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1 Universidade Católica de Brasília-Campus I, Curso de Medicina,
EPCT QS 7 Lote 1, Taguatinga, Distrito Federal, Brazil, 71966-700.
2 Department of Diagnostic Imaging, Federal University of São Paulo,
São Paulo, Brazil.
3 Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical
School, Boston, MA.
Received May 30, 2004;
accepted after revision August 26, 2004.
Address correspondence to A. S. Bezerra
(alexandrebezerra{at}ig.com.br).
Abstract
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MATERIALS AND METHODS. Splenic length, width, and thickness were measured in 249 CT scans and multidimensional indexes were obtained from the multiplication of these measurements. Volume was calculated by summing the volumes of multiple contiguous scans. The relationship of the spleen to the left liver lobe and inferior third of the left kidney was also evaluated. Linear equations were obtained to correlate each measurement to the splenic volume.
RESULTS. The unidimensional measurements with best correlation to volume were splenic length (r = 0.81, p < 0.01) and width (r = 0.804, p < 0.01). Correlation was better for the multidimensional indexes (r = 0.95, p < 0.01). Using a previously described upper limit of normality for splenic volume of 314.5 cm3 in the linear regression equation obtained, a maximum spleen length of 9.76 cm was the upper limit of normality. The relation of the lowest point of the spleen to the inferior third of the kidney also showed that if the spleen reached or extended below this portion of the kidney, it could be used as evidence of splenomegaly (p < 0.005), although it had a low sensitivity.
CONCLUSION. Splenic length and multidimensional indexes correlate well with splenic CT volume. A splenic length of 9.76 cm can be used to accurately diagnose splenomegaly and can replace multiple-measurement, time-consuming methods in the clinical routine.
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Several studies have characterized normal in vivo measurements of the spleen on CT [4–7]. Lackner et al. [4] describe a splenic index using a combination of the width, thickness, and length of the spleen as a numeric measure of splenomegaly, with an upper limit of 480 for normal spleens. Others have used more accurate but more time-consuming methods, such as the sum of the volumes of each cross-sectional image of the spleen [5, 8–10]. Prassopoulos et al. [5] found an upper limit volume of 314.5 cm3 for the normal spleen using this approach. Also, in the pediatric population, attempts have been made to characterize the range of normal measurements that can be used as accurate markers of splenomegaly in these patients [7, 11, 12].
Although these initial reports have described several techniques that have been used to accurately determine splenic size with CT, a need remains for further study comparing these indexes to identify a technique that is practical and accurate. To this end, the purpose of this study was to compare these methods, such as individual linear measures, splenic index and its variations, and the technique of summation of the volume of each axial section, to determine if a simple and clinically practical method exists that can be used in daily routine to reliably diagnose and follow splenomegaly.
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All studies were obtained with a single-detector row helical scanner (Tomoscan AV, Philips Medical Systems) using a 7-mm section thickness, 7-mm reconstructed section thickness, and 7-mm reconstruction interval during a single breath-hold.
A set of unidimensional and multidimensional indexes were evaluated to identify which of these measurements most closely correlated to actual calculated volumes. We also examined if the relationship of the spleen to surrounding organs such as the liver and left kidney can be used as a marker for splenomegaly.
Splenic volume was calculated by the summation-of-volumes technique using a workstation (EasyVision, Philips Medical Systems) to trace the outline of the spleen on every image of the scan and to calculate the area that was enclosed (Fig. 1). This area was multiplied by the section thickness to calculate the volume of that axial section. The sum of the volume of every section represented the total splenic volume and has been shown to represent actual splenic volume within 3–5% accuracy [1–3, 7]. A spleen volume greater than 314.5 cm3 indicates splenomegaly [5].
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Six unidimensional indexes were recorded for every patient, including the length (L), width (W), and several measures of spleen thickness (T1–T4) as described previously [11, 13]. The length (L) was obtained by multiplying the number of sections where the spleen was visualized by the thickness of the sections. For example, if the spleen was seen in 10 contiguous cross-sectional images with 7-mm thickness, the length was recorded as 7 cm. The width (W) was recorded at the point of maximum width of the spleen. Four measures of thickness (T1–T4) were obtained at the following points: T1, the greatest thickness at the section where W was determined; T2, the thickness at the midpoint of the section where W was determined (Fig. 2); T3, the maximum thickness on any section; and T4, the thickness at the midpoint where T3 was determined.
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Using the splenic index previously described [4], four multidimensional indexes were derived from the products of unidimensional measures recorded as described here. The L was multiplied by the W, and this product was then multiplied by each of the four thickness values (L x W x T1–T4) that were obtained, for a total of four multidimensional indexes (indexes 1–4, based on whether T1–T4 was used).
We also examined if there was presence or absence of splenic contact with the left lobe of the liver, including the common variant of the lateral segment extending around the spleen, considering contact as a sign of splenomegaly. The relationship between the lowest point of the spleen and the inferior third of left kidney was recorded. Twelve patients presented with left nephrectomy, renal atrophy, or tumor and were excluded from this evaluation.
