Magnetization transfer using inversion recovery during off-resonance irradiation

Estimation of magnetization transfer (MT) parameters in vivo can be compromised by an inability to drive the magnetization to a steady state using allowable levels of radiofrequency (RF) irradiation, due to safety concerns (tissue heating and specific absorption rate (SAR)). Rather than increasing the RF duration or amplitude, here we propose to circumvent the SAR limitation by sampling the formation of the steady state in separate measurements made with the magnetization initially along the -z and +z axis of the laboratory frame, i.e. with or without an on-resonance inversion pulse prior to the off-resonance irradiation. Results from human brain imaging demonstrate that this choice provides a tremendous benefit in the fitting procedure used to estimate MT parameters. The resulting parametric maps are characterized by notably increased tissue specificity as compared to those obtained with the standard MT acquisition in which magnetization is initially along the +z axis only.     

Magnetization transfer contrast imaging of the human leg at 0.1 T: a preliminary study.

Magnetization transfer contrast imaging is an MR technique that capitalizes on interactions between the protons of mobile and macromolecularly bound water molecules. Studies to date, conducted primarily on 4.7 T and 1.5 T MR systems, have yielded results unique from conventional T1- and T2-weighted imaging studies. In this study, performed on a 0.1 T device, a section of lower leg was imaged in 20 normal human subjects and one patient with muscular dystrophy, using both a standard 500/22 gradient-echo sequence and a 500/22 gradient-echo sequence combined with off-resonance radio frequency irradiation designed to elicit magnetization transfer contrast. Results of the two techniques were compared. Our findings suggest that magnetization transfer contrast imaging is feasible at 0.1 T, and that this technique allows reproducible tissue characterization and improves contrast between certain tissues.     

Dynamics of paramagnetic agents by off-resonance rotating frame technique

Off-resonance rotating frame technique offers a novel tool to explore the dynamics of paramagnetic agents at high magnetic fields (B0 > 3T). Based on the effect of paramagnetic relaxation enhancement in the off-resonance rotating frame, a new method is described here for determining the dynamics of paramagnetic ion chelates from the residual z-magnetizations of water protons. In this method, the dynamics of the chelates are identified by the difference magnetization profiles, which are the subtraction of the residual z-magnetization as a function of frequency offset obtained at two sets of RF amplitude omega(1) and pulse duration tau. The choices of omega(1) and tau are guided by a 2-D magnetization map that is created numerically by plotting the residual z-magnetization as a function of effective field angle theta and off-resonance pulse duration tau. From the region of magnetization map that is the most sensitive to the alteration of the paramagnetic relaxation enhancement efficiency R(1rho)/R1, the ratio of the off-resonance rotating frame relaxation rate constant R(1rho) verse the laboratory frame relaxation rate constant R(1), three types of difference magnetization profiles can be generated. The magnetization map and the difference magnetization profiles are correlated with the rotational correlation time tauR of Gd-DTPA through numerical simulations, and further validated by the experimental data for a series of macromolecule conjugated Gd-DTPA in aqueous solutions. Effects of hydration water number q, diffusion coefficient D, magnetic field strength B0 and multiple rotational correlation times are explored with the simulations of the magnetization map. This method not only provides a simple and reliable approach to determine the dynamics of paramagnetic labeling of molecular/cellular events at high magnetic fields, but also a new strategy for spectral editing in NMR/MRI based on the dynamics of paramagnetic labeling in vivo.     

Effects of off-resonance irradiation, cross-relaxation, and chemical exchange on steady-state magnetization and effective spin-lattice relaxation times

In the presence of an off-resonance radiofrequency field, recovery of longitudinal magnetization to a steady state is not purely monoexponential. Under reasonable conditions with zero initial magnetization, recovery is nearly exponential and an effective relaxation rate constant R_1eff = 1/T_1eff can be obtained. Exact and approximate formulas for R_1eff and steady-state magnetization are derived from the Bloch equations for spins undergoing cross-relaxation and chemical exchange between two sites in the presence of an off-resonance radiofrequency field. The relaxation formulas require that the magnetization of one spin is constant, but not necessarily zero, while the other spin relaxes. Extension to three sites with one radiofrequency field is explained. The special cases of off-resonance effects alone and with cross-relaxation or chemical exchange, cross-relaxation alone, and chemical exchange alone are compared. The inaccuracy in saturation transfer measurements of exchange rate constants by published formulas is discussed for the creatine kinase reaction.     

Magnetization transfer contrast (MTC) in FLASH MR imaging

Magnetization transfer between bound and free protons was used as a source of contrast in high speed MR imaging using the FLASH technique. Contrast in FLASH MR images was found to depend upon the reduced magnetization and the spin lattice relaxation rate of free protons in the presence of bound proton radio-frequency saturation. MTC FLASH imaging was thus used to estimate the variation with saturation frequency of free proton spin-lattice relaxation during magnetization transfer.     

