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1 Division of Pediatric Radiology, 1905 McGovern-Davison Children's Health
Center, Box 3808, Department of Radiology, Duke University Medical Center,
Erwin Rd., Durham, NC 22710.
2 General Electric Medical Systems, 3000 N. Grandview Blvd., Waukesha, WI
53188.
3 Department of Radiology, Children's Hospital and Medical Center, 3333 Burnet
Ave., Cincinnati, OH 45229.
4 Department of Radiology, University of British Columbia and Vancouver Hospital
and Health Sciences Centre, 855 W. 12th Ave., Vancouver, Canada V5Z 1M9.
Received January 9, 2002;
accepted after revision April 12, 2002.
Address correspondence to D. P. Frush.
Abstract
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MATERIALS AND METHODS. After validating the computer-simulation technique with a water phantom, we subjected the original digital scanning data for 26 contrast-enhanced abdominal MDCT scans (120 mA) obtained in infants and children (age range, 1 month-9 years; mean age, 3.1 years) to simulated tube current reduction (100, 80, 60, and 40 mA) by adding noise. this procedure created four additional examinations per child that were identical to the originals except for image noise. The 130 examinations were scored randomly, independently, and without prior knowledge of the children's diagnoses by three radiologists for depiction of high-visibility structures, such as adrenal glands and fat in the intrahepatic falciform ligament, and low-visibility structures, such as the extrahepatic hepatic artery, small intrahepatic vessels, and common bile duct. Aligned rank and Wilcoxon's signed rank tests were used for statistical analyses.
RESULTS. Simulated tube current reduction significantly affected the detection of low-visibility structures (p < 0.001). Reduced detection in low-visibility structures was evident at a level less than or equal to 80 mA. No loss of detection in high-visibility structures was found at any tube current level (p > 0.5).
CONCLUSION. The results of this computer simulation suggest that accurate abdominal MDCT can be performed in pediatric patients using substantially reduced radiation, depending on the indication for imaging. (In our case, the reduction was between 33% and 67%, depending on whether a high-visibility or low-visibility structure was being assessed.) This simulation technology can be applied to MDCT of other organ systems for systematic evaluation of radiation dose reduction.
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A possible explanation for this lack of adjustment, which potentially results in unnecessary radiation exposure, is that relatively few guidelines have been established for performing multidetector CT (MDCT) in children [4, 5]; only recently has attention been given to size-based scanning [6]. Furthermore, little information exists on low-dose or reduced-tube-current scanning in children, with most of the available data being limited to chest CT [7,8,9,10]. To our knowledge, no systematic evaluation of dose reduction in abdominal MDCT in pediatric patients has been conducted, primarily because of ethical issues related to performing serial CT examinations or to experimenting with various CT parameters in scanning children [11].
One novel method of evaluating dose reduction was reported in 1997 by Mayo et al. [11]. In that investigation, data from single-detector helical chest CT scans in adults were modified by adding a controlled amount of noise, providing images that were identical to the actual scans except that the scans appeared as if they had been obtained at varying degrees of lower tube current. We modified the tube current simulation tool to evaluate tube current reductions, and thus radiation dose reduction, for abdominal MDCT in children. The specific paradigm we tested was to determine the effect of simulated dose reduction (increased image noise) on detection of low- and high-visibility structures in the abdomen.
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The indications for MDCT in the patients were cancer (n = 9), inflammation (n = 8), trauma (n = 2), and other (n = 7). The patients were 15 boys and nine girls (two children had two scans). The age range was from 1 month to 9 years (mean, 3.1 years). The difference between the lowest and highest weights was 35 kg (mean weight, 15.5 kg).
All CT scans were obtained on an MDCT scanner (LightSpeed QX/i; General Electric Medical Systems, Milwaukee WI). Scanning parameters were 120 mA, 140 kVp, 2.5-mm detector configuration, 15-mm rotation-1 table speed (high-speed mode), 2.5-mm slice thickness, and 0.8-sec gantry cycle (96 mAs). These parameters were identical for both phantom validation and computer simulations of the clinical CT scans. The dose of low-osmolar IV contrast material for each CT examination was 2.0 mL/kg, delivered by manual injection.
