Volume-selective spectroscopy

Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. 

In the following sections (Sections 5.5.2.2 and 5.5.2.3), two approaches to spatially resolving chemical conversion within a reactor are demonstrated (a) n-dimensional Chemical Shift Imaging (CSI), and (b) volume selective spectroscopy. 

Figure 5.5.8 shows the volume selective spectroscopy pulse sequence used. This pulse sequence combines elements of NMR spectroscopy and MRI pulse sequences three slice selective rf pulses are applied in three orthogonal directions to obtain 1H spectra from pre-determined local volumes within the sample [30]. In this particular application, spectra were recorded from local volumes of dimension 1.5 mm x 1.5 mm x 0.5 mm within the fixed bed the data acquisition time for each spectrum was 3 min. Figure 5.5.9(a) shows the local volumes selected within slice 3 of the bed, as identified in Figure 5.5.7. In Figure 5.5.9(b) the 1H spectra recorded from within these volumes are shown data are presented only for the range of... 

Fig. 5.5.8 The volume selective spectroscopy rf pulse sequence used to acquire the data shown in Figures 5.5.9-5.5.11. Magnetic field gradients applied alongthex, y and zdirections enable localized spectra to be recorded from...

Fig. 5.5.9 Volume selective spectroscopy within the fixed-bed reactor, (a) The location of the ten selected volumes within image slice 3 (see Figure 5.5.7) are identified, (b) It is clearly seen that the chemical shifts of peak B obtained from the spectra recorded from the selected volumes occur at significantly different values...

In this pulse sequence the attributes of NMR spectroscopy and MR imaging are again combined. In this case three slice-selective r.f. pulses are applied in three orthogonal directions to obtain H spectra from predetermined local volumes within the sample.27 A typical volume-selective spectroscopy pulse sequence is shown in... 

Fig. 9 A volume-selective spectroscopy r.f. pulse sequence. Magnetic field gradients applied along x-, y-, and z-directions enable recording of localized spectra from pre-defined local volume elements within the sample.

Fig. 40. 2-D slice through a 3-D RARE image of a fixed bed of ion-exchange resin. The image has an isotropic resolution of 97.7 pm x 97.7 pm x 97.7 pm. The image slice in which the local volumes are located for the volume-selective spectroscopy study is identified. The image was acquired by saturating the bed with pure methanol. r2-contrast was exploited in the data acquisition so that signal was acquired only from the methanol in the inter-particle space. Reprinted from reference (84 with permission of Springer Science and Business Media.

This scheme can be repeated for different gradient directions. Without repetition a slice is selected, with one repetition a line and two repetitions a voxel (Fig. 5.3). Thus, selective excitation is needed to prepare individual volume elements in an extended object for subsequent investigation by NMR spectroscopy [19j. This type of NMR with spatial resolution is called volume-selective spectroscopy. 

Ernst T and Hennig J (1991) Coupling effects in volume-selective spectroscopy of major brain metabolites. Magnetic Resonance in Medicine 21 82-96. 

Ordidge, R.J., Helpern, J.A. Image Guided Volume Selective Spectroscopy A Comparison of Techniques for In-Vivo P.NMR Spectroscopy of Human Brains. Vol. 27, pp. 103-118... 

Cherry Ripening T, Dand volume selective spectroscopy... 

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