Spin tagging

One possibility to visualize fluid transport directly in the fixed bed by means of a series of time-encoded displacement images is given by the spin tagging technique... 

Fig. 4.4.2 The discrete data points represent Taylor-Couette-Poiseuille flow regimes observed with MRI for r = 0.5 [41]. The curved boundaries were obtained for r = 0.77 with optical techniques [38]. The two inserts show MRI spin-tagging FLASH images of the SHV and PTV hydrodynamic modes.

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.

Fig. 4.4.7 (a) Reconstruction of the Stationary Helical Vortex (SHV) mode from MRI data acquired with the spin-tagging spin-echo sequence [27], The axial flow is upwards and the inner cylinder is rotating clockwise. The two helices represent the counter-rotating vortex streamtubes. (b) Construction of Poincare map for SHV [41]. The orbit of a typical particle is... 

Quantitation of the CBF can be an important factor in studies of therapeutic intervention. The method of arterial spin-labeled (ASF) perfusion MRI allows quantitative determination of the perfusion rate in units of ml/100 g tissue/min. However, the technique requires exceptional instrumental stability, as the result depends on accurate determination of the difference between tw o images on the order of a few percent. With the appropriate acquisition parameters to achieve sufficient signal to noise ratio (>100) and current instrument stability, this technique is feasible, especially in high field clinical systems. Currently, arterial spin-tagged perfusion measures are not widely used in clinical settings. 

Arterial spin-tagged perfusion MRI is performed by inverting the v ater protons in the blood of the arteries supplying blood to the tissue of interest. Typically, in brain, the carotid arteries are used as the supply. The original description of the method (Detre et al., 1992 Williams et al., 1992) uses a continuous narrow band RF inversion of the protons in the arterial blood, outside of the imaged slice, before and after excitation to cause a steady-state loss of signal in cerebral tissue. The signal loss is related to tissue perfusion (Detre et al., 1992) by ... 

MRI is also sensitive to motion and can be used to measure velocity and deformations. Three main techniques are used (1) spin tagging, (2) phase contrast, and (3) Fourier velocity encoding. 

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