Generalized equation for describing the magnetization in spoiled gradient-echo imaging

The purpose of this study was to demonstrate a generalized equation for describing the magnetization in spoiled gradient-echo (SPGR) imaging in which the in-pulse relaxation and magnetization transfer (MT) effects are taken into account. First, the time-dependent Bloch equations for the two-pool exchange model with MT effect were reduced to an inhomogeneous linear differential equation, and then a simple equation was derived to solve it using a matrix operation. Second, the equations describing the magnetization before and after the radiofrequency (RF) pulse were derived based on the above solution for the RF-pulse excitation and evolution phases. Finally, a generalized equation describing the steady-state magnetization was derived. The validity of this equation was investigated by comparing with the transverse magnetization obtained by the regular Ernst equation and analytical solution in which the in-pulse transverse relaxation is considered. When the same assumption was made in our method, there were good agreements between them, indicating the validity of our method. The in-pulse transverse and longitudinal relaxations decreased the transverse magnetization compared to the case in which these effects were neglected, whereas MT increased it. In conclusion, we derived a generalized equation for describing the magnetization in SPGR imaging. This equation will provide a suitable basis for understanding the signal intensity in SPGR imaging and/or T₁ measurement using an SPGR sequence in cases in which the effect of in-pulse relaxation and/or MT cannot be neglected.     

Toroid cavity detectors for high-resolution NMR spectroscopy and rotating frame imaging: capabilities and limitations

The capabilities of toroid cavity detectors for simultaneous rotating frame imaging and NMR spectroscopy have been investigated by means of experiments and computer simulations. The following problems are described: (a) magnetic field inhomogeneity and subsequent loss of chemical shift resolution resulting from bulk magnetic susceptibility effects, (b) image distortions resulting from off-resonance excitation and saturation effects, and (c) distortion of lineshapes and images resulting from radiation damping. Also, special features of signal analysis including truncation effects and the propagation of noise are discussed. B(0) inhomogeneity resulting from susceptibility mismatch is a serious problem for applications requiring high spectral resolution. Image distortions resulting from off-resonance excitation are not serious within the rather narrow spectral range permitted by the RF pulse lengths required to read out the image. Incomplete relaxation effects are easily recognized and can be avoided. Also, radiation damping produces unexpectedly small effects because of self-cancellation of magnetization and short free induction decay times. The results are encouraging, but with present designs only modest spectral resolution can be achieved.     

Second-Order Echo Signals in Two-Pulse Delayed Nutation

Nonstationary magnetic nutation signals of the second-order echo that occur during the effect of a resonance radiofrequency pulse of length t ₂ on a two-level inhomogeneously broadened spin system preliminarily excited by a pulse of length t ₁ < t ₂ have been studied theoretically. It has been found that in contrast to the delayed nutation echo whose formation is attributed to the restoration of the longitudinal magnetization that arose by the end of the first pulse, these signals result from the reversal in time of three effective signals of free precession generated after the termination of the first pulse. The theoretical results are in good agreement with the experimental data obtained in NMR of protons in glycerin. A conclusion on the validity of the theorem of coherent transient processes occurring in two-level systems in the presence of an exciting field is drawn.     

H NMR Imaging of Residual Dipolar Couplings in Cross-Linked Elastomers: Dipolar-Encoded Longitudinal Magnetization, Double-Quantum, and Triple-Quantum Filters

Contrastfilters for NMR imaging of residual ¹H dipolar couplings of elastomers are introduced based on dipolar-encoded longitudinal magnetization, as well as double- and triple-quantum coherences. The spin response is discussed in the initial excitation time regime for methylene, methyl, and methine protons applicable to poly(isoprene) and other elastomers, taking into account the hierarchy of dipolar couplings and the associated editing features of multiple-quantum experiments. The efficiency of these filters is investigated for a series of cross-linked poly(isoprene) samples. Spatially resolved dipolar-encoded longitudinal magnetization decays and double-quantum and triple-quantum buildup curves are presented for a phantom made of poly(isoprene) with different cross-link densities. Two-dimensional images representing residual dipolar couplings are presented using dipolar-encoded longitudinal magnetization, double-quantum, and triple-quantum contrast filters. Images from dipolar-encoded longitudinal magnetization and triple-quantum coherences show the highest resolution and contrast, respectively.     

