Can different Catphan phantoms be used in a multi-centre audit of radiotherapy CT image quality?

PurposeTo determine the variation between Catphan image quality CT phantoms, specifically for use in a future multi-centre image quality audit.Method14 Catphan phantoms (models 503, 504 and 604) were scanned on a Canon Aquilion Prime CT scanner using a single scan protocol. Measurements were made of noise in the uniformity section, visibility of low contrast targets and contrast, x-ray attenuation and CT number for 5 materials in the sensitometry section. Scans were also acquired using one phantom and varying reconstruction field of view, image slice thickness, effective tube-current-time product and iterative reconstruction settings to determine how the degree of inter-phantom variability compared with the magnitude of changes from scan parameter alteration.ResultsAcross all phantoms the mean CT value in the uniformity section was 7.0 (SD 0.9) range: 4.9–8.1 HU. For the different materials the CT numbers were air: −1004 ± 5, Polymethylpentene: −190 ± 2, Polystyrene: −42 ± 2, Delrin: 321 ± 5 and Teflon: 898 ± 8 HU. Consistency of low contrast targets through visual scoring was good. Measured contrast was lower (p < 0.001) with more variability for 504 versus 604 models. All phantoms produced identical tube current settings with x-ray tube current modulation, indicating no x-ray attenuation differences. The degree of change in image quality metrics between phantoms was small compared with results when scan parameters were varied.ConclusionCatphan phantoms model 604 showed minimal differences and will be used for multi-centre inter-comparison work, with the consistency between phantoms appropriate for measuring possible variations in image quality.     

Comprehensive quality assurance phantom for the small animal radiation research platform (SARRP)

PurposeTo develop and test the suitability and performance of a comprehensive quality assurance (QA) phantom for the Small Animal Radiation Research Platform (SARRP).Methods and materialsA QA phantom was developed for carrying out daily, monthly and annual QA tasks including: imaging, dosimetry and treatment planning system (TPS) performance evaluation of the SARRP. The QA phantom consists of 15 (60 × 60 × 5 mm³) kV-energy tissue equivalent solid water slabs. The phantom can incorporate optically stimulated luminescence dosimeters (OSLD), Mosfet or film. One slab, with inserts and another slab with hole patterns are particularly designed for image QA.ResultsOutput constancy measurement results showed daily variations within 3%. Using the Mosfet in phantom as target, results showed that the difference between TPS calculations and measurements was within 5%. Annual QA results for the Percentage depth dose (PDD) curves, lateral beam profiles, beam flatness and beam profile symmetry were found consistent with results obtained at commissioning. PDD curves obtained using film and OSLDs showed good agreement. Image QA was performed monthly, with image-quality parameters assessed in terms of CBCT image geometric accuracy, CT number accuracy, image spatial resolution, noise and image uniformity.ConclusionsThe results show that the developed QA phantom can be employed as a tool for comprehensive performance evaluation of the SARRP. The study provides a useful reference for development of a comprehensive quality assurance program for the SARRP and other similar small animal irradiators, with proposed tolerances and frequency of required tests.     

Characterizing 3D printing in the fabrication of variable density phantoms for quality assurance of radiotherapy

PurposeTo present characterization, process flow, and applications of 3D fabricated low density phantoms for radiotherapy quality assurance (QA).Material and methodsA Rostock 3D printer using polystyrene was employed to print slabs of varying relative electron densities (0.18–0.75). A CT scan was used to calibrate infill-to-density and characterize uniformity of the print. Two printed low relative density rods (0.18, 0.52) were benchmarked against a commercial CT-electron-density phantom. Density scaling of Anisotropic Analytical Algorithm (AAA) was tested with EBT3 film for a 0.57 slab. Gamma criterion of 3% and 3 mm was used for analysis.Results3D printed slabs demonstrated uniformity for densities 0.4–0.75. The printed 0.52 rod had close agreement with the commercial phantom. Dosimetric comparison for 0.57 density slab showed >95% agreement between calculation and measurements.Conclusion3D printing allows fabrication of variable density phantoms for QA needs of a small clinic.     

