Stimulated-echo imaging

The popular spin-echo imaging scheme (Fig. 6.2.1(e)) requires execution of a 180° pulse for formation of the Hahn echo. This sequence provides the maximum signal without phase distortions for image construction. However, a 180° pulse also requires considerable rf power, in particular, when it is applied to large diameter coils. As an alternative to Hahn echoes, stimulated echoes can be used for imaging [Burl, Finl, Fral]. They are excited by three instead of two rf pulses (cf. Section 2.2.1). Imaging schemes based on stimulated echoes are also referred to as STEAM (stimulated-echo acquisition mode) images [Fral]. 

As a consequence of having three time periods for the stimulated echo instead of two for the Hahn echo, a considerable variety of imaging schemes can be designed by assigning different functions to the individual pulses. Either one, two, or all three pulses can be made selective, and the selective pulses can be combined with suitable gradients. Thus, imaging sequences which provide more information than those based on Hahn echoes can be designed (cf. Section 7.2.4). 

For optimum signal strength in liquid spin-i systems, all pulses are 90° pulses. In this case, the stimulated-echo sequence can be written as 

A closely related technique can be used for multi-slice imaging (Fig. 6.2.7) [Fral]. The scheme of Fig. 6.2.5(c) is appended by further slice-selective 90° pulses with different centre frequencies, so that the magnetization of other slices is selected [Fral]. In this way, the otherwise necessary recycle delay can effectively be used for acquisition of additional slices. However, the contrast in each slice is affected by a different Ty weight, because is different for each slice. The technique can readily be adapted to line-scan imaging by applying successive slice-selective pulses in orthogonal gradients [Finl]. 

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Figure 10 shows the incorporation of the velocity-encoding field gradient pulses in a stimulated-echo imaging pulse sequence which was used in this work. A remarkable advantage of using the stimulated echo is that the relaxation... 

Flaase A and Frahm J 1985 Multiple chemical-shift-selective NMR imaging using stimulated echoes J. Magn. Reson. 64 94-102... 

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. 

Fig. 4.5.5 Pulsed field gradient sequences to obtain velocity and diffusion data (a) spin-echo (PGSE) and (b) stimulated-echo (PGSTE). The application of imaging gradients C Gy and Gz allows the measurement of velcocity maps and spatially-resolved diffusion coefficients and size distribution in emulsions.

A second imaging gradient (Gy) is added in order to obtain a spatial map of the displacements. A notable feature of the stimulated echo protocol is that during the flow encoding time, A, the magnetization is stored along the z axis and is subject to the longitudinal... 

Cory and Garroway [13] introduced the NMR pulsed gradient stimulated echo method to study compartments which are too small to be observed by conventional NMR imaging. They showed so-called proton displacement profiles of bulk water and dimethyl sulfoxide. The displacements are due to free diffusion and are Gaussian shaped. The profile of water in yeast cells showed restricted diffusion with a characteristic cell width of approximately 5 /xm. 

Fig. 10. The stimulated-echo pulse sequence used in this work for projection imaging of the z component of the velocity over the whole sample.

In our work, the velocity distribution method is used, where the velocity distribution function PA (v, r) is estimated from the velocity imaging, and the average velocity at a voxel located at r, r, is calculated. Consider the one-component ("-direction) velocity imaging in two-dimensional (z- and A-direction) space illustrated in Fig. 11. In this case, the velocity encoding is parallel to the frequency encoding. The signal intensity from the stimulated echo S(kz, kx, q) is converted to the velocity distribution function PA (fi z, x) by combining Eq. (34) and Eq. (36) ... 

Both the Hahn and the stimulated echo are basic elements of many imaging methods. Because the stimulated echo consists of three pulses and three time periods, a greater variety of imaging methods exists for the stimulated echo (cf. Section 6.2.5) [Burl]. 

Fig. 6.2.5 [FRAl] Timing diagrams of basic STEAM imaging sequences. Schemes (a)-

Fig. 7.2.6 [Haa3] Pulse sequences for CHESS-STEAM imaging. The NMR signals are the Hahn or spin echo (HE) and the stimulated echo (STE). (a) Basic sequence for chemical-shift selective measurement of HE and STE images, (b) Sequence for acquisition of n slice-selective images from CHESS stimulated echoes.

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