Larger Voxel Size Research Articles

The Impact of Voxel Size-Based Inaccuracies on the Mechanical Behavior of Thin Bone Structures

Computed tomography (CT)-based measures of skeletal geometry and material properties have been widely used to develop finite element (FE) models of bony structures. However, in the case of thin bone structures, the ability to develop FE models with accurate geometry derived from clinical CT data presents a challenge due to the thinness of the bone and the limited resolution of the imaging devices. The purpose of this study was to quantify the impact of voxel size on the thickness and intensity values of thin bone structure measurements and to assess the effect of voxel size on strains through FE modeling. Cortical bone thickness and material properties in five thin bone specimens were quantified at voxel sizes ranging from 16.4 to 488μm. The measurements derived from large voxel size scans showed large increases in cortical thickness (61.9-252.2%) and large decreases in scan intensity (12.9-49.5%). Maximum principal strains from FE models generated using scans at 488μm were decreased as compared to strains generated at 16.4μm voxel size (8.6-64.2%). A higher level of significance was found in comparing intensity (p=0.0001) vs. thickness (p=0.005) to strain measurements. These findings have implications in developing methods to generate accurate FE models to predict the biomechanical behavior of thin bone structures.

撮像パラメータがＡＤＣ測定に与える影響について

The apparent diffusion coefficient (ADC) values are calculated by using signal intensity in diffusion-weighted images (DWIs) with two or more different b-value. Therefore, the signal to noise ratio (SNR) of DWI with higher b-value may have a big influence on the measured ADC value. We examined the influence of the imaging parameters on the calculated ADC values. The SNR of DWI increased by using a larger voxel size, by means of a decreased number of matrix, an increased slice thickness, and an increased field of view (FOV). However, when the number of excitations was increased to improve the SNR of DWI, the signal intensity of background noise was observed to be slightly increased. It was suggested that the consistency of measured ADC was not preserved when the signal of the DWIs with higher b-value dropped close to the noise level.

Effect of Voxel Size and Computation Method on Tc-99m MAA SPECT/CT-Based Dose Estimation for Y-90 Microsphere Therapy

The use of selective internal radiation therapy for treatment of hepatocellular carcinoma and liver metastases using Y-90 labeled microspheres has become an effective and widely used treatment regimen. However, dosimetric evaluations of this treatment are still primitive as uniform distribution models based only on injected activity are often used. This investigation attempts to quantify the effectiveness of several sophisticated patient-specific techniques which utilize the source distribution of Tc-99m MAA simulation studies to perform voxelized dosimetric computations. Among these techniques are complete Monte-Carlo radiation transport computation in patient-specific CT-based voxel phantoms, local energy deposition in patient specific phantoms and kernel transport techniques in water. Each technique was evaluated using three different phantom voxel dimensions and SPECT reconstruction matrix sizes. Dose evaluation results using all methods were compared to the exact solution, obtained using fully 3-D Monte-Carlo simulations with source distribution based not on SPECT data, but on the injected activity and exact boundaries of the anthropomorphic phantom used in the study. The results of this study show that at large voxel sizes and using SPECT reconstructions with a small matrix size (64 x 64), Monte-Carlo and local deposition methods are nearly equivalent. However, using a large SPECT reconstruction matrix (256 x 256) the local deposition method is significantly more accurate than full 3-D Monte-Carlo transport, and with a negligible computational burden.

Influence of scanning and reconstruction parameters on quality of three-dimensional surface models of the dental arches from cone beam computed tomography

The study aim is to investigate the influence of scan field, mouth opening, voxel size, and segmentation threshold selections on the quality of the three-dimensional (3D) surface models of the dental arches from cone beam computed tomography (CBCT). 3D models of 25 patients scanned with one image intensifier CBCT system (NewTom 3G, QR SLR, Verona, Italy) using three field sizes in open- and closed-mouth positions were created at different voxel size resolutions. Two observers assessed the quality of the models independently on a five-point scale using specified criteria. The results indicate that large-field selection reduced the visibility of the teeth and the interproximal space. Also, large voxel size reduced the visibility of the occlusal surfaces and bone in the anterior region in both maxilla and mandible. Segmentation threshold was more variable in the maxilla than in the mandible. Closed-mouth scan complicated separating the jaws and reduced teeth surfaces visibility. The preliminary results from this image-intensifier system indicate that the use of medium or small scan fields in an open-mouth position with a small voxel is recommended to optimize quality of the 3D surface model reconstructions of the dental arches from CBCT. More research is needed to validate the results with other flat-panel detector-based CBCT systems.

