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SNR
Kinds of SNR Terms modified by SNR Selected AbstractsA simple method to calculate the signal-to-noise ratio of a circular-shaped coil for MRICONCEPTS IN MAGNETIC RESONANCE, Issue 6 2006K. Ocegueda Abstract The introduction of the ultrafast imaging sequences has renewed the interest in development of RF coils. The theoretical frame of the SNR of MRI coils is a challenge because it requires a deep mathematical background to master the associated concepts. Here, a simpler method is proposed based on Legendre polynomials. This approximation method, together with a quasi-static approach, was used to derive a signal-to-noise ratio expression for a circular-shaped coil. Legendre polynomials were used instead of a weighting function to simplify the vector potential of the power loss, and an SNR formula was then derived. The simplified version of the SNR formula of a circular coil was compared with the weighting function-derived SNR expression using the quasi-static approach. SNR-vs.-depth plots were computed to theoretically compare both SNR formulas. Results showed a strong agreement between SNR values for the circular-shaped coil. This approach can be used as a tool to derive SNR expressions for more complex geometries. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part A 28A: 422,429, 2006 [source] Solenoidal microcoil design,Part II: Optimizing winding parameters for maximum signal-to-noise performanceCONCEPTS IN MAGNETIC RESONANCE, Issue 3 2001Kevin R. Minard Abstract In high-field proton NMR, the signal-to-noise ratio (SNR) achieved with a close-fitting solenoidal microcoil is adversely affected by radio frequency (RF) losses in the coil, its leads, the capacitor used to tune it, and finally, the sample. In Part II, a rigorous description of these various losses is presented, and their severity is related to the details of coil design. Results not only provide a rational basis for defining a microcoil's optimal wire diameter and the number of turns, but also for evaluating how the SNR varies with coil size and NMR frequency in high-field proton NMR studies involving either conducting or non-conducting samples. © 2001 John Wiley & Sons, Inc. Concepts Magn Reson 13: 190,210, 2001 [source] Development and validation of a smoothing-splines-based correction method for improving the analysis of CEST-MR imagesCONTRAST MEDIA & MOLECULAR IMAGING, Issue 4 2008J. Stancanello Abstract Chemical exchange saturation transfer (CEST) imaging is an emerging MRI technique relying on the use of endogenous or exogenous molecules containing exchangeable proton pools. The heterogeneity of the water resonance frequency offset plays a key role in the occurrence of artifacts in CEST-MR images. To limit this drawback, a new smoothing-splines-based method for fitting and correcting Z -spectra in order to compensate for low signal-to-noise ratio (SNR) without any a priori model was developed. Global and local voxel-by-voxel Z -spectra were interpolated by smoothing splines with smoothing terms aimed at suppressing noise. Thus, a map of the water frequency offset (,zero' map) was used to correctly calculate the saturation transfer (ST) for each voxel. Simulations were performed to compare the method to polynomials and zero-only-corrected splines on the basis of SNR improvement. In vitro acquisitions of capillaries containing solutions of LIPOCEST agents at different concentrations were performed to experimentally validate the results from simulations. Additionally, ex vivo investigations of bovine muscle mass injected with LIPOCEST agents were performed as a function of increasing pulse power. The results from simulations and experiments highlighted the importance of a proper ,zero' correction (15% decrease of fictitious CEST signal in phantoms and ex vivo preparations) and proved the method to be more accurate compared with the previously published ones, often providing a SNR higher than 5 in different simulated and experimentally noisy conditions. In conclusion, the proposed method offers an accurate tool in CEST investigation. Copyright © 2008 John Wiley & Sons, Ltd. [source] Noise-improved signal detection in cat primary visual cortex via a well-balanced stochastic resonance-like procedureEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 5 2007Klaus Funke Abstract Adding noise to a weak signal can paradoxically improve signal detection, a process called ,stochastic resonance' (SR). In the visual system, noise might be introduced by the image jitter resulting from high-frequency eye movements, like eye microtremor and microsaccades. To test whether this kind of noise might be beneficial or detrimental for cortical signal detection, we performed single-unit recordings from area 17 of anaesthetized cats while jittering the visual stimulus in a frequency and amplitude range resembling the possible range of eye movements. We used weak, sub- and peri-threshold visual stimuli, on top of which we superimposed noise with variable jitter amplitude. In accordance with the typical SR effect, we found that small noise levels actually increased the signal-to-noise ratio (SNR) of previously weak cortical visual responses, while originally strong responses were little affected or even reduced. Above a certain noise level, the SNR dropped a little, but not as a result of increased background activity , as would be proposed by SR theory , but because of a lowered response to signal and noise. Therefore, it seems that the ascending visual pathway optimally utilizes signal detection improvement by a SR-like process, while at the same time preventing spurious noise-induced