selected by the operator. The choice of milliampere-seconds also determines the amount of quantum mottle in the resultant image [9, 10]. High values for milliampere-seconds could result in the patient being overexposed, whereas low values could result in poor image quality, with a risk of the radiologist missing clinically important findings. According to the recommendations of the International Commission on Radiological Protection, patient doses in CT should always be kept as low as reasonably achievable [11].
In this study, we took advantage of CT images generated during biopsy procedures to investigate the relationship between dose and image quality in chest CT examinations. Biopsy procedures have a well-defined imaging task of identifying the biopsy needle relative to an identifiable lesion. This well-defined task permits the reduction of radiographic tube current value, at a constant scan time, for repeated sections acquired at the same patient location. Projection data acquired in this manner can thereby be used to reconstruct the normal contralateral lung to permit the systematic investigation of the effect of the tube current setting on patient dose and image quality.
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CT scans were all obtained at 120 kVp on a CT/i scanner (General Electric Medical Systems, Milwaukee, WI). Initial scanning was performed using 280 mA and a scan time of 1 sec, corresponding to a radiographic technique of 280 mAs. Scans were obtained using 5-mm collimation and a pitch of 1.5:1 to generate a set of four to five helical images using a 5-mm reconstruction interval and a "detail" reconstruction algorithm. The first such set was obtained at 280 mA, which is the standard for chest CT examinations at our institution. Subsequent sets were obtained at decreasing levels of 220, 160, 120, 80, and 40 mA in order to decrease the cumulative radiation dose in a repeatedly imaged region. Each set of images was assessed immediately after reconstruction by the radiologist performing the biopsy, so that if image quality were insufficient the set could be repeated before proceeding with the biopsy.
At the conclusion of the procedure, a representative section was identified at the same anatomic level in each set of images. This section was retrospectively reconstructed at a field of view that included the entire coronal dimension of the thorax. Additional processing was performed to produce an image with a reduced field of view that included the hemithorax contralateral to the site of biopsy, the mediastinum, and as little of the ipsilateral hemithorax as possible. Avoiding displaying the site of the biopsy ensured that an observer would not be able to identify the proper sequence of images by following the progress of the biopsy. These images were printed to a laser camera using a 3 x 4 format, in pairs consisting of lung (window width, 1500 H; level, -500 H) and mediastinal (window width, 450 H; level, 40 H) settings, one group with and another without annotation. Figure 1A,1B,1C shows a series of representative images obtained at 40, 160, and 280 mAs.
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Image Quality Versus Milliampere-Seconds
For each patient examination, six pairs of images lacking any annotation
were generated on two sheets of 14 x 17 film. Each pair was cut out and
placed in random order in an envelope, resulting in 10 envelopes containing
six pairs of images. Five interpreters, all board-certified radiologists who
routinely review chest CT images, were used in this study. The average number
of years of experience (after American Board of Radiology certification) for
the five observers was 29.2 years (range, 10-48 years). Each observer was
asked to order the six pairs of images in each envelope according to image
quality, from best to worst, which was completed in a single sitting. For each
observer, the average rank was computed for 10 images generated at each
technique factor. In this manner, average ranks for each observer were
obtained at each technique factor. Observers were also asked to explicitly
identify any images that were deemed to be of less than diagnostic
quality.
The expected rank of CT images obtained at 280 mAs would be 1, whereas the rank of images obtained at 40 mAs would be 6. An error score was computed as the absolute difference between the expected rank and the actual rank given by an individual observer for each image. In this manner, a rank of 3 for an image generated at 280 mAs would have an error score of 2, because an image generated at 280 mAs would be expected to have a rank of 1. Similarly, a rank of 3 assigned to an image generated at 40 mAs would yield an error score of 3. The total error rate was computed for images generated at each of the six tube current values by summing the errors obtained for each observer. A high error rate would indicate that observers had difficulty differentiating images at that setting, whereas a low error rate would indicate that the image quality could be readily determined.
Patient Doses and Risks
For a given patient undergoing a chest CT examination, the effective dose
depends on the choice of technique factors, including the peak kilovoltage,
milliampere-seconds, section thickness, and number of sections. The total mass
of the patient that is directly irradiated may be modeled as a cylinder of
water on the basis of the dimensions and mean Hounsfield unit values obtained
from representative axial CT images. This procedure permits the computation of
the mean patient dose and total energy imparted for a given chest CT
examination. At a fixed radiographic tube potential, the patient effective
dose is directly proportional to the selected tube current, the section
thickness, and the total number of sections obtained.
An adult patient having 43 sections, each of which is 7 mm thick, generated at 120 kVp and 280 mAs, will receive an effective dose of about 6.0 mSv [7]. Patient doses for conventional film-screen examinations in diagnostic radiology are 0.02-0.05 mSv for chest radiography, 0.1-0.2 mSv for skull radiography, 0.5-1.5 mSv for abdominal radiography, and 2.5-5.0 mSv for excretory urography [6]. Patients undergoing barium enemas receive doses between 3.0 and 7.0 mSv [6]. Head CT has an effective dose between 1 and 2 mSv [12], and the average nuclear medicine procedure has an effective dose of 5 mSv [13]. Therefore, chest CT doses are at the upper end of the range normally encountered in diagnostic radiology. Patient doses in diagnostic radiology can also be compared with the dose from the natural background in the United States (3 mSv/year), as well as with regulatory dose limits for radiation workers (50 mSv/year) and for members of the public (1 mSv/year).
