Query: signal noise ratio in nmr

title: Increase in signal to noise ratio of 10,000 times in liquid state NMR
abstract: A method for obtaining strongly polarized nuclear spins in solution has been developed. The method uses low temperature, high magnetic field, and dynamic nuclear polarization (DNP) to strongly polarize nuclear spins in the solid state. The solid sample is subsequently dissolved rapidly in a suitable solvent to create a solution of molecules with hyperpolarized nuclear spins. The polarization is performed in a DNP polarizer, consisting of a super conducting magnet (3.35 T) and a liquid helium cooled sample space. The sample is irradiated with microwaves at 94 GHz. Subsequent to polarization, the sample is dissolved by an injection system inside the DNP magnet. The dissolution process effectively preserves the nuclear polarization. The resulting hyperpolarized liquid sample can be transferred to a high resolution NMR spectrometer, where an enhanced NMR signal can be acquired, or it may be used as an agent for in vivo imaging or spectroscopy. In this article we describe the use of the method on aqueous solutions of [13C]urea. Polarizations of 37% for 13C and 7.8% for 15N, respectively, were obtained after the dissolution. These polarizations correspond to an enhancement of 44,400 for 13C and 23,500 for 15N, respectively, compared with thermal equilibrium at 9.4 T and room temperature. The method can be used generally for signal enhancement and reduction of measurement time in liquid state NMR and opens up for a variety of in vitro and in vivo applications of DNP enhanced NMR.

title: Ultimate intrinsic signal to noise ratio in MRI
abstract: A method to calculate the ultimate intrinsic signal to noise ratio (SNR) in a magnetic resonance experiment for a point inside an arbitrarily shaped object is presented. The ultimate intrinsic SNR is determined by body noise. A solution is obtained by optimizing the electromagnetic field to minimize total power deposition while maintaining a constant right hand circularly polarized component of the magnetic field at the point of interest. A numerical approximation for the optimal field is found by assuming a superposition of a large number of plane waves. This simulation allowed estimation of the ultimate intrinsic SNR attainable in a human torso model. The performance of six coil configurations was evaluated by comparing the SNR of images obtained by the coils with the ultimate values. In addition, the behavior of ultimate intrinsic SNR was investigated as a function of main field strength. It was found that the ultimate intrinsic SNR increases better than linearly with the main magnetic field up to 10 T for our model. It was observed that for field strengths of 4 T or higher, focusing is required to reach the ultimate intrinsic SNR.

title: A high signal to noise Ca2+ probe composed of a single green fluorescent protein
abstract: Recently, several groups have developed green fluorescent protein (GFP) based Ca2+ probes. When applied in cells, however, these probes are difficult to use because of a low signal to noise ratio. Here we report the development of a high affinity Ca2+ probe composed of a single GFP (named G CaMP) G CaMP showed an apparent Kd for Ca2+ of 235 nM. Association kinetics of Ca2+ binding were faster at higher Ca2+ concentrations, with time constants decreasing from 230 ms at 0.2 mM Ca2+ to 2.5 ms at 1 mM Ca2+ Dissociation kinetics (t ~200 ms) are independent of Ca2+ concentrations. In HEK 293 cells and mouse myotubes expressing G CaMP, large fluorescent changes were observed in response to application of drugs or electrical stimulations. G CaMP will be a useful tool for visualizing intracellular Ca2+ in living cells. Mutational analysis, together with previous structural information, suggests the residues that may alter the fluorescence of GFP.

title: Measuring signal to noise ratio in partially parallel imaging MRI.
abstract: To assess five different methods of signal to noise ratio (SNR) measurement for partially parallel imaging (PPI) acquisitions.  Measurements were performed on a spherical phantom and three volunteers using a multichannel head coil a clinical 3T MRI system to produce echo planar, fast spin echo, gradient echo, and balanced steady state free precession image acquisitions. Two different PPI acquisitions, generalized autocalibrating partially parallel acquisition algorithm and modified sensitivity encoding with acceleration factors (R) of 2 4, were evaluated and compared to nonaccelerated acquisitions. Five standard SNR measurement techniques were investigated and Bland Altman analysis was used to determine agreement between the various SNR methods. The estimated g factor values, associated with each method of SNR calculation and PPI reconstruction method, were also subjected to assessments that considered the effects on SNR due to reconstruction method, phase encoding direction, and R value.  Only two SNR measurement methods produced g factors in agreement with theoretical expectations (g 1) Bland Altman tests demonstrated that these two methods also gave the most similar results relative to the other three measurements. R value was the only factor of the three we considered that showed significant influence on SNR changes.  Non signal methods used in SNR evaluation do not produce results consistent with expectations in the investigated PPI protocols. Two of the methods studied provided the most accurate and useful results. Of these two methods, it is recommended, when evaluating PPI protocols, the image subtraction method be used for SNR calculations due to its relative accuracy and ease of implementation.