Image quality for radiotherapy CT simulators with different scanner bore size

PurposeWe compare image quality parameters derived from phantom images taken on three commercially available radiotherapy CT simulators. To make an unbiased evaluation, we assured images were obtained with the same surface dose measured using XR-QA2 model GafChromic™ film placed at the imaging phantom surface for all three CT-simulators.MethodsRadiotherapy CT simulators GE LS 16, Philips Brilliance Big Bore, and Toshiba Aquilion LB were compared in terms of spatial resolution, low contrast detectability, image uniformity, and contrast to noise ratio using CATPHAN-504 phantom, scanned with Head and Pelvis protocols. Dose was measured at phantom surface, with CT scans repeated until doses on all scanners were within 2%.ResultsIn terms of spatial resolution, the GE simulator appears slightly better, while Philips CT images are superior in terms of SNR for both scanning protocols. The CNR results show that Philips CT images appear to be better, except for high Z material, while Toshiba appears to fit in between the two simulators.ConclusionsWhile the image quality parameters for three RT CT simulators show comparable results, the scanner bore size is of vital importance in various radiotherapy applications. Since the image quality is a function of a large number of confounding parameters, any loss in image quality due to scanner bore size could be compensated by the appropriate choice of scanning parameters, including the exposure and by balancing between the additional imaging dose to the patient and high image quality required in highly conformal RT techniques.     

Absorbed dose and image quality of Varian TrueBeam CBCT compared with OBI CBCT

Purpose: Nowadays, patient positioning and target localization can be verified by using kilovolt cone beam computed tomography (kV-CBCT). There have been various studies on the absorbed doses and image qualities of different kV-CBCT systems. However, the Varian TrueBeam CBCT (TB CBCT) system has not been investigated so far. We assess the image quality and absorbed dose of TB CBCT through comparison with those of on-board imager (OBI) CBCT.Methods: The image quality was evaluated using two phantoms. A CATPHAN phantom measured the image quality parameters of the American Association of Physicists in Medicine Task Group 142 (AAPM TG-142) report. These factors are the pixel value stability and accuracy, noise, high-contrast resolution, low-contrast resolution, and image uniformity. A H₂SO₄ phantom was used to evaluate the image uniformity over a larger region than the CATPHAN phantom. In evaluating the absorbed dose, the radial dose profile and the patient organ doses at the prostate and rectum levels were evaluated.Results: The image quality parameters of AAPM TG-142 using TB CBCT are equal to or greater than those of OBI CBCT. In particular, the contrast-to-noise ratio with TB CBCT is 2.5 times higher than that with OBI CBCT. For the test of a large field uniformity, the maximum difference in the Hounsfield unit (HU) values between the centre and peripheral regions is within 30 HU with TB CBCT and 283 HU with OBI CBCT. The maximum absorbed dose with TB CBCT is decreased by 60%.Conclusions: We find that the image quality improved and the absorbed dose decreased with TB CBCT in comparison to those with OBI CBCT. Its image uniformity is also superior over a larger scanning range.     

An iterative deconvolution algorithm for image recovery in clinical CT: A phantom study

