Magnetization Transfer Pulse Research Articles

Pulsed magnetization transfer contrast in gradient echo imaging: a two-pool analytic description of signal response.

Magnetization transfer (MT) imaging with a rapid gradient-echo sequence and pulsed saturation provides an efficient means of acquiring high resolution three-dimensional data in vivo. This paper presents a derivation of the theoretical steady-state signal equation for this sequence based on the two-site coupled Bloch equations. Numerical simulations are used to validate the derived expression and experiments are performed on an agar gel model and normal brain. Experimental agar data indicate that direct saturation of the liquid component can be a major source of signal attenuation whereas MT normally dominates in brain tissue. The signal equation presented here establishes the necessary theory for sequence design and optimization and provides insight into model parameters and experimental results.

Pulsed magnetization transfer contrast for MR imaging with application to breast.

The relative populations and transverse relaxation times of the solid-like hydrogen pool (PB and T2B) and the magnetization transfer (MT) rates between the solid-like and liquid-like hydrogen pools (kappa) have been determined for three different agar gel concentrations (2%, 4%, and 8% by weight) as well as excised fibroglandular breast tissue specimens. PB was determined to be .003(.001), .01(.002), .02(.01), and .06(.01); T2B was determined to be 13.0(.2), 14.0(.1), 14.5(.1) and 15.2(1.3) microseconds; and kappa was determined to be 0.78(.01), 1.15(.02), 2.00(.02), and 3.55(1.5) sec-1 for the 2%, 4%, and 8% agar gels and the fibroglandular tissue, respectively. The image signal intensities of a pulsed MTC-prepared gradient-echo imaging technique are predicted using these MT parameters and are shown to agree well with experimental data obtained from a clinical MR imaging system. This technique is shown to suppress signal intensity of fibroglandular breast tissue by 40%-50% without exceeding SAR limits (< or = 8 W/kg) and is helpful for visualizing lesions and silicone implants.

Magnetization transfer in hemopoietic bone marrow examined by localized proton spectroscopy

The sensitivity of hemopoietic bone marrow to magnetization transfer is analyzed in 15 healthy volunteers and seven patients with different hematological disorders (inflammation, plasmacytoma, hemopoietic reconstitution after bone marrow transplantation). To obtain sufficient signal-to-noise ratio, a 90 degrees - 180 degrees - 180 degrees double spin echo (PRESS) single voxel spectroscopic method was combined with pulsed magnetization transfer. Several spectra were recorded from each volume element inside the vertebral marrow, alternately with and without prepulses for magnetization transfer. Water signals from marrow with increased content of extracellular water due to inflammation or edema revealed less magnetization transfer effects than marrow with increased intracellular water content due to high cellularity. The preliminary results show magnetization transfer to be a promising tool for the clinically important characterization of the water composition in red bone marrow.

Clinical utility of a new contrast option from magnetization transfer contrast.

Two magnetization transfer (MT) contrast effects, a T2-like effect and the improved contrast observed when gadolinium is used with MT, are combined in a single sequence. Forty patients (22 males:18 females; mean age, 45 years (23-87)) with suspected intracranial pathology underwent MRI on a 1.5 Tesla system. Of 46 lesions; seven were ischemic, five infective, seven neoplastic, four hemorrhagic, four multiple sclerosis, seven human immunodeficiency virus (HIV) leukoencephalopathy, nine normal/miscellaneous, and three gliosis. A conventional spin-echo sequence (TR 900 TE 15) was used with on-resonance binomial MT pulses. The sequence was performed postgadolinium +/- MT. The signal intensity ratios +/- MT were: white matter, 0.62 +/- 0.03; gray matter, 0.75 +/- 0.04; ischemia, edema, and demyelination, 0.75 (0.57-0.86); and gadolinium/methemoglobin, 0.85 (0.81-0.98). Areas which exhibited MT had T2-like contrast and those that did not maintained expected contrast for the given parameters. The result was a combination of T2-like contrast, gadolinium enhancement, and dark cerebrospinal fluid (CSF) providing both increased sensitivity to lesions which exhibited both contrast features and improved delineation of periventricular lesions. Furthermore, the differential signal between T2-like contrast of edema and gadolinium enhancement in neoplastic or infective lesions was maintained.

The application of magnetization transfer to MR angiography with reduced total power.

Magnetization transfer (MT) techniques have been shown to significantly reduce background soft-tissue signal in time-of-flight magnetic resonance angiography. To achieve sufficient suppression, radio frequency (RF) pulses with tip angles on the order of 1000 degrees are typically used, resulting in significant RF power deposition in the patient. Although these power deposition levels do not exceed the FDA guidelines, they are significantly higher than those used in typical imaging techniques. The use of these same magnetization transfer pulses in applications at field strengths higher than 1.5 T will require MT power levels which exceed FDA safety standards. This report demonstrates that the total power deposition required to achieve background tissue suppression can be significantly reduced by the application of the saturation pulses only during the phase-encoding steps corresponding to the central portion of "k space." This technique allows equivalent soft tissue suppression with approximately 10% of the energy deposition of conventional magnetization transfer techniques.

Combined effects of magnetization transfer and gadolinium in cranial MR imaging and MR angiography.

