Minimum Voxel Size Research Articles

Microstructure evolution in laser-based powder bed fusion of metals

Abstract Laser-based powder bed fusion (LPBF) of metals offers the unique possibility of creating the microstructure voxel-by-voxel. The minimum voxel size in each direction is dependent on material dosing accuracy coupled with laser processing parameters. The rapid solidification conditions during LPBF lead to material heterogeneity coupled with hierarchical and non-equilibrium microstructures. The current paper delves into two different pathways available currently to control microstructure in LPBF, namely: in-situ microstructure control through material distribution to form functionally graded components with complex interfaces; application of post-processing thermo-mechanical treatments to control the microstructure. Unlike traditional manufacturing methods, each voxel in LPBF can be further processed multiple times after the first fusion process. Such in-situ processing presents further opportunity for tailoring the microstructure of each voxel in 3D. A future perspective is thus offered on the opportunities to control and engineer LPBF microstructures in metals.

An automated fast neutron computed tomography instrument with on-line focusing for non-destructive evaluation.

A fast neutron tomography imaging instrument has been designed, built, and tested at The Ohio State University 500 kW Research Reactor on a fast neutron beamline with a peak neutron flux ≈5.4 × 107 n·cm-2·s-1 at 1.6MeV median neutron energy. The instrument and beamline are also configurable for thermal neutron imaging. The imaging apparatus is composed of a lens coupled, water-cooled Electron Multiplying Charge Coupled Device camera, a front-surface mirror, and a high light yield plastic Polyvinyl toluene scintillator. The instrument sits on a mobile cart. A total of 5 motion-control stages are built into the system for XYZ and rotational degrees of freedom for sample positioning; the fifth stage fine tunes the focal distance between the camera and the scintillator to achieve on-line focusing. A Python code with a user-friendly graphical user interface controls the fully automated image acquisition, not requiring user interaction, yet facilitating tracking of the image acquisition. A complete fast neutron computed tomography dataset with 360 projections requires less than 3 h, with 30s per projection. On-line focusing is accomplished with a commercial, off-the-shelf, dielectrically actuated liquid lens. Finally, tomographic reconstructions are visualized using the Livermore Tomography Tools software package. The effective pixel size (width and height) is ≈0.1058mm, yielding a minimum voxel size of 0.1058 × 0.1058 × 0.1058mm3, and produces a spatial resolution of 231μm when calculated from knife-edge measurements.

Root and Root Canal Configuration Characterization Using Microcomputed Tomography: A Systematic Review.

This systematic review’s objective was to conduct a complete analysis of the literature on the root canal morphology using advanced micro-computed tomography. The electronic web databases PubMed, Scopus, and Cochrane were examined for research papers concerning the chosen keywords, evaluating the root canal morphology using Micro-CT, published up to 2021. The articles were searched using MeSH keywords and searched digitally on four specialty journal websites. DARE2 extended (Database of Attributes of Reviews of Effects) was used to assess bias risk. The information was gathered from 18 published studies that strictly met the criteria for inclusion. In the included studies, a total of 6696 samples were studied. The studies were conducted on either maxillary (n-2222) or mandibular teeth (n-3760), permanent anteriors (n-625), and Third molars (n-89). To scan samples, a Scanco Medical machine in was used in 10 studies, Bruker Micro-CT in 34, and seven other machines were utilized in the rest. Bruker Micro-CT software from Kontich, Belgium, VG-Studio Max 2.2 software from Volume Graphics, Heidelberg, Germany, was the most commonly used software. The minimum Voxel size (resolution) adopted in the included studies was 11.6 µm. However, 60 µm was the maximum. Most studies classified the root canal morphology using Vertucci’s classification system (n-16) and the four-digit system (n-6).

Image quality assessment of three cone beam computed tomography scanners-an analysis of the visibility of anatomical landmarks.

