Cone-beam CT dose and imaging performance evaluation with a modular, multipurpose phantom.

PURPOSE: A modular phantom for dosimetry and imaging performance evaluation in cone-beam computed tomography (CBCT) is reported, providing a tool for quantitative technical assessment that can be adapted to a broad variety of CBCT imaging configurations and clinical applications. METHODS: The phantom presents a set of modules that can be ordered in various configurations suitable to a particular CBCT system. Modules include slabs containing a uniform medium, low-contrast inserts, line-spread features, and disk features suitable to measurement of image uniformity, noise, noise-power spectrum (NPS), contrast, contrast-to-noise ratio (CNR), Hounsfield (HU) accuracy, linearity, spatial resolution modulation transfer function (MTF), and magnitude of cone-beam artifact. Automated software recognizes the phantom configuration in DICOM images and provides structured reporting of such test measures. In any modular configuration, the phantom permits measurement of air kerma in central and peripheral locations with an air ionization chamber (e.g., Farmer chamber). The utility and adaptability of the phantom were demonstrated across a spectrum of CBCT systems, including scanners for orthopaedic imaging (Carestream OnSight 3D, Rochester NY), breast imaging (Doheny prototype, UC Davis), image-guided surgery (IGS, Medtronic O-arm, Littleton MA), angiography (Siemens Artis Zeego, Forcheim Germany), and image-guided radiation therapy (IGRT, Elekta Synergy XVI, Stockholm Sweden). RESULTS: The phantom provided a consistent platform for quantitative assessment of dose and imaging performance compatible with a broad spectrum of CBCT systems. The purpose of the survey was not to obtain head-to-head performance comparison of systems designed for such distinct clinical applications. Rather, the survey demonstrated the suitability of the phantom to a broad spectrum of systems in a manner that provides characterization pertinent to disparate applications and imaging tasks. For example: the orthopaedic CBCT system (pertinent clinical tasks relating to high-resolution bone imaging) was shown to achieve MTF consistent with imaging of high-contrast trabecular bone structures (i.e., the MTF reduced to 10% at spatial frequency, f 10  = 1.2 mm-1 ); the breast system (even higher-resolution imaging of microcalcifications) exhibited f 10  = 2.2 mm-1 ; the IGS system (tasks including both bone and soft-tissue contrast resolution) provided f 10  = 0.9 mm-1 and soft-tissue CNR  = 1.64; the angiography system (soft-tissue body interventions) demonstrated CNR  = 1.2 in soft tissues approximating liver lesions; and the IGRT system (pertinent tasks emphasizing HU linearity and image uniformity) showed linear response with HU values ( R 2  = 1), with a cupping artifact ( t cup  = 5.8%) due to x-ray scatter. CONCLUSIONS: The phantom provides an adaptable, quantitative basis for CBCT dosimetry and imaging performance evaluation suitable to a broad variety of CBCT systems. The dosimetry and image quality metrics are consistent with up-to-date methods for rigorous, quantitative, physics testing and should be suitable to emerging standards for CBCT quality assurance.

A 3D printed modular phantom for quality assurance of image-guided small animal irradiators: Design, imaging experiments, and Monte Carlo simulations.

