Multi-slice

Fig. 2.1.16 Trabecular bone structure, muscle after removal of the soft tissue (right). Image and tendon of a mouse tail in vitro at 21.14T, parameters multi-slice spin-echo method,...

Electron dynamic scattering must be considered for the interpretation of experimental diffraction intensities because of the strong electron interaction with matter for a crystal of more than 10 nm thick. For a perfect crystal with a relatively small unit cell, the Bloch wave method is the preferred way to calculate dynamic electron diffraction intensities and exit-wave functions because of its flexibility and accuracy. The multi-slice method or other similar methods are best in case of diffraction from crystals containing defects. A recent description of the multislice method can be found in [8]. 

We have developed a software package MSLS [1], in which multi-slice calculation software is combined with least squares refinement software used in X-ray crystallography. With multi-slice calculations which are standardly used for image calculations of HREM images, dynamic diffraction is taken into account explicitly. 

The refinement of the thickness requires a special trick. The derivatives, I (formula (8)), imply that the dependency of the intensity to the thickness is continuous. However, by dividing the crystal in as many slices as in the Multi-slice calculation, the thickness becomes a discrete parameter. If the 6 in formula (8) is smaller than the slice size the resulting derivative will be zero because I(p+8) and I(p) are equal in many cases due to the calculation method. To overcome this problem, in MSLS a crystal is divided in a certain number of equal slices and a last slice having a thickness of a fraction f of the other slices. It is assumed that this slice contains the same scattering potential as that of the other slices but multiplied by f... 

In a second step d5mamical effects must be taken into account. For dynamical refinement we used a multi-slice least squares (MSLS) procedure [21], where the centre of laue circle, crystal thickness and scaling factor were refined for each zone separately with fixed atom positions. In the case of non-centrosymmetric space groups the enantiomorph used for refinement can be chosen for each zone, additionally. After the basic parameters have been derived satisfactorily, the atom positions can be refined. The R-values (see Eq. 2) refined by MSLS usually dropped down to 6-13%. 

Perform three orthogonal sets of multi-slice T2-weighted FSE images to assure appropriate head position and good acoustic coupling. 

Fig. 6.2.7 [Fral] Timing diagram for multi-slice imaging by the STEAM method. The third rf pulse is used for slice selection. It is repeated with different centre frequencies for acquisition of different slices.

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]. 

If the functions of the second and the third pulses are interchanged (Fig. 7.2.6(b)), then the third pulse can be repeated for different frequencies to read out stimulated echoes from different slices. The CHESS multi-slice method is comparatively insensitive to spatial variations in flip angles from B inhomogeneities and makes efficient use of the available rf power. 

Fig. 7.2.7 [Haa3] Multi-CHESS-STEAM imaging, (a) Simultaneous acquisition of one nonselective HE image and n CHESS-STEAM images from one slice, (b) Multi-slice double CHESS-STEAM method for simultaneous acquisition of two CHESS images at n slices.

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