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Rapid MR Imaging Detection of Renal Cysts: Age-based Standards¹

¹ From the Department of Radiology, Thomas Jefferson University Hospital, 132 S 10th St, Suite 1096, Main Bldg, Philadelphia, PA 19107. Received December 18, 2000; revision requested February 5, 2001; revision received February 26; accepted March 23. Address correspondence to D.G.M. (e-mail: donald.mitchell@mail.tju.edu).

	ABSTRACT

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PURPOSE: To establish age-based standards for renal cysts depicted at magnetic resonance (MR) imaging and to compare these standards with existing standards for ultrasonography (US).

MATERIALS AND METHODS: Three radiologists reviewed subsecond T2-weighted single-shot fast spin-echo kidney MR imaging findings in 528 patients (248 men, 280 women) selected from consecutive abdominal MR studies without regard to clinical indication. Age, sex, and number and diameter of cysts were noted. Results were analyzed with nonparametric tests and were compared with published US results.

RESULTS: Men (mean, 2.0; 95% CI: 1.5, 2.5) had more renal cysts than women (mean, 1.2; 95% CI: 0.9, 1.5) (P < .001). Number and diameter of cysts increased with age (P < .001). Of 528 patients, 330 (62.5%) had at least one renal cyst, and 315 (59.7%) had cysts of 10 mm or less. MR imaging findings were comparable to published US criteria for type 1 autosomal dominant polycystic kidney disease (ADPKD) if only cysts larger than 1 cm were considered: Only one subject in the group of 18–29-year-old subjects had at least two renal cysts, and five of 493 subjects aged 30–59 years had at least two cysts in each kidney.

CONCLUSION: Compared with reported US results, MR imaging depicted an increased number of simple renal cysts in healthy individuals because of its increased sensitivity for cysts smaller than 1 cm. If only simple renal cysts larger than 1 cm are considered, US criteria for type 1 ADPKD can be applied to MR imaging.

Index terms: Kidney, cysts, 81.311, 81.312 • Kidney, diseases, 81.3121 • Kidney, MR, 81.121411 • Magnetic resonance (MR), comparative studies, 81.1214 • Magnetic resonance (MR), rapid imaging, 81.121411 • Ultrasound (US), comparative studies, 81.1298

	INTRODUCTION

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Simple renal cysts are important for screening of subjects at risk for inherited renal cystic disease or when considering the presence of occult disease. Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited renal cystic disease, and it is responsible for end-stage renal failure in 6%–10% of patients in North America and Europe (1). Although incurable, its early diagnosis is important in the treatment of complications, such as hypertension and urinary tract infection, and in the screening for intracranial aneurysms in families with increased occurrence (2–4).

Ultrasonography (US) is usually the first-line imaging method for screening family members at risk for ADPKD. In skilled hands, US has a reported sensitivity of 93% for individuals younger than 30 years old and 100% for those older than 30 years, with an overall specificity of 100% (5). However, US can be impaired in patients with thick adipose subcutaneous tissue or bowel gas, which obscure parts of the kidney. Also, careful depiction of small cysts by using US is operator dependent and time-consuming (6).

Subsecond T2-weighted single-shot fast spin-echo (SSFSE) MR images are precise in depicting fluid-filled structures, are not operator dependent, and have little or no motion artifact. A set of these images encompassing the kidneys can be obtained within a single breath hold. Currently, SSFSE MR images are commonly used for evaluating the pancreatobiliary system with MR cholangiopancreatography (7,8). Although MR imaging equipment is more expensive than US equipment, a rapid non–operator-dependent examination may be performed.

Our initial observations were that many small renal cysts are depicted commonly, and we hypothesized that their occurrence is more common than reported with US. We also hypothesized that cysts increase in number and diameter with age, that there is no difference in the number of cysts between kidneys, and that there is no difference in the number of cysts between men and women. Therefore, the purpose of our study was to establish age-based standards for the detection of renal cysts.

