Characterization of tissue magnetic susceptibility-induced distortions for MRIgRT.

PURPOSE: MR image geometric integrity is one of the building blocks of MRI-guided radiotherapy. In particular, tissue magnetic susceptibility-induced effects are patient-dependent and their behavior is difficult to assess and predict. In this study, the authors investigated in detail the characteristics of susceptibility (χ) distortions in the context of MRIgRT, including the case of two common MR-linac system configurations. METHODS: The magnetic field distortions were numerically simulated for several imaging parameters and anatomical sites, i.e., brain, lung, pelvis (with air pockets), and prostate. The simulation process consisted of (a) segmentation of patient CT data into susceptibility relevant anatomical volumes (i.e., soft-tissue, bone and air∕lung), (b) conversion of CT data into susceptibility masks by assigning bulk χ values to the structures defined at (a), (c) numerical computations of the local magnetic fields by using a finite difference algorithm, and (d) generation of the geometric distortion maps from the magnetic field distributions. For each patient anatomy, the distortions were quantified at the interfaces of anatomical structures with significantly different χ values. The analysis was performed for two specific orientations of the external main magnetic field (B(0)) characteristic to the MR-linac systems, specifically along the z-axis for a bore MR scanner and in the (x,y)-plane for a biplanner magnet. The magnetic field local perturbations were reported in ppm. The metrics used to quantify the geometric distortions were the maximum, mean, and range of distortions. The numerical simulation algorithm was validated using phantom data measurements. RESULTS: Susceptibility-induced distortions were determined for both quadratic and patient specific geometries. The numerical simulations showed a good agreement with the experimental data. The measurements were acquired at 1.5 and 3 T and with an encoding gradient varying between 3 and 20 mT∕m by using an annular phantom mimicking the water-air and water-oil χ interfaces. For quadratic geometries, the magnitude of field distortion increased rapidly with the size of the inhomogeneity up to about 10 mm and then tended to plateau. This trend became more evident for materials with a larger Δχ relative to water. The simulations showed only a slight increase in the maximum distortion values when the B(0) orientation was varied with regard to the shape of the χ inhomogeneity. In the case of patient anatomy, the largest distortion values arose at the air-soft-tissue interface. Considering the two MR-linac system configurations and comparing the field distortion values corresponding to all organ structures, the distortions tended to be larger for the biplanar magnet. The authors provide a reference table with ppm values which can be used to easily evaluate the geometric distortions for patient data as a function of B(0) and the strength of the encoding gradient. CONCLUSIONS: The susceptibility distortions were quantified as a function of multiple parameters such as the χ inhomogeneity size and shape, the magnitude of B(0) and the readout gradient, and the orientation of B(0) with respect to the sample geometry. The analysis was performed for several anatomical sites and corresponding to two B(0) orientations as featured by MR-linac systems.

Minimizing dose during fluoroscopic tracking through geometric performance feedback.

PURPOSE: There is a growing concern regarding the dose delivered during x-ray fluoroscopy guided procedures, particularly in interventional cardiology and neuroradiology, and in real-time tumor tracking radiotherapy and radiosurgery. Many of these procedures involve long treatment times, and as such, there is cause for concern regarding the dose delivered and the associated radiation related risks. An insufficient dose, however, may convey less geometric information, which may lead to inaccuracy and imprecision in intervention placement. The purpose of this study is to investigate a method for achieving the required tracking uncertainty for a given interventional procedure using minimal dose. METHODS: A simple model is used to demonstrate that a relationship exists between imaging dose and tracking uncertainty. A feedback framework is introduced that exploits this relationship to modulate the tube current (and hence the dose) in order to maintain the required uncertainty for a given interventional procedure. This framework is evaluated in the context of a fiducial tracking problem associated with image-guided radiotherapy in the lung. A particle filter algorithm is used to robustly track the fiducial as it traverses through regions of high and low quantum noise. Published motion models are incorporated in a tracking test suite to evaluate the dose-localization performance trade-offs. RESULTS: It is shown that using this framework, the entrance surface exposure can be reduced by up to 28.6% when feedback is employed to operate at a geometric tracking uncertainty of 0.3 mm. CONCLUSIONS: The analysis reveals a potentially powerful technique for dynamic optimization of fluoroscopic imaging parameters to control the applied dose by exploiting the trade-off between tracking uncertainty and x-ray exposure per frame.

The effects of compensator and imaging geometry on the distribution of x-ray scatter in CBCT.

