Journal of Magnetic Resonance (1969)
NMR imaging of spin-lattice relaxation using stimulated echoes
Abstract
A new NMR imaging technique is presented which allows a detailed determination of the spin-lattice relaxation behavior. Both the spatial resolution and the measuring time are the same as for a conventional NMR image. The experiment yields a series of cross-sectional images with intensities declining according to the T1 relaxation curve. The technique is based on the principles of stimulated-echo acquisition-mode (STEAM) imaging which have been described in a preceding paper. For T1 imaging the final 90° rf pulse of the basic three-pulse STEAM imaging sequence is split into a variable number of small-angle rf pulses. Each of these pulses should read out the same amount of the longitudinal magnetization which has been prepared by the two leading 90° pulses. They give rise to a series of stimulated echoes with a T1 weighting which depends on the length of the interval between the second 90° pulse and the readout pulse. STEAM T1, imaging is characterized by an extremely low rf power deposition and, therefore, will be suitable for high-field NMR imaging. The method is easily extended to multislice measurements as well as to chemical-shift-selective (CHESS) T1 imaging. 1H NMR images have been obtained at 100 MHz using a 2.3 T magnet with a bore of 40 cm.
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Cited by (36)
A fast 3D look-locker method for volumetric T<inf>1</inf> mapping
1999, Magnetic Resonance ImagingWe introduce a fast technique, based on the principles of the 2D Look-Locker T1 measurement scheme, to rapidly acquire the data for accurate maps of T1 in three dimensions. The acquisition time has been shortened considerably by segmenting the acquisition of the ky phase encode lines. Using this technique, the data for a 256 × 128 × 32 volumetric T1 measurement can be acquired in 7.6 min. T1 measurements made in phantoms with T1s between 200 and 1200 ms had an accuracy of 4% and a reproducibility of 3.5%. Measurements of T1 made in normal brain using the fast 3D sequence corresponded well with inversion-recovery fast spin-echo measurements.
Probing the gelation of polymers within a Bentheimer sandstone by <sup>1</sup>H-PFG n.m.r.
1998, PolymerThe gelation of a polyvinylalcohol–glutaraldehyde–water solution confined in a Bentheimer sandstone was characterized by carrying out 1H-n.m.r. spin-lattice relaxation rate measurement (1/T1) and pulse field gradient diffusion (D) measurements at 67°C. At any time during the gel reaction neither the longitudinal magnetization versus storage time nor the echo-amplitude versus gradient strength (squared) could be described by single exponential functions. In order to characterize these multi-exponential decay curves by a minimum number of parameters a gaussian type of distribution function (Rayleigh distribution) in 1/T1 and D were adopted. When implementing these distribution functions and fitting all spin-lattice relaxation data and diffusion data simultaneously, in order to constrain the fitting more effectively, the two n.m.r. derived parameters (1/T1 and D) were found to give consistent results. During gelation the average relaxation rate and the average diffusion coefficient versus reaction time were found to be described by a first order rate process with a rate constant equal to 18×10−5 s−1. Also, the widths of the two distribution functions were found to decrease with reaction time. Moreover, the gelation rate within the Bentheimer sandstone was found to be significantly faster compared to the gelation rate of the bulk solution.
Proton n.m.r. relaxation times (T1, T2), chemical shift and line width of the solvent water protons in a polyvinylalcohol (PVA)-glutaraldehyde-water solution confined in a porous material (glass beads) revealed no significant changes during crosslinking and gel formation. Also, the self-diffusion coefficient was constant and identical to the self-diffusion coefficient of bulk water (2 × 10−5 cm2 s−1) during the reaction. Due to the smaller self-diffusion coefficient of the polymer molecules the solvent water resonance peak could be completely removed from the spectrum by applying a pulse gradient spin-echo technique, leaving only the signal from the polymer amenable for detection. In spite of the broadening effect caused by susceptibility differences between the solid porous matrix and the confined fluid, the PVA peaks were easily resolved. The observed distribution of self-diffusion coefficients of PVA could be approximated by three single diffusion coefficients ranging from 10−6 to 10−9 cm2 s−1 at 25°C. The slower diffusion coefficient was found to decrease by almost an order of magnitude during the reaction with a rate of change of approximately 3 × 10−5 s−1 at 80°C.
