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DOI: 10.1148/radiol.2412050947
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(Radiology 2006;241:546-553.)
© RSNA, 2006

Technical Developments

Gallstone Detection at CT in Vitro: Effect of Peak Voltage Setting1

Wesley C. Chan, MD, Bonnie N. Joe, MD, PhD, Fergus V. Coakley, MD, Edwin L. Prien, Jr, MD, Robert G. Gould, PhD, Sven Prevrhal, PhD, William C. Barber, PhD, Kimberley S. Kirkwood, MD, Aliya Qayyum, MB, BS and Benjamin M. Yeh, MD

1 From the Departments of Radiology (W.C.C., B.M.Y., F.V.C., R.G.G., S.P., W.C.B., A.Q., B.N.J.) and Surgery (K.S.K.), University of California San Francisco, 505 Parnassus Ave, San Francisco, CA 94143-0628 and the Laboratory for Stone Research (E.L.P.), Waban, Mass. Received June 8, 2005; revision requested August 2; revision received September 29; accepted October 14; final version accepted January 9, 2006. Address correspondence to B.M.Y. (e-mail: ben.yeh{at}radiology.ucsf.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
This study was a retrospective single-institutional study approved by the Committee on Human Research and was HIPAA compliant. A waiver for informed consent was granted. The purpose of the study was to evaluate the effect of four peak voltage settings on the in vitro conspicuity of gallstones in an anthropomorphic phantom at computed tomography (CT). An anthropomorphic phantom was scanned with (n = 86) or without (n = 85) gallstones at CT by using 80, 100, 120, and 140 kVp. The sensitivity for gallstone detection was significantly higher at 140 kVp (86% [74 of 86] for reader 1 and 81% [70 of 86] for reader 2) than at lower voltage settings (up to 67% [58 of 86] for reader 1 and 63% [54 of 86] for reader 2, P < .05 for each reader), regardless of gallstone size (<1.0 cm vs ≥1.0 cm in diameter, P < .05 for each reader). CT attenuation measurements were not useful for determination of gallstone composition. Abdominal CT performed at 140 kVp may be useful when gallstone disease is of clinical concern.

© RSNA, 2006


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Adjustment of computed tomographic (CT) scanning parameters affects the diagnostic quality of CT images. Changes in CT x-ray tube potential (voltage) alters tissue contrast on the resultant images (1), including the measured CT attenuation values of bone (2), calcified pulmonary nodules (3), and kidney stones (4–6). Furthermore, alterations of voltage have been shown to change the conspicuity and CT attenuation of kidney stones (4–7), and such changes in CT attenuation have been shown to be useful in characterizing the composition of kidney stones (8,9).

Gallstones affect approximately 25 million adults in the United States (10). Unfortunately, current diagnostic modalities for depiction of gallstones are imperfect. Although ultrasonography (US) is highly sensitive for depiction of gallstones in the gallbladder, limited acoustic access results in a sensitivity as low as 25% for the direct depiction of stones in the common bile duct (11,12), which often results in a delay in diagnosis and treatment (13). The definitive imaging modality for the diagnosis of common duct stones, endoscopic retrograde cholangiography, is invasive and associated with major complication rates of 1.4% or higher (14,15). Although CT is known to have a limited sensitivity of 25%–88% for direct depiction of gallstones (12,16–19), in the United States CT is often the first imaging examination performed in patients with abdominal pain or pancreatitis (13). Improvement in the sensitivity of CT for gallstone detection would be of great clinical benefit.

To our knowledge, all previously published studies of gallstone evaluation at CT have used a fixed voltage setting. It is unknown whether CT voltage settings influence CT detection and characterization of gallstones. Therefore, we undertook this preliminary study to evaluate the effect of voltage setting on the in vitro conspicuity of gallstones in an anthropomorphic phantom at CT.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Gallstones
Our study was a retrospective single-institutional study approved by our Committee on Human Research and compliant with requirements of the Health Insurance Portability and Accountability Act. Patient consent was not required. Our Committee on Human Research authorized us to retrospectively retrieve gallstones, which are held in temporary storage in our pathology department as a part of routine patient care. No gallstones were retrieved from a special tissue bank or collection. Only the minimum required demographic information (patient age and sex) was recorded for our study.