PurposeTo study the feasibility of using an iterative reconstruction algorithm to improve previously reconstructed CT images which are judged to be non-diagnostic on clinical review. A novel rapidly converging, iterative algorithm (RSEMD) to reduce noise as compared with standard filtered back-projection algorithm has been developed.Materials and methodsThe RSEMD method was tested on in-silico, Catphan^®500, and anthropomorphic 4D XCAT phantoms. The method was applied to noisy CT images previously reconstructed with FBP to determine improvements in SNR and CNR. To test the potential improvement in clinically relevant CT images, 4D XCAT phantom images were used to simulate a small, low contrast lesion placed in the liver.ResultsIn all of the phantom studies the images proved to have higher resolution and lower noise as compared with images reconstructed by conventional FBP. In general, the values of SNR and CNR reached a plateau at around 20 iterations with an improvement factor of about 1.5 for in noisy CT images. Improvements in lesion conspicuity after the application of RSEMD have also been demonstrated. The results obtained with the RSEMD method are in agreement with other iterative algorithms employed either in image space or with hybrid reconstruction algorithms.ConclusionsIn this proof of concept work, a rapidly converging, iterative deconvolution algorithm with a novel resolution subsets-based approach that operates on DICOM CT images has been demonstrated. The RSEMD method can be applied to sub-optimal routine-dose clinical CT images to improve image quality to potentially diagnostically acceptable levels.     

Posterior kV-CBCT scanning of the head and neck region minimizes doses to critical organs with sustained image quality

We evaluated the absorbed dose to critical organs, as well as the image quality, at different partial angles in kV-CBCT (Cone Beam Computed Tomography) scanning of the head and neck region. CBCT images of phantom from a 200° rotation were performed by using three different scanning paths, anterior, posterior, and right lateral with Catphan504 and RANDO phantoms. Critical organ dose was measured using TLD 100H in the RANDO phantom. The image quality of those phantoms was evaluated, using HU uniformity, HU linearity, contrast-to-noise ratio, low contrast visibility and spatial resolution with the Catphan504 dataset; and 5-point grading scales for the RANDO phantom dataset by five radiation oncologists. The image qualities from Catphan504 and RANDO phantom of every scanning path were comparable, with no statistically significant difference (p ≥ 0.05). However, there was a significant difference in the critical organ dose in all paths (p < 0.05), depending on the critical organ location and the scanning direction. Scanning directions show no effects on the image quality. Differences in absorbed dose to critical organs should were evaluated. The posterior scanning path for the CBCT was deemed preferable due because of considerably lower doses to several critical organs of the head and neck region.     

Enhancement of soft-tissue contrast in cone-beam CT using an anti-scatter grid with a sparse sampling approach

PurposeAnti-scatter grids suppress the scatter substantially thus improving image contrast in radiography. However, its active use in cone-beam CT for the purpose of improving contrast-to-noise ratio (CNR) has not been successful mainly due to the increased noise related to Poisson statistics of photons. This paper proposes a sparse-view scanning approach to address the above issue.MethodCompared to the conventional cone-beam CT imaging framework, the proposed method reduces the number of projections and increases exposure in each projection to enhance image quality without an additional cost of radiation dose to patients. For image reconstruction from sparse-view data, an adaptive-steepest-descent projection-onto-convex-sets (ASD POCS) algorithm regularized by total-variation (TV) minimization was adopted. Contrast and CNR with various scattering conditions were evaluated in projection domain by a simulation study using GATE. Then we evaluated contrast, resolution, and image uniformity in CT image domain with Catphan phantom. A head phantom with soft-tissue structures was also employed for demonstrating a realistic application. A virtual grid-based estimation and reduction of scatter has also been implemented for comparison with the real anti-scatter grid.ResultsIn the projection domain evaluation, contrast and CNR enhancement was observed when using an anti-scatter grid compared to the virtual grid. In the CT image domain, the proposed method produced substantially higher contrast and CNR of the low-contrast structures with much improved image uniformity.ConclusionWe have shown that the proposed method can provide high-quality CBCT images particularly with an increased contrast of soft-tissue at a neutral dose for image-guidance.     

