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Genitourinary Imaging |
1 From the Department of Radiology, Asan Medical Center, 388–1 Poongnap-dong, Songpa-gu, Seoul 138–736, Korea. Received January 1, 2003; revision requested March 14; final revision received July 14; accepted August 22. Address correspondence to K.S.C. (e-mail: kscho@amc.seoul.kr).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Eighty-one patients (19 with AML with minimal fat [mean diameter, 2.8 cm; range, 1.5–4.5 cm] and 62 with RCC [mean diameter, 3.1 cm; range, 1.8–4.5 cm]) who had undergone biphasic CT (ie, CT with unenhanced, corticomedullary, and early excretory phase scanning) were evaluated. Two reviewers who were unaware of the diagnosis retrospectively recorded tumor attenuation on unenhanced scans, enhancement characteristics (ie, homogeneity of enhancement, amount of enhancement, enhancement pattern over time), tumor margin, location of tumor center, intratumoral calcification, perinephric changes, and patient age and sex. The predictive value of each CT finding was determined by using multivariate logistic regression analysis.
RESULTS: Homogeneous enhancement (observed in 79% of AMLs vs 5% of RCCs; odds ratio, 37) and prolonged enhancement pattern (observed in 58% of AMLs vs 10% of RCCs; odds ratio, 42) were valuable predictors for differentiating AML with minimal fat from RCC at multivariate analysis (P < .05 for both). When both CT findings were used as a criterion for differentiating AML from RCC, positive and negative predictive values were 91% (10 of 11 tumors) and 87% (61 of 70 tumors), respectively. Fifty-three percent of AMLs versus 13% of RCCs showed high tumor attenuation on unenhanced scans (P = .04), whereas RCCs showed greater mean enhancement than AMLs (114 HU ± 44 [SD] vs 73 HU ± 30 in corticomedullary phase and 66 HU ± 24 vs 49 HU ± 20 in early excretory phase) and a male predominance (male-to-female ratio, 50:12 vs 8:11; P = .001).
CONCLUSION: Biphasic helical CT may be useful in differentiating AML with minimal fat from RCC, with homogeneous tumor enhancement and prolonged enhancement pattern being the most valuable CT findings.
© RSNA, 2004
Index terms: Angiomyolipoma, 81.3141 • Computed tomography (CT), phase imaging, 81.12114 • Kidney neoplasms, CT, 81.12114 • Kidney neoplasms, diagnosis, 81.3141, 81.324 • Lipoma and lipomatosis, 81.3141
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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In the evaluation of AML, one of the most important roles of the radiologist may be to differentiate it from renal cell carcinoma (RCC) because the treatment strategies for the two disease entities are quite different: Asymptomatic AML is not an indication for surgery, whereas RCC should be completely removed. Although typical AMLs can easily be diagnosed with various imaging studies, AML with minimal fat may mimic RCC in various imaging studies, leading to unnecessary surgery.
Previous investigators have described the imaging findings of AML with minimal fat; these include higher attenuation than renal parenchyma on unenhanced CT scans, homogeneous enhancement on contrast material–enhanced CT scans, and homogeneous isoechogenicity on ultrasonographic (US) images (3–8). However, owing to the rarity of AML with minimal fat, previous studies have the limitation of small case populations; to our knowledge, the report of Jinzaki et al (3) described the greatest number of cases (ie, six) previously evaluated in a single study. Furthermore, previous studies did not involve use of modern CT techniques such as multiphasic thin-section helical scanning.
The purpose of our study was to compare the various imaging features of AMLs with minimal fat with those of size-matched RCCs.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Among the 24 patients with AML, 19 who underwent biphasic helical CT at our institution were included in this study. The other five patients were not included in this study because either they did not undergo biphasic CT (n = 4) or CT was performed at another hospital (n = 1). Each of the 19 patients had a single tumor. A staff radiologist (K.S.C.) confirmed that each tumor had no area of negative attenuation on unenhanced CT images. The mean maximum diameter of the tumors was 2.8 cm (range, 1.5–4.5 cm).
From among the 127 patients with RCC, we chose 67 who had tumors smaller than 4.5 cm in maximum diameter (mean, 3.1 cm; range, 1.8–4.5 cm) so that we could perform a size-matched comparison between the two kinds of lesion. Of these 67 patients, five were excluded because they had undergone CT examinations in another hospital (n = 4) or had undergone single-phase CT only (n = 1). Therefore, 62 patients with RCC who had undergone biphasic helical CT were included in this study. Each of the 62 patients had a single tumor.
