Phantom based qualitative and quantitative evaluation of artifacts in MR images of metallic hip prostheses

PurposeTo develop methods for qualitative and quantitative evaluation of MRI artifacts near metallic prostheses, and to compare the efficiency of different artifact suppression techniques with different types of hip prostheses.MethodsThree hip prostheses of cobalt-chromium, stainless steel, and titanium were embedded in agarose gel together with a rectilinear grid. Coronal MR images of the prostheses were acquired on a 1.5T scanner. Three pulse sequences were evaluated; TSE: a high-bandwidth turbo spin echo; VAT: TSE with view angle tilting, SEMAC: TSE with both VAT and slice distortion correction (6, 10 or 16 z-phase-encoding steps). Through-plane distortions were assessed as the length of visible gridlines, in-plane artifacts as the artifact area, and total artifacts by subtraction of an ideal, undistorted image from the actual image.ResultsVAT reduced in-plane artifacts by up to 50% compared to TSE, but did not reduce through-plane artifacts. SEMAC reduced through-plane artifacts by 60–80% compared to TSE and VAT. SEMAC in-plane artifacts were from 20% higher (6 encoding steps) to 50% lower (16 steps) than VAT. Total artifacts were reduced by 60–80% in the best sequence (SEMAC, 16 steps) compared to the worst (TSE). The titanium prosthesis produced 3–4 times lower artifact scores than the other prostheses.ConclusionsA rectilinear grid phantom is useful for qualitative and quantitative evaluation of artifacts provoked by different MRI protocols and prosthesis models. VAT and SEMAC were superior to TSE with high bandwidth. A proper number of z-encoding steps in SEMAC was critical. The titanium prosthesis caused least artifacts.     

A Cache Of Scrapers Near Crow Butte,Nebraska

Abstract

A cache of scrapers and other, possibly associated artifacts found near Crow Butte in northwestern Nebraska is of undetermined affiliation but probably can be placed in the pre-ceramic period.     

Iterative image reconstruction using modified non-local means filtering for limited-angle computed tomography

PurposeLimited-angle CT imaging is an effective technique to reduce radiation. However, existing image reconstruction methods can effectively reduce streak artifacts but fail to suppress those artifacts around edges due to incomplete projection data. Thus, a modified NLM (mNLM) based reconstruction method is proposed.MethodsSince the artifacts around edges mainly exist in local position, it is possible to restore the true pixels in artifacts using pixels located in artifacts-free regions. In each iteration, mNLM is performed on image reconstructed by ART followed by positivity constraint. To solve the problem caused by ART-mNLM that there is undesirable information that may appear in the image, ART-TV is then utilized in the following iterative process after ART-mNLM iterates for a number of iterations. The proposed algorithm is named as ART-mNLM/TV.ResultsSimulation experiments are performed to validate the feasibility of algorithm. When the scanning range is [0, 150°], our algorithm outperforms the ART-NLM and ART-TV with more than 40% and 29% improvement in terms of SNR and with more than 58% and 49% reduction in terms of MAE. Consistently, reconstructed images from real projection data also demonstrate the effectiveness of presented algorithm.ConclusionThis paper uses mNLM which benefits from redundancy of information across the whole image, to recover the true value of pixels in artifacts region by utilizing pixels from artifact-free regions, and artifacts around the edges can be mitigated effectively. Experiments show that the proposed ART-mNLM/TV is able to achieve better performances compared to traditional methods.     

Investigation of the Swift Bird House (39DW233) in the Oahe Reservoir,South Dakota

Abstract

The Swift Bird House (39DW233) is a prehistoric structure of indeterminate characteristics. The associated artifacts are directed to a Northern Plains horticulture and hunting complex. A tenuous relationship to the Anoka Focus and a possible occupation date of about A. D. 1500 is suggested for the site     

On the Ewald Artifacts in Computer Simulations. The Test-Case of the Octaalanine Peptide With Charged Termini

Abstract

The treatment of electrostatic interactions in molecular simulations is of fundamental importance. Ewald and related methods are being increasingly used to the detriment of cutoff schemes, which are known to produce several artifacts. A potential drawback of the Ewald method is the spatial periodicity that is imposed to the system, which could produce artifacts when applied in the simulation of liquids. In this work we analyze the octaalanine peptide with charged termini in explicit solvent, for which severe effects due to the use of Ewald sums were predicted using continuum electrostatics. Molecular Dynamics simulations for a total of 158 nanoseconds were performed in cells of different sizes. From the comparison of the results of different system sizes, no significant periodicity-induced artifacts were observed. It is argued that in current biomolecular simulations, the incomplete sampling is likely to affect the results to a larger extent than the artifacts induced by the use of Ewald sums.     

The Bentzen-Kaufmann Cave Site 48 SH 301

Abstract

A small cave in northeastern Wyoming yielded a number of bone fragments and artifacts representing an intermittent cultural deposition since the Early Period. A radiocarbon age of 6975 ± 275 years was obtained for one of the oldest components in the site.     

