The optimal dose reduction level using iterative reconstruction with prospective ECG-triggered coronary CTA using 256-slice MDCT.

AIM

To assess the image quality (IQ) of an iterative reconstruction (IR) technique (iDose(4)) from prospective electrocardiography (ECG)-triggered coronary computed tomography angiography (coronary CTA) on a 256-slice multi-detector CT (MDCT) scanner and determine the optimal dose reduction using IR that can provide IQ comparable to filtered back projection (FBP).

METHOD AND MATERIALS

110 consecutive patients (69 men, 41 women; age: 54 ± 10 years) underwent coronary CTA on a 256-slice MDCT (Brilliance iCT, Philips Healthcare). The control group (Group A, n=21) were scanned using the conventional tube output (120 kVp, 210 mAs) and reconstructed using FBP. The other 4 groups were scanned with the same kVp but successively reduced tube output as follows: B[n=15]: 125 mAs; C[n=22]: 105 mAs; D[n=36]: 84 mAs: E[n=16]: 65 mAs) and reconstructed using IR levels of L3 (Group B), L4 (Group C) and L5 (Groups D and E), to compensate for the noise increase. All images were reconstructed using the same kernel (XCB). Two radiologists graded IQ in a blinded fashion on a 4-point scale (4 - excellent, 3 - good, 2 - fair and 1 - poor). Quantitative measurements of CT values, image noise and contrast-to-noise (CNR) were measured in each group. A receiver-operating characteristic (ROC) analysis was performed to determine a radiation reduction threshold up to which excellent IQ was maintained.

RESULTS

There were no significant differences in objective noise, SNR and CNR values among Groups A, B, C, D, and E (P=0.14, 0.09, 0.17, respectively). There were no significant differences in the scores of the subjective IQ between Group A, and Groups B, C, D, E (P=0.23-0.97). Significant differences in image sharpness and study acceptability were observed between groups A and E (P<0.05). Using the criterion of excellent IQ (score 4), the ROC curve of dose levels and IQ acceptability established a reduction of 60% of tube output (Group D) as optimum cutoff point (AUC: 0.72, 95% CI: 0.59-0.86). Group D (84 mAs with L5) provided equivalent subjective image ranking (with lumen sharpness taken into account) and objective IQ measurements (noise: 36.5 ± 10.7; SNR: 13.6 ± 4.9; CNR: 16.28 ± 5.4) compared with FBP images in Group A (noise: 35.5 ± 9.4; SNR: 12.4 ± 2.5; CNR: 15.4 ± 3.2) (P=0.14, 0.09, 0.17, respectively). The effective dose (ED) of Group D was 63% lower than that of Group A (1.2 ± 0.1 mSv versus 3.2 ± 0.6 mSv).

CONCLUSION

Iterative reconstruction techniques can provide 63% ED reduction in prospectively-triggered coronary CTA using 256-slice MDCT while maintaining excellent image quality.