An MR imaging method for simultaneous measurement of gaseous diffusion constant and longitudinal relaxation time

A magnetic resonance imaging method for simultaneous and accurate determination of gaseous diffusion constant and longitudinal relaxation time is presented. The method is based on direct observation of diffusive motion. Initially, a slice-selective saturation of helium-3 (3He) spins was performed on a 3He/O2 phantom (9 atm/2 atm). A time-delay interval was introduced after saturation, allowing spins to diffuse in and out of the labeled slice. Following the delay interval a one-dimensional (1-D) projection image of the phantom was acquired. A series of 21 images was collected, each subsequent image having been acquired with an increased delay interval. Gradual spreading of the slice boundaries due to diffusion was thus observed. The projection profiles were fit to a solution of the Bloch equation corrected for diffusive motion. The fitting procedure yielded a value of D3He = 0.1562+/-0.0013 cm2/s, in good agreement with a measurement obtained with a modified version of the standard pulsed-field gradient technique. The method also enabled us to accurately measure the longitudinal relaxation of 3He spins by fitting the change of the total area under the projection profiles to an exponential. A value of T1 = 1.67 s (2 T field) was recorded, in excellent agreement with an inversion recovery measurement.     

Cyclic variation of T1 in the myocardium at 3 T

Standard methods of longitudinal relaxation (T1) measurements in the heart produce only one T1 map of the myocardium, usually at the end diastole (ED). In this article, we investigated the feasibility of using a dual flip angle fast gradient echo technique in the steady state to generate a movie of T1 maps in the myocardium during the cardiac cycle. The effects of nonideal slice profile and transient steady state on the T1 measurements were evaluated by Bloch simulations. Based on these results, we introduce a linear correction to the measured T1 values, which was validated by phantom experiments. In vivo T1 cine maps in healthy volunteers show 70+/-7% drop in T1 from the ED to the end systole in the septum and a 43+/-13% drop in the left ventricular lateral wall. With further improvements, this technique could be used to assess the myocardial blood volume changes during the cardiac cycle.     

Magnetic Resonance Fingerprinting with short relaxation intervals

PurposeThe aim of this study was to investigate a technique for improving the performance of Magnetic Resonance Fingerprinting (MRF) in repetitive sampling schemes, in particular for 3D MRF acquisition, by shortening relaxation intervals between MRF pulse train repetitions.Material and methodsA calculation method for MRF dictionaries adapted to short relaxation intervals and non-relaxed initial spin states is presented, based on the concept of stationary fingerprints. The method is applicable to many different k-space sampling schemes in 2D and 3D. For accuracy analysis, T₁ and T₂ values of a phantom are determined by single-slice Cartesian MRF for different relaxation intervals and are compared with quantitative reference measurements. The relevance of slice profile effects is also investigated in this case. To further illustrate the capabilities of the method, an application to in-vivo spiral 3D MRF measurements is demonstrated.ResultsThe proposed computation method enables accurate parameter estimation even for the shortest relaxation intervals, as investigated for different sampling patterns in 2D and 3D. In 2D Cartesian measurements, we achieved a scan acceleration of more than a factor of two, while maintaining acceptable accuracy: The largest T₁ values of a sample set deviated from their reference values by 0.3% (longest relaxation interval) and 2.4% (shortest relaxation interval). The largest T₂ values showed systematic deviations of up to 10% for all relaxation intervals, which is discussed. The influence of slice profile effects for multislice acquisition is shown to become increasingly relevant for short relaxation intervals. In 3D spiral measurements, a scan time reduction of 36% was achieved, maintaining the quality of in-vivo T1 and T2 maps.ConclusionsReducing the relaxation interval between MRF sequence repetitions using stationary fingerprint dictionaries is a feasible method to improve the scan efficiency of MRF sequences. The method enables fast implementations of 3D spatially resolved MRF.     

Echo planar imaging of perfluorocarbons

Emulsions of perfluorotributylamine (FTBA) and perflubron were evaluated for their utility in ¹⁹F echo planar imaging. Fluorine images of the emulsions were obtained in a phantom and two mice that had been predosed. Both agents, but particularly perflubron, show potential for fluorine echo planar studies because of the long spin-spin relaxation times of the CF₃ resonances. High resolution thin slice images obtained in as little as 26.6 ms are presented.     

