單邊可攜帶式核磁共振裝置的製作

Abstract

單邊核磁共振是近年興起的一種技術，其原理是利用永久磁鐵產生的靜磁場和射頻線圈產生的射頻磁場在單邊核磁共振裝置外面的某區域內互相垂直而產生一個收訊敏感區域，透過射頻線圈的射頻激發和信號接收，置於此敏感區域內的物體的核磁共振信號即可以被裝置偵測到。在這種架構中，由於待測物不需要被磁鐵包圍，所以單邊核磁共振裝置可以被用來測量任何體積大小的待測物的核磁共振信號，達到完全非破壞性檢測的目的。近來許多學者提出各種單邊核磁共振裝置的設計，然而大多數裝置的體積或重量仍稍大，降低了裝置的可攜帶性，同時大多數裝置並無報告可以於單次射頻激發後即取得核磁共振信號，顯示出大多數裝置的信雜比的不足。 這份研究提出了兩種新的單邊可攜帶式核磁共振裝置的設計，一種設計是使用 Halbach 磁鐵，另一種設計使用了單一根的橫向充磁的柱狀磁鐵，作成的裝置可以用單手操作，重量小於2公斤，而且製作成本低廉(少於1.5萬元台幣)，待測物放在裝置頂面之後就可以進行測量，是屬於開放空間式的架構。作成的裝置可以在單次射頻激發後取得橡皮擦的核磁共振信號，顯示出裝置具有適當的信雜比。裝置可以用來測量物體的遲豫參數，以及利用裝置表面中心上方大於10T/m的靜磁場梯度做厚度數百μm 的薄樣品的一維成像，使用單根柱狀磁鐵設計的裝置還可以測量液體的擴散係數。裝置的應用有評估木板受熱損害以後氫原子的活動性，觀測塑鋼補土的硬化過程、觀測餅乾的吸水過程、測量奶粉的脂肪含量等等。我們也用數值軟體模擬尋找出一些裝置可能的改進，包括沿磁鐵磁化方向切分磁鐵和加上微調磁鐵等，可以調整裝置上方靜磁場的分布，進而產生更好的一維成像空間解析度或較長的偵測距離。

與先前被發表的大部份單邊核磁共振裝置相比，這份研究提出的兩種裝置的體積及重量較小而且能提供較高的信雜比，因此提出的裝置具有較佳的可攜帶性並且能減少執行實驗所需的時間。幾種應用試驗也展現了提出的裝置可以透過測量聚合物或食品樣品的遲豫參數或回訊訊號強度來鑑別樣品的狀態，因此提出的裝置具有進行聚合物或食品等材料的品質管理的潛力。研究最後所提出的裝置改進方式則可以被期望提高裝置在非破壞性檢測的功能。

Single-sided NMR is technique which emerges recently. Its principle is to use permanent magnets to generate a static magnetic field B0 and a radiofrequency (RF) coil to generate an RF magnetic field B1; B0 and B1 fields are made perpendicular to each other in some space outside a single-sided NMR apparatus, where a sensitive volume of the apparatus is generated. The NMR signal of an object situated in the sensitive volume of the apparatus can be acquired by the RF excitation and signal detection of the RF coil. In this architecture, the experimental object does not need to be encompassed by the magnet. Consequently, the single-sided NMR apparatus can be utilized to measure the NMR signal of an experimental object with arbitrary dimensions, attaining a totally non-destructive inspection. Many researchers proposed various designs of the single-sided NMR apparatus recently; however, most reported apparatuses have relatively large sizes and heavy weights, decreasing the portability of the apparatus. Meanwhile, the ability to collect an NMR signal in one shot is not reported for most apparatuses, which reflects the insufficient signal-to-noise ratio of the most apparatuses. This research presents two new designs of the single-sided mobile NMR apparatus. One design employs a Halbach magnet and the other design uses a bar magnet with transverse magnetization. Each constructed apparatus can be held in one’s palm, weighs less than 2 KG and the production cost is low (less than 15000 NTD). An experimental object can be measured after it is placed on the apparatus top; consequently the apparatus has an open structure. Each apparatus can measure an NMR signal of a pencil eraser block in one shot and demonstrates the adequate SNR of the apparatus. Each apparatus can measure the NMR relaxation parameters of an experimental object, as well as perform one-dimensional imaging of a thin sample with a thickness of about several hundreds of microns by utilizing the static magnetic field gradient of > 10 T/m directly above the apparatus top center. The single-sided mobile NMR apparatus with the bar-magnet design also can measure the self-diffusion coefficients of liquids. Applications of the apparatuses include evaluating the proton mobility of a wood plate after undergoing thermal damage, monitoring the hardening process of epoxy putties, monitoring the moisture absorption of a cracker and measuring the fat content of milk powders. We also employ numerical software simulations to find several possible modifications of the proposed apparatus, including halving the magnet along its magnetized direction and adding shim magnets. These approaches can adjust the static magnetic field profiles above the apparatus to generate better spatial resolution for one-dimensional imaging or a longer detection distance compared with the unmodified apparatus. Compared with most previously published single-sided NMR apparatuses, the proposed apparatuses in this research have relatively small dimensions and light weights and provide a higher SNR; consequently the proposed apparatuses have better portability and can reduce the experimental time. Several application experiments demonstrate that the proposed apparatuses can inspect the status of the polymer or food samples by accessing their relaxation parameters or echo signal amplitudes; therefore, the proposed apparatuses have the potential to conduct quality control of polymers and foods. We expect that the modification approaches presented in the final chapter can enhance the performance of the apparatus for non-destructive inspection.