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

Publication History

Received: 13 September 2019

Accepted: 03 August 2020

Article published online:
03 September 2020

• References

• 1 Smyth M, Sutton D, Houston J. Evaluation of the quality of CT-like images obtained using a commercial flat panel detector system. Biomed Imaging Interv J 2006; 2: e48
• 2 Dörfler A, Struffert T, Engelhorn T. et al. Rotational flat-panel computed tomography in diagnostic and interventional neuroradiology. Fortschr Röntgenstr 2008; 180: 891-898
• 3 Braun H, Kyriakou Y, Kachelrieß M. et al. The influence of the heel effect in cone-beam computed tomography: artifacts in standard and novel geometries and their correction. Phys Med Biol 2010; 55: 6005-6021
• 4 Ott S, Struffert T, Saake M. et al. Influence of different reconstruction parameters in the visualization of intracranial stents using c-arm flat panel CT angiography: experience in an animal model. Acta Radiol 2015; 57: 233-240
• 5 Werncke T, Sonnow L, Meyer BC. et al. Ultra-high resolution C-Arm CT arthrography of the wrist: Radiationdose and image quality compared to conventional multidetectorcomputed tomography. Euro J Radiol 2017; 89: 191-199
• 6 Jones AK, Mahvash A. Evaluation of the potential utility of flat panel CT for quantifying relative contrast enhancement. Med Phys 2012; 39: 4149-4154
• 7 Orth RC, Wallace MJ, Kuo MD. C-arm Cone-beam CT: General Principles and Technical Considerations for Use in Interventional Radiology. Journal of Vascular and Interventional Radiology 2008; 19: 814-820
• 8 Fahrig R, Dixon R, Payne T. et al. Dose and image quality for a cone-beam C-arm CT system. Med Phys 2006; 33: 4541-4550
• 9 Rana N, Rawat D, Parmar M. et al. Evaluation of external beam hardening filters on image quality of computed tomography and single photon emission computed tomography. Journal of Medical Physics 2015; 40: 198-206
• 10 Zhang G, Marshall N, Jacobs R. et al. Bowtie filtration for dedicated cone beam CT of the head and neck: a simulation study. Br J Radiol 2013; 86: 20130002
• 11 Mori S, Endo M, Nishizawa K. et al. Prototype heel effect compensation filter for cone-beam CT. Phys Med Biol 2005; 50: N359-N370
• 12 Kusano Y, Uesaka S, Yajima K. et al. Positional dependence of the CT number with use of a cone-beam CT scanner for an electron density phantom in particle beam therapy. Radiol Phys Tech 2013; 6: 241-247
• 13 McCollough CH, Bruesewitz MR, McNitt-Gray MF. et al. The phantom portion of the American college of radiology (ACR) Computed Tomography (CT) accreditation program: Practical tips, artifact examples, and pitfalls to avoid. Med Phys 2004; 31: 2423-2442
• 14 Hinrichs JB, von Falck C, Hoeper MM. et al. Pulmonary Artery Imaging in Patients with Chronic Thromboembolic Pulmonary Hypertension: Comparison of Cone-Beam CT and 64-Row Multidetector CT. J Vasc Inter Radiol 2016; 27: 361-368
• 15 Dixon RL, Munley MT, Bayram E. An improved analytical model for CT dose simulation with a new look at the theory of CT dose. Med Phys 2005; 32: 3712-3728
• 16 Dixon RL, Boone JM. Analytical equations for CT dose profiles derived using a scatter kernel of Monte Carlo parentage with broad applicability to CT dosimetry problems. Med Phys 2011; 38: 4251-4264
• 17 Droege RT, Morin RL. A practical method to measure the MTF of CT scanners. Med Phys 1982; 9: 758-760
• 18 Grimmer R, Kachelrieß M. Empirical binary tomography calibration (EBTC) for the precorrection of beam hardening and scatter for flat panel CT. Med Phys 2011; 38: 2233-2240
• 19 Hunter AK, McDavid WD. Characterization and correction of cupping effect artifacts in cone beam CT. Dentomaxillofacial Radiol 2012; 41: 217-223
• 20 Nagarajappa AK, Dwivedi N, Tiwari R. Artifacts: The downturn of CBCT image. J Inter Soc of Prev Comm Dentis 2015; 5: 440-445
• 21 Barrett JF, Keat N. Artifacts in CT: Recognition and Avoidance. RadioGraphics 2004; 24: 1679-1691
• 22 Bai M, Liu B, Mu H. et al. The comparison of radiation dose between C-arm flat-detector CT (DynaCT) and multi-slice CT (MSCT): A phantom study. Euro J Radiol 2012; 81: 3577-3580
• 23 Schegerer AA, Lechel U, Ritter M. et al. Dose and image quality of cone-beam computed tomography as compared with conventional multislice computed tomography in abdominal imaging. Invest Radiol 2014; 49: 675-684
• 24 Jones AK, Odisio BC. Comparison of radiation dose and image quality between flat panel computed tomography and multidetector computed tomography in a hybrid CT-angiography suite. Journal of Applied Clinical Medical Physics 2020; 21: 121-127