W146 AJR:195, August 2010

that does not contain functioning renal tissue and does vary from patient to patient. These factors likely contributed to the findings in a recent study that showed a positive, but weak association between sonographically deter-mined kidney volume and various indices of glomerular filtration rate (GFR) [6].

A recent study showed renal volume, and specifically cortical volume, measured at CT had a strong positive relationship with renal function [7]. Additional studies have shown total renal volumes obtained at CT relate to renal function [8, 9]. However, there are drawbacks to using CT, including increased cost and radiation exposure. In addition, Wi-djaja et al. [8] showed a significant corre-lation between ultrasound-measured renal length and CT-measured renal volume.

In patients with CKD, the renal cortical echogenicity increases at ultrasound [10].

Renal Cortical Thickness Measured at Ultrasound: Is It Better Than Renal Length as an Indicator of Renal Function in Chronic Kidney Disease?

Michael D. Beland1

Nicholas L. Walle1 Jason T. Machan2 John J. Cronan1

Beland MD, Walle NL, Machan JT, Cronan JJ

Genitour inar y Imaging • Or ig ina l Research

AJR 2010; 195:W146–W149

0361–803X/10/1952–W146

© American Roentgen Ray Society

Traditional teaching is that renal length correlates with renal func-tion in chronic kidney disease (CKD), and therefore bipolar re-

nal lengths are almost always reported at renal ultrasound [1]. Previous studies have shown that renal volume calculated at ultrasound is a more exact measurement of a functioning kid-ney than renal length [2, 3]. A more recent study showed that kidney length and volume significantly correlated with estimated glomer-ular filtration rate (eGFR) in the elderly, but kidney length has a low specificity in predict-ing renal impairment [4]. However, measuring the true kidney volume at ultrasound is diffi-cult. Estimates of volume can be made on the basis of the ellipsoid formula, but this method has an inherent defect because the kidney is not actually ellipsoid [5]. In addition, the ellip-soid volume would include the central sinus fat

Keywords: chronic kidney disease, estimated glomerular filtration rate, renal cortex, renal failure, ultrasound

DOI:10.2214/AJR.09.4104

Received December 10, 2009; accepted after revision January 21, 2010.

WEBThis is a Web exclusive article.

1Department of Diagnostic Imaging, Rhode Island Hospital, Warren Alpert Medical School of Brown University, 593 Eddy St., Providence, RI 02903. Address correspondence to M. D. Beland (mbeland@lifespan.org).

2Departments of Biostatistics and Research, Orthopaedics, and Surgery, Rhode Island Hospital, Warren Alpert Medical School of Brown University, Providence, RI.

OBJECTIVE. The purpose of our study was to determine whether there is a relationship between renal cortical thickness or length measured on ultrasound and the degree of renal impairment in chronic kidney disease (CKD).

MATERIALS AND METHODS. From October to December 2007, 25 patients (13 men and 12 women, mean age 73 years) were identified who had CKD but were not on di-alysis. The patients were from a single institution and had undergone renal ultrasound and at least three serum creatinines within 90 days. The lowest creatinine was used for estimated glomerular filtration rate (eGFR) calculation using both the Cockcroft-Gault (CG) and the Modification of Diet in Renal Disease Study (MDRD) equations. Ultrasounds were consen-sus reviewed by three radiologists (2 attendings and a resident) blinded to specific renal func-tion. Cortical thickness was measured in the sagittal plane over a medullary pyramid, per-pendicular to the capsule. Length was measured pole-to-pole. Linear regression was used for statistical analysis.

RESULTS. Mean cortical thickness was 5.9 mm (range, 3.2–11.0 mm). Mean length was 10 cm (7.2–12.4 cm). Mean minimum serum creatinine was 2.1 mg/dL (1.1–6.1 mg/dL). Mean eGFR using CG was 34.8 mL/min (10.6–99.4 mL/min) and 36 mL/min (8–66 mL/min) using MDRD. There was a statistically significant relationship between eGFR and cor-tical thickness using both CG (p < 0.0001) and MDRD (p = 0.005). There was a statistical-ly significant relationship between CG and length (p = 0.003) but not between MDRD and length (p = 0.08).

CONCLUSION. Cortical thickness measured on ultrasound appears to be more closely related to eGFR than renal length. Reporting cortical thickness in patients with CKD who are not on dialysis should be considered.