Effective dose and image quality for intraoperative imaging with a cone-beam CT and a mobile multi-slice CT in spinal surgery: A phantom study

PurposeTo compare the effective dose (ED) and image quality (IQ) of O-arm cone-beam CT (Medtronic, Minneapolis, MN, USA) and Airo multi-slice CT (Brainlab AG, Munich, Germany) for intraoperative-CT (i-CT) in spinal surgery.MethodsThe manufacturer-defined protocols available in the O-arm and Airo systems for three-dimensional lumbar spine imaging were compared.Organ dose was measured both with thermo-luminescent dosimeters and GafChromic films in the Alderson Radiation Therapy anthropomorphic phantom.A subjective analysis was performed by neurosurgeons to compare the clinical IQ of the anthropomorphic phantom images acquired with the different i-CT systems and imaging protocols.Image uniformity, noise, contrast-to-noise-ratio (CNR), and spatial resolution were additionally assessed with the Catphan 504 phantom.ResultsO-arm i-CT caused 56% larger ED than Airo due to the high definition (HD) imaging protocol.The noise was larger for O-arm images leading to a lower CNR than that measured for Airo. Moreover, scattering and beam hardening effects were observed in the O-arm images. Better spatial resolution was measured for the O-arm system (9 lp/cm) than for Airo (4 lp/cm).For all the investigated protocols, O-arm was found to be better for identifying anatomical features important for accurate pedicle screw positioning.ConclusionsAccording to phantom measurements, the HD protocol of O-arm offered better clinical IQ than Airo but larger ED. The larger noise of O-arm images did not compromise the clinical IQ while the superior spatial resolution of this system allowed a better visibility of anatomical features important for pedicle screw positioning in the lumbar region.     

Comparison between new-generation SiPM-based and conventional PMT-based TOF-PET/CT

PurposeThis study aimed to determine whether the SiPM-PET/CT, Discovery MI (DMI) performs better than the PMT-PET/CT system, Discovery 710 (D710).MethodsThe physical performance of both systems was evaluated using NEMA NU 2 standards. Contrast (%), uniformity and image noise (%) are criteria proposed by the Japanese Society of Nuclear Medicine (JSNM) for phantom tests and were determined in images acquired from Hoffman and uniform phantoms using the DMI and D710. Brain and whole-body [¹⁸F]FDG images were also acquired from a healthy male using the DMI and D710.ResultsThe spatial resolution at 1.0 cm off-center in the DMI and D710 was 3.91 and 4.52 mm, respectively. The sensitivity of the DMI and D710 was 12.62 and 7.50 cps/kBq, respectively. The observed peak noise-equivalent count rates were 185.6 kcps at 22.5 kBq/mL and 137.0 kcps at 29.0 kBq/mL, and the scatter fractions were 42.1% and 37.9% in the DMI and D710, respectively. The D710 had better contrast recovery and lower background variability. Contrast, uniformity and image noise in the DMI were 61.0%, 0.0225, and 7.85%, respectively. These outcomes were better than those derived from the D710 and satisfied the JSNM criteria. Brain images acquired by the DMI had better grey-to-white matter contrast and lower image noise at the edge of axial field of view.ConclusionsThe DMI offers better sensitivity, performance under conditions of high count rates and image quality than the conventional PMT-PET/CT system, D710.     

Image quality characteristics for virtual monoenergetic images using dual-layer spectral detector CT: Comparison with conventional tube-voltage images

PurposeTo investigate the image quality characteristics for virtual monoenergetic images compared with conventional tube-voltage image with dual-layer spectral CT (DLCT).MethodsHelical scans were performed using a first-generation DLCT scanner, two different sizes of acrylic cylindrical phantoms, and a Catphan phantom. Three different iodine concentrations were inserted into the phantom center. The single-tube voltage for obtaining virtual monoenergetic images was set to 120 or 140 kVp. Conventional 120- and 140-kVp images and virtual monoenergetic images (40–200-keV images) were reconstructed from slice thicknesses of 1.0 mm. The CT number and image noise were measured for each iodine concentration and water on the 120-kVp images and virtual monoenergetic images. The noise power spectrum (NPS) was also calculated.ResultsThe iodine CT numbers for the iodinated enhancing materials were similar regardless of phantom size and acquisition method. Compared with the iodine CT numbers of the conventional 120-kVp images, those for the monoenergetic 40-, 50-, and 60-keV images increased by approximately 3.0-, 1.9-, and 1.3-fold, respectively. The image noise values for each virtual monoenergetic image were similar (for example, 24.6 HU at 40 keV and 23.3 HU at 200 keV obtained at 120 kVp and 30-cm phantom size). The NPS curves of the 70-keV and 120-kVp images for a 1.0-mm slice thickness over the entire frequency range were similar.ConclusionVirtual monoenergetic images represent stable image noise over the entire energy spectrum and improved the contrast-to-noise ratio than conventional tube voltage using the dual-layer spectral detector CT.     