In total, 86 gallstones were obtained from 36 patients who had undergone laparoscopic cholecystectomy between March and June 2004. No more than four gallstones were taken from any individual patient. The patients consisted of 22 women and 14 men aged 25–91 years (mean age, 50.5 years). The gallstones were washed with normal saline and stored in airtight tubes filled with 10% neutral buffered formalin. Each gallstone was measured in three orthogonal dimensions by using measurement calipers, and the mean of these three measurements was considered to be the physical mean diameter of that gallstone. The physical mean diameter of the gallstones ranged from 0.33 to 2.43 cm (mean ± standard deviation, 1.05 cm ± 0.48). Gallstones were classified as small (physical mean diameter, <1.0 cm) or large (physical mean diameter, ≥1.0 cm). The gallstones were also determined to be of cholesterol (n = 71; mean diameter, 1.13 cm) or pigment (n = 15; mean diameter, 0.66 cm) composition by using Fourier transform infrared spectroscopy (E.L.P., more than 20 years of experience with the technique) (Fig 1).


Figure 1
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  		Figure 1: Assortment of gallstones scanned in our study. Marked gallstones (*) are pigment gallstones. All others are cholesterol gallstones.

 
CT Phantom
The gallstones were washed with normal saline and individually placed into a water-filled elliptical plastic phantom (QCT PRO QA; Mindways Software, Austin, Tex), which measured 27 x 21 cm. This phantom contains three airtight chambers (one posterior and two lateral) and one open chamber (anterior) (Fig 2). The posterior chamber was filled with dipotassium hydrogen phosphate at a concentration of 50 mg/mL water to simulate bone (20). The two lateral chambers were filled partly with water and partly with air to simulate bowel. The phantom was fixed to the CT table with medical tape.


Figure 2
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  		Figure 2: Transverse CT image of anthropomorphic phantom obtained at 120 kVp. Anterior chamber (1) was filled with saline, and gallstones could be placed within. Lateral chambers (2) were sealed and filled partly with water and partly with air to simulate bowel, and posterior chamber (3) was sealed and filled with dipotassium hydrogen phosphate solution (50 mg/mL) to simulate bone.

 
The open anterior chamber was 4.5 cm long, was filled with saline, and served as the chamber into which gallstones were placed. This chamber was divided into three 1.5-cm sections (superior, middle, and inferior). Into each section, zero or one gallstone(s) was placed at any one time. Individual gallstones were placed in the phantom such that a minimum of zero and a maximum of three gallstones were present in the phantom for any given scan. Only gallstones from the same patient were placed inside the phantom simultaneously. Gallstones from different patients were not placed in the phantom at the same time. In cases in which fewer than three gallstones were in the phantom, the sections of the chamber to contain gallstones and the sections to remain empty were chosen at random.

CT Protocol
The phantom was scanned with an eight–detector row CT scanner (LightSpeed; GE Healthcare, Milwaukee, Wis) with a 5-mm section thickness, 5-mm intervals, a table-top speed of 27.5 mm per rotation, a gantry rotation time of 0.8 second, reconstruction type set to standard, and reconstruction mode set to full. Each set of gallstones in the CT phantom was scanned four times with x-ray tube potential (kilovolt-peak) and tube current (milliampere) set sequentially as follows: 80 kVp and 400 mA, 100 kVp and 220 mA, 120 kVp and 205 mA, and 140 kVp and 165 mA. Accordingly, each stone was scanned four times, once with each of the four different settings and with identical table positions for each scan.