PURPOSE: X-ray scatter contributes significantly to image degradation in cone-beam CT (CBCT) reconstructed images in the form of CT number inaccuracy image artifacts and loss of contrast. The need for an understanding of the relationship between the scatter distribution and common imaging parameters (cone angle, air gap, filtration, object size) is an essential element to developing methods for efficiently correcting for the effects of scatter in CBCT. The first objective of this study is to validate the scatter distributions calculated using a CBCT Monte Carlo (MC) model against measured scatter estimates. The second objective is to use the CBCT MC model to investigate the effects of common imaging parameters and bowtie compensators on the resulting scatter distribution. METHODS: This investigation employs the use of a CBCT MC model, developed using the EGSnrc code, to simulate the primary and scatter fluence arriving at the detector. The simulation is validated against projection images, scatter-to-open field ratio (SOR), and scatter-to-primary ratio (SPR) measurements taken using a bench-top CBCT system. The CBCT MC model is used to simulate the scatter distribution arriving at the detector for different cone angles {1.4 degrees, 2.8 degrees, 5.7 degrees, and 11.3 degrees}, source-to-axis distances (SADs) {50, 75 and 100 cm}, and axis-to-detector distances (ADDs) {9, 18, 30, 44, 56 cm} for both a 16.4 and 30.6 cm diameter water cylinder. The effects of different bowtie filters are also simulated using the CBCT MC model for the aforementioned cylinder sizes. RESULTS: Profiles of the simulated and measured projection images agree within 6%. The measurements of the SOR and SPR, taken using beam stop techniques, show good agreement with the simulated results. Limitations of current beam stop techniques in estimating scatter profiles of objects with varying thickness are also demonstrated. A functional relationship between scatter (SOR, SPR) and air gap, cone angle is reported. The bowtie filter was found to have the beneficial effect of decreasing the magnitude of scatter in the projection images (SPR decreased by 56%) as well as altering the spatial distribution of the scatter. CONCLUSIONS: The CBCT MC model accurately simulates scatter and primary fluences in the CBCT imaging components and geometry. The CBCT MC model provides a useful tool in investigating the effects of varying CBCT imaging parameters on the scatter distribution. Increasing the air gap, decreasing the cone angle, and the use of bowtie filtration were all found to be effective ways to minimize scatter in CBCT. The bowtie filter was particularly effective in both minimizing the magnitude and modifying the spatial distribution of the scattered photons. This observation directs further research in optimizing bowtie filter design in an effort to minimize scatter induced artifacts.

Characterization of image quality and image-guidance performance of a preclinical microirradiator.

PURPOSE: To assess image quality and image-guidance capabilities of a cone-beam CT based small-animal image-guided irradiation unit (micro-IGRT). METHODS: A micro-IGRT system has been developed in collaboration with the authors' laboratory as a means to study the radiobiological effects of conformal radiation dose distributions in small animals. The system, the X-Rad 225Cx, consists of a 225 kVp x-ray tube and a flat-panel amorphous silicon detector mounted on a rotational C-arm gantry and is capable of both fluoroscopic x-ray and cone-beam CT imaging, as well as image-guided placement of the radiation beams. Image quality (voxel noise, modulation transfer, CT number accuracy, and geometric accuracy characteristics) was assessed using water cylinder and micro-CT test phantoms. Image guidance was tested by analyzing the dose delivered to radiochromic films fixed to BB's through the end-to-end process of imaging, targeting the center of the BB, and irradiation of the film/BB in order to compare the offset between the center of the field and the center of the BB. Image quality and geometric studies were repeated over a 5-7 month period to assess stability. RESULTS: CT numbers reported were found to be linear (R2 0.998) and the noise for images of homogeneous water phantom was 30 HU at imaging doses of approximately 1 cGy (to water). The presampled MTF at 50% and 10% reached 0.64 and 1.35 mm(-1), respectively. Targeting accuracy by means of film irradiations was shown to have a mean displacement error of [deltax, deltay, deltaz] = [-0.12, -0.05, -0.02] mm, with standard deviations of [0.02, 0.20, 0.17] mm. The system has proven to be stable over time, with both the image quality and image-guidance performance being reproducible for the duration of the studies. CONCLUSIONS: The micro-IGRT unit provides soft-tissue imaging of small-animal anatomy at acceptable imaging doses (< or =1 cGy). The geometric accuracy and targeting systems permit dose placement with submillimeter accuracy and precision. The system has proven itself to be stable over 2 yr of routine laboratory use (>1800 irradiations) and provides a platform for the exploration of targeted radiation effects in small-animal models.

Improving superficial target delineation in radiation therapy with endoscopic tracking and registration.