Development of an x-ray-opaque-marker system for quantitative phantom positioning in patient-specific quality assurance

PurposeWe developed an x-ray-opaque-marker (XOM) system with inserted fiducial markers for patient-specific quality assurance (QA) in CyberKnife (Accuray) and a general-purpose linear accelerator (linac). The XOM system can be easily inserted or removed from the existing patient-specific QA phantom. Our study aimed to assess the utility of the XOM system by evaluating the recognition accuracy of the phantom position error and estimating the dose perturbation around a marker.MethodsThe recognition accuracy of the phantom position error was evaluated by comparing the known error values of the phantom position with the values measured by matching the images with target locating system (TLS; Accuray) and on-board imager (OBI; Varian). The dose perturbation was evaluated for 6 and 10 MV single-photon beams through experimental measurements and Monte Carlo simulations.ResultsThe root mean squares (RMSs) of the residual position errors for the recognition accuracy evaluation in translations were 0.07 mm with TLS and 0.30 mm with OBI, and those in rotations were 0.13° with TLS and 0.15° with OBI. The dose perturbation was observed within 1.5 mm for 6 MV and 2.0 mm for 10 MV from the marker.ConclusionsSufficient recognition accuracy of the phantom position error was achieved using our system. It is unnecessary to consider the dose perturbation in actual patient-specific QA. We concluded that the XOM system can be utilized to ensure quantitative and accurate phantom positioning in patient-specific QA with CyberKnife and a general-purpose linac.     

Prospective image planning in radiation therapy for optimization of image quality and reduction of patient dose

IntroductionCT simulation data in image-guided radiation therapy (IGRT) provides patient-specific subject contrast. This information can be exploited to establish, a priori, a suitable imaging goal and to select patient-specific imaging acquisition parameters that optimize the similarity between reference and daily set-up images and reduce imaging dose. This study aims to describe and clinically validate a computerized algorithm designed to provide such optimization.Material and methodsAn image planning system (IPS) was developed to assist in planar kV imaging technique selection for radiation therapy. The system's patient-specific image quality and dose reduction capabilities were validated herein. Anthropomorphic phantom and clinical data were acquired. Mutual information (MI) was used to compare simulated and measured images in both phantom and clinical tests. Variations in contrast resolution resulting from imaging panel underexposure, saturation and a contrast plateau were investigated. For evaluation of patient-specific imaging dose reduction, the IPS was used to modify acquisition settings for six patients.ResultsPhantom data confirmed the IPS's predictive capability regarding image contrast. Measured and simulated images showed similar progressions from under-exposure, image quality peak, and loss of contrast due to detector saturation. Clinical data demonstrated that contrast resolution and imaging dose could be prospectively improved without loss of image contrast. The algorithm reduced imaging dose by an average of 47%, and a maximum of 80%.ConclusionsLoss of image contrast resulting from under-exposure or over-exposure, as well as a contrast plateau can be predicted by use of a prospective image planning algorithm. Image acquisition parameters can be predicted that reduce patient dose without loss of useful contrast.     

