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Acceleration Factors (acceleration + factor)

Distribution by Scientific Domains

Medical Sciences	78%

Selected Abstracts

Re-calibration of Engelmaier's Model for Leadless, Lead-free Solder Attachments

QUALITY AND RELIABILITY ENGINEERING INTERNATIONAL, Issue 4 2007
Olli Salmela
Abstract In this paper, the solder attachment fatigue model created by Werner Engelmaier is re-calibrated in order to make it applicable in conjunction with leadless, lead-free solder attachments. Sn3.8Ag0.7Cu solder attached ball-grid-array components are addressed to three thermal cycling test profiles. Based on the results, both physical and statistical parameters are obtained and compared with the values relevant to tin,lead solder assemblies. The validity of the statistical distribution selection (two-parameter Weibull) is studied. Acceleration factors correlating different test profiles are obtained, and they are found to be only weakly dependent on the test vehicle type. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Could accelerated aging explain the excess mortality in patients with seropositive rheumatoid arthritis?

ARTHRITIS & RHEUMATISM, Issue 2 2010
Cynthia S. Crowson
Objective To determine whether the mortality pattern in patients with seropositive rheumatoid arthritis (RA) is consistent with the concept of accelerated aging, by comparing the observed mortality rates in patients with RA with the age-accelerated mortality rates from the general population. Methods A population-based inception cohort of patients with seropositive RA (according to the American College of Rheumatology 1987 criteria) was assembled and followed up for vital status until July 1, 2008. The expected mortality rate was obtained by applying the death rates from the general population to the age, sex, and calendar year distribution of the RA population. The observed mortality was estimated using Kaplan-Meier methods. Acceleration factors for the expected mortality were estimated in accelerated failure time models. Results A total of 755 patients with seropositive RA (mean age 55.6 years, 69% women) were followed up for a mean of 12.5 years, during which 315 patients died. The expected median survival was age 82.4 years, whereas the median survival of the RA patients was age 76.7 years. Results of statistical modeling suggested that, in terms of mortality rates, patients with RA were effectively 2 years older than actual age at RA incidence, and thereafter the patients underwent 11.4 effective years of aging for each 10 years of calendar time. Conclusion The overall observed mortality experience of patients with seropositive RA is consistent with the hypothesis of accelerated aging. The causes of accelerated aging in RA deserve further investigation. [source]

A graphical generalized implementation of SENSE reconstruction using Matlab

CONCEPTS IN MAGNETIC RESONANCE, Issue 3 2010
Hammad Omer
Abstract Parallel acquisition of Magnetic Resonance Imaging (MRI) has the potential to significantly reduce the scan time. SENSE is one of the many techniques for the reconstruction of parallel MRI images. A generalized algorithm for SENSE reconstruction and theoretical background is presented. This algorithm can be used for SENSE reconstruction for any acceleration factor between 2 and 8, for any Phase Encode direction (Horizontal or Vertical), with or without Regularization. The user can select a particular type of Regularization. A GUI based implementation of the algorithm is also given. Signal-to-noise ratio, artefact power, and g -factor map are used to quantify the quality of reconstruction. The effects of different acceleration factors on these parameters are also discussed. The GUI based implementation of SENSE reconstruction provides an easy selection of various parameters needed for reconstruction of parallel MRI images and helps in an efficient reconstruction and analysis of the quality of reconstruction. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 178,186, 2010. [source]

Parallel imaging of knee cartilage at 3 Tesla

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2007
Jin Zuo PhD
Abstract Purpose To evaluate the feasibility and reproducibility of quantitative cartilage imaging with parallel imaging at 3T and to determine the impact of the acceleration factor (AF) on morphological and relaxation measurements. Materials and Methods An eight-channel phased-array knee coil was employed for conventional and parallel imaging on a 3T scanner. The imaging protocol consisted of a T2-weighted fast spin echo (FSE), a 3D-spoiled gradient echo (SPGR), a custom 3D-SPGR T1rho, and a 3D-SPGR T2 sequence. Parallel imaging was performed with an array spatial sensitivity technique (ASSET). The left knees of six healthy volunteers were scanned with both conventional and parallel imaging (AF = 2). Results Morphological parameters and relaxation maps from parallel imaging methods (AF = 2) showed comparable results with conventional method. The intraclass correlation coefficient (ICC) of the two methods for cartilage volume, mean cartilage thickness, T1rho, and T2 were 0.999, 0.977, 0.964, and 0.969, respectively, while demonstrating excellent reproducibility. No significant measurement differences were found when AF reached 3 despite the low signal-to-noise ratio (SNR). Conclusion The study demonstrated that parallel imaging can be applied to current knee cartilage quantification at AF = 2 without degrading measurement accuracy with good reproducibility while effectively reducing scan time. Shorter imaging times can be achieved with higher AF at the cost of SNR. J. Magn. Reson. Imaging 2007;26:1001,1009. © 2007 Wiley-Liss, Inc. [source]