PURPOSE: The goal of this work was to develop and test a cylindrical tissue-equivalent quality assurance (QA) phantom for micro computed tomography (microCT) image-guided small animal irradiators that overcomes deficiencies of existing phantoms due to its mouse-like dimensions and composition. METHODS: The 8.6-cm-long and 2.4-cm-diameter phantom was three-dimensionally (3D) printed out of Somos NeXt plastic on a stereolithography (SLA) printer. The modular phantom consisted of four sections: (a) CT number evaluation section, (b) spatial resolution with slanted edge (for the assessment of longitudinal resolution) and targeting section, (c) spatial resolution with hole pattern (for the assessment of radial direction) section, and (d) uniformity and geometry section. A Python-based graphical user interface (GUI) was developed for automated analysis of microCT images and evaluated CT number consistency, longitudinal and radial modulation transfer function (MTF), image uniformity, noise, and geometric accuracy. The phantom was placed at the imaging isocenter and scanned with the small animal radiation research platform (SARRP) in the pancake geometry (long axis of the phantom perpendicular to the axis of rotation) with a variety of imaging protocols. Tube voltage was set to 60 and 70 kV, tube current was set to 0.5 and 1.2 mA, voxel size was set to 200 and 275 µm, imaging times of 1, 2, and 4 min were used, and frame rates of 6 and 12 frames per second (fps) were used. The phantom was also scanned in the standard (long axis of the phantom parallel to the axis of rotation) orientation. The quality of microCT images was analyzed and compared to recommendations presented in our previous work that was derived from a multi-institutional study. Additionally, a targeting accuracy test with a film placed in the phantom was performed. MicroCT imaging of the phantom was also simulated in a modified version of the EGSnrc/DOSXYZnrc code. Images of the resolution section with the hole pattern were acquired experimentally as well as simulated in both the pancake and the standard imaging geometries. The radial spatial resolution of the experimental and simulated images was evaluated and compared to experimental data. RESULTS: For the centered phantom images acquired in the pancake geometry, all imaging protocols passed the spatial resolution criterion in the radial direction (>1.5 lp/mm @ 0.2 MTF), the geometric accuracy criterion (<200 µm), and the noise criterion (<55 HU). Only the imaging protocol with 200-µm voxel size passed the criterion for spatial resolution in the longitudinal direction (>1.5 lp/mm @ 0.2 MTF). The 70-kV tube voltage dataset failed the bone CT number consistency test (<55 HU). Due to cupping artifacts, none of the imaging protocols passed the uniformity test of <55 HU. When the phantom was scanned in the standard imaging geometry, image uniformity and longitudinal MTF were satisfactory; however, the CT number consistency failed the recommended limit. A targeting accuracy of 282 and 251 µm along the x- and z-direction was observed. Monte Carlo simulations confirmed that the radial spatial resolution for images acquired in the pancake geometry was higher than the one acquired in the standard geometry. CONCLUSIONS: The new 3D-printed phantom presents a useful tool for microCT image analysis as it closely mimics a mouse. In order to image mouse-sized animals with acceptable image quality, the standard protocol with a 200-µm voxel size should be chosen and cupping artifacts need to be resolved.

Simplificando o tratamento de paciente em uso de rivaroxabana através da cirurgia guiada ­ relato de caso / Simplifying treatment for patient using rivaroxaban through guided surgery ­ case report

O número de pacientes em tratamento com anticoagulantes necessitando de tratamento odontológico cresceu consideravelmente nos últimos anos. Dessa forma, os clínicos que trabalham com Implantodontia devem estar preparados para esses pacientes e possíveis complicações. O presente estudo objetiva fazer um relato de caso sobre o manejo clínico de paciente de 70 anos de idade com histórico de hemorragia severa associada ao uso do anticoagulante rivaroxabana, mostrando como o uso da cirurgia guiada pode simplificar e viabilizar reabilitações com implantes dentários em pacientes com essa condição. Para isso, foi realizado escaneamento intraoral e planejamento virtual em software específico para instalação de dois implantes Straumann em região posterior direita, onde já havia sido realizado um levantamento de seio maxilar. O procedimento cirúrgico transcorreu de forma rápida e sem complicações, com alta aceitação por parte do paciente, que recebeu a reabilitação protética final aproximadamente 3 meses após a instalação dos implantes. Concluiu-se que a cirurgia guiada é uma excelente alternativa para reduzir morbidade e risco de hemorragia em pacientes fazendo uso de rivaroxabana (AU).

The number of patients receiving anticoagulants requiring dental treatment has increased considerably in recent years. Thus, clinicians working with Implantology should be prepared to treat these patients and possible complications. This study aims to present a case report on the clinical management of a 70-year-old patient with a history of severe hemorrhage associated with the use of rivaroxaban anticoagulant, showing how the use of guided surgery can simplify and enable rehabilitation with dental implants in patients with this condition. In order to do this, we performed intraoral scan and virtual planning in specific software for the placement of two Straumann implants in the right posterior region, where a maxillary sinus lift had already been performed. The surgical procedure was performed quickly and without complications, with high acceptance by the patient, who received the final prosthetic rehabilitation approximately 3 months after implant placemenet. It is concluded that guided surgery is an excellent alternative to reduce morbidity and risk of hemorrhage in patients taking rivaroxaban (AU).

Stray light in cone beam optical computed tomography: III. Evaluation of a redesigned large-volume commercial scanner based on a convergent light source.