The aim of this study was to evaluate large field of view (FoV) subjective image quality of three cone-beam computed tomography (CBCT) machines to assess the threshold for reliable diagnostic perceptibility when lowering exposure settings. One entire cadaver's head was scanned using three CBCT scanners. The largest available FoV of each device, imaging orthodontic relevant structures, was applied. CBCT datasets with different image quality based on standard to minimum scanning parameters were acquired. Five dentists evaluated the visibility of selected anatomical structures of the upper and lower jaw using a 5-point rating scale. Image quality depends on parameters such as a minimum voxel size of 0.2 to 0.4mm or a minimum of 4mA. A reduction in number of images (scanning protocol) reduces image quality. Visualization of different anatomical structures for orthodontic treatment planning requires distinct scanning protocols to support adequate perception of these structures. This study does not account for the evaluation of paediatric anatomical structures due to the availability of cadaver's head. CBCT scans performed for orthodontic purposes using a large FoV with reduced parameters (400 µm, 2 to 4 mA and low dose protocols) are acceptable for visualization of large anatomical structures. Further lowering these parameters will not be sufficient to view small anatomical structures. Orthodontic indications will have to define specific anatomical structures to choose adequate scanning protocols to reduce dose and ensure reliable diagnostic visibility.

3D virtual pathohistology of lung tissue from Covid-19 patients based on phase contrast X-ray tomography.

We present a three-dimensional (3D) approach for virtual histology and histopathology based on multi-scale phase contrast x-ray tomography, and use this to investigate the parenchymal architecture of unstained lung tissue from patients who succumbed to Covid-19. Based on this first proof-of-concept study, we propose multi-scale phase contrast x-ray tomography as a tool to unravel the pathophysiology of Covid-19, extending conventional histology by a third dimension and allowing for full quantification of tissue remodeling. By combining parallel and cone beam geometry, autopsy samples with a maximum cross section of 8 mm are scanned and reconstructed at a resolution and image quality, which allows for the segmentation of individual cells. Using the zoom capability of the cone beam geometry, regions-of-interest are reconstructed with a minimum voxel size of 167 nm. We exemplify the capability of this approach by 3D visualization of diffuse alveolar damage (DAD) with its prominent hyaline membrane formation, by mapping the 3D distribution and density of lymphocytes infiltrating the tissue, and by providing histograms of characteristic distances from tissue interior to the closest air compartment.

Assembly of Materials Building Blocks into Integrated Complex Functional Systems

Assembly of Materials Building Blocks into Integrated Complex Functional Systems

Rapid Assembly of Small Materials Building Blocks (Voxels) into Large Functional 3D Metamaterials

AbstractHerein, various 3D additive manufacturing approaches are reviewed in terms of two important figures of merit: maximum voxel printing rate and minimum voxel size. Voxel sizes from several 100 µm down to the 100 nm scale are covered. Original results on multifocus two‐photon printing at around voxel printing rates of 107 voxels s−1 are presented in this context, which significantly surpass previous best values. These advances are illustrated by and applied to the making of microstructured 3D (chiral) mechanical metamaterials that are composed of more than one‐hundred‐thousand unit cells in three dimensions. Previous best values for unit cells of similar complexity are smaller by two orders of magnitude.

Surface reaction-diffusion kinetics on lattice at the microscopic scale.

Microscopic models of reaction-diffusion processes on the cell membrane can link local spatiotemporal effects to macroscopic self-organized patterns often observed on the membrane. Simulation schemes based on the microscopic lattice method (MLM) can model these processes at the microscopic scale by tracking individual molecules, represented as hard spheres, on fine lattice voxels. Although MLM is simple to implement and is generally less computationally demanding than off-lattice approaches, its accuracy and consistency in modeling surface reactions have not been fully verified. Using the Spatiocyte scheme, we study the accuracy of MLM in diffusion-influenced surface reactions. We derive the lattice-based bimolecular association rates for two-dimensional (2D) surface-surface reaction and one-dimensional (1D) volume-surface adsorption according to the Smoluchowski-Collins-Kimball model and random walk theory. We match the time-dependent rates on lattice with off-lattice counterparts to obtain the correct expressions for MLM parameters in terms of physical constants. The expressions indicate that the voxel size needs to be at least 0.6% larger than the molecule to accurately simulate surface reactions on triangular lattice. On square lattice, the minimum voxel size should be even larger, at 5%. We also demonstrate the ability of MLM-based schemes such as Spatiocyte to simulate a reaction-diffusion model that involves all dimensions: three-dimensional (3D) diffusion in the cytoplasm, 2D diffusion on the cell membrane, and 1D cytoplasm-membrane adsorption. With the model, we examine the contribution of the 2D reaction pathway to the overall reaction rate at different reactant diffusivity, reactivity, and concentrations.

