Volume selective imaging

Exploring Spatial Heterogeneity in Conversion Volume Selective Imaging... 

Studies of this reaction have recently been extended to acquisition of a 3(4)D CSI dataset, shown in Figure 5.5.12 the grey scale indicates the extent of conversion. As expected from the 1(2)D CSI and volume selective imaging studies discussed earlier, conversion is seen to be heterogeneous within transverse sections through the bed at any position along the direction of superficial flow. 

Volume-selective imaging. In this pulse sequence the attributes of MR spectroscopy and MRI are again combined. In this case three slice-selective r.f. pulses are applied in three orthogonal directions to obtain H spectra from pre-determined local volumes within the sample (24). A typical volume-selective spectroscopy pulse sequence is shown in Fig. 13. An example of this pulse sequence used in application... 

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. 

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...

Fig. 29. Thirteen-year-old boy with Duchenne Dystrophy, (a) The fat selective image shows a fatty degeneration of all muscles in a cross-section of the lower limb. The fat tissue is orientated in the direction of the muscle fibres, and is present between the fibres and in the septa, (b, c) The spectra from both volume elements indicated in (a) reveal signals from EMCL. Muscle specific metabolites (TMA and creatine) are clearly reduced and not visible in the proton spectra due to the reduction of muscle tissue in favour of adipose tissue.

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.

From each spectrum it is possible to determine, directly and quantitatively, the extent of conversion within that local volume. In Fig. 41, the regions within which the individual volume-selective spectra were acquired are identified and color coded by the extent of conversion as determined from the spectrum recorded from that particular local volume. All images in Fig. 41 show that there is significant heterogeneity in conversion within a single transverse section through the bed, perpendicular to the direction of superficial flow fractional variations in conversion of up to approximately 20% were found to be typical under steady-state operating conditions. 

Magnetic resonance spectroscopy (MRS) This technique is also referred to as magnetic resonance spectroscopy imaging (MRS ) or volume-selective NMR spectroscopy, which basically combines MRI with NMR spectroscopy. Thus MRS allows conventional MRI study of a body region, while NMR spectroscopy provides information on the biomolecules in that same region. This is a powerful tool, which can be used to provide spectral data on metabolites including lactate, choline, creatinine and myoinositol. 

Volume selection is particularly useful for spatially localized 2D spectroscopy, because acquisition times for 2D spectroscopic imaging would be prohibitively long. Localized 2D spectroscopy is of interest, for instance, to detect information hidden beneath overlapping signals in ID spectra [Alol, Berl, Cohl, Sotl] as well as to follow reaction kinetics [Ball]. 

Spatially resolved NMR is concerned with unraveling the spatial distribution of Mo(x, y, z) and measuring associated NMR parameters at individual volume cells , or voxels at space coordinates r = (x, y, z) - If many contingent voxels are investigated on a plane, or if a projection is investigated, one refers to NMR imaging-, if individual voxels are investigated one refers to volume-selective NMR. 

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