Simulation Technique
Our simulation technique uses a reduction model that adds a random gaussian
noise distribution to existing scanning data to simulate a reduction in tube
current (mA) [11]. This
simulation model does not alter the mean signal level. We added the noise on a
sample-by-sample basis, rather than using one factor for the entire
projection. In addition, the simulation model does not account for the
electronics noise floor and assumes that the projection noise is X-ray quantum
limited. However, this impact was negligible, given the relatively small
object size and tube current range in these pediatric examinations. The
equation used is
 |
is the ratio of the simulated tube current to the
original tube current (<1). Our simulations represented serial tube current reductions to 100, 80, 60, and 40 mA. All simulations were performed on an independent console at the manufacturer's site. Although tube current and gantry cycle time (mAs) are common descriptors for CT, we have chosen to discuss tube current alone because the simulations were based on changes in tube current only. The corresponding milliampere-second values were 96 (original CT data), 80, 64, 48, and 32, and the percentage of difference between tube current and milliampere-seconds remained the same. The lowest value chosen was 40 mA, which, according to recent weight-adjusted guidelines, represented the lowest tube current [6]. Combined with the original 26 CT scans obtained at 120 mA, these four simulated scans yielded five images per child, or a total of 130 (26 original + 4 simulations x 26 originals) CT scans for review.
Phantom Validation of Simulation Technique
Before applying the simulation technique to the original CT scans, we
performed a validation study using a 25-cm water phantom. The CT parameters
were identical to those used for the clinical MDCT scans. Field of view was 25
cm, with a 512 x 512 matrix. The phantom was scanned at 120, 100, 80,
60, and 40 mA. For the simulations, the data from the 120-mA scan was
processed to create simulated examinations at 100, 80, 60, and 40 mA (Fig.
1A,1B,1C,1D,1E).
A single mid-phantom slice obtained at the center point of the MDCT data set
was analyzed for the amount of noise (Table
1). A single circular region of interest of 10,000 mm2
was used at the center of the slice to minimize nonuniformity. Three separate
phantom CT scans were obtained at each tube current, and three simulations
were performed at each of the values (100, 80, 60, and 40 mA). Data from these
three data stes combined to yield a mean SD as a measure of noise.
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Clinical Evaluation of Simulation Technique
Each of the clinical MDCT scans was obtained using identical filtering and
an identical 12:1 format, with all annotation removed except the assigned
study code. The window and level settings were 350 and 40 H, respectively.
Simulated images were filmed at a 5-mm reconstruction interval, beginning with
the same initial image to be sure the image sequences within the set of
original and simulated scans were identical. This technique provided five
examinations per patient that were identical in every way except for the
addition of a predetermined amount of noise (Fig.
2A,2B,2C,2D,2E,2F,2G,2H).
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All CT examinations were randomized and reviewed independently by three pediatric radiologists who had expertise in pediatric body CT and who were unaware of the diagnoses of the patients. A scoring sheet was divided into categories of high-visibility and low-visibility structures. We developed our scoring method after reviewing another collection of CT images (Fig. 2A,2B,2C,2D,2E,2F,2G,2H) to arbitrarily define high visibility and low visibility. The term "high visibility" was used to designate a relatively large difference in attenuation between the structure under study and adjacent structures. The high-visibility structures consisted of fat in the intrahepatic course of the falciform ligament (intralobar fissure) and the adrenal glands (fat and soft-tissue attenuation differences). The term "low visibility" was used to describe either structures with a small difference in attenuation between these structures and the adjacent tissue, or structures of relatively small size, or both. Low-visibility structures consisted of the extraparenchymal course of the hepatic artery, the common bile duct, and the intraparenchymal hepatic vessels.
