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. 2020 Jan 20;6(1):015025.
doi: 10.1088/2057-1976/ab693a.

Long-term Inter-protocol kV CBCT image quality assessment for a ring-gantry linac via automated QA approach

Jiayuan Peng  1 , Hua Li, Eric Laugeman, Thomas Mazur, Dao Lam, Taoran Li, Baozhou Sun, Weigang Hu, Lei Dong, Geoffrey D Hugo, Sasa Mutic, Bin Cai
Affiliations

Long-term Inter-protocol kV CBCT image quality assessment for a ring-gantry linac via automated QA approach

Jiayuan Peng et al. Biomed Phys Eng Express. .

Abstract

We develop a fully automated QA process to compare the image quality of all kV CBCT protocols on a Halcyon linac with ring gantry design, and evaluate image quality stability over a 10-month period. A total of 19 imaging scan and reconstruction protocols were characterized with measurement on a newly released QUART phantom. A set of image analysis algorithms were developed and integrated into an automated analysis suite to derive key image quality metrics, including HU value accuracy on density inserts, HU uniformity using the background plate, high contrast resolution with the modulation transfer function (MTF) from the edge profiles, low contrast resolution using the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), slice thickness with the air gap modules, and geometric accuracy with the diameter of the phantom. Image quality data over 10 months was tracked and analyzed to evaluate the stability of the Halcyon kV imaging system. The HU accuracy over all 19 protocols is within tolerance (±50HU). The maximum uniformity deviation is 12.2 HU. The SNR and CNR, depending on the protocol selected, range from 18.5-911.9 and 1.9-102.8, respectively. A much-improved SNR and CNR were observed for iterative reconstruction (iCBCT) modes and protocols designed for large subjects over low dose and fast scanning modes. The Head and Image Gently protocols have the greatest high contrast resolution with MTF10% over 1 lp/mm and MTF50% over 0.6 lp/mm. The iCBCT mode slightly improved the MTF10% and MTF50% compared to the Feldkamp-Davis-Kress approach. The slice thickness (maximum error of 0.31 mm) and geometry metrics (maximum error of 0.7 mm) are all within tolerance (±0.5 mm for slice thickness and ±1 mm for geometry metrics). The long-term study over 10-month showed no significant drift for all key image quality metrics, which indicated the kV CBCT image quality is stable over time.

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Conflict of interest statement

Conflict of interest

This study receives funding from Varian Medical Systems.

Figures

Figure 1.
Figure 1.
The diagram of the front view (a), transverse view of the middle block (b), and 3D air gap (c) of the QUART phantom.
Figure 2.
Figure 2.
Diagram of the center slice and off-center slices of the phantom.
Figure 3.
Figure 3.
The location and the illustration of different image modules including: detection of the Air (top and bottom in red), Polystyrene (right in red), and Teflon inserts (left in red), along with four acrylic ROIs (blue) for HU accuracy measurement (a), and ROIs (b) specified in Acrylic for uniformity measurement.
Figure 4.
Figure 4.
Edge profiles are collected in the red and blue areas for horizontal and vertical high contrast resolution measurement respectively (a). With averaged edge profiles, a line spread function (b) is calculated which is used for obtaining the MTF (c).
Figure 5.
Figure 5.
The location of the air gap boxes (blue and yellow rectangular) and placement of profiles (red lines) (a) along with the plot of an averaged profile with relevant definitions for FWHM calculation(b). Inset figure shows the air gap stripe and artifacts.
Figure 6.
Figure 6.
Visual comparisons of kV CBCT image quality of different scanning protocols for iCBCT (bottom half of each figure) and FDK (upper half of each figure) reconstruction for the first eight subfigures. The last sub-figure compares Thorax with the Thorax Fast protocols both reconstructed with FDK algorithm.
Figure 7.
Figure 7.
10-month trends for HU accuracy and Uniformity deviation. Left column for Head and Image gently protocols, right column for Pelvis, Thorax and Breast protocols. For the suffixes in the figure, ‘i’ means iCBCT, ‘l’ means large, ‘f’ means fast.
Figure 8.
Figure 8.
10-month trends for SNR, CNR and MTF. Left column for Head and Image gently protocols, right column for Pelvis, Thorax and Breast protocols. For the suffixes in the figure, ‘i’ means iCBCT, ‘l’ means large, ‘f’ means fast.
Figure 9.
Figure 9.
10-month trends for geometry and slice thickness. Left column for Head and Image gently protocols, right column for Pelvis, Thorax and Breast protocols. For the suffixes in the figure, ‘i’ means iCBCT, ‘l’ means large, ‘f’ means fast.
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