A Pearson correlation coefficient test was used to determine the relationship between the different unidimensional and multidimensional indexes with the volume of the spleen, and a linear regression equation was obtained for each of the indexes [14].
A chi-square test was used to determine correlation between an enlarged spleen and the relationship it has with the liver and the left kidney. A p value of less than 0.05 was considered statistically significant.
The volume of spleen calculated by the summation-of-volumes technique at CT was considered to be the reference test. Sensitivity and specificity values were calculated for the unidimensional index with the best correlation with the splenic volume value (the upper limit of normality value was calculated using a linear regression equation) and for the relationship between the splenic edge and the inferior third of the kidney as predictors of splenomegaly. Presence (sensitivity) and absence (specificity) of splenomegaly were considered when the unidimensional index or the relationship between splenic edge and the inferior third of the kidney was concordant with the reference test.
Statistical calculations were performed using the SPSS statistical package version 11.5 for Windows and Microsoft Excel 2003 for Windows.
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Table 1 shows the correlation index and linear regression equations between splenic volume and the unidimensional and multidimensional indexes.
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Among the unidimensional indexes (Fig.
3), the splenic L showed the best correlation with the splenic
volume (r = 0.816, p < 0.01) with a resulting linear
regression expressed as:
 | (1) |
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Among the multidimensional indexes (Fig.
4), the best correlation with splenic volume was observed for
Index 3 (r = 0.95, p < 0.01), with a resulting linear
regression equation expressed as:
 | (2) |
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Previous reports have shown that 314.5 cm3 is the upper volume limit of the normal spleen [5]. Using this value in equation 1, we obtained a value of 9.76 cm as the upper limit of normality for L.
When comparing the number of spleens considered enlarged using this unidimensional index with those considered enlarged by measurement of the splenic volume, we found that using the value of 9.76 cm as a single parameter for splenomegaly provides a sensitivity of 85.40%, a specificity of 81.80%, and an accuracy of 82.73% (p < 0.001) (Tables 2 and 3).
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The second best unidimensional index with correlation to splenic volume was the width. Using this measurement's linear regression equation, we obtained a value of 11.1 cm as the upper limit of normality for W. If this value is used to determine splenomegaly, a sensitivity of 82.5%, a specificity of 81.1% and an accuracy of 81.5% (p < 0.001) are obtained (Tables 2 and 3).
Evaluation of the relationship between splenic margins and surrounding structures showed that extension of the splenic edge to or beyond the inferior third of the kidney was also a predictor of splenomegaly (p < 0.005). This parameter has a sensitivity of 19.60%, a specificity of 93.30%, and an accuracy of 75.95% (Table 4).
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The relationship between splenomegaly and contact between the spleen and the left liver lobe was not statistically significant (p = 0.611).
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The results of our study underscore the strong correlation that can be achieved to actual spleen volume by using indexes that combine several measurements. Indeed, no significant difference was observed among the multidimensional indexes, suggesting that any of these indexes can be used reliably in estimating splenic volume.
Our results concerning the use of individual unidimensional measurements show a strong correlation between spleen length and volume over other individual measurements. These results are in accordance with a study by Lamb et al. [16] in 50 adult patients comparing sonography and CT for the evaluation of splenomegaly, in which they conclude that the use of a single spleen length measurement by sonogram correlates well with spleen volume. Our research confirms these data in CT and suggests that this practical and convenient measure can be used reliably in a clinical setting on a daily basis. Splenic length and width have the best correlation indexes, but we think the first measurement is easier to obtain since it involves more simple calculations, such as multiplying the number of sections where the spleen was visualized by the thickness of the sections; determination of width requires manual measurement of the distance itself in several images and is therefore more time consuming. Perhaps the use of coronal or sagittal reformations for more accurate measurements could slightly modify the correlation indexes, but they would require more time for evaluation and would be less practical for daily use. In a similar study by Prassopoulos and Cavouras [12] in 153 children correlating individual measures to the splenic volume, the authors found that the splenic thickness measured at the hilum of the organ could be used for determination of splenomegaly, although the absolute value of this measure varied with the different age groups.
For daily routine, a value of 10 cm could also be used as the upper limit of normality for the splenic length since it is a number that is easier to remember and simplifies the calculation one may use for determining this measurement. Using this value, we obtained a sensitivity of 80.6%, a specificity of 90.3%, and an accuracy of 87.9% (Table 4).
Contact of the spleen to the left lobe of the liver was shown not to be statistically significant for determining splenomegaly; thus, its use should be avoided in daily practice, although this parameter is used as an indicator of splenic enlargement by some radiologists [19].
The relationship of the splenic margin to surrounding organs can also function as a convenient and simple measure of splenomegaly. Our results show that the extension of the splenic margin beyond the inferior third of the left kidney was highly specific for splenomegaly, despite its low sensitivity.
In summary, our results indicate that simple measurements obtained at CT studies of the splenic length or the relationship between the splenic edge and the inferior third of the left kidney are strong predictors of splenomegaly and can be easily used.
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