	MATERIALS AND METHODS

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Patients
In our retrospective review, we identified 1,708 consecutive MR imaging examinations of the abdomen that were performed from January 1998 to January 2000 for a variety of reasons. None of these examinations were performed to screen for ADPKD or other occult renal disease. Of these examinations, 685 were performed in patients between 18 and 59 years old and included T2-weighted SSFSE MR imaging as one of the sequences. Patients older than 60 years were not included because we were targeting a younger population to be used as a baseline for possible diagnosis of ADPKD. Patients younger than 18 years old were not included because of the small size of this population at our institution. We excluded 55 examinations because of signs of renal disease (eg, atrophy, hydronephrosis, renal neoplasm, nephrectomy) and 102 examinations because of suboptimal or incomplete images. Chronic renal disease, if not manifested as renal atrophy, was not a criterion for exclusion, since this information often was not provided. Four of the excluded patients had MR imaging signs of advanced ADPKD, which was verified with clinical history data. In the final sample of 528 patients, 35 were 18–29 years old, 190 were 30–44 years old, and 303 were 45–59 years old.

Renal atrophy was considered when the longest diameter of the kidney was less than 8 cm. Examinations with findings of solid renal masses more than 5 cm in diameter were excluded because bulky tumors made the evaluation of the normal renal parenchyma difficult. In the 528 examinations selected, the kidneys had been imaged in their entire length with SSFSE T2-weighted MR sequences in the coronal and/or transverse planes and, except for the possible presence of cysts, were grossly normal. Renal US, even if performed, was not reviewed for this study.

MR Imaging
MR imaging examinations were performed with a 1.5-T unit (Signa Horizon; GE Medical Systems, Milwaukee, Wis) with a phased-array torso coil. SSFSE T2-weighted MR images (effective echo time msec, 90–180) were obtained in the following planes: coronal, 308 examinations; transverse, 45 examinations; and both planes, 175 examinations. The images were acquired with 7- or 8-mm section thickness, no gap, and a 256 x 160 matrix. When images in two planes were acquired, both were analyzed.

Image Interpretation
Three radiologists (A.B.N., X.M.Z., T.K.), with 3–5 years experience reading MR images of the body, retrospectively and individually reviewed the examination findings in the search for simple renal cysts. They complied with the approved policy of our institutional review board for retrospective review of medical records by the department of radiology. When images in two planes were included, both were reviewed. When two MR examinations were performed in a patient, results of only the more recent one were analyzed. Images were viewed independently at a workstation (Canon Medical Systems, Irvine, Calif) during several separate sessions.

A simple cyst was diagnosed when it was sharply demarcated from the surrounding parenchyma, with a smooth and thin wall, with no internal septations, and with signal intensity similar to that of simple fluid (Figure) (9). If the cyst was oval, the longest diameter was measured. The cysts were classified according to diameter (0.1–1.0 cm [small], 1.1–2.0 cm [medium], and 2.1 cm or more [large]), number, affected kidney, and age and sex of the patient. If one of the kidneys showed more than 10 cysts, the medical record was reviewed for a history of ADPKD. When the criteria for simple cysts were not met, the lesions were separately classified as non–simple cysts, images obtained with other pulse sequences were viewed, and the discharge history and pathology reports for the patients were reviewed. Findings of other imaging examinations were not reviewed.

View larger version (119K): [in this window] [in a new window] [Download PPT slide]   Figure 1. Images show a subcentimeter cortical cyst (arrow) in the interpolar region of the right kidney. Top: Coronal SSFSE MR image (/100). Bottom: Transverse SSFSE MR image (/185).

	RESULTS

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Paired comparisons regarding the number of cysts showed very good reliability among the three raters, respectively, for small cysts ( value = 0.66, 0.66, 0.65), medium cysts ( value = 0.64, 0.68, 0.76), large cysts ( value = 0.75, 0.70, 0.87), and total cysts ( value = 0.68, 0.72, 0.68). The intraclass correlations for the number of small, medium, large and total cysts were 0.89, 0.76, 0.91, and 0.92, respectively.