High-resolution renal MRA: Comparison of image quality and vessel depiction with different parallel imaging acceleration factors

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2006
Henrik J. Michaely MD
Abstract Purpose To investigate the image quality and vessel depiction of renal MRA with integrated parallel imaging techniques (iPAT) using acceleration factors of 2 and 3. Materials and Methods In this prospective study renal MRA was performed on 14 and 12 patients with acceleration factors of 3 and 2, respectively. For the MRA a 3D-GRE sequence with an acquired spatial resolution of 0.9 × 0.8 × 1.0 mm3 was applied (TR/TE = 3.79 msec/1.39 msec, FOV = 400 mm × 320 mm, acquired matrix = 512 × 384, flip angle = 25°) on a 32-channel 1.5T MR scanner. The acquisition time was 26 seconds with iPAT 2, and 19 seconds with iPAT3. All parameters other than acquisition time and acceleration factor were kept constant. To assess the signal-to-noise ratio (SNR) we performed repetitive phantom measurements using iPAT 2 and 3. The images were rated by two radiologists in terms of noise, artifacts, and the quality of vessel depiction for the proximal, segmental, and subsegmental renal artery. A Mann-Whitney U-test and kappa-test were used for statistical analysis. Results SNR decreased significantly with iPAT 3 in the phantom measurements. The two readers found no difference in noise, but significantly fewer artifacts with iPAT 3. The depiction of segmental vessels was significantly better for both readers with iPAT 3, and the subsegmental vessels were rated significantly better by one reader. iPAT 3 also resulted in a better interreader agreement. Conclusion The use of iPAT 3 for renal MRA enables a better depiction of the distal parts of the renal artery. The decrease in SNR is not diagnostically impairing. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc. [source]

Quiescent-interval single-shot unenhanced magnetic resonance angiography of peripheral vascular disease: Technical considerations and clinical feasibility

MAGNETIC RESONANCE IN MEDICINE, Issue 4 2010
Robert R. Edelman
Abstract We performed technical optimization followed by a pilot clinical study of quiescent-interval single-shot MR angiography for peripheral vascular disease. Quiescent-interval single-shot MR angiography acquires data using a modified electrocardiographic (ECG)-triggered, fat suppressed, two-dimensional, balanced steady-state, free precession pulse sequence incorporating slice-selective saturation and a quiescent interval for maximal enhancement of inflowing blood. Following optimization at 1.5 T, a pilot study was performed in patients with peripheral vascular disease, using contrast-enhanced MR angiography as the reference standard. The optimized sequence used a quiescent interval of 228 ms, ,/2 catalyzation of the steady-state magnetization, and center-to-out partial Fourier acquisition with parallel acceleration factor of 2. Spatial resolution was 2-3mm along the slice direction and 0.7-1mm in-plane before interpolation. Excluding stented arterial segments, the sensitivity, specificity, and positive and negative predictive values of quiescent-interval single-shot MR angiography for arterial narrowing greater than 50% or occlusion were 92.2%, 94.9%, 83.9%, and 97.7%, respectively. Quiescent-interval single-shot MR angiography provided robust depiction of normal peripheral arterial anatomy and peripheral vascular disease in less than 10 min, without the need to tailor the technique for individual patients. Moreover, the technique provides consistent image quality in the pelvic region despite the presence of respiratory and bowel motion. Magn Reson Med 63:951,958, 2010. © 2010 Wiley-Liss, Inc. [source]