activity and keeping the SNR sufficiently high. [source] Opportunistic multiuser scheduling with reduced feedback loadEUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 4 2010Yahya S. Al-HarthiArticle first published online: 27 MAY 2010 In this paper, we propose a reduced feedback opportunistic scheduling (RFOS) algorithm that reduces the feedback load while preserving the performance of opportunistic scheduling (OS). The RFOS algorithm is a modified version of our previously proposed algorithm, the DSMUDiv algorithm. The main difference is that RFOS consists of a probing process (search process) and a requesting feedback process based on a threshold. The threshold value is variable, and it depends on the probing process. To reduce the feedback rate, a quantised value indicating the modulation level is fed back, instead of the full value of the signal-to-noise ratio (SNR), which we call quantised SNR. The paper includes the closed-form expressions of the probing load, feedback load and spectral efficiency. In addition, we investigate the effect of the scheduling delay on the system throughput (STH). Under slow Rayleigh fading assumption, we compare RFOS algorithm with the DSMUDiv and optimal (full feedback load) selective diversity scheduling algorithms. Copyright © 2010 John Wiley & Sons, Ltd. [source] Adaptive resource allocation in OFDMA systems with fairness and QoS constraints,EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 6 2007Liang Chen This paper describes several practical and efficient adaptive subchannel, power and bit allocation algorithms for orthogonal frequency-division multiple-access (OFDMA) systems. Assuming perfect knowledge of channel state information (CSI) at the transmitter, we look at the problem of minimising the total power consumption while maintaining individual rate requirements and QoS constraints. An average signal-to-noise ratio (SNR) approximation is used to determine the allocation while substantially reducing the computational complexity. The proposed algorithms guarantee improvement through each iteration and converge quickly to stable suboptimal solutions. Numerical results and complexity analysis show that the proposed algorithms offer beneficial cost versus performance trade-offs compared to existing approaches. Copyright © 2007 John Wiley & Sons, Ltd. [source] Gaussian inputs: performance limits over non-coherent SISO and MIMO channelsEUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 3 2007Rasika R. Perera Performance limits of information transfer over a discrete time memoryless Rayleigh fading channel with neither the receiver nor the transmitter knowing the fading coefficients except its statistics is an important problem in information theory. We derive closed form expressions for the mutual information of single input single output (SISO) and multiple input multiple output (MIMO) Rayleigh fading channels for any antenna number at any signal to noise ratio (SNR). Using these expressions, we show that the maximum mutual information of non-coherent Rayleigh fading MIMO channels is achieved with a single transmitter and multiple receivers when the input distribution is Gaussian. We show that the addition of transmit antennas for a fixed number of receivers result in a reduction of mutual information. Furthermore, we argue that the mutual information is bounded by the SNR in both SISO and MIMO systems showing the sub-optimality of Gaussian signalling in non-coherent Rayleigh fading channels. Copyright © 2006 AEIT [source] Trellis coded quantization/trellis coded continuous phase modulation over Rician channel with imperfect phase referenceEUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 5 2004Osman Nuri Ucan In this paper, to improve bandwidth efficiency and error performance, trellis coded quantization/trellis continuous phase modulation (TCQ/TCPM) are combined and a trellis coded quantization/trellis coded continuous phase modulation (TCQ/TCCPM) scheme is introduced. Here, we use TCQ/TCM system as source coding because of its advantage over classical joint systems in terms of decoding time and complexity. We also present CPM for TCQ/TCM signals, since CPM provides low spectral occupancy and is suitable for power and bandwidth limited channels. The bit error performance of TCQ/TCCPM schemes is derived taking into account quantization noise over Rician channel with imperfect phase reference. The analytical upper bounds are obtained using Chernoff bounding technique, combined with the modified generating functional approach with no channel state information (CSI) and no side information for the phase noise process. As an example, TCQ/TCCPM scheme for 16CPFSK with modulation index h,=,1/2 (16CPFSK-TCQ/TCCPM) is investigated and compared to TCQ/TCM for 16PSK (16PSK-TCQ/TCM). It is shown that 16CPFSK-TCQ/TCCPM has better bit error performance than 16PSK-TCQ/TCM in all signal-to-noise-ratio (SNR) and quantization noise effect increases at high SNR values for both uniform and optimum quantization. Copyright © 2004 AEI. [source] Multicanier Modulation with Multistage Encoding/Decoding for a Nakagami Fading ChannelsEUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 5 2000Lev Goldfeld The Multi Carrier Modulation (MCM) system with a multistage encoding/decoding scheme based on repetition and erasures-correcting decoding of block codes applied for a Nakagami fading channel is considered. Bit Error Rate (BER) as a function of Signal-to-Noise Ratio (SNR) has been found to agree well with the simulated results. It is shown that for low SNR the proposed system has a lower BER than both the MCM with Forward Error Correction (FEC) and MCM with optimal diversity reception and FEC. [source] An analysis of P times reported in the Reviewed Event Bulletin for Chinese underground explosionsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2005A. Douglas SUMMARY Analysis of variance is used to estimate the measurement error and path effects in the P times reported in the Reviewed Event Bulletins (REBs, produced by the provisional International Data Center, Arlington, USA) and in times we have read, for explosions at the Chinese Test Site. Path effects are those differences between traveltimes calculated from tables and the true times that result in epicentre error. The main conclusions of the study are: (1) the estimated variance of the measurement error for P times reported in the REB at large signal-to-noise ratio (SNR) is 0.04 s2, the bulk of the readings being analyst-adjusted automatic-detections, whereas for our times the variance is 0.01 s2 and (2) the standard deviation of the path effects for both sets of observations is about 0.6 s. The study shows that measurement error is about twice (,0.2 s rather than ,0.1 s) and path effects about half the values assumed for the REB times. However, uncertainties in the estimated epicentres are poorly described by treating path effects as a random variable with a normal distribution. Only by estimating path effects and using these to correct onset times can reliable estimates of epicentre uncertainty be obtained. There is currently an international programme to do just this. The results imply that with P times from explosions at three or four stations with good SNR (so that the measurement error is around 0.1 s) and well distributed in azimuth, then with correction for path effects the area of the 90 per cent coverage ellipse should be much less than 1000 km2,the area allowed for an on-site inspection under the Comprehensive Test Ban Treaty,and should cover the true epicentre with the given probability. [source] Improved EEG source analysis using low-resolution conductivity estimation in a four-compartment finite element head modelHUMAN BRAIN MAPPING, Issue 9 2009Seok Lew Abstract Bioelectric source analysis in the human brain from scalp electroencephalography (EEG) signals is sensitive to geometry and conductivity properties of the different head tissues. We propose a low-resolution conductivity estimation (LRCE) method using simulated annealing optimization on high-resolution finite element models that individually optimizes a realistically shaped four-layer volume conductor with regard to the brain and skull compartment conductivities. As input data, the method needs T1- and PD-weighted magnetic resonance images for an improved modeling of the skull and the cerebrospinal fluid compartment and evoked potential data with high signal-to-noise ratio (SNR). Our simulation studies showed that for EEG data with realistic SNR, the LRCE method was able to simultaneously reconstruct both the brain and the skull conductivity together with the underlying dipole source and provided an improved source analysis result. We have also demonstrated the feasibility and applicability of the new method to simultaneously estimate brain and skull conductivity and a somatosensory source from measured tactile somatosensory-evoked potentials of a human subject. Our results show the viability of an approach that computes its own conductivity values and thus reduces the dependence on assigning values from the literature and likely produces a more robust estimate of current sources. Using the LRCE method, the individually optimized four-compartment volume conductor model can, in a second step, be used for the analysis of clinical or cognitive data acquired from the same subject. Hum Brain Mapp, 2009. © 2008 Wiley-Liss, Inc. [source] Low-Noise Fully Differential Amplifiers Using JFET-CMOS Integration Technology for Smart SensorsIEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING, Issue 3 2008Hidekuni Takao Member Abstract In this paper, CMOS-based low-noise amplifiers with JFET-CMOS technology for high-resolution sensor interface circuits are presented. A differential difference amplifier (DDA) configuration is employed to realize differential signal amplification with very high input impedance, which is required for the front-end circuit in many sensor applications. Low-noise JFET devices are used as input pair of the input differential stages or source-grounded output load devices, which are dominant in the total noise floor of DDA circuits. A fully differential amplifier circuit with pure CMOS DDA and three types of JFET-CMOS DDAs were fabricated and their noise performances were compared. The results show that the total noise floor of the JFET-CMOS amplifier was much lower compared to that of the pure CMOS configuration. The noise-reduction effect of JFET replacement depends on the circuit configuration. The noise reduction effect by JFET device was maximum of about , 18 dB at 2.5 Hz. JFET-CMOS technology is very effective in improving the signal-to-noise ratio (SNR) of a sensor interface circuit with CMOS-based sensing systems. © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [source] A general framework for evaluating nonlinearity, noise and dynamic range in continuous-time OTA-C filters for computer-aided design and optimizationINTERNATIONAL JOURNAL OF CIRCUIT THEORY AND APPLICATIONS, Issue 4 2007S. Koziel Abstract Efficient procedures for evaluating nonlinear distortion and noise valid for any OTA-C filter of arbitrary order are developed based on matrix description of a general OTA-C filter model. Since those procedures use OTA macromodels, they allow us to obtain the