Optical cone beam computed tomography (CT), using a digital camera to acquire 2D projection images, provides a fast, mechanically simple method for 3D radiation dosimetry. However, original cone beam designs had poor accuracy as a result of considerable scatter/stray light reaching the camera. Previously, our group presented a redesigned convergent light source for optical cone beam CT that considerably reduced stray light contribution and improved accuracy (Dekker et al 2016 Phys. Med. Biol. 61 2910). Here, we performed an evaluation of a newly updated commercial optical cone beam CT scanner (VistaTM, ModusQA, London, Canada) based on that design. Two different light source configurations were examined: the manufacturer's default configuration which uses a 10 cm wide, 5 cm high diffuser light source, and a smaller, 1.5 cm diameter diffuser light source that more closely aligns with our previously described design. We imaged large volume (15 cm diameter cylinders) absorbing and scattering solution phantoms as well as a 1.25 cm diameter absorber placed within 15 cm diameter gel-like scattering phantom. Optical CT reconstructions were compared against narrow-beam measurements of attenuation made by placing an aperture in the optical CT system. Our results show that considerable stray light is present when using the manufacturer's default configuration, as cupping artifacts and large (⩾10%) discrepancies between optical CT and narrow-beam attenuation measurements occur when imaging scattering phantoms. However, when imaging is performed using the 1.5 cm diameter source, optical CT measurements agree with narrow-beam measurements within ∼3% for both absorbing and scattering objects, as well as the small absorber in a scattering medium. Using this light source will require higher optical quality vessels than are currently provided by the manufacturer.

Mobile C-Arm with a CMOS detector: Technical assessment of fluoroscopy and Cone-Beam CT imaging performance.

PURPOSE: Indirect-detection CMOS flat-panel detectors (FPDs) offer fine pixel pitch, fast readout, and low electronic noise in comparison to current a-Si:H FPDs. This work investigates the extent to which these potential advantages affect imaging performance in mobile C-arm fluoroscopy and cone-beam CT (CBCT). METHODS: FPDs based on CMOS (Xineos 3030HS, 0.151 mm pixel pitch) or a-Si:H (PaxScan 3030X, 0.194 mm pixel pitch) sensors were outfitted on equivalent mobile C-arms for fluoroscopy and CBCT. Technical assessment of 2D and 3D imaging performance included measurement of electronic noise, gain, lag, modulation transfer function (MTF), noise-power spectrum (NPS), detective quantum efficiency (DQE), and noise-equivalent quanta (NEQ) in fluoroscopy (with entrance air kerma ranging 5-800 nGy per frame) and cone-beam CT (with weighted CT dose index, CTDIw , ranging 0.08-1 mGy). Image quality was evaluated by clinicians in vascular, orthopaedic, and neurological surgery in realistic interventional scenarios with cadaver subjects emulating a variety of 2D and 3D imaging tasks. RESULTS: The CMOS FPD exhibited ~2-3× lower electronic noise and ~7× lower image lag than the a-Si:H FPD. The 2D (projection) DQE was superior for CMOS at ≤50 nGy per frame, especially at high spatial frequencies (~2% improvement at 0.5 mm-1 and ≥50% improvement at 2.3 mm-1 ) and was somewhat inferior at moderate-high doses (up to 18% lower DQE for CMOS at 0.5 mm-1 ). For smooth CBCT reconstructions (low-frequency imaging tasks), CMOS exhibited ~10%-20% higher NEQ (at 0.1-0.5 mm-1 ) at the lowest dose levels (CTDIw ≤0.1 mGy), while the a-Si:H system yielded slightly (~5%) improved NEQ (at 0.1-0.5 lp/mm) at higher dose levels (CTDIw ≥0.6 mGy). For sharp CBCT reconstructions (high-frequency imaging tasks), NEQ was ~32% higher above 1 mm-1 for the CMOS system at mid-high-dose levels and ≥75% higher at the lowest dose levels (CTDIw ≤0.1 mGy). Observer assessment of 2D and 3D cadaver images corroborated the objective metrics with respect to a variety of pertinent interventional imaging tasks. CONCLUSION: Measurements of image noise, spatial resolution, DQE, and NEQ indicate improved low-dose performance for the CMOS-based system, particularly at lower doses and higher spatial frequencies. Assessment in realistic imaging scenarios confirmed improved visibility of fine details in low-dose fluoroscopy and CBCT. The results quantitate the extent to which CMOS detectors improve mobile C-arm imaging performance, especially in 2D and 3D imaging scenarios involving high-resolution tasks and low-dose conditions.

Beam Hardening Correction Using Cone Beam Consistency Conditions.

