Magnetization Transfer

Water provides the largest signal contribution to the MRI signal in brain tissues. While estimates of conductivity can be calculated from diffusion tensor data (Tuch et al. 2001), a more ideal probe of the effectiveness of white matter conduction properties would be obtained from images of myelin components. The problem is that the signals from protein and lipid molecules associated with myelin are essentially undetectable in an MRI experiment because they have ultrashort T2 values (10s of microseconds). However, the magnetization (sum of dipole moments) of free water does interact with the macromolecules through chemical exchange and dipolar coupling. This exchange of magnetic moments is referred to as magnetization transfer (Balaban and Ceckler 1992).

Magnetization transfer (MT) effects may be detected in an MRI experiment by applying strong radio-frequency (RF) pulses at a frequency shifted by roughly 1000 Hz or more from the resonance frequency of free water. The RF pulse energy will partially saturate the magnetization of the protons bound to macromolecules, which have a very broad frequency spectrum relative to that of free water (width inversely proportional to T2). The fast exchange of magnetization between the macromolecular and free water pools will indirectly attenuate the signal from the free water. The process is illustrated in Fig. 20. The attenuation is a function of the amplitude, rate, and frequency offset of the RF attenuation pulses, and the concentration of macromolecules and exchange rate of the magnetization between the free water and bound macromolecular pools.

The most common approach for characterizing MT is to acquire two sets of images - one with the off-resonance saturation MT pulses (Ms) and one set without (Mo). The MT contrast (MTC) is the difference between the images, MTC = Mo — Ms. Since absolute signal intensities are arbitrary, the MTC is typically normalized by the signal without MT saturation, which is the MT ratio

The MTR is the most commonly used measure of magnetization transfer and example maps are shown in Fig. 21. Increased MTR values may correspond to increased macromolecular concentrations in the tissue. The MTR values in healthy WM and GM are roughly 0.4-0.55 and 0.25-0.35, respectively. The higher MTR in WM is believed to be associated with the proteins and lipids associated with myelinated axons (Stanisz et al. 1999). Consequently, the MTR in WM is reduced in demyelinating diseases such as multiple sclerosis

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