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Experimental Studies |
1 From the Department of Radiology, New York University Medical Center, 560 First Ave, New York, NY 10016. From the 2005 RSNA Annual Meeting. Received September 5, 2006; revision requested November 8; revision received December 7; final version accepted January 8, 2007. Address correspondence to B.A.B. (e-mail: bernard.birnbaum{at}nyumc.org).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEIMPLICATION FOR PATIENT CAREReferences |
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Materials and Methods: A customized anthropomorphic phantom was created to accept interchangeable 40-, 140-, and 240-HU renal inserts that contained stacked 0- and 50-HU cylindric cysts measuring 7, 10, and 15 mm in diameter. Each phantom and insert was scanned with five different multidetector CT scanners on five separate occasions by using 120 kVp, low and high tube current settings, 3.00–3.75-mm collimation, and standard and high-spatial-resolution kernels. A total of 2340 CT attenuation measurements were obtained by using standardized regions of interest. The effect of multidetector CT imaging regimen, tube current, cyst diameter, and renal attenuation on pseudoenhancement incidence was assessed by using generalized estimating equations based on a binary logistic regression model. Within this framework, a Bonferroni multiple comparison correction was used to assess pseudoenhancement frequency differences among imaging regimens.
Results: Pseudoenhancement occurred in both 0- and 50-HU cysts; was significantly correlated with multidetector CT imaging regimen (P < .0001), cyst diameter (P < .0001), and renal attenuation (P 
.032); and was independent of tube current (P > .3). When convolution kernels on specific scanners were compared, significant differences (P < .04) between kernels were identified with all five scanners in terms of observed pseudoenhancement incidence. Generational differences in equipment were noted, with pseudoenhancement incidence ranging from 1.7% to 8.3%, 1.7% to 16.7%, and 18.3% to 56.7% across relevant kernels for three scanners from one manufacturer.
Conclusion: Pseudoenhancement is strongly dependent on multidetector CT convolution kernel. Varying this parameter may mitigate this phenomenon, which is independent of volume-averaging effects.
Supplemental material: http://radiology.rsnajnls.org/cgi/content/full/244/3/767/DC1.
© RSNA, 2007
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEIMPLICATION FOR PATIENT CAREReferences |
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10 HU) after intravenous contrast material administration (1,2). The absence of tissue enhancement is a critical diagnostic criterion, because enhancement of a renal lesion indicates that the lesion is vascular, and tumor neovascularity is assumed unless available imaging findings or clinical information suggest otherwise. Small, benign intrarenal cysts may demonstrate an artifactual increase in computed tomographic (CT) attenuation when helical CT scanning is performed during high levels of renal parenchymal enhancement (3–9). While this increase in cyst attenuation is most commonly caused by partial volume averaging with adjacent enhanced renal parenchyma, results of phantom studies (3,5,6) that incorporate cylindric renal cyst design have shown that this pseudoenhancement effect may be attributable to an artifact other than volume averaging alone. As a result, renal cyst pseudoenhancement may be observed despite the use of optimal thin-section helical CT data acquisition techniques in which section thickness measures less than 50% of lesion diameter (7). This phenomenon is clinically relevant because it may complicate helical CT differentiation of small renal cysts from neoplasms.
It has been hypothesized that the pseudoenhancement effect is primarily related to an inadequate CT algorithmic correction for beam hardening (3). Thus, the purpose of our study was to prospectively determine the dependence of renal cyst pseudoenhancement on multidetector CT scanner type and convolution kernel in a phantom model.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEIMPLICATION FOR PATIENT CAREReferences |
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Phantom Design
Renal cyst pseudoenhancement was studied by using a solid anthropomorphic CT phantom (Computerized Imaging Reference Systems, Norfolk, Va) that was specifically designed to accept "variably enhanced" interchangeable renal inserts that comprised the left kidney and a small amount of perinephric fat (Fig 1). The phantom core measured 30 x 23 x 7.3 cm and was constructed by using multiple permanent "tissue-equivalent" visceral, soft-tissue, and osseous components. The phantom components were designed to simulate potential parenchymal enhancement if a moderate-sized (<90 kg) patient underwent helical CT of the abdomen performed with both intravenous and oral contrast material (7). The phantom core was designed to accept three newly designed renal inserts that measured background attenuation values of 40 HU (unenhanced kidney), 140 HU (moderately enhanced kidney), and 240 HU (maximally enhanced kidney). Each customized insert contained tissue-equivalent material of appropriate cylindric shape and mass-attenuation coefficient to simulate stacked 7-, 10-, and 15-mm-diameter cylindric cysts that measured 50 and 0 HU at the medial and lateral aspects of the kidney, respectively (Fig 1). The hyperattenuating (50-HU) and simple (0-HU) cysts were arranged in cyst pairs (intrarenal cyst cylinders of the same diameter) that were equally distributed along the z-axis of the renal insert.