The polychromatic X-ray spectrum and the energy-dependent attenuation coefficient of materials cause beam hardening artifacts in CT reconstructed volumes. These artifacts appear as cupping and streak artifacts depending on the material composition and the geometry of the imaged object. CT scanners employ projection linearization to transform polychromatic attenuation to monochromatic attenuation using a polynomial model. Polynomial coefficients are computed during calibration or using prior information such as X-ray spectrum and attenuation properties of the materials. In this paper, we are presenting a novel method to correct beam hardening artifacts by enforcing cone beam consistency conditions on the projection data. We used consistency conditions derived from Grangeat's fundamental relation between cone beam projection data and 3-D Radon transform. The optimal polynomial coefficients for artifact reduction are iteratively estimated by minimizing the inconsistency of a set of projection pairs. The results from simulated and real datasets show the visible reduction of artifacts. Our studies also demonstrate the robustness of the algorithm when the projections are perturbed with other physical measurement and geometrical errors. The proposed method requires neither calibration nor prior information like X-ray spectrum, attenuation properties of the materials and detector response. The algorithm can be used for beam hardening correction in clinical, pre-clinical, and industrial CT systems.

Cone Beam X-Ray Luminescence Tomography Imaging Based on KA-FEM Method for Small Animals.

Cone beam X-ray luminescence tomography can realize fast X-ray luminescence tomography imaging with relatively low scanning time compared with narrow beam X-ray luminescence tomography. However, cone beam X-ray luminescence tomography suffers from an ill-posed reconstruction problem. First, the feasibility of experiments with different penetration and multispectra in small animal has been tested using nanophosphor material. Then, the hybrid reconstruction algorithm with KA-FEM method has been applied in cone beam X-ray luminescence tomography for small animals to overcome the ill-posed reconstruction problem, whose advantage and property have been demonstrated in fluorescence tomography imaging. The in vivo mouse experiment proved the feasibility of the proposed method.

X-Ray Scatter Correction on Soft Tissue Images for Portable Cone Beam CT.

Soft tissue images from portable cone beam computed tomography (CBCT) scanners can be used for diagnosis and detection of tumor, cancer, intracerebral hemorrhage, and so forth. Due to large field of view, X-ray scattering which is the main cause of artifacts degrades image quality, such as cupping artifacts, CT number inaccuracy, and low contrast, especially on soft tissue images. In this work, we propose the X-ray scatter correction method for improving soft tissue images. The X-ray scatter correction scheme to estimate X-ray scatter signals is based on the deconvolution technique using the maximum likelihood estimation maximization (MLEM) method. The scatter kernels are obtained by simulating the PMMA sheet on the Monte Carlo simulation (MCS) software. In the experiment, we used the QRM phantom to quantitatively compare with fan-beam CT (FBCT) data in terms of CT number values, contrast to noise ratio, cupping artifacts, and low contrast detectability. Moreover, the PH3 angiography phantom was also used to mimic human soft tissues in the brain. The reconstructed images with our proposed scatter correction show significant improvement on image quality. Thus the proposed scatter correction technique has high potential to detect soft tissues in the brain.

Analytic image reconstruction from partial data for a single-scan cone-beam CT with scatter correction.

PURPOSE: A beam-blocker composed of multiple strips is a useful gadget for scatter correction and/or for dose reduction in cone-beam CT (CBCT). However, the use of such a beam-blocker would yield cone-beam data that can be challenging for accurate image reconstruction from a single scan in the filtered-backprojection framework. The focus of the work was to develop an analytic image reconstruction method for CBCT that can be directly applied to partially blocked cone-beam data in conjunction with the scatter correction. METHODS: The authors developed a rebinned backprojection-filteration (BPF) algorithm for reconstructing images from the partially blocked cone-beam data in a circular scan. The authors also proposed a beam-blocking geometry considering data redundancy such that an efficient scatter estimate can be acquired and sufficient data for BPF image reconstruction can be secured at the same time from a single scan without using any blocker motion. Additionally, scatter correction method and noise reduction scheme have been developed. The authors have performed both simulation and experimental studies to validate the rebinned BPF algorithm for image reconstruction from partially blocked cone-beam data. Quantitative evaluations of the reconstructed image quality were performed in the experimental studies. RESULTS: The simulation study revealed that the developed reconstruction algorithm successfully reconstructs the images from the partial cone-beam data. In the experimental study, the proposed method effectively corrected for the scatter in each projection and reconstructed scatter-corrected images from a single scan. Reduction of cupping artifacts and an enhancement of the image contrast have been demonstrated. The image contrast has increased by a factor of about 2, and the image accuracy in terms of root-mean-square-error with respect to the fan-beam CT image has increased by more than 30%. CONCLUSIONS: The authors have successfully demonstrated that the proposed scanning method and image reconstruction algorithm can effectively estimate the scatter in cone-beam projections and produce tomographic images of nearly scatter-free quality. The authors believe that the proposed method would provide a fast and efficient CBCT scanning option to various applications particularly including head-and-neck scan.