CT Examinations
CT examinations were performed in 53 patients (11 of whom had AML with minimal fat and 42 of whom had RCC) by using a single–detector row helical CT scanner (Somatom Plus-S; Siemens Medical Systems, Erlangen, Germany). A multi–detector row helical CT scanner (LightSpeed QX/i; GE Medical Systems, Milwaukee, Wis) was used to examine the remaining 28 patients (eight of whom had AML with minimal fat and 20 of whom had RCC).
All patients received 500–900 mL of oral contrast material (2% barium sulfate suspension, E-Z-CAT; E-Z-Em, Westbury, NY) 30 minutes before the CT examination. Intravenous contrast material (iopromide, Ultravist 300, Schering, Berlin, Germany; or iopamidol, Iopamiro 300, Bracco, Milan, Italy) was administered into an antecubital vein by using a power injector at a dose of 2 mL per kilogram of body weight and a rate of 3 mL/sec to a maximum of 160 mL.
All patients underwent biphasic CT scanning that included unenhanced, corticomedullary, and early excretory phase scanning. Unenhanced scanning was performed after administration of the oral contrast agent but before administration of the intravenous agent. The scan delay was 30 seconds for corticomedullary phase scanning and 120–150 seconds for early excretory phase scanning. Unenhanced and corticomedullary phase scanning covered the entire volume of the kidneys, and early excretory phase scanning covered the area from the diaphragmatic dome to the ischial tuberosities.
The scanning parameters for single–detector row helical CT were as follows for all phases: section collimation, 5 mm; pitch, 1.5; table speed, 7.5 mm per rotation (10 mm/sec); reconstruction interval, 5 mm; 120 kV; and 210 mA.
With multi–detector row helical CT, a beam pitch of 1.5 (equivalent to a section pitch of 6 in high-speed mode), an x-ray tube voltage of 120 kV, and a tube current of 210–240 mA were used for scanning in all phases. The other parameters differed somewhat among the scanning phases: A detector array of 4 x 5 mm, a table speed of 30 mm per rotation (37.5 mm/sec), and a reconstruction interval of 5 mm were used for unenhanced and early excretory phase scanning, while a detector array of 4 x 2.5 mm, a table speed of 15 mm per rotation (18.75 mm/sec), and a reconstruction interval of 2.5 mm were used for corticomedullary phase scanning.
Image Analysis
Two radiologists (J.K.K., S.Y.P.) who were unaware of the final diagnoses reviewed the CT images in consensus at a picture archiving and communication system, or PACS, monitor (Radpia; Hyundai Information & Technology, Seoul, Korea), at which it was possible to measure tumor diameter and attenuation in a particular region of interest.
The radiologists evaluated tumor attenuation (as low, similar to that of surrounding renal parenchyma, or high) on the unenhanced scans; homogeneity of tumor enhancement and amount of tumor enhancement (in Hounsfield units) on the corticomedullary and early excretory phase scans; and enhancement pattern over time (as an early washout pattern or a gradual or prolonged enhancement pattern), tumor margin (as smooth or irregular), location of the tumor center (as extracapsular or intracapsular), and presence or absence of intratumoral calcification and perinephric changes (ie, strands of soft-tissue attenuation in the perinephric area and thickening of the Gerota fascia) on all three kinds of scans. Tumor attenuation on the unenhanced scans was determined by visual inspection and classified by comparing it with the attenuation of the surrounding renal parenchyma. When a tumor showed heterogeneous attenuation, the area with the greatest attenuation was used for classification.
The homogeneity of tumor enhancement was also determined by visual inspection; homogeneous enhancement was considered to be present when most areas of a tumor showed uniform enhancement on both corticomedullary and early excretory phase scans. Otherwise, a tumor was regarded to have heterogeneous enhancement.
To evaluate the amount of tumor enhancement on the corticomedullary and early excretory phase scans, the first radiologist (J.K.K.) measured the attenuation (in Hounsfield units) at a region of interest, the location of which was chosen by the two radiologists in consensus. An area that appeared as an enhancing solid area on corticomedullary phase images was agreed to be a suitable place for measuring the attenuation value. A round or elliptic region-of-interest cursor was placed over the enhancing solid area, which had to be at least 1 cm2 and consistent in location and size on images obtained during all three scanning passes.
