Magnetization transfer in MR imaging

Abstract

THE MANY DIFFERENT local environments of hydrogen nuclei within tissue give rise to complicated magnetic relaxation behavior while ensuring the rich and diverse contrast seen in magnetic resonance (MR) imaging. From an operational point of view, it has proved convenient to classify the multiple environments into two basic types: a solid-like environment and a liquid-like environment. Protons in the solid-like environmentmostly associated with macromolecules-do not experience sufficiently fast rotational or translational motions to average out mutual magnetic dipole-dipole interactions. As a consequence, the MR spectrum from the solid-like pool is represented by a broad distribution of Larmor frequencies (several kilohertz). Such spins dephase quickly, with T2 values generally less than 1 msec (1). In contrast, hydrogen nuclei in the liquid-like environment-mostly associated with small mobile molecules such as water-experience rapid rotational and translational motions that serve to substantially average out mutual magnetic dipoledipole interactions. Thus, the MR spectrum of these protons is represented by a relatively narrow distribution of Larmor frequencies (several hertz), with consequently long T2 values.In conventional MR imaging, the liquid-like pool produces the bulk of observable MR signal. However, magnetization transfer between the solid-like pool and the liquid-like pool has a con-

Magnetization transfer MRS

“…19 Several authors have compared methodological aspects of cw off-resonance and pulsed on-resonance techniques. [20][21][22] Hua and Hurst showed that nominally optimized versions of each of these methods provide essentially similar MT effects at a given level of applied rf power. 21 Thus, the choice between cw or pulsed MT methods is often made on the basis of experimental considerations (available hardware, ease of implementation, possible combination with localization techniques).…”

Magnetization transfer attenuates metabolite signals in tumorous and contralateral animal brain: In vivo observations by proton NMR spectroscopy

“…CW irradiation saturates the B spins more efficiently than RF bursts set apart by a delay ?llrx While the equilibrium of A and B magnetization is constantly unbalanced during CW irradiation, as compared with the CW case, higher A magnetization is established quickly during the RF-free period of the pulsed method. (2) Even at a moderate speed of exchange of r, = 0.1 s-', a B/A concentration ratio as small as f = 1/100 is sufficient to cause a detectable MT effect.…”

“…(2) Even at a moderate speed of exchange of r, = 0.1 s-', a B/A concentration ratio as small as f = 1/100 is sufficient to cause a detectable MT effect.…”

“…Since this type of magnetization transfer (MT) experiment has been combined with in vivo MR imaging (I), the relevance of MTC exhibited by water protons has been proved for various clinical applications (2). Likewise, MT attenuation exhibited by metabolite signals may provide supplementary information for understanding (patho)physiological processes.…”

Magnetization transfer studies of the fast and slow tissue water diffusion components in the human brain

“…[15][16][17][18][19][20][21][22] We hypothesize that if the physical basis of empirically useful biexponential decay analyses is two distinct water compartments with widely disparate diffusion coefficients, then it would reasonably follow that the two compartments might have substantially different magnetization transfer ratios. [23][24][25][26] For example, within the context of an intra-vs extracellular water compartmentation model, the intracellular water molecules responsible for the 'slow' diffusion component might reasonably be expected to experience more interactions with macromolecules, intracellular organelles, etc, than those responsible for the 'fast' diffusion component within the extracellular matrix. This in turn would lead to a larger 'bound' water fraction and so an enhanced MTR for the slow vs the fast tissue water diffusion component.…”