The x-ray tube current was chosen for each voltage setting such that the noise in the resulting image was similar for each of the voltage settings. At 80 kVp and 400 mA, the noise (the standard deviation of voxel attenuation values in the anterior chamber) was 10.50 HU; at 100 kVp and 220 mA, the noise was 10.20 HU; at 120 kVp and 205 mA, the noise was 9.57 HU; and at 140 kVp and 165 mA, the noise was 9.62 HU. There was no significantly statistical difference in the level of background noise between the four groups (analysis of variance t test, P = .60). We chose to adjust the tube current to maintain similar image noise so as not to unblind readers to the voltage setting with obvious differences in image quality and because software in most modern CT scanners modulates tube currents automatically to approximate a defined level of image noise and thereby decrease patient radiation dose without unduly sacrificing image quality (21). The dose-length product (surrogate measure of radiation dose) and the dose index volume for each CT setting were recorded from the CT scanner console (LightSpeed; GE Healthcare) by two authors (W.C.C., B.M.Y.). Each time a new peak voltage was selected, a different CT series was created.

In total, for each voltage setting, the phantom was scanned 57 times. The phantom contained zero stones at seven scans, one stone at 24 scans, two stones at 16 scans, and three stones at 10 scans. Empty chamber sections, as well as phantom scans with zero stones, served as negative controls. As there were three sections for possible gallstone placement each time the phantom was scanned, there were 171 total sections for gallstone placement per voltage setting. Eighty-six of these sections contained gallstones, and 85 contained only saline.

Image Analysis
The 228 CT series (57 scans for each of the four voltage settings) were randomized by assigning each series a random number then re-sorting the series by using the assigned numbers. Two attending radiologists (B.M.Y., B.N.J., 4 years and 3 years of subspecialty body CT experience, respectively) independently viewed all images on a picture archiving and communication system workstation (IMPAX, version 4.5; Agfa, Mortsel, Belgium). Both radiologists were fellowship trained in cross-sectional abdominal imaging. The readers were unaware of CT acquisition settings, such as voltage setting, tube current, and time of examination. All images were initially screened at a window setting of 350 HU and level of 50 HU, but the readers were allowed to dynamically adjust the window and level settings of the images. The readers evaluated the CT images of each of the three sections (superior, middle, inferior) of the anterior chamber in the phantom for each of the 228 CT scans and recorded the presence or absence of a visible gallstone. For each observed gallstone, the readers also recorded the CT attenuation of the gallstone (in Hounsfield units) by using a large elliptical region of interest contained within the gallstone and encompassing as much of the gallstone as possible without including the walls of the phantom in the region of interest.

Statistical Analysis
Statistical analysis was performed with software (Stata, version 8.0; Stata, College Station, Tex) for Windows (Microsoft, Redmond, Wash). Intraobserver agreement for the detection of stones was analyzed with the {kappa} statistic, which was interpreted as follows: 0–0.2, slight agreement; 0.21–0.4, fair agreement; 0.41–0.6, moderate agreement; 0.61–0.8, substantial agreement; and 0.81–1, almost perfect agreement (22). CT attenuation measurements of readers 1 and 2 were correlated by using the pairwise correlation coefficients.

The sensitivity and specificity for gallstone detection was determined for each reader at each voltage setting, and subgroup analysis was performed for gallstones of different size and composition. Ninety-five percent confidence intervals were determined by using exact intervals. The random effects model for binary data used to assess for determinants of true-positive and true-negative gallstone detection for each reader accounted for voltage setting, the number of stones in each phantom, and the fact that some gallstones were from the same patients. This model for estimation was accomplished by using generalized estimating equations. Because the total numbers of gallstones and of empty sections were the same for each voltage setting, the true-positive findings were proportional to the sensitivity and true-negative findings were proportional to the specificity. A test for linear trend in proportions was used to assess for an increase or decrease in true-positive findings (sensitivity) or true-negative findings (specificity) as voltage setting increased. For each voltage setting and each reader, mean CT attenuation of cholesterol gallstones was compared with that of pigment gallstones by using the paired t test. Multiple linear regression analyses incorporating voltage setting and gallstone composition in the models were performed to determine predictors of CT gallstone attenuation. For all tests, P ≤ .05 was considered to indicate a statistically significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Gallstone Detection
For readers 1 and 2, the sensitivity for detection of gallstones (Fig 3, Table 1)—measured by using generalized estimating equations with a test for linear trend in proportions—was greater at higher voltage settings (P < .005 for reader 1, P < .001 for reader 2) than at lower voltage settings. Results of similar analysis showed that the specificity for gallstone detection was greater at higher voltage settings (P < .01 for reader 1, P < .05 for reader 2) than at lower voltage settings.


