Distortion Artifacts Research Articles

Importance of k-space trajectory in echo-planar myocardial tagging at rest and during dobutamine stress.

Hybrid fast gradient echo/echo-planar imaging (FGRE-EPI) can be used to increase temporal resolution, enhance tag contrast, and/or decrease scan time for breathhold myocardial tagging. However, off-resonance effects and motion can lead to local phase discontinuities in FGRE-EPI raw data when a conventional interleaved bottom-up k-space trajectory is used. These discontinuities can be particularly problematic for myocardial tagging, where the image energy is not only concentrated near the k-space origin, but is also concentrated in multiple spectral peaks centered throughout k-space. In this study, tag distortion artifacts in FGRE-EPI tagging due to off-resonance and velocity-induced phase discontinuities were characterized at rest and dobutamine stress, and the flyback and gradient moment smoothing (GMS) methods were shown to reduce these artifacts. For the specific parameters used in this study, flyback and GMS resulted in improved image quality at rest and stress, increased myocardium-tag contrast-to-noise ratio (11.4 +/- 2.1 vs. 10.0 +/- 2.9, P < 0.01 at rest; 11.1 +/- 1.8 vs. 8.1 +/- 2.4, P < 0.01 at stress), and reduced full width at half maximum of the tag profile (3.6 vs. 3.8 pixels at rest; 4.0 vs. 5.1 pixels at stress) compared to the conventional trajectory. A limitation of the improved trajectory is a parameter-dependent decrease in data acquisition efficiency. For the specific imaging protocol used, the repetition time of the improved trajectory increased by 36% compared to the conventional trajectory.

Time-series analysis of MRI intensity patterns in multiple sclerosis

In progressive neurological disorders, such as multiple sclerosis (MS), magnetic resonance imaging (MRI) follow-up is used to monitor disease activity and progression and to understand the underlying pathogenic mechanisms. This article presents image postprocessing methods and validation for integrating multiple serial MRI scans into a spatiotemporal volume for direct quantitative evaluation of the temporal intensity profiles. This temporal intensity signal and its dynamics have thus far not been exploited in the study of MS pathogenesis and the search for MRI surrogates of disease activity and progression. The integration into a four-dimensional data set comprises stages of tissue classification, followed by spatial and intensity normalization and partial volume filtering. Spatial normalization corrects for variations in head positioning and distortion artifacts via fully automated intensity-based registration algorithms, both rigid and nonrigid. Intensity normalization includes separate stages of correcting intra- and interscan variations based on the prior tissue class segmentation. Different approaches to image registration, partial volume correction, and intensity normalization were validated and compared. Validation included a scan–rescan experiment as well as a natural-history study on MS patients, imaged in weekly to monthly intervals over a 1-year follow-up. Significant error reduction was observed by applying tissue-specific intensity normalization and partial volume filtering. Example temporal profiles within evolving multiple sclerosis lesions are presented. An overall residual signal variance of 1.4% ± 0.5% was observed across multiple subjects and time points, indicating an overall sensitivity of 3% (for axial dual echo images with 3-mm slice thickness) for longitudinal study of signal dynamics from serial brain MRI.

Bite mark documentation and analysis: the forensic 3D/CAD supported photogrammetry approach

Bite mark identification is based on the individuality of a dentition, which is used to match a bite mark to a suspected perpetrator. This matching is based on a tooth-by-tooth and arch-to-arch comparison utilising parameters of size, shape and alignment. The most common method used to analyse bite mark are carried out in 2D space. That means that the 3D information is preserved only two dimensionally with distortions. This paper presents a new 3D documentation, analysis and visualisation approach based on forensic 3D/CAD supported photogrammetry (FPHG) and the use of a 3D surface scanner. Our photogrammetric approach and the used visualisation method is, to the best to our knowledge, the first 3D approach for bite mark analysis in an actual case. The documentation has no distortion artifacts as can be found with standard photography. All the data are documented with a metric 3D measurement, orientation and subsequent analysis in 3D space. Beside the metrical analysis between bite mark and cast, it is possible using our method to utilise the topographical 3D feature of each individual tooth. This means that the 3D features of the biting surfaces and edges of each teeth are respected which is—as shown in our case—very important especially in the front teeth which have the first contact to the skin. Based upon the 3D detailed representation of the cast with the 3D topographic characteristics of the teeth, the interaction with the 3D documented skin can be visualised and analysed on the computer screen.

