Direct measurement and correction of both megavoltage and kilovoltage scattered x-rays for orthogonal kilovoltage imaging subsystems with dual flat panel detectors

Abstract

Purpose: To measure the scattered x-rays of megavoltage (MV) and kilovoltage (kV) beams (MV scatter and kV scatter, respectively) on the orthogonal kV imaging subsystems of Vero4DRT.

Methods: Images containing MV- and kV-scatter from another source only (i.e., MV- and kV-scatter maps) were acquired for each investigated flat panel detector. The reference scatterer was a water-equivalent cuboid phantom. The maps were acquired by changing one of the following parameters from the reference conditions while keeping the others fixed: field size: 10.0 × 10.0 cm² ; dose rate: 400 MU/min; gantry and ring angles: 0°; kV collimator aperture size at isocenter: 10.0 × 10.0 cm² : tube voltage: 110 kV; and exposure: 0.8 mAs. The average pixel values of MV- and kV-scatter (i.e., the MV- and kV-scatter values) at the center of each map were calculated and normalized to the MV-scatter value under the reference conditions (MV- and kV-scatter value factor, respectively). In addition, an MV- and kV-scatter correction experiment with intensity-modulated beams was performed using a phantom with four gold markers (GMs). The ratios between the intensities of the GMs and those of their surroundings were calculated.

Results: The measurements showed a strong dependency of the MV-scatter on the field size and dose rate. The maximum MV-scatter value factors were 2.0 at a field size of 15.0 × 15.0 cm² and 2.5 at a dose rate of 500 MU/min. The maximum kV-scatter value was 0.48 with a fully open kV collimator aperture. In the phantom experiment, the intensity ratios of kV images with MV- and kV-scatter were decreased from the reference ones. After correction of kV-scatter only, MV-scatter only, and both MV- and kV-scatter, the intensity ratios gradually improved.

Conclusions: MV- and kV-scatter could be corrected by subtracting the scatter maps from the projections, and the correction improved the intensity ratios of the GMs.

Keywords: MV- and kV-scatter; Vero4DRT; dynamic tumor tracking; orthogonal imaging; radiotherapy.

Fig. 1

Example of treatment console during dynamic tumor tracking treatment using Vero4DRT. The projection images from flat panel detectors1 (FPD1) and FPD2, kV x‐ray and tracking parameters, infrared reflective marker motion, and absolute difference between the detected and predicted radiopaque marker positions are displayed. (a) If the radiopaque marker is detected without problems, the absolute difference can be calculated and shown. (b) If not, the absolute difference cannot be calculated and shown on the console (orange arrows). In addition, the contrast of the radiopaque marker is degraded by noise, especially in the image from FPD2.

Fig. 2

(a) Frontal view of Vero4DRT. (b) Experimental setup for this study. (c) Setup of the QUASAR phantom. (d) Schematic drawing of the wooden rod.

Fig. 3

MV‐ and kV‐scatter maps for field sizes of 2.0 × 2.0, 6.0 × 6.0, 10.0 × 10.0, 14.0 × 14.0, and 15.0 × 15.0 cm² and kV collimator aperture sizes of 10.0 × 10.0, 12.0 × 12.0, 14.0 × 14.0, 16.0 × 16.0, and 22.0 × 17.0 cm² at the isocenter, respectively. The window levels and widths are 300 and 600.

Fig. 4

Pixel value profiles for the MV‐scatter maps acquired by (a) flat panel detectors1 (FPD1) and (b) FPD2, and those of kV‐scatter maps acquired by (c) FPD1 and (d) FPD2.

Fig. 5

Field size dependencies of MV‐scatter acquired by (a) flat panel detectors1 (FPD1) and (b) FPD2. Dose rate dependencies of MV‐scatter acquired by (c) FPD1 and (d) FPD2. Gantry and ring angle dependencies of MV‐scatter acquired by (e) FPD1 and (f) FPD2.

Fig. 6

kV collimator aperture size dependencies of kV‐scatter acquired by (a) flat panel detectors1 (FPD1) and (b) FPD2. Tube voltage dependencies of kV‐scatter acquired by (c) FPD1 and (d) FPD2. Exposure dependencies of kV‐scatter acquired by (e) FPD1 and (f) FPD2. Gantry and ring angle dependencies of kV‐scatter acquired by (g) FPD1 and (h) FPD2.

Fig. 7

Reference kV image without MV beam irradiation (kV only), concurrent kV image during MV beam irradiation (MV + kV), kV‐scatter‐corrected image (kVScorr), MV‐scatter‐corrected image (MVScorr), MV‐ and kV‐scatter‐corrected image (MVkVScorr) for flat panel detectors1 (FPD1) and FPD2 obtained at segments 2 and 11, and EPID images of Field 4. x‐ray path of FPD2 was longer than that of FPD1. The window levels and widths for EPID images are 3500 and 7000. The window levels and widths for FPD images are 300 and 600.

Fig. 8

Boxplots of intensity ratios of gold markers (GMs) on reference kV images without MV beam irradiation (kV only) and in concurrent kV images acquired during MV beam irradiation (MV + kV), kV‐scatter‐corrected images (kVScorr), MV‐scatter‐corrected images (MVScorr), and MV‐ and kV‐scatter‐corrected images (MVkVScorr) acquired by flat panel detectors1 (FPD1) and FPD2 at (a) Field 1, (b) Field 2, (c) Field 3, (d) Field 4, (e) Field 5, and (f) Field 6. *P < 0.05, N.S.: not significant.