Focus on lasers, imaging, and microscopy

Focus on lasers, imaging, and microscopy

Development of Chinese Female Computational Phantom Rad-Human and Its Application in Radiation Dosimetry Assessment

Dose conversion coefficients are important physical quantities in radiation dosimetry assessment and can be derived from Monte Carlo simulation based on human computational phantoms. In order to accurately evaluate the dose to a human body especially for a Chinese female, a precise whole-body Chinese female computational phantom named Rad-Human was constructed based on high-resolution digital color slice images of an adult female body. Rad-Human includes 46 tissues and organs with a minimum voxel size of 0.15 × 0.15 × 0.25 mm for head and neck and 0.15 × 0.15 × 0.5 mm for other regions, and it contains more than 28.8 billion voxels. Conversion coefficients and effective doses of external radiation, specific absorbed fractions, and S values of internal radiation for different energies for Rad-Human were calculated. The calculated dose conversion coefficients were reasonable comparing and analyzing the relationship between dose and organ characteristics with those values of the International Commission on Radiological Protection (ICRP) reference phantom. Based on the information and simulation results of Rad-Human, a set of more complete data of dose conversion coefficients in the radiation field was constructed for a Chinese adult female. Dose discrepancies that were observed were due to differences of body structures between the two phantoms. The differences of dose conversion coefficients between Rad-Human and the ICRP reference phantom demonstrate that Rad-Human can more accurately assess the exposure dose especially for a Chinese female.

A volumetric three-dimensional digital light photoactivatable dye display

Volumetric three-dimensional displays offer spatially accurate representations of images with a 360° view, but have been difficult to implement due to complex fabrication requirements. Herein, a chemically enabled volumetric 3D digital light photoactivatable dye display (3D Light PAD) is reported. The operating principle relies on photoactivatable dyes that become reversibly fluorescent upon illumination with ultraviolet light. Proper tuning of kinetics and emission wavelengths enables the generation of a spatial pattern of fluorescent emission at the intersection of two structured light beams. A first-generation 3D Light PAD was fabricated using the photoactivatable dye N-phenyl spirolactam rhodamine B, a commercial picoprojector, an ultraviolet projector and a custom quartz imaging chamber. The system displays a minimum voxel size of 0.68 mm3, 200 μm resolution and good stability over repeated ‘on-off’ cycles. A range of high-resolution 3D images and animations can be projected, setting the foundation for widely accessible volumetric 3D displays.

Uncertainty in 3D gel dosimetry

Three-dimensional (3D) gel dosimetry has a unique role to play in safeguarding conformal radiotherapy treatments as the technique can cover the full treatment chain and provides the radiation oncologist with the integrated dose distribution in 3D. It can also be applied to benchmark new treatment strategies such as image guided and tracking radiotherapy techniques. A major obstacle that has hindered the wider dissemination of gel dosimetry in radiotherapy centres is a lack of confidence in the reliability of the measured dose distribution. Uncertainties in 3D dosimeters are attributed to both dosimeter properties and scanning performance. In polymer gel dosimetry with MRI readout, discrepancies in dose response of large polymer gel dosimeters versus small calibration phantoms have been reported which can lead to significant inaccuracies in the dose maps. The sources of error in polymer gel dosimetry with MRI readout are well understood and it has been demonstrated that with a carefully designed scanning protocol, the overall uncertainty in absolute dose that can currently be obtained falls within 5% on an individual voxel basis, for a minimum voxel size of 5 mm3. However, several research groups have chosen to use polymer gel dosimetry in a relative manner by normalizing the dose distribution towards an internal reference dose within the gel dosimeter phantom. 3D dosimetry with optical scanning has also been mostly applied in a relative way, although in principle absolute calibration is possible. As the optical absorption in 3D dosimeters is less dependent on temperature it can be expected that the achievable accuracy is higher with optical CT. The precision in optical scanning of 3D dosimeters depends to a large extend on the performance of the detector. 3D dosimetry with X-ray CT readout is a low contrast imaging modality for polymer gel dosimetry. Sources of error in x-ray CT polymer gel dosimetry (XCT) are currently under investigation and include inherent limitations in dosimeter homogeneity, imaging performance, and errors induced through post-acquisition processing. This overview highlights a number of aspects relating to uncertainties in polymer gel dosimetry.

