Evaluation of gantry speed on image quality and imaging dose for 4D cone-beam CT acquisition
© The Author(s). 2016
Received: 9 January 2016
Accepted: 22 July 2016
Published: 29 July 2016
This study investigates the effect of gantry speed on 4DCBCT image quality and dose for the Varian On-Board Imager®.
A thoracic 4DCBCT protocol was designed using a 125 kVp spectrum. Image quality parameters were evaluated for 4DCBCT acquisition using Catphan® phantom with real-time position management™ system for gantry speeds varying between 1.0 to 6.0°/s. Superior-inferior motion of the phantom was executed using a sinusoidal waveform with five second period. Scans were retrospectively sorted into 4 phases (CBCT-4 ph) and 10 phases (CBCT-10 ph); average 4DCBCT (CBCT-ave), using all image data from the 4DCBCT acquisitions was also evaluated. The 4DCBCT images were evaluated using the following image quality metrics: spatial resolution, contrast-to-noise ratio (CNR), and uniformity index (UI). Additionally, Hounsfield unit (HU) sensitivity compared to a baseline CBCT and percent differences and RMS errors (RMSE) of excursion were also determined. Imaging dose was evaluated using an IBA CC13 ion chamber placed within CIRS Thorax phantom using the same sinusoidal motion and image acquisition settings as mentioned above.
Spatial resolution decreased linearly from 5.93 to 3.82 lp/cm as gantry speed increased from 1.0 to 6.0°/s. CNR decreased linearly from 4.80 to 1.82 with gantry speed increasing from 1.0 to 6.0°/s, respectively. No noteworthy variations in UI, HU sensitivity, or excursion metrics were observed with changes in gantry speed. Ion chamber dose rates measured ranged from 2.30 (lung) to 5.18 (bone) E-3 cGy/mAs.
A quantitative analysis of the Varian OBI’s 4DCBCT capabilities was explored. Changing gantry speed changes the number of projections used for reconstruction, affecting both image quality and imaging dose if x-ray tube current is held constant. From the results of this study, a gantry speed between 2 and 3°/s was optimal when considering image quality, dose, and reconstruction time. The future of 4DCBCT clinical utility relies on further investigation of image acquisition and reconstruction optimization.
Stereotactic body radiotherapy (SBRT) has become a form of treating inoperable non-small cell lung cancer (NSCLC) in its early stages . Work conducted by Onishi et al. showed that patients staged with T1 and T2 NSCLC treated with SBRT had cumulative local control rates better than 70 % at 5 years; this is in contrast to local control rates of 50 % with a 5-year survival of approximately 15–30 % for patients treated with conventionally fractionated radiotherapy . Five-year relative survival rates for lung cancers remains somewhat low at 18 %, attributed to more than one-half of diagnoses made at a distant stage .
Though initial clinical results of SBRT for lung cancer are promising, there remain technical complexities that must be addressed. In particular, localization error associated with the treatment of moving targets in lung SBRT must be minimized . The ability of four-dimensional CT (4DCT) to map motion and tissue deformation during respiration, while reducing artifacts, allows for accurate targeting of tumors in the thorax [4, 5]. Strategies to improve target coverage, such as breath hold treatments, gated delivery, and mid-position treatments can also be utilized as part of an approach for respiratory motion management [4, 6, 7]. Margin reduction is also possible for SBRT, having subsequent implications for reducing mean lung dose .
Interventional imaging techniques such as cone-beam CT (CBCT) have become clinical standards in image-guided radiotherapy for soft tissue-based target localization and positioning . Linear accelerators can be mounted with a kV source and flat panel detector, allowing for volumetric image acquisition. These volumetric images allow for accurate soft tissue localization and retrospective dose calculation . However, large magnitudes of intra-scan motion in the reconstructed image can lead to clinically dosimetric discrepancies due to poor image quality.
Tumor motion at the treatment position can be assessed using 4DCBCT . Conventional free-breathing CBCT tends to underestimate the tumor extent (i.e., the internal target volume) by as much as 24.2 to 40.1 % depending on tumor size and interfraction variability [3, 12, 13]. To allow for binning of projection images, a surrogate respiratory signal is used to determine amplitude, phase, or temporal information; various methods have been utilized for the respiratory cycle surrogate, including tracking the motion of the diaphragm, mapping changes of the skin surface, thoracic transducer belts, or infrared reflective markers . The 4D reconstruction involves a retrospective correlation of the timing of projection images to the breathing cycle surrogate parameter of interest (i.e., phase or amplitude of respiratory cycle) [4, 15].
