Scanning Conditions Research Articles

Intramuscular ketamine provides better sedation and scan conditions in children undergoing magnetic resonance imaging: A single-blinded observational study.

The objectives of this study were to compare the quality of sedation provided by intravenous (i.v.) and intramuscular (im) ketamine for pediatric magnetic resonance imaging (MRI). This study was a non-randomized, single-blinded, and prospective observational study. After receiving approval from the Institutional Ethics Committee, a total of 108 children aged 2-7 years were divided into two groups, with 54 children in each group. In the i.v. group, children received ketamine at a dose of 1.5 mg/kg intravenously, while in the im group, children received ketamine at a dose of 4 mg/kg intramuscularly. If a Ramsay sedation score of 6 (RSS-6) was not achieved, half of the loading dose of ketamine was repeated. In both groups, rescue propofol boluses of 1 mg/kg intravenously were administered whenever the child moved. The primary outcome measure was the quality of sedation, which was assessed by a blinded radiologist. The time taken to reach RSS-6, the number of rescue propofol boluses, the total time wasted in taking repeat sequences, and the time required to achieve a modified Aldrete score of 9 (MAS-9) were recorded. The im group demonstrated significantly better sedation quality. In the i.v. group, the time to achieve RSS-6 was significantly shorter, but it required more rescue propofol boluses to maintain sedation. The i.v. group also experienced a notable increase in the total time wasted during repeat sequences. On the other hand, the i.v. group exhibited a shorter time to reach MAS-9 compared to the im group. The im group showed superior sedation quality when compared to the i.v. group. However, it is important to consider that the im group experienced a longer recovery time.

Use of scanners for colorimetric analysis of microfluidic paper-based analytical devices (µPADs): A practical guide

Office style scanners are often used by researchers for the colorimetric detection in the development of microfluidic paper-based analytical devices (µPADs). However, little attention has been paid to nuances in calibration curve responses caused by varying settings of a single image-capturing device. Hence, in this study, the effect of parameters, such as the scanner operation conditions, the method of data analysis used (e.g., choice of colour space system, suitable channel), and the influence of the physical characteristics of the µPAD (e.g., template colour, type of lamination pouches used) on the colorimetric detection were evaluated for the first time.Combinations of the scanning parameters were thoroughly examined and their impact on the quality of a mock calibration curve assessed. The most sensitive calibration curve was achieved only by the selection of the optimal scanning conditions combined with an appropriate choice of colorimetric system. A flatbed scanner, used in this study, showed greater variation of signal with varying scanning setup, whereas a portable scanner provided nearly identical responses under diverse scanning conditions. Gamma correction, scanning resolution, region of interest (ROI) surrounding colour (i.e., template colour), bit-depth, image file type, and the number of analysed pixels were found not to markedly affect the dynamics of the calibration curve when colour in the ROI is homogeneous. Parameters that had an effect on the colorimetric detection were the scanner settings, colour of the blank, image acquisition mode (colour vs greyscale), and µPAD covering (e.g., lamination). Based on these findings, generic guidelines for optimal data acquisition and µPAD manipulation with respect to µPAD calibration have been formulated.

CROSS: Cross-Domain Residual-Optimization-Based Structure Strengthening Reconstruction for Limited-Angle CT

Limited-angle CT is an effective way for practical scenario due to its flexibility in various complex scanning conditions. However, incomplete projection data will lead to severe wedge artifacts and degraded images, which significantly lower the diagnostic values. To overcome this problem, we propose a novel method termed Cross-domain Residual-Optimization Based Structure Strengthening (CROSS) Reconstruction for limited-angle CT. The proposed CROSS framework consists of three steps, which are conducted on the image domain and measurement domain alternatively. Differing from traditional dual-domain-based algorithms, our CROSS method not only regularizes the reconstruction results on the image space but also the residual-error space, which boosts organ recovery where the area has a larger attenuation coefficient. Besides, the structure-strengthening network is adopted to enhance tissue preservation. Simulated and preclinical datasets are conducted to evaluate the proposed CROSS method. Experiments show that the proposed framework could produce a better performance in artifact removal and edge preservation.

