The Impact of Parameter Selection and Setup Conditions on Image Quality of an On-Board Helical Kilovoltage Computed Tomography System

Abstract

To evaluate the imaging performance of an on-board helical kilovoltage computed tomography (kVCT) system mounted on a helical tomotherapy unit for various imaging parameters and setup conditions. MethodsImages of a commonly used computed tomography (CT) image quality phantom were acquired while varying the selection of available parameters (anatomy, mode, body size) as well as phantom positioning and size. Image quality metrics (IQM) including noise, uniformity, contrast, CT number constancy, and spatial resolution were compared for parameter and setup variations. ResultsThe use of fine mode improved noise and contrast metrics by 20-30% compared to normal mode and by nearly a factor of two compared to the coarse mode for otherwise identical protocols. Uniformity, CT number constancy, and spatial resolution were also improved for fine mode. Thorax and pelvis anatomy protocols improved noise, uniformity, and contrast metrics by 10-20% compared to images acquired with head protocols. No significant differences in CT number constancy or spatial resolution were observed regardless of anatomy choice. Increasing body size (milliampere second (mAs)/rotation) improved each image quality metric. Vertical and lateral phantom shifts of up to ±6 cm degraded noise and contrast metrics by up to 30% relative to the isocenter while also worsening uniformity and CT number constancy. IQM were also degraded substantially with the use of annuli to increase the phantom diameter (32 cm vs. 20 cm). Despite variations in image characteristics among the investigated changes, most metrics were within manufacturer specifications when applicable. ConclusionThis work demonstrates the dependence of image quality on parameter selection and setup conditions for a helical kVCT system utilized in image-guided and adaptive helical tomotherapy treatments. While the overall image quality is robust to variations in imaging parameters, care should be taken when selecting parameters as patient size increases or positioning moves from the isocenter to ensure adequate image quality is still achieved.

Evaluating on-board kVCT- and MVCT-based dose calculation accuracy using a thorax phantom for helical tomotherapy treatments

“…In a clinical setting, it is also important to consider the trade-off between image quality, patient dose, and scan acquisition time when selecting protocols for use in ART. The impact of parameter selection on these considerations has been noted by Tegtmeier et al [22,23].…”

“…Variation in this parameter has been observed to have the largest bearing on overall image quality, as application of Fine mode decreases noise and improves contrast relative to Normal and Coarse modes due to a reduction in the longitudinal beam width at isocenter (50 mm for Fine, 100 mm for Normal, ∼140 mm for Coarse) and an increase in views per rotation (which results in a higher mAs for this system-note that body size also impacts the mAs). For additional information on the impact of parameter selection on image quality and patient dose, the reader is referred to the literature [21][22][23] while for specific values associated with these parameter selections as well as additional information on system specifications, the reader is referred to the Radixact Physics Essentials Guide [20].…”

“…noticeable variation in image quality based on changes to this imaging parameter [21][22][23]. In a clinical setting, it may prove beneficial to limit the mode used in ART implementation to that which produces the best image quality (Fine) if a single calibration is to be used or provide separate calibrations to minimize uncertainty in HU variation if it is intended that mode be varied from treatment to treatment.…”

“…This system, released in Spring 2021, is designed to improve overall image quality for the treatment planning and delivery process compared to the MV-based imaging approach [20] and is available for use in research applications at this institution. The performance of this system has recently been characterized in the literature [21][22][23].…”