MO-D-351-04: A Hybrid Inverse Planning Algorithm of IMPT Optimization

Purpose: Although intensity modulated proton therapy (IMPT) is a flexible and powerful treatment technique, its inverse planning can be computationally challenging in terms of memory and speed. Here, we propose a novel optimization algorithm with the hybrid strategy to overcome those challenges. The algorithm worked very well in our tested phantoms and patients. Method and Materials: A common strategy to speed up the dose calculation during optimization is to use a Kij matrix which stores precalculated dose contributed by the scanning spot j to voxel i. However, this often requires large amount of memory due to the large dose voxel size in patients and multiple energy layers as the extra dimension of data for proton scanning beams. To speed up the dose calculation while keeping the memory usage low, our algorithm separately caches only major dose contributions from scanning spots with full intensities to each dose voxel; the sum of contributions from the remaining scanning spots is cached but unchanged except when they are accurately updated at some specified moment. A lower cut-off threshold can be adjusted to establish the set of spots considered to make major dose contributions. We developed a variant of Newton's method, which automatically determines the patient-customized damping parameter to attain convergence efficiency. Results: In addition to the greatly improved memory usage by the partial Kij, our tests showed that each iteration is also much faster than that using the full Kij matrix. The algorithm worked well for patients with tumors at different sites, including one with a large tumor in the lung. It showed that significant improvements of critical structure sparing were achieved. Conclusion: The proposed algorithm was computationally efficient in terms of computing time and memory usage. It worked well in complicated situations involving different treatment sites.

Real-time 3 T fMRI data of brain tumour patients for intra-operative localization of primary motor areas

Introduction In patients with tumours in or near the motor cortex reliable intra-operative identification of the precentral gyrus can be difficult due to anatomical dislocation. Maps of functional magnetic resonance imaging (fMRI) based on the blood oxygen level dependent (BOLD) effect are used to localize eloquent functional areas of the brain but require postprocessing for reduction of false positive activations. We set the focus of this study on the evaluation of feasibility and clinical usefulness of using real-time fMRI t-maps without postprocessing for pre-operative planning and intra-operative localization of functional motor areas. Methods Real-time fMRI t-maps from a 3-T MRI scanner were coregistered with MRI data. Ten patients were operated under general anaesthesia using 3D neuronavigation with integrated real-time fMRI t-maps. Surgical and functional outcome was compared to results of 12 patients who previously underwent wake surgeries. Results Good neurological outcome was achieved in all treated patients. Main activation clusters on fMRI real-time maps were easily identified. Coregistered real-time fMRI data without additional postprocessing were useful in planning the surgical approach. However, due to brain shift and large voxel size of BOLD contrast signals on t-maps exact localization of borders between tumours and functional areas was not possible intra-operatively. Conclusion Our method is very simple to use and effective in guiding the neurosurgeon safely through minimally invasive craniotomies to tumours in eloquent areas without setting lesions to functional areas. Furthermore, the neurosurgeon is more independent when tumour location requires acquisition of fMRI data for pre-operative planning and intra-operative navigation.