Real-time autoshimming for echo planar timecourse imaging.

Head motion within an applied magnetic field alters the effective shim within the brain, causing geometric distortions in echo planar imaging (EPI). Even if subtle, change in shim can lead to artifactual signal changes in timecourse EPI acquisitions, which are typically performed for functional MRI (fMRI) or diffusion tensor imaging. Magnetic field maps acquired before and after head motions of clinically realistic magnitude indicate that motion-induced changes in magnetic field may cause translations exceeding 3 mm in the phase-encoding direction of the EPI images. The field maps also demonstrate a trend toward linear variations in shim changes as a function of position within the head, suggesting that a real-time, first-order correction may compensate for motion-induced changes in magnetic field. This article presents a navigator pulse sequence and processing method, termed a "shim NAV," for real-time detection of linear shim changes, and a shim-compensated EPI pulse sequence for dynamic correction of linear shim changes. In vivo and phantom experiments demonstrate the detection accuracy of shim NAVs in the presence of applied gradient shims. Phantom experiments demonstrate reduction of geometric distortion and image artifact using shim-compensated EPI in the presence of applied gradient shims. In vivo experiments with intentional interimage subject motion demonstrate improved alignment of timecourse EPI images when using the shim NAV-detected values to update the shim-compensated EPI acquisition in real time.

Artifact Removal Procedure Distorts Multifocal Electroretinogram

Purpose: To study whether the Artifact Removal procedure available for eliminating artifacts in multifocal electroretinograms (mERG) works correctly or not. Methods: A test response was made using a photo-diode circuit. mERGs were recorded from 3 well-trained normal subjects using the Veris III system, and were then analyzed by the procedure that is included in the Veris Science (Artifact Removal) software program. The stimuli consisted of densely arranged arrays of 103 or 37 hexagonal elements. It took a total of 8 minutes to obtain one mERG record, and 16 sessions were required to complete this record. The first-order as well as the second-order kernel response components were extracted by Veris Science software, and the Artifact Removal procedure was used for both components. Results: The Artifact Removal procedure influenced both the test response on the center element as well as the neighboring traces just around the test response. After the repetitions of the Artifact Removal procedure, the shape of the test response changed considerably. Some of the traces of the second-order kernel response components elicited from a normal subject changed irregularly when the Artifact Removal procedure was repeatedly used. The noise increased at the first iteration of the Artifact Removal procedure. Conclusion: This procedure has been considered useful for eliminating artifact distortion in mERG, but should be carefully checked by well-established testing methods before clinical use.

Evaluation of Contrast Enhancement by Digital Equalization in Digital Mammography

Purpose: This study evaluated an algorithm based on a method of contrast enhancement by digital equalization (CEDE). Method: The algorithm was designed to enhance image contrast by employing digital equalization of digital mammograms. The CEDE algorithm was tested using ten mammograms with cancer (13 lesions) taken the University of South Florida data base, together with eight mammograms which only contained benign lesions. Three readers compared the processed images with the original mammograms for lesion conspicuity. A five point ranking scale was employed where a score of 3 corresponded to equal lesion visibility, ranks > 3 corresponded to superior lesion visibility, whereas ranks < 3 corresponded to markedly inferior lesion visibility. Results: The mean observer score for all lesions was always at least equal to that of the original digital mammogram (i.e., 3 or greater), and there was no evidence of any image distortion or other image processing artefacts. The mean rank (± standard deviation) for the 13 malignant lesions was 3.52 ± 0.38. The corresponding rank for the eight benign lesions was 3.33 ± 0.26. These differences were statistically significant in terms of standard error. Conclusion: The CEDE algorithm is capable of significantly enhancing lesion contrast in digital mammograms and our preliminary results indicate that this algorithm merits additional refinement and further (objective) evaluation.