Simulations of Modified Shallow Water Equation and Reaction Kinetics of Poly Etching on Single Wafer Process

With the downscaling of semiconductor feature size, single wafer wet etching has shown better within-wafer and wafer-to-wafer uniformity. According to previous research and analysis, liquid film thickness has a close relationship with etch loss, and numerical simulations have been applied to calculate liquid film thickness with fixed nozzle dispensing. This study investigated liquid film thickness with scanning nozzle processing and a modeled aqueous tetramethylammonium hydroxide (TMAH) poly etching method. A modified shallow water equation was used to calculate the liquid film thickness under nozzle scanning conditions. Additionally, the liquid film thickness and liquid velocity were coupled with a chemical reaction kinetic model to calculate the TMAH poly etching reaction rate on the wafer surface. Scan dispensing etch loss was calculated by integrating the reaction rate. Both the experimental and simulation results showed that the scan speed can affect within-wafer etch loss profile and that a fast scan speed could increase etch loss in wafer donut area. The simulation model could be a predictive tool for improving within-wafer etching profile.

Evaluation of Renal Fibrosis Using Magnetization Transfer Imaging at 1.5T and 3T in a Porcine Model of Renal Artery Stenosis.

Renal fibrosis is an important marker in the progression of chronic kidney disease, and renal biopsy is the current reference standard for detecting its presence. Currently, non-invasive methods have only been partially successful in detecting renal fibrosis. Magnetization transfer imaging (MTI) allows estimates of renal fibrosis but may vary with scanning conditions. We hypothesized that MTI-derived renal fibrosis would be reproducible at 1.5T and 3T MRI and over time in fibrotic kidneys. Fifteen pigs with unilateral renal artery stenosis (RAS, n = 9) or age-matched sham controls (n = 6) underwent MTI-MRI at both 1.5T and 3T 6 weeks post-surgery and again 4 weeks later. Magnetization transfer ratio (MTR) measurements of fibrosis in both kidneys were compared between 1.5T and 3T, and the reproducibility of MTI at the two timepoints was evaluated at 1.5T and 3T. MTR at 3T with 600 Hz offset frequency successfully distinguished between normal, stenotic, and contralateral kidneys. There was excellent reproducibility of MTI at 1.5T and 3T over the two timepoints and no significant differences between MTR measurements at 1.5T and 3T. Therefore, MTI is a highly reproducible technique which is sensitive to detect changes in fibrotic compared to normal kidneys in the RAS porcine model at 3T.

Feasibility of size-specific organ-dose estimates based on water equivalent diameter for common head CT examinations: a Monte Carlo study

Computed tomography dose index (CTDI) is an unreliable dose estimate outside of the standard CTDI phantom diameters (16 and 32 cm). Size-specific dose estimate (SSDE) for head computed tomography (CT) examination was studied in the American Association of Physicists in Medicine Report 293 to provide SSDE coefficient factors based on water equivalent diameter as size metrics. However, it is limited to one protocol and for a fully irradiated organ. This study aimed to evaluate the dependency of normalized organ dose (ND) on water equivalent diameter as a size metric in three common protocols: routine head, paranasal sinus, and temporal bone. CTDIw measurements were performed for outlined protocols in the Siemens Emotion 16-slice-configuration scanner. Geant4 Application for Tomographic Emission Monte Carlo simulation platform, coupled with ten GSF patient models, was used to estimate organ doses. CT scanner system was modeled. Helical CT scans were simulated using constructor scan parameters and calculated scan lengths of each patient model. Organ doses provided by simulations were normalized to CTDIvol. The water equivalent diameters (D w) of patient models were obtained via relationships between D w and both effective diameter for a sample of patients’ data.NDs received by fully, partially, and non-directly irradiated organs were then reported as a function of D w. For fully irradiated organs, brain (R 2 > 0.92), eyes (R 2 > 0.88), and eye lens (R 2 > 0.89) correlate well with D w. For the rest of the results, a poor correlation was observed. For partially irradiated organs, the exception was scalp (R 2 = 0.93) in temporal bone CT. For non-directly irradiated organs, the exception was thyroid (R 2 > 0.90) and lungs (R 2 > 0.91) in routine head CT. ND correlates well in routine head CT than other protocols. For the most part, no relationship seems to exist between R 2 and scan percentage coverage. The results have revealed additional factors that may influence the ND and D w relationship, which explains the need for more studies in the future to investigate the effect of scan conditions and organ anatomy variation.

