Dual source dual energy MDCT: comparison of 80 kVp and weighted average 120 kVp data for conspicuity of hypo-vascular liver metastases

Abstract

Purpose: To determine whether liver metastases conspicuity is improved at 80 kVp when compared with weighted average (WA) simulated 120 kVp data using dual source dual energy CT.

Methods: A total of 11 patients with 44 hypo-vascular liver metastases underwent contrast enhanced Dual Energy CT (DECT). In all cases the subject's abdominal diameter measured <or=35 cm. Data were reconstructed as a WA of the 140 kVp and 80 kVp acquisitions (simulating 120 kVp) and as a pure 80 kVp data set. A region of interest cursor was placed within the metastasis and adjacent normal parenchyma and attenuation differences and contrast to noise ratios (CNR) were calculated for the metastases at 80 kVp and on the WA acquisition. A mixed model 2-way analysis of variance was used to test whether the attenuation difference between metastases and normal liver was higher at 80 kVp than 120 kVp. An exact Wilcoxon matched-pairs signed rank test was used to test whether CNR was higher at 80 kVp. Cases were retrospectively reviewed to determine whether lesions could be seen on only one or both data sets. As the 80 kVp tube has a smaller detector than the 140 kVp tube, we also noted whether any of the liver lesions were not included on the 80 kVp dataset. Two radiologists in consensus evaluated the 80 kVp data and WA data and subjectively rated hepatic metastases conspicuity on a 4 point scale; with 1 being excellent, 2 good, 3 poor, and 4 not seen.

Results: The mean size of the metastases was 2.6 cm. The mean +/- SD of the attenuation difference between the metastases and the normal liver was 78.37 +/- 24.6 at 80 kVp and 56.89 +/- 17.9 at 120 kVp. The mean difference in attenuation was significantly higher at 80 kVp (P < 0.001). In 2 cases, a metastases was only seen at 80 kVp. The difference between 80 and 120 kVp in terms of CNR was statistically significant (P = 0.042). In one patient, 11 lesions were not included in the smaller field of view of the 80 kVp detector. The conspicuity scores were rated as significantly better at 80 kv than at 120 kVp (P < 0.0001).

Conclusion: When compared with 120 kVp data, pure 80 kVp data acquired from a dual source dual energy MDCT scanner demonstrates greater attenuation differences and improved contrast to noise between metastatic disease and normal liver.