results significantly faster than transistor-level simulation. On the other hand, the general OTA-C filter model allows us to apply matrix transforms that manipulate (rescale) filter element values and/or change topology without changing its transfer function. Due to this, the proposed procedures can be used in direct optimization of OTA-C filters with respect to important characteristics such as noise performance, THD, IM3, DR or SNR. As an example, a simple optimization procedure using equivalence transformations is discussed. An application example of the proposed approach to optimal block sequencing and gain distribution of 8th order cascade Butterworth filter is given. Accuracy of the theoretical tools has been verified by comparing to transistor-level simulation results and to experimental results. Copyright © 2006 John Wiley & Sons, Ltd. [source] Performance evaluation for asynchronous MC-CDMA systems with a symbol timing offsetINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 4 2009Myonghee Park Abstract This paper models a symbol timing offset (STO) with respect to the guard period and the maximum access delay time for asynchronous multicarrier-code division multiple access systems over frequency-selective multipath fading channels. Analytical derivation shows that STO causes desired signal power degradation and generates self-interferences. This effect of the STO on the average bit error rate (BER) and the effective signal-to-noise ratio (SNR) is evaluated using the semi-analytical method, and the approximated BER and the SNR loss caused by STO are then obtained as closed-form expressions. The tightness between the semi-analytical result and the approximated one is verified for the different STOs and SNRs. Furthermore, the derived analytical results are verified via Monte Carlo simulations. Copyright © 2008 John Wiley & Sons, Ltd. [source] Performance analysis of wireless multihop diversity systemsINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 9 2008Diomidis S. Michalopoulos Abstract We study the performance of multihop diversity systems with non-regenerative relays over independent and non-identical Rayleigh fading channels. The analysis is based on the evaluation of the instantaneous end-to-end signal-to-noise ratio (SNR), depending on the type of the relay and the diversity scheme used. A closed-form expression is derived for the average end-to-end SNR, when fixed-gain relays and a maximal ratio combiner are used; also, an analytical expression formula for the average symbol-error rate (ASER) for the above case is presented. The results show that, as expected, multihop diversity systems outperform conventional telecommunication systems in terms of ASER when the same amount of energy is assumed to be consumed in both cases. Copyright © 2008 John Wiley & Sons, Ltd. [source] Multiple spot diffusing geometries for indoor optical wireless communication systemsINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 10 2003A. G. Al-Ghamdi Abstract In order to improve the performance of indoor optical wireless communication links, two multispot diffusing geometries based on diamond and line strip spot distribution geometries are proposed, analysed and compared to the known uniform spot distribution. Such geometries combine the advantages of the diffuse and the line-of-sight systems, giving great robustness and ease of use. The novel line strip multibeam transmitter geometry has resulted in a receiver signal-to-noise ratio (SNR) improvement of about 4 dB compared to the conventional diffuse system as well as a significant reduction in the pulse spread. Simulation and comparison results for both the conventional diffuse system and the three multispot diffusing geometries are presented. Further, pulse responses, SNR, and delay spread results at various locations are presented. Copyright © 2003 John Wiley & Sons, Ltd. [source] Robust partial least squares regression: Part II, new algorithm and benchmark studiesJOURNAL OF CHEMOMETRICS, Issue 1 2008Uwe Kruger Abstract This paper presents the second part of the work on robust partial least squares (RPLS) regression and develops a new RPLS algorithm based on the concept laid out in Part I. The paper also contrasts the new algorithm with existing work using two simulation examples. This comparison highlights (i) the impact of the flaws in existing RPLS work and (ii) the compromised sensitivity resulting from introducing simplifications to the determination of the Stahel,Donoho estimator (SDE). The paper finally presents an evaluation of the computational complexity of RPLS algorithms and examines the impact of the signal-to-noise ratio (SNR) upon the sensitivity of detecting outliers. The third part of this work will examine practical aspects of RPLS applications based on the analysis of experimental data. Copyright © 2007 John Wiley & Sons, Ltd. [source] Can MR fluoroscopic triggering technique and slow rate injection provide appropriate arterial phase images with reducing artifacts on gadoxetic acid-DTPA (Gd-EOB-DTPA)-enhanced hepatic MR imaging?JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2010Hiroki Haradome MD Abstract Purpose: To evaluate whether using MR fluoroscopic triggering technique and slow rate injection improves the quality of arterial phase images in gadoxetic acid-DTPA-enhanced (Gd-EOB-DTPA) MR imaging because of proper acquisition timing and reduction of artifacts. Materials and Methods: Two hundred sixteen patients undergoing examination for liver diseases were retrospectively reviewed. All MR images were obtained with two Gd-EOB-DTPA injection protocols: (i) a combination protocol, in which the MR