Upgrade to lterative Image Reconstruction (lR) in MDCT lmaging: A Clinical Study for Detailed Parameter Optimization Beyond Vendor Recommendations Using the Adaptive Statistical lterative Reconstruction Environment (ASIR) Part2: The Chest

Abstract

Purpose: To compare the image quality in dose-reduced 64-row CT of the chest at different levels of adaptive statistical iterative reconstruction (ASIR) to full-dose baseline examinations reconstructed solely with filtered back projection (FBP) in a realistic upgrade scenario.

Materials and Methods: A waiver of consent was granted by the institutional review board (IRB). The noise index (NI) relates to the standard deviation of Hounsfield units in a water phantom. Baseline exams of the chest (NI = 29; LightSpeed VCT XT, GE Healthcare) were intra-individually compared to follow-up studies on a CT with ASIR after system upgrade (NI = 45; Discovery HD750, GE Healthcare), n = 46. Images were calculated in slice and volume mode with ASIR levels of 0 – 100 % in the standard and lung kernel. Three radiologists independently compared the image quality to the corresponding full-dose baseline examinations (-2: diagnostically inferior, -1: inferior, 0: equal, + 1: superior, + 2: diagnostically superior). Statistical analysis used Wilcoxon’s test, Mann-Whitney U test and the intraclass correlation coefficient (ICC).

Results: The mean CTDIvol decreased by 53 % from the FBP baseline to 8.0 ± 2.3 mGy for ASIR follow-ups; p < 0.001. The ICC was 0.70. Regarding the standard kernel, the image quality in dose-reduced studies was comparable to the baseline at ASIR 70 % in volume mode (-0.07 ± 0.29, p = 0.29). Concerning the lung kernel, every ASIR level outperformed the baseline image quality (p < 0.001), with ASIR 30 % rated best (slice: 0.70 ± 0.6, volume: 0.74 ± 0.61).

Conclusion: Vendors’ recommendation of 50 % ASIR is fair. In detail, the ASIR 70 % in volume mode for the standard kernel and ASIR 30 % for the lung kernel performed best, allowing for a dose reduction of approximately 50 %.

Zusammenfassung

Ziel: Zielsetzung war im Rahmen eines System-Upgrades ein Vergleich der Bildqualität dosis-reduzierter 64-Zeilen CT´s des Thorax, die mit unterschiedlichem Einfluss von Adaptiver Statistischer Iterativer Rekonstruktion (ASIR) rekonstruiert wurden, mit Voll-Dosis-Voruntersuchungen, welche mittels gefilterter Rückprojektion (FBP) rekonstruiert wurden.

Material und Methoden: Die Ethikkommission hatte keine Bedenken. Der Rauschindex (NI) bezieht sich auf die Standardabweichung in einem Wasserphantom. Baseline-Untersuchungen des Thorax (NI = 29; LightSpeed VCT XT, GE) wurden nach System-Upgrade intra-individuell mit Follow-Up-Untersuchungen eines CT mit ASIR verglichen (NI = 45; Discovery HD750, GE), n = 46. Im Standard- und Lung-Kernel wurden Bilder mit 0 – 100 % ASIR-Einfluss sowohl im Slice- als auch im Volumen-Modus berechnet. Drei Radiologen verglichen die Bildqualität mit der jeweiligen Voll-Dosis-Baseline (-2: diagnostisch schlechter, -1: schlechter, 0: gleich, + 1: besser, + 2: diagnostisch besser). Zur Analyse wurden der Wilcoxon, der Mann-Whitney-U-Test und die Intra-Class-Correlation’s Koeffizient (ICC) verwendet.

Ergebnisse: Im Vergleich zur FBP-Baseline verringerte sich der durchschnittliche CTDIvol der ASIR-Follow-Ups um 53 % auf 8,0 ± 2,3 mGy (p < 0,001, ICC = 0,70). Mit Standard-Kernel war die durchschnittliche Bildqualität der dosis-reduzierten Studien mit denen der Baseline bei ASIR 70 % im Volumen-Modus (-0.07 ± 0.29, p = 0.29) vergleichbar. Für den Lung-Kernel war die Bildqualität jeder ASIR-Stufe besser als die der Baseline (p < 0.001), mit den besten Ergebnissen bei ASIR 30 % (slice: 0.70 ± 0.6, volume: 0.74 ± 0.61).

Schlussfolgerung: Die Herstellerempfehlung von 50 % ASIR ist in Ordnung. Im Detail waren aber mit dem Standard-Kernel und ASIR 70 % im Volumen-Modus und mit dem Lung-Kernel und ASIR 30 % die besten Ergebnisse zu erreichen und erlauben eine Dosiseinsparung von etwa 50 %.

Key words

thorax - CT - iterative image reconstruction - statistical image noise - dose reduction

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