The number of images (usually sequential images) in an examination in which the structure was judged to be present were tabulated (except in the case of the intraparenchymal vessels). We instructed the reviewers to count only those images in which they were more than 50% certain that the structure under study was present. For evaluation of the intraparenchymal hepatic vessels, we initially selected a single image obtained at the inferior aspect of the liver where the course of the vessels tends to be predominantly vertical and the vascular structures are thus manifested on MDCT as round (rather than linear or branching) structures. In this largely vertical course, small punctate vessels are more difficult to identify than branching or linear vessels of the same caliber and attenuation. In addition, the vessels in this inferiorly obtained image tended to be more peripheral, hence smaller and less distinct than if the vessels were closer to either the portal vein or main hepatic veins. We selected this single image after subjectively assessing the original 120-mA MDCT examinations and before the pediatric radiologists made their blinded review of the original and simulated examinations. The image selected contained a range of vessel visibilities. The corresponding image was marked for each of the simulations, and the reviewers counted the number of vessels for that single image. Image and vessel counts for the three reviewers were combined, and a single mean value calculated for statistical analyses.
Statistical analyses consisted of the aligned rank test for investigating the differences in outcome variables (high- and low-visibility structures) as a function of tube current. Wilcoxon's signed rank tests were used to compare each of the individual simulated tube currents with the original 120-mA scanning data to determine the threshold tube current at which a significant difference for the variables was found.
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For the clinical evaluation, 130 MDCT scans (26 original examinations and four simulations for each original examination) were analyzed. The mean data on structure detection for the three reviewers are found in Table 2. No association of tube current reductions and detection of structures with high visibility (p > 0.05) was observed; the high-visibility structures were seen equally well at any level of tube current simulation. However, a significant association between tube current and detection of low-visibility structures was found.
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More important is the threshold computer-simulated tube current at which the individual structures become substantially more difficult to detect (Table 3). This threshold was 60 mA for the extraparenchymal hepatic artery and 80 mA for the common bile duct and for a number of intraparenchymal vessels. For example, at some point between 80 and 60 mA, the radiologists' ability to see the extraparenchymal hepatic artery as well as they could at 120 mA diminished. For purposes of clarity in our discussion, we will assume that a structure first became difficult to see at the level at which the difference was first noted—60 mA for the hepatic artery and 80 mA for the common bile duct and the intraparenchymal vessels.
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Another potential experimental paradigm for evaluating low-dose CT of the abdomen would be to randomly assign children to receive CT using one of several different tube currents. An experiment in which children are scanned using potentially suboptimal parameters poses an ethical problem. Additional problems with this study design are the potential contributions of differences in anatomy, physiology (with respect to IV contrast enhancement), and diseases or normal anatomic manifestations. Such factors would necessitate obtaining a large sample size for evaluating subtle differences attributable to the lower tube current alone. Although animals could be used, the results would not necessarily be directly applicable to CT in children. Animal investigations are also logistically difficult to perform on clinical CT scanners. The paucity of animal data applied to clinical CT supports the difficulties and limitations with this type of design. Our simulation technique allows these factors to be controlled, and its only effect is on the changes in image appearance that result from a lower radiation dose.
With the computer-simulation tool, we were able to systematically investigate the effect of lower tube current on structure detection in pediatric abdominal MDCT. We found that lowering tube current (and, correspondingly, radiation dose) to 67% of the tube current of the original MDCT scan did not affect the ability to detect high-visibility structures. Even tube current reductions of 33-50% were acceptable for detection of low-visibility structures.
To the best of our knowledge, ours is the only evaluation of low-dose CT of the abdomen in children. Only one other investigation concerning CT of a part of the body other than the chest in children was been conducted [15]. In that study, the impact of a decrease in tube current on image quality and diagnostic confidence was assessed for conventional pelvic CT in children. The authors concluded that pelvic CT can be performed at a substantially reduced radiation dose without loss of diagnostic quality. However, the two tube currents studied were 240 and 80 mA. The lower current was near the tube current level used for our highest tube current (96 mAs). We examined greater tube current reductions than those in the pelvic CT study. Our investigation is also, as far as we know, the first in pediatric CT literature to address the question of tube current reduction thresholds based on a measure of clinical interpretation, that of low- and high-visibility of structures.