fluoroscopic triggering technique and slow rate injection (1 mL/s) were used; and for comparison, (ii) a conventional protocol, in which adjusted fixed scan delay and ordinary rate injection (2 mL/s) were adopted. Signal-to-noise ratio (SNR) of aorta, portal vein, and liver parenchyma on arterial phase images were calculated. Two blinded readers independently evaluated the obtained arterial phase images in terms of acquisition timing and degree of artifacts. Results: The SNRs of aorta and portal vein on arterial phase images were significantly higher in the combination protocol group (aorta/portal: 221.9 ± 91.9/197.1 ± 89.8) than that in the conventional protocol group (aorta/portal: 169.8 ± 97.4/92.7 ± 48.5) (P < 0.05). The acquisition timing for arterial phase images with the combination protocol was significantly better than that with the conventional protocol (P < 0.01). The image quality of the combination protocol was significantly higher than that of the conventional protocol (P < 0.01). The occurrence rate of moderate or severe degree of artifacts in the conventional protocol (38.0%) was more prominent than that in the combination protocol (18.5%). Conclusion: The combination of the MR fluoroscopic triggering technique and slow rate injection provides proper arterial phase images and reduces the artifacts in Gd-EOB-DTPA MR imaging. J. Magn. Reson. Imaging 2010;32:334,340. © 2010 Wiley-Liss, Inc. [source] 3D flow-independent peripheral vessel wall imaging using T2 -prepared phase-sensitive inversion-recovery steady-state free precessionJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2010Jingsi Xie BS Abstract Purpose: To develop a 3D flow-independent peripheral vessel wall imaging method using T2 -prepared phase-sensitive inversion-recovery (T2PSIR) steady-state free precession (SSFP). Materials and Methods: A 3D T2 -prepared and nonselective inversion-recovery SSFP sequence was designed to achieve flow-independent blood suppression for vessel wall imaging based on T1 and T2 properties of the vessel wall and blood. To maximize image contrast and reduce its dependence on the inversion time (TI), phase-sensitive reconstruction was used to restore the true signal difference between vessel wall and blood. The feasibility of this technique for peripheral artery wall imaging was tested in 13 healthy subjects. Image signal-to-noise ratio (SNR), wall/lumen contrast-to-noise ratio (CNR), and scan efficiency were compared between this technique and conventional 2D double inversion recovery , turbo spin echo (DIR-TSE) in eight subjects. Results: 3D T2PSIR SSFP provided more efficient data acquisition (32 slices and 64 mm in 4 minutes, 7.5 seconds per slice) than 2D DIR-TSE (2,3 minutes per slice). SNR of the vessel wall and CNR between vessel wall and lumen were significantly increased as compared to those of DIR-TSE (P < 0.001). Vessel wall and lumen areas of the two techniques are strongly correlated (intraclass correlation coefficients: 0.975 and 0.937, respectively; P < 0.001 for both). The lumen area of T2PSIR SSFP is slightly larger than that of DIR-TSE (P = 0.008). The difference in vessel wall area between the two techniques is not statistically significant. Conclusion: T2PSIR SSFP is a promising technique for peripheral vessel wall imaging. It provides excellent blood signal suppression and vessel wall/lumen contrast. It can cover a 3D volume efficiently and is flow- and TI-independent. J. Magn. Reson. Imaging 2010;32:399,408. © 2010 Wiley-Liss, Inc. [source] Smoothing that does not blur: Effects of the anisotropic approach for evaluating diffusion tensor imaging data in the clinicJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2010Marta Moraschi MS Abstract Purpose: To compare the effects of anisotropic and Gaussian smoothing on the outcomes of diffusion tensor imaging (DTI) voxel-based (VB) analyses in the clinic, in terms of signal-to-noise ratio (SNR) enhancement and directional information and boundary structures preservation. Materials and Methods: DTI data of 30 Alzheimer's disease (AD) patients and 30 matched control subjects were obtained at 3T. Fractional anisotropy (FA) maps with variable degrees and quality (Gaussian and anisotropic) of smoothing were created and compared with an unsmoothed dataset. The two smoothing approaches were evaluated in terms of SNR improvements, capability to separate differential effects between patients and controls by a standard VB analysis, and level of artifacts introduced by the preprocessing. Results: Gaussian smoothing regionally biased the FA values and introduced a high variability of results in clinical analysis, greatly dependent on the kernel size. On the contrary, anisotropic smoothing proved itself capable of enhancing the SNR of images and maintaining boundary structures, with only moderate dependence of results on smoothing parameters. Conclusion: Our study suggests that anisotropic smoothing is more suitable in DTI studies; however, regardless of technique, a moderate level of smoothing seems to be preferable considering the artifacts introduced by this manipulation. J. Magn. Reson. Imaging 2010;31:690,697. © 2010 Wiley-Liss, Inc. [source] 7 Tesla MR imaging of the human eye in vivoJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 5 2009Kathryn Richdale OD Abstract Purpose: To develop a protocol which optimizes contrast, resolution and scan time for three-dimensional (3D) imaging of the human eye in vivo using a 7 Tesla (T) scanner and custom radio frequency (RF) coil. Materials and Methods: Initial testing was conducted to reduce motion and susceptibility artifacts. Three-dimensional FFE and IR-TFE images were