PURPOSE: Target delineation within volumetric imaging is a critical step in the planning process of intensity modulated radiation therapy. In endoluminal cancers, endoscopy often reveals superficial areas of visible disease beyond what is seen on volumetric imaging. Quantitatively relating these findings to the volumetric imaging is prone to human error during the recall and contouring of the target. We have developed a method to improve target delineation in the radiation therapy planning process by quantitatively registering endoscopic findings contours traced on endoscopic images to volumetric imaging. METHODS: Using electromagnetic sensors embedded in an endoscope, 2D endoscopic images were registered to computed tomography (CT) volumetric images by tracking the position and orientation of the endoscope relative to a CT image set. Regions-of-interest (ROI) in the 2D endoscopic view were delineated. A mesh created within the boundary of the ROI was projected onto the 3D image data, registering the ROI with the volumetric image. This 3D ROI was exported to clinical radiation treatment planning software. The precision and accuracy of the procedure was tested on two solid phantoms with superficial markings visible on both endoscopy and CT images. The first phantom was T-shaped tube with X-marks etched on the interior. The second phantom was an anatomically correct skull phantom with a phantom superficial lesion placed on the pharyngeal surface. Markings were contoured on the endoscope images and compared with contours delineated in the treatment planning system based on the CT images. Clinical feasibility was tested on three patients with early stage glottic cancer. Image-based rendering using manually identified landmarks was used to improve the registration. RESULTS: Using the T-shaped phantom with X-markings, the 2D to 3D registration accuracy was 1.5-3.5 mm, depending on the endoscope position relative to the markings. Intraobserver standard variation was 0.5 mm. Rotational accuracy was within 2°. Using the skull phantom, registration accuracy was assessed by calculating the average surface minimum distance between the endoscopy and treatment planning contours. The average surface distance was 0.92 mm with 93% of all points in the 2D-endoscopy ROI within 1.5 mm of any point within the ROI contoured in the treatment planning software. This accuracy is limited by the CT imaging resolution and the electromagnetic (EM) sensor accuracy. The clinical testing demonstrated that endoscopic contouring is feasible. With registration based on em tracking only, accuracy was 5.6-8.4 mm. Image-based registration reduced this error to less than 3.5 mm and enabled endoscopic contouring in all cases. CONCLUSIONS: Registration of contours generated on 2D endoscopic images to 3D planning space is feasible, with accuracy smaller than typical set-up margins. Used in addition to standard 3D contouring methods in radiation planning, the technology may improve gross tumour volume (GTV) delineation for superficial tumors in luminal sites that are only visible in endoscopy.

Monte Carlo kilovoltage X-ray tube simulation: A statistical analysis and compact simulation method.

INTRODUCTION: Monte Carlo (MC) simulations are a powerful tool for improving image quality in X-ray based imaging modalities. An accurate X-ray source model is essential to MC modeling for CBCT but can be difficult to implement on a GPU while maintaining efficiency and memory limitations. A statistical analysis of the photon distribution from a MC X-ray tube simulation is conducted in hopes of building a compact source model. MATERIALS & METHODS: MC simulations of an X-ray tube were carried out using BEAMnrc. The resulting photons were sorted into four categories: primary, scatter, off-focal radiation (OFR), and both (scatter and OFR). A statistical analysis of the photon components (energy, position, direction) was completed. A novel method for a compact (memory efficient) representation of the PHSP data was implemented and tested using different statistical based linear transformations (PCA, ZCA, ICA), as well as a geometrical transformation. RESULTS: The statistical analysis showed all photon groupings had strong correlations between position and direction, with the largest correlation in the primary data. The novel method was successful in compactly representing the primary (error < 2%) and scatter (error < 6%) photon groupings by reducing the component correlations. DISCUSSION & CONCLUSION: Statistical linear transforms provide a method of reducing the memory required to accurately simulate an X-ray source in a GPU MC system. If all photon types are required, the proposed method reduces the memory requirements by 3.8 times. When only primary and scatter data is needed, the memory requirement is reduced from gigabytes to kilobytes.

Bridging the gap between micro- and macro-scales in medical imaging with textural analysis - A biological basis for CT radiomics classifiers?