Experimental Artifacts and the Analysis of Ligand Binding Data: Results of a Computer Simulation

Abstract

Use of computerized analysis techniques for ligand binding data have recently become generally available, and are now used quite routinely. When used appropriately, these tools can improve the precision of the estimated parameters for binding affinity, K, and capacity, R. Furthermore, such programs can also calculate the uncertainty of the estimates e.g., as a percent coefficient of variation (%CV). However, because of unmeasured variability in specific activity, tracer purity, counting efficiency, counter background, efficiency of separation, etc., the actual uncertainty in the parameters K and R is usually much larger than stated. In an attempt to examine the effects of such artifacts, we have developed a computer program which simulates data arising from a number of commonly used experimental designs, and then intentionally distorted with each of these artifacts. Finally, the data are converted to B/F and B and plotted in the conventional Scatchard plot. Distortions revealed in this graph are indicative of the effect each artifact has on the parameter estimates. The computer program is generally written to simulate the binding of 2 or more ligands to one, two or many classes of independent or cooperative specific sites as well as to nonspecific sites. Thus, the program is applicable in a wide variety of situations. Results show that low tracer purity (“bindability”) or low filtration efficiency will significantly alter the measured R value. Poorly determined specific radioactivity may significantly alter the measured K value as well. Imprecise measurement of machine background may result in the specious appearance of positive cooperativity, or of additional high or low affinity classes of binding sites. Finally, under some circumstances, it is possible to detect and correct for the presence of these artifacts.     

Mathematical Modeling and Numerical Simulation of Magnetic Susceptibility Artifacts in Magnetic Resonance IMAGING*

Abstract

The technique used to recognise information in Magnetic Resonance Imaging (MRI) is based on electromagnetic fields. A linearly varying field (around 10^?2 Tesla per meter) is added to a strong homogeneous magnetic field (order of magnitude of approximately one Tesla). When these fields are disturbed by the presence of a paramagnetic material, in the sample for instance, the resulting image is usually distorted, these distortions being termed artifacts. Our goal is to present a method, assuming the field disturbances are known, to construct the resulting images. A mathematical model of the MRI process is developed. The way the images are distorted in intensity and shape is explained and an algorithm to simulate magnetic susceptibility artifacts is deduced.     

Effect of number of views on cross-sectional Compton imaging: A fundamental study with backprojection

IntroductionWe have been developing a medical imaging technique using a Compton camera, which is expected to reconstruct three-dimensional images. If the number of views is not sufficient, star-shaped artifacts (streak artifacts) could arise in cross-sectional images. Therefore, we estimated the point spread function (PSF) of cross-sectional Compton images and the effect of the number of views by Monte Carlo simulations and experimental studies.Materials and methodsA cross-sectional Compton image was reconstructed using a dataset comprising 719 view directions and PSF was analyzed using a radial distribution. The peak height, full width at half maximum (FWHM), background (BG), and residual sum of squares (RSS) were calculated from the obtained PSF. In addition, RSSs were plotted against the number of views to estimate the required number to suppress star-shaped artifacts.ResultsThere was no correlation found between the number of views and both FWHM (16 mm) and peak/BG ratio (∼1 × 10⁴). RSSs were reduced with the number of views and approached the minimum asymptotically. Correlation was observed between the required number of views and the number of Compton events used for image reconstruction.ConclusionWe determined the PSF of cross-sectional Compton images and the effect of the number of views on the images. The required number of views to suppress the star-shaped artifact is related to the square root of the number of Compton events used to reconstruct the image. From this study, we concluded that 21 or more views are required for clinical purposes.     

Salvage Archaeology at a Site Near Fort Thompson. South Dakota

Abstract

Construction of dams and reservoirs along the Missouri River has resulted in an accelerated program of archaeological field-work. Many sites have been surveyed, numerous others tested, and a lesser number extensively excavated. Surveys and test excavations will add to previously gathered data in terms of the geographical distribution of diagnostic artifacts, speculative routes of migration and cultural diffusion. The minutiae from the tested sites may offer reasons for the re-evaluation of established concepts or they may strengthen current archaeological interpretations.

The Farm School site, reported here, received limited tests. The artifact inventory suggests the presence of 2 ceramic components and contributes to information regarding the Fort Thompson and Campbell Creek foci in South Dakota.     

The West Island Site, 14PH10, A Keith Focus Plains Woodland Site in Kirwin Reservoir Phillips County,Kansas

Abstract

Salvage excavations at a badly eroded site, now an island in the Kirwin Reservoir of west-central Kansas, produced pottery and other artifacts indicative of the Keith Focus. Although the site remains undated, it should fall at ca. A. D. 600.     

Big-Game Hunting Artifacts from Minnesota

Abstract

A Browns Valley type point, a Scottsbluff type point, and four Cody knives, recovered as surface finds, give evidence of the presence of the Big-Game Hunting Tradition in east-central Minnesota. These artifacts, their location, and similar finds in the Minnesota-Wisconsin area are described.     