Imaging in radiotherapy treatment planning and delivery

Recent years have seen major developments in radiotherapy techniques and equipment, focusing on conformal external beam techniques with the emphasis on intensity-modulated radiotherapy (IMRT). The progress of IMRT implementation has been summarized in a series of review articles in the British Journal of Radiology during 2003 and 2004. One of the fundamental prerequisites for conformal radiotherapy is the localization of the target, starting with the gross tumour volume (GTV) and moving outwards to the planning target volume (PTV). The judgement of how to define these volumes is crucial. The future value of IMRT and related techniques may depend on the underpinning images of these volumes and how they are interpreted. Fortunately, in parallel with the developments in radiotherapy delivery techniques has come an equally impressive and timely development in associated imaging. Although CT remains the gold standard for radiotherapy planning, MRI has much to offer. Superior soft tissue characterization and new developments in dynamic contrast enhanced, diffusion-weighted and diffusion tensor imaging are making their mark in target delineation. Magnetic resonance spectroscopy (MRS) promises insights into tissue metabolism and, although currently limited by large voxel sizes, may provide information on tumour response and recurrence to complement the structural information of MRI and CT. The traditional role of the simulator is now complemented by the use of the CT scanner as a virtual simulator, thus making 3D image datasets available. Verification on a fraction-by-fraction basis is now possible by several means. CT scanners within the treatment room have been used to compare images at treatment time with planning CT scans and also to measure and correct for interfraction variations in anatomy or positioning. Portal imaging has seen equally impressive developments as a result of the introduction of flat panel imaging systems. As well as their improved image quality for traditional radiographic projection imaging, when used in conjunction with cone beam reconstruction algorithms, they have made megavoltage CT a reality. Alternatively, if used with a kilovoltage X-ray source and cone beam CT algorithm, the CT reconstructions can harness the improved tissue contrast associated with the lower X-ray energies. The development of PET/CT imaging for radiotherapy planning provides, like MRS, another example of the imaging of metabolic function, but in this case the functional information is displayed within the same coordinate system as the familiar structure provided by CT. This is an unusual luxury, however, and the problems of extracting and overlaying data from different imaging studies and modalities has prompted several advances in image registration and fusion algorithms. So these are exciting times in radiotherapy imaging and this special issue reflects both the current practice and future opportunities for exploiting advances in several imaging modalities in order to increase the confidence with which modern conformal radiotherapy treatments may be planned and delivered. The British Journal of Radiology, 79 (2006), S1

The effect of microcomputed tomography scanning and reconstruction voxel size on the accuracy of stereological measurements in human cancellous bone

Stereological parameters have been used as an approximation for the architecture of trabecular bone. Structural indices such as bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), bone surface-to-volume ratio (BS/BV), degree of anisotropy (MIL1/MIL3), and connectivity density (−Euler/Vol) have been widely studied to investigate pathological conditions in bone. Due to its high resolution and nondestructiveness, microcomputed tomography (micro-CT) has been utilized to take precise three-dimensional (3D) images of trabecular microstructures. However, spatial limitations for applying micro-CT-based analyses to large specimens, such as whole vertebral bodies, require using larger scanning and reconstruction voxel sizes. In this study, combinations of three different scanning and reconstruction voxel size were used to represent best possible voxel size (21 μm; best in our scanner for the specimen size used) relative to other voxel sizes used in this study, commonly used intermediate voxel sizes (50 μm), and those applicable to scans of whole human vertebral bodies (110 μm) in order to examine the effect of scanning and reconstruction voxel size on stereological measures for human cancellous bone. The error in stereological parameters calculated using combinations of large voxel sizes compared to the gold standard (best possible case) ranged from 0.1% to 102%. The signed magnitude of the error in other cases relative to the gold standard was a function of either scanning or reconstruction voxel size or both ( r 2 = 0.55–0.95). For most of the structural indices, the results from analysis of images with larger voxel sizes were correlated with those from the gold standard ( r 2 = 0.55–0.99) except for Tb.N at 110/110 μm, MIL1/MIL3 at larger than 110 μm reconstruction voxel size, and −Euler/Vol at any combination of voxel sizes. Overall, it was observed that resampling a high resolution image at lower resolutions (corresponding to increasing reconstruction voxel size in this study) had different effects on the calculated parameters than scanning at the same low resolution (corresponding to increasing scanning voxel size in this study). Our results show that investigations of image resolution should include actual scans at the resolution of interest rather than simply coarsening of high-resolution images as is customarily done.

Evaluation of the first commercial Monte Carlo dose calculation engine for electron beam treatment planning.

