Influence of patient alignment on image quality provided by a C-arm flat-panel detector computer tomography: a phantom study

TL;DR

This phantom study investigated how patient positioning affects image quality in C-arm flat-panel detector CT, finding minor effects mainly due to non-symmetrical rotation rather than the anode heel effect.

Contribution

It provides new insights into the minimal impact of patient alignment on CACT image quality and clarifies the underlying causes.

Findings

Patient alignment has a minor influence on image quality.

The anode heel effect does not significantly affect image quality in this context.

Non-symmetrical rotation of the CACT is the main factor affecting image quality.

Abstract

Purpose: The aim of this phantom study was to evaluate the influence of patient alignment on the image quality by a C-arm flat-panel detector computer tomography (CACT). Materials and Methods: An ACR phantom placed in two opposite directions along the z-Axis was imaged using a CACT. Image acquisition was performed by three different image acquisition protocols using fixed X-ray tube voltages of 81, 102 and 125 kVp. The images were reconstructed with four different convolution kernels, i.e. normal, sharp, soft and very soft. Image quality was assessed in terms of high contrast image quality using the modulation transfer function (MTF) and low contrast image quality by assessing the signal-to-noise ratio (SNR) and contrast-to-noise ratios (CNR) as well as reliability of density measurements. Furthermore, the dose intensity profiles parallel and perpendicular to the patient support were measured free-in-air. Results: The intensity profile of the CACT measured by the detector system free-in-air showed that the anode heel effect is not in the longitudinal direction to the z-axis. The image noises measured in Setup A for the air and bone inserts were systematically higher compared to those measured in Setup B, in average about 3% and 4% for the air and bone inserts, respectively. An opposite behavior has been observed for the polyethylene, water-equivalent and acrylic inserts. The corresponding image noises were in average about 4%, 6% and 2% lower measured in Setup A compared to those measured in Setup B. SNR for all inserts behaves inversely to the image noise. Conclusion: The patient alignment has a minor influence on the image quality of CACT. This effect is not based on the X-ray anode heel effect. It is caused mainly. This effect is caused mainly by the non-symmetrical rotation of the CACT.