Quantitative interpretation of magnetization transfer

Abstract

Magnetization transfer contrast (MTC) experiments using off-resonance irradiation have been performed with an agar gel model by systematically varying offset frequency, amplitude of the RF irradiation and gel concentration. The experimental results are shown to be quantitatively modelled by a two-pool system consisting of a liquid pool with a Lorentzian line shape and a small semisolid pool with a Gaussian lineshape. The fitted model yields physically realistic fundamental parameters with a T2 of the semisolid pool of 13 microseconds. Further analysis shows that the off-resonance irradiation MTC experiment had significant limitations in its ability to saturate the semisolid pool without directly affecting the liquid component.

Detection of thrombus size and protein content by ex vivo magnetization transfer and diffusion weighted MRI

“…The importance of collagen in creating an MT effect is evident by the fact that although this single macromolecule comprises only ~ 40% of the macromolecules in cartilage, it is responsible for nearly all of the MT effect observed [39,41]. Moreover, several other mechanisms contribute to the rate of MT, including the mobility, the hydration state and the number of hydroxyl and amide groups [40], as well as the concentration of the macromolecules and/or water [51]. …”

Magnetization transfer in MRI: a review

“…2 can be readily expressed by a set of coupled differential equations. 4 The effect of off-resonance irradiation on this system is different for the two pools. For pool B, the protons in the macromolecules are strongly coupled to each other resulting in a homogeneously broadened absorption lineshape as is shown in Fig.…”

“…7 that the MTR for a tissue would be expected to increase as field strength increases. This is not a result of the exchange rate, which has been shown to be independent of field strength, 4 but is due to diminished competition from recovery of longitudinal magnetization at higher fields. Although the discussion so far has focused on CW offresonance irradiation for the production of magnetization transfer, the same theoretical analysis can be applied to generation of MT by pulsed irradiation.…”

“…Figure 2 shows a two-pool model that is simple yet sufficient for quantitative interpretation of MT. 4 Pool A represents the liquid spins. The number of spins in this compartment is by convention normalized to unity (M 0A = 1).…”

“…Agar was the first model system to be studied in detail. 4 Figure 4 shows representative MT measurements for an aqueous gel sample containing 4% agar by weight. The data show the fraction of longitudinal magnetization remaining after CW irradiation to a steady-state condition (7 s) as a function of the frequency offset (D) for seven different irradiation amplitudes (expressed as the nutational frequency of B 1 , !…”