Evaluation of Low-contrast Detectability of Photon-Counting-Detector CT Using Channelized Hotelling Observer and an ACR Accreditation Phantom.

The purpose of this work is to evaluate the low-contrast detectability on a clinical whole-body photon-counting-detector (PCD)-CT scanner and compare it with an energy-integrating-detector (EID) CT scanner, using an efficient Channelized Hotelling observer (CHO)-based method previously developed and optimized on the American College of Radiology (ACR) CT accreditation phantom for routine quality control (QC) purpose. The low-contrast module of an ACR CT phantom was scanned on both the PCD-CT and EID-CT scanners, each with 10 different positionings. For PCD-CT, data were acquired at 120 kV with two major scan modes, standard resolution (SR) (collimation: 144×0.4 mm) and ultra-high-resolution (UHR) (120×0.2 mm). Images were reconstructed with two major modes: virtual monochromatic energy at 70 keV and low-energy threshold (T3D), each with filtered-backprojection (Br44) and iterative reconstruction (Br44-3) kernels. For each positioning, 3 repeated scans were acquired for each scan mode at a fixed radiation dose setting (CTDIvol = 12 mGy). For EID-CT, scans (10 positionings × 3 repeated scans) were performed at a matched CTDIvol, and images were reconstructed using the same kernels with FBP and IR. A recently developed CHO-based method dedicated for QC of low-contrast performance on the ACR phantom was applied to calculate the low-contrast detectability (d') for each scan and reconstruction condition. Results showed that there was no significant difference in low-contrast detectability (d') between the UHR mode and SR mode (p = 0.360-0.942), and the T3D reconstruction resulted in 7.7%-14.6% higher d' than 70keV (p < 0.0016). Similar detectability levels were observed on PCD-CT and EID-CT. The PCD-CT: UHR-T3D had 6.2% higher d' than EID-CT with IR (p = 0.047) and 4.1% lower d' without IR (p = 0.122).

Method of Suppressing Solidification Cracking by Laser Surface Melting and Epitaxial Growth Behavior for Directionally Solidified 247LC Superalloy

In this study, the relationship between solidification cracking and epitaxial growth behavior with the high-speed laser surface melting of a directionally solidified 247LC superalloys was fundamentally and metallurgically investigated, to develop a successful welding procedure for the next generation of gas turbine blades. Under typical laser surface melting conditions (scan speed: 50 mm/s, heat input: 40 J/mm), severe solidification cracking phenomena occurs. The key metallurgical factors of solidification cracking have been identified as solidification segregation-assisted pipeline diffusion behavior at the solidification grain boundary, and in the randomly formed polycrystalline melting zone microstructure. In addition, under extremely low heat input and high-speed laser beam scan conditions (scan speed: 1000 mm/s, heat input: 2 J/mm), an effective surface melting zone can be obtained within a single directionally solidified grain under a relatively high-energy beam density (65 J/mm2) using the characteristics of single-mode fiber lasers. Results reveal that the laser melting zone successfully shows a 99.9% epitaxial growth achievement ratio. Because of the superior epitaxial growth ratio within the laser surface melting zone, and the rapid solidification phenomena, formation of a solidification grain boundary and solidification segregation-assisted pipeline diffusion behavior can be suppressed. Finally, a solidification crack-free laser melting zone can thus be achieved.