INTRODUCTION: Studies suggest there is utility in computed tomography (CT) radiomics for pancreatic disease; however, the precise biological interpretation of its features is unclear. In this manuscript, we present a novel approach towards this interpretation by investigating sub-micron tissue structure using digital pathology. METHODS: A classification-to attenuation (CAT) function was developed and applied to digital pathology images to create sub-micron linear attenuation maps. From these maps, grey level co-occurrence matrix (GLCM) features were extracted and compared to pathology features. To simulate the spatial frequency loss in a CT scanner, the attenuation maps were convolved with a point spread function (PSF) and subsequently down-sampled. GLCM features were extracted from these down-sampled maps to assess feature stability as a function of spatial frequency loss. RESULTS: Two GLCM features were shown to be strongly and positively correlated (r = 0.8) with underlying characteristics of the tumor microenvironment, namely percent pimonidazole staining in the tumor. All features underwent marked change as a function of spatial frequency loss; progressively larger spatial frequency losses resulted in progressively larger inter-tumor standard deviations; two GLCM features exhibited stability up to a 100 µm pixel size. CONCLUSION: This work represents a necessary step towards understanding the biological significance of radiomics. Our preliminary results suggest that cellular metrics of pimonidazole-detectable hypoxia correlate with sub-micron attenuation coefficient texture; however, the consistency of these textures in face of spatial frequency loss is detrimental for robust radiomics. Further study in larger data sets may elucidate additional, potentially more robust features of biologic and clinical relevance.

The influence of bowtie filtration on cone-beam CT image quality.

The large variation of x-ray fluence at the detector in cone-beam CT (CBCT) poses a significant challenge to detectors' limited dynamic range, resulting in the loss of skinline as well as reduction of CT number accuracy, contrast-to-noise ratio, and image uniformity. The authors investigate the performance of a bowtie filter implemented in a system for image-guided radiation therapy (Elekta oncology system, XVI) as a compensator for improved image quality through fluence modulation, reduction in x-ray scatter, and reduction in patient dose. Dose measurements with and without the bowtie filter were performed on a CTDI Dose phantom and an empirical fit was made to calculate dose for any radial distance from the central axis of the phantom. Regardless of patient size, shape, anatomical site, and field of view, the bowtie filter results in an overall improvement in CT number accuracy, image uniformity, low-contrast detectability, and imaging dose. The implemented bowtie filter offers a significant improvement in imaging performance and is compatible with the current clinical system for image-guided radiation therapy.

Adapting liver motion models using a navigator channel technique.

Deformable registration can improve the accuracy of tumor targeting; however for online applications, efficiency as well as accuracy is important. A navigator channel technique has been developed to combine a biomechanical model-based deformable registration algorithm with a population motion model and patient specific motion information to perform fast deformable registration for application in image-guided radiation therapy. A respiratory population-based liver motion model was generated from breath-hold CT data sets of ten patients using a finite element model as a framework. The population model provides a biomechanical reference template of the average liver motions, which were found to be (absolute mean +/-SD) 0.12 +/- 0.10, 0.84 +/- 0.13, and 1.24 +/- 0.18 cm in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. The population motion model was then adapted to the specific liver motion of 13 patients based on their exhale and inhale CT images. The patient motion was calculated using a navigator channel (a narrow region of interest window) on liver boundaries in the images. The absolute average accuracy of the navigator channel to predict the 1D SI and AP motions of the liver was less than 0.11, which is less than the out-of-plane image voxel size, 0.25 cm. This 1D information was then used to adapt the 4D population motion model in the SI and AP directions to predict the patient specific liver motion. The absolute average residual error of the navigator channel technique to adapt the population motion to the patients' specific motion was verified using three verification methods: (1) vessel bifurcation, (2) tumor center of mass, and (3) MORFEUS deformable algorithm. All three verification methods showed statistically similar results where the technique's accuracy was approximately on the order of the voxel image sizes. This method has potential applications in online assessment of motion at the time of treatment to improve image-guided radiotherapy and monitoring of intrafraction motion.

Implementation and characterization of a 320-slice volumetric CT scanner for simulation in radiation oncology.

PURPOSE: Effective target definition and broad employment of treatment response assessment with dynamic contrast-enhanced CT in radiation oncology requires increased speed and coverage for use within a single bolus injection. To this end, a novel volumetric CT scanner (Aquilion One, Toshiba, Tochigi Pref., Japan) has been installed at the Princess Margaret Hospital for implementation into routine CT simulation. This technology offers great advantages for anatomical and functional imaging in both scan speed and coverage. The aim of this work is to investigate the system's imaging performance and quality as well as CT quantification accuracy which is important for radiotherapy dose calculations. METHODS: The 320-slice CT scanner uses a 160 mm wide-area (2D) solid-state detector design which provides the possibility to acquire a volumetric axial length of 160 mm without moving the CT couch. This is referred to as "volume" and can be scanned with a rotation speed of 0.35-3 s. The scanner can also be used as a 64-slice CT scanner and perform conventional (axial) and helical acquisitions with collimation ranges of 1-32 and 16-32 mm, respectively. Commissioning was performed according to AAPM Reports TG 66 and 39 for both helical and volumetric imaging. Defrise and other cone-beam image analysis tests were performed. RESULTS: Overall, the imaging spatial resolution and geometric efficiency (GE) were found to be very good (>10 lp/mm, <1 mm spatial integrity and GE160 mm=85%) and within the AAPM guidelines as well as IEC recommendations. Although there is evidence of some cone-beam artifacts when scanning the Defrise phantom, image quality was found to be good and sufficient for treatment planning (soft tissue noise <10 HU). Measurements of CT number stability and contrast-to-noise values across the volume indicate clinically acceptable scan accuracy even at the field edge. CONCLUSIONS: Initial experience with this exciting new technology confirms its accuracy for routine CT simulation within radiation oncology and allows for future investigations into specialized dynamic volumetric imaging applications.