obtained with variable flip angles and TI times. T1 measurements were made and numerical simulations were performed to determine the ideal contrast of certain ocular structures. Studies were performed to optimize resolution and signal-to-noise ratio (SNR) with scan times from 20 s to 5 min. Results: Motion and susceptibility artifacts were reduced through careful subject preparation. T1 values of the ocular structures are in line with previous work at 1.5T. A voxel size of 0.15 × 0.25 × 1.0 mm3 was obtained with a scan time of approximately 35 s for both 3D FFE and IR-TFE sequences. Multiple images were registered in 3D to produce final SNRs over 40. Conclusion: Optimization of pulse sequences and avoidance of susceptibility and motion artifacts led to high quality images with spatial resolution and SNR exceeding prior work. Ocular imaging at 7T with a dedicated coil improves the ability to make measurements of the fine structures of the eye. J. Magn. Reson. Imaging 2009;30:924,932. © 2009 Wiley-Liss, Inc. [source] Left ventricular infarct size, peri-infarct zone, and papillary scar measurements: A comparison of high-resolution 3D and conventional 2D late gadolinium enhancement cardiac MRJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2009Dana C. Peters PhD Abstract Purpose To compare higher spatial resolution 3D late gadolinium enhancement (LGE) cardiovascular magnetic resonance (Cardiac MR) with 2D LGE in patients with prior myocardial infarction. Materials and Methods Fourteen patients were studied using high spatial resolution 3D LGE (1.3 × 1.3 × 5.0 mm3) and conventional 2D LGE (2 × 2 × 8 mm3) scans. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured. Total infarct volume, peri-infarct volume measured in a limited slab, and papillary muscle scar volume were compared using Bland,Altman analysis. Image quality was graded. Results 3D LGE had higher scar SNR (P < 0.001), higher myocardial SNR (P = 0.001), higher papillary scar-blood CNR (P = 0.01), and greater sharpness (P = 0.01). The scar volumes agreed (14.5 ± 8.2 for 2D, vs. 13.2 ± 8.8 for 3D), with bias ± 2 standard deviations (SDs) of 0.5 ± 6.8 mL, P = 0.59 R = 0.91. The peri-infarct volumes correlated but less strongly than scar (P = 0.40, R = 0.77). For patients with more heterogeneous scar, larger peri-infarct volumes were measured by 3D (1.9 ± 1.1 mL for 2D vs. 2.4 ± 1.6 mL for 3D, P = 0.15, in the matched region). Papillary scar, present in 6/14 (42%) patients, was more confidently identified on 3D LGE. Conclusion Higher spatial resolution 3D LGE provides sharper images and higher SNR, but less myocardial nulling. Scar volumes agree well, with peri-infarct volumes correlating less well. 3D LGE may be superior in visualization of papillary muscle scar. J. Magn. Reson. Imaging 2009;30:794,800. © 2009 Wiley-Liss, Inc. [source] 1H spectroscopic imaging of human brain at 3 Tesla: Comparison of fast three-dimensional magnetic resonance spectroscopic imaging techniquesJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2009Matthew L. Zierhut PhD Abstract Purpose To investigate the signal-to-noise-ratio (SNR) and data quality of time-reduced three-dimensional (3D) proton magnetic resonance spectroscopic imaging (1H MRSI) techniques in the human brain at 3 Tesla. Materials and Methods Techniques that were investigated included ellipsoidal k -space sampling, parallel imaging, and echo-planar spectroscopic imaging (EPSI). The SNR values for N-acetyl aspartate, choline, creatine, and lactate or lipid peaks were compared after correcting for effective spatial resolution and acquisition time in a phantom and in the brains of human volunteers. Other factors considered were linewidths, metabolite ratios, partial volume effects, and subcutaneous lipid contamination. Results In volunteers, the median normalized SNR for parallel imaging data decreased by 34,42%, but could be significantly improved using regularization. The normalized signal to noise loss in flyback EPSI data was 11,18%. The effective spatial resolutions of the traditional, ellipsoidal, sensitivity encoding (SENSE) sampling scheme, and EPSI data were 1.02, 2.43, 1.03, and 1.01 cm3, respectively. As expected, lipid contamination was variable between subjects but was highest for the SENSE data. Patient data obtained using the flyback EPSI method were of excellent quality. Conclusion Data from all 1H 3D-MRSI techniques were qualitatively acceptable, based upon SNR, linewidths, and metabolite ratios. The larger field of view obtained with the EPSI methods showed negligible lipid aliasing with acceptable SNR values in less than 9.5 min without compromising the point-spread function. J. Magn. Reson. Imaging 2009;30:473,480. © 2009 Wiley-Liss, Inc. [source] Diffusion-weighted imaging of the liver: Comparison of navigator triggered and breathhold acquisitionsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2009Bachir Taouli MD Abstract Purpose To compare a free breathing navigator triggered single shot echoplanar imaging (SS EPI) diffusion-weighted imaging (DWI) sequence with prospective acquisition correction (PACE) with a breathhold (BH) DWI sequence for liver imaging. Materials and Methods Thirty-four patients were evaluated with PACE-DWI and BH DWI of the liver using b-values of 0, 50, and 500 s/mm2. There were 29 focal liver lesions in 18 patients. Qualitative evaluation was performed on a 3-point scale (1,3) by two independent observers (maximum score 9). Quantitative evaluation included estimated SNR (signal to noise ratio), lesion-to-liver contrast ratio, liver and lesion apparent diffusion coefficients (ADCs), and coefficient of variation (CV) of ADC in