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Genitourinary Imaging |
1 From the Mallinckrodt Institute of Radiology, Washington University School of Medicine, 510 S Kingshighway Blvd, St Louis, MO 63110. From the 1999 RSNA scientific assembly. Received September 14, 1999; revision requested October 8; revision received November 16; accepted December 7. Address correspondence to K.T.B. (e-mail: baet@mir.wustl.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Dual-phase renal spiral CT studies (5-mm collimation; pitch, 1.0) were retrospectively analyzed in 24 consecutive patients who had ultrasonographic (US) documentation of simple renal cysts. Forty-eight cysts were identified. The attenuation values of each cyst were measured on nonenhanced, cortical phase, and nephrographic phase images. The size and the location of each cyst in relation to the renal parenchyma were also recorded.
RESULTS: The cysts were 0.6–10.8 cm in diameter (mean, 2.6 cm; SD, 2.0). The mean attenuation change in the cysts from nonenhanced to contrast-enhanced images was statistically significant in a comparison of cortical phase and nephrographic phase images (P < .01): +1.8 HU (SD, 3.8) for cortical phase and +3.6 HU (SD, 5.6) for nephrographic phase images. Renal cysts 1.0 cm or smaller showed a higher attenuation increase (mean, +4.0 HU for cortical phase and +11.0 HU for nephrographic phase). None of the renal cysts larger than 1.0 cm demonstrated an increase greater than 10 HU (mean, +1.4 HU for cortical phase and +2.3 HU for nephrographic phase). Intraparenchymal cysts showed higher mean attenuation changes than the exophytic cysts.
CONCLUSION: Attenuation values in the renal cysts increased artifactually on contrast-enhanced images, but this pseudoenhancement was not substantial and was less than 10 HU when the cyst was larger than 1.0 cm in diameter.
Index terms: Computed tomography (CT), contrast enhancement, 81.12112 • Computed tomography (CT), helical, 81.12115 • Kidney, CT, 81.12111, 81.12112 • Kidney, cysts, 81.3111
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Despite advances in computed tomographic (CT) techniques, the evaluation of renal cystic lesions remains difficult (2–4). One of the important CT criteria used to distinguish a cystic neoplasm from a benign cyst is an increase in the attenuation of the mass after intravenous contrast medium administration. However, a host of technical factors such as beam hardening and partial volume averaging can cause the attenuation increase and result in "pseudoenhancement." Because of these potentially confounding effects, an increase in attenuation of at least 10 HU is suggested as evidence of enhancement (1). After the administration of intravenous contrast medium, an attenuation increase less than 10 HU is considered to be within the limits of artifactual pseudoenhancement. Thus, the CT diagnosis of a benign renal cyst versus a potentially malignant renal tumor depends largely on whether the radiologist can document a postcontrast increase in the attenuation of more than 10 HU.
There are two important problems with this method of diagnosing cystic renal masses with CT: (a) The threshold of 10 HU is based on anecdotal observations made with nonspiral CT technique and, to our knowledge, has not been validated with a systematic analysis; and (b) in a recent phantom study, Maki et al (5) questioned the validity of this enhancement criterion for cystic renal lesions. Their study findings demonstrated that the attenuation of simple renal cysts can increase by 10–28 HU on postcontrast images.
In our study, we evaluated the enhancement characteristics of renal cysts detected on a series of clinically acquired spiral CT images to determine the magnitude of pseudoenhancement and the factors that affect it. The objectives of this study were (a) to measure on dual-phase, dedicated renal spiral CT studies the change in attenuation of documented simple cysts after contrast medium administration and (b) to investigate whether the size and location of the cysts and the patient’s body habitus affect cyst enhancement.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Dual-phase renal spiral CT studies were performed with a Somatom Plus S or Plus 4 scanner (Siemens Medical Systems, Iselin, NJ). Image acquisition and reconstruction parameters were as follows: 120 kVp, 240 or 292 mAs, 5-mm collimation, 5 mm/sec table speed, and 3-mm reconstruction interval. These parameters remained constant between enhancement phases. Precontrast scans were acquired first. After the administration of 125 mL of Optiray 320 (ioversol; Mallinckrodt, St Louis, Mo; 320 mg of iodine per milliliter) into an antecubital vein at 3 mL/sec, postcontrast scans were acquired in the cortical enhancement phase (30–60 seconds after the start of injection) and in the nephrographic enhancement phase (100–130 seconds after the start of injection).
Image Data Measurement
CT images were reviewed and were compared with the corresponding US report by one of the authors (K.T.B.). Simple cysts were identified on pre- and postcontrast CT images, and their locations were documented (K.T.B.). Cyst location was classified according to the following: (a) side (right or left kidney), (b) vertical position (upper pole, interpolar, lower pole), (c) horizontal position (anterior, lateral, medial, posterior), and (d) relationship to renal parenchyma (intraparenchymal or exophytic). Exophytic cysts were further classified according to the percentage of cyst extending beyond the renal parenchyma (<25%, 25%–50%, 51%–75%, >75%). The patient’s body habitus was assessed qualitatively (thin, medium, obese) by evaluating the abdomen size and the amount of body fat (K.T.B.).