To minimize partial volume averaging with surrounding renal parenchyma, we tried to place the region of interest near the center of a tumor. When a tumor contained multiple areas of enhancement, we placed two or three separate regions of interest over the enhancing areas that were greater than 1 cm in short-axis diameter and then calculated the mean values. We tried to include as much of the enhancing area in the region of interest as possible and to exclude the surrounding renal parenchyma and any area of intratumoral cyst or calcification. The amount of tumor enhancement on corticomedullary and early excretory phase scans was measured by calculating the difference in tumor attenuation value from that on the unenhanced scans.
The enhancement pattern over time was classified as follows: An early washout pattern was considered to be present when a tumor showed peak enhancement in the corticomedullary phase and then demonstrated a washout of at least 20 HU in the early excretory phase, a gradual enhancement pattern was considered to be present when the tumor attenuation value in the early excretory phase was at least 20 HU greater than it was in the corticomedullary phase, and a prolonged enhancement pattern was considered to be present when the difference in tumor attenuation between the corticomedullary and early excretory phases ranged from -20 to 20 HU.
The location of the tumor center was classified as extracapsular when the tumor epicenter was located beyond the outline of the kidney and as intracapsular when it was located below the outline of the kidney.
Statistical Analysis
Statistical analysis was performed by using SPSS version 10.0.7 (SPSS, Chicago, Ill). The 
2 test was used to compare AML with minimal fat and RCC in terms of tumor attenuation on unenhanced scans, homogeneity of tumor enhancement, enhancement pattern over time, tumor margin, location of tumor center, presence or absence of intratumoral calcification and perinephric changes, and patient age and sex. We used the independent-samples t test to compare amount of tumor enhancement and patient age between the two diseases.
In the evaluation of the amount of tumor enhancement and the tumor margin, there may have been a risk that the use of two different CT scanners (ie, one single–detector row helical CT scanner and one multi–detector row helical CT scanner) resulted in different data. Therefore, we stratified patient data according to the scanner that was used and separately compared those imaging findings between AML with minimal fat and RCC.
To assess the predictive value of each CT finding that was found at univariate analysis to have a statistically significant role in the differentiation of AML with minimal fat from RCC, multivariate logistic regression analysis was performed. For multivariate logistic regression analysis, data concerning the amount of tumor enhancement in the corticomedullary and early excretory phases were dichotomized into "suggesting AML with minimal fat" or "suggesting RCC" by using a threshold determined by a review of receiver operating characteristic curve analysis results. For the parameters that were found at multivariate logistic regression analysis to have a statistically significant role in the differentiation of AML with minimal fat from RCC, we calculated the positive and negative predictive values. A P value of less than .05 was considered to indicate a statistically significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Tumor Enhancement
The amounts of tumor enhancement on the corticomedullary and early excretory phase scans are summarized in Table 2. Both AML with minimal fat and RCC showed a wide range in the amount of tumor enhancement in both the corticomedullary phase (AML with minimal fat, 36–141 HU; RCC, 33–195 HU) and early excretory phase (AML with minimal fat, 15–86 HU; RCC, 21–140 HU). The mean amount of tumor enhancement was greater in RCC than in AML with minimal fat in both the corticomedullary and the early excretory phases (P < .05). At both single–detector row and multi–detector row helical CT, RCC showed greater enhancement than AML with minimal fat (P < .05).
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Tumor Enhancement Pattern
Results of the evaluation of tumor enhancement patterns over time are summarized in Table 3. The enhancement pattern over time was also different between the two diseases: A prolonged enhancement pattern was observed in more than half (58%) of the patients with AML with minimal fat, whereas an early washout pattern was observed in most (85%) of the patients with RCC (P < .001) (Figs 1, 2).
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Location of the tumor center was also not significantly different (P > .05) between patients with AML with minimal fat and those with RCC: The tumor center had an extracapsular location in nine (47%) of the patients with AML with minimal fat and an intracapsular location in the other 10 patients (53%), while it had an extracapsular location in 21 (34%) of the patients with RCC and an intracapsular location in the other 41 patients (66%).
Calcification
Only patients with RCC—not those with AML with minimal fat—were observed to have intratumoral calcification (n = 8, 13%) (Fig 3) and perinephric changes (n = 6, 10%), although the frequency of these findings was not significantly different between the two diseases (P > .05).
