Quantitative interpretation of magnetization transfer

Magnetization transfer in cartilage and its constituent macromolecules

The goal of this work was to investigate magnetization transfer (MT) in cartilage by measuring water proton signals Ms/Mo, as an indicator of MT, in (i) single-component systems of the tissue's constituent macromolecules and (ii) intact cartilage under control conditions and after two pathomimetic interventions. Ms/Mo was quantified with a 12-microT saturation pulse applied 6 kHz off resonance. Both glycosaminoglycans (GAG) and collagen exhibited concentration dependent effects on Ms/Mo, being approximately linear for GAG solutions (Ms/Mo = -0.0137[% GAG] + 1.02) and exponential for collagen suspensions (Ms/Mo = 0.80 x exp[-(%collagen)/6.66] + 0.20); the direct saturation of water could not account for the measured Ms/Mo. Although the effect of collagen on Ms/Mo is much stronger than for a corresponding concentration of GAG, Ms/Mo is not very sensitive to changes in collagen concentration in the physiological range. Tissue degradation with 25 mg/ml trypsin led to an increase in Ms/Mo from the baseline value of 0.2 (final/initial values = 1.15 +/- 0.13, n = 11, P < 0.001). In contrast, a 10-day treatment of cartilage with 100 ng/ml of interleukin-1 beta (IL-1 beta) caused a 19% decrease in Ms/Mo (final/initial values = 0.81 +/- 0.08, n = 3, P = 0.085). The changes in hydration and macromolecular content for the two treatments were comparable, suggesting that Ms/Mo is sensitive to macromolecular structure as well as concentration. In conclusion, whereas the baseline Ms/Mo value in cartilage may be primarily due to the tissue collagen concentration, changes in Ms/Mo may be due to physiological or pathophysiological changes in GAG concentration and tissue structure, and the measured Ms/Mo may differentiate between various pathomimetic degradative procedures.

Optimization of T2-selective binomial pulses for magnetization transfer

Accurate rules have been established to build binomial pulses up to fifth order, for selectively saturating protons at any given T2 with minimum power deposition. Pulse performance and sensitivity to experimental defects have been evaluated; the third order is generally found to be best suited. It is shown, by combining theory and experiment performed at 0.1 T, that matching the saturation pulse to T2 of the motionally restricted pool is essential to reveal exchange with free water protons. It is emphasized that, to date, lack of magnetization transfer contrast with binomial pulses is due mainly to insufficient RF level available with most MR imaging systems, especially at high magnetic field.

Magnetization transfer by simple sequences of rectangular pulses

On-resonant radio frequency (RF) sequences composed of a train of short rectangular pulses of the same kind were optimized in order to obtain selective saturation of protons with short transverse relaxation times for magnetization transfer purposes. It is demonstrated that the sequences regarded allow a good adaptation to different requirements for magnetization transfer examinations on whole-body imagers. The sequences presented here provide relatively strong saturation of protons with very short transverse relaxation times T2 approximately less than 50 microseconds, whereas signals from protons with long T2 to be recorded are hardly influenced in a broad frequency range. The sequences are especially advantageous for applications in pulse files with limited numbers of support points.

A model for magnetization transfer in tissues

Magnetization transfer in several tissues is measured and successfully modeled using a two-pool model of exchange. The line shape for the semi-solid pool is characterized by a superLorentzian and the liquid pool by a Lorentzian. The tissues investigated were white and gray matter, optic nerve, muscle, and liver. All tissues the authors studied are characterized by the same model but differ in the parameter values of the model. Blood and cerebral spinal fluid (CSF) were also investigated. The two-pool model with a Lorentzian line shape for both the semi-solid and liquid pools modeled the magnetization transfer in blood. In CSF, as expected, there is no measurable exchange of magnetization. The T2B associated with the semi-solid pool was short (approximately 10 microseconds) for all tissues indicating a fairly rigid semi-solid pool. In addition characterization of the line shape as superLorentzian indicates molecules such as integral membrane proteins or lipids in membranes are likely molecules participating in the exchange. Conversely, in blood large globular proteins are indicated due to the Lorentzian nature of the semi-solid pool and a T2B approximately 300 microseconds.

Magnetization transfer and T2 relaxation components in tissue

T2 relaxation makes an important contribution to tissue contrast in magnetic resonance (MR) imaging. Many tissues are known to exhibit multicomponent T2 relaxation that suggests some compartmental segregation of mobile protons on a T2 timescale. Magnetization transfer (MT) is another relaxation mechanism that can be used to produce tissue contrast in MR imaging. The MT process depends strongly on water-macromolecular interactions. To investigate the relationship between multicomponent T2 relaxation and the MT process, multiecho T2 measurements have been combined with MT measurements for freshly excised samples of cardiac muscle, striated muscle, and white matter. For muscle, short T2 components show greater MT than long T2 components, consistent with the belief that they represent distinct water environments. For white matter, quantitative MT measurements were identical for the two major T2 components, apparently because of exchange between the T2 compartments on a time-scale characteristic of the MT experiment. Implications for accurate modeling of MT in tissue and the use of MT for MR image contrast are discussed.

