Xe-133 retention ratio: A useful index for fatty liver quantification

This study was undertaken to develop an index, Xe-133 retention ratio, useful for quantitating fatty liver (FL). Data were acquired in frame mode in the hepatic region and both lung bases for 5 min after rebreathing 20 mCi of gaseous Xe-133 and for another 5 min during washout. Static Tc-99m sulfur colloid liver imaging was performed with the patient in the identical position immediately after the ventilation study and data were stored for liver localization. A hepatic time-activity curve corrected for background activity was generated. The Xe-133 retention ratio (RR) was derived by dividing the activity at 3.5 min after washout by the peak activity. Sixteen normal subjects (N) and 15 patients (pts) with proved FL were studied. RRR (mean +- SD) was greatly increased in pts with FL (0.38 +- 0.20 vs 0.04 +- 0.08 in N, p < 0.005). The sensitivity and specificity for diagnosing FL were 0.93 and 0.94, respectively. There was a strong positive correlation between RR's and percentage of fat on biopsy (r = 0.92, SEE = 0.08, p < 0.005) in 15 pts. Intra-and interobserver variations for RR procedure were very small (r = 0.98 by each of 3 observers; r = 0.97, 0.98, 0.98 for interobserver correlation). The authors' preliminary results indicate that Xe-133 retention ratio provides a simple, accurate, and useful index of quantifying and evaluating fatty infiltration of the liver