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1 Department of Radiology, Partners HealthCare System, Massachusetts General
Hospital and Harvard Medical School, 55 Fruit St., Boston, MA 02114.
2 Department of Radiology, Partners HealthCare System, Brigham and Women's
Hospital and Harvard Medical School, 75 Francis St., Boston, MA 02115.
Received February 27, 2002;
accepted after revision May 17, 2000.
Presented at the annual meeting of the American Roentgen Ray Society,
Atlanta, April—May 2002.
Abstract
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MATERIALS AND METHODS. A kidney phantom with varying attenuation was created to simulate different levels of renal parenchymal enhancement (150 and 250 H). Two water-filled cylinders simulating renal cysts—one with a 5-mm diameter and one with a 15-mm diameter—were suspended in the "kidney." After validating the pseudoenhancement effect produced in our phantom model with a single-detector helical CT scanner, we investigated the effect with matrix array and adaptive array MDCT scanners using detector configurations of 1.25 and 2.5 mm and beam pitches of 0.75:1.0 and 1.5:1.0 at an effective reconstructed slice thickness of approximately 3 mm. Three sets of experiments were performed at each setting, and mean cyst density was measured. Data were statistically analyzed using the Student's t test and multiple logistic regression analysis when appropriate.
RESULTS. Although pseudoenhancement was observed with MDCT scanners, the effect was statistically significant only for scans depicting the smaller cyst at a background renal density of 250 H on the matrix array MDCT. Modulation of scanning parameters did not alter these findings. Pseudoenhancement was significantly higher with the matrix array MDCT scanner than with the adaptive array MDCT scanner (p < 0.05).
CONCLUSION. In our phantom model, high levels of renal enhancement produced pseudoenhancement in small renal cysts with different models of MDCT scanners, irrespective of pitch or detector configuration.
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The aim of our study was to determine whether pseudoenhancement occurs on images obtained with MDCT scanners made by different manufacturers. In addition, we also assessed whether the effect was influenced by detector configuration and scanning parameters with matrix array and adaptive array MDCT scanners.
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A kidney phantom was constructed with a plastic container and two plastic tubes to simulate a kidney containing two cysts of different sizes (Fig. 1A,1B). An 11-cm diameter cylinder (length, 16 cm) with a removable top represented the kidney. On one end of this container, two plastic tubes representing cysts—the cross-sectional diameter of one tube was 5 mm and that of the other was 15 mm—were firmly attached to maintain a constant position with respect to the kidney and to prevent any movement of the cysts during scanning. Neither the cylinder nor the tubes contained highly attenuating materials that could cause beam attenuation. The tubes were filled with water to simulate intrarenal cysts and were sealed to prevent water leakage or iodine contamination from the container. We chose a tubular configuration for the cysts to eliminate the contribution of partial volume averaging (in the z-axis) to artifactual elevations of attenuation. The plastic container serving as the kidney was filled with water and was submerged in a rectangular water bath (20 x 30 x 35 cm) that simulated the abdomen.
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The density of the water in the kidney-simulating container was varied by adding contrast material to achieve an attenuation of 45 H, representing unenhanced kidney, and attentuations of 150 and 250 H, representing moderately and markedly enhanced renal parenchyma in a contrast-enhanced CT study [2]. The solutions in the kidney container were created by diluting 300 mg I/mL of iodinated contrast medium (Ultravist [iopromide]; Berlex Laboratory, Wayne, NJ) in water. The desired attenuation values of 45, 150, and 250 H were obtained by scanning serial dilutions of contrast material with a single-detector helical CT scanner. The dilutions of solutions obtained in this manner were used in studies with both MDCT scanners. The entire phantom model was imaged in each CT scanner.
To confirm the validity of the phantom for renal cyst pseudoenhancement, we performed the initial scan on the same model of single-detector CT scanner (HiSpeed CT/i; General Electric Medical Systems) previously used in another study [7]. Single-detector CT scans of the phantom model were obtained with a 3-mm slice thickness and a pitch of 2.0 at 140 kVp [3, 4, 6]. Pseudoenhancement of renal cysts seen on single-detector CT confirmed the validity of the phantom for use in subsequent experimentation with the MDCT scanners.