Spectral phase-corrected GRAPPA reconstruction of three-dimensional echo-planar spectroscopic imaging (3D-EPSI)

MAGNETIC RESONANCE IN MEDICINE, Issue 5 2007
Xiaoping Zhu
Abstract MR spectroscopic (MRS) images from a large volume of brain can be obtained using a 3D echo-planar spectroscopic imaging (3D-EPSI) sequence. However, routine applications of 3D-EPSI are still limited by a long scan time. In this communication, a new approach termed "spectral phase-corrected generalized autocalibrating partially parallel acquisitions" (SPC-GRAPPA) is introduced for the reconstruction of 3D-EPSI data to accelerate data acquisition while preserving the accuracy of quantitation of brain metabolites. In SPC-GRAPPA, voxel-by-voxel spectral phase alignment between metabolite 3D-EPSI from individual coil elements is performed in the frequency domain, utilizing the whole spectrum from interleaved water reference 3D-EPSI for robust estimation of the zero-order phase correction. The performance of SPC-GRAPPA was compared with that of fully encoded 3D-EPSI and conventional GRAPPA. Analysis of whole-brain 3D-EPSI data reconstructed by SPC-GRAPPA demonstrates that SPC-GRAPPA with an acceleration factor of 1.5 yields results very similar to those obtained by fully encoded 3D-EPSI, and is more accurate than conventional GRAPPA. Magn Reson Med 57:815,820, 2007. © 2007 Wiley-Liss, Inc. [source]

Effects of inductive coupling on parallel MR image reconstructions

MAGNETIC RESONANCE IN MEDICINE, Issue 3 2004
Michael A. Ohliger
Abstract Theoretical arguments and experimental results are presented that characterize the impact of inductive coupling on the performance of parallel MRI reconstructions. A simple model of MR signal and noise reception suggests that the intrinsic amount of spatial information available from a given coil array is unchanged in the presence of inductive coupling, as long as the sample remains the dominant source of noise for the coupled array. Any loss of distinctness in the measured coil sensitivities is compensated by information stored in the measured noise correlations. Adjustments to the theory are described to account for preamplifier noise contributions. Results are presented from an experimental system in which preamplifier input impedances are systematically adjusted in order to vary the level of coupling between array elements. Parallel image reconstructions using an array with four different levels of coupling and an acceleration factor up to six show average SNR changes of ,7.6% to +7.5%. The modest changes in overall SNR are accompanied by similarly small changes in g-factor. These initial results suggest that moderate amounts of inductive coupling should not have a prohibitive effect on the use of a given coil array for parallel MRI. Magn Reson Med 52:628,639, 2004. © 2004 Wiley-Liss, Inc. [source]

Real-time accelerated interactive MRI with adaptive TSENSE and UNFOLD,

MAGNETIC RESONANCE IN MEDICINE, Issue 2 2003
Michael A. Guttman
Abstract Reduced field-of-view (FOV) acceleration using time-adaptive sensitivity encoding (TSENSE) or unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD) can improve the depiction of motion in real-time MRI. However, increased computational resources are required to maintain a high frame rate and low latency in image reconstruction and display. A high-performance software system has been implemented to perform TSENSE and UNFOLD reconstructions for real-time MRI with interactive, on-line display. Images were displayed in the scanner room to investigate image-guided procedures. Examples are shown for normal volunteers and cardiac interventional experiments in animals using a steady-state free precession (SSFP) sequence. In order to maintain adequate image quality for interventional procedures, the imaging rate was limited to seven frames per second after an acceleration factor of 2 with a voxel size of 1.8 × 3.5 × 8 mm. Initial experiences suggest that TSENSE and UNFOLD can each improve the compromise between spatial and temporal resolution in real-time imaging, and can function well in interactive imaging. UNFOLD places no additional constraints on receiver coils, and is therefore more flexible than SENSE methods; however, the temporal image filtering can blur motion and reduce the effective acceleration. Methods are proposed to overcome the challenges presented by the use of TSENSE in interactive imaging. TSENSE may be temporarily disabled after changing the imaging plane to avoid transient artifacts as the sensitivity coefficients adapt. For imaging with a combination of surface and interventional coils, a hybrid reconstruction approach is proposed whereby UNFOLD is used for the interventional coils, and TSENSE with or without UNFOLD is used for the surface coils. Magn Reson Med 50:315,321, 2003. Published 2003 Wiley-Liss, Inc. [source]