Consideration of random errors in the quantitative imaging of NMR relaxation
1992, Journal of Magnetic Resonance (1969)Quantitative magnetic resonance imaging is a field whose development is at an early stage. Relatively little work has been done to assess the reliability of the values obtained from the quantitative maps previously presented. Here, the problem of spatial mapping of T1 relaxation is studied. The effects of random errors on the results of imaging experiments incorporating saturation-recovery and fast inversion-recovery measurements are examined. Consideration is given to various factors affecting experimental design, common to all implementations of these techniques: the ratio, the number of relaxation points sampled, and their distribution. No attempt has been made to assess different fitting algorithms. It is demonstrated that quantitative image data can be well modeled by simulations, leading to the conclusion that a T1-imaging experiment can be regarded simply as a set of independentT1 measurements, one on each image voxel. Results from previous work on bulk T1 measurements are therefore applicable to quantitative imaging and are used to design an efficient imaging experiment, under a typical clinical time constraint of approximately one hour.
Suppression of artifacts in multiple-echo magnetic resonance
1989, Journal of Magnetic Resonance (1969)Many techniques in both magnetic resonance imaging and magnetic resonance spectroscopy use two or more RF pulses to excite the spin system and detect the echo signals which form between or after the pulses. In general many different echoes form during each acquisition interval, only one of which carries the information required. The others lead to distortion of peak heights and lineshapes in MRS, and to ghost images and similar artifacts in MRI. The “coherence transfer pathway” formalism of Bodenhausen et al. allows the evolution of each echo to be studied and suggests methods of removing the unwanted signals. The general phase-cycling methods described by Bodenhausen et al. require a degree of flexibility in the control of RF pulses which is not available on all spectrometers, however, so simpler schemes requiring only 180° phase shifts have been investigated. With certain restrictions, these schemes give cancellation of the unwanted echoes during any particular acquisition interval, and in certain cases can be extended to cancel the unwanted echoes in all acquisition intervals of a multiple-echo sequence. All such schemes, however, require a large number of transients to be collected, so a second method has been investigated whereby the systematic application of magnetic field gradients can produce similar results within a single transient. Both of these approaches have been reported previously, but we introduce a novel formalism which allows the required pulse phases and gradient magnitudes to be systematically calculated, rather than empirically determined, for any pulse sequence. Examples of the application of each method to the spin-echo and TART imaging sequences are given, although both methods are equally applicable to many pulse sequences used in spectroscopy.
Slice selection and T<inf>1</inf> contrast in FLASH NMR imaging
1988, Journal of Magnetic Resonance (1969)This paper describes the signal intensity in rapid FLASH NMR imaging as a function of the repetition time, the NMR relaxation times, the flip angle, and the shape of the tailored RF pulses used for slice selection. In the absence or after elimination of signal contributions from transverse coherences the theoretical treatment may be confined to a steady state of the longitudinal magnetization. It turns out that deviations from a rectangular excitation profile due to imperfect pulse shapes strongly alter both the dynamic approach to steady-state conditions and the resulting saturation behavior as expected from theoretical expressions. As a consequence the signal-to-noise and image contrast become dependent on the actual slice profile. In T1 images calculated from series of FLASH images with different flip angles or repetition times qualitative relations between tissues with different T1 values are borne out correctly, whereas the accuracy of T1 relaxation times may not be satisfactory. No restrictions are expected for 3D imaging using a spatially homogeneous RF excitation. Experiments have been carried out on phantoms and human volunteers using a Bruker 2.35 T 40 cm NMR system.