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Patient Age and Sex
The age distribution was similar between the two patient groups: The mean age of patients with AML with minimal fat was 57 years ± 10 (SD) (age range, 28–79 years), and the mean age of patients with RCC was 54 years ± 12 (age range, 22–66 years) (P > .05). However, the sex distribution was significantly different between the two patient groups: Most patients with RCC were male (male-to-female ratio, 50:12), whereas there was an approximately even sex distribution in the group of patients with AML with minimal fat (male-to-female ratio, 8:11) (P = .001).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Results of our study showed that, despite considerable overlap, some CT findings were useful for the differentiation of AML with minimal fat from RCC. According to results of our multivariate logistic regression analysis, homogeneous enhancement and a prolonged enhancement pattern were the most valuable CT findings for differentiating AML with minimal fat from RCC, with positive and negative predictive values, respectively, as high as 91% and 87%. Fifty-three percent of patients with AML with minimal fat had tumors that showed these enhancement characteristics; thus, our data suggest that unnecessary surgery and further preoperative evaluations such as biopsy could be avoided in more than half of patients with AML with minimal fat.
Although our multivariate analysis did not reveal the amount of tumor enhancement in the corticomedullary phase to be a significant predictor, we suggest that it could nevertheless be considered a helpful CT finding in differentiating AML with minimal fat from RCC. In our study, only one (5%) patient with AML with minimal fat had an amount of tumor enhancement greater than 115 HU, whereas 50% of patients with RCC met this criterion. We suggest that the wide range in the amount of tumor enhancement in the corticomedullary phase (AML with minimal fat, 36–141 HU; RCC, 33–195 HU) in our study may explain the lack of statistical significance.
High tumor attenuation on unenhanced scans has been presented as a unique finding in AML with minimal fat in previous reports (3–6). The definition of AML with minimal fat is generally established on the basis of findings on unenhanced CT scans, although such tumors actually have intratumoral fat at microscopic examination. Thus, it is somewhat ironic that AML with minimal fat can show high attenuation on unenhanced scans despite the actual presence of intratumoral fat. In previous studies, almost all cases of AML with minimal fat were observed to have high attenuation on unenhanced scans (3–6,10). However, in our study, only 53% of AMLs with minimal fat showed high attenuation. Moreover, according to our data, 22% of RCCs also showed high attenuation on unenhanced scans. Thus, we believe that results of visual inspection of tumor attenuation on unenhanced scans may vary, and high tumor attenuation on unenhanced scans should not be used as a definitive finding for AML with minimal fat.
We suggest that, despite the statistical nonsignificance of the finding of intratumoral calcification in our study (which might have been due to the small number of RCCs with this feature in our series), its presence may be helpful for differentiation because it was observed only in cases of RCC. In general, intratumoral calcification is not an uncommon finding in RCCs and may be seen in about 30% of cases (11), whereas it is rare in AML (12).
Patient sex distribution was significantly different between the two disease groups in our study; RCC had an apparent male predominance. However, because we selected only patients who underwent surgery, rather than a population of people with renal masses who were not necessarily candidates for surgery, this finding may be limited to use only in the discrimination between RCC and AML with minimal fat.
In our series, only CT images were evaluated because we could not identify a sufficient number of cases in which US or magnetic resonance (MR) imaging had been performed. For MR imaging, two case reports of AML with minimal fat indicated that chemical shift gradient imaging was helpful for detecting a small amount of fat not detected on conventional images (13,14). However, according to another report (15), RCC can also show intratumoral fat on chemical shift gradient images.
There may have been some limitations to our study. First, RCC can show different features according to the tumor subtype. In a recent study (16), papillary RCCs and chromophobic RCCs were observed to show homogeneous and weaker enhancement than conventional RCCs at helical CT. However, those RCC subtypes also tended to have an early washout enhancement pattern and intratumoral calcification. Therefore, we expect that the integration of various CT findings may make it possible to differentiate AML with minimal fat from those RCC subtypes.
Second, the CT findings of high attenuation on unenhanced scans and homogeneous enhancement that predicted AML with minimal fat in our series may be similar to CT findings of other benign solid tumors such as metanephric adenoma and oncocytoma. According to some reports of two or three cases of metanephric adenoma or oncocytoma, those tumors tend to show high attenuation on unenhanced scans and homogeneous enhancement (17–20). Thus, those CT findings may passively be considered to be findings seen in benign solid tumors, but we caution that the numbers of cases in the aforementioned studies were small. Further evaluation with a larger number of patients is needed.
Last, our study had a potential limitation in terms of the acquisition of CT data. In obtaining and comparing attenuation values in different phases, image acquisition parameters—in particular, section thickness and scanning time—ideally should be constant. In this regard, our study results are limited because two different CT techniques, single–detector row and multi–detector row helical scanning, were used with different section thicknesses and table speeds in the corticomedullary phase. In addition, the scan delay for early excretory phase scanning was somewhat variable (120–150 seconds). Therefore, our measurements of the amount of enhancement and our determinations of the enhancement patterns might be biased.
