Читать книгу Magnetic Resonance Microscopy - Группа авторов - Страница 18

In Equation 1.2, d represents the sensitivity of the NMR detector, which has been proven to increase with decreasing d [18]. Numerous reports have been published on how to improve the NMR detection sensitivity by miniaturizing the detection coil [19–22]. The vast majority of those papers targeted NMR spectroscopy applications, and far fewer talked about microscopy [23–25]. One of the papers that targeted both applications though is the work reported by [26]. In this paper a novel high-resolution NMR/MRI Helmholtz microcoil was introduced (Figure 1.1). The coil that was manufactured using a combination of standard and home-developed micro fabrication technologies featured an extremely user-friendly sample-handling approach that allows easy loading/unloading of the sample.

Figure 1.1 A micro Helmholtz coil manufactured using a mixture of standard and home-developed micro-manufacturing technologies. [26] N. Spengler et al. (2014), figure 04 [p.05]/with permission from IOP Publishing Ltd.

The coil was manufactured by stacking three layers, the top and the bottom of which are made of glass, each featuring a wire-bonded 1.5-winding coil using a 25-µm diameter copper wire. The coils were encapsulated in SU-8 epoxy-based photoresist. The glass layers also contained copper traces for the feed and return paths of the current. These layers were spaced by a poly(methyl methacrylate) (PMMA) layer, which was U-shaped to allow the sample-handling microfluidic chip to slide in the sensor. The Helmholtz microcoil was designed and optimized to achieve an extremely uniform B₁ field (92% ratio of signal intensity at flip angles of 810/90) while maintaining the high B₀ homogeneity (1.79 Hz achieved linewidth of a water sample).

The exceptional performance of the microcoil in terms of B₁ uniformity and local field homogeneity allowed high-resolution microimaging. Figure 1.2 shows the optical (left) and MR (right) microimages of a deionized water sample of 154 nl volume. The sample contains 50-µm diameter polymer beads to show some contrast. The MR image that is a sum of 80 acquisition was recorded over a total scan time of 11 h, 22 min, and 40 s. With a matrix size of 256 × 256 and covering a 2.5 × 2.5 mm field of view (FoV), the MR image exhibited an in-plane resolution of 10 × 10 µm for a slice with a 100-µm thickness.

Figure 1.2 MR microimaging of a 154-nl deionized water sample with 50-µm diameter polymer beads. (left) Optical micrograph. (right) MR image. [26] N. Spengler et al. (2014), figure 10 [p.07]/with permission from IOP Publishing Ltd.