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Development of clinically relevant QA procedures for the BrainLab ExacTrac imaging system

Abstract Purpose The aim of this study was to develop Quality Assurance procedures for the BrainLab ExacTrac (ET) imaging system following the TG 142 recommendations for planar kV imaging systems. Materials and Methods A custom‐designed 3D printed holder was used to position the Standard Imaging QCkV‐1 phantom at isocenter, facing the ET X ray tubes. The linac's light field (collimator at 45⁰) was used to position the phantom holder. The ET images were exported to ARIA where geometric distortion was checked. The DICOM images were analyzed in the PIPSpro software. The following parameters were recorded (technique 80 kV/2mAs): spatial resolution (Modulated Transfer Function (MTF) F50/F40/F30), contrast‐to‐noise ratio (CNR), and noise. A baseline was generated for future image analysis. Beam quality and exposure were measured using the Unfors R/F detector. Using a rod holder, the detector was placed at isocenter, facing each ET X‐ray tube. The measurements were performed for all preset protocols ranging from cranial low (80 kV/6.3 mAs) to abdomen high (145 kV/25 mAs). The total exposure was converted to dose. Results and Discussion The image quality parameters were close for the two tubes. A common baseline was therefore generated. The average baseline values (both tubes, both images/tube) were 1.06/1.18/1.30, 1.32, and 67.3 for the MTF F50/F40/F30, noise, and CNR respectively. The procedure described here was used for another 24 sets. Using a positioning template and 3D printed phantom holder, experimental reproducibility has been acceptably high. The measured phantom dimensions were within 1 mm from the nominal values. The measured kV values were within 2% of the nominal values. The exposure values for the two tubes were comparable. The range of total measured dose was 0.099 mGy (cranial low) to 1.353 mGy (abdomen high). Conclusions A reliable process has been implemented for QA of the ET imaging system by characterizing the system's performance at isocenter, consistent with clinical conventions.


| INTRODUCTION
The BrainLab ExacTrac (ET) X ray 6D stereotactic IGRT system (BrainLab AG, Feldkirchen, Germany) could play an important role in image-guided radiosurgery and radiotherapy, 1,2 on the assumption that the image quality is reliably good. This system is an integration of two subsystems: an infrared-based optical positioning system for initial patient setup and couch movement control and a radiographic kV X ray imaging system for position verification and readjustments based on internal anatomy or implanted fiducials. This kV imaging system is configured with X ray sources in the floor and amorphous silicon flat-panel detectors mounted near the ceiling, forming an oblique imaging geometry.
The AAPM Task Group Report 142 (TG 142) 3 recommends checking monthly the image quality (geometric distortion, spatial resolution, contrast, uniformity, and noise) and measuring annually the beam quality/energy and imaging dose for the planar kV imaging systems. The measured values should be compared with an institution/system specific baseline generated a priori. The BrainLab Novalis ExacTrac system is listed in the AAPM Task Group Report 75 (TG 75) 4 focusing on image dose management during image-guided radiotherapy, and the dose for two extreme techniques (cranium/Cspine and body-thoracic/lumbar spine) are tabulated, without describing the measurement methodology.
The ET system's oblique geometry presents some challenges to conducting image quality and dose tests in the normal linear accelerator "treatment space". Currently there is only one publication focused on specific quality assurance (QA) tests for the imaging component of the BrainLab ET system. 5  The plan was scheduled for multiple fractions, and imported into the ET system using the reference array. An ET machine with beam energy 80 kV was generated in the PIPSpro software V 5.4 (Standard Imaging, Inc., Middleton, WI, USA). The 80 kV was selected to match the ET clinical imaging technique for the stereotactic cranial cases, also ensuring a good image quality for the phantom we used.
A custom-designed 3D printed holder was used to position the Standard Imaging QCkV-1 phantom at isocenter, facing the ET X ray tubes (i.e., rotated 45°in both horizontal and vertical planes). The linac's light field (collimator at 45°) was used to position the phantom holder. A paper template was designed for consistent phantom placement at isocenter (see Fig. 1a-c).
To ensure the phantom on the holder is positioned at isocenter, a mathematical calculation was performed a priori to determine the couch vertical setting (v): where H is the phantom height, t 1 is the holder thickness corresponding to the bottom side of the phantom mid-plane, and h 1 is given by: and the distance (d) from the inside part of the holder bottom plate to the laser intersection/cross-hair is given by: where d 0 is the distance from the inside bottom plate of the holder and the bottom front plane of the phantom and T is the phantom thickness (see schematic diagrams in Fig. 2a-c). The angle a = 45°( i.e., equal to the tilt of the ET detectors relative to a vertical axis).
For the holder-phantom combination used here the values for the