Inter-observer variability of prostate delineation on cone beam computerised tomography images.

AIM: To determine the inter-observer variability of defining the prostate gland on cone beam computerised tomography images for the purposes of image-guided radiotherapy. MATERIALS AND METHODS: Five genitourinary oncologists contoured the prostate gland on five cone beam computerised tomography datasets. The variations in prostate boundary delineation and consequent isocentre placement between observers were measured. Variations in volume and centre of mass were calculated. The variation in boundary definition was determined with finite element modelling. RESULTS: The average standard deviation for centre of mass displacements was small, measuring 0.7, 1.8 and 2.8mm in the left-right, anterior-posterior and superior-inferior directions, respectively. The standard deviation for volume determination was 8.93 cm(3) with large variability (3.98-19.00 cm(3)). The mean difference between the computerised tomography-derived volume and the mean cone beam-derived volume was 16% (range 0-23.7%). The mean standard deviations for left-right, anterior-posterior and superior-inferior boundary displacements were, respectively, 1.8, 2.1 and 3.6 mm. The maximum deviation seen was 9.7 mm in the superior direction. CONCLUSION: Expert observers had difficulty agreeing upon the location of the prostate peri-prostatic interface on the images provided. The effect on the centre of mass determination was small, and inter-observer variability for prostate detection on cone beam computerised tomography images is not prohibitive to the use of soft tissue guidance protocols. Potential exists for significant systematic matching errors, and points to the need for rigorous therapist image recognition training and development of guidance protocols before clinical implementation of soft tissue cone beam image guidance.

Navigated non-contact fluorescence tomography.

A non-contact approach for diffuse optical tomography (DOT) has been developed for on-demand image updates using surgical navigation technology. A stereoscopic optical tracker provides real-time localization of reflective spheres mounted to a laser diode and near-infrared camera. Standard camera calibration is combined with tracking data to determine the intrinsic camera parameters (focal length, principal point and non-linear lens distortion) and the tracker-to-camera transform. Tracker-to-laser calibration is performed using images of laser beam intersection with a tracked calibration surface. Source and detector positions for a finite-element DOT implementation are projected onto the boundary elements of the tissue mesh by finding ray-triangle intersections. A multi-stage model converts camera counts to surface flux by accounting for lens aperture settings, fluorescence filter transmittance, photodetector quantum efficiency, photon energy, exposure time, readout offset and camera gain. The image-guidance framework was applied to an in-house optical tomography system configured for indocyanine green (ICG) fluorescence. Mean target registration errors for camera and laser calibration were less than 1 mm. Surface flux measurements of total reflectance and fluorescence in Intralipid-based fluorescence phantoms (0-2 µg ml-1) had mean errors of 3.1% and 4.4%, respectively, relative to diffusion theory predictions. Spatially-resolved reflectance measurements in a calibrated optical phantom agreed with theory for radial distances up to 25 mm from the laser source. Inverse fluorescence reconstructions of a sub-surface fluorescence target confirmed the localization accuracy (average target centroid error of 0.44 mm). This translational research system is under investigation for clinical applications in head and neck surgery, including oral cavity tumor resection, lymph node mapping and free-flap perforator assessment.

Intraoperative cone-beam CT spatial priors for diffuse optical fluorescence tomography.