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Multidetector CT Imaging
The abdominal phantom was imaged with each of five multidetector CT scanners on five separate occasions (25 total scanning sessions; temporal scanning range, 6–36 days). The CT systems included three four-section multidetector scanners (Volume Zoom, Siemens, Forcheim, Germany; LightSpeed QX/i, GE Healthcare; and MX8000 Quad, Philips Medical Systems, Best, the Netherlands), a 16-section multidetector scanner (Sensation 16; Siemens), and a 64-section multidetector scanner (Sensation 64; Siemens). All five scanners were calibrated according to the manufacturer's specifications immediately before each phantom data acquisition with a complete calibration scan.
The phantom was appropriately centered and positioned on the CT table before each scanning session. Data acquisition sessions included three groups of scan sequences to enable the 40-, 140-, and 240-HU renal inserts to be individually scanned within the phantom for a given set of scan parameters (Table 1). We chose to evaluate the B40, B46, and B70 convolution kernels on the three Siemens scanners, the standard and Bone Plus kernels on the GE scanner, and the B and EC kernels on the Philips scanner. Tube current varied between 50 and 200 mAs on the Siemens and Philips scanners and from 26.7 mAs (50 mA) to 106.7 (200 mA) on the GE scanner. Automatic dose modulation software was not used in this experiment. A total of 130 phantom data sets were acquired. This number represents the product of scanning the phantom with 13 imaging regimens (fixed combinations of multidetector CT scanner and convolution kernel) at two tube current settings and studying each multidetector CT scanner five times.
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A total of 2340 individual ROI measurements were acquired for this study. This was accomplished by generating 90 tissue-specific ROIs for both the 0- and the 50-HU cylindric cysts, which were studied across 13 imaging regimens. Cyst enhancement was determined at background renal attenuation levels of 140 and 240 HU by measuring the difference in cyst attenuation values between the 140- and 40-HU renal insert images and between the 240- and 40-HU renal insert images, respectively. This method ultimately provided 60 enhancement level observations for each imaging regimen for both the simple and the hyperattenuating cysts.
Statistical Analysis
Generalized estimating equations based on a binary logistic regression model were used to evaluate the effect of renal cyst diameter, tube current, background attenuation, and multidetector CT imaging regimen on the incidence of pseudoenhancement (the percentage of times pseudoenhancement was observed). Data for the 0- and 50-HU renal cysts were analyzed separately. In each case, the binary indicator of pseudoenhancement (ie, whether enhancement of 10 HU occurred) was the dependent variable and the model included cyst diameter (7, 10, and 15 mm), tube current (low and high settings), background attenuation (140 and 240 HU), and imaging regimen as fixed classification factors. The covariance structure was modeled by assuming data to be correlated or independent when derived from the same or from different imaging sessions, respectively. Wald type 3 P values were used to assess the effect of each fixed-effect factor on pseudoenhancement incidence. Results were considered to be statistically significant at a two-sided 5% significance level. All statistical computations were performed by using software (SAS for Windows, version 9.0, 2002; SAS Institute, Cary, NC).
Within the generalized estimating equations framework, a point estimate and 95% confidence interval for the incidence of pseudoenhancement associated with each cyst diameter, tube current setting, background renal attenuation, and imaging regimen were computed for both the 0- and 50-HU renal cysts. Pairwise comparisons among imaging regimens with respect to the incidence of pseudoenhancement were made after the application of a Bonferroni correction to maintain the family-wise type I error rate for the set of comparisons at or below the 5% level.
Summary statistics for the distribution of renal cyst enhancement were determined by assessing the number of times cyst enhancement was observed within selected intervals for both the 0- and the 50-HU renal cysts at each imaging regimen. Enhancement thresholds that provided 98% specificity for renal cyst pseudoenhancement were determined for each imaging regimen. Specificity curves were plotted for each imaging regimen with both 0- and 50-HU cyst enhancement data. Specificity was determined as follows: A renal cyst was classified as an enhancing cyst if the level of pseudoenhancement observed for the cyst was greater than that of some specified threshold T. The specificity associated with each possible choice of threshold T was defined as 100% minus the percentage of times a cyst was classified as enhancing as a result of having an observed level of pseudoenhancement greater than that of T. Specificity curves show the observed specificity, averaged across the 0- and 50-HU cysts, as a function of the threshold T.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEIMPLICATION FOR PATIENT CAREReferences |
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.06).
