Purpose: The goal of this work was to develop a method of calculating blood flow and xenon solubility coefficient (λ) in the hepatic tissue by xenon-enhanced computed tomography (Xe-CT) and to demonstrate λ can be used as a measure of fat content in the human liver.
Methods: A new blood supply model is introduced which incorporates both arterial blood and portal venous blood which join and together flow into hepatic tissue. We applied Fick's law to the model. It was theoretically derived that the time course of xenon concentration in the inflow blood (the mixture of the arterial blood and the portal venous blood) can be approximated by a monoexponential function. This approximation made it possible to obtain the time-course change rate (K(I)) of xenon concentration in the inflow blood using the time course of xenon concentration in the hepatic tissue by applying the algorithm we had reported previously. K(I) was used to calculate blood flow and λ for each pixel in the CT image of the liver. Twenty-six patients (49.2 ± 18.3 years) with nonalcoholic steatohepatitis underwent Xe-CT abdominal studies and liver biopsies. Steatosis of the liver was evaluated using the biopsy specimen and its severity was divided into ten grades according to the fat deposition percentage [(severity 1) ≤ 10%, 10 % <(severity 2) ≤ 20%, [ellipsis (horizontal)], 90% < (severity 10) ≤ 100%]. For each patient, blood flow and λ maps of the liver were created, and the average λ value (λ) was compared with steatosis severity and with the CT value ratio of the liver to the spleen (liver∕spleen ratio).
Results: There were good correlations between λ and steatosis severity (r = 0.914, P < 0.0001), and between λ and liver∕spleen ratio (r = -0.881, P < 0.0001). Ostwald solubility for xenon in the hepatic tissue (tissue Xe solubility), which is calculated using λ and the hematocrit value of the patient, also showed a good correlation with steatosis severity (r = 0.910, P < 0.0001). λ ranged from 0.86 to 7.81, and tissue Xe solubility ranged from 0.12 to 1.16. This range of solubility is reasonable considering the reported Ostwald solubility coefficients for xenon in the normal liver and in the fat tissue are 0.10 and 1.3, respectively, at 37 °C. The average blood flow value ranged from 15.3 to 53.5 ml∕100 ml tissue∕min.
Conclusions: A method of calculating blood flow and λ in the hepatic tissue was developed by means of Xe-CT. This method would be valid even if portosystemic shunts exist; it is shown that λ maps can be used to deduce fat content in the liver. As a noninvasive modality, Xe-CT would be applicable to the quantitative study of fatty change in the human liver.