A hybrid system for intraoperative cone-beam CT (CBCT) imaging and continuous-wave fluorescence tomography (FT) has been developed using an image-guidance framework. Intraoperative CBCT images with sub-millimeter spatial resolution are acquired with a flat-panel C-Arm. Tetrahedral meshes are generated from CBCT for finite element method implementation of diffuse optical tomography (NIRFAST). Structural data from CBCT is incorporated directly into the optical reconstruction process using Laplacian-type regularization ('soft spatial priors'). Experiments were performed using an in-house optical system designed for indocyanine green (ICG) fluorescence. A dynamic non-contact geometry was achieved using a stereoscopic optical tracker for real-time localization of a laser diode and CCD camera. Source and detector positions were projected onto the boundary elements of the tissue mesh using algorithms for ray-triangle intersection and camera lens calibration. Simulation studies showed the capabilities of a soft-prior approach, even in the presence of segmentation uncertainties. Experiments with ICG targets embedded in liquid phantoms determined the improvements in the quantification of the fluorophore yield, with errors of 85% and <20% for no priors and spatial priors, respectively. Similar results were observed with the ICG target embedded in ex vivo porcine loin, with errors of 52% and 12%, respectively. A proof-of-principal animal study was performed in a VX2-tumor in vivo rabbit model using liposomal nanoparticles co-encapsulating contrast for CT (iohexol) and fluorescence (ICG) imaging. Fusion of CBCT and FT reconstructions demonstrated concurrent anatomical and functional delineations of contrast enhancement around the periphery of the buccal tumor. These developments motivate future clinical translation of the FT system into an ongoing CBCT-guided head and neck surgery trial.

Dosimetric evaluation of a dedicated stereotactic linear accelerator using measurement and Monte Carlo simulation.

Dosimetric parameters of a dedicated stereotactic linear accelerator have been investigated using measurements and Monte Carlo simulations. This linac has a unique built in multileaf collimation (MLC) system with the maximum opening of 16 x 21 cm2 and 4 mm leaf width at the isocenter and has successfully been modeled for the first time using the Monte Carlo simulation. The high resolution MLC, combined with its relatively large maximum field size, opens up a new opportunity for expanding stereotactic radiation treatment techniques from traditionally treating smaller targets to larger ones for both cranial and extracranial lesions. Dosimetric parameters of this linac such as accuracy of leaf positioning and field shaping, leakage and transmission, percentage depth doses, off-axes dose profiles, and dose penumbras were measured and calculated for different field sizes, depths, and source to surface distances. In addition, the ability of the linac in accurate dose delivery of several treatment plans, including intensity modulated radiation therapy (IMRT), performed on phantom and patients was determined. Ionization chamber, photon diode detector, films, several solid water phantoms, and a water tank were used for the measurements. The MLC leaf positioning to any particular point in the maximum aperture was accurate with a standard deviation of 0.29 mm. Maximum and average leakages were 1.7% and 1.1% for the reference field of 10.4 x 9.6 cm2. Measured penumbra widths (80%-20%) for this field at source axis distance (SAD) of 100 cm at a depth of 1.5 cm (dmax) were 3.2 and 4 mm for the leaf-sides and leaf-ends, respectively. The corresponding results at 10 cm depth and SAD =100 cm were 5.4 and 6.3 mm. Monte Carlo results generally agreed with the measurements to within 1% and or 1 mm, with respective uncertainties of 0.5% and 0.2 mm. The linac accuracy in delivering non-IMRT treatment plans was better than 1%. Ionization chamber dosimetry results for a phantom IMRT plan in the high dose and low dose regions were -0.5% and +3.6%, respectively. Dosimetry results at isocenter for three patients' IMRT plans were measured to be within 3% of their corresponding treatment plans. Film dosimetry was also used to compare dose distributions of IMRT treatment plans and delivered cumulative doses at different cross sectional planes.

An empirical method for lag correction in cone-beam CT.

Image lag degrades image quality in cone-beam CT (CBCT), resulting in contrast reduction, lack of CT number accuracy and uniformity, and skin-line artifacts. The magnitude of such degradation and the extent to which imaging performance can be improved by means of a lag correction method were investigated. Measurements were performed using a radiotherapy CBCT guidance system (Elekta Synergy XVI, Elekta Oncology Systems, Atlanta, GA), for which the imaging system is based upon a RID1640-AL1 flat-panel imager (Perkin Elmer, Wiesbaden, Germany). Image lag and its relationship to various parameters including signal magnitude, photon energy, and frame number were investigated, and an empirical lag correction method was developed to manage lag artifacts. The correction method was simply the subtraction from the current frame by previous frames weighted by the temporal response function. The CatPhan 500 phantom (The Phantom Laboratory, Salem, NY) within an irregularly shaped body annulus was used to demonstrate the magnitude of artifacts with and without lag correction. CBCT images after correction demonstrated improvement in skin-line reconstruction, CT number accuracy, image uniformity, and contrast-to-noise ratio. Lag artifacts can be reduced by means of algorithmic correction of the projection images. Lag correction is most important for all shapes of objects having contrast inserts. For circular/cylindrical objects, lag correction does not improve the skin-line artifact but can improve low contrast visibility adjacent to high contrast objects.

Geometric calibration of a mobile C-arm for intraoperative cone-beam CT.

