Magnetization Transfer Contrast
Water molecules exist either free or loosely attached to macromolecules, the two populations maintaining an equilibrium (Fig. 64). Water in hydration layers is not visible by current MRI due to extremely rapid T2 decay: the emitted signal is so short-live
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Magnetization Transfer Contrast
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Free (bulk) water vs loosely bound or hydration layer › Transfer of longitudinal magnetization
Water molecules exist either free or loosely attached to macromolecules, the two populations maintaining an equilibrium (Fig. 64). Water in hydration layers is not visible by current MRI due to extremely rapid T2 decay: the emitted signal is so short-lived that current echo times are too long to detect it. Thus, if we affect the magnetization of either pool we will affect the total signal. We can temporarily inactivate the bound fraction with saturation pulses. Some of these water molecules will jump free and displace an equal number of molecules to the bound compartment. The effect is the reduction of active spins in the free pool and thus attenuation of the final signal (Fig. 65).
This type of contrast is called magnetization transfer contrast (MTC), since it results from the exchange of water molecules (and their signal ability) between the free and the bound compartments. It is evident that magnetization transfer (MT) is not applicable to pure tissues such as fat or CSF. Brain white and gray matter differ in their relative water, lipid, and protein content, and thus have different susceptibility to MT pulses (Fig. 66). In routine practice, MTC is used primarily as an additional method of background suppression to “reinforce” either contrast enhancement in T1-w sequences or the blood signal in MR angiography (Figs. 67, 68).
M a gn e tiz atio n Tra n sfe r Cont rast
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Fig. 64 Hydration layer. Weak electrostatic forces attract
water molecules close to the surface of the macromolecules, DNA, and other cellular constituents, forming a water “cloak” of variable thickness (called the hydration layer). The free and bound members can easily trade places, carrying over their current magnetization to the new site
Fig. 65a,b Mechanism of magnetization transfer
contrast (MTC). a The resonance frequency of free water is a sharp tall spike. The bound pool has a broad distribution around the central peak. b With appropriate pulses on either side of the central peak, we affect only the bound component. c The result is a reduction of the longitudinal magnetization of the free pool. Thus, the maximum signal of the affected tissue is also reduced
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Ch a pter 2 2
M a gn e tiz atio n Tra n sfe r Cont rast
Fig. 66a–f Effect of MT pulses on brain parenchyma.
T1-w images from a a standard sequence and b after application of MT pulses. Spatial resolution, TR and TE (532/15) are the same in a and b. In b note the overall decrease in signal intensity and greater suppression of the white matter: the signal intensity of the white matter and cortical gray matter have become similar in the paramedian frontal areas. Note the hyperintensity of the thalami (T) and the basal ganglia (white arrows) relative to the internal capsules (black arrow). Incidentally, an arachnoid
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cyst is present in the left temporal area (A). c, d A pair of PD images (TR/TE: 1,750/25) from a healthy volu
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