A segmentation-based method improving the performance of N4 bias field correction on T2weighted MR imaging data of the prostate

Magnetic Resonance (MR) images suffer from spatial inhomogeneity, known as bias field corruption. The N4ITK filter is a state-of-the-art method used for correcting the bias field to optimize MR-based quantification. In this study, a novel approach is presented to quantitatively evaluate the performance of N4 bias field correction for pelvic prostate imaging. An exploratory analysis, regarding the different values of convergence threshold, shrink factor, fitting level, number of iterations and use of mask, is performed to quantify the performance of N4 filter in pelvic MR images. The performance of a total of 240 different N4 configurations is examined using the Full Width at Half Maximum (FWHM) of the segmented periprostatic fat distribution as evaluation metric. Phantom T2weighted images were used to assess the performance of N4 for a uniform test tissue mimicking material, excluding factors such as patient related susceptibility and anatomy heterogeneity. Moreover, 89 and 204 T2weighted patient images from two public datasets acquired by scanners with a combined surface and endorectal coil at 1.5 T and a surface coil at 3 T, respectively, were utilized and corrected with a variable set of N4 parameters. Furthermore, two external public datasets were used to validate the performance of the N4 filter in T2weighted patient images acquired by various scanning conditions with different magnetic field strengths and coils. The results show that the set of N4 parameters, converging to optimal representations of fat in the image, were: convergence threshold 0.001, shrink factor 2, fitting level 6, number of iterations 100 and the use of default mask for prostate images acquired by a combined surface and endorectal coil at both 1.5 T and 3 T. The corresponding optimal N4 configuration for MR prostate images acquired by a surface coil at 1.5 T or 3 T was: convergence threshold 0.001, shrink factor 2, fitting level 5, number of iterations 25 and the use of default mask. Hence, periprostatic fat segmentation can be used to define the optimal settings for achieving T2weighted prostate images free from bias field corruption to provide robust input for further analysis.

Multi-scale dilated dense reconstruction network for limited-angle computed tomography

Objective. Due to the incomplete projection data, the image reconstructed by limited-angle computed tomography (CT) usually suffers from significant artifacts, by which the structural details are heavily blurred. In this study, we aim to develop a novel approach to improve the limited-angle CT reconstruction performance, especially for the narrow scanning angular range. Approach. A deep learning based iterative framework for limited-angle tomography is proposed, which is named multi-scale dilated dense reconstruction network (MSDDRNet). The MSDDRNet utilizes a multi-scale dilated dense convolution neural network (MSDD-CNN) with conventional reconstruction algorithm for predicting image from incomplete projection data. The MSDD-CNN enhances the image features in the network by merging the DenseNet-Like structure, which serves to restore invisible singularities and reduce artifacts, as well as introducing constraints on the projection domain data into the iterative process to achieve better image detail recovery. Additionally, to improve the training speed of the network, we use a strategy of pre-training and model migration. Main results. Numerical experiments demonstrate that the proposed MSDDRNet performs well in terms of artifact correction, noise reduction and structure recovery compared to existing methods with limited scan angles, and we also extend the proposed method to more general scanning condition and other application such as dental CT data. Significance. The proposed method is a general framework, which can be applied to other CT problems, such as low dose CT, sparse-data CT and spectral CT.

Liquation crack-free welding strategy for 247LC DS superalloy by control of pipeline diffusion via ultra-high-speed laser beam scanning

Under completely suppressed constitutional liquation of MC carbide, unexplored 247LC superalloy single-mode fibre laser welds experience unexpected liquation cracking. These cracks continuously propagate from a solidification crack, particularly at highly misoriented and epitaxially grown solidification grain boundaries, and dominated by pipeline diffusion of impurities. Two novel metallurgical preconditions were identified which, if satisfied, ensure complete suppression of liquation cracking of 247LC superalloy welds. These preconditions are (i) the constitutional liquation of MC carbide at the heat-affected zone and (ii) pipeline diffusion of impurities at the fusion zone. It is also highly recommended that the welding is carried out in a single directionally solidified grain under scanning conditions, avoiding the formation of highly misoriented solidification grain boundaries.