Dynamics of paramagnetic agents by off-resonance rotating frame technique in the presence of magnetization transfer effect

The simple method for measuring the rotational correlation time of paramagnetic ion chelates via off-resonance rotating frame technique is challenged in vivo by the magnetization transfer effect. A theoretical model for the spin relaxation of water protons in the presence of paramagnetic ion chelates and magnetization transfer effect is described. This model considers the competitive relaxations of water protons by the paramagnetic relaxation pathway and the magnetization transfer pathway. The influence of magnetization transfer on the total residual z-magnetization has been quantitatively evaluated in the context of the magnetization map and various difference magnetization profiles for the macromolecule conjugated Gd-DTPA in cross-linked protein gels. The numerical simulations and experimental validations confirm that the rotational correlation time for the paramagnetic ion chelates can be measured even in the presence of strong magnetization transfer. This spin relaxation model also provides novel approaches to enhance the detection sensitivity for paramagnetic labeling by suppressing the spin relaxations caused by the magnetization transfer. The inclusion of the magnetization transfer effect allows us to use the magnetization map as a simulation tool to design efficient paramagnetic labeling targeting at specific tissues, to design experiments running at low RF power depositions, and to optimize the sensitivity for detecting paramagnetic labeling. Thus, the presented method will be a very useful tool for the in vivo applications such as molecular imaging via paramagnetic labeling.     

High-Speed Magic-Angle SpinningC MAS NMR Spectra of Adamantane: Self-Decoupling of the Heteronuclear Scalar Interaction and Proton Spin Diffusion

We have investigated the carbon line shape of solid adamantane under high-speed magic-angle sample spinning (MAS) acquired without proton decoupling. The CH-group shows a spinning-speed-dependent line broadening while the CH₂-group consists of a spinning-speed-independent sharp component and a spinning-speed-dependent broader part. These phenomena can be explained by self-decoupling of theJ-interaction due to proton spin diffusion. Such a self-decoupling process can be described by a magnetization exchange process between the multiplet lines. Changing the spin-diffusion rate constant by off-resonance irradiation of the protons allows us to observe the full range from slow exchange to coalescence to fast exchange of the carbon spectra. One of the multiplet components in the CH₂-group corresponds to a group spin of the protons of zero and therefore does not couple to the other protons. This gives rise to the sharp central line. The magnetization exchange rate constant between the different multiplet lines can be determined from the spectra and is a measure for the spinning-speed-dependent proton spin-diffusion rate constant. Even at an MAS speed of 30 kHz, proton spin diffusion is still observable despite the relatively weak intermolecular proton dipolar-coupling network in adamantane which results in a static proton line width of only 14 kHz (full width at half height).     

Free Induction Signals after Multipulse Excitation of a Spin System

A free induction signal (FIS) after excitation of a two-level inhomogeneously broadened spin system by a sequence of n electromagnetic pulses of the same duration t ₁ and with the same intervals between them is investigated theoretically and experimentally. It has been established that an FIS develops n coherent oscillations of magnetization, whose phases change in time following definite laws. These oscillations arise after cessation of the nth pulse and terminate successively at times that are multiple of t ₁, thus increasing the FIS duration. The conclusion has been drawn on the validity of the theorem of coherent transient processes on multipulse excitation of two-level spin systems with a finite inhomogeneous line width. The theoretical results are in fairly good agreement with the experimental data obtained in the NMR of protons in glycerin.     

T1 and T2 effects during radio-frequency pulses in spoiled gradient echo sequences

Finite pulse durations in diverse pulse schemes lead to the reduction of the magnitude of the magnetization vector due to T₁ and T₂ effects during the radio-frequency pulses. This paper presents an analysis of the steady state signal in the presence of relaxation effects during radio-frequency pulses in MRI spoiled gradient echo sequences. It is shown that minor attenuations of the magnetization vector can have dramatic consequences on the measured signal, and may thus entail a loss in SNR benefits at high static magnetic fields if a careful analysis is not performed. It is emphasized that it is the time-integrated magnetization vector trajectory that matters for these effects and not only the pulse duration. Some experimental results obtained on a phantom at 3 T verify this analysis.     

Spin-locking in one pulse NMR experiment

The response of a spin system to a long (in comparison to spin–spin relaxation time T₂) radiofrequency pulse has been studied. We observed that the magnetization after the long pulse does not fall to zero at time tT₂ for both on-resonance and off-resonance conditions. The dependencies of the magnetization on frequency offset, linewidth and radiofrequency power are investigated, both theoretically and experimentally. The question of the effective field direction is also discussed.     

1H NMR imaging of residual dipolar couplings in cross-linked elastomers: dipolar-encoded longitudinal magnetization, double-quantum, and triple-quantum filters.