Figure 3
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  		Figure 3: Transverse CT images of gallstones scanned at 80, 100, 120, and 140 kVp in phantom with window setting of 350 HU and level of 50 HU. Images were cropped to focus on anterior chamber. A, Images of pigment gallstone demonstrate higher CT attenuation at 140 kVp than at lower settings. B, Images of cholesterol gallstone demonstrate CT attenuation that is lower than that of saline at 80 kVp, but higher than that of saline at 140 kVp. C, Images depict pigment gallstone (4 mm) at 140 kVp but not at lower settings. Gallstones in 140-kVp column are indicated by arrowheads.

 

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  		Table 1. Sensitivity and Specificity for Detection of Gallstones at Four Peak Voltage Settings at CT

 
Table 2 shows the sensitivity and specificity for detection of gallstones with a physical mean diameter of 1.0 cm or larger and of those with a physical mean diameter smaller than 1.0 cm. Results of generalized estimating equations demonstrated that sensitivity for large gallstones was higher than that for small gallstones for reader 1 (P < .05) but not for reader 2 (P = .113) and that gallstone composition (Table 3) was not predictive of sensitivity for either reader (P = .086 for reader 1, P = .826 for reader 2).


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  		Table 2. Sensitivity and Specificity for Detection of Gallstones at Four Peak Voltage Settings at CT according to Gallstone Size

 

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  		Table 3. Sensitivity and Specificity for Detection of Gallstones at Four Peak Voltage Settings at CT according to Gallstone Composition

 
Interobserver Agreement
Overall interobserver agreement between readers 1 and 2 for determination of the presence or absence of a gallstone was substantial ({kappa} = 0.66; 95% confidence interval: 0.601, 0.717). The interobserver agreement for individual voltage settings was moderate to substantial ({kappa} = 0.61 and 95% confidence interval of 0.50, 0.73 for 80 kVp, {kappa} = 0.67 and 95% confidence interval of 0.55, 0.79 for 100 kVp, {kappa} = 0.62 and 95% confidence interval of 0.50, 0.75 for 120 kVp, and {kappa} = 0.71 and 95% confidence interval of 0.60, 0.81 for 140 kVp). The correlation between gallstone CT attenuation for reader 1 (mean, 44.3 HU; range, –37.2–299.2 HU) and that for reader 2 (mean, 42.6 HU; range, –32.0–297.0 HU) was excellent (r = 0.83, P < .001).

Gallstone CT Attenuation and Composition
The mean CT attenuation of cholesterol gallstones was significantly lower than that of pigment gallstones at all voltage settings (P < .01 for each reader for each voltage setting), but substantial overlap of CT attenuation values between cholesterol and pigment gallstones was seen (Fig 4). Results of multiple linear regression showed that increasing voltage settings (P < .05 for reader 1, P < .01 for reader 2) and pigment gallstone composition (P < .005 for both readers) were both independent predictors of higher CT attenuation of gallstones.


Figure 4
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  		Figure 4: Graphs of CT attenuation and gallstone composition. Mean CT attenuation of cholesterol gallstones was lower than that of pigment gallstones for all peak voltages (P < .01 for each pairwise comparison for each reader), but extensive overlap of CT attenuation values was seen between cholesterol and pigment gallstones.

 
Radiation Dose
The CT dose index volume was 6.88, 6.58, 11.37, and 11.47 mGy, and the dose-length product was 264.43, 252.88, 437.17, and 440.75 mGy · cm for CT x-ray tube potential settings of 80 kVp and 400 mA, 100 kVp and 220 mA, 120 kVp and 205 mA, and 140 kVp and 165 mA, respectively.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 References
 
We found that the sensitivity for in vitro gallstone detection is significantly higher (81%–86% vs 52%–67%) when CT images of an anthropomorphic phantom are obtained with a high x-ray tube potential of 140 kVp rather than with a lower x-ray tube potential. To our knowledge, our study is the first to demonstrate the effect of adjusting voltage settings for gallstone detection at CT. This finding is of potential clinical interest because most centers routinely perform CT examinations of the abdomen by using a fixed x-ray tube potential of 120 kVp (16,23), whereas other institutions routinely use 140 kVp (24,25). Potentially, a minor adjustment in CT technique may allow for improved detection of cholelithiasis or choledocholithiasis at CT.

