Volume-selective excitation

Fig. 9.1.2 [Aue2] Radio-frequency field gradients and rf pulses for volume-selective excitation. Each composite 45°-90°-45° pulse is applied in a different field gradient, so that the space dimension of the magnetization to be investigated is reduced by one in each step.

Fig. 9.1.8 [Bri2] Schematic illustration of the VOISINER pulse sequence for volume-selective excitation. It consists of three excitation modules, a SPACE module, a selective 90° pulse, and a selective 180° pulse.

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. 

The E,Z-photoisomerization of previtamin D to tachysterol has also received recent attention. Jacobs and coworkers examined the process in various solvents at 92 K and found evidence for the formation of a triene intermediate which converts thermally (Ea ca 6.5 kcal mol 1) to the more stable tEc rotamer of tachysterol (tEc-T equation 58)230. The rate of this conversion is viscosity dependent. They identified this intermediate as the cEc rotamer, produced by selective excitation of the cZc rotamer of previtamin D. In a re-examination of the low temperature ,Z-photoisomerization of previtamin D as a function of excitation wavelength, Fuss and coworkers have suggested an alternative mechanism, in which tEc-1 is produced directly from cZc-P and cEc-T directly from tZc-P (equation 59)103. This mechanism involves isomerization about both the central double bond and one of its associated single bonds—the hula-twist mechanism of Liu and Browne101 — and involves a smaller volume change than the conventional mechanism for ,Z-isomerization. The vitamin D system has also been the subject of recent theoretical study by Bemardi, Robb and Olivucci and their co workers232. 

Fig. 12. Sequences for volume selective single voxel spectroscopy. Both techniques work with three slice-selective RF-pulses. (a) The Point RESolved Spectroscopy (PRESS) sequence generates a volume selective double spin-echo. The entire time delay between the initial 90° excitation and the echo is sensitive to transverse relaxation, (b) The Stimulated Echo Acquisition Mode (STEAM) sequence generates a stimulated echo. Maximal signal intensity (without relaxation effects) is only half the signal intensity of PRESS under comparable conditions, but slice profiles are often better (only 90° pulses instead of 180° pulses) and the TM interval is not susceptible to transverse relaxation, (c) The recorded echo signal is only generated in a volume corresponding to the intersection of all three slices.

Most types of selective excitation can be modified for simultaneous excitation of n slices or volume elements. Such an approach is advantageous when a limited number of slices or volume elements, but not the entire 3D object, needs to be investigated. By suitable coding of the volume information in n experiments an improvement in signal-to-noise ratio of can be gained [Boll, Miill]. Compared to 3D volume imaging, multislice and multi-volume techniques (cf. Section 9.1) suffer from the lack of achieving well-defined boundaries. 

Apart from preparation of magnetization in slices and lines, selective excitation can also be used for point selection. Here the objective normally is not to scan an image in a pointwise fashion, but rather to localize a selected volume element to acquire a spectroscopic response from it [Auel] (cf. Section 9.1). 

The volume of interest by selective inversion, excitation and refocusing (VOISINER) technique [Bril] is a hybrid volume localization method (Fig. 9.1.8). It consists of three selective excitation modules. The first one is an inversion module like VSE, SPACE, or... 

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. 

In atomic laser spectroscopy, the laser radiation, which is tuned to a strong dipole transition of the atoms under investigation, penetrates the volume of species evaporated from the sample. The presence of analyte atoms can be measmed by means of the specific interaction between atoms and laser photons, such as by absorption techniques (laser atomic absorption spectrometry, LAAS), by fluorescence detection (laser-induced fluorescence spectroscopy, LIFS), or by means of ionization products (electrons or ions) of the selectively excited analyte atoms after an appropriate ionization process (Figures lA and IB). Ionization can be achieved in different ways (1) by interaction with an additional photon of the exciting laser or of a second laser (resonance ionization spectroscopy, RIS, or resonance ionization mass spectrometry, RIMS, respectively, if combined with a mass detection system) (2) by an electric field applied to the atomization volume (field-ionization laser spectroscopy, FILS) or (3) by collisional ionization by surrounding atoms (laser-enhanced ionization spectroscopy, LEIS). 

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