Assessment of scanner performance and normalization of estimated relaxation rate values

The purpose of this study was to develop and test a method for the assessment of Magnetic Resonance (MR) scanner performance suitable for routine brain MR studies and for normalization of calculated relaxation times. We hypothesized that regular monitoring of machine performance changes could provide a helpful normalization tool for calculating tissue MR parameters, thus contributing to support their use for longitudinal and comparative studies of both normal and diseased tissues.The method is based on the acquisition of phantom images during routine brain studies with standard spin-echo sequences. MR phantom and brain tissue parameters were used to assess the influence of machine related changes on relaxation parameter estimates. Experimental results showed that scanner performance may affect relaxation rate estimates. Phantom and in vivo results indicate that the correction method yields a reduction in variability of estimated phantom R1 values up to 29% and of R1 for different brain structures up to 17%. These findings support the validity of using brain coil phantoms for routine system monitoring and correction of tissue relaxation rates.     

Maximizing MR signal for 2D UTE slice selection in the presence of rapid transverse relaxation

Ultrashort TE (UTE) sequences allow direct visualization of tissues with very short T2 relaxation times, such as tendons, ligaments, menisci, and cortical bone. In this work, theoretical calculations, simulations, and phantom studies, as well as in vivo imaging were performed to maximize signal-to-noise ratio (SNR) for slice selective RF excitation for 2D UTE sequences. The theoretical calculations and simulations were based on the Bloch equations, which lead to analytic expressions for the optimal RF pulse duration and amplitude to maximize magnetic resonance signal in the presence of rapid transverse relaxation. In steady state, it was found that the maximum signal amplitude was not obtained at the classical Ernst angle, but at an either lower or higher flip angle, depending on whether the RF pulse duration or amplitude was varied, respectively.     

Bayesian Modeling of NMR Data: Quantifying Longitudinal Relaxation in Vivo,and in Vitro with a Tissue-Water-Relaxation Mimic (Crosslinked Bovine Serum Albumin)

Recently, a number of magnetic resonance imaging protocols have been reported that seek to exploit the effect of dissolved oxygen (O₂, paramagnetic) on the longitudinal ¹H relaxation of tissue water, thus providing image contrast related to tissue oxygen content. However, tissue water relaxation is dependent on a number of mechanisms and this raises the issue of how best to model the relaxation data. This problem, the model selection problem, occurs in many branches of science and is optimally addressed by Bayesian probability theory. High signal-to-noise, densely sampled, longitudinal ¹H relaxation data were acquired from rat brain in vivo and from a cross-linked bovine serum albumin (xBSA) phantom, a sample that recapitulates the relaxation characteristics of tissue water in vivo. Bayesian-based model selection was applied to a cohort of five competing relaxation models: (1) monoexponential, (2) stretched-exponential, (3) biexponential, (4) Gaussian (normal) R ₁-distribution, and (5) gamma R ₁-distribution. Bayesian joint analysis of multiple replicate datasets revealed that water relaxation of both the xBSA phantom and in vivo rat brain was best described by a biexponential model, while xBSA relaxation datasets truncated to remove evidence of the fast relaxation component were best modeled as a stretched exponential. In all cases, estimated model parameters were compared to the commonly used monoexponential model. Reducing the sampling density of the relaxation data and adding Gaussian-distributed noise served to simulate cases in which the data are acquisition-time or signal-to-noise restricted, respectively. As expected, reducing either the number of data points or the signal-to-noise increases the uncertainty in estimated parameters and, ultimately, reduces support for more complex relaxation models.     

A Simple Phantom to Locate the Origin of MRI Ghost Artefacts

A simple pyramidal tube phantom has been designed to allow accurate location of a selected slice along the desired axis of a magnetic resonance scanner and to provide a check of the quality of slice selection. It is particularly helpful in locating the origin of any out-of-slice ghost artefacts. The geometry of the phantom allows the slice positions to be calculated readily.     