Microimaging at 14 Tesla Using GESEPI for Removal of Magnetic Susceptibility Artifacts in T2*-Weighted Image Contrast

Inmagnetic resonance imaging (MRI), T2*-weighted contrast is significantly enhanced by extremely high magnetic field strength, offering broad potential applications. However, the T2*-weighted image contrast distortion and signal loss artifact arising from discontinuities of magnetic susceptibility within and around the sample are also increased, limiting utilization of high field systems for T2*-weighted contrast applications. Due to the B0 dependence of the contrast distortions and signal losses, and the heterogeneity of magnetic susceptibility in biological samples, magnetic susceptibility artifacts worsen dramatically for in vivo microimaging at higher fields. Practical applications of T2*-sensitive techniques enhanced by higher magnetic fields are therefore challenged. This report shows that magnetic susceptibility artifacts dominate T2*-weighted image contrast at 14 T, and demonstrates that the GESEPI (gradient echo slice excitation profile imaging) technique effectively reduces or eliminates these artifacts at long TE in the highest field (14 T) currently available for 1H imaging.

Effect of silicone breast implants on myocardial imaging.

The authors investigated the effect of silicone breast implants (SBIs) on thallium-201 myocardial perfusion scintigraphy. Twenty-nine patients with SBIs and 14 control subjects were included in the study. All of the participants underwent a resting thallium-201 myocardial perfusion study. Comparison of the thallium images between patients with SBI and control subjects was done both visually and semiquantitatively. On visual analysis, neither image distortion nor image artifact attributable to the SBIs was noted. Semiquantitative analysis revealed that in patients with SBIs, the uptake values of the anteroseptal, anterolateral, lateroanterior, and lateroinferior myocardial walls was slightly, but significantly, lower than those of control subjects: anteroseptal, 87.7+/-6.0% vs. 92.8+/-5.1%; anterolateral, 88.0+/-5% vs. 92.6+/-4.5%; lateroanterior, 87.4+/-5% vs. 91.4+/-4.7%; and lateroinferior, 86.0+/-6.0% vs. 91.7+/-7.0%. The uptake value of the remaining walls in patients with SBIs were not significantly different than the control subjects. In conclusion, SBIs did cause a significant decrease in uptake values of the myocardial walls, which should be considered during interpretation of the images.

An Optimized Pulse Sequence for Isotropically Weighted Diffusion Imaging

Single-shot echo-planar imaging is becoming the most widely used technique for magnetic resonance diffusion imaging, since it enables measurement of diffusion coefficients in human brain without motion artifacts. However, its reliability is limited by geometrical distortions due to eddy currents. In this report, an isotropically weighted echo-planar pulse sequence, optimized to give the maximum signal-to-noise ratio in the computed trace image and designed to produce inherently low distortions, is presented. It is also shown how the residual translational distortion can be easily characterized and removed by postprocessing. A full characterization of the distortion artifact involves a few measurements on a phantom, in order to estimate the distortion as a function of slice orientation, which can then be used to correct any slice orientation. Results of applying the image translation correction to data collected from a patient are presented.

The Effects of Digitization on Nonstationary Stochastic Signals with Applications to Pulsar Signal Baseband Recording

ABSTRACTThe process of digitizing a stochastic signal introduces systematic distortions into the resulting digitized data. Further processing of these data may result in the appearance of unwanted artifacts, especially when the input signal was generated by a nonstationary stochastic process. In this paper the magnitude of these distortions are calculated analytically and the results are applied to a specific example found in pulsar signal processing. A pulsar signal is an excellent example of a nonstationary stochastic process. When analyzing pulsar data, the effects of interstellar medium (ISM) dispersion must be removed by digitally filtering the received signal. The distortions introduced through the digitization process cause unwanted artifacts to appear in the final “dedispersed” signal. These artifacts are demonstrated using actual 2‐bit (4‐level) digitized data of the pulsar PSR B0833−45 (Vela). Techniques are introduced that simultaneously minimize these artifacts and maximize the signal‐to‐noise ratio of the digitized data. The distortion analysis and artifact removal techniques described in this paper hold for an arbitrary number of input digitization thresholds (i.e., number of bits). Also presented are tables of the optimum digitizer thresholds for both uniform and nonuniform input threshold digitizers.