liver parenchyma and focal liver lesions (estimate of noise contamination in ADC). Results PACE-DWI showed significantly better image quality, higher SNR and lesion-to-liver contrast ratio when compared with BH DWI. ADCs of liver and focal lesions with both sequences were significantly correlated (r = 0.838 for liver parenchyma, and 0.904 for lesions, P < 0.0001), but lower with the BH sequence (P < 0.02). There was higher noise contamination in ADC measurement obtained with BH DWI (with a significantly higher SD and CV of ADC). Conclusion The use of a navigator echo to trigger SS EPI DWI improves image quality and liver to lesion contrast, and enables a more precise ADC quantification compared with BH DWI acquisition. J. Magn. Reson. Imaging 2009;30:561,568. © 2009 Wiley-Liss, Inc. [source] Cine cardiac imaging using black-blood steady-state free precession (BB-SSFP) at 3TJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2009Tamer A. Basha MSE Abstract Purpose To propose a new black-blood (BB) pulse sequence that provides BB cine cardiac images with high blood-myocardium contrast. The proposed technique is based on the conventional steady-state free precession (SSFP) sequence. Materials and Methods Numerical simulations of the Bloch equation were conducted to compare the resulting signal-to-noise ratio (SNR) to that of conventional BB imaging, including the effects of changing the imaging flip angle and heart rates. Simulation results were verified using a gel phantom experiment and five normal volunteers were scanned using the proposed technique. Results The new sequence showed higher SNR and contrast-to-noise ratio (CNR) (,100%) compared to the conventional BB imaging. Also, the borders of the left ventricle (LV) and right ventricle (RV) appear more distinguishable than the conventional SSFP. We were also able to cover about 80% of the cardiac cycle with short breath-hold time (,10 cardiac cycles) and with reasonable SNR and CNR. Conclusion Based on an SSFP conventional sequence, the new sequence provides BB cines that cover most of the cardiac cycle and with higher SNR and CNR than the conventional BB sequences. J. Magn. Reson. Imaging 2009;30:94,103. © 2009 Wiley-Liss, Inc. [source] Quantitative contrast-enhanced perfusion measurements of the human lung using the prebolus approachJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2009Markus Oechsner MS Abstract Purpose To investigate dynamic contrast-enhanced MRI (DCE-MRI) for quantification of pulmonary blood flow (PBF) and blood volume (PBV) using the prebolus approach and to compare the results to the global lung perfusion (GLP). Materials and Methods Eleven volunteers were examined by applying different contrast agent doses (0.5, 1.0, 2.0, and 3.0 mL gadolinium diethylene triamine pentaacetic acid [Gd-DTPA]), using a saturation-recovery (SR) true fast imaging with steady precession (TrueFISP) sequence. PBF and PBV were determined for single bolus and prebolus. Region of interest (ROI) evaluation was performed and parameter maps were calculated. Additionally, cardiac output (CO) and lung volume were determined and GLP was calculated as a contrast agent,independent reference value. Results The prebolus results showed good agreement with low-dose single-bolus and GLP: PBF (mean ± SD in units of mL/minute/100 mL) = single bolus 190 ± 73 (0.5-mL dose) and 193 ± 63 (1.0-mL dose); prebolus 192 ± 70 (1.0,2.0-mL dose) and 165 ± 52 (1.0,3.0-mL dose); GLP (mL/minute/100 mL) = 187 ± 34. Higher single-bolus resulted in overestimated values due to arterial input function (AIF) saturation. Conclusion The prebolus approach enables independent determination of appropriate doses for AIF and tissue signal. Using this technique, the signal-to-noise ratio (SNR) from lung parenchyma can be increased, resulting in improved PBF and PBV quantification, which is especially useful for the generation of parameter maps. J. Magn. Reson. Imaging 2009;30:104,111. © 2009 Wiley-Liss, Inc. [source] Apparent wall thickening of cystic renal lesions on MRIJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2008Vikas Gulani MD Abstract Purpose To show that cystic renal lesions that would otherwise meet criteria for simple cysts can demonstrate perceptible walls or increased wall thickness on MRI, sometimes causing these lesions to be "upgraded." It was hypothesized that thickening of cyst walls on MRI can be artifactual, due to data truncation, applied filtering, and low signal-to-noise ratio (SNR). Materials and Methods k-Space data for a 4-cm cyst were created in a 40-cm field of view (FOV) (512 × 512 matrix). Additional data sets were created using the central 512 × 256 and 512 × 128 points. Noise was simulated so that the cyst SNR was approximately 7, 14, and 20, respectively. Actual wall thickness was set at 0.25 mm, and cyst:wall signal at 1:4. An inverse two-dimensional (2D) fast Fourier transform (FFT) yielded simulated images. A Fermi filter was applied to reduce ringing. Images/projections were examined for wall thickening. Seven patients with initially thick-walled cysts on fat-saturated spoiled gradient-echo (FS-SPGR) images were scanned with increasing resolution (256 × 128 and 256 × 256; four patients were also scanned with 512 × 512). Average wall thickness at each resolution was compared using a two-tailed paired Student's t -test. Results Simulations showed apparent wall thickening at low resolution, improving with higher resolutions. Low SNR and application of the Fermi filter made it difficult to identify ringing as the cause of this thickening. The simulation results were confirmed on seven patients, whose cyst walls proved to be artifactually