Low-tube-voltage selection for non-contrast-enhanced CT: Comparison of the radiation dose in pediatric and adult phantoms

PurposeWe used pediatric and adult anthropomorphic phantoms to compare the radiation dose of low- and standard tube voltage chest and abdominal non-contrast-enhanced computed tomography (CT) scans. We also discuss the optimal low tube voltage for non-contrast-enhanced CT.MethodsUsing a female adult- and three differently-sized pediatric anthropomorphic phantoms we acquired chest and abdominal non-contrast-enhanced scans on a 320-multidetector CT volume scanner. The tube voltage was set at 80-, 100-, and 120 kVp. The tube current was automatically assigned on the CT scanner in response to the set image noise level. On each phantom and at each tube voltage we measured the surface and center dose using high-sensitivity metal-oxide-semiconductor field-effect transistor detectors.ResultsThe mean surface dose of chest and abdominal CT scans in 5-year olds was 4.4 and 5.3 mGy at 80 kVp, 4.5 and 5.4 mGy at 100 kV, and 4.0 and 5.0 mGy at 120 kVp, respectively. These values were similar in our 3-pediatric phantoms (p > 0.05). The mean surface dose in the adult phantom increased from 14.7 to 19.4 mGy for chest- and from 18.7 to 24.8 mGy for abdominal CT as the tube voltage decreased from 120 to 80 kVp (p < 0.01).ConclusionCompared to adults, the surface and center dose for pediatric patients is almost the same despite a decrease in the tube voltage and the low tube voltage technique can be used for non-contrast-enhanced chest- and abdominal scanning.     

Toward standardized quantitative image quality (IQ) assessment in computed tomography (CT): A comprehensive framework for automated and comparative IQ analysis based on ICRU Report 87

PurposeBased on the guidelines from “Report 87: Radiation Dose and Image-quality Assessment in Computed Tomography” of the International Commission on Radiation Units and Measurements (ICRU), a software framework for automated quantitative image quality analysis was developed and its usability for a variety of scientific questions demonstrated.MethodsThe extendable framework currently implements the calculation of the recommended Fourier image quality (IQ) metrics modulation transfer function (MTF) and noise-power spectrum (NPS), and additional IQ quantities such as noise magnitude, CT number accuracy, uniformity across the field-of-view, contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) of simulated lesions for a commercially available cone-beam phantom. Sample image data were acquired with different scan and reconstruction settings on CT systems from different manufacturers.ResultsSpatial resolution is analyzed in terms of edge-spread function, line-spread-function, and MTF. 3D NPS is calculated according to ICRU Report 87, and condensed to 2D and radially averaged 1D representations. Noise magnitude, CT numbers, and uniformity of these quantities are assessed on large samples of ROIs. Low-contrast resolution (CNR, SNR) is quantitatively evaluated as a function of lesion contrast and diameter. Simultaneous automated processing of several image datasets allows for straightforward comparative assessment.ConclusionsThe presented framework enables systematic, reproducible, automated and time-efficient quantitative IQ analysis. Consistent application of the ICRU guidelines facilitates standardization of quantitative assessment not only for routine quality assurance, but for a number of research questions, e.g. the comparison of different scanner models or acquisition protocols, and the evaluation of new technology or reconstruction methods.     

Assessment of the variation in CT scanner performance (image quality and Hounsfield units) with scan parameters,for image optimisation in radiotherapy treatment planning

