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Echo Time (echo + time)
Kinds of Echo Time Selected AbstractsFunctional magnetic resonance imaging of the human brain and spinal cord by means of signal enhancement by extravascular protonsCONCEPTS IN MAGNETIC RESONANCE, Issue 1 2003P.W. Stroman Abstract A review of functional magnetic resonance imaging (fMRI) signal changes in spin,echo image data is presented. Spin,echo fMRI data from the human brain and spinal cord show a consistent departure from that expected with blood oxygen level dependent (BOLD) contrast. Studies to investigate this finding demonstrate fMRI signal changes of 2.5% in the spinal cord and 0.7% in the brain at 1.5 T, which is extrapolated to an echo time of zero. Consistent evidence of a non-BOLD contrast mechanism arising from a proton-density change at sites of neuronal activation is demonstrated. A mathematical model and physiological explanation for signal enhancement by extravascular protons is also presented. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson 16A: 28,34, 2003 [source] Single-cell detection by gradient echo 9.4 T MRI: a parametric studyCONTRAST MEDIA & MOLECULAR IMAGING, Issue 4 2006P. Smirnov Abstract Recent studies have shown that cell migration can be monitored in vivo by magnetic resonance imaging after intracellular contrast agent incorporation. This is due to the dephasing effect on proton magnetization of the local magnetic field created by a labelled cell. Anionic iron oxide nanoparticles (AMNP) are among the most efficient and non-toxic contrast agents to be spontaneously taken up by a wide variety of cells. Here we measured the iron load and magnetization of HeLa tumour cells labelled with AMNP, as a function of the external magnetic field. High-resolution gradient echo 9.4,T MRI detected individual labelled cells, whereas spin echo sequences were poorly sensitive. We then conducted a systematic study in order to determine the gradient echo sequence parameters (echo time, cell magnetization and resolution) most suitable for in vivo identification of single cells. Copyright © 2006 John Wiley & Sons, Ltd. [source] MRA of intracranial aneurysms embolized with platinum coils: A vascular phantom study at 1.5T and 3TJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2008Shingo Kakeda Abstract Purpose To analyze the influence of matrix and echo time (TE) of three-dimensional time-of-flight (3D TOF) magnetic resonance angiography (MRA) on the depiction of residual flow in aneurysms embolized with platinum coils at 1.5T and 3T. Materials and Methods A simulated intracranial aneurysm of the vascular phantom was loosely packed to maintain the patency of some residual aneurysmal lumen with platinum coils and connected to an electromagnetic flow pump with pulsatile flow. MRAs were obtained altering the matrix and TE of 3D TOF sequences at 1.5T and 3T. Results The increased spatial resolution and the shorter TE offered better image quality at 3T. For the depiction of an aneurysm remnant, the high-spatial-resolution 3T MRA (matrix size of 384 × 224 and 512 × 256) with a short TE of ,3.3 msec were superior to the 1.5T MRA obtained with any sequences. Conclusion 3T MRA is superior to 1.5T MRA for the assessment of aneurysms embolized with platinum coils; the combination of the 512 × 256 matrix and short TE (3.3 msec or less) seems feasible at 3T. J. Magn. Reson. Imaging 2008;28:13,20. © 2008 Wiley-Liss, Inc. [source] Improved slice selection for R2* mapping during cryoablation with eddy current compensation,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2008Aiming Lu PhD Abstract Purpose To improve the slice profile and image quality of R2* mapping in the iceball during cryoablation with ultrashort echo time (UTE) imaging by compensating for eddy currents induced by the selective gradient when half-pulse radiofrequency (RF) excitation is employed to achieve UTEs. Materials and Methods A method to measure both B0 and linear eddy currents simultaneously is first presented. This is done with a least-square fitting process on calibration data collected on a phantom. Eddy currents during excitation are compensated by redesigning the RF pulse and the selective gradient accordingly, while that resultant from the readout gradient are compensated for during image reconstruction. In vivo data were obtained continuously during the cryoablation experiments to calculate the R2* values in the iceball and to correlate them with the freezing process. Results Image quality degradation due to eddy currents is significantly reduced with the proposed approaches. R2* maps of iceball throughout the cryoablation experiments were achieved with improved quality. Conclusion The proposed approaches are effective for compensating eddy currents during half-pulse RF excitation as well as readout. TEs as short as 100 ,sec were obtained, allowing R2* maps to be obtained from frozen tissues with improved quality. J. Magn. Reson. Imaging 2008;28:190,198. © 2008 Wiley-Liss, Inc. [source] Echo combination to reduce proton resonance frequency (PRF) thermometry errors from fatJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2008Viola