自旋-晶格弛豫

本文讨论了工作[3]中决定耦合系统磁化系数的方法,并用来分析自旋-晶格弛豫过程。在弱耦合的情况下,得出了决定磁化系数的耦合方程组,并求出纵向非共振吸收和横向共振吸收线型的表达式。得出的纵向和横向弛豫时间是外加交变场频率的函数,这反映了高频场对弛豫的影响是由于所用密度矩阵方程为非马尔科夫型的直接结果。在远离共振点处,所得的线型公式和德拜型或洛仑兹型差别较大。一般说来,非马尔科夫效应是不能忽略的。在自旋S=1的情况,我们系统地分析了纵向及横向弛豫的基本过程。其中包含与通常讨论的过程相应的项,如单声子过程,Raman过程,Orbach过程等等,但现在都有外加交变场频率ω参与进去。最后讨论了声子的寿命对横向弛豫时间的影响。     

Consideration of slice profiles in inversion recovery Look–Locker relaxation parameter mapping

Purpose

To include the flip angle distribution caused by the slice profile into the model used for describing the relaxation curves observed in inversion recovery Look–Locker FLASH T₁ mapping for a more accurate determination of the relaxation parameters.

Materials and methods

For each inversion time, the flip angle dependent signal of the mono-exponential relaxation model is integrated across the slice profile. The resulting Consideration of Slice Profiles (CSP) relaxation curves are compared to the mono-exponential signal model in numerical simulations as well as in phantom and in-vivo experiments.

Results

All measured relaxation curves showed systematic deviations from a mono-exponential curve increasing with flip angle and T₁ but decreasing with repetition time. Additionally, the accuracy of T₁ was found to be largely dependent on the temporal coverage of the relaxation curve. All these systematic errors were largely reduced by the CSP model.

Conclusion

The proposed CSP model represents a useful extension of the conventionally used mono-exponential relaxation model. Despite inherent model inaccuracies, the mono-exponential model was found to be sufficient for many T₁ mapping situations. However, if only a poor temporal coverage of the relaxation process is achievable or a very precise modeling of the relaxation course is needed as in model-based techniques, the mono-exponential model leads to systematic errors and the CSP model should be used instead.     

In vivo oximetry by a pulsed longitudinally detected ESR spectrometer

By using a narrow single electron spin resonance (ESR) line agent, triarylmethyl, tris(8-carboxy-2,2,6,6-tetrahydroxyethylbenzo[1,2-d:4,5-d′] bis(1,3)dithiole-4-yl)methyl sodium salt (TAM OX063), pulsed longitudinally detected ESR (LODESR) measurements of a phantom or the chest of a living mouse at the operating frequency of ca. 300 MHz were taken and the effective longitudinal relaxation time (T ₁^*) was estimated for oximetry. Under irradiation of a pair of π-pulses with a variable interval between pulses (τ), in-phase LODESR signal intensities were obtained from the phantoms containing TAM dissolved in a physiological saline solution at a concentration of 1 mM and various concentrations of oxygen. TheT ₁^* of the phantom was calculated from the plotted curve of the LODESR signal intensity against τ. It was found that the reciprocal ofT ₁^*, i.e., the longitudinal relaxation rate, increased with the concentration of oxygen. In vivo pulsed LODESR measurements of the chest of living mice that had received a TAM injection via the intraperitoneal route were made. While the LODESR measurements were being made, the mice in one group breathed normal air and those in another group breathed 100% oxygen. It was found that the longitudinal relaxation rate of the mice breathing 100% oxygen was significantly greater than that of mice breathing normal air, indicating that breathing 100% oxygen elevates the thoracic longitudinal relaxation rate.     

Partial volume effects in MRI studies of multiple sclerosis

We have estimated the accuracy of volume measurements of multiple sclerosis (MS) lesions made using magnetic resonance imaging (MRI) for lesions of comparable diameter to the image slice thickness. We used a phantom containing objects of known volume and obtained images using a range of slice thicknesses. Measurements on the phantom were used to assess a theoretical model, which was then employed to investigate the effects of image dimensions and geometry upon volume measurement accuracy. We observed measured volume to be dependent upon slice thickness. Thin slices gave the most accurate estimate of volume. As slice thickness increased relative to object diameter, the error in the volume measurement increased (to as much as 100%), the volume measured being dependent on the position of the object relative to the slice center. Using a signal intensity threshold value of 50% to outline objects gave results closest to the actual volume. As expected, a lower threshold value tended to give higher volume estimates (up to 100% larger), as did a semi-automated local edge detection technique. For accurate volume measurement, the slice thickness should be no more than a fifth of anticipated object diameter. For typical MS lesions (7 mm in diameter), this implies using a 1.5-mm slice thickness. For serial studies, a repositioning error of 1 mm could lead to differences in the volume measurement of individual lesions of up to 12% between studies for lesions of typical MS size and 5-mm slice thickness. These results emphasize the need for accurate patient repositioning, relatively thin slices, for regular quality assurance checks to ensure that pixel size and slice position are correct and stable over time, and that lesion outlining is performed in a consistent fashion. We would recommend the use of a 3D sequence with 1 mm cubic voxels for accurate measurements of MS lesions.     