A piecewise-focused high DQE detector for MV imaging.

PURPOSE: Electronic portal imagers (EPIDs) with high detective quantum efficiencies (DQEs) are sought to facilitate the use of the megavoltage (MV) radiotherapy treatment beam for image guidance. Potential advantages include high quality (treatment) beam's eye view imaging, and improved cone-beam computed tomography (CBCT) generating images with more accurate electron density maps with immunity to metal artifacts. One approach to increasing detector sensitivity is to couple a thick pixelated scintillator array to an active matrix flat panel imager (AMFPI) incorporating amorphous silicon thin film electronics. Cadmium tungstate (CWO) has many desirable scintillation properties including good light output, a high index of refraction, high optical transparency, and reasonable cost. However, due to the 0 1 0 cleave plane inherent in its crystalline structure, the difficulty of cutting and polishing CWO has, in part, limited its study relative to other scintillators such as cesium iodide and bismuth germanate (BGO). The goal of this work was to build and test a focused large-area pixelated "strip" CWO detector. METHODS: A 361 × 52 mm scintillator assembly that contained a total of 28 072 pixels was constructed. The assembly comprised seven subarrays, each 15 mm thick. Six of the subarrays were fabricated from CWO with a pixel pitch of 0.784 mm, while one array was constructed from BGO for comparison. Focusing was achieved by coupling the arrays to the Varian AS1000 AMFPI through a piecewise linear arc-shaped fiber optic plate. Simulation and experimental studies of modulation transfer function (MTF) and DQE were undertaken using a 6 MV beam, and comparisons were made between the performance of the pixelated strip assembly and the most common EPID configuration comprising a 1 mm-thick copper build-up plate attached to a 133 mg/cm(2) gadolinium oxysulfide scintillator screen (Cu-GOS). Projection radiographs and CBCT images of phantoms were acquired. The work also introduces the use of a lightweight edge phantom to generate MTF measurements at MV energies and shows its functional equivalence to the more cumbersome slit-based method. RESULTS: Measured and simulated DQE(0)'s of the pixelated CWO detector were 22% and 26%, respectively. The average measured and simulated ratios of CWO DQE(f) to Cu-GOS DQE(f) across the frequency range of 0.0-0.62 mm(-1) were 23 and 29, respectively. 2D and 3D imaging studies confirmed the large dose efficiency improvement and that focus was maintained across the field of view. In the CWO CBCT images, the measured spatial resolution was 7 lp/cm. The contrast-to-noise ratio was dramatically improved reflecting a 22 × sensitivity increase relative to Cu-GOS. The CWO scintillator material showed significantly higher stability and light yield than the BGO material. CONCLUSIONS: An efficient piecewise-focused pixelated strip scintillator for MV imaging is described that offers more than a 20-fold dose efficiency improvement over Cu-GOS.

A level set method for cupping artifact correction in cone-beam CT.

PURPOSE: To reduce cupping artifacts and improve the contrast-to-noise ratio in cone-beam computed tomography (CBCT). METHODS: A level set method is proposed to reduce cupping artifacts in the reconstructed image of CBCT. The authors derive a local intensity clustering property of the CBCT image and define a local clustering criterion function of the image intensities in a neighborhood of each point. This criterion function defines an energy in terms of the level set functions, which represent a segmentation result and the cupping artifacts. The cupping artifacts are estimated as a result of minimizing this energy. RESULTS: The cupping artifacts in CBCT are reduced by an average of 90%. The results indicate that the level set-based algorithm is practical and effective for reducing the cupping artifacts and preserving the quality of the reconstructed image. CONCLUSIONS: The proposed method focuses on the reconstructed image without requiring any additional physical equipment, is easily implemented, and provides cupping correction through a single-scan acquisition. The experimental results demonstrate that the proposed method successfully reduces the cupping artifacts.

[Application possibilities and initial experience with digital volume tomography in hand and wrist imaging]. / Digitale Volumentomografie (DVT) des knöchernen Handskeletts: Erste Erfahrungen und Anwendungsmöglichkeiten.

