Impact of Patient Alignment on Image Quality in C-Arm Computed Tomography – Evaluation Using an ACR Phantom

Abstract

Purpose To evaluate the influence of patient alignment and thereby heel effect on the image quality (IQ) of C-arm flat-panel detector computed tomography (CACT).

Materials and Methods An ACR phantom placed in opposite directions along the z-axis (setup A and B) on the patient support was imaged using CACT. Image acquisition was performed with three different image acquisition protocols. The images were reconstructed with four convolution kernels. IQ was assessed in terms of high contrast using the modulation transfer function (MTF) and low contrast by assessing the image noise, signal-to-noise ratio (SNR) and contrast-to-noise ratios (CNR) as well as the reliability of density measurements. Furthermore, the dose intensity profiles were measured free-in-air.

Results The MTF in setup B is higher than the MTF measured in setup A (p < 0.01). The image noises measured in setup A for the air and bone inserts were higher compared to those measured in setup B (p > 0.05). Opposite behavior has been observed for the polyethylene, water-equivalent and acrylic inserts. The SNR for all inserts is inversely related to the image noise. A systematically increasing or decreasing trend of CNR could not be observed (p > 0.05). The intensity profile measured by the detector system free-in-air showed that the anode heel effect is perpendicular to the z-axis.

Conclusion The patient alignment has a minor influence on the IQ of CACT. This effect is not based on the X-ray anode heel effect but is caused mainly by the non-symmetrical rotation of CACT.

Key Points: 

• The impact of patient alignment and thereby the heel effect on the image quality of CACT was analyzed.

• The patient alignment has a minor influence on the physical parameters related to image quality, such as noise, SNR, and MTF.

• This effect is based mainly on the non-symmetrical rotation of CACT.

Citation Format

• Alikhani B, Renne J, Maschke S et al. Impact of Patient Alignment on Image Quality in C-Arm Computed Tomography – Evaluation Using an ACR Phantom. Fortschr Röntgenstr 2021; 193: 417 – 426

Zusammenfassung

Ziel Der Einfluss der Patientenausrichtung und des Heel-Effekts auf die Bildqualität einer C-Bogen-Flachbilddetektor-Durchleuchtungseinheit (CACT) wurde untersucht.

Material und Methoden Ein ACR-Phantom wurde in entgegengesetzten Richtungen entlang der z-Achse auf dem Patiententisch platziert (Setup A und B). Die Messungen wurden an einem CACT mit neuartigen Flachdetektoren durchgeführt. Die Bilddaten wurden mit 3 verschiedenen Untersuchungsprotokollen und 4 Faltungskernen rekonstruiert. Die Hochkontrastauflösung mithilfe der Modulationsübertragungsfunktion (MTF) und die Niederkontrastsauflösung unter Verwendung des Bildrauschens, des Signal-Rausch-Verhältnisses (SNR) und des Kontrast-Rausch-Verhältnisses (CNR) wurden bestimmt. Außerdem wurden die Dosisintensitätsprofile „frei in Luft“ gemessen.

Ergebnisse MTF in Setup B ist höher als in Setup A (p < 0,01). Das Rauschen in Setup A für die Luft- und Knocheneinsätze war höher als in Set-up B (p > 0,05). Ein umgekehrter Verlauf wurde für die Einsätze aus Polyethylen, Wasseräquivalent und Acryl beobachtet. SNR für alle Einsätze ist umgekehrt proportional zum Bildrauschen. Ein systematisch ansteigender oder abnehmender Trend von CNR konnte nicht beobachtet werden (p > 0,05). Das vom Detektorsystem gemessene Intensitätsprofil „frei in Luft“ zeigte, dass der Heel-Effekt senkrecht zur z-Achse und nicht parallel dazu verläuft.

Schlussfolgerung Die Patientenausrichtung hat einen geringen Einfluss auf die Bildqualität von CACT. Dieser Effekt beruht nicht auf dem Hell-Effekt, sondern durch die asymmetrische Rotation des CACT-Arms.

Kernaussagen: 

• Der Einfluss der Patientenausrichtung und des Heel-Effektes auf die Bildqualität von CACT wurde analysiert.

• Die Patientenausrichtung hat einen geringen Einfluss auf die physikalischen Bildqualitätsparameter wie Rauschen, SNR und MTF.

• Dieser Effekt beruht hauptsächlich auf der asymmetrischen Rotation des CACT-Arms.

Key words

C-arm flat-panel detector computed tomography - flat-panel detector - patient alignment - heel effect - image quality

Publikationsverlauf

Eingereicht: 13. September 2019

Angenommen: 03. August 2020

Artikel online veröffentlicht:
03. September 2020

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