Beland et al.Renal Function in Chronic Kidney Disease

Genitourinary ImagingOriginal Research

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In addition, the renal cortex often becomes thinned [11]. Often this finding occurs with a normal bipolar renal length and an increase in the relative amount of central sinus fat. To the best of our knowledge, a relationship be-tween renal function and cortical thickness has not been well established at ultrasound. The purpose of our study was to determine whether there is a relationship between renal cortical thickness or length measured at ul-trasound and the degree of renal impairment in CKD using two widely accepted computa-tional methods of estimating GFR.

Materials and MethodsPatient Selection

This retrospective study was approved by our institutional review board and is HIPAA compliant. A search was performed of our hospital electronic medical record for patients with the clinical diagnosis of CKD who were not on dialysis and also had undergone renal ultrasound over a 3-month period from October 2007 to December 2007. This search was further narrowed to patients who had at least three serum creatinines and weight recorded within 90 days of the ultrasound. Patients with hydronephrosis were excluded. Twenty-five patients (13 men and 12 women, mean age, 73 years; age range 26–90 years) met these criteria and constitute the study population.

Estimation of Renal FunctionThe lowest creatinine performed within 90 days

of the ultrasound was used for eGFR calculations. The lowest creatinine was chosen because it represents the best recorded renal function during the study period and helps to minimize the influence of superimposed acute on chronic renal insufficiency [10]. The Cockcroft-Gault (CG) and the Modification of Diet in Renal Disease Study (MDRD) equations were used for eGFR calculation, as follows [12]:

The CG equation is eGFR = (140 − age) × (Weight in kg) × (0.85 if female) / (72 × Cr) where Cr is creatinine.

The equation for Modification of Diet in Renal Disease (MDRD) for isotope dilution–mass spectrometry (ID–MS)-traceable creatinine measurements is GFR (mL/min/1.73 m2) = 175 × (Scr)

−1.154 × (Age)−0.203 × (0.742 if female) × (1.212 if African American) (conventional units). Scr is serum creatinine.

Ultrasound InterpretationAll renal ultrasound studies were performed in

the inpatient setting at our tertiary care hospital. The examinations were performed using standard gray-scale B-mode imaging with a 3.5-MHz

curvilinear transducer (Logiq 9, GE Healthcare). The examinations were retrospectively reviewed at a PACS workstation (Centricity, GE Healthcare)

by three authors (a radiology resident (PGY-4) and two radiology attending physicians specializing in ultrasound with 3 and 27 years of experience). All

TABLE 1: Study Population

Patient No. Sex

Age (y)

Minimum Creatinine

(mg/dL)

Maximum Creatinine

(mg/dL)

Mean Creatinine

(mg/dL)MDRD eGFR CG eGFR

Mean Renal Length (cm)

Mean Cortical Thickness (cm)