Rieke PhD Abstract Purpose To validate echo combination as a means to reduce errors caused by fat in temperature measurements with the proton resonance frequency (PRF) shift method. Materials and Methods Computer simulations were performed to study the behavior of temperature measurement errors introduced by fat as a function of echo time. Error reduction by combining temperature images acquired at different echo times was investigated. For experimental verification, three echoes were acquired in a refocused gradient echo acquisition. Temperature images were reconstructed with the PRF shift method for the three echoes and then combined in a weighted average. Temperature measurement errors in the combined image and the individual echoes were compared for pure water and different fractions of fat in a computer simulation and for a phantom containing a homogenous mixture with 20% fat in an MR experiment. Results In both simulation and MR measurement, the presence of fat caused severe temperature underestimation or overestimation in the individual echoes. The errors were substantially reduced after echo combination. Residual errors were about 0.3°C for 10% fat and 1°C for 20% fat. Conclusion Echo combination substantially reduces temperature measurement errors caused by small fractions of fat. This technique then eliminates the need for fat suppression in tissues such as the liver. J. Magn. Reson. Imaging 2007. © 2007 Wiley-Liss, Inc. [source] In vivo and in vitro MR spectroscopic profile of central neurocytomasJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2003Rama Jayasundar PhD Abstract This study reports in vivo and in vitro magnetic resonance spectroscopic findings in two cases of central neurocytomas (CNC) confirmed by immunohistochemistry. Volume localized in vivo proton magnetic resonance spectroscopy (MRS) was carried out before surgery using a point resolved spectroscopy (PRESS) sequence with a repetition time of six seconds and an echo time of 135 msec. Normal spectrum was obtained from gray matter from a volunteer for comparison. 1H and 31P in vitro MRS studies were carried out at 9.4 T on the extracts prepared from the surgically excised tumors. The in vivo spectra showed prominent glycine (Gly) and choline (Cho) and low N-acetyl aspartate compared to the normal. The Gly peak was assigned using the in vitro studies. These studies showed that the major contribution to the Cho peak observed in vivo is from phosphocholine. A combination of the presence of NAA and an increased Gly in the proton MR spectrum could be a characteristic feature of CNCs, which are rare intraventricular tumors of neuronal origin. J. Magn. Reson. Imaging 2003;17:256,260. © 2003 Wiley-Liss, Inc. [source] Applications of neural network analyses to in vivo 1H magnetic resonance spectroscopy of Parkinson disease patientsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2002David Axelson PhD Abstract Purpose To apply neural network analyses to in vivo magnetic resonance spectra of controls and Parkinson disease (PD) patients for the purpose of classification. Materials and Methods Ninety-seven in vivo proton magnetic resonance spectra of the basal ganglia were recorded from 31 patients with (PD) and 14 age-matched healthy volunteers on a 1.5-T imager. The PD patients were grouped as follows: probable PD (N = 15), possible PD (N = 11), and atypical PD (N = 5). Total acquisition times of approximately five minutes were achieved with a TE (echo time) of 135 msec, a TR (repetition time) of 2000 msec, and 128 scan averages. Neural network (back propagation, Kohonen, probabilistic, and radial basis function) and related (generative topographic mapping) data analyses were performed. Results Conventional data analysis showed no statistically significant differences in metabolite ratios based on measuring signal intensities. The trained networks could distinguish control from PD with considerable accuracy (true positive fraction 0.971, true negative fraction 0.933). When four classes were defined, approximately 88% of the predictions were correct. The multivariate analysis indicated metabolic changes in the basal ganglia in PD. Conclusion A variety of neural network and related approaches can be successfully applied to both qualitative visualization and classification of in vivo spectra of PD patients. J. Magn. Reson. Imaging 2002;16:13,20. © 2002 Wiley -Liss, Inc. [source] Diffusion-weighted spectroscopy: A novel approach to determine macromolecule resonances in short-echo time 1H-MRSMAGNETIC RESONANCE IN MEDICINE, Issue 4 2010N. Kunz Abstract Quantification of short-echo time proton magnetic resonance spectroscopy results in >18 metabolite concentrations (neurochemical profile). Their quantification accuracy depends on the assessment of the contribution of macromolecule (MM) resonances, previously experimentally achieved by exploiting the several fold difference in T1. To minimize effects of heterogeneities in metabolites T1, the aim of the study was to assess MM signal contributions by combining inversion recovery (IR) and diffusion-weighted proton spectroscopy at high-magnetic field (14.1 T) and short echo time (=8 msec) in the rat brain. IR combined with diffusion weighting experiments (with ,/, = 1.5/200 msec and b -value = 11.8 msec/,m2) showed that the metabolite nulled spectrum (inversion time = 740 msec) was affected by residuals attributed to creatine, inositol, taurine, choline, N -acetylaspartate as well as glutamine and glutamate. While the metabolite residuals were significantly attenuated by 50%, the MM signals were almost not affected (<8%). The combination of metabolite-nulled IR spectra with diffusion weighting allows a specific characterization of MM resonances with minimal metabolite signal contributions and is expected to lead to a more precise quantification of the neurochemical profile. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source] Ultrashort TE T1rho (UTE T1rho) imaging of the Achilles tendon and meniscusMAGNETIC RESONANCE IN MEDICINE, Issue 3 2010Jiang Du Abstract In this study, we report the use of a novel ultrashort echo time T1rhoT1 sequence that combines a spin-lock preparation pulse with a two-dimensional ultrashort echo time sequence of a nominal echo time 8 ,sec. The ultrashort echo time- T1rho sequence was employed to quantify T1rho in short T2 tissues including the Achilles tendon and the meniscus. T1rho dispersion was investigated by varying the spin-lock field strength. Preliminary results on six cadaveric ankle specimens and five healthy volunteers show that the ultrashort echo time- T1rho sequence provides high signal and contrast for both the Achilles tendon and the meniscus. The mean T1rho of the Achilles tendon ranged from 3.06 ± 0.51 msec for healthy volunteers to 5.22 ± 0.58 msec for cadaveric specimens. T1rho increased to 8.99 ± 0.24 msec in one specimen with tendon degeneration. A mean T1rho of 7.98 ± 1.43 msec was observed in the meniscus of the healthy volunteers. There was significant T1rho dispersion in both the Achilles tendon and the meniscus. Mean T1rho increased from 2.06 ± 0.23 to 7.85 ± 0.74 msec in normal Achilles tendon and from 7.08 ± 0.64 to 13.42 ± 0.93 msec in normal meniscus when the spin-lock field was increased from 250 to 1,000 Hz. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source] A technique for rapid single-echo spin-echo T2 mappingMAGNETIC RESONANCE IN MEDICINE, Issue 2 2010Marshall S. Sussman Abstract A rapid technique for mapping of T2 relaxation times is presented. The method is based on the conventional single-echo spin echo approach but uses a much shorter pulse repetition time to accelerate data acquisition. The premise of the new method is the use of a constant difference between the echo time and pulse repetition time, which removes the conventional and restrictive requirement of pulse repetition time , T1. Theoretical and simulation investigations were performed to evaluate the criteria for accurate T2 measurements. Measured T2s were shown to be within 1% error as long as the key criterion of pulse repetition time/T2 ,3 is met. Strictly, a second condition of echo time/T1 , 1 is also required. However, violations of this condition were found to have minimal impact in most clinical scenarios. Validation was conducted in phantoms and in vivo T2 mapping of healthy cartilage and brain. The proposed method offers all the advantages of single-echo spin echo imaging (e.g., immunity to stimulated echo effects, robustness to static field inhomogeneity, flexibility in the number and choice of echo times) in a considerably reduced amount of time and is readily implemented on any clinical scanner. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source] In vivo detection of serine in the human brain by proton magnetic resonance spectroscopy (1H-MRS) at 7 TeslaMAGNETIC RESONANCE IN MEDICINE, Issue 4 2009Changho Choi Abstract A single-voxel proton magnetic resonance spectroscopy (1H-MRS) filtering strategy for in vivo detection of serine (Ser) in human brain at 7T is proposed. Spectral difference of coupled resonances arising from different subecho times of triple refocusing at a constant total echo time (TE) was utilized to detect the Ser multiplet and cancel the overlapping creatine (Cr) 3.92-ppm singlet via difference editing. Dependence of the Ser signal on subecho times was investigated using density-matrix simulation incorporating the slice-selective radio frequency (RF) pulses. The simulation indicated that the difference-edited Ser CH2 multiplet at ,3.96 ppm is maximized with (TE1, TE2, TE3) = (54, 78, 78) and (36, 152, 22) ms. The edited Ser peak amplitude was estimated, with both numerical and phantom analyses of the performance, as 83% with respect to 90° acquisition for a localized volume, ignoring relaxation effects. From the area ratio of the edited Ser and unedited Cr 3.03-ppm peaks, assuming identical T1 and T2 between Ser and Cr, the Ser-to-Cr concentration ratio for the frontal cortex of healthy adults was estimated to be 0.8 ± 0.2 (mean ± SD; N = 6). Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc. [source] Spin-echo MRI using ,/2 and , hyperbolic secant pulses,MAGNETIC RESONANCE IN MEDICINE, Issue 1 2009Jang-Yeon Park Abstract Frequency-modulated (FM) pulses have practical advantages for spin-echo experiments, such as the ability to produce a broadband , rotation, with an inhomogeneous radiofrequency (RF) field. However, such use leads to a nonlinear phase of the transverse magnetization, which is why FM pulses like the hyperbolic secant (HS) pulse are not commonly used for multislice spin-echo magnetic resonance imaging (MRI). Here, a general theory and methods are described for conventional spin-echo imaging using a , HS pulse for refocusing. Phase profiles produced by the HS pulse are analytically described. The analysis is extended to yield the specific relationships between pulse parameters and gradients, which must be satisfied to compensate the nonlinear phase variation produced with a spin-echo sequence composed of ,/2 and , HS pulses (the ,/2 HS , , HS sequence). The latter offers advantages for multislice spin-echo MRI, including excellent slice-selection and partial compensation for RF inhomogeneity. Furthermore, it can be implemented with a shorter echo time and lower power deposition than a previously described method using a pair of , HS pulses. Magn Reson Med 61:175,187, 2009. © 2008 Wiley-Liss, Inc. [source] BOLD contrast sensitivity enhancement and artifact reduction with multiecho EPI: Parallel-acquired inhomogeneity-desensitized fMRIMAGNETIC RESONANCE IN MEDICINE, Issue 6 2006Benedikt A. Poser Abstract Functional MRI (fMRI) generally employs gradient-echo echo-planar imaging (GE-EPI) to measure blood oxygen level-dependent (BOLD) signal changes that result from changes in tissue relaxation time T between activation and rest. Since T strongly varies across the brain and BOLD contrast is maximal only where the echo time (TE) equals the local T, imaging at a single TE is a compromise in terms of overall sensitivity. Furthermore, the long echo train makes EPI very sensitive to main field inhomogeneities, causing strong image distortion. A method is presented that uses accelerated parallel imaging to reduce image artifacts and acquire images at multiple TEs following a single excitation, with no need to increase TR. Sensitivity gains from the broadened T coverage are optimized by pixelwise weighted echo summation based on local T or contrast-to-noise ratio (CNR) measurements. The method was evaluated using an approach that allows differential BOLD CNR to be calculated without stimulation, as well as with a Stroop experiment. Results obtained at 3 T showed that BOLD sensitivity improved by 11% or more in all brain regions, with larger gains in areas typically affected by strong susceptibility artifacts. The use of parallel imaging markedly reduces image distortion, and hence the method should find widespread application in functional brain imaging. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] Fast CT-PRESS-based spiral chemical shift imaging at 3 TeslaMAGNETIC RESONANCE IN MEDICINE, Issue 5 2006Dirk Mayer Abstract A new sequence is presented that combines constant-time point-resolved spectroscopy (CT-PRESS) with fast spiral chemical shift imaging. It allows the acquisition of multivoxel spectra without line splitting with a minimum total measurement time of less than 5 min for a field of view of 24 cm and a nominal 1.5 × 1.5-cm2 in-plane resolution. Measurements were performed with 17 CS encoding steps in t1 (,t1 = 12.8 ms) and an average echo time of 151 ms, which was determined by simulating the CT-PRESS experiment for the spin systems of glutamate (Glu) and myo -inositol (mI). Signals from N-acetyl-aspartate, total creatine, choline-containing compounds (Cho), Glu, and mI were detected in a healthy volunteer with no or only minor baseline distortions within 14 min on a 3 T MR scanner. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] Eliminating spurious lipid sidebands in 1H MRS of breast lesionsMAGNETIC RESONANCE IN MEDICINE, Issue 2 2002Patrick J. Bolan Abstract Detecting metabolites in breast lesions by in vivo 1H MR spectroscopy can be difficult due to the abundance of mobile lipids in the breast which can produce spurious sidebands that interfere with the metabolite signals. Two-dimensional J -resolved spectroscopy has been demonstrated in the brain as a means to eliminate these artifacts from a large water signal; coherent sidebands are resolved at their natural frequencies, leaving the noncoupled metabolite resonances in the zero-frequency trace of the 2D spectrum. This work demonstrates that using the zero-frequency trace,or equivalently the average of spectra acquired with different echo times,can be used to separate noncoupled metabolite signals from the lipid-induced sidebands. This technique is demonstrated with simulations, phantom studies, and in several breast lesions. Compared to the conventional approach using a single echo time, echo time averaging provides increased sensitivity for the study of small and irregularly shaped lesions. Magn Reson Med 48:215,222, 2002. © 2002 Wiley-Liss, Inc. [source] Toward direct mapping of neuronal activity: MRI detection of ultraweak, transient magnetic field changes,MAGNETIC RESONANCE IN MEDICINE, Issue 6 2002Jerzy Bodurka Abstract A novel method based on selective detection of rapidly changing ,B0 magnetic fields and suppression of slowly changing ,B0 fields is presented. The ultimate goal of this work is to present a method that may allow detection of transient and subtle changes in B0 in cortical tissue associated with electrical currents produced by neuronal activity. The method involves the detection of NMR phase changes that occur during a single-shot spin-echo (SE) echo-planar sequence (EPI) echo time. SE EPI effectively rephases all changes in B0 that occur on a time scale longer than the echo time (TE) and amplifies all ,B0 changes that occur during TE/2. The method was tested on a phantom that contains wires in which current can be modulated. The sensitivity and flexibility of the technique was demonstrated by modulation of the temporal position and duration of the stimuli-evoked transient magnetic field relative to the 180 RF pulse in the imaging sequence,requiring precise stimulus timing. Currently, with this method magnetic field changes as small as 2 × 10,10 T (200 pT) and lasting for 40 msec can be detected. Implications for direct mapping of brain neuronal activity with MRI are discussed. Magn Reson Med 47:1052,1058, 2002. Published 2002 Wiley-Liss, Inc. [source] Compartmental relaxation and diffusion tensor imaging measurements in vivo in ,-carrageenan-induced edema in rat skeletal muscle,NMR IN BIOMEDICINE, Issue 6 2008Reuben H. Fan Abstract Integrated diffusion tensor T2 measurements were made on normal and edematous rat muscle, and the data were fitted with one- and two-compartment models, respectively. Edematous muscle exhibited a short-lived component (T2,=,28,±,6,ms), with diffusion characteristics similar to that of normal muscle, and a long-lived component (T2,=,96,±,27,ms), with greater mean apparent diffusion coefficient (ADC) and lower fractional anisotropy (FA). With this two-component description of diffusion and relaxation, values of ADC and FA estimated with a conventional pulsed-gradient spin-echo sequence will depend on the echo time, relative fraction of short-lived and long-lived water signals, and the intrinsic ADC and FA values within the tissue. On the basis of the relative differences in water diffusion properties between long-lived and short-lived water signals, as well as the similarities between the short-lived component and normal tissue, it is postulated that these two signal components largely reflect intracellular and extracellular water. Copyright © 2007 John Wiley & Sons, Ltd. [source] An automated quantitation of short echo time MRS spectra in an open source software environment: AQSESNMR IN BIOMEDICINE, Issue 5 2007Jean-Baptiste Poullet Abstract This paper describes a new quantitation method called AQSES for short echo time magnetic resonance spectra. This method is embedded in a software package available online from www.esat.kuleuven.be/sista/members/biomed/new/ with a graphical user interface, under an open source license, which means that the source code is freely available and easy to adapt to specific needs of the user. The quantitation problem is mathematically formulated as a separable nonlinear least-squares fitting problem, which is numerically solved using a modified variable-projection procedure. A macromolecular baseline is incorporated into the fit via nonparametric modelling, efficiently implemented using penalized splines. Unwanted components such as residual water are removed with a maximum-phase FIR filter. Constraints on the phases, dampings and frequencies of the metabolites can be imposed. AQSES has been tested on simulated MR spectra with several types of disturbance and on short echo time in vivo proton MR spectra. Results show that AQSES is robust, easy to use and very flexible. Copyright © 2006 John Wiley & Sons, Ltd. [source] Characterization of BOLD activation in multi-echo fMRI data using fuzzy cluster analysis and a comparison with quantitative modelingNMR IN BIOMEDICINE, Issue 7-8 2001Markus Barth Abstract A combination of multiple gradient-echo imaging and exploratory data analysis (EDA), i.e. fuzzy cluster analysis (FCA), is proposed for separation and characterization of BOLD activation in single-shot spiral functional magnetic resonance imaging (fMRI) experiments at 3 T. Differentiation of functional activation using FCA is performed by clustering pixel signal changes (,S) as a function of echo time (TE). Further vascular classification is supported by the localization of activation and the comparison with a single-exponential decay model. In some subjects, an additional indication for large vessels within a voxel was found as oscillation of the fMRI signal difference vs echo time (TE). Such large vessels may be separated from small vessel activation and, therefore, our proposed procedure might prove useful if a more specific functional localization is desired in fMRI. In addition to the signal change ,S, ,T2*/T2* is significantly different between activated regions. Averaged over all eight subjects ,T2* is 1.7,±,0.2,ms in ROIs with the highest signal change characterized as containing large vessels, whereas in ROIs corresponding to microvascular environment average ,T2* values are 0.8,±,0.1,ms. Copyright © 2001 John Wiley & Sons, Ltd. [source] 1H and 19F nuclear magnetic resonance microimaging of water and chemical distribution in soil columnsENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 7 2007Myrna J. Simpson Abstract Nuclear magnetic resonance (NMR) microimaging is a noninvasive and nondestructive technique that has great potential for the study of soil processes. Hydrogen-1 NMR microimaging techniques were used to examine the distribution of water in four different soil cores. Fluorine-19 NMR microimaging is also used to study the transport of three model contaminants (hexafluorobenzene, sodium fluoride, and trifluralin) in soil columns. The 1H water distribution studies demonstrate that NMR microimaging can provide unique detail regarding the nature and location of water in soils. Image distortion (magnetic susceptibility) was observed for soil samples low in water (20,28% by weight) and that contained an iron content of 0.73 to 0.99%. Highly resolved images were obtained for the organic-rich soil (Croatan sample) and also facilitated the analysis of bound and unbound soil water through varying spin echo times. The contaminant studies with 19F NMR demonstrated that preferential flow processes can be observed in soil cores in as little as 16 h. Studies with hexafluorobenzene produced the highest quality images whereas the definition decreased over time with both trifluralin and sodium fluoride as the compounds penetrated the soil. Nonetheless, both 1H and 19F NMR microimaging techniques demonstrate great promise for studying soil processes. [source] Echo combination to reduce proton resonance frequency (PRF) thermometry errors from fatJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2008Viola Rieke PhD Abstract Purpose To validate echo combination as a means to reduce errors caused by fat in temperature measurements with the proton resonance frequency (PRF) shift method. Materials and Methods Computer simulations were performed to study the behavior of temperature measurement errors introduced by fat as a function of echo time. Error reduction by combining temperature images acquired at different echo times was investigated. For experimental verification, three echoes were acquired in a refocused gradient echo acquisition. Temperature images were reconstructed with the PRF shift method for the three echoes and then combined in a weighted average. Temperature measurement errors in the combined image and the individual echoes were compared for pure water and different fractions of fat in a computer simulation and for a phantom containing a homogenous mixture with 20% fat in an MR experiment. Results In both simulation and MR measurement, the presence of fat caused severe temperature underestimation or overestimation in the individual echoes. The errors were substantially reduced after echo combination. Residual errors were about 0.3°C for 10% fat and 1°C for 20% fat. Conclusion Echo combination substantially reduces temperature measurement errors caused by small fractions of fat. This technique then eliminates the need for fat suppression in tissues such as the liver. J. Magn. Reson. Imaging 2007. © 2007 Wiley-Liss, Inc. [source] Multiecho reconstruction for simultaneous water-fat decomposition and T2* estimation,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2007Huanzhou Yu PhD Abstract Purpose To describe and demonstrate the feasibility of a novel multiecho reconstruction technique that achieves simultaneous water-fat decomposition and T2* estimation. The method removes interference of water-fat separation with iron-induced T2* effects and therefore has potential for the simultaneous characterization of hepatic steatosis (fatty infiltration) and iron overload. Materials and Methods The algorithm called "T2*-IDEAL" is based on the IDEAL water-fat decomposition method. A novel "complex field map" construct is used to estimate both R2* (1/T2*) and local B0 field inhomogeneities using an iterative least-squares estimation method. Water and fat are then decomposed from source images that are corrected for both T2* and B0 field inhomogeneity. Results It was found that a six-echo multiecho acquisition using the shortest possible echo times achieves an excellent balance of short scan and reliable R2* measurement. Phantom experiments demonstrate the feasibility with high accuracy in R2* measurement. Promising preliminary in vivo results are also shown. Conclusion The T2*-IDEAL technique has potential applications in imaging of diffuse liver disease for evaluation of both hepatic steatosis and iron overload in a single breath-hold. J. Magn. Reson. Imaging 2007;26:1153,1161. © 2007 Wiley-Liss, Inc. [source] Focal liver lesions: Breathhold gradient- and spin-echo T2-weighted imaging for detection and characterization ,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2006Takeshi Yoshikawa MD Abstract Purpose To evaluate breathhold gradient- and spin-echo (GRASE) T2-weighted imaging for the detection and characterization of focal liver lesions. Materials and Methods Two GRASE sequences with different echo times (75 and 90 msec, GRASE75 and GRASE90) were compared with respiratory-triggered