Contrastfilters for NMR imaging of residual 1H dipolar couplings of elastomers are introduced based on dipolar-encoded longitudinal magnetization, as well as double- and triple-quantum coherences. The spin response is discussed in the initial excitation time regime for methylene, methyl, and methine protons applicable to poly(isoprene) and other elastomers, taking into account the hierarchy of dipolar couplings and the associated editing features of multiple-quantum experiments. The efficiency of these filters is investigated for a series of cross-linked poly(isoprene) samples. Spatially resolved dipolar-encoded longitudinal magnetization decays and double-quantum and triple-quantum buildup curves are presented for a phantom made of poly(isoprene) with different cross-link densities. Two-dimensional images representing residual dipolar couplings are presented using dipolar-encoded longitudinal magnetization, double-quantum, and triple-quantum contrast filters. Images from dipolar-encoded longitudinal magnetization and triple-quantum coherences show the highest resolution and contrast, respectively.     

Dual-acquisition phase-sensitive fat-water separation using balanced steady-state free precession

Balanced steady-state free precession (SSFP) sequences use fully re-focussed gradient waveforms to achieve a high signal and useful image contrast in short scan times. Despite these strengths, the clinical feasibility of balanced SSFP is still limited both by bright fat signal and by the signal voids that result from off-resonance effects such as field or susceptibility variations. A new method, dual-acquisition phase-sensitive SSFP, combines the signals from two standard balanced SSFP acquisitions to separate fat and water while simultaneously reducing the signal voids. The acquisitions are added in quadrature and then phase corrected using a simple algorithm before fat and water can be identified simply by the sign of the signal. This method is especially useful for applications at high field, where the RF power deposition, spatial resolution requirements and gradient strength limit the minimum repetition times. Finally, dual-acquisition phase-sensitive SSFP can be combined with other magnetization preparation schemes to produce specific image contrast in addition to separating fat and water signals.     

Transient Decay of Longitudinal Magnetization in Heterogeneous Spin Systems under Selective Saturation. IV. Reformulation of the Spin-Bath-Model Equations by the Redfield-Provotorov Theory

Alternative formulations to the conventional Bloch equations for the RF saturation of the solid component in heterogeneous spin systems according to a spin-bath model are derived using the concept of spin temperature as suggested by Redfield and Provotorov. These formulations and the resulting equations derived by the projection-operator technique provide an analytical and explicit solution to the general problem of solid saturation under continuous RF irradiation. Using the Provotorov theory, a set of generalized (non-Markovian) equations of motions is derived. The solutions to these generalized equations approach those of the conventional Bloch formulation at one extreme when the applied RF is weak and the Redfield formulation at another when the applied RF is strong. In short, this development provides a simple alternative which removes the restriction of the lineshape function used to represent the solid component; the latter is well known to be non-Lorentzian, contrary to the tacit assumption made in the conventional Bloch formulation. Experimental verification of the generalized theory is provided by transient and steady-state longitudinal magnetization data acquired from cross-linked bovine serum albumin under selective saturation by continuous off-resonance RF irradiation.     

Dipolar contrast for dense tissues imaging

A new type of contrast called dipolar contrast is obtained by a decrease in the dipolar line broadening of protons. This contrast is usable for dense tissue NMR imaging and more generally for the study of dipolar linked protons in biological tissues. The sequence used is based on a variant of the Magic Sandwich Echoes (MSE) technique. In vitro experiments on a tendon sample are used to reinforce the image intensity of regions where the direct proton dipolar interaction exists.     

Frequency-Selective Fat Suppression Radiofrequency Pulse Train to Remove Olefinic Fats

CHESS pulse can suppress the signal originating from aliphatic fat protons but cannot suppress the signal from olefinic fat protons, which is near the resonance frequency of water protons. Adipose tissue contains various fat species; aliphatic fat comprises about 90 % and olefinic fat about 10 % of adipose tissue. Thus, CHESS pulse cannot be used to suppress the signal from adipose tissue completely. The purpose of this study was to find a method to suppress the signal from adipose tissue completely. The Fatsat train pulse, created with an arbitrary flip angle and insensitive to B1 inhomogeneity, was used. Because B1 inhomogeneity is larger on higher field magnetic resonance imaging, the fat suppression radiofrequency pulse needs to be B1-insensitive. To investigate a percentage of olefinic fat in adipose tissues, the excitation frequency of the Fatsat train pulse was varied from ?240 to +400 Hz and the images and fat-suppressed images were obtained. The presence of olefinic fat comprising about 10 % of abdominal adipose tissue was identified. The result agreed with some previous papers. Complete fat suppression could be achieved by partial (10 %) inversion of longitudinal aliphatic fat magnetization and by canceling out the two fat magnetizations. The flip angle was identified to about 95°. In conclusion, the cause that the signal from adipose tissues cannot be suppressed completely has been found. Improved images that signals from adipose tissues were suppressed completely have been demonstrated. This technique can also be applied to several pulse sequences.