In conclusion, our study findings have shown that biphasic helical CT may be useful in the differentiation of AML with minimal fat from RCC. Homogeneity of tumor enhancement and a prolonged enhancement pattern are the most valuable CT findings for differentiating between AML with minimal fat and RCC. Other findings, including tumor attenuation on unenhanced scans, amount of tumor enhancement, intratumoral calcification, and patient sex provide supplementary information.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, J.K.K.; study concepts, J.K.K.; study design, J.K.K., K.S.C.; literature research, J.K.K.; clinical studies, J.K.K., S.Y.P., K.S.C.; data acquisition, J.K.K., S.Y.P.; data analysis/interpretation, J.K.K.; statistical analysis, J.K.K.; manuscript preparation and revision/review, J.K.K., S.Y.P., J.H.S.; manuscript definition of intellectual content and editing, J.K.K.; manuscript final version approval, J.K.K., S.Y.P.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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3 cm) hyperechoic renal masses: comparison of helical and conventional CT for diagnosing angiomyolipoma. AJR Am J Roentgenol 1996; 167:877-881.This article has been cited by other articles:
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  O. A. Catalano, A. E. Samir, D. V. Sahani, and P. F. Hahn Pixel Distribution Analysis: Can It be Used to Distinguish Clear Cell Carcinomas from Angiomyolipomas with Minimal Fat? Radiology, June 1, 2008; 247(3): 738 - 746. [Abstract] [Full Text] [PDF]
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R. Dyer, D. J. DiSantis, and B. L. McClennan Simplified Imaging Approach for Evaluation of the Solid Renal Mass in Adults Radiology, May 1, 2008; 247(2): 331 - 343. [Abstract] [Full Text] [PDF]
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 |
|
 S. R. Prasad, V. R. Surabhi, C. O. Menias, A. A. Raut, and K. N. Chintapalli Benign Renal Neoplasms in Adults: Cross-Sectional Imaging Findings Am. J. Roentgenol., January 1, 2008; 190(1): 158 - 164. [Abstract] [Full Text] [PDF]
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J. Y. Kim, J. K. Kim, N. Kim, and K.-S. Cho CT Histogram Analysis: Differentiation of Angiomyolipoma without Visible Fat from Renal Cell Carcinoma at CT Imaging Radiology, December 19, 2007; (2007) 2462061312. [Abstract] [Full Text]
|
|
|
|
 |
|
 S. G. Silverman, K. J. Mortele, K. Tuncali, M. Jinzaki, and E. S. Cibas Hyperattenuating Renal Masses: Etiologies, Pathogenesis, and Imaging Evaluation RadioGraphics, July 1, 2007; 27(4): 1131 - 1143. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. G. Silverman, Y. U. Gan, K. J. Mortele, K. Tuncali, and E. S. Cibas Renal Masses in the Adult Patient: The Role of Percutaneous Biopsy Radiology, July 1, 2006; 240(1): 6 - 22. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. K. Kim, S. H. Kim, Y. J. Jang, H. Ahn, C.-S. Kim, H. Park, J. W. Lee, S. Kim, and K.-S. Cho Renal Angiomyolipoma with Minimal Fat: Differentiation from Other Neoplasms at Double-Echo Chemical Shift FLASH MR Imaging Radiology, April 1, 2006; 239(1): 174 - 180. [Abstract] [Full Text] [PDF]
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|
|
|
|
|
G. M. Israel and M. A. Bosniak How I Do It: Evaluating Renal Masses Radiology, August 1, 2005; 236(2): 441 - 450. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Jinzaki, S. G. Silverman, A. Tanimoto, H. Shinmoto, S. Kuribayashi, J. K. Kim, and K.-S. Cho Angiomyolipomas That Do Not Contain Fat Attenuation at Unenhanced CT * Drs Kim and Cho respond: Radiology, January 1, 2005; 234(1): 311 - 312. [Full Text] [PDF]
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J. M. Pereira, C. B. Sirlin, P. S. Pinto, and G. Casola CT and MR Imaging of Extrahepatic Fatty Masses of the Abdomen and Pelvis: Techniques, Diagnosis, Differential Diagnosis, and Pitfalls RadioGraphics, January 1, 2005; 25(1): 69 - 85. [Abstract] [Full Text] [PDF]
|
|
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|
 |
|
 D. Neal Franz Non-Neurologic Manifestations of Tuberous Sclerosis Complex J Child Neurol, September 1, 2004; 19(9): 690 - 698. [Abstract] [PDF]
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