During the last decade, DVT (digital volume tomography) imaging has become a widely used standard technique in head and neck imaging. Lower radiation exposure compared to conventional computed tomography (MDCT) has been described. Recently, DVT has been developed as an extremity scanner and as such represents a new imaging technique for hand surgery. We here describe the first 24 months experience with this new imaging modality in hand and wrist imaging by presenting representative cases and by describing the technical background. Furthermore, the method's advantages and disadvantages are discussed with reference to the given literature.

Artefact expression associated with several cone-beam computed tomographic machines when imaging root filled teeth.

AIM: To evaluate the characteristic artefact patterns associated with teeth root filled with Gutta-percha when scanned with four cone-beam CT devices. METHODOLOGY: Whilst using soft tissue simulation, ten root filled human premolars were placed in empty sockets in a dry human skull. Subsequently, the skull was scanned using 3D Accuitomo 170(®) , WhiteFox(®) , Cranex 3D(®) and Scanora 3D(®) following clinical protocols with the highest resolution and artefact reduction. After proper image registration in OnDemand3D(®) software (Cybermed, Seoul, Korea), each image slice was evaluated by three trained and calibrated dentomaxillofacial radiologists, which scored absence (0) and presence (1) of cupping artefact, hypodense halos and streak artefacts. Kappa test was performed for intra- and interobserver agreement. RESULTS: A moderate to perfect agreement for each observer (intra-observer κ = 0.5-1.0) was found. Agreement between the different observers was moderate to almost perfect for the different artefact patterns (interobserver κ = 0.55-0.9). Cupping artefact was the most prevalent (70%), followed by a hypodense halo (35%) and streak artefacts (16%). The Chi-squared test revealed significantly more streaks in axial slices (P < 0.0001), with some CBCT systems yielding significantly inferior results to others (P < 0.05). The dedicated EndoMode and artefact reduction did not improve the result significantly. CONCLUSIONS: The variation of artefact expression was significantly different amongst CBCT machines for root filled teeth. Continuous efforts are needed to improve CBCT reconstruction algorithms, with a specific focus on reducing artefacts induced by dense dental materials, whilst striving for enhanced image quality at low-radiation doses.

The effect of surrounding conditions on pixel value of cone beam computed tomography.

OBJECTIVE: The purpose of this study was to evaluate the reliability of pixel value in CBCT, especially with regard to the effect of surrounding objects that are presented outside the field of view (FOV). MATERIALS AND METHODS: This experiment used the GE Hi-Speed QXi, a multidetector helical computed tomography (MDCT) scanner, and the 3D Accuitomo FPD 8, a cone beam computed tomography (CBCT) scanner. Two types of phantoms were used, both of which contained Lipiodol Ultra Fluid (Lipiodol UF). The type A phantom was a target phantom for pixel value measurement while type B was used for the surrounding environment. For CBCT, the type A phantom was placed in a water bath, and 4 types of surrounding environmental conditions were created: (1) no other phantom present, (2) phantom type B also within the FOV, (3) half of phantom type B within the FOV, (4) phantom type B entirely outside the FOV but within the path of x-rays aimed at phantom A. RESULTS: In MDCT, pixel value (CT number) showed an almost linear correlation with the concentration of Lipiodol UF. In CBCT, on the other hand, pixel value was not linearly correlated with Lipiodol UF concentration. The position of the type B phantom affected pixel values in images of the type A phantom. CONCLUSIONS: Pixel value in CBCT may be affected by various conditions such as beam hardening and surrounding materials, and is therefore not reliable. Caution is essential when pixel values in CBCT are used to estimate bone density at potential implant sites.

Cone-beam breast computed tomography with a displaced flat panel detector array.