thickened (P < 0.01). Conclusion Thickening of cyst walls on MRI can be artifactual. Upon encountering thick-walled cystic renal lesions, high-resolution images can be acquired to exclude apparent thickening. J. Magn. Reson. Imaging 2008;28103,110. © 2008 Wiley-Liss, Inc. [source] Conspicuity of zones of ablation after radiofrequency ablation in porcine livers: Comparison of an extracellular and an SPIO contrast agentJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2008Christopher Bangard MD Abstract Purpose To compare conspicuity of zones of ablation on nonenhanced, gadopentetate dimeglumine-(Gd-DTPA) and ferucarbotran-(SPIO)-enhanced magnetic resonance (MR) images. Materials and Methods In all, 33 radiofrequency ablations (RFA) were performed in 17 healthy porcine livers at 1.5T MR imaging 1 day and 2 and 4 weeks after RFA: T2-weighted (w) ultra turbo spin echo (UTSE), proton density (PD)-w UTSE, T1-w gradient echo (GRE) pre- and 5 minutes postcontrast administration, dynamic T1-w GRE during Gd-DTPA (Magnevist) or SPIO (Resovist) administration, T2-w UTSE, and PD-w UTSE sequences 10 minutes after SPIO administration. Regions of interest (ROIs) for contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) were drawn in consensus by two radiologists. Results PD-w SPIO-enhanced images (23.5 ± 5.5) showed higher liver-to-lesion CNR than T1-w GRE Gd-DTPA-enhanced images (13.5 ± 6.1) 1 day after RFA (P , 0.05). At all other timepoints, liver-to-lesion CNR of PD-w and T2-w SPIO-enhanced images did not differ significantly from T1-w GRE Gd-DTPA-enhanced images (P , 0.05). Nonenhanced T2-w images revealed lower liver-to-lesion CNR (7.0 ± 7.5/6.5 ± 5.9/6.8 ± 5.0, 1 day/2 weeks/4 weeks, respectively) than T2-w SPIO-enhanced (17.4 ± 4.8/15.3 ± 4.5/14.2 ± 5.7), PD-w SPIO-enhanced (23.5 ± 5.5/16.9 ± 3.6, 1 day/2 weeks), and T1-w Gd-DTPA-enhanced (15.3 ± 3.6/12.7 ± 3.5, 2/4 weeks) images (P , 0.05). Liver-to-lesion CNR of SPIO-enhanced dynamic T1-w GRE images after 30, 80, 150, and 240 seconds did not change significantly over time (P , 0.05). Conclusion One day after RFA lesion conspicuity on PD-w ferucarbotran-enhanced images is better than on T1-w GRE Gd-DTPA-enhanced images. At all other timepoints, ferucarbotran is not superior to gadolinium. Ferucarbotran- and gadolinium-enhanced images improve lesion conspicuity compared with nonenhanced T2-w images at all timepoints. J. Magn. Reson. Imaging 2008;28:263,270. © 2008 Wiley-Liss, Inc. [source] Evaluation of optimized inversion-recovery fat-suppression techniques for T2-weighted abdominal MR imagingJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2008Thomas C. Lauenstein MD Abstract Purpose To test the theoretical benefits of a spectral attenuated inversion-recovery (SPAIR) fat-suppression (FS) technique in clinical abdominal MRI by comparison to conventional inversion-recovery (IR) FS combined with T2-weighted (T2W) partial Fourier single shot fast spin echo (SSFSE). Materials and Methods 1.5T MRI studies of the abdomen were performed in 28 patients with liver lesions (hemangiomas n = 14; metastases n = 14). T2W sequences were acquired using IR and SPAIR SSFSE. Measurements included retroperitoneal and mesenteric fat signal-to-noise (SNR) to evaluate FS; liver lesion contrast-to-noise (CNR) to evaluate bulk water signal recovery effects; and bowel wall delineation to evaluate susceptibility and physiological motion effects. Results SPAIR-SSFSE images produce significantly improved FS and liver lesion CNR. The mean SNR of the retroperitoneal and mesenteric fat for SPAIR SSFSE was 20.5 ± 10.2 (±1 SD) and 12.7 ± 6.2, compared to 43.2 ± 24.1 (P = 0.000006) and 29.3 ± 16.8 (P = 0.0000005) for IR-SSFSE. SPAIR-SSFSE images produced higher CNR for both hemangiomas CNR = 164 ± 88 vs. 126 ± 83 (P = 0.00005) and metastases CNR = 75 ± 27 vs. 53 ± 19 (P = 0.007). Bowel wall visualization was significantly improved using SPAIR-SSFSE (P = 0.002). Conclusion The theoretical benefits of SPAIR over conventional IR FS translate into significant multiple improvements that can be measured on clinical abdominal MRI scans. J. Magn. Reson. Imaging 2008;27:1448,1454. © 2008 Wiley-Liss, Inc. [source] Detection of choline signal in human breast lesions with chemical-shift imagingJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 5 2008Hyeon-Man Baek PhD Abstract Purpose To investigate the application of MR spectroscopy using chemical-shift imaging (CSI) for characterizing human breast lesions at 1.5T, and to evaluate the diagnostic performance using ROC (receiver operating characteristics) analysis. Materials and Methods Thirty-six patients (35,73 years old, mean 52), with 27 malignant and 9 benign lesions, underwent anatomical imaging, dynamic contrast-enhanced MR imaging, and CSI. The ROC analysis was performed and the cutoff point yielding the highest accuracy was found to be a choline (Cho) signal-to-noise ratio (SNR) >3.2. Results The mean Cho SNR was 2.8 ± 0.8 (range, 1.8,4.3) for the benign group and 5.9 ± 3.4 (2.1,17.5) for the malignant group (P = 0.01). Based on the criterion of Cho SNR >3.2 as malignant, CSI correctly diagnosed 22 of 27 malignant lesions and 7 of 9 benign lesions, resulting in a sensitivity of 81%, specificity of 78%, and overall accuracy of 81%. If the criterion was set higher at Cho SNR >4.0 the specificity improved to 89% but sensitivity was lowered to 67%. Conclusion The ROC analysis presented in this work could be used to set an objective diagnostic criterion depending on preferred emphasis on sensitivity or specificity. J. Magn. Reson. Imaging 2008;27:1114,1121. © 2008 Wiley-Liss, Inc. [source] | ||||||||||||||||||||||