The predominant risk to patients undergoing chest CT is the induction of cancer. Although some uncertainty exists about the radiation risks at the exposure levels normally encountered in diagnostic radiology [14], the best estimate currently in use for the general population is a 5% risk per sievert for cancer mortality [15]. An effective dose of 6 mSv for a chest CT scan thus corresponds to a nominal cancer fatality risk of approximately 3 per 10,000 patients. The risk from a typical chest CT scan can be compared with other everyday risks. An effective dose of 6 mSv is comparable to the risk of dying from lung cancer after smoking approximately 100 packs of cigarettes, or the risk of dying in an automobile when driving a distance of approximately 5000 miles [16]. Table 1 summarizes the patient effective doses for the range of technique factors used in this study and the corresponding radiation risk factors.
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Image Quality Versus Dose
For each patient, the image obtained at 160 mAs was used as a reference
image against which observers judged the image quality of the five remaining
images. The five remaining image pairs (280, 220, 120, 80, and 40 mAs) for
each patient were randomly selected for presentation to the observer. Each CT
image was clearly marked with the settings for milliampere-seconds and the
estimated patient dose for a full chest CT scan (see
Table 1). The observers were
explicitly informed that the pertinent image quality issue was mottle, which
depends only on the total number of X-ray photons used to generate the CT.
Each observer indicated whether any observed image quality difference between
the reference image and the selected image would merit the use of additional
radiation for a routine (unenhanced) CT examination to achieve the perceived
improvement in image quality. Observers were asked to make this assessment
with regard to the diagnostic quality of the images rather than the aesthetic
appearance. The six-point scoring scale used in this evaluation is listed in
Table 2. All observers
participating in this study were provided the summary statement about
radiation doses and risks in chest CT scans described previously in the
Patient Doses and Risks section.
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Radiologists participating in this study varied widely in their ability to differentiate the image quality of CT studies performed at different technique factors. Observer 1, for example, was almost perfect in ranking the images obtained at the six tube current values, whereas observers 3 and 4 clearly could not differentiate between images generated at 280 mAs and those generated at 220 mAs. All observers, however, had no difficulty in identifying the images obtained at 40 mAs as being the worst.
The average error rate for the five observers is plotted as a function of milliampere-seconds in Figure 3. The solid line is a least squares fit to a second order polynomial (r2 = 0.95), showing that the error rate reaches a plateau at values for milliampere-seconds greater than or equal to 160. The constant error rate at greater than or equal to 160 mAs implies that observers cannot differentiate between images generated at 160, 220, and 280 mAs. On the other hand, these data show that below 160 mAs, a monotonic reduction of the error rate is seen, implying that observers can differentiate between these images.
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Image Quality Versus Dose
On the basis of the ranking and error data we have described, 160 mAs for
the generation of a chest CT image was identified as the borderline technique
value. At a technique factor equal to or greater than 160 mAs, image quality
is similar; at less than 160 mAs, image quality is progressively identifiable
as inferior. Consequently, images generated at 160 mAs were used as reference
images in the second part of this study.
Figure 4 shows the average score for the five observers plotted against the technique factor for the five observers when they evaluated the quality of images obtained at specified values against the quality of images obtained at 160 mAs. Images generated at 120 mAs or greater were deemed to be similar in terms of mottle to the reference image obtained at 160 mAs. The radiologists in this study would not use additional radiation exposure to achieve this slight improvement in image quality at greater tube current values.
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At 80 mAs, a minor reduction in image quality was perceived, but the radiologists in this study did not generally consider that the difference in image quality would justify any increase in patient radiation exposure. Only on CT images obtained at 40 mAs was unambiguous evidence seen that the deterioration in image quality relative to the reference image was sufficient to warrant additional radiation exposure. A reduction of the tube current to 120 mAs for routine unenhanced chest CT examinations would appear justified and would reduce the typical adult effective dose from 6 to 2.6 mSv. This decrease corresponds to a 57% reduction in patient dose with no adverse impact on image quality.
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Image quality was ranked on a relative scale, an approach that has been shown to be an efficient method for identifying small differences in image quality [17]. No attempt was made to generate an absolute assessment of image quality except for a general assessment as to whether the images were of diagnostic quality. Evaluating CT image quality with more objective criteria can assess the ability to see second and third order bronchi and vessels on lung windows and the differentiation of soft-tissue structures (blood vessels and lymph nodes) within fat on mediastinal windows [18]. This approach was not used in this study because the approach does not explicitly refer to image mottle, and because this type of image quality metric often shows little correlation with patient radiation dose [19]. The use of a relative scale is further justified by the fact that the images being ranked were from the same patient. In addition, our study did not contain any pathologic abnormalities, and no attempt was made to assess the delineation of pathologic abnormalities.