One important limitation of our investigation is that, because it was a study of both a research tool and initial clinical applications, we did not evaluate actual lesion conspicuity. Although we reported only on children with normal anatomy, our results concerning high and low visibility of structures apply to abnormalities as well. Our findings that detection of high-visibility structures was not compromised—even with substantial reductions in tube current—show that MDCT of high-visibility structures, or gross abnormalities, could be performed at tube currents at least as low as 40 mA. For example, CT for a follow-up of an abscess or pancreatic pseudocyst in a child might be performed with a reduced radiation dose. Conversely, if low-visibility abnormalities are suspected, CT at 60-80 mA could be performed. These speculative scenarios might include suspected fungal microabscesses or metastases in solid organs [6]. An argument could be made that image quality may be compromised by lowering tube current and increasing noise; however, no data indicate that an unsuspected low-visibility disease occurs with sufficient frequency to warrant performing CT of the abdomen with a high tube current and high radiation dose in children. Moreover, existing guidelines support adjusting the CT parameters to fit the clinical indication in children [6]. Just as weight- or sized-based adjustments of CT parameters are appropriate in children, adjustments of parameters based on the clinical indications are appropriate.
Among the other limitations to our study is the fact that our technique is an investigational tool not available for routine use. It is only applicable to the type of scanner and the specific single set of MDCT parameters that we used. Work currently in progress indicates that similar mathematic constructs make possible simulation examinations using a wider variety of MDCT parameters, including different tube currents and kilovoltages. This development would allow more flexibility in studying different clinical applications in the pediatric population. We chose to perform MDCT at 120 mA (96 mAs), which is in line with tube current guidelines for single-slice abdominal CT in children in our study (mean age, 3.1 years; approximate weight, 15 kg) [6]. Therefore, we felt it unnecessary to begin the simulations using higher tube currents than were used for the original MDCT scans from which the simulations were obtained.
Another limitation is that we did not take into account differences in patient size for our simulations. An MDCT simulation at 40 mA of a 4-kg infant looks different in terms of noise than the same tube current MDCT simulation of a 40-kg child. Improved methods of addressing the size-specific appearances of MDCT simulations and lesion detection are currently being developed. An argument could also be made that differences in the administration of contrast medium or in breathing-related artifacts can affect the visibility of structures. However, the simulation model controls for these factors because the same enhancement and motion artifacts are present in each of the simulated groups; the only difference is noise.
An additional limitation is that determining which structures were high visibility and which were low visibility was an arbitrary distinction, and other indicators of scanning quality could be used [16]. We have been discussing radiation dose reduction only as it relates to tube current reduction. Obviously other parameters, such as slice thickness, table speed, kilovoltage, and gantry cycle time, can also can affect the radiation dose [17, 18]. Given these limitations, we believe that our simulation tool provides a carefully controlled and systematic method for investigation of radiation dose reduction for MDCT of the abdomen in children. Our study provides data that can be used to determine guidelines about detection thresholds.
In conclusion, computer-simulated tube current reduction is useful for the systematic evaluation of radiation dose reduction and structure visibility for abdominal MDCT in pediatric patients. Perhaps one of the more important applications of this simulation tool is its potential usefulness in other investigations. For example, this technology could be used in conjunction with simulation of solid-organ (e.g., liver) lesions. Variables such as lesion size or contrast enhancement could be controlled, and the effect of tube current reductions using simulations could be assessed. The simulation technology can also be applied to other organs, such as the chest (Fig. 3), or to organ systems, such as the musculoskeletal and central nervous systems, in an effort to help determine guidelines for tube currents and lesion detection in these organ systems. For example, with the increasing use of MDCT in screening for coronary artery disease, simulated tube current reductions may provide useful guidelines that could reduce radiation dose for a substantial segment of the population. With these applications, a comprehensive and systematic effort at controlling the amount of radiation that children (or adults) receive from MDCT can be addressed.
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