The number of renal cysts increased with age, with a mean of 0.71 cysts for 18–29-year-old patients, of 1.27 for 30–44-year-old patients, and of 1.87 for 45–59-year-old patients (P < .001). The number of cysts differed according to sex only in patients in the 45–59-year group, in which it was significantly greater (P < .001) in men than in women (Table 1). The number of cysts in both kidneys is stratified by age group and sex in Table 2.

Patients within each age group were classified according to the diameter of cysts (Table 3). For comparison with the current US criteria for detection of type 1 ADPKD (9), patients were also classified by age according to the distribution of total cysts (Table 4) and the distribution of cysts larger than 1 cm (Table 5).

There were 44 non–simple cysts among the 528 patients, which represented 8% of the sample. We diagnosed two cases of confirmed renal cell carcinoma; one septate, hemorrhagic lesion; one lesion of uncertain cause, which required follow-up; and one pancreatic pseudocyst that mimicked a renal lesion. There was no confirmation of the other non–simple cysts, most of which had MR imaging characteristics of hemorrhagic cysts (9).

	DISCUSSION

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MR imaging findings showed a much greater occurrence of simple renal cysts than was reported for US, and MR imaging helped to identify at least one renal cyst in 330 (62.5%) of 528 patients. Although the patient selection was not identical, the reported occurrence of cysts that were visible at US was much less: 188 (15.1%) of 1,241 subjects (10) and 69 (9.5%) of 729 subjects (11).

Our results confirmed that simple renal cysts increase in number and in diameter with age; however, they are markedly more common in men than in women among patients aged 45–59 years. This datum agrees with that in earlier articles (10–13) in which occurrence of renal cysts examined with US and CT was reported. The larger number of cysts in older patients could be related to degenerative phenomena of the basal membranes in the distal tubule of the nephron, which are commonly seen in elderly people (14). This would lead to the development of diverticula, which is considered as the starting point (14). The cause for the greater occurrence of simple renal cysts in men is not yet clear. We found no differences in the number of cysts between the kidneys, findings which also agree with those in previous works (12). We also confirmed that there is no difference in cyst diameter between the kidneys.

We categorized the number of cysts in each age group according to the current US criteria for type 1 ADPKD, which were initially described by Ravine et al (15) and modified and verified by Nicolau et al (5). These authors indicated that renal cystic disease should be suspected if two or more cysts are noted in individuals 30 years or younger, or two or more cysts are noted in each kidney in those aged 30–59 years, or four cysts are noted in each kidney in those older than 60 years. Although most of our patients presumably did not have the disorder, several would meet these criteria: at least two renal cysts were detected in six (17.1%) of 35 subjects aged 18–29 years, and at least two cysts in each kidney were found in 50 (10.1%) of 493 subjects aged 30–59 years. In a similar study with US in a healthy population, no case of two cysts or more was detected in those 15–29 years old, nor was any case of at least two cysts in each kidney detected in individuals younger than 69 years old (11).

Since we found many individuals with cysts 1 cm or less in diameter (Figure), 315 (59.7%) of the total sample, and because a substantial proportion of renal lesions (both solid and cystic) less than 1 cm are not detected by using US (6), we also applied the US criteria for type 1 ADPKD restricted to cysts larger than 1 cm. Only one (2.8%) of 35 subjects aged 18–29 years had at least two renal cysts, and five (1%) of 493 subjects aged 30–59 years had at least two cysts in each kidney. This is an acceptably small percentage of potential false-positive examinations, which is comparable to reported results with US that showed 0% of individuals aged 15–29 years with at least two renal cysts and 0% of individuals aged 30–59 years with at least two cysts in each kidney (11).