The extent of representation,the relationships of artifacts and the anthropology of art

This paper provides an account of the potential roles that artifacts can occupy and distinguishes their occurrence as instruments of representation from other types of relationship of artifact use. When artifacts are employed as instruments of representation there is a deliberately constructed causal connection between the artifact and what it represents. This is in contrast to relationships of association in which the artifact takes on features of the context of its occurrence. Anthropological studies of artifact use in the native societies of Africa, America, the Indian Subcontinent and the Pacific are reviewed by comparing “dominant functions” that artifacts can serve.     

A Description of Certain Engraved ar Tif Acts From the utz Oneota Site

Abstract

Engraved objects of various types have been found at the Utz Oneota Site in Saline County, Missouri. A number of these artifacts are described and illustrated in this article, with comparative data from other localities. They indicate a line of cultural contact and/ or trade running northwest and southeast, along the Missouri-Mississippi River System.     

A Progress Report on a Pre-Ceramic Site at Rattlesnake Draw Eastern New Mexico

Abstract

The Rattlesnake Draw site has produced both Paleo-Indian and Archaic artifacts in a concentrated area centering about a presently dry lake basin. In addition to the artifacts discovered, the investigators have found evidence of the construction of artificial wells during the archaic period. These wells are similar to those earlier discovered at Blackwater Draw, Locality One.     

Characterization of displacement forces and image artifacts in the presence of passive medical implants in low-field (<100 mT) permanent magnet-based MRI systems,and comparisons with clinical MRI systems

PurposeTo investigate the displacement forces and image artifacts associated with passive medical implants for recently-developed low-field (<100 mT) MRI systems, and to compare these with values from higher field strengths used for clinical diagnosis.MethodsSetups were constructed to measure displacement forces in a permanent magnet-based Halbach array used for in vivo MRI at 50 mT, and results compared with measurements at 7 T. Image artifacts were assessed using turbo (fast) spin echo imaging sequences for four different passive medical implants: a septal occluder, iliac stent, pedicle screw and (ferromagnetic) endoscopic clip. Comparisons were made with artifacts produced at 1.5, 3 and 7 T. Finally, specific absorption rate (SAR) simulations were performed to determine under what operating conditions the limits might be approached at low-field.ResultsDisplacement forces at 50 mT on all but the ferromagnetic implant were between 1 and 10 mN. Image artifacts at 50 mT were much less than at clinical field strengths for all passive devices, and with the exception of the ferromagnetic clip. SAR simulations show that very long echo train (>128) turbo spin echo sequences can be run with short inter-pulse times (5–10 ms) within SAR limits.ConclusionsThis work presents the first evaluation of the effects of passive implants at field strengths less than 100 mT in terms of displacement forces, image artifacts and SAR. The results support previous claims that such systems can be used safely and usefully in challenging enviroments such as the intensive care unit.     

Cherokee Mountain Rock Shelter

Abstract

Cherokee Mountain Rock Shelter is south of Denver, in Douglas County, Colorado, and overlooks Plum Creek near the small town of Sedalia. The shelter was formed in a sandstone conglomerate in an area of high bluffs and gently rolling hills. Three intermixed levels were excavated, with artifacts from each dating from the Late Prehistoric Period. A few sherds, suggestive of a Shoshonean occupation, were in the top two levels.     

Quantifying near metal visibility using dual energy computed tomography and iterative metal artifact reduction in a fracture phantom

PurposeTo quantitatively assess CT image quality and fracture visibility using virtual monochromatic imaging and iterative metal artifact reduction (iMAR) in a femoral bone fracture phantom with different fixation implants.MethodsA custom made phantom was scanned at 120-kVp and 140-kVp single-energy and 100/150-kVp dual-energy. Three stainless steel and two titanium implants with different thicknesses were placed on the phantom containing simulated one and two mm fractures. Single-energy CT images were reconstructed with and without iMAR, while DECT images were reconstructed at monochromatic energies between 70 and 190 keV. Non-metal scans were used as a reference. A Fourier power spectrum method and fracture model were used to analyze several anatomical areas.ResultsCT-value deviations of titanium implants were much lower compared to stainless steel implants. These deviations decreased for both DECT and iMAR. Fracture visibility, measured with the fracture model, improved the most when DECT was used while artifact reduction benefitted more from iMAR. The optimal monochromatic energy for metal artifact reduction, based on CT-value deviation, varied for each metal between 130 and 150 keV. The fracture model provided a signal-to-noise ratio for the near metal fracture visibility, providing the optimal keV.ConclusioniMAR and high keV monochromatic images extracted from DECT both reduce metal artifacts caused by different metal fixation implants. Quantitative femoral phantom results show that DECT is superior to iMAR regarding fracture visualization adjacent to metal fixation implants. The introduction of new artifacts when using iMAR impedes its value in near metal fixation implant imaging.