1 F 43 6.1 11.5 8.63 7.53 10.72 9.8 0.56

2 M 90 1.2 1.8 1.42 56.91 43.29 10.7 0.86

3 M 83 2.7 7.1 4.67 22.70 16.51 10.15 0.61

4 F 69 3.5 4.1 3.89 12.95 21.57 9.7 0.70

5 F 84 2.1 3.1 2.59 22.45 19.43 9.7 0.56

6 F 68 1.7 1.9 1.82 29.93 25.43 7.15 0.39

7 M 59 1.8 3.8 2.76 38.77 55.79 9.55 0.82

8 F 65 1.2 1.9 1.60 45.16 51.03 11.85 0.77

9 F 85 1.1 3.3 2.07 47.24 35.67 9.45 0.64

10 M 26 1.7 7.3 3.25 48.96 99.38 12.4 1.10

11 M 82 1.5 5.6 3.39 44.78 39.07 11.25 0.64

12 F 90 1.2 2.6 1.79 42.17 21.81 9.5 0.46

13 M 83 1.5 2.7 2.15 44.67 56.80 11.45 0.75

14 M 74 1.6 2.6 2.03 42.42 46.01 10.1 0.86

15 F 75 1.9 2.9 2.42 25.76 25.33 8.5 0.47

16 F 84 2.8 3.4 3.16 16.10 18.50 7.7 0.28

17 F 78 1.8 6.4 3.97 27.23 37.29 9.55 0.49

18 M 83 1.3 2.0 1.53 52.68 40.77 11.9 0.46

19 M 75 1.7 3.1 2.53 39.45 33.78 10.15 0.54

20 F 70 5.7 6.6 6.20 7.35 12.33 10.9 0.34

21 M 76 2.6 5.7 3.92 24.12 19.01 8.45 0.41

22 M 87 1.2 3.2 1.77 57.29 48.06 8.75 0.77

23 M 42 1.2 4.6 2.77 66.33 45.73 10.65 0.67

24 F 85 1.1 2.2 1.68 47.21 34.00 10.0 0.61

25 M 69 2.9 3.5 3.24 21.68 17.85 10.0 0.38

Note—eGFR = estimated glomerular filtration rate, MDRD eGFR = Modification of Diet in Renal Disease study for isotope dilution–mass spectometry traceable creatinine measurements, CG eGFR = Cockcroft-Gault equation. See Materials and Methods section for calculation of MDRD eGFR and CG eGFR.

Fig. 1—75-year-old woman with chronic kidney disease. Longitudinal ultrasound image of right kidney shows cortical thickness measured perpendicularly from outer margin of kidney to corticomedullary junction (arrow). Measurement is 0.46 cm.

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W148 AJR:195, August 2010

Beland et al.

measurements were made by consensus agreement. Renal lengths were measured as the greatest pole-to-pole distance in the sagittal plane. The renal cortical thickness was measured in the sagittal plane at the level of the mid kidney as described by Moghazi et al. [11]. The measurement was taken over a medullary pyramid, perpendicular to the capsule as the shortest distance from the base of the medullary pyramid to the renal capsule (Fig. 1). The readers were blinded to specific renal function, additional imaging, or any additional clinical information at the time of image review. Cortical thickness and length were measured bilaterally.

Statistical AnalysisThe data were entered and stored on a spread-

sheet (Excel, Microsoft). Mean cortical thickness and length were used in analyses. Statistical analysis and visualization were performed using Matlab (MathWorks). The relationship between ultrasound measurements and renal function was tested using linear regression. Significance was considered at a p value < 0.05.

ResultsDetails regarding the study population are

given in Table 1. The mean cortical thick-ness was 5.9 mm (range, 3.2–11.0 mm). The mean length was 10 cm (range, 7.2–12.4 cm). A statistically significant positive re-lationship was observed between eGFR and mean cortical thickness using both the CG and the MDRD equations (CG, p < 0.0001; MDRD, p = 0.0050). There also was a statis-tically significant relationship between CG eGFR and mean renal length (p = 0.0029) but not MDRD eGFR (p = 0.0756) (Fig. 2). The strongest relationship, as evidenced by the highest r2 value, was for mean cortical thickness and CG eGFR (r2 = 66%).

DiscussionOur series showed a statistically significant

relationship between cortical thickness mea-sured at ultrasound and renal function in pa-tients with CKD. Although there was also a significant relationship between CG eGFR and renal length, there was not for MDRD eGFR. Renal length has traditionally been consid-ered a surrogate marker of renal function be-cause renal length decreases with decreasing renal function. Renal lengths are universally reported and are usually the only measure-ments given at renal ultrasound [1]. However, on the basis of our study, it appears that corti-cal thickness measured at ultrasound may be related more closely to eGFR than renal length in patients with chronic renal failure.

As the burden of CKD continues to in-crease, efforts to reduce the cost of monitor-ing and managing this disease are needed. Our study attempted to evaluate the usefulness of a generally obtainable measurement at ultra-sound in the setting of CKD as a correlate to kidney function (eGFR). Prior studies also have evaluated imaging measurements as sur-rogate markers of renal function. A study eval-uating 69 patients with suspected unilateral re-nal artery stenosis showed renal volume was a better predictor of single-kidney GFR than re-nal length. They also showed the addition of re-nal area and parenchymal thickness measured at ultrasound to length was a better predictor of both single-kidney GFR and renal volume than length measured at ultrasound alone [8]. Another study showed a correlation between eGFR and renal volumes measured at ultra-sound in 116 healthy children [13]. Other au-thors have described kidney volume as a bet-ter predictor of renal function than renal length [2, 3]. This was further supported by a study in 2009 by Sanusi et al. [6] showing a weak but positive correlation between kidney volume and various indices of GFR, best with mea-sured creatinine clearance, in 40 patients with CKD. Their results also showed a significant correlation with the measured creatinine clear-

ance and the CG and MDRD equations, further validating these estimates of GFR in CKD [6].

