Macromolecule

Fig. 20. Schematic of the MT saturation process. An intense RF pulse is applied off-resonance, which saturates the magnetization of the macromolecule pool. Rapid exchange between magnetization of the macromolecule pool and the free water pool causes the free water signal to be partially attenuated

Fig. 21. Example images from an MTR experiment. The image on the left was obtained without any MT saturation. The MT-weighted image in the middle was obtained by applying a 90° pulse 3000 Hz off-resonance (TR = 30 ms). The image on the right is the estimated MT ratio (MTR) map using Equation 16. Images courtesy of A. Samsonov and A. Field

Fig. 21. Example images from an MTR experiment. The image on the left was obtained without any MT saturation. The MT-weighted image in the middle was obtained by applying a 90° pulse 3000 Hz off-resonance (TR = 30 ms). The image on the right is the estimated MT ratio (MTR) map using Equation 16. Images courtesy of A. Samsonov and A. Field although the MTR can also be influenced by overall water content and other macromolecules in processes such as neuroinflammation (Stanisz et al. 2004).

MT saturation is achieved using an RF pre-pulse, which may be applied in combination with any RF pulse sequence o. An example spin-echo CPMG pulse sequence with RF saturation is shown in Fig. 22. There has been considerable variation of reported MTR properties in the literature, which is likely caused by inconsistencies in the pulse sequence protocols. The exact MTR measurement will depend upon the pulse sequence parameters (e.g., TR, TE, excitation flip angle), the magnetic field strength, as well as the shape, amplitude and frequency offset of the saturation pulses. Consequently, within a single MTR study, the imaging parameters should be fixed to maximize consistency. Common problems with MTR experiments include spatial inhomo-geneities in both the static magnetic field (B0) and the RF magnetic field (B1).

Fig. 22. Measurement of T2 relaxation in the presence and absence of an RF saturation pulse. Courtesy of G.J. Stanisz

B0 inhomogeneities are caused by poor shimming and spatial variations in the magnetic susceptibilities in soft tissue, bone and air, which lead to shifts (errors) in the saturation frequency offsets. Inhomogeneities in the B1 field, which are common using volume head coils particularly at high magnetic fields (B0 > 1.5T) will affect the saturation pulse amplitude and consequently alter the level of MT saturation. Both B0 and B1 fields may be measured and used to retrospectively correct MTR measurements (Sled and Pike 2000; Ropele et al. 2005) Another source of MT saturation is the application of RF excitation pulses for slice selection in 2D pulse sequences (Santyr 1993). The slice selective RF pulses of other slices shifted relative to the current one will cause MT saturation. This is more problematic for multi-slice 2D pulse sequences with many 180° pulses (e.g., fast spin echo, and T1-weighted spin echo); therefore, 3D scans are generally preferable for MTR measurements. Other considerations for MTR measurements are discussed in two excellent review papers (Henkelman et al. 2001; Horsfield et al. 2003).

As discussed above, the MTR measurement is highly dependent upon a broad range of pulse sequence and scanner factors. Consequently, several research groups have been developing models and imaging protocols for quantitative measurements of MT properties (Henkelman et al. 1993; Stanisz et al. 1999; Sled and Pike 2001; Yarnykh 2002; Tozer et al. 2003; Yarnykh and Yuan 2004). these techniques typically require measurements at multiple frequency offsets and saturation pulse amplitudes. Since MT saturation is performed using RF pulses, the MT models are usually based upon a two-pool model (free water and macromolecule) with continuous RF saturation approximated by regular RF saturation pulses. By using these models, it is possible to estimate the macromolecular concentration (bound pool fraction), the exchange rate between the free and bound pools, and the T2 of the bound pool (Fig. 23). Unfortunately, the acquisition of the required images can be quite time consuming, which has limited the overall applicability of the technique. Nonetheless, quantitative MT methods are much more specific than the conventional MTR methods.

Fig. 23. Quantitative MT maps obtained by acquiring data at multiple frequency offsets and flip angles and using a two pool (free water and macromolecule) model with exchange. The images from left to right are: no MT contrast, T1 map, exchange rate (k), bound pool fraction (fb), and the T2 of the bound pool (T2b). The images demonstrate the wide range of quantitative imaging measures that can be obtained in a quantitative MT experiment. Images courtesy of A. Samsonov and A. Field

Fig. 23. Quantitative MT maps obtained by acquiring data at multiple frequency offsets and flip angles and using a two pool (free water and macromolecule) model with exchange. The images from left to right are: no MT contrast, T1 map, exchange rate (k), bound pool fraction (fb), and the T2 of the bound pool (T2b). The images demonstrate the wide range of quantitative imaging measures that can be obtained in a quantitative MT experiment. Images courtesy of A. Samsonov and A. Field

0 0

Post a comment