Long-term Reliability Prediction of 935 nm LEDs Using Failure Laws and Low Acceleration Factor Ageing Tests

QUALITY AND RELIABILITY ENGINEERING INTERNATIONAL, Issue 6 2005
Y. Deshayes
Abstract Numerous papers have already reported various results on electrical and optical performances of GaAs-based materials for optoelectronic applications. Other papers have proposed some methodologies for a classical estimation of reliability of GaAs compounds using life testing methods on a few thousand samples over 10,000 hours of testing. In contrast, fewer papers have studied the complete relation between degradation laws in relation to failure mechanisms and the estimation of lifetime distribution using accelerated ageing tests considering a short test duration, low acceleration factor and analytical extrapolation. In this paper, we report the results for commercial InGaAs/GaAs 935 nm packaged light emitting diodes (LEDs) using electrical and optical measurements versus ageing time. Cumulative failure distributions are calculated using degradation laws and process distribution data of optical power. A complete methodology is described proposing an accurate reliability model from experimental determination of the failure mechanisms (defect diffusion) for this technology. Electrical and optical characterizations are used with temperature dependence, short-duration accelerated tests (less than 1500 h) with an increase in bias current (up to 50%), a small number of samples (less than 20) and weak acceleration factors (up to 240). Copyright © 2005 John Wiley & Sons, Ltd. [source]

A graphical generalized implementation of SENSE reconstruction using Matlab

Parallel transmit and receive technology in high-field magnetic resonance neuroimaging

INTERNATIONAL JOURNAL OF IMAGING SYSTEMS AND TECHNOLOGY, Issue 1 2010
Andrew G. Webb
Abstract The major radiofrequency engineering challenges of high-field MR neuroimaging are as follows: (1) to produce a strong, homogeneous transmit B1 field, while remaining within regulatory guidelines for tissue power deposition and (2) to receive the signal with the maximum signal-to-noise and the greatest flexibility in terms of utilizing the benefits of parallel imaging. Borrowing from developments in electromagnetic hyperthermia, the first challenge has been met by the use of transmit arrays, in which the input power to each element of the array can be varied in terms of magnitude and phase. Optimization of these parameters, as well as the form of the applied RF pulse, leads to very homogeneous B1 fields throughout the brain. The design of large receive arrays, using impedance-mismatched preamplifiers and geometrical overlap for interelement isolation, has resulted in significant sensitivity improvements as well as large acceleration factors in parallel imaging. © 2010 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 20, 2,13, 2010 [source]

Accelerating the analyses of 3-way and 4-way PARAFAC models utilizing multi-dimensional wavelet compression

JOURNAL OF CHEMOMETRICS, Issue 11-12 2005
Jeff Cramer
Abstract Parallel factor analysis (PARAFAC) is one of the most popular methods for evaluating multi-way data sets, such as those typically acquired by hyphenated measurement techniques. One of the reasons for PARAFAC popularity is the ability to extract directly interpretable chemometric models with little a priori information and the capability to handle unknown interferents and missing values. However, PARAFAC requires long computation times that often prohibit sufficiently fast analyses for applications such as online sensing. An additional challenge faced by PARAFAC users is the handling and storage of very large, high-dimensional data sets. Accelerating computations and reducing storage requirements in multi-way analyses are the topics of this manuscript. This study introduces a data pre-processing method based on multi-dimensional wavelet transforms (WTs), which enables highly efficient data compression applied prior to data evaluation. Because multi-dimensional WTs are linear, the intrinsic underlying linear data construction is preserved in the wavelet domain. In almost all studied examples, computation times for analyzing the much smaller, compressed data sets could be reduced so much that the additional effort for wavelet compression was more than recompensated. For 3-way and 4-way synthetic and experimental data sets, acceleration factors up to 50 have been achieved; these data sets could be compressed down to a few per cent of the original size. Despite the high compression, accurate and interpretable models were derived, which are in good agreement with conventionally determined PARAFAC models. This study also found that the wavelet type used for compression is an important factor determining acceleration factors, data compression ratios and model quality. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Fast four-dimensional coronary MR angiography with k-t GRAPPA