Effect of radiographic positioning on interpretation of cubital joint congruity in dogs

Abstract Objective To ascertain effects of x-ray beam centering and limb position on apparent congruity of a normal cubital joint (elbow). Animals 6 skeletally mature male Treeing Walker Coonhounds without physical, radiographic, or gross evidence of elbow abnormalities. Procedure Relative movement among humerus, radius, and ulna and measured joint space width on mediolateral and craniocaudal radiographic views was compared, using various x-ray beam centering and limb positions. Results Highest agreement and greatest certainty on subjective determination of congruity was for the flexed 90° mediolateral radiographic view with the x-ray beam centered on the elbow. Distortion artifact of the proximal ulnar measurements was significant when the x-ray beam was centered on the midpoint of the radius. On the mediolateral view, the humeroradial joint space became significantly wide when the elbow was flexed. On the craniocaudal view, maximal humeroradial joint space width was obtained when the x-ray beam bisected the angle of the joint or was angled +30° toward the humerus. Conclusions Artifact distortion of joint width affected objective and subjective assessment of elbow congruity when the limb was placed in extreme flexion or extension or when the x-ray beam was not centered over the area of interest. Optimal visualization of the humeroradial joint space on the craniocaudal view was achieved when the x-ray beam bisected the angle of the elbow or was slightly angled toward the humerus. Clinical Relevance Elbow congruity was best assessed on the flexed 90° lateral radiographic view with the x-ray beam centered on the joint. (Am J Vet Res 1998;59:1351–1357)

Abstracts of Literature

Abstracts of Literature

Macromolecular intravenous contrast agent for MR lymphography: characterization and efficacy studies.

To determine the pharmacokinetic and magnetic resonance (MR) imaging properties of diethylenetriaminepentaacetic acid (DTPA) conjugated with a polyglucose-associated macrocomplex (PGM), which accumulates in lymph nodes. In 124 normal and 20 tumor-bearing rats, Gd-DTPA PGM was administered intravenously in doses of 2, 10, 20 mumol gadolinium per kilogram of tissue. Mean blood half-life was 2 hours. Maximum accumulation in peripheral (33.0% injected dose [ID]/g +/- 16.2 [standard deviation]) and central lymph nodes (63.2% ID/g +/- 16.5) was observed within 24 hours after administration. The optimum dose range was 10-20 mumol Gd/kg in rats. At 24 hours after administration of 20 mumol Gd/kg, the signal-to-noise ratio increased from 30.9 +/- 0.4 to 83.2 +/- 5.2 in normal lymph nodes (P < .001). Differentiation between normal and metastatic lymph nodes was improved. When labeled with Gd-DTPA, the PGM-based graft copolymer significantly increases signal intensity at MR imaging of normal but not metastatic lymph nodes without causing distortion artifacts.

An Improved Stimulation Technique for the Human Frequency-Following Response (FFR)

ABSTRACT.A critical evaluation of shielded headphones to be used for stimulation in order to elicit the frequency-following response (FFR) revealed a tradeoff between stimulus distortion and stimulus-induced artifact. Hence an audio delay line was developed, which avoids stimulus artifact and minimizes stimulus distortions for moderate to high intensity stimuli with frequencies between 100 and 1,000 Hz. Caution is advocated if headphone stimulation is used for FFR recording.

Wavefront amplitude distortion and image artifacts: Qing Zhu and Bernard D. Steinberg, Valley Forge Research Center, The Moore School of Electrical Engineering, Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104

Wavefront amplitude distortion and image artifacts: Qing Zhu and Bernard D. Steinberg, Valley Forge Research Center, The Moore School of Electrical Engineering, Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104

The Effect of Suppression of the Distortion Artifact during Transcutaneous Pacing on the Shape of the QRS Complex

The quality of the ECG recording during transcutaneous pacing was evaluated in six healthy volunteers. The transcutaneous pacing stimulator was an NP4D special unit to which was attached an electronic suppressor of artifact generated by the transcutaneous stimulating impulse. The relationship between this suppression of artifact distortion and the resulting QRS complex was analyzed. The results revealed that the suppression time (described as a 2-mm oscillation from 100-Hz frequency) was required for ECG distortion elimination and that this is dependent on the threshold of ventricular pacing. The width of the resulting QRS complex diminishes as the suppression is extended over 110 ms. These results suggest the necessity of individual adjustment of the suppression time so that the efficacy of transcutaneous pacing is adequately assessed.