Our study is limited by the small study sample. We hope the results presented here will serve as a pilot study prompting fur-ther studies with larger patient samples to validate the results. Future areas of investi-gation using larger patient samples may in-clude development of a predictive range of renal function given a particular cortical thickness. Alternatively, a determination of a threshold cortical thickness above which renal function is preserved may be identi-fied. Because of the retrospective design of our study, measurements were made on the images after they were obtained. Renal cor-tex measurements were taken perpendicular to the renal capsule from the capsule to the corticomedullary interface. This interface can be difficult to identify in some patients in whom there is poor corticomedullary dif-ferentiation. To ensure accuracy, these mea-surements ideally would be made prospec-tively at the time of the examination. This also would allow real-time image optimiza-tion to possibly make the corticomedullary interface more apparent. Additional mea-surements in the transverse plane or an av-erage cortical thickness including the upper

y = –13.5 + 80.6x, p = 0.0001r 2 = 0.66

100

50

00 0.5

Mean Cortical Thickness (cm)

CG

1 1.5

y = –46.8 + 8.2x, p = 0.0029r 2 = 0.3

100

50

06 8

Mean Length (cm)

CG

1210 14

y = 9.1 + 44.7x, p = 0.0050r 2 = 0.29

100

50

00 0.5

Mean Cortical Thickness (cm)

MD

RD

1 1.5

y = –8.7 + 4.5x, p = 0.0756r 2 = 0.13

100

50

06 8

Mean Length (cm)

MD

RD

10 12 14

Fig. 2—Graphs show Cockcroft-Gault (CG) (top) and Modification of Diet in Renal Disease (MDRD) study (bottom) estimated glomerular filtration rate (eGFR) plotted as function of mean cortical thickness (left) and mean length (right) with best-fit straight line from regression.

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Renal Function in Chronic Kidney Disease

and lower poles may prove to be a better rep-resentation of functioning parenchymal vol-ume in future studies. Despite these limita-tions, we found the cortical thickness was usually easy to measure on the PACS work-station. The method used in our study also likely reflects common practice in which ul-trasound images are obtained by a technolo-gist and then interpreted by the radiologist after the patient has left the department.

Another potential limitation of our study is the use of computational estimates of re-nal function, rather than measured GFR. Al-though both formulas have been previously validated and are widely used, there is contin-ued debate over which formula is best [14, 15]. A study by Rule et al. [16] in 2004 showed that the MDRD equation systematically underes-timated renal function. The underestimation was much more pronounced for healthy volun-teers (29% underestimation) than in patients with CKD (6.2% underestimation). Howev-er, their study was performed with a version of the MDRD equation before standardiza-tion of creatinine estimates between labora-tories. The development of the ID–MS-trace-able MDRD study equation that was used in our study has allowed the use of standardized creatinine measurements and should mini-mize measurement differences [12]. A slight-ly more recent study directly comparing CG and MDRD estimates of GFR in patients with CKD showed the MDRD equation to be more accurate than the CG equation in patients with moderate to advanced kidney disease and dia-betic nephropathy [17]. Our study only evalu-ated patients with CKD, which would mini-mize any potential underestimation of GFR using the MDRD equation relative to patients without known kidney disease and suggests MDRD may be the more accurate estimate applied to our study population [16].

Patients on dialysis were necessarily exclud-ed from this study. Examining the relationship between renal function on the basis of serum creatinine and cortical thickness would be in-herently flawed in this group because the creati-nine used for calculation would be a measure of

dialysis efficacy rather than native renal func-tion. Patients without known kidney disease also were not included in this study. It would be interesting to see if the correlation between cortical thickness and eGFR is also identified in these patients. Although this would be a more difficult study to perform, evaluating the applicability of measuring cortical thickness to healthy kidneys would be valuable.

In summary, we have shown renal cortical thickness measured at ultrasound appears to relate to the degree of renal impairment in patients with CKD, and routine reporting of cortical thickness should be considered in such patients who are not on dialysis.

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