The purpose of this study is to perform a clinical evaluation of the first commercial (MDS Nordion, now Nucletron) treatment planning system for electron beams incorporating Monte Carlo dose calculation module. This software implements Kawrakow's VMC++ voxel-based Monte Carlo calculation algorithm. The accuracy of the dose distribution calculations is evaluated by direct comparisons with extensive sets of measured data in homogeneous and heterogeneous phantoms at different source-to-surface distances (SSDs) and gantry angles. We also verify the accuracy of the Monte Carlo module for monitor unit calculations in comparison with independent hand calculations for homogeneous water phantom at two different SSDs. All electron beams in the range 6-20 MeV are from a Siemens KD-2 linear accelerator. We used 10,000 or 50,000 histories/cm2 in our Monte Carlo calculations, which led to about 2.5% and 1% relative standard error of the mean of the calculated dose. The dose calculation time depends on the number of histories, the number of voxels used to map the patient anatomy, the field size, and the beam energy. The typical run time of the Monte Carlo calculations (10,000 histories/cm2) is 1.02 min on a 2.2 GHz Pentium 4 Xeon computer for a 9 MeV beam, 10 x 10 cm2 field size, incident on the phantom 15 x 15 x 10 cm3 consisting of 31 CT slices and voxels size of 3 x 3 x 3 mm3 (total of 486,720 voxels). We find good agreement (discrepancies smaller than 5%) for most of the tested dose distributions. We also find excellent agreement (discrepancies of 2.5% or less) for the monitor unit calculations relative to the independent manual calculations. The accuracy of monitor unit calculations does not depend on the SSD used, which allows the use of one virtual machine for each beam energy for all arbitrary SSDs. In some cases the test results are found to be sensitive to the voxel size applied such that bigger systematic errors (>5%) occur when large voxel sizes interfere with the extensions of heterogeneities or dose gradients because of differences between the experimental and calculated geometries. Therefore, user control over voxelization is important for high accuracy electron dose calculations.

Ultra-high field (7T) mapping of the human ventral visual area for “head-from-motion”

Introduction The visual system of the primates consists of a “what” (ventral) and a “where” (dorsal) pathway (1). Recent studies (2) indicate that areas in the human ventral stream respond to perceived object shapes induced solely by coherent motion of random dot patterns (“structure-from-motion” @FM)). The important question of content-specificity of those SFM areas, however, remains elusive due to the limitations of functional imaging using conventional low-field scanners (low signal-to-noise, large voxel size). In the present study, we have utilized the unique advantages of the ultra-high field scanners to map the detailed layout of human ventral visual areas with respect to the perception of different object categories that are induced by almost identical physical stimuli.

Introduction and evaluation of a gray-value voxel conversion technique

In micro finite element analyses (μFEA) of cancellous bone, the 3D-imaging data that the FEA-models are based on, contain a range of gray-values. In the construction of the eventual FEA-model, these gray-values are commonly thresholded. Although thresholding is successful at small voxel sizes, at larger voxel sizes there is substantial loss of trabecular connectivity. We propose a new method: the gray-value method, where the μFEA-models use the information within the 3D-imaging data directly, without prior thresholding. Our question was twofold. First, how does the gray-value method compare to both plain and mass-compensated thresholding? Second, what is the effect of element size on the results obtained with the gray-value method? We used nine μCT-scans of human vertebral cancellous bone. These were degraded to represent different resolutions, and converted into μFEA-models using plain thresholding, mass-compensated thresholding, and the gray-value method. The apparent elastic moduli of the specimens were determined using μFEA. The different methods were compared on the basis of the apparent elastic moduli, compared to those calculated for a 28 μm reference model. The results showed that the gray-value method greatly improves the results relative to other methods. The gray-value method gives accurate predictions of the apparent elastic moduli, for voxel sizes up to one trabecular thickness (Tb.Th.). For voxel sizes greater than one Tb.Th. the accuracy, although still better than for both thresholding methods, becomes increasingly worse.

Magnetic resonance imaging of microbubbles in a superheated emulsion chamber for brachytherapy dosimetry

This paper describes development of magnetic resonance imaging (MRI) techniques for three-dimensional (3D) imaging of a position-sensitive detector for brachytherapy dosimetry. The detector is a 0.5 l chamber containing an emulsion of halocarbon-115 droplets in a tissue-equivalent glycerin-based gel. The halocarbon droplets are highly superheated and expand into vapor microbubbles upon irradiation. Brachytherapy sources can be inserted into the superheated emulsion chamber to create distributions of bubbles. Three-dimensional MRI of the chamber is then performed. A 3D gradient-echo technique was optimized for spatial resolution and contrast between bubbles and gel. Susceptibility gradients at the interfaces between bubbles and gel are exploited to enhance contrast so microscopic bubbles can be imaged using relatively large voxel sizes. Three-dimensional gradient-echo images are obtained with an isotropic resolution of 300 microns over a 77 mm x 77 mm x 9.6 mm field-of-view in an imaging time of 14 min. A post-processing technique was developed to semi-automatically segment the bubbles from the images and to assess dose distributions based on the measured bubble densities. Relative dose distributions are computed from MR images for a 125I brachytherapy source and the results compare favorably to relative radial dose distributions calculated as recommended by Task Group 43 of the American Association of Physicists in Medicine.