Tape-transfer Technique for Large Paraffin Blocks

AbstractA tape-transfer technique for reducing technical problems associated with the sectioning of large paraffin blocks is described. Ten different tapes were attached to the surface of tissue blocks containing decalcified bone, and sections were cut with a microtome. The tapes were evaluated for adhesion to the block face, ease of handling during sectioning, and removal before staining without disruption of the tissue. 3M Company tape numbers 3710–2 and 3750-G were found to be the most satisfactory. This tape-transfer technique yields excellent quality sections with greater ease, less distortion, and fewer artifacts than unsupported sections do. ( The J Histotechnol 13:207, 1990)

Image restoration by Wiener deconvolution in limited-view computed tomography

In many applications of computed tomography, we cannot acquire the projection data at all angles evenly spaced over 360°. In such cases, the computed tomography images reconstructed using a limited number of projections, measured over a narrow angle range, are characterized by approximately elliptical distortion along the view angles used and poor contrast at angles not used (anisotropic resolution). This systematic geometric distortion is caused by the 2-D point spread function of the reconstruction process. In this paper, we show that such geometric distortion and other artifacts introduced in the reconstruction process can be reduced substantially by deconvolution performed via Wiener filtering using a priori knowledge derived from the given projections. The 2-D system transfer function used in the deconvolution is obtained from the reconstruction of a test image by the same reconstruction algorithm which has been used for reconstructing the unknown object.

Light Microscopic Histochemistry on Plastic Sections

As compared with conventional paraffin, celloidin, and frozen sections, semithin plastic sections offer a superior quality of the light microscopic image in terms of better resolution, absence of distortion and shrinkage artifacts, and suitability for calcified tissues. Application of histochemical methods to such sections often encounters, however, serious difficulties resulting from a considerably reduced reactivity of plastic-embedded biological material. Factors involved include a poor penetration of reagents into plastic embedding media due to a steric or hydrophobic hindrance, as well as a blockade of the reactive chemical groups in the sample due to interactions with fixatives and plastics. Embedding in polar (hydrophilic) plastics, such as glycol methacrylate, permits carrying out a large number of histochemical reactions, including the demonstration of enzymatic activities, directly on sections, but is less suitable for combined light/electron microscopic studies because of an imperfect ultrastructural preservation of tissues. Embedding in nonpolar epoxy resins, particularly if combined with a double aldehyde-osmium fixation, results in a high quality ultrastructure but almost fully inhibits the histochemical reactivity of the embedded material. In order to restore this reactivity, i.e. to unmask chemical groups bound by the polymerized resin, semithin epoxy sections require the removal of the embedding matrix by alkoxides prior to the histochemical procedure. Additional steps are also often necessary: treatment of osmium-fixed sections with oxidative agents, e.g., hydrogen peroxide or periodate which reoxidize the bound osmium and remove it from tissue, and a controlled proteolytic digestion, especially useful in immunocytochemical studies, which probably cleaves the bonds between the primary aldehyde fixative, and the reactive sites. This article reviews histochemical methods which have been successfully applied to plastic-embedded material. Using polar methacrylates and/or nonpolar epoxy resins as embedding media, it has been possible to demonstrate proteins and aminoacid residues, carbohydrates, lipids, nucleic acids, biogenic amines, inorganic ions, and some enzymes, although the spectrum of methods found as suitable for plastic-embedded material is far narrower than that available for paraffin or frozen sections.(ABSTRACT TRUNCATED AT 400 WORDS)

Backscatter and Attenuation Imaging from Ultrasonic Scanning in Medicine

Images of backscatter, attenuation, and frequency dependence of attenuation are obtained based on a three-parameter model and computation techniques described in this paper. There are several critical sources of error: backscatter speckle, beamwidth distortion, and cross-coupling artifacts between attenuation and backscatter. An iterative method of imaging and filtering is developed which effectively reduces these errors. Stability of the numerical solution involving the large number of unknowns is obtained by image iteration as opposed to parameter iteration along individual transmitter rays. This method incorporates three basic functional aspects: (1) multiple scans to reduce speckle, beamwidth distortions, and certain cross-coupling artifacts, (2) pre-image filtering to decrease beam distortions and post-image filtering to reduce cross-coupling artifacts, and (3) proper sequencing of image reconstruction and filtering. Backscatter images formed by this image iteration method are significantly superior to standard B-scan images. Further, the image iteration method yields three images of the same scan field. The present investigation is based on simulated echo data from cyst-like and complex targets.