Estimation of the Minimum Dose Required to Measure Ventricular Width in Follow-Up Cranial Computed Tomography (CCT) in Children with Hydrocephalus

Zusammenfassung

Ziel: Die für computertomografische Verlaufskontrollen der Ventrikelweite bei Kindern mit Hydrozephalus erforderliche Mindestdosis abzuschätzen. Material und Methoden: Als Messphantom verwendeten wir eine mit Gelatine gefüllte kindliche Kalotte, in die als Seitenventrikel die zentralen Faserbündel von 2 Möhren eingebettet waren. Das Phantom wurde mit 2 Mehrschicht-Computertomografen (LightSpeed Ultra, GE und Somatom Sensation, Siemens) jeweils 10-mal untersucht, wobei ein Röhrenstrom von 400, 350, 300, 250, 200, 150 und 100 mAs sowie eine Röhrenspannung von 140, 120, 100 und 80 kV verwendet wurden. Die Ventrikelweite wurde an 4 Stellen gemessen. Die bei 380 mAs/ 400 mAs und 140 kV (LightSpeed/Somatom) gemessenen Ventrikelweiten dienten als Referenz. Messwerte erhielten einen Punkt, wenn sie um maximal 0,5 mm von der Referenz abwichen. Ergebnisse: Die Strahlendosis konnte beim LightSpeed von 60,9 mGy auf 9,2 mGy (15,1 %) und beim Somatom von 55,0 mGy auf 8,0 mGy (14,6 %) reduziert werden, ohne die Verlässlichkeit der Messungen zu beeinträchtigen. Allerdings gab es bei beiden Geräten bestimmte Kombinationen von Röhrenstrom und -spannung, die trotz höherer Dosis und geringeren Pixelrauschens weniger verlässliche Messergebnisse lieferten. Schlussfolgerung: Es gibt keine einheitliche Mindestdosis oder bestimmte Kombination von Röhrenstrom und -spannung, die für beide Computertomografen verlässliche Messungen der Ventrikelweite bei geringst möglicher Strahlendosis garantiert. Als Faustregel kann der Röhrenstrom des Standardprotokolls auf 100 kV reduziert werden, ohne die Messgenauigkeit zu beeinträchtigen.

Abstract

Purpose: To estimate the minimum dose needed at follow-up cranial computed tomography (CCT) to reliably determine ventricular width in children with hydrocephalus. Materials and Methods: For the study, a phantom was created using the calvarium of an infant which was filled with gelatin and the shaped inner cones of two carrots serving as lateral ventricles. The phantom was scanned ten times with two multi-slice CTs (LightSpeed Ultra, GE, and Somatom Sensation, Siemens), using a tube current of 400, 350, 300, 250, 200, 150, and 100 mA, and a tube voltage of 140, 120, 100, and 80 kV. The width of both lateral ventricles was measured at 4 sites. The values derived from scans performed at 380 / 400 mA and 140 kV (LightSpeed/Somatom) served as a reference. Measurements scored 1 point if they did not differ by more than 0.5 mm from the reference values. Results: The radiation dose can be reduced from 61.0 mGy to 9.2 mGy (15.1 %) with LightSpeed and from 55.0 mGy to 8.0 mGy (14.6 %) with Somatom without impairing the reliability of ventricular width measurements. However, in the case of both scanners, certain combinations of tube voltage and current yielded less reliable measurements although the dose was higher and the pixel noise was lower. Conclusion: There is no single cut-off dose or setting for tube voltage and current which guarantees reliable ventricular width measurements with the least radiation exposure for both scanners. As a guideline, it is safe to use the standard protocols with a reduced tube current of 100 kV.

Key words

brain - CT spiral - hydrocephalus - ventricles - radiation dose - children

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