2.B | ExacTrac Annual Quality Assurance (ET AQA)
The beam quality/energy and exposure were measured using the Unfors RaySafe Xi R/F detector (Unfors RaySafe AB, Billdal, Sweden). Using a rod holder, the detector was placed at isocenter, facing in turn each ET X ray tube (see Fig. 3). The gantry and couch were set to 0°. The collimator was set to 45°, such that the light field projection could help to set the detector at isocenter, rotated 45°relative to the couch long axis in a horizontal plane. After that, the rod with the detector attached was rotated 45°relative to a vertical axis passing through the isocenter (i.e., facing an X ray tube). The measurements were performed for all preset protocols ranging from cranial low (80 kV/6.3mAs) to abdomen high (145 kV/25mAs), exposing only one tube at a time and re-orienting the detector for the second tube. The kV was recorded for each tube and compared with the nominal values.
The total exposure (summed for both tubes) was converted to dose assuming an exposure to "in-air" dose conversion factor of 0.87. For the ET energy range (80-145 kV) the exposure-to-dose conversion factor may be about 8-10% higher for water (tissue equivalent) media, 6 consequently patient dose could be somewhat larger than estimated in this work. Setup errors and tube output fluctuations were assessed to evaluate the uncertainties of the measured dose and kV values.

3.A | ET MQA
Five pairs of images were initially acquired for each tube and analyzed. The image quality parameters were close for the two tubes, consequently a common database was generated in PIPSpro. The procedure described above was used for 24 sets (see Section 2.A).   imaged with 80-100 kV, the mAs is in the range 6.3-12.5, therefore the noise would be reduced comparative to our in-phantom measurements. Furthermore, for patients the contrast level is adjusted as needed, while for our measurement we kept that at its lowest position for setup reproducibility.
The image quality parameters were similar for the measurements performed in the same day, keeping the same setup or redoing it, and for those performed months later. Both the setup errors and the tube output fluctuations contribute to the variation in image quality parameters, but the former contribution seems minimal.

3.B | ET AQA
Two AQA tests were performed to date using the measurement approach described above (ET system version 6.2.0.). Table 4  Accurate alignment of the Unfors detector with the ET X ray beam is critical for accurate measurement of kV. The current method of rotating the rod holder is limited in precision due to the lack of an angle indicator. We are in the process of redesigning the rod holder setup in order to increase its consistency.

| CONCLUSIONS
A reliable methodology has been implemented for QA of the ET imaging system by characterizing the system's performance at isocenter, in agreement with clinical conventions. The ExacTrac system showed stable image quality parameters (high-contrast spatial resolution, CNR, and geometrical accuracy) and dose/kVp over a year. The parameters were comparable for the two tubes, consequently a common baseline could be generated for the MQA testing. Separate baselines may be considered for the two tubes in order to tighten the tolerances further.
Based on individual tube image test results to date the warning/fail thresholds can be set to 3%/5% for the MTF and to 10%/15% for the CNR. The low mAs values necessary for these tests may be responsible for the level of noise/noise fluctuation. Similar with clinical procedure, we run the beams for both tubes simultaneously, and then we analyze in turn the image for the tube facing the phantom. The scatter radiation from one tube may contribute to the noise fluctuation for the other one, though a simple test with one tube covered by 1.0 mm lead did not show a significant difference.
For the MQA, future work will focus on the PIPSpro "single image method", requiring one phantom and one flood field image. In addition to image quality parameters as currently obtained via the dual-image method, this method will provide uniformity calculations, per the AAPM TG 142 recommendations.
For the AQA, future work will focus on rod holder setup redesign to improve consistency, avoiding variation in the attenuation/ scatter from the detector lead back coating.

CONFLI CT OF INTEREST
None.