An important drawback of the use of CT for gallstone evaluation is the need for ionizing radiation exposure. In particular, use of higher voltage settings without a corresponding decrease in tube current (in milliamperes) results in improved image noise but excessive radiation exposure because radiation dose increases exponentially with increases in voltage setting and linearly with increases in the tube current (21). This consideration is especially relevant for the pediatric population, for which higher radiation doses are more harmful than those for adults (26). To address this concern of radiation dose, all major CT manufacturers routinely provide the option for automatic tube current modulation that dynamically adjusts the tube current to maintain a relatively constant image noise level throughout the scanning and between patients as a means to avoid excessive radiation exposure (21). Similarly, the tube currents in our study were adjusted such that images obtained at each of the four voltage settings had a similar noise of approximately 10 HU, which has been shown to be that of a good quality abdominal CT image (21). Had we not adjusted the tube current with the voltage, the quality of the images obtained with high voltage settings would have been substantially better than that of the images obtained with low voltage settings and would have introduced image noise and reader bias as confounding factors.

Although our adjustment of tube current eliminated noise as an explanation for differences in gallstone detection, we noted a nearly twofold increase in estimated radiation dose with an increase in x-ray tube potential from 80 to 140 kVp. Notably, however, we found little difference in radiation dose when the phantom was scanned at 120 kVp and 205 mA compared with that at 140 kVp and 165 mA (dose-length product, 437.17 and 440.75 mGy · cm, respectively). The nonlinear change in dose between 100 and 120 kVp (252.88 to 437.17 mGy · cm) may be the result of broader differences in the noise index between these two potentials relative to that between the other potentials. Our results, which are similar to previously published results of studies with kidney stones (7), show that use of higher tube potential with appropriate reduction in tube current can improve the sensitivity of CT for gallstone detection without substantially increasing radiation.

A second concern about the use of a higher x-ray tube potential at abdominal CT is that such settings may decrease image contrast for soft-tissue organs (1). However, the assumption that clinical CT diagnostic accuracy is improved at lower voltage settings has not, to our knowledge, been supported with empiric evidence, and some medical centers perform all routine adult abdominal CT examinations at 140 kVp (24,25). Therefore, when possible stone disease is of clinical concern, use of a 140-kVp setting may be considered. Further research into the poorly understood effect of CT x-ray tube potential for evaluation of clinical disease should also be encouraged.

Our finding that gallstones are best detected at higher voltage settings may seem counterintuitive because tissue contrast generally worsens as voltage increases (1), and because CT attenuation of other stones such as kidney stones has been shown to decrease as voltage increases (4–6). This latter phenomenon occurs because as the mean incident x-ray energy (in kiloelectron volts) increases from 80 to 140 kVp and moves away from the k-edge of calcium (4.0 keV), the frequency of photoelectric effects in a calcified stone decreases, which decreases the stone's CT attenuation (7). When the effect of adjusting the voltage setting on CT detection of kidney stones was studied in vitro, however, a result similar to ours was found in that the sensitivity of CT for detection of kidney stones was higher at 140 kVp than at lower CT x-ray tube potentials (7).

This increase in detection of stones at higher voltage settings may be explained on the basis of the broader range of x-ray energies delivered with higher mean x-ray energy. In general, higher energy x-rays are more likely to pass through water, cholesterol, calcium phosphate, and calcium carbonate, which decreases the absolute x-ray attenuation of all of these substances for higher energy x-rays. The absolute attenuation of higher energy x-rays of water, however, decreases more than that of a gallstone because water blocks relatively fewer higher energy x-rays than does the gallstone; both water and a gallstone become darker, but water becomes relatively more dark. The measured CT attenuation of water does not change appreciably because all clinical CT scanners are calibrated by the manufacturers such that water is assigned an attenuation value of approximately 0 HU for all voltage settings.