Information Value of the Results of Examining Metallic Products by the X-Ray Computer Tomography Method

The information value of X-ray computer tomography (X-CT) lies in the quality and accuracy of its results. Despite indisputable advantages of X-CT in comparison with other non-destructive examination methods, it has not received widespread use in the industry in view of its high cost and low throughput capacity. In addition, in Russia there is no regulatory and technical framework that would stipulate a standard procedure for selecting the tomographic examination parameters, nor are there standard techniques for estimating the quality of X-CT results. In view of this circumstance, the article analyses the US standard ASTM E 1695 Standard Test Method for Measurement of Computed Tomography (CT) System Performance. This standard stipulates a procedure for quantitatively assessing the quality of tomograms of a cylindrical test specimen, namely, spatial resolution and contrast sensitivity. The standard describes methods and a mathematical tool for calculating the modulation transfer function (MTF), contrast discrimination function (CDF), and constructing a contrast-detail diagram&nbsp;(CDD), which takes into account the influence of both unsharpness and noise on the possibility of detecting a flaw of certain size and morphology. The MTF values obtained under specific scanning and reconstruction conditions are a quality indicator of the X-CT results for objects examined under the same scanning conditions and made of materials similar in terms of attenuation coefficient, and can be used to assess the sensitivity of examination by the X-CT method. The article defines artifacts as any discrepancy between the object’s reconstructed volume and real attenuation factors. The reasons why artifacts appear are explained, their images on tomograms are shown, and recommendations for minimizing them are given. Factors affecting the information value of the X-ray tomographic examination results are pointed out.

CBCT Images to an STL Model: Exploring the "Critical Factors" to Binarization Thresholds in STL Data Creation.

In-house fabrication of three-dimensional (3D) models for medical use has become easier in recent years. Cone beam computed tomography (CBCT) images are increasingly used as source data for fabricating osseous 3D models. The creation of a 3D CAD model begins with the segmentation of hard and soft tissues of the DICOM images and the creation of an STL model; however, it can be difficult to determine the binarization threshold in CBCT images. In this study, how the different CBCT scanning and imaging conditions of two different CBCT scanners affect the determination of the binarization threshold was evaluated. The key to efficient STL creation through voxel intensity distribution analysis was then explored. It was found that determination of the binarization threshold is easy for image datasets with a large number of voxels, sharp peak shapes, and narrow intensity distributions. Although the intensity distribution of voxels varied greatly among the image datasets, it was difficult to find correlations between different X-ray tube currents or image reconstruction filters that explained the differences. The objective observation of voxel intensity distribution may contribute to the determination of the binarization threshold for 3D model creation.

Construction of a computational MDCT model for simulations of the detector signals

ObjectiveTo develop a computational model of a multi-detector CT scanner (MDCT), which could be used to simulate the signal of each detector element in the MDCT by using the Monte Carlo method. MethodsThe CT scanner was modelled, including the X-ray source, the bowtie filter, the collimator, the couch and the detector panel. Under a general scanning condition, the signal in each detector element was simulated based on the model by using the MCNPX code. Both the energy spectra at different tube voltages and energy deposition in the detector panel at different collimations were simulated to test the robustness of the MDCT model built in this study. Furthermore, the simulated signals in each detector element were compared with their recorded signals. The accuracies were evaluated by the relative root mean square error (RRMSE) and the structural similarity (SSIM) for each detector element and the whole detector panel, respectively. ResultsThe simulated energy spectra before and after passing through the phantom and simulated energy deposition in the detector panel were rational. In the scan range from the apex of lung to pubic symphysis, the RRMSE of the 18 axial projections ranged from 0.02 to 0.17, with an average of 0.08. And the SSIMs were calculated to be 0.979 and 0.976 for projections with the largest peak signal and the smallest peak signal, respectively. ConclusionsThe computational model of the MDCT developed in this study is accurate and successful, it is helpful for further accurate simulations of the radiation dose and image quality of the MDCT.

Evaluation of In-room Volumetric Imaging Doses for Image-guided Radiotherapy: A Multi-institutional Study.