A research radiology technologist independently measured the sizes and attenuation coefficients of the cysts by loading the CT images onto a CT scanner console. This technologist was trained and experienced in measuring CT data and to reduce a potential bias in measurement was blinded to any information about the purpose of the study. The maximum diameter of each cyst was measured electronically. The attenuation of each cyst was measured on nonenhanced, cortical phase, and nephrographic phase images (Fig 1). To minimize partial volume averaging with surrounding tissues, the diameter of the region of interest (ROI) used for attenuation measurement was standardized at approximately one-half the diameter of the cyst being evaluated. The ROIs used for the pre- and postcontrast evaluation of a given cyst were the same size and were obtained at the same location in the kidney. The mean attenuation coefficient and SD of each cyst were recorded. Enhancement of the cyst was calculated as the absolute difference between the mean postcontrast and precontrast attenuation values.
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The effects of cyst location and patient body habitus on attenuation were investigated with a one-way analysis of variance, or ANOVA, test. P values were computed in each test, and those less than .05 were considered to indicate a statistically significant difference. Two scattergrams of the attenuation changes, the cortical phase minus nonenhanced CT and the nephrographic phase minus nonenhanced CT attenuation, versus the cyst size were plotted.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Mean changes in attenuation were correlated with the percentage of the cyst located within the renal parenchyma. Completely intraparenchymal cysts showed higher mean changes in attenuation (11 of 48 cysts; +3.5 HU for cortical phase and +8.4 HU for nephrographic phase) than the exophytic cysts (37 of 48 cysts; +1.3 HU for cortical phase and +2.1 HU for nephrographic phase). This increase was statistically significant (P < .01) for the nephrographic phase images but not statistically significant (P = .10) for the cortical phase images.
The completely intraparenchymal cysts ranged from 0.6 to 5.5 cm in diameter (mean, 1.6 cm; SD, 1.4; only one cyst was larger than 2 cm). The range of the attenuation changes in these cysts was -5.7 to +11.1 HU for the cortical phase and -0.8 to +30.4 HU for the nephrographic phase. Exophytic cysts with a larger intraparenchymal component tended to demonstrate larger mean changes in attenuation, but these differences were not statistically significant. For cysts with intraparenchymal components of greater than 75%, 51%–75%, 25%–50%, and less than 25%, the mean changes were +3.4, +0.2, +1.2, and -0.2 HU (P = .06) for the cortical phase images and +2.8, +0.6, +3.3, and +1.3 HU (P = .30) for the nephrographic phase images, respectively.
No statistically significant correlation was observed in the attenuation changes with respect to the right or left kidney location (P = .91 for cortical phase and .32 for nephrographic phase), the vertical location (P = .92 for cortical phase and .77 for nephrographic phase), the horizontal location (P = .92 for cortical phase and .43 for nephrographic phase), and patient body habitus (P = .26 for cortical phase and .55 for nephrographic phase).
Figure 3 shows a scattergram of the attenuation changes plotted against the renal cyst size. No definite global trend was observed, but the cysts less than or equal to 1.0 cm showed a higher mean attenuation increase: +4.0 HU (SD, 4.4; 95% CI: 8.0, 0.0) for the cortical phase and +11.0 HU (SD, 9.9; 95% CI: 20.2, 1.9) for the nephrographic phase images. None of the renal cysts larger than 1.0 cm in diameter (41 of 48 cysts) demonstrated an increase above 10 HU. The mean attenuation increase in cysts larger than 1.0 cm was +1.4 HU (SD, 3.7; 95% CI: 2.6, 0.3) for the cortical phase and +2.3 HU (SD, 3.3; 95% CI: 3.4, 1.3) for the nephrographic phase images.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Although simple cysts do not enhance, they may show an increase in attenuation after contrast medium administration because of technical factors (1,5,8). An increase in attenuation less than 10 HU is considered to be within the technical limits of the study and is not considered to represent enhancement (1). To minimize the effect of technical variations on the attenuation coefficient, the scanning technique used for pre- and postcontrast scans (eg, kilovolt peak; milliampere seconds; collimation; and the size, shape, and location of ROI) should be held constant. Other technical and anatomic variations such as the part of the cyst measured, the location of the cyst within the kidney, and adjacent renal or extrarenal structures might affect the attenuation measured (4,8,9).
To our knowledge, no systematic evaluation has been performed to assess the effect of these technical or anatomic factors on the variation in the attenuation of renal cysts. The only published report is an abstract by Hopper et al (8), who concluded that it would be inappropriate to use an absolute CT number of less than 30 HU or an increase of less than 15 HU following contrast medium administration to define a complex or abnormal renal cyst. Unfortunately, these authors did not describe their methods in this abstract.