fast spin-echo (SE) and breathhold fast SE in 64 patients with 103 malignant and 51 benign lesions. Compared with respiratory-triggered and breathhold fast SE, GRASE reduced scan time by 77% to 82% and 21% to 27%, respectively. Two independent readers evaluated image quality and reviewed 504 liver segments on a segment-by-segment basis. Observer performance was evaluated with receiver operating characteristic (ROC) curve analysis. The signal-to-noise ratio (SNR) of liver and spleen, and lesion-to-liver contrast-to-noise ratio (CNR) were also measured. Results The overall quality of the GRASE images was higher than that of the respiratory-triggered and breathhold fast SE images, although signal inhomogeneities were more frequently observed with GRASE. No significant difference in the values of the area under the ROC curve (Az) for malignant lesion detection was found. The mean SNR and CNR were highest for respiratory-triggered fast SE. Conclusion T2-weighted breathhold GRASE has the potential to provide faster liver imaging. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc. [source] A simple method for rectified noise floor suppression: Phase-corrected real data reconstruction with application to diffusion-weighted imagingMAGNETIC RESONANCE IN MEDICINE, Issue 2 2010Douglas E. Prah Abstract Diffusion-weighted MRI is an intrinsically low signal-to-noise ratio application due to the application of diffusion-weighting gradients and the consequent longer echo times. The signal-to-noise ratio worsens with increasing image resolution and diffusion imaging methods that use multiple and higher b-values. At low signal-to-noise ratios, standard magnitude reconstructed diffusion-weighted images are confounded by the existence of a rectified noise floor, producing poor estimates of diffusion metrics. Herein, we present a simple method of rectified noise floor suppression that involves phase correction of the real data. This approach was evaluated for diffusion-weighted imaging data, obtained from ethanol and water phantoms and the brain of a healthy volunteer. The parameter fits from monoexponential, biexponential, and stretched-exponential diffusion models were computed using phase-corrected real data and magnitude data. The results demonstrate that this newly developed simple approach of using phase-corrected real images acts to reduce or even suppress the confounding effects of a rectified noise floor, thereby producing more accurate estimates of diffusion parameters. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source] A technique for rapid single-echo spin-echo T2 mappingMAGNETIC RESONANCE IN MEDICINE, Issue 2 2010Marshall S. Sussman Abstract A rapid technique for mapping of T2 relaxation times is presented. The method is based on the conventional single-echo spin echo approach but uses a much shorter pulse repetition time to accelerate data acquisition. The premise of the new method is the use of a constant difference between the echo time and pulse repetition time, which removes the conventional and restrictive requirement of pulse repetition time , T1. Theoretical and simulation investigations were performed to evaluate the criteria for accurate T2 measurements. Measured T2s were shown to be within 1% error as long as the key criterion of pulse repetition time/T2 ,3 is met. Strictly, a second condition of echo time/T1 , 1 is also required. However, violations of this condition were found to have minimal impact in most clinical scenarios. Validation was conducted in phantoms and in vivo T2 mapping of healthy cartilage and brain. The proposed method offers all the advantages of single-echo spin echo imaging (e.g., immunity to stimulated echo effects, robustness to static field inhomogeneity, flexibility in the number and choice of echo times) in a considerably reduced amount of time and is readily implemented on any clinical scanner. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source] Field-cycling NMR relaxometry with spatial selectionMAGNETIC RESONANCE IN MEDICINE, Issue 6 2010Kerrin J. Pine Abstract Fast field-cycling MRI offers access to sources of endogenous information not available from conventional fixed-field imagers. One example is the T1 dispersion curve: a plot of T1 versus field strength. We present a pulse sequence that combines saturation-recovery/inversion-recovery T1 determination with field cycling and point-resolved spectroscopy localization, enabling the measurement of dispersion curves from volumes selected from a pilot image. Compared with a nonselective sequence, our method of volume selection does not influence measurement accuracy, even for relatively long echo times and in the presence of radiofrequency field nonuniformity. The measured voxel profile, while not ideal, corresponds with that expected from the image slice profile. On a whole-body fast field-cycling scanner with 59-mT detection, the sensitivity of the experiment is sufficient to reveal distinctive "quadrupole dips" in dispersion curves of protein-rich human tissue in vivo. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source] Use of tissue water as a concentration reference for proton spectroscopic imagingMAGNETIC RESONANCE IN MEDICINE, Issue 6 2006Charles Gasparovic Abstract A strategy for using tissue water as a concentration standard in 1H magnetic resonance spectroscopic imaging studies on the brain is presented, and the potential errors that may arise when the method is used are examined. The sensitivity of the method to errors in estimates of the different water compartment relaxation times is shown to be small at short echo times (TEs). Using data from healthy human subjects, it is shown that different image segmentation approaches that are commonly used to account for partial volume effects (SPM2, FSL's FAST, and K-means) lead to different estimates of metabolite levels, particularly in gray matter (GM), owing primarily to variability in the estimates of the cerebrospinal fluid (CSF) fraction. While consistency does not necessarily validate a method, a multispectral segmentation approach using FAST yielded the lowest intersubject variability in the estimates of GM metabolites. The mean GM and white matter (WM) levels of N-acetyl groups (NAc, primarily N-acetylaspartate), choline (Ch), and creatine (Cr) obtained in these subjects using the described method with FAST multispectral segmentation are reported: GM [NAc] = 17.16 ± 1.19 mM; WM [NAc] = 14.26 ± 1.38 mM; GM [Ch] = 3.27 ± 0.47 mM; WM [Ch] = 2.65 ± 0.25 mM; GM [Cr] = 13.98 ± 1.20 mM; and WM [Cr] = 7.10 ± 0.67 mM. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] Centering the projection reconstruction trajectory: Reducing gradient delay errors,MAGNETIC RESONANCE IN MEDICINE, Issue 1 2003Dana C. Peters Abstract The projection reconstruction (PR) trajectory was investigated for the effect of gradient timing delays between the actual and requested start time of each physical gradient. Radial trajectories constructed with delayed gradients miss the center of k -space in an angularly dependent manner, causing effective echo times to vary with projection angle. The gradient timing delays were measured in phantoms, revealing delays on the x, y, and z gradients which differed by as much as 5 ,sec. Using this one-time calibration measurement, the trajectories were corrected for gradient delays by addition of compensatory gradient areas to the prephasers of the logical x and y readout gradients. Effective projection-to-projection echo time variability was reduced to less than 1 ,sec for all imaging orientations. Using corrected trajectories, artifacts were reduced in phantom images and in volunteer studies. This correction should potentiate greater clinical use of the PR trajectory. Magn Reson Med 50:1,6, 2003. Published 2003 Wiley-Liss, Inc. [source] Changes in the proton T2 relaxation times of cerebral water and metabolites during forebrain ischemia in rat at 9.4 TMAGNETIC RESONANCE IN MEDICINE, Issue 6 2003Hao Lei Abstract Proton T2 relaxation times of cerebral water and metabolites were measured before, during, and after transient forebrain ischemia in rat at 9.4 T using localized proton magnetic resonance spectroscopy (1H-MRS) with Hahn echoes formed at different echo times (TEs). It was found that the T2 values of water and N-acetyl aspartate (NAA) methyl, but not total creatine (tCr) methyl, decrease significantly (,10%) during ischemia, and this T2 reduction is reversed by reperfusion. The T2 reduction observed for NAA was most likely caused by the extravascular component of the blood oxygenation level-dependent (BOLD) effect induced by a drastically increased deoxyhemoglobin content during ischemia. The absence of T2 changes for tCr can probably be explained by the fact that the BOLD-related T2 decrease was counterbalanced by the conversion of phosphocreatine (PCr) to creatine (Cr), which has a longer T2 than PCr, during ischemia. The changes in T2 should be taken into account for the quantification of metabolite concentrations during ischemia. Magn Reson Med 49:979,984, 2003. © 2003 Wiley-Liss, Inc. [source] Eliminating spurious lipid sidebands in 1H MRS of breast lesionsMAGNETIC RESONANCE IN MEDICINE, Issue 2 2002Patrick J. Bolan Abstract Detecting metabolites in breast lesions by in vivo 1H MR spectroscopy can be difficult due to the abundance of mobile lipids in the breast which can produce spurious sidebands that interfere with the metabolite signals. Two-dimensional J -resolved spectroscopy has been demonstrated in the brain as a means to eliminate these artifacts from a large water signal; coherent sidebands are resolved at their natural frequencies, leaving the noncoupled metabolite resonances in the zero-frequency trace of the 2D spectrum. This work demonstrates that using the zero-frequency trace,or equivalently the average of spectra acquired with different echo times,can be used to separate noncoupled metabolite signals from the lipid-induced sidebands. This technique is demonstrated with simulations, phantom studies, and in several breast lesions. Compared to the conventional approach using a single echo time, echo time averaging provides increased sensitivity for the study of small and irregularly shaped lesions. Magn Reson Med 48:215,222, 2002. © 2002 Wiley-Liss, Inc. [source] | |||||||||||||