We investigated imaging dose and noise under clinical scan conditions at multiple institutions using a simple and unified method, and demonstrated the need for diagnostic reference levels in image-guided radiotherapy (IGRT). Nine cone-beam and helical computed tomography (CT) scanners (Varian, Elekta, Accuray Inc., and BrainLAB) from seven institutions were investigated in this study. The weighted cone-beam dose index (CBDIw) was calculated for head and pelvic protocols using a 100 mm pencil chamber under the conditions used in actual clinical practice at each institution. Cone-beam CT image noise was evaluated using polymethylmethacrylate head and body phantoms with diameters of 16 and 32 cm, respectively. For head and pelvic protocols, CBDIw values ranged from 0.94-6.59 and 1.47-20.9 mGy, respectively. Similarly, standard deviation (SD) values ranged from 9.3-34.0 and 26.9-97.4 HU, respectively. The SD values tended to increase with decreasing imaging dose (r = -0.33 and -0.61 for the head and pelvic protocols, respectively). Among the nine machines, the imaging dose for high imaging dose institutions was approximately 20 mGy to the pelvic phantom, and there was a 14-fold difference in dose compared with the other institutions. These results suggest the need to establish DRLs for IGRT to guide clinical decision-making.

LRIP-Net: Low-Resolution Image Prior-Based Network for Limited-Angle CT Reconstruction

In the practical applications of computed tomography (CT) imaging, the projection data may be acquired within a limited-angle range and corrupted by noises due to the limitation of scanning conditions. The noisy incomplete projection data results in the ill-posedness of the inverse problems. Based on the observation that the low-resolution reconstruction problem has better numerical stability, we propose a novel low-resolution image prior-based CT reconstruction model for limited-angle reconstruction. More specifically, we build up a low-resolution reconstruction problem on the down-sampled projection data, and use the reconstructed low-resolution image as prior knowledge for the high-resolution limited-angle CT problem. The constrained minimization problem is then solved by the alternating direction method with all subminimization problems approximated by the convolutional neural networks. Numerical experiments demonstrate that our double-resolution network outperforms both the variational method and popular learning-based reconstruction methods on noisy limited-angle reconstruction problems.

Metal Artifact Reduction in Dental CBCT Images Using Direct Sinogram Correction Combined with Metal Path-Length Weighting.

Metal artifacts in dental computed tomography (CT) images, caused by highly X-ray absorbing objects, such as dental implants or crowns, often more severely compromise image readability than in medical CT images. Since lower tube voltages are used for dental CTs in spite of the more frequent presence of metallic objects in the patient, metal artifacts appear more severely in dental CT images, and the artifacts often persist even after metal artifact correction. The direct sinogram correction (DSC) method, which directly corrects the sinogram using the mapping function derived by minimizing the sinogram inconsistency, works well in the case of mild metal artifacts, but it often fails to correct severe metal artifacts. We propose a modified DSC method to reduce severe metal artifacts, and we have tested it on human dental images. We first segment the metallic objects in the CT image, and then we forward-project the segmented metal mask to identify the metal traces in the projection data with computing the metal path length for the rays penetrating the metal mask. In the sinogram correction with the DSC mapping function, we apply the weighting proportional to the metal path length. We have applied the proposed method to the phantom and patient images taken at the X-ray tube voltage of 90 kVp. We observed that the proposed method outperforms the original DSC method when metal artifacts were severe. However, we need further extensive studies to verify the proposed method for various CT scan conditions with many more patient images.

Contextualizing the Role of Volumetric Analysis in Pulmonary Nodule Assessment: AJR Expert Panel Narrative Review.