A geometric calibration method that determines a complete description of source-detector geometry was adapted to a mobile C-arm for cone-beam computed tomography (CBCT). The non-iterative calibration algorithm calculates a unique solution for the positions of the source (X(s), Y(s), Z(s)), detector (X(d), Y(d), Z(d)), piercing point (U(o), V(o)), and detector rotation angles (phi, theta, eta) based on projections of a phantom consisting of two plane-parallel circles of ball bearings encased in a cylindrical acrylic tube. The prototype C-arm system was based on a Siemens PowerMobil modified to provide flat-panel CBCT for image-guided interventions. The magnitude of geometric nonidealities in the source-detector orbit was measured, and the short-term (approximately 4 h) and long-term (approximately 6 months) reproducibility of the calibration was evaluated. The C-arm exhibits large geometric nonidealities due to mechanical flex, with maximum departures from the average semicircular orbit of deltaU(o) = 15.8 mm and deltaV(o) = 9.8 mm (for the piercing point), deltaX and deltaY = 6-8 mm and deltaZ = 1 mm (for the source and detector), and deltaphi approximately 2.9 degrees, deltatheta approximately 1.9 degrees, and delta eta approximately 0.8 degrees (for the detector tilt/rotation). Despite such significant departures from a semicircular orbit, these system parameters were found to be reproducible, and therefore correctable by geometric calibration. Short-term reproducibility was < 0.16 mm (subpixel) for the piercing point coordinates, < 0.25 mm for the source-detector X and Y, < 0.035 mm for the source-detector Z, and < 0.02 degrees for the detector angles. Long-term reproducibility was similarly high, demonstrated by image quality and spatial resolution measurements over a period of 6 months. For example, the full-width at half-maximum (FWHM) in axial images of a thin steel wire increased slightly as a function of the time (delta) between calibration and image acquisition: FWHM=0.62, 0.63, 0.66, 0.71, and 0.72 mm at delta = 0 s, 1 h, 1 day, 1 month, and 6 months, respectively. For ongoing clinical trials in CBCT-guided surgery at our institution, geometric calibration is conducted monthly to provide sufficient three-dimensional (3D) image quality while managing time and workflow considerations of the calibration and quality assurance process. The sensitivity of 3D image quality to each of the system parameters was investigated, as was the tolerance to systematic and random errors in the geometric parameters, showing the most sensitive parameters to be the piercing point coordinates (U(o), V(o)) and in-plane positions of the source (X(s), Y(s)) and detector (X(d), Y(d)). Errors in the out-of-plane position of the source (Z(s)) and detector (Z(d)) and the detector angles (phi, theta, eta) were shown to have subtler effects on 3D image quality.

Design and implementation of a 3D-MR/CT geometric image distortion phantom/analysis system for stereotactic radiosurgery.

The design, construction and application of a multimodality, 3D magnetic resonance/computed tomography (MR/CT) image distortion phantom and analysis system for stereotactic radiosurgery (SRS) is presented. The phantom is characterized by (1) a 1 × 1 × 1 (cm)3 MRI/CT-visible 3D-Cartesian grid; (2) 2002 grid vertices that are 3D-intersections of MR-/CT-visible 'lines' in all three orthogonal planes; (3) a 3D-grid that is MR-signal positive/CT-signal negative; (4) a vertex distribution sufficiently 'dense' to characterize geometrical parameters properly, and (5) a grid/vertex resolution consistent with SRS localization accuracy. When positioned correctly, successive 3D-vertex planes along any orthogonal axis of the phantom appear as 1 × 1 (cm)2-2D grids, whereas between vertex planes, images are defined by 1 × 1 (cm)2-2D arrays of signal points. Image distortion is evaluated using a centroid algorithm that automatically identifies the center of each 3D-intersection and then calculates the deviations dx, dy, dz and dr for each vertex point; the results are presented as a color-coded 2D or 3D distribution of deviations. The phantom components and 3D-grid are machined to sub-millimeter accuracy, making the device uniquely suited to SRS applications; as such, we present it here in a form adapted for use with a Leksell stereotactic frame. Imaging reproducibility was assessed via repeated phantom imaging across ten back-to-back scans; 80%-90% of the differences in vertex deviations dx, dy, dz and dr between successive 3 T MRI scans were found to be ⩽0.05 mm for both axial and coronal acquisitions, and over >95% of the differences were observed to be ⩽0.05 mm for repeated CT scans, clearly demonstrating excellent reproducibility. Applications of the 3D-phantom/analysis system are presented, using a 32-month time-course assessment of image distortion/gradient stability and statistical control chart for 1.5 T and 3 T GE TwinSpeed MRI systems.

Quantification accuracy for dynamic contrast enhanced (DCE) CT imaging: phantom and quality assurance framework.