Water-macromolecular proton magnetization transfer in infarcted myocardium: a method to enhance magnetic resonance image contrast

Water proton nuclear magnetic resonance relaxation times and magnetization transfer (MT) parameters of rat hearts were studied 24 h or 4 weeks after ligation of the left coronary artery or sham operation. Compared with sham-operated controls, measured relaxation times (T1sat and T2) of both acute and chronic myocardial infarction increased. The MT effect significantly decreased in the infarcted myocardium. The changes in relaxation times and MT effect were significantly greater in chronic infarcts compared with acute infarcts. Improvements in calculated image contrast between normal and infarcted tissue were supported by images of ex vivo hearts with chronic infarction. Image contrast was increased at short echo times in the presence of macromolecular proton pool irradiation. Exploiting changes in tissue MT following myocardial infarction to enhance contrast between normal and infarcted tissue should allow improved identification and characterization of infarcted myocardium.

Novel MR image contrast mechanisms in epilepsy

A range of magnetic resonance (MR) parameters are introduced, which can give rise to image contrast by using suitable pulse sequences, and that can be measured quantitatively. Their relationship to tissue pathology is given as far as possible. Techniques for their measurement, and results from multiple sclerosis, stroke, and epilepsy are given. The parameters are proton density, T1, T2, transverse magnetisation decay, which gives estimates of extracellular water and myelin concentrations, magnetisation transfer ratio and T1sat, and diffusion (including trace and anisotropy measured from the tensor matrix).

Near-resonance spin-lock contrast

Spin-lock and spin-tip excitations are the two magnetization components created by the preparatory RF pulse of an MRI contrast enhancement sequence. Only spin-lock is inherently adiabatic, preserving spin alignment so that tissue-specific relaxation can generate desired saturation contrasts. Spin-tip is the rotating-frame oscillating excitation, and generally causes nonadiabatic loss of all detectable magnetization. Relative levels of spin-lock and spin-tip are important to understand as a function of the preparatory B1 delta amplitude, resonance frequency offset, delta, and the pulse waveform. These MR responses can be accurately analyzed theoretically and numerically by using Torrey's tipped coordinates to formulate Bloch's equations. At near-resonance offsets, (delta/gamma B1) less than 2.0, spin-lock contrast (SLC) depends strongly on T2, due to the nature of spin-lock T1 rho relaxation in the RF pulse interval. The relaxation rates 1/T1 rho and 1/T2 rho apply for active B1 delta, but remain linear combinations of ordinary (1/T1) and 1/T2) for motionally narrowed MR. The SLC increases rapidly as delta decreases below 2000 Hz; carefully chosen B1 delta rise times avoid spin-tip losses down to 150 Hz or less. The SL saturation enhances or multiplies any other indirect saturation effects that may be also present, such as magnetization transfer. A strong near-resonance SLC multiplier is advantageous for clinically practical MRI sequences that use short B1 delta pulses and fast SE multislice scan modes. Simulations based upon spin-lock/spin-tip theory and measured (T1,T2) yield excellent agreement with real MRI results for clinically practical fast multislice scans.

Analysis of on- and off-resonance magnetization transfer techniques

Three methods of performing magnetization transfer (MT) MR imaging are analyzed: (a) off-resonance continuous wave, (b) off-resonance shaped pulses, and (c) on-resonance binomial pulses. With two-pool Bloch-model simulations, signal levels from "MT active" spin systems were calculated, with reference to direct saturation of "MT inactive" systems, allowing calculation of contrast due to MT. Simulations demonstrate several trends with variation of excitation amplitude and offset frequency for the off-resonance methods and with variation of excitation amplitude and pulse shape "order" for binomial pulses. The simulations show that nominally optimized versions of each of these approaches provide essentially equivalent contrast at a given level of applied MT power, contrary to previous claims. Experiments with an MT-inactive phantom, with a whole-body system, show results with off-resonance pulses to be in good agreement with simulations, whereas binomial-pulse experiments show anomalously large direct saturation.