The clinical utility of 4DCBCT is dependent on both image quality and imaging dose. Dosimetric studies have been conducted for patient skin dose using thermoluminescent dosimeters as well as Gafchromic film inserted into homogeneous phantoms for different CBCT protocols [16, 17]. Wen et al. reported cumulative dose levels to the left femoral head from daily kV CBCT of pelvic sites can be upwards of 400 cGy. Since 4DCBCT has great potential in localizing lesions in the thorax, it would be worthwhile to understand the dose delivered from 4DCBCT in a heterogeneous medium and the effect of changing practical variables in 4D protocols such as gantry speed due to gantry speed’s inverse relationship with the number of projections. This study investigates the effect of gantry speed on 4DCBCT image quality and dose using the On-Board Imager® (OBI) on the Edge™ radiosurgery system (Varian Medical System, Palo Alto, CA).
Materials and methods
Technique factors and parameters set with a variable gantry speed. All images were acquired with 125 kVp x-ray tube setting
Gantry speed (°/s)
Field of view (cm)
Pixel size (mm)
Slice thickness (mm)
Tube voltage (kVp)
512 × 512
512 × 512
512 × 512
512 × 512
512 × 512
512 × 512
where A 0 represents an amplitude constant in cm. The RPM system™ tracks the block’s motion with an infrared camera and assigns projections to appropriate phase bins used for retrospective reconstruction. The Catphan® 504 phantom was used to evaluate CBCT image quality (Phantom Laboratory, Salem, NY) by placing it on a lower moving superior-inferior (SI) platform of the gating phantom that moved in phase with the infrared marker block’s platform.
Image quality analysis
Low contrast detectability
UI was calculated for ten individual slices and then averaged producing a single UI for every protocol. Variation of UI normalized to the square root of mAs was also evaluated with respect to mAs.
An imaging system’s ability to accurately characterize a given material’s electron density (ED) is the essence of HU sensitivity. This was accomplished via measurement of various cylinders found in the Catphan’s sensitometry module (CTP404), as seen in Fig. 3d. The mean HU value of each cylinder was measured over ten individual slices and then averaged producing a single mean HU for each cylinder for every protocol. The difference in mean HU from a baseline CBCT scan without motion performed at the same kVp was calculated for every cylinder. Ground truth of a given material’s HU value is represented by the baseline scan . This was used to construct a curve for quantification of HU sensitivity.
This was done for both 4D reconstruction techniques.
A. Image quality evaluation
Maximal spatial resolution (lp/cm) for CBCT-ave at specified gantry speeds
Gantry speed (°/s)
Percent differences in excursion for the various protocols
Gantry speed (°/s)
Imaging dose evaluation
Dose rates (E-3 cGy/mAs) measured in different tissue-mimicking materials
CBCT finds great utility in radiation oncology. CBCT image acquisition prior to delivery of radiation can be registered to previously acquired CT scans used for treatment planning and provides images for retrospective dose accumulation studies. However, standard 3D acquisition and reconstruction does not allow visualization of mobile lesions as a function of respiratory phase and is susceptible to motion artifacts such as blurring and distortion. Moreover, average 3D reconstructions may result in underestimates in excursion due to short periods of time spent at peak inhalation/exhalation. The use of 4DCBCT has the potential to mitigate these issues by assigning projections to specific breathing cycle phases. This is feasible due to changes in the diaphragm, causing organ motion with the diaphragm’s dynamics. This in turn justifies the use of surrogate signals such as skin surface mapping or infrared markers, as the position of the surrogate is variable with respect to time [4, 7].
A major difficulty in 4DCBCT is the limited number of projections per phase bin. Insufficient projection data leads to aliasing artifacts. Li and Xing proposed slow gantry rotation (SGR) and multiple gantry rotation (MGR) acquisition techniques to increase the number of projections for a given phase . They showed SGR produced superior images to MGR for the same mAs when compared to 3DCBCT counterparts.
Time-averaged 4DCBCT images over the respiratory cycle are used for patient localization compared against time-averaged 4DCT images . The image quality of 4DCBCT images plays an important role in 4D dose accumulation. Anatomical voxels are mapped from the 4DCBCT at treatment position to a reference 4DCT. However, the low number of projections per phase in the 4DCBCT image may result in degraded image quality, specifically streaking at high contrast boundaries and blurring, in turn leading to unrealistic dose calculations .
HU sensitivity and excursion did not exhibit noteworthy differences in CBCT-ave with variable gantry speed, especially when accounting for error in the case HU sensitivity. With respect to excursion, across all the protocols, percent differences were less than 2.0 %. In comparing 4 phase against 10 phase reconstructions, both RMSE are less than half a millimeter. This indicates that the 4D reconstructions reproduce realistic excursion. In this particular study, the 15 fps acquisition and characteristics of the waveform motion mitigate issues of latency. Based on the motion waveform used, acquisition may miss 0.8 mm of motion between frames on average. For more irregular breathing patterns, this may not be the case resulting in errors in the phase sorting process.