Magnetization transfer (MT) imaging is an MR technique in which image contrast is altered by applying RF pulses that saturate a restricted pool of hydrogen protons associated with cell membranes, proteins, and other macromolecules. Protons in this restricted pool, unlike those in tissue-free water, are not visible on MR due to their short T2 relaxation times. However, these restricted protons modulate the observed signal from free water by dipolar and chemical exchange interactions. In MT imaging, specifically tailored RF pulses are applied to saturate selectively the restricted macromolecular pool. This saturation is "transferred" to the free protons, causing their signal amplitude to decrease [1]. Increased signal intensity due to T1 shortening caused by gadolinium administration does not depend upon macromolecular interactions and is not appreciably suppressed by MT pulses (Fig. 1). Consequently, MT pulses act synergistically with gadolinium to increase the visibility of enhancing lesions by preferentially suppressing nonenhancing background tissue [2]. The purpose of this paper is to demonstrate the principles underlying the synergistic effects of MT saturation and paramagnetic contrast agents and to illustrate these effects in clinical MR imaging and MR angiography.

Pulsed magnetization transfer for imaging and spectroscopic applications on whole-body imagers

Magnetization transfer contrast is a tool to obtain additional information on tissue using standard whole-body magnetic resonance (MR) units without any hardware extension. Short hard pulse sequences with a total duration of about 1 ms can provide selective saturation of protons with very short relaxation timesT2 of 1–10 µs. Comparatively high amplitude of the radio-frequency field is advantageous; on the other hand, this amplitude is limited for whole-body units. The presented approach to achieve adapted hard pulse sequences is mainly based on maximum RF amplitudes corresponding with pulse angles per millisecond of 360 °, or 720°. The pulse sequences must not influence the magnetization of free protons with longer relaxation timesT2>10 ms in a frequency range that depends on the circumstances of the application. This frequency range has to be markedly broader for imaging techniques than for localized spectroscopy. The number of pulses, the pulse durations and pulse angles, and the interpulse delays were systematically varied. The time intervals between repetitions of the hard pulse sequences in order to obtain stronger magnetization transfer contrast were also optimized experimentally for human skeletal muscle and brain.

Numerically optimized hard pulse sequences for fast frequency selective excitation and inversion

Numerically optimized hard pulse sequences were developed providing a frequency selective response of the transverse and longitudinal magnetization for several applicationsin vivo. The sequences are based on pulse trains with binomial ratios of the pulse angles and constant time intervals between the pulses. These pulse trains were systematically optimized to obtain broad Larmor frequency ranges in which the magnetization is not markedly influenced by the pulse sequence. In addition, the sequences had to provide maximum transverse magnetization or complete inversion of the magnetization beside the suppression range. Such behaviour is needed for chemical shift selective imaging, pulsed magnetization transfer, and frequency selective spectroscopyin vivo. The phase of the magnetization response is shown and adequate rephasing conditions are discussed as well as the actual phase duration. Short optimized hard pulse sequences for water and fat-selective imaging provide lowT2-sensitivity of the resulting magnetization. The new optimized hard pulse sequences are suitable, if optimum suppression of signals at one single point in the Larmor frequency spectrum is less important than good suppression in a larger frequency range.

Pulsed magnetization transfer spin-echo MR imaging.

Cross relaxation between macromolecular protons and water protons is known to be important in biologic tissue. In magnetic resonance (MR) imaging sequences, selective saturation of the characteristically short T2 macromolecular proton pool can produce contrast called magnetization transfer contrast, based on the cross-relaxation process. Selective saturation can be achieved with continuous wave irradiation several kilohertz off resonance or short, intense 0 degree pulses on resonance. The authors analyze 0 degree binomial pulses for T2 selective saturation, present design guidelines, and demonstrate the use of these pulses in spin-echo imaging sequences in healthy volunteers and patients. Using the phenomenologic Bloch equations modified for two-site exchange, the authors derive the analytic expressions for water proton relaxation under periodic pulsed saturation of the macromolecular protons. This relaxation is shown to be monoexponential, with a rate constant dependent on the saturation pulse repetition rate and the individual and cross-relaxation rates.

Improving the contrast in rapid imaging sequences with pulsed magnetization transfer contrast

Improving the contrast in rapid imaging sequences with pulsed magnetization transfer contrast

Design and Implementation of Magnetization Transfer Pulse Sequences for Clinical Use

The transfer of magnetization between a free and a bound pool of spins is described in terms of the respective longitudinal relaxation times and the life times of spins in each pool. The effect of an off resonance radiofrequency (RF) pulse in producing saturation in the bound pool and a consequent decrease in both the available longitudinal magnetization and the T1 of spins in the free pool is described. The effects of increasing duration of the saturating RF pulse on image pixel signal intensity were used to determine values for the decrease in both T1 and the available magnetization in gray and white matter of the brain as well as in muscle, fat, and CSF. At 0.15 T the available magnetization of muscle was reduced by approximately 60% and its T1 was decreased from 350 to 150 ms. The available magnetization of white and gray matter was reduced by 40% and their values of T1 were reduced by 80-110 ms. The reduction in available magnetization was used to increase contrast on proton density weighted or T2-weighted SE pulse sequences. These changes were also used to design inversion recovery (IR) pulse sequences with particular contrast properties. A short inversion time (TI) magnetization transfer (MT) IR (MT-STIR) pulse sequence was used to reduce the signal from normal muscle to zero to produce an angiographic effect in the leg. Increased tissue contrast was observed with a T2-weighted (MT-SE) sequence in a patient with bilateral cerebral infarction and with an MT-IR pulse sequence in a patient who had an intracranial hematoma. Three patients with cerebral tumors showed high lesion contrast with MT-STIR sequences. Components within two tumors were changed to different degrees by MT and in one case change in the brain attributable to recent radiotherapy treatment was only identified with an MT-STIR sequence. Magnetization transfer can be used to manipulate both the available longitudinal magnetization and the T1 of normal and abnormal tissues. The changes in tissue contrast produced by this can be very substantial and are likely to be of importance in clinical imaging.