Alternatively, if all simple renal cysts are to be counted at MR imaging for screening of patients at risk for ADPKD, we suggest use of the following criteria: five cysts or more for subjects aged 18–29 years, six cysts or more for subjects aged 30–44 years, six cysts or more for women aged 45–59 years, and nine cysts or more for men aged 45–59 years. These criteria would yield positive results in 5.7% (two of 35), 4.2% (eight of 190), 4% (six of 150), and 5.2% (eight of 153) of cases for each of these groups, respectively.

Investigations with use of older CT techniques showed an occurrence of simple renal cysts of 25 (24%) in 103 subjects (12) and 108 (20%) in 542 subjects (13). Spiral CT eliminates respiratory misregistration and, therefore, avoids "skipping" of areas (16), so the organ is completely imaged. Additionally, thinner image sections with more optimal contrast enhancement are obtained, especially with multi–detector row instruments. However, we are not aware of any updated data on the occurrence of renal cysts depicted with spiral or multi–detector row CT. The increased occurrence of renal cysts at MR imaging when compared with that in studies with other imaging techniques can be attributed to the contrast of simple fluid versus tissue on T2-weighted SSFSE MR images, which leads to the increased number of cysts smaller than 1 cm detected at MR imaging.

The limitations of this work are that mild forms of occult ADPKD may have been included. Also, the group sample of the youngest subjects is smaller than are the group samples of the older subjects. Most of our data, 308 (58%) of the 528 examinations, were analyzed in the coronal plane with a large field of view. It is possible that even more tiny cysts might have been detected with thinner sections and a smaller field of view. We did not obtain independent confirmation that all cysts depicted on T2-weighted SSFSE MR images were truly simple cysts. In our experience, hemorrhagic cysts that have high signal intensity on T1-weighted MR images are depicted with intermediate or low signal intensity on T2-weighted SSFSE MR images. Although we cannot exclude microscopic tumor within nonhemorrhagic cystic tumors, we do not believe that this would be common or that this would affect the clinical importance of our findings.

In conclusion, renal cysts are more common on MR images than on US scans. Subsecond SSFSE T2-weighted MR imaging is a safe, quick, non-operator-dependent method with increased sensitivity for detecting small renal cysts. Because of the increased precision of MR imaging, many cysts, mostly smaller than 1 cm, are detected in healthy individuals. Although MR imaging might be useful to screen for cysts, confirmatory findings of a randomized trial must be determined before this method can be advocated. If only simple renal cysts larger than 1 cm are considered, the US criteria for type 1 ADPKD can be applied to MR imaging. If all cysts are to be counted, we suggest the following criteria if MR imaging is used to screen patients at risk for ADPKD: five cysts or more for 18–29-year-old subjects, six cysts or more for 30–44-year-old subjects, six cysts or more for women 45–59 years old, and nine cysts or more for men 45–59 years old.

	FOOTNOTES

 
Abbreviations: ADPKD = autosomal dominant polycystic kidney disease, SSFSE = single-shot fast spin-echo

Author contributions: Guarantor of integrity of entire study, D.G.M.; study concepts and design, D.G.M., A.B.N.; literature research, A.B.N.; clinical studies, A.B.N., X.M.Z., T.K.; data acquisition, A.B.N., X.M.Z., T.K.; data analysis/interpretation, D.G.M., A.B.N.; statistical analysis, L.P., A.B.N.; manuscript preparation and definition of intellectual content, D.G.M., A.B.N.; manuscript editing, A.B.N.; manuscript revision/review, all authors; manuscript final version approval, D.G.M., G.A.H.

	REFERENCES

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This Article Abstract Figures Only Full Text (PDF) All Versions of this Article: 2213010178v1 221/3/628    most recent Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Nascimento, A. B. Articles by Holland, G. A. Search for Related Content PubMed PubMed Citation Articles by Nascimento, A. B. Articles by Holland, G. A.