In our study, we found that while pigment gallstones have a significantly higher mean CT attenuation than that of cholesterol gallstones, there is extensive overlap in the CT attenuation values between the two types of gallstones. The inability of CT to reliably differentiate cholesterol from pigment gallstones was likely because of the presence of varying amounts of calcium in both types of gallstones (7,27–30). Rarely are gallstones composed of purely cholesterol or pigment. It is likely that magnetic resonance (MR) determination of gallstone composition, which has been reported to be up to 95% specific (31,32), is better than that of CT determination (30).

Our study has several limitations. First, although we evaluated gallstones in an anthropomorphic phantom, our study remains an in vitro rather than an in vivo study. As such, evaluation of gallstones in our study was not impaired by motion artifact, dense biliary sludge, or a nonuniform shape of the gallbladder (28). The position and size of the gallstones in our phantom study were known and were scanned in the same configuration for each of the four x-ray tube potential settings. This eliminated partial volume averaging as a concern because the extent of partial volume averaging was identical for all settings—a controlled scenario not feasible in a study of living patients.

Second, our study analysis was on a per stone basis rather than on a per patient basis, and this likely decreased our observed sensitivity for gallstone detection. In an in vivo setting, patients are likely to have multiple stones, which would increase the likelihood that at least one of the multiple stones would be seen.

Third, our study was not restricted to gallstones of pure composition (pure cholesterol vs pure pigment gallstones), which limited our ability to evaluate whether CT is able to distinguish gallstones of pure composition. Accurate detection of gallstones, however, is of much greater clinical value than determination of composition because current medical and interventional therapy is largely independent of gallstone composition.

Fourth, the gallstones of our study were stored in 10% neutral buffered formalin, which may have changed the gallstone CT attenuation. Native bile, which is the storage medium of choice, was not used because of lack of availability. Nevertheless, past studies have shown stones stored in formalin to have fewer changes in CT attenuation than those stored in saline or air (33).

Fifth, our study was performed in only one phantom; hence the influence of body habitus was not evaluated. Last, our study did not evaluate the effects of intravenous or oral contrast material on gallstone detection at higher voltage settings, and therefore our results are most applicable to CT without contrast material. In this regard, results of prior reports have shown that nonenhanced CT is superior to enhanced CT with intravenous contrast material for gallstone detection (13). Further work will be required to determine whether changes in voltage setting would improve rates of gallstone detection when intravenous contrast material is given.

Notwithstanding these limitations, our findings show that the sensitivity of CT for detection of gallstones is highest when CT is performed at 140 kVp rather than at lower voltage settings. Because the first examination performed for upper abdominal pain is often CT rather than US or MR imaging, a small adjustment in a nonenhanced CT scan parameter may be useful in increasing sensitivity for gallstones. In vivo studies will be needed to confirm that our findings are of clinical benefit.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
References
 


   ACKNOWLEDGMENTS
 
We thank Bruce H. Hasegawa, PhD, and Mindways Software, Austin, Tex, for invaluable assistance with building the CT phantom and the Laboratory for Stone Research, Waban, Mass, for expertise and use of facilities for determination of gallstone composition. We thank Yanjun Fu, PhD, and the laboratory of Robert C. Brasch, MD, at the University of California San Francisco for assistance with handling of gallstones.


   FOOTNOTES
 
Author contributions: Guarantors of integrity of entire study, W.C.C., B.M.Y.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, W.C.C., B.M.Y., R.G.G.; clinical studies, W.C.C., B.M.Y., K.S.K.; experimental studies, W.C.C., B.M.Y., E.L.P., R.G.G., W.C.B., B.N.J.; statistical analysis, W.C.C., B.M.Y.; and manuscript editing, W.C.C., B.M.Y., F.V.C., E.L.P., R.G.G., S.P., A.Q., B.N.J.

Authors stated no financial relationship to disclose.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 

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S. W. Anderson, E. Rho, and J. A. Soto
Detection of Biliary Duct Narrowing and Choledocholithiasis: Accuracy of Portal Venous Phase Multidetector CT
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