PURPOSE: In cone-beam computed tomography (CBCT), and in particular in cone-beam breast computed tomography (CBBCT), an important issue is the reduction of the image artifacts produced by photon scatter and the reduction of patient dose. In this work, the authors propose to apply the detector displacement technique (also known as asymmetric detector or "extended view" geometry) to approach this goal. Potentially, this type of geometry, and the accompanying use of a beam collimator to mask the unirradiated half-object in each projection, permits some reduction of radiation dose with respect to conventional CBBCT and a sizeable reduction of the overall amount of scatter in the object, for a fixed contrast-to-noise ratio (CNR). METHODS: The authors consider a scan configuration in which the projection data are acquired from an asymmetrically positioned detector that covers only one half of the scan field of view. Monte Carlo simulations and measurements, with their CBBCT laboratory scanner, were performed using PMMA phantoms of cylindrical (70-mm diameter) and hemiellipsoidal (140-mm diameter) shape simulating the average pendant breast, at 80 kVp. Image quality was evaluated in terms of contrast, noise, CNR, contrast-to-noise ratio per unit of dose (CNRD), and spatial resolution as width of line spread function for high contrast details. RESULTS: Reconstructed images with the asymmetric detector technique deviate less than 1% from reconstruction with a conventional symmetric detector (detector view) and indicate a reduction of the cupping artifact in CT slices. The maximum scatter-to-primary ratio at the center of the phantom decreases by about 50% for both small and large diameter phantoms (e.g., from 0.75 in detector view to 0.40 in extended view geometry at the central axis of the 140-mm diameter PMMA phantom). Less cupping produces an increase of the CT number accuracy and an improved image detail contrast, but the associated increase of noise observed may produce a decrease of detail CNR. By simulating the energy deposited inside the phantoms, the authors evaluated a maximum 50% reduction of the absorbed dose at the expense of a decrease of CNR, for the half beam irradiation of the object performed with the displaced detector technique with respect to full beam irradiation. The decrease in CNR, and in absorbed dose as well, translates into a detail CNRD showing values comparable to or higher than the ones obtained for a conventional symmetric detector technique, attributed to the effect of decreased scatter in particular at the axis of the irradiated object. An estimate is provided (about 12%) for the average dose reduction possible in CBBCT at constant CNR for the average uncompressed breast (14 cm diameter, 50% glandularity), in case of minimum image overlapping in extended view. CONCLUSIONS: Simulations and experiments show that CBCT reconstructions with the displaced detector technique and with a half beam collimator are less affected by scatter artifacts, which could lead to some decrease of the radiation dose to the irradiated object with respect to a conventional reconstruction. This dose reduction is associated with increase of noise, decrease of CNR, but equal or improved CNRD values. The use of a small area detector would allow also to reduce the apparatus cost and to improve the data transfer speed with a corresponding increment of frame rate.

Characterization and correction of cupping effect artefacts in cone beam CT.

OBJECTIVE: The purpose of this study was to demonstrate and correct the cupping effect artefact that occurs owing to the presence of beam hardening and scatter radiation during image acquisition in cone beam CT (CBCT). METHODS: A uniform aluminium cylinder (6061) was used to demonstrate the cupping effect artefact on the Planmeca Promax 3D CBCT unit (Planmeca OY, Helsinki, Finland). The cupping effect was studied using a line profile plot of the grey level values using ImageJ software (National Institutes of Health, Bethesda, MD). A hardware-based correction method using copper pre-filtration was used to address this artefact caused by beam hardening and a software-based subtraction algorithm was used to address scatter contamination. RESULTS: The hardware-based correction used to address the effects of beam hardening suppressed the cupping effect artefact but did not eliminate it. The software-based correction used to address the effects of scatter resulted in elimination of the cupping effect artefact. CONCLUSION: Compensating for the presence of beam hardening and scatter radiation improves grey level uniformity in CBCT.

Evaluation of image quality for different kV cone-beam CT acquisition and reconstruction methods in the head and neck region.

PURPOSE: To evaluate the image quality obtained in a standard QA phantom with both clinical and non-clinical cone-beam computed tomography (CBCT) acquisition modes for the head and neck (HN) region as a step towards CBCT-based treatment planning. The impact of deteriorated Hounsfield unit (HU) accuracy was investigated by comparing results from clinical CBCT image reconstructions to those obtained from a pre-clinical scatter correction algorithm. METHODS: Five different CBCT acquisition modes on a clinical system for kV CBCT-guided radiotherapy were investigated. Image reconstruction was performed in both standard clinical software and with an experimental reconstruction algorithm with improved beam hardening and scatter correction. Using the Catphan 504 phantom, quantitative measures of HU uniformity, HU verification and linearity, contrast-to-noise ratio (CNR), and spatial resolution using modulation transfer function (MTF) estimation were assessed. To benchmark the CBCT image properties, comparison to standard HN protocols on conventional CT scanners was performed by similar measures. RESULTS: The HU uniformity within a water-equivalent homogeneous region was considerably improved using experimental vs. standard reconstruction, by factors of two for partial scans and four for full scans. Similarly, the amount of capping/cupping artifact was reduced by more than 1.5%. With mode and reconstruction specific HU calibration using seven inhomogeneity inserts comparable HU linearity was observed. CNR was on average 5% higher for experimental reconstruction (scaled with the square-root of dose between modes for both reconstruction methods). CONCLUSIONS: Judged on parameters affecting the common diagnostic image properties, improved beam hardening and scatter correction diminishes the difference between CBCT and CT image quality considerably. In the pursuit of CBCT-based treatment adaptation, dedicated imaging protocols may be required.