Definition of an acceptable threshold value for pseudoenhancement of a cyst remains controversial. The most commonly suggested value is 10 HU (1), but other authors have proposed 15 HU (8). Birnbaum et al (10) reported no statistically significant enhancement in 15 benign cysts during dual-phase CT (mean, 1 HU in the cortical phase and 3 HU in the nephrographic phase). But the same group observed pseudoenhancement as high as 18–28 HU in small cysts in a phantom study (5). Another point of controversy regarding CT attenuation measurements is the technique used for image acquisition, that is, spiral versus conventional. Some investigators have reported that spiral CT scanners demonstrate greater variability in absolute CT numbers than conventional scanners (2,3), although others have found no significant variability between the two scanning techniques in controlled phantom experiments (5,11).
Our study of renal cysts from clinically acquired spiral CT images confirms the phenomenon of pseudoenhancement that was reported in a phantom study (5). In our study, the mean attenuation change in cysts between nonenhanced and contrast-enhanced images was +1.8 HU (SD, 3.8) for the cortical phase and +3.6 HU (SD, 5.6) for the nephrographic phase images. These values are similar to those obtained in a previous clinical study by Birnbaum et al (10). The larger increase in attenuation in the nephrographic phase was statistically significant in our study and is likely related to the fact that the overall enhancement of the renal parenchyma is greater during the nephrographic phase than the cortical phase. Thus, our result concurs with the finding observed by Maki et al (5) that an increase in background attenuation by contrast medium results in a higher degree of pseudoenhancement. The pseudoenhancement phenomenon was more apparent in small renal cysts (0.6–1.0 cm) than in large cysts (>1.0 cm).
Less pseudoenhancement was demonstrated in our study than in the phantom study. We postulate that beam hardening, which was proposed as the explanation of the pseudoenhancement in the phantom study, would have less of an effect in vivo, because in the body the contrast-enhanced kidney makes a smaller contribution to the total beam attenuation than the completely radiopaque cylindrical "kidney" surrounding the "cysts" in the phantom. Another possible explanation for less pseudoenhancement in our study is that only 11 of the 48 cysts were completely intraparenchymal in location. The exophytic cysts were likely less affected than the intraparenchymal cysts by the background renal enhancement. In the phantom study, the cysts were completely surrounded by radiopaque fluid. Another difference between our and phantom studies is that different CT scanners were used. The variation between CT scanners or manufacturers was not addressed in our study.
Our study has some limitations. The selection of cysts was based on US findings diagnostic of simple cysts. Because lesions considered to be simple cysts were not resected or imaged further, we do not have histopathologic confirmation that all lesions in our study were in fact simple cysts. Another limitation is that more than one cyst was included in the study in patients with multiple cysts (total of 48 cysts in 24 patients). Although one may need to consider intrapatient dependence when evaluating each cyst independently, this factor is not likely to have had an effect in our investigation. Intrapatient dependence could be further explored by randomly choosing a single cyst in each patient and performing a separate statistical analysis. Despite this potential bias, our results largely agreed with those of previously reported clinical (10) and phantom (5) studies.
The enhancement of the renal parenchyma as a probable cause of pseudoenhancement also was supported by our result that the intraparenchymal cysts showed a greater degree of pseudoenhancement than the exophytic cysts. The attenuation changes were not affected by other factors such as the location in the right or left kidney, the vertical or horizontal location, or patient body habitus.
One major problem in measuring the attenuation of small renal cysts is partial volume averaging with adjacent renal parenchyma, which increases the attenuation of the cysts, particularly on the contrast-enhanced images. The effect of volume averaging can be minimized by using narrow collimation and reduced pitch or by using overlapping reconstructions. In our study, small renal cysts (0.6–1.0 cm) showed a larger mean attenuation increase than large cysts (> 1.0 cm) on both the cortical phase (+4.0 vs +1.4 HU) and the nephrographic phase (+11.0 vs +2.3 HU) images. However, no definite global trend was observed between the attenuation change and the size of cyst, as shown in Figure 3. None of the cysts larger than 1.0 cm in diameter demonstrated an increase greater than 10 HU.
In conclusion, our study findings have confirmed that the attenuation of a renal cyst can be artifactually increased on contrast-enhanced CT images. The degree of the increase is positively associated with intraparenchymal location and a small size of the cysts. The cause of the increase is likely an inappropriate correction of beam hardening and partial volume averaging with adjacent enhanced renal parenchyma. In our study, pseudoenhancement was not substantial for cysts larger than 1.0 cm in diameter, none of which demonstrated an increase in attenuation above 10 HU. However, small renal cysts exhibited a higher degree of pseudoenhancement, as they were more subject to partial volume averaging. Thus, postcontrast enhancement less than 10 HU in a renal mass larger than 1.0 cm can be considered as evidence that the mass is a cyst if (a) pre- and postcontrast image acquisition parameters are held constant, (b) ROIs are carefully placed by using approximately one-half the diameter of the mass, and (c) the section thickness is no more than half the diameter of the mass being evaluated.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, K.T.B.; study concepts and design, K.T.B., J.P.H.; definition of intellectual content, K.T.B.; literature research, K.T.B., C.L.S.; clinical studies, C.L.S., H.F.B.; data acquisition, C.L.S., H.F.B.; data analysis, K.T.B.; statistical analysis, K.T.B.; manuscript preparation, K.T.B.; manuscript editing, J.P.H., C.L.S., H.F.B.; manuscript review, K.T.B., J.P.H.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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