Characterization of image quality for given tube current settings (mAs) for 4DCBCT has been previously performed for systems including the Varian’s Acuity™ simulator and Trilogy™ OBI (Varian Medical Systems, Palo Alto, CA) [25, 29]. These studies focused primarily on a comparison of image quality relative to some reference using SGR or MGR acquisition techniques. Dose was characterized via mAs, as dose is directly proportional to mAs .
The CTDI100 metric is not appropriate for dosimetric evaluation of CBCT scans due to its inability to accommodate and record the whole primary beam and scattered radiation . Hence 4DCBCT was evaluated dosimetrically on an absolute scale. Ion chamber measurements from this study produced dose rates of 3.0 × 10−03 cGy/mAs at isocenter in a water equivalent medium. Li et al. reported for the Varian Acuity and Trilogy systems, a 125 kVp spectrum and tube current of 80 mA produced dose rates at isocenter of 2.92 × 10−03 and 2.79 × 10−03 cGy/mAs, respectively . Similarly, Gardner et al. reported isocenter doses using film in a Wellhofer phantom of (2.72 ± 0.11) × 10−03 cGy/mAs using TrueBeam OBI and 125 kVp setting. McMillan et al. compared measured dose rates to dose rates determined using Monte Carlo methods for the OBI kV imaging system integrated into the Varian Novalis Tx radiosurgery platform (Varian Medical Systems, Palo Alto, CA) . A dose rate of 2.23 × 10−03 cGy/mAs at isocenter was calculated for a comparable pelvic CBCT protocol using a 125 kVp spectrum and a 32 cm diameter cylindrical CTDI phantom composed of homogeneous polymethyl methacrylate. Variation in dose rate in this study from others is attributed to spectral differences and variations in phantom composition and geometry.
While increasing the cumulative mAs increases the quality of spatial resolution and low contrast detectability, the cost of linearly increasing dose and increased 4D image acquisition and reconstruction time must be considered. Artifacts such as streaking in 4DCBCT are largely attributed to the lack of projection data being used for reconstruction. In this particular study, the incremental improvements in image quality above 2000 projections become unjustified compared to the linear increases in dose. This corresponds to a gantry speed between 2 and 3°/s. Clinical use of 4DCBCT systems for patients relies entirely on intended application.
A quantitative analysis of the Varian OBI’s 4DCBCT capabilities was explored for making clinical decisions. The effects of gantry speed on 4DCBCT image quality and dose have been investigated yielding anticipated results. When using the same technique settings (kVp), variations in gantry speed change the number of projections used for reconstruction but maintain the mAs per projection. Unsurprisingly, contrast-based image quality metrics were found to decrease linearly with increasing gantry speed but show marginal improvements when appropriately normalized to the square root of mAs. The benefit of increased contrast comes at the cost of increased dose, slower acquisition time, and longer 4D reconstruction time; the clinical benefits of improved image quality must be weighed against the costs of decreased efficiency and increased imaging dose. The future of 4DCBCT’s clinical utility relies on further investigation of image optimization, requiring more than just large-scale increases in mAs to improve image quality.
4DCT, four-dimensional CT; AP, anterior-posterior; CBCT, cone-beam CT; CBCT-10 ph, ten phase 4DCBCT reconstruction; CBCT-4 ph, four phase 4DCBCT reconstruction; CBCT-ave, average CBCT reconstruction; CNR, contrast-to-noise ratio; ED, electron density; FDK, Feldkamp-Davis-Kress; fps, frames per second; HU, Hounsfield unit; HVL, half-value layer; MGR, multiple gantry rotation; MTF, modulation transfer function; NSCLC, non-small cell lung cancer; OBI, On-Board Imager®; RMSE, root-mean square error; ROI, region-of-interest; RPM, real-time position management; SBRT, stereotactic body radiotherapy; SGR, single gantry rotation; SI, superior-inferior; UI, uniformity index
The authors thank American Cancer Society for funding this project. The assistance from Richard Cattaneo II, MD, Radiation Oncologist at St. Joseph Mercy Hospital is especially appreciated.
This work was supported by a research scholar grant (RSG-15-137-01-CCE) from the American Cancer Society.
Availability of data and materials
All data and materials in the manuscript are freely available to any scientist.
APS, KHS, SJG, and NW designed the project. APS, KHS, and YQ contributed to acquisition of data. APS, KHS, SJG, and CL performed data analysis. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Consent for publication
Ethics approval and consent to participate
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