[Periinterventional cone-beam-CT: application at transarterial chemoembolization of liver tumors]. / Periinterventionelle Cone-Beam-CT: Anwendung bei transarterieller Chemoembolisation von Lebertumoren.

Periinterventional Cone-Beam CT (CBCT) today is a valuable tool in complex radiological interventions. Only little experience exists about CBCT in transarterial chemoembolisations (TACE) of liver tumors. 25 patients underwent periinterventional CBCT. We used a C-arc DSA with 30 × 40 cm flat panel detector. Image data with axial, coronal and 3D-reconstruction were acquired by 217° rotation in 8 seconds. In all 25 cases CBCT had an influence on the TACE regarding the decision which vessels to catheterize, the amount of retention of the embolisation agent or an abort because of insufficient vascularisation. In comparison with DSA alone, CBCT allows a better visualisation of tumour vessels, simplifies selective catheterisation, the decision whether an embolisation is possible and enables a good visualisation of Lipiodol retention. Hence, CBCT is a helpful periinterventional tool but cannot substitute CT and MRI in follow up.

Scatter correction for cone-beam computed tomography using moving blocker strips: a preliminary study.

PURPOSE: One well-recognized challenge of cone-beam computed tomography (CBCT) is the presence of scatter contamination within the projection images. Scatter degrades the CBCT image quality by decreasing the contrast, introducing shading artifacts, and leading to inaccuracies in the reconstructed CT number. The authors propose a blocker-based approach to simultaneously estimate scatter signal and reconstruct the complete volume within the field of view (FOV) from a single CBCT scan. METHODS: A physical strip attenuator (i.e., "blocker"), consisting of lead strips, is inserted between the x-ray source and the patient. The blocker moves back and forth along the z-axis during the gantry rotation. With such a design, the data required for the filtering step of the Feldkamp-Davis-Kress (FDK) algorithm are complete in the unblocked region and the entire volume within the FOV has the measurements at different projection views. The two-dimensional scatter fluence is estimated by interpolating the signal from the blocked regions. A modified FDK algorithm and an iterative reconstruction based on the constraint optimization are used to reconstruct CBCT images from unblocked projection data after the scatter signal is subtracted. A simulation study and an experimental study are performed to evaluate the performance of the proposed scatter correction scheme. RESULTS: The scatter-induced shading/cupping artifacts are substantially reduced in CBCT using the proposed strategy. In the simulation study, the mean relative error is reduced from 25% to 3% and 2% in the images reconstructed by the modified FDK and constraint optimization, respectively. In the experimental study using a CatPhan 600 phantom, CT number errors in the selected regions of interest are reduced from 256 to less than 20. CONCLUSIONS: An effective scatter correction scheme is proposed for CBCT. A moving blocker consisting of lead strips is inserted between the x-ray source and the patient during CBCT acquisition. The proposed method allows the authors to simultaneously estimate the scatter signal in projection data, reduce the imaging dose, and obtain complete volumetric information within the FOV.

A study of image-guided radiotherapy of bladder cancer based on lipiodol injection in the bladder wall.

PURPOSE: We have tested a procedure of focal injection of the contrast medium Lipiodol as a fiducial marker for image-guided boost of the tumor in bladder cancer radiotherapy (RT). In this study, we have evaluated the feasibility and the safety of the method as well as the inter- and intra-fraction shift of the bladder tumor. MATERIALS AND METHODS: Five patients with muscle invasive urinary bladder cancer were included in the study. Lipiodol was injected during flexible cystoscopy into the submucosa of the bladder wall at the periphery of the tumor or the post resection tumor-bed. Cone-beam CT (CBCT) scans were acquired daily throughout the course of RT. RESULTS: Lipiodol demarcation of the bladder tumor was feasible and safe with only a minimum of side effects related to the procedure. The Lipiodol spots were visible on CT and CBCT scans for the duration of the RT course. More than half of all the treatment fractions required a geometric shift of 5 mm or more to match on the Lipiodol spots. The mean intra-fraction shift (3D) of the tumor was 3 mm, largest in the anterior-posterior and cranial-caudal directions. CONCLUSION: This study demonstrates that Lipiodol can be injected into the bladder mucosa and subsequently visualized on CT and CBCT as a fiducial marker. The relatively large inter-fraction shifts in the positions of Lipiodol spots compared to the intra-fraction movement indicates that image-guided RT based on radio-opaque markers is important for RT of the bladder cancer tumor.