PurposeTo define a method and investigate how the adjustment of scan parameters affected the image quality and Hounsfield units (HUs) on a CT scanner used for radiotherapy treatment planning. A lack of similar investigations in the literature may be a contributing factor in the apparent reluctance to optimise radiotherapy CT protocols.MethodA Catphan phantom was used to assess how image quality on a Toshiba Aquilion LB scanner changed with scan parameters. Acquisition and reconstruction field-of-view (FOV), collimation, image slice thickness, effective mAs per rotation and reconstruction algorithm were varied. Changes were assessed for HUs of different materials, high contrast spatial resolution (HCSR), contrast-noise ratio (CNR), HU uniformity, scan direction low contrast and CT dose-index.ResultsCNR and HCSR varied most with reconstruction algorithm, reconstruction FOV and effective mAs. Collimation, but not image slice width, had a significant effect on CT dose-index with narrower collimation giving higher doses. Dose increased with effective mAs. Highest HU differences were seen when changing reconstruction algorithm: 56 HU for densities close to water and 117 HU for bone-like materials. Acquisition FOV affected the HUs but reconstruction FOV and effective mAs did not.ConclusionsAll the scan parameters investigated affected the image quality metrics. Reconstruction algorithm, reconstruction FOV, collimation and effective mAs were most important. Reconstruction algorithm and acquisition FOV had significant effect on HU. The methodology is applicable to radiotherapy CT scanners when investigating image quality optimisation, prior to assessing the impact of scan protocol changes on clinical CT images and treatment plans.     

Geometric inaccuracy and co-registration errors for CT,DynaCT and MRI images used in robotic stereotactic radiosurgery treatment planning

PurposeTo measure the combined errors due to geometric inaccuracy and image co-registration on secondary images (dynamic CT angiography (dCTA), 3D DynaCT angiography (DynaCTA), and magnetic resonance images (MRI)) that are routinely used to aid in target delineation and planning for stereotactic radiosurgery (SRS).MethodsThree phantoms (one commercial and two in-house built) and two different analysis approaches (commercial and MATLAB based) were used to quantify the magnitude of geometric image distortion and co-registration errors for different imaging modalities within CyberKnife’s MultiPlan treatment planning software. For each phantom, the combined errors were reported as a mean target registration error (TRE). The mean TRE’s for different intramodality imaging parameters (e.g., mAs, kVp, and phantom set-ups) and for dCTA, DynaCTA, and MRI systems were measured.ResultsOnly X-ray based imaging can be performed with the commercial phantom, and the mean TRE ± standard deviation values were large compared to the in-house analysis using MATLAB. With the 3D printed phantom, even drastic changes in treatment planning CT imaging protocols did not greatly influence the mean TRE (<0.5 mm for a 1 mm slice thickness CT). For all imaging modalities, the largest mean TRE was found on DynaCT, followed by T2-weighted MR images (albeit all <1 mm).ConclusionsThe user may overestimate the mean TRE if the commercial phantom and MultiPlan were used solely. The 3D printed phantom design is a sensitive and suitable quality assurance tool for measuring 3D geometric inaccuracy and co-registration errors across all imaging modalities.     

Evaluation of a no-reference image quality metric for projection X-ray imaging using a 3D printed patient-specific phantom

PurposeFeasability of a no-reference image quality metric was assessed on patient-like images using a patient-specific phantom simulating a frame of a coronary angiogram.MethodsOne background and one contrast-filled frame of a coronary angiogram, acquired using a clinical imaging protocol, were selected from a Philips Integris Allura FD (Philips Healthcare, Best, The Netherlands). The background frame’s pixels were extruded to a thickness proportional to their grey value. One phantom was 3D printed using composite 80% bronze filament (max. thickness of 5.1 mm), the other was a custom PMMA cast (max thickness of 8.5 cm). A vessel mold was created from the contrast-filled frame and injected with a solution of 320 mg I/ml contrast fluid (75%), water and gelatin. Still X-ray frames of the vessel mold + background phantom + 16 cm PMMA were acquired at manually selected different exposure settings using a Philips Azurion (Philips Healthcare, Best, The Netherlands) in User Quality Control Mode and were exported as RAW images. The signal-difference-to-noise-ratio-squared (SDNR²) and a spatial-domain-equivalent of the noise equivalent quanta (NEQ_SDE) were calculated. The Spearman’s correlation of the latter parameters with a no-reference perceptual image quality metric (NIQE) was investigated.ResultsThe bronze phantom showed better resemblance to the original patient frame selected from a coronary angiogram of an actual patient, with better contrast and less blur than the PMMA phantom. Both phantoms were imaged using a comparable imaging protocol to the one used to acquire the original frame. The bronze phantom was hence used together with the vessel mold for image quality measurements on the 165 still phantom frames. A strong correlation was noted between NEQ_SDE and NIQE (SROCC = –0.99, p < 0.0005) and between SDNR² and NIQE (SROCC = –0.97, p < 0.0005).ConclusionUsing a cost-effective and easy to realize patient-specific phantom we were able to generate patient-like X-ray frames. NIQE as a no-reference image quality model has the potential to predict physical image quality from patient images.     

