Low-Dose CT for Renal Calculi Detection Using Spectral Shaping of High Tube Voltage

Abstract

Purpose To investigate reduction of radiation exposure in unenhanced CT in suspicion of renal calculi using a tin-filtered high tube voltage protocol compared to a standard low-dose protocol without spectral shaping.

Materials and Methods A phantom study using 7 human renal calculi was performed to test both protocols. 120 consecutive unenhanced CT examinations performed due to suspicion of renal calculi were included in this retrospective, monocentric study. 60 examinations were included with the standard-dose protocol (SP) (100 kV/130 mAs), whereas another 60 studies were included using a low-dose protocol (LD) applying spectral shaping with tin filtration of high tube voltages (Sn150 kV/80 mAs). Image quality was assessed by two radiologists in consensus blinded to technical parameters using an equidistant Likert scale ranging from 1–5 with 5 being the highest score. Quantitative image quality was assessed using regions of interest in abdominal organs, muscles, and adipose tissue to analyze image noise and signal-to-noise ratios (SNR). Commercially available dosimetry software was used to determine and compare effective dose (ED) and size-specific dose estimates (SSDEmean).

Results All seven renal calculi of the phantom could be detected with both protocols. There was no difference regarding calcluli size between the two protocols except for the smallest one. The smallest concretion measured 1.5 mm in LD and 1.0 mm in SP (ground truth 1.5 mm). CTDIvol was 3.36 mGy in LD (DLP: 119.3 mGycm) and 8.27 mGy in SP (DLP: 293.6 mGycm). The mean patient age in SP was 47 ± 17 years and in LD 49 ± 13 years. Ureterolithiasis was found in 33 cases in SP and 32 cases in LD. The median concretion size was 3 mm in SP and 4 mm in LD. The median ED in LD was 1.3 mSv (interquartile range (IQR) 0.3 mSv) compared to 2.3 mSv (IQR 0.9 mSv) in SP (p < 0.001). The SSDEmean of LD was also significantly lower compared to SP with 2.4 mGy (IQR 0.4 mGy) vs. 4.8 mGy (IQR 2.3 mGy) (p < 0.001). The SNR was significantly lower in LD compared to SP (p < 0.001). However, there was no significant difference between SP and LD regarding the qualitative assessment of image quality with a median of 4 (IQR 1) for both groups (p = 0.648).

Conclusion Tin-filtered unenhanced abdominal CT for the detection of renal calculi using high tube voltages leads to a significant reduction of radiation exposure and yields high diagnostic image quality without a significant difference compared to the institution’s standard of care low-dose protocol without tin filtration.

Key Points: 

• Tin-filtered CT for the detection of renal calculi significantly reduces radiation dose.

• The application of tin filtration provides comparable diagnostic image quality to that of SP protocols.

• An increase in image noise does not hamper diagnostic image quality.

Citation Format

• Gassenmaier S, Winkelmann MT, Magnus J et al. Low-Dose CT for Renal Calculi Detection Using Spectral Shaping of High Tube Voltage. Fortschr Röntgenstr 2022; 194: 1012 – 1019

Zusammenfassung

Ziel Ziel dieser Studie war die Untersuchung der Strahlendosisreduktion in der nativen Computertomografie (CT) bei Verdacht auf Nephro- und Urolithiasis mittels additiver Zinn-Filterung im Vergleich zum Standardprotokoll ohne spektrale Filterung.

Material und Methoden Es wurde eine Phantomstudie mit 7 humanen Nierensteinen durchgeführt. Zusätzlich wurden 120 konsekutive, native CT-Untersuchungen, die aufgrund des Verdachts auf Nierensteine durchgeführt wurden, in diese retrospektive, monozentrische Studie aufgenommen. 60 Untersuchungen wurden mit dem Standarddosisprotokoll (SP) (100 kV/130 mAs) durchgeführt, während weitere 60 Untersuchungen mit einem Niedrigdosisprotokoll (LD) mit additiver Zinn-Filterung (Sn150 kV/80 mAs) durchgeführt wurden. Die Bildqualität wurde in consensus durch 2 Radiologen (verblindet für die Akquisitionstechnik) anhand einer äquidistanten Likert-Skala von 1 bis 5 bewertet (5 = sehr gut). Die quantitative Bildqualität wurde mittels Region-of-interest-Analysen in den Bauchorganen sowie im Muskel- und Fettgewebe beurteilt und in Form des Bildrauschens und des Signal-Rausch-Verhältnisses (SNR) verglichen. Zur Analyse der Strahlendosis-Exposition kam eine kommerziell erhältliche Dosimetrie-Software zum Einsatz.

Ergebnisse Alle 7 Nierensteine des Phantoms konnten mit beiden Protokollen nachgewiesen werden. Hinsichtlich der Größe der Konkremente gab es keinen Unterschied zwischen den beiden Protokollen, mit Ausnahme des kleinsten Konkrements. Das kleinste Konkrement maß 1,5 mm in LD und 1,0 mm in SP (Ground Truth 1,5 mm). CTDIvol betrug 3,36 mGy in LD (DLP: 119,3 mGycm) und 8,27 mGy in SP (DLP: 293,6 mGycm). Das mittlere Patientenalter bei SP betrug 47 ± 17 Jahre und bei LD 49 ± 13 Jahre. Eine Ureterolithiasis wurde in 33 Fällen bei SP und in 32 Fällen bei LD gefunden. Die mediane Größe des Konkrements betrug 3 mm bei SP und 4 mm bei LD. Die mediane effektive Dosis (ED) bei LD betrug 1,3 mSv (Interquartilenabstand (IQR) 0,3 mSv) im Vergleich zu 2,3 mSv (IQR 0,9 mSv) bei SP (p < 0,001). Die Schätzung der diameterkorrigierten Dosis (SSDEmean) bei LD war mit 2,4 mGy (IQR 0,4 mGy) im Vergleich zu 4,8 mGy (IQR 2,3 mGy) bei SP ebenfalls signifikant niedriger (p < 0,001). SNR war bei LD im Vergleich zur SP signifikant niedriger (p < 0,001). Hinsichtlich der qualitativen Beurteilung der Bildqualität gab es jedoch keinen signifikanten Unterschied zwischen SP und LD mit einem Median von 4 (IQR 1) für beide Gruppen (p = 0,648).

Schlussfolgerung Die native CT mit additiver Zinn-Filterung zur Detektion von Nierensteinen führt zu einer signifikanten Reduktion der Strahlendosis im Vergleich zum Standard Low-Dose-Protokoll bei gleichbleibender diagnostischer Aussagekraft.

Kernaussagen: 

• Signifikante Reduktion der Strahlendosis mittels Zinn-gefilterter CT zur Detektion von Nierensteinen

• Vergleichbare diagnostische Aussagekraft der Niedrigdosis-CT trotz Anwendung von Zinn-Filterung

• Keine Kompromittierung der diagnostischen Bildqualität durch Anstieg des Bildrauschens

Key words

CT - calcifications/calculi - nephrolithiasis - tin filtration - kidney - radiation

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