Multisliced ultrafast 2D relaxometry

By associating each slice in a spatially homogeneous sample with a different inversion-recovery delay time, multislice methods are used to reduce the acquisition times of 2D inversion-recovery T₁-T₂ relaxation spectra to just a few minutes. The increased speed comes at the expense of reduced signal/noise and this is reflected most noticeably in shifts in the component longitudinal relaxation times. Nevertheless, the major features of the 2D relaxation spectra are reproduced.     

Quantitative quality assurance in a multicenter HARDI clinical trial at 3 T

A phantom-based quality assurance (QA) protocol was developed for a multicenter clinical trial including high angular resolution diffusion imaging (HARDI). A total of 27 3 T MR scanners from 2 major manufacturers, GE (Discovery and Signa scanners) and Siemens (Trio and Skyra scanners), were included in this trial. With this protocol, agar phantoms doped to mimic relaxation properties of brain tissue are scanned on a monthly basis, and quantitative procedures are used to detect spiking and to evaluate eddy current and Nyquist ghosting artifacts. In this study, simulations were used to determine alarm thresholds for minimal acceptable signal-to-noise ratio (SNR). Our results showed that spiking artifact was the most frequently observed type of artifact. Overall, Trio scanners exhibited less eddy current distortion than GE scanners, which in turn showed less distortion than Skyra scanners. This difference was mainly caused by the different sequences used on these scanners. The SNR for phantom scans was closely correlated with the SNR from volunteers. Nearly all of the phantom measurements with artifact-free images were above the alarm threshold, suggesting that the scanners are stable longitudinally. Software upgrades and hardware replacement sometimes affected SNR substantially but sometimes did not. In light of these results, it is important to monitor longitudinal SNR with phantom QA to help interpret potential effects on in vivo measurements. Our phantom QA procedure for HARDI scans was successful in tracking scanner performance and detecting unwanted artifacts.     

Single-slice mapping of ultrashort T(2)

In this communication we present a method for single-slice mapping of ultrashort transverse relaxation times T(2). The RF pulse sequence consists of a spin echo preparation of the magnetization followed by slice-selective ultrashort echo time (UTE) imaging with radial k-space sampling. In order to keep the minimum echo time as small as possible, avoid out-of-slice contamination and signal contamination due to unwanted echoes, the implemented pulse sequence employs a slice-selective 180° RF refocusing pulse and a 4-step phase cycle. The slice overlap of the two slice-selective RF pulses was investigated. An acceptable Gaussian slice profile could be achieved by adjusting the strength of the two slice-selection gradients. The method was tested on a short T(2) phantom consisting of an arrangement of a roll of adhesive tape, an eraser, a piece of modeling dough made of Plasticine?, and a 10% w/w agar gel. The T(2) measurements on the phantom revealed exponential signal decays for all samples with T(2)(adhesive tape)=(0.5 ± 0.1)ms, T(2)(eraser)=(2.33 ± 0.07)ms, T(2)(Plasticine?)=(2.8 ± 0.06)ms, and T(2)(10%agar)=(9.5 ± 0.83)ms. The T(2) values obtained by the mapping method show good agreement with the T(2) values obtained by a non-selective T(2) measurement. For all samples, except the adhesive tape, the effective transverse relaxation time T(2)(?) was significantly shorter than T(2). Depending on the scanner hardware the presented method allows mapping of T(2) down to a few hundreds of microseconds. Besides investigating material samples, the presented method can be used to study the rapidly decaying MR-signal from biological tissue (e.g.: bone, cartilage, and tendon) and quadrupolar nuclei (e.g.: (23)Na, (35)Cl, and (17)O).     

