Ex situ velocity imaging
Pulsed-Field-Gradient (PFG) NMR has proved to be a powerful tool to characterize molecular motion in a non-invasive fashion [1]. It has been widely used in biology, medicine, and material science offering for example, the possibility to measure vascular flow in steams and petioles, as well as vascular blood flow at various stages in the cardiac cycle. Also numerous applications in porous media as natural sandstone have made possible the elucidation of models that describe the transport of fluids within the porous structure. A large family of PFG sequences have been developed for different applications, but they have been always implemented and tested in the homogeneous field of conventional magnets. These magnets offer limited volume for housing the flow setup restricting the application field of this technique. During the last years increasing interest has been observed in developing open sensors for ex situ experiments [2]. The extreme versatility of this type of devices gives to NMR access to a large number of applications hindered to the conventional closed magnet geometries (Fig. 1).
In this work the so-called 13-interval PFG stimulated spin-echo (SSE) sequence proposed by Cotts [3] was implement on a single-sided sensor to encode displacement along the pulsed gradient direction. A proper phase cycling is proposed to remove the distortions introduced by the off resonance excitation, and it is shown that the sequence effectively reduces the signal attenuation due to diffusion under the strong static gradient of the unilateral magnet. To improve the signal-to-noise ratio a novel multi-echo acquisition scheme was implemented (Fig. 2). Adding the generated echo train a reduction in the experimental time up to two orders of magnitude was achieved. The experiments were performed on a single-sided sensor equipped with an optimized U-shaped magnet that provides a uniform gradient along the depth direction optimum for slice selection, and with a suitable gradient coil system suitable to apply pulsed gradients along the two lateral dimensions.
To show how the velocity distribution in a selected slice is encoded by this method, the velocity inside a circular pipe was measured. The method was combined with the depth selection obtained by retuning the probe frequency to provide velocity profiles spatially resolved along the depth direction. Figure 1 shows a circular pipe 30 cm long and 3 mm in diameter positioned with its center at about 8 mm from the surface of the sensor. The velocity distribution inside each plane goes from zero to a vmax that depends in a quadratic way, being maximum in the center of the pipe. Figure 3 shows the velocity distribution spatially resolved along the depth as well as the addition of all of them, which approach the theoretical constant velocity probability characteristic of a cylindrical pipe.
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References
- P. T. Callaghan, Principles of nuclear magnetic resoance microscopy, Clarendon Press 1991.
- E. O. Stejkal, and J. E. Tanner, J. Chem. Phys. 42, 288 (1965).
- R.M. Cotts, M.J.R. Hoch, T. Sun, J. T. Markert, J. Magn. Reson., 83, 252 (1989).
- F.Casanova, J. Perlo, and B. Blümich, Velocity distributions remotely measured with a single-sided NMR sensor, J. Magn. Reson. 171, 124-130 (2004).
- J. Perlo, F. Casanova, and B. Blümich, Velocity imaging by ex situ NMR, J. Magn. Reson (2005) 173 (2005) 254–258.
Figure 1: Drawing of the single-sided sensor scanning the velocity inside a circular .
Figure 2: 13-interval PFG-STE sequence combined with a multi-echo sequence applied to sample the complete echo decay train.
Figure 3: a) Velocity distribution of a circular pipe spatially resolved along the depth direction by selecting different slices across the tube. The velocity distribution inside each selected slice was obtained in a experimental time of about 3 minutes. The velocity resolution of these experiments is 1.5 mm/s. b) Integrated distribution of a circular pipe obtained by integrating the differnt slices.
