Technical evaluation of the cone-beam computed tomography imaging performance of a novel, mobile, gantry-based X-ray system for brachytherapy

Abstract

Purpose: A novel, mobile cone-beam computed tomography (CBCT) system for image-guided adaptive brachytherapy was recently deployed at our hospital as worldwide first site. Prior to the device's clinical operation, a profound characterization of its imaging performance was conducted. This was essential to optimize both the imaging workflow and image quality for achieving the best possible clinical outcomes. We present the results of our investigations.

Methods: The novel CBCT-system features a ring gantry with 121 cm clearance as well as a 43.2 × 43.2 cm² flat-panel detector, and is controlled via a tablet-personal computer (PC). For evaluating its imaging performance, the geometric reproducibility as well as imaging fidelity, computed tomography (CT)-number accuracy, uniformity, contrast-noise-ratio (CNR), noise characteristics, and spatial resolution as fundamental image quality parameters were assessed. As dose metric the weighted cone-beam dose index (CBDI_w ) was measured. Image quality was evaluated using standard quality assurance (QA) as well as anthropomorphic upper torso and breast phantoms. Both in-house and manufacturer protocols for abdomen, pelvis, and breast imaging were examined.

Results: Using the in-house protocols, the QA phantom scans showed altogether a high image quality, with high CT-number accuracy (R² > 0.97) and uniformity (<12 Hounsfield Unit (HU) cupping), reasonable noise and imaging fidelity, and good CNR at bone-tissue transitions of up to 28:1. Spatial resolution was strongly limited by geometric instabilities of the device. The breast phantom scans fulfilled clinical requirements, whereas the abdomen and pelvis scans showed severe artifacts, particularly at air/bone-tissue transitions.

Conclusion: With the novel CBCT-system, achieving a high image quality appears possible in principle. However, adaptations of the standard protocols, performance enhancements in image reconstruction referring to artifact reductions, as well as the extinction of geometric instabilities are imperative.

Keywords: cone-beam computed tomography (CBCT); image quality; image-guided adaptive brachytherapy; imaging performance analysis; mobile X-ray system.

FIGURE 1

Shown is the technical setup of the IRm (a), which is constructed from an X‐ray source and flat‐panel detector (FPD) mounted on a gantry with 121 cm clearance and controlled via the portable Human Machine Interface (HMI). Motorized wheels integrated to the system's legs ensure high mobility. Particularly, an Alderson® upper torso as well as a CIRS breast phantom (b), in which plastic catheters were implanted, were used for imaging assessment of the IRm. (c) A zoom image of the breast phantom is shown

FIGURE 2

The CatPhan modules CTP404 (a), CTP486 (b), and CTP528 (c) used for physical image analysis. For uniformity assessment, computed tomography (CT)‐number profiles were drawn within the CTP486 along the indexed directions (b). The four rods used for the assessment of imaging fidelity are labeled with the numbers 1–4. Images from our archive, acquired with the conventional CT‐system SOMATOM go.Open Pro (Siemens Healthineers, Erlangen, Germany)

FIGURE 3

The three translational corrections of focal spot (a) and detector midpoint (b) as well as the detector rotation corrections (c) obtained for clockwise gantry rotation in all eight calibration procedures. Furthermore, the differences of the corrections obtained for clockwise and counter‐clockwise gantry rotation were calculated for each calibration and then averaged per angle over all eight measurements. The absolute values of these averages are illustrated in (d‐f), where no error bars are shown for clarity

FIGURE 4

Double contours observed at the inner CTP486 edge, exemplary presented for the region marked black on the left image. The appearance of the double contours varied as illustrated between the eight considered PH scans acquired in direct succession, thus indicating geometric instabilities varying from scan‐to‐scan. Windowing—Level: ‐40 HU, Width: 500 HU

FIGURE 5

The anterior–posterior (AP) computed tomography (CT)‐number profiles obtained for the AS, BS, and PO protocols (a). The AS protocol showed significant cupping, whereas the phantom edges were blurred for the PO protocol. Furthermore, the CT‐numbers measured for the CTP404 inserts are plotted exemplarily for BS and AS (b), where for BS a high compliance (R ² = 0.99) with the expectations was achieved. Furthermore, non‐uniformities (determined for both the lateral and AP image direction) depended distinctly on the distance to the longitudinal zero position (c), revealing significant variations within the scan range

FIGURE 6

(a–c) The 1D noise power spectrum (NPS) calculated for all protocols. The overall noise obtained by appropriate integration is indicated in each case. For the manufacturer protocols, a significant amplification of the noise was found, particularly also at higher frequencies. The calculated modulation transfer functions (MTFs) are shown exemplarily for the PS, PH, BS, and BH protocols in (d). The rapid drop in the MTF particularly of the PH protocol demonstrated the significant impact of the geometric instabilities of the IRm on spatial edge resolution. Note, that the resolution is mainly determined by the sampling pixel size of the initial image reconstruction

FIGURE 7

Two representative axial slices (bottom and top rows, respectively) acquired with the AL (a and d), AS (b and e), and AO (c and f) protocols. The AL and AS protocols were particularly characterized by high image noise. On all scans, significant artifacts occurred at the transitions between different phantom structures. Windowing—Level: 60 HU, Width: 400 HU

FIGURE 8

Axial slices of the upper and lower pelvis (top and bottom rows, respectively), acquired with the pelvis protocols PL (a and e), PS (b and f), PO (c and g), and PH (d and h). Again, significant artifacts appeared at structural interfaces which, however, could be reduced with increasing prefilter strength. The PH protocol also showed comparatively more pronounced blurring and double structures. Windowing—Level: 60 HU, Width: 400 HU

FIGURE 9

Scans of the CIRS breast phantom acquired with the BS (a and d), BO (b and e), and BH (c and f) protocols in axial (top row) and coronal (bottom row) views. Significant artifact reduction was achieved with the BH protocol. However, all scans fulfilled the clinical requirements of catheter reconstruction in the treatment planning system. Windowing—Level: ‐40 HU, Width: 900 HU