Design and clinical use of a rotational phantom for dosimetric verification of IMRT/VMAT treatments

PurposeTo describe the design and clinical use of a rotational phantom for dosimetric verification of IMRT/VMAT treatment plans using radiochromic film.MethodsA solid water cylindrical phantom was designed with separable upper and lower halves and rests on plastic bearings allowing for 360° rotation about its central axis. The phantom accommodates a half sheet of radiochromic film, and by rotating the cylinder, the film can be placed in any plane between coronal and sagittal. Calculated dose planes coinciding with rotated film measurements are exported by rotating the CT image and dose distribution within the treatment planning system. The process is illustrated with 2 rotated film measurements of an SRS treatment plan involving 4 separate targets. Additionally, 276 patient specific QA measurements were obtained with the phantom and analyzed with a 2%/2 mm gamma criterion.ResultsThe average 2%/2 mm gamma passing rate for all 276 plans was 99.3%. Seventy-two of the 276 plans were measured with the plane of the film rotated between the coronal and sagittal planes and had an average passing rate of 99.4%.ConclusionsThe rotational phantom allows for accurate film measurements in any plane. With this technique, regions of a dose distribution which might otherwise require multiple sagittal or coronal measurements can be verified with as few as a single measurement. This increases efficiency and, in combination with the high spatial resolution inherent to film dosimetry, makes the rotational technique an attractive option for patient-specific QA.     

Tridimensional dose evaluation of the respiratory motion influence on breast radiotherapy treatments using conformal radiotherapy,forward IMRT,and inverse IMRT planning techniques

PurposeTo evaluate the respiratory motion influence on the tridimensional (3D) dose delivery to breast-shaped phantoms using conformal radiotherapy (3D-RT), Field-in Field (FiF), and IMRT planning techniques.MethodsThis study used breast-shaped phantoms filled with MAGIC-f gel dosimeter to simulate the breast, and an oscillation platform to simulate the respiratory motion. The platform allowed motion in the anterior-posterior direction with oscillation amplitudes of 0.34 cm, 0.88 cm, and 1.22 cm. CT images of the static phantom were used for the 3D-RT, FiF, and IMRT treatment planning. Five phantoms were prepared and irradiated for each planning technique evaluated. Phantom 1 was irradiated static, phantoms 2–4 were irradiated moving with the three different motion amplitudes, and phantom 5 was used as a reference. The 3D dose distributions were obtained by relaxometry of magnetic resonance imaging, and the respiratory motion influence in the doses distribution was accessed by gamma evaluations (3%/3mm/15% threshold) comparing the measurements of the phantoms irradiated under movement with the static ones.ResultsThe mean gamma approvals for three oscillatory amplitudes were 96.44%, 93.23%, and 91.65%; 98.42%, 95.66%, and 94.31%; and 94.49%, 93.51%, and 86.62% respectively for 3D-RT, FiF and IMRT treatments. A gamma results profile per slice along the phantom showed that for FiF and IMRT irradiations, most of the failures occurred in the central region of the phantom.ConclusionsBy increasing the respiratory motion movement, the dose distribution variations for the three planning techniques were more pronounced, being the FiF technique variations the smallest one.