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.032). Evaluation of the observed pseudoenhancement data for each cyst type across all relevant imaging regimens revealed that the incidence of pseudoenhancement increased as renal attenuation increased (Table 2). As background renal attenuation increased from 140 to 240 HU, pseudoenhancement frequency significantly increased from 13.1% to 17.9% for 0-HU cysts (P = .032) and from 11.0% to 20.5% for 50-HU cysts (P < .0001).
Tube Current
Pseudoenhancement data provided in Table 2 reveal that the incidence of pseudoenhancement was not influenced by tube current setting. As tube current varied from low to high settings, pseudoenhancement incidence increased from 14.9% to 16.2% for 0-HU cysts and decreased from 16.9% to 14.6% for 50-HU cysts; however, neither change was statistically significant (P > .3).
Imaging Regimen
Imaging regimen was significantly associated (P < .0001) with pseudoenhancement incidence for both simple and hyperattenuating renal cysts. When convolution kernels on specific multidetector CT scanners were compared, significant differences (P < .04) between kernels in terms of the observed incidence of pseudoenhancement for 0-HU cysts were identified on all five scanners. Significant differences (P < .001) between kernels in terms of the observed incidence of pseudoenhancement for 50-HU cysts were identified on the Sensation 64 and MX8000 scanners (Table 3). Varying the convolution kernel between the B40, B46, and B70 settings on the Volume Zoom and Sensation 16 scanners and between the standard and Bone Plus settings on the QX/i scanner had no significant effect on the observed incidence of pseudoenhancement for 50-HU renal cysts (P > .2).
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The B46 and B70 kernels on the Sensation 64 scanner were associated with the highest incidence of pseudoenhancement observed in this study. These results were significantly higher (P < .024) than those observed for all other imaging regimens for the 50-HU renal cysts and across all other regimens (P < .01), except the QX/i scanner with the Bone Plus kernel (P > .7), for 0-HU renal cysts. For both the 0- and the 50-HU renal cysts, the B40 kernel on the Sensation 64 scanner was associated with a significantly higher incidence of pseudoenhancement (P < .04) than was the B40 kernel on the Volume Zoom scanner. For 50-HU cysts, the B40 kernel on the Sensation 64 scanner was associated with a significantly higher incidence of pseudoenhancement (P = .047) than was the B40 kernel on the Sensation 16 scanner.
The EC kernel on the MX8000 scanner was the only convolution kernel that was not associated with pseudoenhancement (Tables 3 and 4). While the MX8000 imaging regimen with the EC kernel was associated with a significantly lower incidence of pseudoenhancement (P < .001) than was the MX8000 regimen with the B kernel, the overall performance of the MX8000 regimen with the EC kernel was statistically indistinguishable from that of other imaging regimens that performed well in this study (eg, Volume Zoom regimens with B40 or B46 kernels). With regard to the distribution of renal cyst enhancement for both the 0- and the 50-HU cylindric cysts (Table 4), data reveal that pseudoenhancement frequency and magnitude were greatest with the Sensation 64 with the B46 kernel, Sensation 64 with the B70 kernel, and QX/i with the Bone Plus kernel imaging regimens.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEIMPLICATION FOR PATIENT CAREReferences |
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Statistical analysis of our data revealed that imaging regimen was significantly associated with pseudoenhancement incidence. Of note, convolution kernel modification often resulted in significant differences in the incidence of renal cyst pseudoenhancement when kernels on specific multidetector CT scanners were compared. This finding indicates that certain convolution kernels are superior to others in their ability to compensate for beam hardening at the boundary of the renal cyst and renal parenchyma and that pseudoenhancement may be minimized in patients by selecting convolution kernels that provide the best CT algorithmic correction for beam-hardening effects.