All experiments on each of the MDCT scanners were performed during the same session, so variation in the scanner calibration was minimized. The aim was to study the pseudoenhancement effect at a slice thickness of approximately 3 mm, which has been recommended as the appropriate slice thickness for evaluating small renal masses [7]. Hence, on the matrix array MDCT scanner, images were obtained at a detector configuration of 1.25 mm and a beam pitch of 0.75:1.0, resulting in a slice profile (full width at half maximum) of 2.5 mm; a detector configuration of 1.25 mm and a beam pitch of 1.5:1.0, resulting in a slice profile of 2.5 mm; a detector configuration of 2.5 mm and a beam pitch of 0.75:1.0, resulting in a slice profile of 2.6 mm; and a detector configuration of 2.5 mm and a beam pitch of 1.5:1.0, resulting in a slice profile of 3.2 mm. On the adaptive array MDCT scanner, images were obtained at a detector configuration of 2.5 mm and a beam pitch of 0.75:1.0, resulting in a slice profile (full width at half maximum) of 3 mm; a detector configuration of 2.5 mm and a beam pitch of 1.5:1.0, resulting in a slice profile of 3 mm; a detector configuration of 1.25 mm and a beam pitch of 0.75:1.0, resulting in a slice profile of 3 mm; and a detector configuration of 1.25 mm and a beam pitch of 1.5:1.0, resulting in a slice profile of 3 mm. The images were obtained in a four-slice mode at 140 kVp and 230 mA. Beam pitch was uniformly defined as the ratio of the gantry table speed to the beam width for both MDCT scanners [8]. Three sets of independent acquisitions were obtained for each setting.
Images were analyzed by measuring the densities of the phantom cysts and kidney with uniformly sized circular regions of interest drawn at constant positions within the kidney and each renal cyst. In the cysts, the size of the regions of interest was maximized without touching the walls (for the small cyst, the region of interest was approximately 20 square pixels; for the large cyst, approximately 40 square pixels). Mean cyst density and standard deviation of these values were calculated. Statistical analysis was performed using the Student's t test and multiple logistic regression analysis when appropriate. Our study had four degrees of freedom, for a cutoff critical t value of 2.203.
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Pseudoenhancement of the cysts was seen on scans obtained with matrix array and adaptive array MDCT scanners. Although the effect was higher with the former, no significant attenuation difference was found with modulation of scanning parameters, such as detector configuration, pitch, and slice profile. The findings are summarized in Tables 1 and 2. Pseudoenhancement was greater in the small cyst than in the large cyst and was more pronounced at higher levels of background renal attenuation. Statistically significant pseudoenhancement of both kidney cysts was found at the 250-H renal background attenuation with the matrix array MDCT scanner using almost every combination of protocol parameters. In contrast to the pseudoenhancement found with the matrix array MDCT scanner, the degree of pseudoenhancement found with the adaptive array MDCT scanner was not significant (p > 0.05).
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Although simple renal cysts are avascular and do not enhance, they may show artifactually increased attenuation after administration of a contrast medium. This phenomenon presents a potential difficulty in differentiating a simple cyst from a neoplasm [2,3,4]. The misleading difference in attenuation is thought to be a consequence of beam-hardening effects of the enhancing renal parenchyma in conjunction with the image-reconstruction algorithm. Pseudoenhancement occurs when a cyst is surrounded by materials that attenuate a poly-chromatic X-ray beam to a greater degree than water [12]. When the lower energy rays are attenuated, beam-hardening artifacts result. CT scanners perform the image reconstruction from the raw data using an algorithm that compensates for the beam hardening that would occur if the body scanned had the homogeneous density of water [13]. Maki et al. [2] showed that partial volume-averaging effects were not responsible for pseudoenhancement. The enhancement threshold used to determine the boundaries of pseudoenhancement has been defined differently in studies by Bae et al. [3], Coulam et al. [4], and Bosniak and Rofsky [10]. Prior studies have been performed with conventional CT and single-detector helical CT scanners on both phantoms and in clinical settings [2,3,4, 7]. To the best of our knowledge, renal cyst pseudoenhancement with MDCT has not been adequately investigated previously. Our study was performed to assess pseudoenhancement with MDCT scanners having different detector designs under simulated physiologic conditions of renal enhancement. The effects of varying the parameters of collimation, pitch, and slice profile were also investigated.