Spin-lattice relaxation and magnetization transfer in intracranial tumors in vivo: effects of Gd-DTPA on relaxation parameters

Spin-lattice relaxation time T1 and relaxation parameters in magnetization transfer (MT) imaging were measured in 11 intracranial tumors before and after injection of Gd-DTPA at 0.1 T by using the inversion recovery method and the saturation transfer technique, respectively. Preinjection T1 relaxation times of the tumors were longer than those of white matter, but after Gd-enhancement the relaxation times of most tumors were in the same range as those of white matter. Gd-DTPA shortened the apparent relaxation time in the presence of off-resonance saturation pulse (T1a) due to marked shortening of the relaxation time of mobile water (T1w). Gd-DTPA decreased the magnetization transfer contrast (MTC) but did not influence on the magnetization transfer rate (Rwm). The parameters MTC and Rwm differed clearly between Gd-enhanced tumors and normal brain, whereas the relaxation time T1a was in many Gd-enhanced tumors in the same range as in normal brain.

Magnetization transfer contrast MRI of musculoskeletal neoplasms

Magnetic resonance imaging (MRI) examinations were performed in 15 patients with musculoskeletal neoplasms to assess the value of magnetization transfer contrast in tumor characterization. Multiplanar gradient-recalled echo sequences (TR 500-600/TE 15-20/flip angle 20-30 degrees) were performed first without and then with magnetization transfer contrast generated by a zero degree binomial pulse (MPGR and MTMPGR). Standard T1-weighted spin echo images (SE; TR 300-400/TE 12-20) and either T2-weighted SE (TR 2000-2900/TE 70-80) or T2-weighted fast spin echo (FSE; TR 4000-5000/TE 100-119 effective) images were also obtained. Signal intensities on MTMPGR scans were compared to those on MPGR scans for both tumors and normal tissues. Signal intensity ratios (SIR) and contrast-to-noise ratios (CNR) were also compared for all sequences. MTMPGR images provided better contrast between pathologic tissues and muscle than did standard MPGR images, increasing both conspicuity of lesions and definition of tumor/muscle interfaces. Benign and malignant tumors, with the exception of lipoma, underwent similar degrees of magnetization transfer and could not be distinguished by this technique.

Magnetization transfer, cross-relaxation, and chemical exchange in rotationally immobilized protein gels

Water proton spin-lattice relaxation rates are reported as a function of the magnetic field strength for cross-linked bovine serum albumin samples. The relaxation dispersion profile is analyzed using a relaxation model where the solid components have the magnetic field dependence proportional to v-0.5 which may result from a defect diffusion model with two degrees of freedom. If the cross-linking agent concentration is not sufficiently high, the relaxation dispersion curve may have significant contributions from freely rotating protein. The magnetic field dependence of the relaxation rates studied as a function of the proton mole fraction in the sample show that approximately 30% of the magnetization transfer rate is directly proportional to the proton mole fraction. This contribution is identified with the magnetization transfer from exchange of whole water molecules with buried binding sites on the protein. The second order magnetization transfer rate constant is 388 s-1 assuming unit water spin concentration. The solid component relaxation obeys an Arrhenius activation law, but the overall temperature dependence of the cross-relaxation is complicated by chemical exchange processes which enter with opposite sign.

Anisotropy of NMR properties of tissues

Orientational anisotropy of T2 and T1 relaxation times, diffusion, and magnetization transfer has been investigated for six different tissues: tendon, cartilage, kidney, muscle, white matter, and optic nerve. Relaxation anisotropy was observed for tendon and cartilage, and diffusional anisotropy was measured in kidney, muscle, white matter, and optic nerve. All other NMR measurements of these tissues showed no orientational dependence. This pattern of NMR anisotropies can be interpreted from the underlying geometrical structures of the tissues.

Transition from Lorentzian to Gaussian line shape of magnetization transfer spectrum in bovine serum albumin solutions

Magnetization transfer experiments using an off-resonance irradiation technique were performed on bovine serum albumin solutions by varying the irradiation frequency and the concentration. A transition of the magnetization transfer spectrum from Lorentzian to Gaussian line shape was observed around the critical concentration of 6.2 mmoles of protein to kg of solution. Observed magnetizations were well expressed by the rate equations of populations for spins below and above the transition, which yielded the magnetization transfer rates, the intrinsic relaxation rates of both protein and water protons, and the effective tumbling time and the rigid line width of the protein. The result showed that the estimates of the values for magnetization transfer rate do not change once the critical concentration is reached.

Dynamic properties of bound water studied through macroscopic water relaxations in concentrated protein solutions

Magnetization transfer experiments using an off-resonance irradiation technique were performed on bovine serum albumin solutions by varying the irradiation frequency and concentration. Observed macroscopic magnetizations of water protons were well expressed by the rate equations of populations for spins, which gave the tumbling time of protein protons and the intrinsic relaxation rates of water and protein protons. These parameters conformed to a model of rapid-exchange water system with bound water molecules in the interior of the protein that interact with protein protons. Analysis of the data enabled the separation of relaxation rates into the respective contributions by the interior bound water and water in the hydration layer at the protein surface, and determined the amounts and the average correlation times of these water fractions. The average residence time of the interior bound water with respect to exchange with the bulk water was found to be (5 +/- 2).10(-6) s. The estimation of the hydration layer showed excellent agreement with the amount measured by a thermodynamical method.