The influence of collective effects on quantum fluctuations of laser radiation

Quantum fluctuations in a laser with two different relaxation times are considered, i.e., transverse (polarization relaxation) and longitudinal (population relaxation) in the case in which the cavity transmission band half-width is much smaller than the transverse width and much larger than the longitudinal one. The lasing frequency detuning from the transition frequency of a two-level system is assumed to be arbitrary in this case, and it is necessary to take into account the contribution of two-particle correlators into the dispersion and laser linewidth. The results are considered as applied to a semiconductor laser.     

Effects of RF profile on precision of quantitative T2 mapping using dual-echo steady-state acquisition

The dual echo steady-state (DESS) sequence has been shown successful in achieving fast T2 mapping with good precision. Under-estimation of T2, however, becomes increasingly prominent as the flip angle decreases. In 3D DESS imaging, therefore, the derived T2 values would become a function of the slice location in the presence of non-ideal slice profile of the excitation RF pulse. Furthermore, the pattern of slice-dependent variation in T2 estimates is dependent on the RF pulse waveform. Multi-slice 2D DESS imaging provides better inter-slice consistency, but the signal intensity is subject to integrated effects of within-slice distribution of the actual flip angle. Consequently, T2 measured using 2D DESS is prone to inaccuracy even at the designated flip angle of 90°. In this study, both phantom and human experiments demonstrate the above phenomena in good agreement with model prediction.     

RF slice profile effects in magnetic resonance fingerprinting

The radio frequency (RF) slice profile effects on T1 and T2 estimation in magnetic resonance fingerprinting (MRF) are investigated with respect to time-bandwidth product (TBW), flip angle (FA) level and field inhomogeneities. Signal evolutions are generated incorporating the non-ideal slice selective excitation process using Bloch simulation and matched to the original dictionary with and without the non-ideal slice profile taken into account. For validation, phantom and in vivo experiments are performed at 3T. Both simulations and experiments results show that T1 and T2 error from non-ideal slice profile increases with increasing FA level, off-resonance, and low TBW values. Therefore, RF slice profile effects should be compensated for accurate determination of the MR parameters.     

Spin-diffusion NMR at low field for the study of multiphase solids

The use of spin-diffusion NMR for the measurement of domain sizes in multiphase materials is becoming increasingly popular, in particular for the study of heterogeneous polymers. Under conditions where T(1) relaxation can be neglected, which is mostly the case at high field, analytical and approximate solutions to the evolution of spin diffusion are available. In order to extend the technique to more general conditions, we performed a comprehensive study of the diffusion of magnetization in a model copolymer at low field, where T(1) tends to be of the same order of magnitude as the typical spin-diffusion time. In order to study the effects of T(1) and to delineate the optimal T(1) values for back correction prior to applying the initial-rate approximation, we developed a numerical simulation based on the diffusion equation and including longitudinal relaxation. We present and discuss the limits of simple correction strategies for initial-slope analysis based on apparent relaxation times from saturation-recovery experiments or the spin-diffusion experiments themselves. Our best strategy faithfully reproduces domain sizes obtained by both TEM investigations and full simultaneous fitting of spin-diffusion and saturation-recovery curves. Full fitting of such independent data sets not only yields correct domain sizes, but also the true longitudinal relaxation times, as well as spin-diffusion coefficients. Effects of interphases with distinct mobility on spin-diffusion curves, as well as practical hints concerning the reliable component decomposition of the detected low-resolution FID signal by help of different magnetization filters are also discussed in detail.     

Optimised in vivo visualisation of cortical structures in the human brain at 3 T using IR-TSE

The primary visual cortex in humans can be identified using magnetic resonance imaging (MRI) in vivo by detection of the stria of Gennari. To fully characterize this area, high spatial resolution is essential, including the use of very thin image slices to avoid loss of definition due to partial volume effects. A three-dimensional magnetization-prepared turbo spin-echo sequence, with appropriate parameter optimization, provided high-resolution imaging (0.4 x 0.4 x 0.5 mm3) on a clinical 3-T scanner with adequate contrast to noise ratio. These images allowed visualisation of the stria of Gennari in every slice of a volume covering most of the occipital cortex, in each of six healthy volunteers. The effective longitudinal relaxation time was measured with the isotropic resolution turbo spin echo sequence and found to be substantially shorter than values measured with a dedicated relaxometric sequence. The shortening was attributed to magnetization transfer effects, as supported by the investigation of its slab and turbo-factor dependence.