We found that pseudoenhancement of the phantom renal cyst occurred with MDCT scanners. The matrix array MDCT scanner consistently produced higher attenuation values than did the adaptive array MDCT scanner. The difference in pseudoenhancement between the two MDCT scanners could be due to variation in the detector design or scanning calibration. Alternatively, the discrepancy in the attenuation values could be a result of the difference in the reconstruction algorithm that each manufacturer used to correct beam hardening. The pronounced pseudoenhancement with MDCT scanners as compared with the single-detector CT scanner may be due to an associated beam-hardening effect or different reconstruction algorithms used in MDCT scanners. Levi et al. [14] found a significant difference in absolute CT density numbers of a simulated body phantom among most conventional CT scanners even when scanning with identical parameters. These researchers also found significant differences in attenuation values when using two different units of the same model of a conventional CT scanner from the same manufacturer.
Our findings agree with those of Levi et al. [14]. Our results suggest a need to develop a higher limit of pseudoenhancement for MDCT scanners than the limits used for the studies performed with conventional and single-detector helical CT scanners. The 5-mm-diameter phantom cyst showed maximal attenuation differences of 33 H at a background renal attenuation of 250 H and of 10 H at a background renal attenuation of 150 H. In the 15-mm cyst, maximal attenuation differences were 19 H at a background renal attenuation of 250 H and 8 H at a background renal attenuation of 150 H. The results showed consistently higher pseudoenhancement for the smaller phantom cyst. More pronounced changes in pseudoenhancements of both cysts were found at the higher background renal attenuation of 250 H.
Past studies have variably established the limits of significant renal cyst enhancement in clinical studies and simulated phantom experiments [2, 6, 9]. According to Bosniak and Rofsky [10], the cutoff point for simple renal cysts should be changed to 20 H on contrast-enhanced CT. Helical CT requires higher threshold value for "true" enhancement than does conventional CT. Accordingly, Bosniak and Rofsky proposed that findings of attenuation values above 20 H be considered cause for suspicion of a possible neoplasm.
Our study also supports the need for a higher threshold level for false renal cyst enhancement, as suggested by Maki et al. [2] and Heneghan et al. [6]. Maki et al. have reported that increasing the attenuation-difference limit for small renal cysts to 15-20 H is a necessity. Recently, Heneghan et al. reported finding pseudoen-hancement exceeding 20 H with MDCT scanners. In addition, Heneghan et al. found higher pseudoenhancement attenuation values with MDCT scanners than with single-detector scanners, as we observed in our study. For small renal cysts, pseudoenhancement of as much as 18-28 H was described by Birnbaum et al. [5]. For a cyst smaller than 1 cm in diameter, the range of attenuation differences in our study was 12-33 H at the higher background attenuation (250 H).
Our study did not find any significant correlation between pseudoenhancement and the variation of scanning parameters such as detector configuration, slice profile, and pitch. In addition, because we noted less pseudoenhancement at the lower background attenuation (150 H), measurements of contrast enhancement taken during the tubular or delayed phase may be more accurate assessments of enhancement than measurements taken during the corticomedullary phase. In a recent study, Herts et al. [15] reported a case of well-differentiated papillary renal cell carcinoma that showed a relatively low contrast enhancement. Hence, raising the threshold for enhancement must be cautiously investigated to minimize the possibility of misdiagnosing hypovascular malignant tumors as simple renal cysts.
Our study has limitations. Our phantom was a simplified model. Ideally, a phantom should have similar surrounding structures such as vertebrae, retroperitoneal fat, and other enhancing structures. However, we validated the phenomenon of pseudoenhancement for our phantom with a single-detector CT scanner before performing the experiment with MDCT scanners. Also, we minimized pseudoenhancement resulting from partial volume averaging by using thinner slice collimation and cylindrical containers to represent the cysts. To minimize the variation in attenuation values, we calibrated all scanners before scanning the phantom model. The interscanner differences in pseudoenhancement thresholds may have resulted from the differences in the slice thickness used for scanning of our phantom with the two MDCT scanners.
Further clinical studies are needed to assess renal cyst pseudoenhancement with other MDCT scanners as well as the temporal variation in the attenuation difference on the same MDCT scanner. On the basis of our data, we believe that the limit of attenuation difference for renal cysts should be redefined and standardized for contrast-enhanced MDCT scanning. Ultimately, clinical studies must be undertaken to assess and define the exact limit of attenuation differences between unenhanced and contrast-enhanced MDCT scans that is consistent with renal benign cysts.
Acknowledgments
We thank Karen Flynn, Jeanne Haddad, Janet Matthews, and Ann McGinnis for
their help in scanning the phantom.
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3 cm) renal masses: correlation of spiral CT features and
pathologic findings. AJR
1994;163:597
-605
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