Blood Oxygen Level–Dependent Magnetic Resonance Imaging of the Kidneys

The aim of this study was to quantify the influence of image resolution on the apparent transverse relaxivity (R2*) of the magnetic resonance (MR) signal in human renal tissue in vivo and in phantom measurements. This prospective study included 17 healthy volunteers (age, 32 ± 8 years, 6 women). Parametrical R2* maps were computed via monoexponential fitting of multiecho 2-dimensional fast-field echo data measured at 1.5 T (repetition time [TR], 150 milliseconds; flip angle [FA], 40°; minimum echo time [TE], 4.6 milliseconds; ΔTE, 5 milliseconds; 16 echoes) and at 3 T (TR, 140 milliseconds; FA, 70°; minimum TE, 2 milliseconds; ΔTE, 5 milliseconds; 16 echoes) with varying nominal volumes of the encoded voxels (from 5.76 to 36.0 mm). For each voxel size, mean R2* values were computed in regions of interest drawn in the left and right renal parenchyma. For data acquired using minimum voxel size, the mean R2* values were computed over the cortex and medulla separately. The squared 2-norm of the residuals was computed to evaluate the goodness of the pixel-wise exponential fits. Six multiecho MR images of a water phantom were acquired using a 2-dimensional fast-field echo sequence (FA, 50°; TR, 108 milliseconds; TE, 4 milliseconds; ΔTE, 20 milliseconds) at 3 T after shim adjustment and in the presence of a uniform background gradient of 40 μT/m. The nominal voxel size was varied in a range between 2 and 12.5 mm. Mean R2* values of 13.04 ± 0.71 s (right renal cortex) and 16.47 ± 1.92 s (right renal medulla) were computed at 1.5 T. At 3 T, the R2* of the right medulla was 28.27 ± 1.52 s and the cortical R2* was 19.22 ± 2.32 s. Comparable relaxivity values were found over the left kidney at both field strengths. Increasing R2* values were observed for increasing voxel volume in both the water phantom and renal tissue data. At a constant slice thickness of 4 mm, the decrease in the in-plane resolution from 1.2 × 1.2 mm to 3.0 × 3.0 mm led to a maximum increase of the renal R2* of 15% at 1.5 T and of 12% at 3 T. Increasing the slice thickness from 3 to 8 mm at a constant in-plane resolution of 1.5 × 1.5 mm resulted in a maximum increase of the renal R2* of 30% at 1.5 T and of 26% at 3 T. On the other hand, increasing the voxel size improved the goodness of the fit implied by the smaller residuals. The phantom experiments and in vivo acquisitions of healthy renal tissue documented a significant dependence of the apparent R2* relaxation rate on the spatial resolution of the MR imaging data. In clinical practice, the voxel volume for the quantification of renal R2* should be optimized in a compromise between minimizing the effects of macroscopic field inhomogeneity and maintaining a sufficiently high signal-to-noise ratio and goodness of fit. When comparing quantitative R2* among different publications, the influence of the spatial resolution should be taken into account.

SU‐FF‐I‐28: Effects of Voxel Size On Visibility of Microcalcifications in Cone Beam Breast CT

Purpose: To investigate the effects of voxel size on the visibility of simulated microcalcifications (MCs) in cone bream breast CT. Method and materials: Calcium carbonate grains (ranging from 140–150 to 200–212 μm), were used to simulate MCs. 25 MCs from same size group were arranged into a 5 × 5 cluster and embedded at the center of a 15 mm diameter paraffin rod. Rods with MCs from various size groups were constructed and scanned with a bench top experimental CBCT system. A CCD detector (C4742‐56‐12ER, Hamamatsu Photonics K.K) operated in 2×2 binning mode was used to acquire 300 projection images with a pixel size of 48 μm over 360°. Imaging geometry was configured with a magnification factor of 1.16 to result in a voxel size of 42 μm to match the spatial resolution limit. The acquired images were further binned into image sets of larger pixels (96, 144, 192, 240 and 288 μm). The image sets were then reconstructed with Feldkamp's backprojection algorithm using a ramp filter. The reconstructed image sets were reviewed by 6 readers independently. The visibility, computed as the ratio of visible MCs and averaged over all readers, was plotted as the function of voxel size to quantify the effects of voxel size on the visibility of MCs. Results: Based on the visibility versus voxel size plot, 90% visibility was achieved for 200 – 212, 180 – 200, 160 – 180, 150 – 160 and 140 – 150 μm calcifications at a voxel size of 249, 249, 206, 166 and 125 μm, respectively. The visibility versus voxel size plot may be used to determine the minimum pixel size required to achieve a specific visibility level for MCs of certain sizes. Conclusions: We have demonstrated that the minimum voxel size required to achieve a specific visibility level decreased with the MC size.

