Hahn-echo imaging

Figure 7.5 [15] Susceptibility contrast in EPDM samples at 363 K (a) 2D Hahn-echo image acquired with the sequence of Figure 7.4a (b) 2D gradient-echo image acquired...

Figure 7.13 Hahn-echo images of a terpolymer (a) Unswollen sample, (b) sample after swelling in cyclohexane. The images were taken from a 15 mm diameter cylinder (adapted from [47] with permission of the authors)...

Figure 7.16 T2 weighted Hahn-echo image (a) and quotient image (b) computed as the quotient of two Hahn echo images acquired with different echo times of an unfilled SBR/NR covulcanisate with dimensions 9 mm x 30 mm. The acquired signal has been integrated over the sample thickness of 1 mm. Contrast in the quotient image is determined mainly by transverse relaxation. Contrast in the Hahn-echo image is formed by a mixture of spin density and relaxation...

A mixture of spin density and relaxation forms contrast in the Hahn-echo image. Contrast in the quotient image is determined only by transverse relaxation. In this image, the interface appears abrupt and well defined. The image of the interface shows a transition from the hard SBR component to the soft NR component with a width of the order of 0-5 mm (184,185). This is the space scale on which the modulus changes. The shape and dimension of the interface are defined by the concentration differences in the vulcanizing agents, which diffuse at elevated temperatures across the interface until their diffusion is hampered by their role in the vulcanization reaction. Thus, the interface arises from a delicate balance between diffusion, reaction, heat supply, and removal. 

Usually the rf pulses applied in an imaging sequence are not perfect. The pulses may differ in shape, phase, and amplitude from the required values. As a result, unwanted transverse magnetization may arise which leads to erroneous signals. This is so, in particular, for the 180° refocusing pulse of the Hahn echo. These transverse magnetization components can be dephased by application of homogeneity-spoil or crusher gradient pulses. 

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). 

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

Fig. 6.2.16 [Cho2] 2D DANTE-imaging. (a) Timing of signals. The rf excitation is a DANTE pulse train applied in a read gradient Gx- The Hahn echo of the response is detected while both the phase-encoding gradient Gy and the read gradient Gx are turned on. The slice-selection gradient is Gj. (b) Traces in k space.

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