Pulmonary nodules are managed based on their size and morphologic characteristics. Radiologists are familiar with assessing nodule size by diameter measured using manually deployed electronic calipers. Size may also be assessed by 3D volumetric measurements (referred to as volumetry) using software. Nodule size and growth are more accurately assessed by volumetry than by diameter, and the evidence supporting clinical use of volumetry has expanded, driven by its use in lung cancer screening (LCS) nodule management algorithms in Europe. The application of volumetry has the potential to reduce recommendations for imaging follow-up of indeterminate solid nodules without impacting cancer detection. Although changes in scanning conditions or volumetry software package may lead to variation in volumetry results, ongoing technical advances have improved the reliability of calculated volumes. Volumetry is now the primary method for determining size of solid nodules in the European LCS position statement and British Thoracic Society recommendations. In this article, we review technical aspects, advantages, and limitations of volumetry, and contextualize its use with respect to its importance relative to morphologic evaluation, role for predicting malignancy in risk models, and practical impact on nodule management, by considering specific scenarios. Implementation challenges and areas requiring further evidence are also highlighted.

Optimization of smoothing factor for fast non-local means algorithm in high pitch based low-dose computed tomography images with tin-filter

In this study, we applied the fast non-local means (FNLM) algorithm to high-pitch low-dose computed tomography (CT) images using a tin-filter. This study was conducted via a simulation study using a Shepp-Logan phantom and an experimental approach using American association of physicists in medicine (AAPM) phantom images. To optimize the smoothing factor of the FNLM algorithm, the values were adjusted from 0.01 to 0.05 at intervals of 0.001 and applied to the CT image. The smoothing factor of the optimal FNLM algorithm, derived through various quantitative evaluations, was 0.03. According to the results, when the optimized FNLM algorithm was applied to the acquired CT image with a low-dose protocol based on a high pitch condition, the estimated coefficient of variation, contrast-to-noise ratio, and signal-to-noise ratio were improved by 8.7, 6.7, and 8.0 times, respectively, compared to the general CT scanning condition (none). In addition, compared with the noise filtering methods mainly used in the past, both simulation and real experimental results proved that the application efficiency of the FNLM algorithm was high in the obtained low-dose-based CT images. In summary, the applicability of the FNLM algorithm in CT images obtained under low-dose conditions with high pitch using a tin-filter was demonstrated, and the derived research results are expected to be actively utilized in the diagnostic medical field to help reduce exposure doses and improve image quality.

Rigorous Density Correction Model for Single-Scan TLS Point Clouds

Terrestrial laser scanning (TLS) can acquire high-precision and high-resolution 3D point clouds of the scanned targets, and the common technical challenge is how to accurately and efficiently process the massive unorganized point clouds. Density is one of the most fundamental and significant quantities that can be derived for each point by counting the number of neighbors in a given 3D space. It contains key information of the target geometric features and thus plays an important role in TLS data processing (e.g., classification and feature extraction). However, density usually varies remarkably for different scenes with varied scan geometry (e.g., distance and incidence angle), scan resolution (e.g., horizontal and vertical angular resolutions), and object geometry (e.g., slope, size, and spatial attitude), thereby greatly hindering its full capability in reflecting the geometric discrepancies of different targets. The density must be corrected before reliably used as a proxy for target geometric features. In this study, a generalized rigorous model is proposed to correct the density of single-scan TLS point clouds based on the mathematically deduced relation between density and related influencing factors. The superiority of the proposed model over existing models is that it is rigorously developed based on the instrumental scanning principles and target spatial geometry and can be used for different instruments, scanning scenes, targets, and scanning parameters. The proposed model is verified by a series of indoor quantitative control experiments and three natural scenes with totally different instruments and scanning conditions. The results show that the proposed model can accurately simulate the density variation at different scanning geometries with an average correlation coefficient of 0.9957 between the calculated and actual densities. The effects of all influencing factors on density are significantly removed in complex natural scenes by the proposed model where the coefficient of variation of the density from a homogeneous surface after correction is reduced by 72.31%, on the average. The proposed model exhibits good performance in terms of the feasibility, effectiveness, and generality and can derive a corrected density value that is a proxy of target geometric size. Additionally, the proposed model can be conducted on each scan individually before the co-registration in the actual TLS campaign with a number of single scans, which has tremendous application value in facilitating TLS data interpretation.