Evaluating CMR

Measuring Ectopic Fat Using CT

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Figure 4: Measuring liver fat by computed tomography (CT): normal vs. fatty liver

Figure 4: Measuring liver fat by computed tomography (CT): normal vs. fatty liver

CT can also be used to assess ectopic fat in the muscle and liver (36-39). As measured by CT, liver fat infiltration is calculated by determining the attenuation values for each voxel within a region of interest in the liver. CT attenuation values depend on the molecular composition of the tissues within each voxel. Fat has a lower density than water and protein, and liver fat infiltration is reflected by a lower liver density and thus lower attenuation values (37). However, normal and fatty liver values overlap (40). As such, it has been suggested that liver density (CTL) should be expressed relative to the spleen attenuation value (CTS), which is not infiltrated with fat (Figure 4) (40). In normal individuals, liver and spleen attenuation values have a constant relationship. The liver, however, is a denser organ and therefore has a higher attenuation value. A liver-to-spleen attenuation (CTL/CTS) ratio of less than one therefore indicates fatty infiltration (37, 40). Normally, the liver and spleen mean attenuation values are based on two or three regions of interest within the liver and spleen. However, due to the small area of interest and subjectivity involved in determining the regions of interest, the whole liver and spleen surface areas should be used to determine respective mean attenuation values (Figure 4) (41). Attenuation values within the liver and spleen are fairly homogeneous throughout, and using the whole surface area can slightly reduce the inter-observer coefficient of variation from 5.1% (42) to 2.9% (41). However, it is difficult to obtain a CT image that contains both liver and spleen. Not only does the spleen’s vertical position vary relative to the liver, both organs also vary in terms of their position within the abdominal cavity. As a multi-image approach is not feasible because of excess radiation exposure (43), a single axial image at the T12-L1 inter-vertebral space may be the best landmark for assessing both liver and spleen attenuation, given that liver and spleen can be identified at that level in approximately 90% of the men and women studied (41).

Reference

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41. Davidson LE, Kuk JL, Church TS, et al. Protocol for Measurement of Liver Fat by Computed Tomography. J Appl Physiol 2005.

42. Nguyen-Duy TB, Nichaman MZ, Church TS, et al. Visceral fat and liver fat are independent predictors of metabolic risk factors in men. Am J Physiol Endocrinol Metab 2003; 284: E1065-71.

43. Joy D, Thava VR and Scott BB. Diagnosis of fatty liver disease: is biopsy necessary? Eur J Gastroenterol Hepatol 2003; 15: 539-43.

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