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An evaluation of the stability of image‐quality parameters of Varian on‐board imaging (OBI) and EPID imaging systems

ABSTRACT: Quality assurance (QA) of the image quality for image‐guided localization systems is crucial to ensure accurate visualization and localization of regions of interest within the patient. In this study, the temporal stability of selected image parameters was assessed and evaluated for kV CBCT mode, planar radiographic k V, and MV modes. The motivation of the study was to better characterize the temporal variability in specific image‐quality parameters. The CATPHAN, QckV‐1, and QC‐3 phantoms were used to evaluate the image‐quality parameters of the imaging systems on a Varian Novalis Tx linear accelerator. The planar radiographic images were analyzed in PIPSpro with high‐contrast spatial resolution (f30,f40,f50lp/mm) being recorded. For OBI kV CBCT, high‐quality head full‐fan acquisition and pelvis half‐fan acquisition modes were evaluated for uniformity, noise, spatial resolution, HU constancy, and geometric distortion. Dose and X‐ray energy for the OBI were recorded using the Unfors RaySafe Xi system with the R/F High Detector for kV planar radiographic and the CT detector for kV CBCT. Dose for the MV EPID was recorded using a PTW975 Semiflex ion chamber, PTW UNIDOS electrometer, and CNMC Plastic Water. For each image‐quality parameter, values were normalized to the mean, and the normalized standard deviations were recorded to evaluate the parameter's temporal variability. For planar radiographic modes, the normalized standard deviations of the spatial resolution (f30,f40,& f50) were 0.015, 0.008, 0.004 lp/mm and 0.006, 0.009, 0.018 lp/mm for the kV and MV, respectively. The normalized standard deviation of dose for kV and MV were 0.010 mGy and 0.005 mGy, respectively. The standard deviations for full‐and half‐fan kV CBCT modes were averaged together. The following normalized standard deviations for each kV CBCT parameter were: 0.075 HU (uniformity), 0.071 HU (noise), 0.006 mm (AP‐geometric distortion), 0.005 mm (LAT‐geometric distortion), 0.058 mm (slice thickness), 0.124 (f50), 0.031 (HU constancy – Lung), 0.063 (HU constancy – Water), 0.020 (HU constancy – Bone), 0.006 mGy (Dose – Center), 0.004 mGy (Dose –Periphery). Using control chart analysis, institutional QA tolerances were reported as warning and action thresholds based on 1σ and 2σ thresholds. A study was performed to characterize the stability of image‐quality parameters recommended by AAPM Task Group‐142 for the Varian OBI and EPID imaging systems. Both imaging systems show consistent imaging and dosimetric properties over the evaluated time frame. PACS number: 87.10.‐e

SUBMITTER: Stanley D

PROVIDER: S-EPMC5690094 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The purposes of this study is to measure the low frequency drop (LFD) of the modulation transfer function (MTF), associated with the long tails of the detector point spread function (PSF) of an on-board flat panel imager and study its impact on cone-beam CT (CBCT) image quality and scatter measurement accuracy.Two different experimental methods were used to characterize LFD and its associated PSF of a Varian OBI flat-panel detector system: the edge response function (ERF) method and the disk transfer function (DTF) method. PSF was estimated by fitting parametric models to these measurements for four values of the applied voltage (kVp). The resultant PSF was used to demonstrate the effect of LFD on image contrast and CT number accuracy in CBCT images reconstructed from synthetic datasets, as well as, accuracy of scatter measurements with the beam-stop method.The MTFs derived from the measured ERF data revealed LFDs varying from 8% (at 60 kVp) to 10.5% (at 120 kVp), while the intensity of the long PSF tails was found to increase with increasing kVp. The veiling glare line spread functions derived from the ERF and DTF methods were in excellent agreement. Uncorrected veiling glare reduced contrast and the image intensity in CBCT reconstruction, near the phantom periphery (by 67 Hounsfield units in a 20 cm-in-diameter water phantom) and (to a smaller degree) near inhomogeneities. Use of the bow-tie filter mitigated these effects. Veiling glare also resulted in about 10%-15% overestimation of the scatter-to-primary ratio when measured with the beam-stop or beam-stop array method.The long tails of the detector PSF were found to have a modest dependence of beam spectrum, which is reflected on the MTF curve LFD. Our findings show that uncorrected veiling glare can affect quantitative accuracy and contrast in CBCT imaging, based on flat panel imager. In addition, it results in overestimation of the scatter-to-primary ratio, measured with the beam-stop methods.

Project description:OBJECTIVE::To evaluate the exposure parameters, radiation protection, absorbed dose and radiographic image quality of the DIOX® intraoral portable radiography device. METHODS::The exposure parameters were measured using the Xi UNFORS detector. Operator exposure to secondary radiation was measured using the 1800cc ionization chamber coupled to the electrometer. The absorbed dose (D) in the patient was calculated using TLD-100H positioned in the Alderson RANDO anthropomorphic simulator. The quality of the radiographic digital image was assessed by comparing radiographic images obtained from two conventional devices (CS 2200®, Carestream Health; Heliodent plus®, Sirona Dental Systems GMbH) with the radiological simulator of the upper molar region RMI (Radiation Measurements Instruments), using three acquisition sensors: Kodak RVG 5000® and Kodak PSP®, Eastman Kodak Company, Rochester, NY; EVO Micro Image®, Brazil. RESULTS::The DIOX intraoral portable radiographic device demonstrated reliability in relation to the performance of the standard evaluated parameters, except for the diameter of the radiation field (5.8 ?mm) less or greater. No evidence of device head radiation was detected. The Pb lead protection of the apparatus attenuates the secondary radiation, thus protecting the operator. However, it was observed that the region of the operator's gonads was the most exposed during the measurements. In the Alderson RANDO anthropomorphic simulator, the highest value of D was in the region corresponding to the submandibular and lingual glands of the left side (0.568 mGy). The image quality of the DIOX portable radiographic apparatus presented quality standards equivalent to those produced by the two conventional radiographic devices. CONCLUSION::The DIOX intraoral portable radiography device demonstrated reliability in relation to the quality control and radioprotection criteria, according to international standards. Results obtained demonstrated the safe use of the DIOX intraoral portable radiography device and indicated the need for debate and change in international sanitary oversight standards regarding the use of portable XR devices in dentistry.

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An evaluation of the stability of image‐quality parameters of Varian on‐board imaging (OBI) and EPID imaging systems

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