Previous studies have clearly suggested that the radiographic exposure level as expressed by the tube current can be reduced when performing chest CT examinations [20]. The findings of our study are in good agreement with those of Mayo et al. [21], who found that reducing the tube current from 400 to 140 mAs did not significantly change subjective image quality or the detection of mediastinal or lung abnormalities. However, performance is generally task-dependent, and this study evaluated only the performance for unenhanced chest CT examinations. For contrast-enhanced images, further study would be required before any reduction in the level of X-ray exposure is considered.
Further reductions in tube current may be possible for cancer screening and high-resolution CT. In general, screening tasks use only lung window settings, where the perception of image mottle would be markedly lower than on the corresponding mediastinal window settings. Two recent studies indicate that it may be possible to use tube currents and scanning times that result in values between 20 mAs and 40 mAs when performing screening studies [22,23]. However, other experimental and clinical studies for the detection of pulmonary nodules have also shown that low-dose CT performed using a tube current of 50 or 25 mA could significantly impair the detection of nodules 5.5 mm or smaller [24].
Image quality per se is not the critical issue in diagnostic radiology, for which correct diagnosis is the ultimate goal. Additional radiation to improve image quality will not necessarily add to the diagnostic quality of the examination. Image quality may be perceptibly worse, particularly with increased noise, but if normal and abnormal structures can be easily identified, then the image is still of diagnostic quality. As an example, a recent study showed that although reducing the CT technique from 250 to 50 mAs resulted in noisy images, no significant difference was seen in the detection of lung and mediastinal abnormalities [25]. Also, the minimum tube current required for screening may be different for different locations in the lung [26].
In this study, the radiographic tube potential was kept constant, because a constant voltage is currently the normal mode of operating clinical CT scanners in North America. Changing the CT radiographic tube potential also needs to be considered in defining CT imaging protocols, which will affect both image contrast and noise [27]. The radiographic tube potential would ideally be altered to ensure that the contrast-to-noise ratio is adequate for a given imaging task. Changing the radiographic tube potential, however, will also have a significant effect on the patient dose. If the patient radiation dose is not a major factor, the optimum tube voltage should be the one that produces good quality images to avoid missing important diagnostic information. For high-dose procedures, or when exposing radiosensitive patients such as infants and pregnant women, a more careful evaluation of image quality and patient dose should be undertaken.
The risk estimates in Table 1 were computed on the basis of the accepted nominal risk coefficients used by the International Commission on Radiological Protection for radiation protection purposes [15]. Considerable uncertainty exists about the radiation risks associated with low-dose CT. Most published data for radiation-induced carcinogenesis have been obtained at relatively high organ doses, typically greater than 0.25 Gy, whereas individual organ doses in chest CT are much lower (<0.02 Gy). Current national and international bodies assume that low doses of radiation are associated with a radiation risk, and that the use of radiation must take this into account. As a result, any use of radiation in patients needs to be formally justified by ensuring that the individual patient will be expected to benefit from the information gathered. In addition, any radiation exposure should be minimized to ensure that the patient is not subjected to unnecessary exposure.
The most important aspect of this study was the attempt to explicitly address the issue of whether any perceived improvement of image quality at a higher radiation level is deemed to be worth the additional radiation dose received by the patient. Radiologists and other imaging professionals who are responsible for developing scanning protocols need to address the trade-off between patient dose and image quality for each diagnostic imaging study. This balancing of dose and image quality should be performed explicitly to ensure that patient doses are kept as low as reasonably achievable. Selection of technique factors in this manner must take into account the scanned population, because subgroups such as infants and pregnant women may be more radiosensitive than others. In addition, the optimum technique factor will depend on the specific task at hand. For example, low-dose CT is being promoted for general screening applications, whereas high-dose CT may be appropriate for the detection of some subtle diseases.
At our institution, we perform all chest CT at a fixed radiographic tube potential (120 kVp) and a fixed radiographic tube output (280 mA), together with a constant scan time (1 sec) and a section thickness of 7 mm. In practice, minor changes are made to the actual tube current because of the commercial system used (SmartScan; General Electric), which automatically adjusts the tube current depending on the dimensions of the body part being scanned. Our study results indicate that reducing the radiographic tube current to no more than 140 mA would be justified for performing adult chest CT examinations. The section thickness for routine chest CT examinations is 7 mm, whereas the section thickness in this study was 5 mm, which would imply that greater reductions in technique factors may be possible. We are therefore planning to reduce the chest CT technique factors used clinically in an incremental manner while the resulting image quality for all types of studies is monitored. In this manner, we will empirically determine whether the results obtained in this study are applicable for the whole range of CT chest studies, including contrast-enhanced and unenhanced studies. As a result, we expect to implement CT protocols that explicitly attempt to ensure that patient doses are kept as low as reasonably achievable without compromising diagnostic performance.
Acknowledgments
We appreciate the assistance of E. R. Heitzman, S. A. Groskin, A. S. Berne,
and P. A. Randall, who interpreted the CT images
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