title: Signal to noise measurements in magnitude images from NMR phased arrays
abstract: A method is proposed to estimate signal to noise ratio (SNR) values in phased array magnitude images, based on a region of interest (ROI) analysis. It is shown that the SNR can be found by correcting the measured signal intensity for the noise bias effects and by evaluating the noise variance as the mean square value of all the pixel intensities in a chosen background ROI, divided by twice the number of receivers used. Estimated SNR values are shown to vary spatially within a bound of 20% with respect to the true SNR values as a result of noise correlations between receivers.

title: Ultimate intrinsic signal to noise ratio for parallel MRI: Electromagnetic field considerations
abstract: A method is described for establishing an upper bound on the spatial encoding capabilities of coil arrays in parallel MRI. Ultimate intrinsic signal to noise ratio (SNR) independent of any particular conductor arrangement, is calculated by expressing arbitrary coil sensitivities in terms of a complete set of basis functions that satisfy Maxwell's equations within the sample and performing parallel imaging reconstructions using these basis functions. The dependence of the ultimate intrinsic SNR on a variety of experimental conditions is explored and a physically intuitive explanation for the observed behavior is provided based on a comparison between the electromagnetic wavelength and the distance between aliasing points. Imaging at high field strength, with correspondingly short wavelength, is shown to offer advantages for parallel imaging beyond those already expected due to the larger available spin polarization. One dimensional undersampling of k space yields a steep drop in attainable SNR for more than a 5 fold reduction of scan time, while 2D undersampling permits access to much higher degrees of acceleration. Increased tissue conductivity decreases baseline SNR, but improves parallel imaging performance. A procedure is also provided for generating the optimal coil sensitivity pattern for a given acceleration, which will serve as a useful guide for future coil designs.  50:1018 1030, 2003.

title: Noise in MRI
abstract: This study analyzes the signal to noise ratio (SNR) in magnetic resonance imaging. The factors that determine the SNR are derived starting from basic principles. The SNR, for a given object, is shown to be proportional to the voxel volume and the square root of the acquisition time. The noise generated by the body is derived using a cylindrical model and is shown to be proportional to the square of the radius and the square root of the length.

title: 7T vs. 4T: RF power, homogeneity, and signal to noise comparison in head images
abstract: Signal to noise ratio (SNR) RF field (B1) and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths. The variation in B1 magnitude was nearly twofold higher at 7T than at 4T ~42% compared to ~23% The power required for a 90deg pulse in the center of the head at 7T was approximately twice that at 4T. The SNR averaged over the brain was at least 1.6 times higher at 7T compared to 4T. These experimental results were consistent with calculations performed using a human head model and Maxwell's equations. Magn Reson Med 46:24 30, 2001.

title: Comprehensive quantification of signal to noise ratio and g factor for image based and k space based parallel imaging reconstructions
abstract: Parallel imaging reconstructions result in spatially varying noise amplification characterized by the g factor, precluding conventional measurements of noise from the final image. A simple Monte Carlo based method is proposed for all linear image reconstruction algorithms, which allows measurement of signal to noise ratio and g factor and is demonstrated for SENSE and GRAPPA reconstructions for accelerated acquisitions that have not previously been amenable to such assessment. Only a simple "prescan" measurement of noise amplitude and correlation in the phased array receiver, and a single accelerated image acquisition are required, allowing robust assessment of signal to noise ratio and g factor. The "pseudo multiple replica" method has been rigorously validated in phantoms and in vivo, showing excellent agreement with true multiple replica and analytical methods. This method is universally applicable to the parallel imaging reconstruction techniques used in clinical applications and will allow pixel by pixel image noise measurements for all parallel imaging strategies, allowing quantitative comparison between arbitrary k space trajectories, image reconstruction, or noise conditioning techniques. Magn Reson Med 60:895 907, 2008.

title: Reducing SAR and enhancing cerebral signal to noise ratio with high permittivity padding at 3 T
abstract: Previous works have shown that placement of a high dielectric pad can improve image intensity in a region adjacent to the pad, or that placement of dielectric pads around a large surface of the head can improve image homogeneity on an entire plane through the head in high field MRI. Here, experimental results show that use of high dielectric pads around the human head can reduce the required input radiofrequency power by 50% while enhancing image signal to noise ratio by 20 40% throughout the cerebrum at 3 T. Thus, dielectric pads may be used to provide a relatively simple and low cost method for improving quality and safety of MRI in a variety of applications at 3 T. Magn Reson Med, 2011.