Accuracy and Precision of Dual-Energy CT for Measuring Iodine Concentration: Validation in Phantom Studies

Purpose
Dual-energy CT (DECT) has been proposed as a method for evaluating organ perfusion at contrast enhanced CT. We analyzed the DECT signal characteristics of iodine with the goal of establishing the accuracy and precision of DECT for quantifying different iodine concentrations in a tissue phantom.

Method and materials
In vitro experiments were carried out using 1st and 2nd generation dual-source CT (DSCT) scanners (Somatom Definition and Flash, Siemens) in DECT mode and a dual-energy phantom (QRM) with inserts containing varying, known iodine concentrations. The phantom can be fitted with two fat-equivalent rings to simulate different body types. Both, spiral and sequential acquisition modes were evaluated. Both, the 180° half-rotation and 360° full-rotation reconstruction algorithms were tested. 2nd generation DSCT was evaluated in 64- and 128-slice modes. 1st generation DSCT was operated at 80 kVp and 100 kVp. All measurements were performed in triplicate (phantom alone and with one and two fat rings added). The Perfusion Blood Volume software was used to measure the iodine-specific DECT signal within the tissue phantoms.

Results
Calibration curves were established correlating the DECT iodine signal with the known iodine contents of the phantom inserts. There was excellent overall correlation (r=0.97) between known and measured iodine concentrations across all scan protocols with a mean overall standard deviation of 0.4mg/ml. The standard deviation measured 0.3mg/ml at the lowest iodine concentration (5mg/ml) and doubled to 0.6mg/ml at the highest iodine concentration (20mg/ml). Standard deviation was lowest (0.15mg/ml) with no fat rings and increased to 0.4mg/ml when two fat rings were added. Across all body types, 2nd generation DSCT in 64-slice spiral mode with 180° and 360° reconstruction as well as the 360° 64-slice sequential mode showed the lowest standard deviation (0.2mg/ml for all). The standard deviation was highest (0.6mg/ml) when 1st generation DSCT at 80 kVp in 360° 64-slice sequential and spiral modes was used.

Conclusion
DECT enables accurate and precise measurements of iodine concentrations in a tissue phantom. The performance is stable across platforms and scan protocols, although standard deviations are lower with 2nd generation DSCT.

Clinical relevance/application
Based on quantification of iodine within tissues, DECT may be able to estimate blood volume as a surrogate of organ perfusion at contrast enhanced CT.

Submission Type: Scientific Presentations RSNA 2010

Authors: J D Koonce, MD, Charleston, SC; B Schmidt, PhD; M Weininger, MD; G Bastarrika, MD; T G Flohr, PhD; U J Schoepf, MD