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2008
Peng Lai MS
Abstract Purpose To investigate the effectiveness of k-t GRAPPA for accelerating four-dimensional (4D) coronary MRA in comparison with GRAPPA and the feasibility of combining variable density undersampling with conventional k-t GRAPPA (k-t2 GRAPPA) to alleviate the overhead of acquiring autocalibration signals. Materials and Methods The right coronary artery of nine healthy volunteers was scanned at 1.5 Tesla. The 4D k -space datasets were fully acquired and subsequently undersampled to simulate partially parallel acquisitions, namely, GRAPPA, k-t GRAPPA, and k-t2 GRAPPA. Comparisons were made between the images reconstructed from full k-space datasets and those reconstructed from undersampled k-space datasets. Results k-t GRAPPA significantly reduced artifacts compared with GRAPPA and high acceleration factors were achieved with only minimal sacrifices in vessel depiction. k-t2 GRAPPA could further increase imaging speed without significant losses in image quality. Conclusion By exploiting high-degree spatiotemporal correlations during the rest period of a cardiac cycle, k-t GRAPPA and k-t2 GRAPPA can greatly increase data acquisition efficiency and, therefore, are promising solutions for fast 4D coronary MRA. J. Magn. Reson. Imaging 2008. © 2008 Wiley-Liss, Inc. [source]

High-resolution renal MRA: Comparison of image quality and vessel depiction with different parallel imaging acceleration factors

Feasibility of k-t BLAST technique for measuring "seven-dimensional" fluid flow

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2006
Ian Marshall PhD
Abstract Purpose To investigate the feasibility of rapid MR measurement of "seven-dimensional" (three velocity components, three dimensions, and time) fluid flow using the k-t Broad-use Linear Acquisition Speed-Up Technique (BLAST). Materials and Methods Complete k -space data were acquired for pulsatile fluid flow in a model of a stenosed carotid bifurcation. The data was subsampled to simulate "training" and "accelerated acquisition" data for reconstruction using k-t BLAST. Results Flow waveforms estimated from k-t BLAST reconstructions were in good agreement with those measured from the full data set for overall speedup factors up to approximately four times when slice-by-slice undersampling in ky was used. Accuracy was better than 25 mm/second or 7% (root-mean-square error) for individual time frames under these conditions. Flow patterns in the plane of symmetry, near the bifurcation, and in the stenosis were also in good agreement with those reconstructed from the full data set. Improved performance was obtained from undersampling in both ky and kz, when acceleration factors up to 12 times gave acceptable results. Conclusion The k-t BLAST technique can be applied to flow quantification, and may make feasible the acquisition of time-resolved blood flow from extended arterial regions within acceptable examination times. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc. [source]

Rapid 3D-T1, mapping of the knee joint at 3.0T with parallel imaging

MAGNETIC RESONANCE IN MEDICINE, Issue 3 2006
S. Kubilay Pakin
Abstract Three-dimensional spin-lattice relaxation time in the rotating frame (3D-T1,) with parallel imaging at 3.0T was implemented on a whole-body clinical scanner. A 3D gradient-echo sequence with a self-compensating spin-lock pulse cluster was combined with generalized autocalibrating partially parallel acquisitions (GRAPPA) to acquire T1, -weighted images. 3D-T1, maps of an agarose phantom and three healthy subjects were constructed using an eight-channel phased-array coil without parallel imaging and with parallel imaging acceleration factors of 2 and 3, in order to assess the reproducibility of the method. The coefficient of variation (CV) of the median T1, of the agarose phantom was 0.44%, which shows excellent reproducibility. The reproducibility of in vivo 3D-T1, maps was also investigated in three healthy subjects. The CV of the median T1, of the patellar cartilage varied between ,1.1% and 4.3%. Similarly, the CV varied between ,2.1,5.8%, ,1.4,8.7%, and ,1.5,4.1% for the biceps femoris and lateral and medial gastrocnemius muscles, respectively. The preliminary results demonstrate that 3D-T1, maps can be constructed with good reproducibility using parallel imaging. 3D-T1, with parallel imaging capability is an important clinical tool for reducing both the total acquisition time and RF energy deposition at 3T. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source]