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1School of Physical and Health Education and 2Division of Endocrinology and Metabolism, Department of Medicine, Queen's University, Kingston, Ontario, Canada; and 3Centers for Integrated Health Research, The Cooper Institute, Dallas, Texas
Submitted 12 August 2005 ; accepted in final form 6 November 2005
| ABSTRACT |
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liver attenuation; spleen; fatty liver; hepatic steatosis
The identification of liver fat by CT as a predictor of health risk was first described by Banerji et al. (1) and Goto et al. (7) in 1995. The CT method employed realizes that the lower the mean liver attenuation or CT number in Hounsfield units (HU), the lower the tissue density and hence the greater the fat content. Therefore, liver density (e.g., attenuation in HUs) is inversely related to liver fat and thus is a surrogate for it (15). However, although extremely low HU values have been measured in livers infiltrated with fat, an overlap exists between normal and abnormal liver HU values (13). Therefore, the absolute liver density determined by CT may not be sensitive for predicting abnormal liver fat content. Because a constant relationship exists between liver and spleen attenuation in individuals with normal livers, the ratio of mean liver to spleen attenuation values is used as an index of liver fat, as originally described by Piekarski et al. (14) in 1980. Obtaining a CT image that contains both liver and spleen presents a challenge; variations exist not only in the vertical positioning of the spleen relative to the liver but also in positioning of both organs within the abdominal cavity. A multi-image approach is not feasible because of excess exposure (8). This implicates a single CT image approach; however, a single-image protocol at the level of the abdomen that routinely identifies the liver and spleen has yet to be firmly established. Furthermore, once obtained, it is important to determine whether the distribution of fat throughout the liver is uniform: an observation with direct implications for those that determine liver fat by measuring only small portions or regions of interest of the liver on the CT image (e.g., biopsy, magnetic resonance spectroscopy, and CT).
The aim of this study was twofold: first, to determine an optimal location for simultaneous imaging of both liver and spleen, and second, to document liver and spleen attenuation characteristics and determine whether variation exists, and if so, whether it is of a magnitude that should alter the protocol employed to measure liver fat.
| METHODS |
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98% of attendees at the Cooper Institute during this time were Caucasian. None of the subjects were smokers or had a history of diabetes mellitus, cardiovascular disease, stroke, or cancer. All gave informed consent before participation in the examination according to the ethical guidelines of The Cooper Institute Institutional Review Board. Medical examinations included an abdominal CT scan, comprising 40 contiguous images with 6-mm thickness, from at least T11 through L5, including all or most of the liver. CT images were obtained using an electron-beam CT scanner (Imatron, General Electric, Milwaukee, WI). The image-acquisition protocol used 130 kV and 630 mA with a 48-cm field of view and a 512 x 512 matrix. The full abdominal scan results in a patient radiation exposure of
640 mrem. Body weight and height were measured using a standard physician's scale and stadiometer, and body mass index (BMI) was calculated using the weight in kilograms divided by the height in meters squared. Waist circumference was measured at the level of the umbilicus using a plastic tape measure. Determination of optimal image location. A preliminary investigation was undertaken to determine the location of an easily identifiable, single axial image that would most frequently provide simultaneous visualization of liver and spleen. Axial images at the intervertebral spaces and the midpoints of the vertebral bodies within the region of T11T12 and L1L2 were visually inspected for the presence or absence of liver and spleen in a sample of 118 male and 76 female subjects (Table 1).
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Effect of image level on liver and spleen attenuation. On the cross-validation sample of 130 men and 113 women, the three images identified at T12L1, 12 mm superior, and 12 mm inferior were analyzed for liver and spleen attenuation characteristics using specialized image-analysis software (Tomovision, Montreal, Canada). Lines were manually drawn around the perimeter of the liver and spleen on each image to calculate the mean HU value for each organ. Mean HU values were obtained at each level to compare the differences in attenuation that might occur with subtle differences in locating T12L1.
Regional variation in tissue attenuation. We sought to further investigate liver and spleen tissue attenuation characteristics by dividing the liver and spleen at the T12L1 level into quartiles from anterior to posterior, and then subdividing each of the quartiles into medial and lateral regions, as depicted in Fig. 1. Our primary purpose in subdividing the images into these eight regions was to map regional variation in liver and spleen attenuation so that a recommendation could be made as to whether one region or another better represented the whole, and thus it be a better-suited location for placement of a region of interest when assessing liver fat by CT. The images 12 mm superior and 12 mm inferior to T12L1 were analyzed in a similar fashion to determine whether attenuation patterning differed in regions slightly above or below the targeted image.
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Statistical analyses. Data are presented as means (SD). Independent t-tests were used to assess gender differences. Univariate general linear modeling with repeated measures was used to determine the effect of landmarking on mean liver and spleen attenuation and to determine regional variation in organ attenuation compared with the whole. Bonferroni adjustment was used post hoc to correct for the multiple comparisons. Logistic regression was used to determine whether variance in anthropometric measures could explain potential differences in optimal image location. All statistical procedures were performed using SPSS version 12.0 (SPSS, Chicago, IL).
| RESULTS |
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The two landmarks with the highest percentage of liver and spleen appearance were the midpoint of T12 and the T12L1 intervertebral space. Because appearance rates were comparable in men and women at both sites, and because an intervertebral space is more readily identified than the midpoint of a vertebral body, the T12L1 intervertebral space was selected as the landmark for subsequent analyses.