SU-FF-T-275: Octree Based Compression Method of DICOM Images for Voxel Number Reduction and Faster Monte Carlo Simulations

Purpose: Diminish the number of voxels created from a set of DICOM images. Keep critical information at tissue interfaces needed in a Monte Carlo simulation without compromising the physical quality of the voxelized image. Method and Materials: An algorithm was developed to apply an octree compression to DICOM images. The algorithm work as follow: the whole set of DICOM image is assumed as a cube. It is then split in eight equal smaller cubes. Each of the cubes is check for density homogeneity. If a high density gradient is encountered in a cube, it is also split in height equal parts. This goes on until minimum voxel size is reach. The resulting image is composed of various voxels size. To test this approach, images from a CT calibration phantom which contains various densities were used. To verify precision, Monte Carlo simulation with GEANT4 using a narrow beam passing through several high density gradient was done. Results: The resulting number of voxels range from 5 to 20% of the original size depending on configurations. The voxel area on a typical slice is 1 square voxel (i.e. no compression) at high density gradient to 64 square voxel for homogeneous sections. Mean volume was about 5.43 voxels for the phantom used. The octree has the particularity to smooth digital noise present in homogenous area. The density difference between the original and the octree images are directly related to this noise. Monte Carlo simulation shows less then 1% difference in dose at the high gradient interface. Conclusion: The octree compression is an excellent method to compress DICOM information for Monte Carlo treatment planning without losing precision at the interfaces in homogeneous regions. Given good parameters the algorithm also has the particularity to smooth digital noise in homogenous area.

Two-dimensional MR Spectroscopic Characterization of Breast Cancer In Vivo

The major goal of this work was to characterize invasive ductal carcinoma and healthy fatty breast tissues noninvasively using the classification and regression tree analysis (CART) of 2D MR spectral data. 2D L-COSY spectra were acquired in 14 invasive breast carcinoma and 21 healthy fatty breasts using a GE 1.5 Tesla MRI/MRS scanner equipped with a 2-channel phased-array breast MR coil. The 2D spectra were recorded in approximately 10 minutes using a minimum voxel size of 1 ml without any water suppression technique. For healthy breasts, spectra were acquired from at least one fatty region. 2D L-COSY spectra were recorded in a total of 43 voxels. Five diagonal and six cross peak volumes were integrated and at least eighteen ratios were selected as potential features for the statistical method, namely CART. The 2D L-COSY data showed a significant increase for the majority of these ratios in invasive breast carcinomas compared to healthy fatty tissues. Better accuracy of identifying carcinomas and fatty tissues is reported using CART analysis of different combinations of ratios calculated from the relative levels of water, choline, and saturated and unsaturated lipids. This is a first report on the statistical classification of 2D L-COSY in human breast carcinomas in vivo.

Homonuclear J-refocused spectral editing technique for quantification of glutamine and glutamate by 1H NMR spectroscopy.

Quantification of the excitatory amino acid of glutamine and glutamate is extremely important partly because, when in excessive amounts, they can produce neuronal damage. However, quantification of such metabolites seems to be not straightforward owing to a large number of metabolic signals, particularly between the 2.0- and 3.0-ppm chemical shift range. The authors investigated a quantification method by using a homonuclear J-refocused spectral editing technique, namely the J-refocused PRESS (JPRESS), using both 1.5- and 4-T whole body NMR systems. At first, the J-coupling modulation of each metabolite at different TE values using the PRESS and JPRESS pulse sequences was compared. The suppression of J modulations with the JPRESS is well demonstrated on condition that TE values are less than 1/4J. Then, the accuracy of glutamate quantification after subtracting the two spectra obtained at two different echo times and integrating the difference spectral area at different concentrations was evaluated. The results exhibit an excellent linear relationship between the actual concentration versus the difference spectral peak area at both 1.5 and 4 T. The achievable minimum voxel size was also considered and a 1-ml voxel size seems to be possible at the typical glutamate concentration in vivo at a field strength of 4 T.