Off-resonance spin locking for MR imaging

Off-resonance spin locking is investigated as a low power method for achieving low field spin-lattice relaxation contrast using high field clinical MR imaging systems (e.g., 1.5 tesla). Spin-lattice relaxation times and equilibrium magnetizations in the off-resonance rotating frame (T1 rho(off), beta) were measured for tissue-mimicking phantom materials as a function of the ratio of the amplitude to the resonance offset of the spin-locking pulse (f1/delta). The phantom materials consisted of vegetable oil to simulate fat and two different gels containing 2% and 4% agar to simulate nonfatty tissues with different macromolecular compositions. These measurements were used to verify a signal strength equation for a multislice off-resonance spin-locking technique implemented on a clinical MR imaging system operating at 1.5 tesla. Although the oil showed little change in image contrast with increasing f1/delta, the two gels demonstrated a strong variation which provided improved discrimination compared to T1-weighted imaging. Off-resonance spin locking is suggested as a method for improving delineation of breast lesions and a preliminary clinical example from a patient volunteer is presented.

T1 discrimination contributions to proton magnetization transfer in heterogeneous biological systems

Water proton spin-lattice relaxation times are commonly used as a guide in establishing the off-resonance irradiation time as well as the repetition time of the magnetization transfer experiment. T1 discrimination effects occur if the motionally restricted spin bath longitudinal magnetization does not reach thermal equilibrium. In this study we developed the formalism necessary for the evaluation of T1 discrimination contributions to proton magnetization transfer arising from the use of a short repetition time relative to the spin-lattice relaxation time of the motionally restricted spin bath. The results of computer simulation indicate that T1 discrimination contributions occur when the repetition time is small relative to the spin-lattice relaxation time of the motionally restricted spin bath, and when the off-resonance irradiation is weak and far off-resonance. For somewhat longer repetition times, T1 discrimination contributions become important only when the cross relaxation rate is small, and the fractional amount of motionally restricted component large. The occurrence of T1 discrimination effects results in distortion of water proton intensity ratio dispersion curves thereby resulting in the estimation of erroneous magnetization transfer parameters, whereas in magnetization transfer contrast enhanced imaging, such contributions are manifested by a decrease in image contrast.

Determination of proton magnetization transfer rate constants in heterogeneous biological systems

Two procedures are currently in use for the determination of proton magnetization transfer rate constants between macromolecular tissue components and water. The first method assumes that there are only two spin baths (macromolecular plus solvent) and that during off-resonance irradiation complete saturation of the "immobile" proton spin bath occurs (S. H. Koenig, R. D. Brown, III, R. Ugolini, Magn. Reson. Med. 29, 311 (1993)). This approach neglects the possibility of incomplete saturation and polydispersity, and yields an apparent magnetization transfer rate constant, Kapp. The second approach utilizes a formalism which can account for polydispersity and incomplete saturation of the immobile spin bath (K. Kuwata, D. Brooks, H. Yang, T. Schleich, J. Magn. Reson., in press). In this work magnetization transfer rate constants derived by the use of both methods for two systems, ocular lens tissue and cross-linked bovine serum albumin (BSA) were compared. For both samples Kapp was dependent on B2 off-resonance irradiation frequency and power when the first method was used. The second method provided values of the magnetization transfer rate constant that were similar to the values obtained by the first method, as the limit of complete saturation was approached.

Estimation of Bloch model MT spin system parameters from Z-spectral data

Previous studies have described magnetization transfer (MT) Z-spectra in terms of a two-pool Bloch model, with six spin-system parameters KA, F, T1A, T1B, T2A, and T2B. By simulation, we show that a process including nonlinear constrained optimization can be used to accurately and uniquely estimate spin-system parameters from MT Z-spectra prepared by continuous wave (CW) RF irradiation. Experiments producing Z-spectra by pulsed RF irradiation give substantially different magnetization values, relative to MT acquisitions obtained by CW RF irradiation, at small offset frequencies, with a consequence that only T2B can be uniquely determined. However, several equalities and bounds involving four of the other parameters (KA, F, T1A, and T1B) are derived, which are applicable to pulsed data. These relationships allow calculation of "free pool" magnetization corresponding to complete saturation of the restricted pool, without requiring that this complete saturation be experimentally achieved. MT experimental data from pulsed RF irradiation on boiled egg albumin, obtained using a clinical whole-body MRI system, are analyzed using an optimization algorithm and the derived expressions.