Cross-validation of optimal image location. A separate sample of 130 men and 113 women, for which scans of the entire liver were available, was selected from a larger database. There was a relatively wide range of age and obesity in the sample selected, and the men differed from the women for most variables (Table 2).
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Effect of image level on liver and spleen attenuation. Mean attenuation values (e.g., HU) from liver and spleen derived from the three images analyzed in the cross-validation sample were compared to observe any differences that may be a function of obtaining an image slightly above or below the T12L1 level. A consistent, stepwise increase in tissue density of both liver and spleen was evident from the image 12 mm inferior to T12L1, and then from T12L1 to the image 12 mm superior (Fig. 2). The increases from level to level were statistically significant (P < 0.001) independent of sex, and they were an average magnitude of 1.5 HU (2.5%) in liver and 2.0 HU (3.9%) in spleen.
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For the images 12 mm above and below the T12L1 level, with few exceptions, the pattern of attenuation for the respective regions mirrored those of T12L1 in relation to the whole (data not shown).
| DISCUSSION |
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The accuracy of CT to estimate liver fat in vivo by comparison to histological determination of fat from liver biopsies was first described in the early 1980s (3, 4). In those studies, the CT number (attenuation values in HUs) was a strong, inverse correlate of liver fat from biopsy samples. However, because an overlap exists between normal and abnormal liver HU values (13), the absolute liver density determined by CT may not be sensitive for predicting abnormal liver fat content. Because a constant relationship exists between liver and spleen attenuation in individuals with normal livers, it was shown that the ratio of mean liver to spleen attenuation values provides a useful index of liver fat (14). Simultaneous measurement of liver and spleen attenuation to characterize liver fat in obesity was first reported by Goto et al. (7) in a study wherein the difference in the ratio of liver to spleen attenuation values were reported to be associated with insulin clearance and insulin sensitivity. A limitation of this study is that the authors did not clearly identify the landmark employed for acquisition of the CT image. That we observed a stepwise increase in both liver and spleen attenuation values from the images acquired 12 mm inferior and superior to T12L1 underscores the importance of proper landmarking to avoid differences in attenuation attributable to positioning error. Recently, Kelley et al. (9) used T11T12 as a landmark for imaging liver and spleen in men and women with Type 2 diabetes mellitus. In that study, the authors did not report the frequency for which the liver and spleen were observed nor whether any gender difference existed. The results of our study confirm that T11T12 is a useful landmark in men. In women, however, the ability of the image at T11T12 to identify the liver and spleen was substantially less by comparison to T12L1. Hence, our recommendation is that a single axial CT image at the T12L1 level is extremely useful for simultaneous identification of liver and spleen for both men and women with wide variation in age, visceral adiposity, and obesity. However, it is noteworthy that, while neither age nor visceral adiposity had any effect on liver and spleen appearance within the T12L1 image, subjects with an elevated BMI had a slightly lower success rate at that level. This point may be important for future studies to consider, especially when acquiring images in a morbidly obese sample of men and women. Because our sample contained relatively few subjects with a BMI in excess of 35 kg/m2, further research is required to establish whether T12L1 remains the optimal location in these individuals.
The region-by-region analysis of liver and spleen performed in this study sheds light on patterning of tissue attenuation in both organs. To our knowledge, this is the first study to employ a systematic sampling of liver and spleen that includes a comparison from medial to lateral, quartiles from anterior to posterior, and replication of these regions on images above and below to identify attenuation patterning in liver and spleen. Our objective in characterizing these regions was to identify specific areas of the liver and spleen that may best represent the whole image so that optimal locations for regions of interest could be recommended. However, although many of the regions varied statistically from the mean attenuation of the whole liver image, the magnitude was on average <2.0 HU. From a clinical perspective, this is a minor variation in light of reports suggesting that a 14-HU increase in liver attenuation is observed as a consequence of a modest 6% reduction in body weight (12). Furthermore, our findings suggest that methodologies that measure liver fat by assessment of regions of interest, including biopsy and magnetic resonance spectroscopy, need not be particular with respect to placement of the regions of interest, provided there is consistency in placement for serial measurements. Accordingly, with CT, it seems reasonable to recommend that the index of liver and spleen attenuation be derived from a mean score of the entire image. This would avoid potential bias introduced by variable placement of a region of interest and would gain the advantage of a much larger sample of tissue for estimating liver fat.
It is also important to note that with CT attenuation alone it is not possible to quantify liver fat. Because the molecular composition of lipid, water, and lean tissue within each voxel influences the resulting attenuation value, small variations in one may mask changes in the other. For example, it is entirely possible that elevations in the water component of a voxel may alter (increase) the measured attenuation, leading to the erroneous conclusion that the absolute lipid content was altered (decreased). This represents a challenge to the researcher when trying to interpret subtle changes in attenuation that may result from a given perturbation.
The findings of our study extend current knowledge with respect to CT-measured tissue characteristics of the liver and spleen and offers evidence for the improvement of methodology in CT image acquisition. Indeed, that the attenuation values within the liver are relatively homogeneous lends support to protocols that acquire liver fat measures within only a small region of interest. Furthermore, our results provide a compelling argument for the use of a single CT image at T12L1 as a practical, reliable method for routine measurement of liver fat in research and clinical settings.
| GRANTS |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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