Analysis of the observed incidence of renal cyst pseudoenhancement with the Siemens scanners revealed important generational differences in equipment that could not be explained on the basis of convolution kernel alone. As this family of scanners evolved from the Volume Zoom (four-section multidetector CT, adaptive array technology) to the Sensation 64 (64-section multidetector CT, matrix array technology), the B40 and B70 kernels remained constant while the B46 kernel was modified on the Sensation 64 with an edge-preserving filter that provided similar image sharpness but reduced image noise compared with that generated by previous generations of scanners. Statistical analysis of our data revealed that for both 0- and 50-HU renal cysts, the Sensation 64 with the B40 kernel regimen was associated with a significantly higher incidence of renal cyst pseudoenhancement than was the Volume Zoom with the B40 kernel regimen, and the Sensation 64 with the B70 kernel regimen was associated with a significantly higher incidence of renal cyst pseudoenhancement than were both the Volume Zoom with B70 kernel and the Sensation 16 with B70 kernel regimens. In view of the fact that the B40 and B70 kernels were not modified by the manufacturer across the equipment tested, our results indicate that convolution kernel setting is not the only factor that influences scanner compensation for beam hardening at the boundary of the renal cyst and renal parenchyma. Additional research is necessary to better understand how innovations in CT technology and image reconstruction processes affect the ability of multidetector CT scanners to control beam hardening.
Our data demonstrated substantial variability in the observed incidence of renal cyst pseudoenhancement among the multidetector CT imaging regimens tested. As an example, pseudoenhancement incidence varied by a factor of approximately 11 when the Sensation 64 with B40 kernel and the Volume Zoom with B40 kernel regimens were compared. As a result, we believe it is not appropriate to directly extrapolate findings of previously published pseudoenhancement experiments performed with single-section or early generation multidetector helical CT scanners to all manufacturers' multidetector CT scanners, to varying generations of scanning equipment, or even to the same scanner operating with different convolution kernels.
The enhancement threshold values that provided 98% specificity for renal cyst pseudoenhancement varied widely across the imaging regimens tested. On the basis of our results, we believe that radiologists should adopt renal mass enhancement threshold criteria that are specific to the combination of multidetector CT scanner and convolution kernel used in their practice. This may not always be feasible, however, because many radiology practices interpret renal CT studies that are acquired with different multidetector CT scanners, and specific scan parameters may not always be available at the time of study interpretation, depending on the picture archiving and communication system display configuration used. In this study, the 98% specificity threshold for renal cyst pseudoenhancement across all imaging regimens that incorporated standard convolution kernels was 15 HU. If one were to translate our phantom results to clinical care, we believe that this enhancement threshold should represent the lowest universal threshold criterion used for this imaging application.
Our phantom study had several limitations. Scan acquisition parameters were not uniformly standardized because of differences in multidetector CT design. Although this prevented us from constructing thin sections of uniform thickness, we avoided volume-averaging effects by using a slab phantom design in which appropriately sized ROIs were used to interrogate the attenuation values of cylindric renal cyst data generated from overlapping thin sections. Section profile and beam pitch both varied; however, pseudoenhancement has been shown to be independent of these parameters (6). Two investigators shared responsibility for ROI placement and interpretation. We compensated for this by training these investigators to use a consistent ROI placement technique to minimize individual performance variation. Most important, it may not be possible to directly extrapolate the results of our phantom study to patient care. The pseudoenhancement data generated were based on studying cylindric cysts of particular sizes that were placed in specified locations within renal inserts that had background attenuation values of only 140 and 240 HU. Our results, particularly the enhancement threshold values that provided 98% specificity for renal cyst pseudoenhancement, may not be directly applicable to patients, who have different levels of renal enhancement or who may have a pathologic renal condition that varies from what we simulated in this experiment.
Practical application: Both simple and hyperattenuating renal cysts may be subjected to the pseudoenhancement effect, which is significantly dependent on imaging regimen (multidetector CT scanner and convolution kernel combination), cyst diameter, and background renal attenuation. Pseudoenhancement of both simple and hyperattenuating renal cysts may be minimized in patients by selecting convolution kernels that provide the best CT algorithmic correction for beam hardening and by adjusting contrast material dose according to patient weight to prevent renal superenhancement. We believe renal mass enhancement threshold criteria should be imaging-regimen specific because the incidence of pseudoenhancement may vary significantly between different manufacturers' scanners and across generations of equipment.
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ADVANCES IN KNOWLEDGE |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEIMPLICATION FOR PATIENT CAREReferences |
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IMPLICATION FOR PATIENT CARE |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEIMPLICATION FOR PATIENT CAREReferences |
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FOOTNOTES |
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Abbreviations: ROI = region of interest
2 Current address: Department of Radiologic Sciences, UCLA Medical Center, Los Angeles, Calif. 
See Materials and Methods for pertinent disclosures.
Author contributions: Guarantor of integrity of entire study, B.A.B.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, B.A.B., N.H.; experimental studies, B.A.B., N.H., J.L.; statistical analysis, J.S.B.; and manuscript editing, B.A.B., N.H.
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References |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCES IN KNOWLEDGEIMPLICATION FOR PATIENT CAREReferences |
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