Spin-echo imaging

Figure Bl.14.1. Spin warp spin-echo imaging pulse sequence. A spin echo is refocused by a non-selective 180° pulse. A slice is selected perpendicular to the z-direction. To frequency-encode the v-coordinate the echo SE is acquired in the presence of the readout gradient. Phase-encoding of the > -dimension is achieved by incrementmg the gradient pulse G...

Figure Bl.14.9. Imaging pulse sequence including flow and/or diflfiision encoding. Gradient pulses before and after the inversion pulse are supplemented in any of the spatial dimensions of the standard spin-echo imaging sequence. Motion weighting is achieved by switching a strong gradient pulse pair G, (see solid black line). The steady-state distribution of flow (coherent motion) as well as diffusion (spatially...

Fig. 3.4.12 Three-dimensional rendered spin-echo image of water filled cracks in a cement paste specimen [13]. Three cracks are visible in the image a large triangular crack in the forefront, a smaller crack in the bottom left corner and a sheet-like structure at the top of the image. Water droplets can also be observed condensing on the cement paste surfaces. The measurement parameters were FOV 20 x 20 x 20 mm, acquisition points 128 x 128 x 64, nominal resolution 156 x 156 x 312 pm, echo time 2.7 ms, repetition time 500 ms and acquisition time 270 min.

Fig. 4.4.5 Gradual blurring (staring on locations marked by arrow) of MRI spin-tagging spin-echo images of Taylor—Couette—Poiseuille flow as the axial flow is increased (from left to right). The images correspond to longitudinal sections of the flow and the axial flow is upwards. The dashed line marks the location of one of the stationary helical vortices which characterize the SHV mode. This flow regime corresponds to the transition from the SHV (steady) to partial PTV (unsteady) regimes as Re increases, as shown in Figure 4.4.2.

This section introduces the reader to the basic principles of MRI and the concept of the k-space raster. The basic MRI pulse sequence, the spin-echo imaging sequence, is described at this point. For more detailed discussion of the background theory of MRI the interested reader should refer to texts by Callaghan5 and Kimmich.6... 

II.2 The simplest MRI pulse sequence spin-echo imaging... 

Fig. 5. Standard fast spin-echo imaging of the pelvis and the lower leg. Typical contrasts between musculature and other tissues are demonstrated. Bl = bladder, Fe = femur. Gluteus = gluteus muscle. Original recording parameters matrix 192 x 256, slice thickness 6 mm, a-c field of view (fov) = 380 mm, d-f fov = 180 mm. (a) and (d) Proton density weighting TR = 5000 ms, TE = 12 ms. (b) and (e) Ti-weighting TR = 500 ms, TE = 12 ms. (c) and (f) 7 2-weighting TR = 5000 ms, TE = 100 ms.

Fig. 7. Double spin-echo imaging sequence for chemical shift selective imaging. The sequence applies spectral-spatial excitation pulses as presented in Fig. 6. Refocusing is performed with standard slice selective 180° pulses. For the examples in Fig. 8 echo times were chosen to TEi = 20 ms and TE2 = 60 ms.

Fig. 32. The 30-year-old male patient with acquired generalized lipodystrophy (AGE) shows lacking lipids in the subcutaneous layer and in the musculature in a Ti-weighted spin-echo image (a) and in a fat selective image (b). Only marrow fat in the tibia and fibula of the patient seems to be not affected by the disease. Comparison between a spectrum from a insulin resistant subject without AGE (c) and the spectrum from the AGE patient (d) reveals that the AGE patient has nearly lacking EMCE and markedly reduced IMCE content.

Durian is one of the most commercially important fresh fruits in S.E. Asia, yet sorting immature from mature fruit by external measures is very difficult, so it is a prime candidate for non-invasive methods. Yantarasri and co-workers have sought correlations between soluble solids and sensory estimates of maturity with X-ray CT and NIRS measurements with moderate success but more recently they observed that MRI spin-echo image contrast at 0.5 T varied with the degree of maturity. Unfortunately no attempt was made to quantify relaxation time changes or separate oil-water peaks. However it was suggested that the contrast differences indicated signihcantly lower oil content in unripe durian compared to the ripe and overripe fruit. 

To facilitate international export, mangos are usually disinfested from insect larvae by either chemical or heat treatment. Unfortunately, heat treatments severe enough to kill the larvae can also damage the skin and pulp (mesocarp). Pulp symptoms include impaired starch degradation and development of internal cavities, which are manifest in MRI. Spin-echo image contrast showed the initiation of heat injury around vascular traces in the mesocarp possibly because they form a network for rapid heat transfer and/or retain heat... 

The spin-echo images showed no obvious differences between ripe and overripe samples, but the gradient-echo images of the over-ripe melons showed decreased intensity. It was surmised that the increased air spaces in the overripe tissue intensified the internal magnetic susceptibility gradients, whose dephasing effects can be refocused by a spin-echo but not by a gradient-echo. 

Fig. 6. Spin-echo Image of diseased potato showing hollow heart (dark area) surrounded by brown tissue (bright area). Printed with permission, from Ref. 92.

As we have seen, conventional spin-echo imaging (Section II.A.3) typically takes the order of a few minutes. As shown in Fig. 6, an independent r.f. excitation is required for acquisition of each row of k-space data. Hence sampling of the complete raster is limited by the repetition/recycle time of the pulse sequence used, which in turn is governed by the inherent relaxation time(s) of the system. In... 

In spin echo imaging, the signal strength of an imaging element is given by... 

FIGURE 4.22 Spin echo images of four raw potato varieties, scale bar 1 cm. (From Thybo et al., 2003.)... 

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