PURPOSE: Standardization and protocol optimization is essential for quantification of Dynamic Contrast Enhanced CT as an imaging biomarker. Currently, no commercially available quality assurance (QA) phantoms can provide for testing a complete set of imaging parameters pertaining to routine quality control for contrast-enhanced (CE) CT, as well as spatiotemporal accuracy. The purpose of this work was, therefore: (a) developing a solid calibration phantom for routine CE CT quality assurance; (b) investigating the sensitivity of CECT to organ motion, and (c) characterizing a volumetric CT scanner for CECT. METHODS: CECT calibration phantom consisting of an acrylic uniform cylinder containing multiple capsules of varying diameters and orientations was designed and built. The capsules contain different solid density materials mimicking iodine contrast enhancement. Sensitivity and accuracy of CECT measurements on all capsules was performed using a 320-slice CT scanner for a range of scan parameters both with and without phantom motion along the transaxial axis of the scanner. RESULTS: Routine commissioning tests such as uniformity, spatial resolution and image noise were successfully determined using the CECT phantom. Partial volume effect and motion blurring both contribute to a general decrease in contrast enhancement and this was further dependent on capsule orientation (least pronounced for the transaxial orientation). Scanning with a rotation time of less than 0.5 s, the effect of blurring is less than 3% for all orientations and phantom speeds. CONCLUSION: A new robust contrast calibration phantom was developed and used to evaluate the performance of a 320-slice volumetric CT scanner for DCE-CT.

Compensators for dose and scatter management in cone-beam computed tomography.

The ability of compensators (e.g., bow-tie filters) designed for kV cone-beam computed tomography (CT) to reduce both scatter reaching the detector and dose to the patient is investigated. Scattered x rays reaching the detector are widely recognized as one of the most significant challenges to cone-beam CT imaging performance. With cone-beam CT gaining popularity as a method of guiding treatments in radiation therapy, any methods that have the potential to reduce the dose to patients and/or improve image quality should be investigated. Simple compensators with a design that could realistically be implemented on a cone-beam CT imaging system have been constructed to determine the magnitude of reduction of scatter and/or dose for various cone-beam CT imaging conditions. Depending on the situation, the compensators were shown to reduce x-ray scatter at the detector and dose to the patient by more than a factor of 2. Further optimization of the compensators is a possibility to achieve greater reductions in both scatter and dose.

Image quality and localization accuracy in C-arm tomosynthesis-guided head and neck surgery.

The image quality and localization accuracy for C-arm tomosynthesis and cone-beam computed tomography (CBCT) guidance of head and neck surgery were investigated. A continuum in image acquisition was explored, ranging from a single exposure (radiograph) to multiple projections acquired over a limited arc (tomosynthesis) to a full semicircular trajectory (CBCT). Experiments were performed using a prototype mobile C-arm modified to perform 3D image acquisition (a modified Siemens PowerMobil). The tradeoffs in image quality associated with the extent of the source-detector arc (theta(tot)), the number of projection views, and the total imaging dose were evaluated in phantom and cadaver studies. Surgical localization performance was evaluated using three cadaver heads imaged as a function of theta(tot). Six localization tasks were considered, ranging from high-contrast feature identification (e.g., tip of a K-wire pointer) to more challenging soft-tissue delineation (e.g., junction of the hard and soft palate). Five head and neck surgeons and one radiologist participated as observers. For each localization task, the 3D coordinates of landmarks pinpointed by each observer were analyzed as a function of theta(tot). For all tomosynthesis angles, image quality was highest in the coronal plane, whereas sagittal and axial planes exhibited a substantial decrease in spatial resolution associated with out-of-plane blur and distortion. Tasks involving complex, lower-contrast features demonstrated steeper degradation with smaller tomosynthetic arc. Localization accuracy in the coronal plane was correspondingly high, maintained to < 3 mm down to theta(tot) approximately 30 degrees, whereas sagittal and axial localization degraded rapidly below theta(tot) approximately 60 degrees. Similarly, localization precision was better than approximately 1 mm within the coronal plane, compared to approximately 2-3 mm out-of-plane for tomosynthesis angles below theta(tot) approximately 45 degrees. An overall 3D localization accuracy of approximately 2.5 mm was achieved with theta(tot) approximately 90 degrees for most tasks. The high in-plane spatial resolution, short scanning time, and low radiation dose characteristic of tomosynthesis may enable the surgeon to collect near real-time images throughout the procedure with minimal interference to surgical workflow. Therefore, tomosynthesis could provide a useful addition to the image-guided surgery arsenal, providing on-demand, high quality image updates, complemented by CBCT at critical milestones in the surgical procedure.