Volume-selective NMR

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. 

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. 

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. 

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... 

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... 

As the volume of literature relating to NMR expands annually it is essential, for people applying this technique to various problems, to have access to authoritative reviews on selected NMR topics. It is the aim of Annual Reports to provide such reviews particularly in those areas which are of widespread current interest. Thus the general review on NMR which has been featured in this series will be removed and a continuing emphasis given to the NMR of nuclei other than protons. 

Fig. 13. ITie WET sequence modified for use with ID LC NMR The four SEDUCE RF pulses are 98.2°, 80.0°, 75.0° and 152.2° for B,-insensitive WET. Other angles are used in the case of B - and Tj-insensitive V T. The gradient pulses are each 2 ms long with amplitudes of 32, 16, 8 and 4Gcm , respectively, " ere is a delay of 2 ms between each gradient pulse and the next RF pulse. Carbon decoupling with a field strength of 100 Hz is applied during the H-selective pulses. Instead of a single (hard) irU excitation pulse, a volume selective composite hard ir/2 pulse scheme (i.e. 90°90 90°- 90°,) can be used which affords a flatter baseline.

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. 

Volume-selective MRI using an adjustable, single-sided, portable sensor This sensor can probe the interior of samples nondestructively by collecting images and measuring relaxation and dilfusion constants, and even perform chemical analysis under conditions inaccessible to conventional NMR. MRI in axial and sagittal planes at dilferent depths of the sensitive volume and T-1-weighted contrast in a tissue sample are introduced. 

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... 

Spatially localized NMR spectroscopy was performed using a sample composed of a monoolein-D20 dispersion forming a cubic liquid-crystalline phase [41]. The spectra were selected from a slice of 300-pm thickness. The direction of the gradient was along the spinning axis. A spectral resolution was obtained that had never been observed before when selecting the whole volume of the sample. 

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