Hahn spin-echo imaging

Fig. 2.9.7 Hahn spin-echo rf pulse sequence combined with bipolar magnetic field gradient pulses for hydrodynamic-dispersion mapping experiments. The lower left box indicates field-gradient pulses for the attenuation of spin coherences by incoherent displacements while phase shifts due to coherent displacements on the time scale of the experiment are compensated. The box on the right-hand side represents the usual gradient pulses for ordinary two-dimensional imaging. The latter is equivalent to the sequence shown in Figure 2.9.2(a).

The rf part of the pulse sequence generates a Hahn spin echo at time 2x. In imaging jargon the time from the center of the 90° pulse to the center of the echo is called the echo time (or time to echo) TE = 2x (where x is the spectroscopist s usual symbol for the time from the 90° pulse to the 180° pulse). The time from the center of the 90° pulse to the center of the next 90° pulse is called the repetition time (or time to repeat) TR. Spectroscopists know TR as the time equal to the recycle delay plus the time taken by the pulsing and data sampling. 

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]. 

Fig. 2.9.2 Radiofrequency, field gradient and current distributions requires a three-dimen-ionic current pulse sequences for two-dimen- sional imaging sequence [see Figure 2.9.1(a)] sional current density mapping. TE is the Hahn and multiple experiments with the orientation spin-echo time, Tc is the total application time of the sample relative to the magnetic field of ionic currents through the sample. The 180°- incremented until a full 360°-revolution is pulse combined with the z gradient is slice reached. The polarity of the current pulses...

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...

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.

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. 

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