Small nuclear magnetic resonance (NMR) systems rely heavily on the uniformity of their magnetic fields to produce accurate and high-quality images. This article provides an overview of how small NMR devices achieve magnetic field homogeneity, focusing on shimming techniques used in these systems.
Magnetic field uniformity refers to the consistency of the magnetic field within a defined volume. In simpler terms, it measures whether the number of magnetic field lines passing through a given area is the same across that space. In MRI, this uniformity is typically measured in parts per million (ppm) relative to the main magnetic field strength. For example, a 1.0 T MRI system has a ppm unit that translates to about 42 Hz for hydrogen protons. The smaller the deviation, the more uniform the field.
The quality of the magnetic field directly affects the T2* relaxation time of tissues. A less uniform field results in a shorter T2*, leading to faster signal decay and a shorter FID (Free Induction Decay) tail. Conversely, a more uniform field leads to a longer T2*, slower relaxation, and a longer FID tail. Ideally, if the magnetic field were perfectly uniform, T2* would equal T2, and the FID signal would decay based solely on the tissue’s natural T2 relaxation.
To optimize the magnetic field, small NMR devices monitor the FID signal's decay on a display. By adjusting the parallelism between the two magnetic poles, they can fine-tune the field uniformity. The longer the FID tail, the better the magnetic field uniformity.
For permanent magnet-based NMR systems, the field uniformity depends largely on the alignment of the magnetic poles. Adjusting the parallelism between them helps achieve basic shimming. However, this alone may not be sufficient for advanced MRI applications, so additional shimming methods are often necessary.
Passive shimming involves placing small magnets or magnetic pieces on the inner or outer surfaces of the magnetic poles to correct local field distortions. While effective, this method is rarely used in experimental setups due to the already good uniformity achieved through mechanical adjustments. Instead, passive shimming is often used as a supplementary technique.
Active shimming, on the other hand, uses coils to generate controlled magnetic fields that counteract inhomogeneities. By applying precise currents to different coils, the system can dynamically adjust the magnetic field to improve its uniformity.
Understanding and achieving magnetic field homogeneity is essential for producing clear and reliable MRI images. Whether through mechanical adjustments, passive shimming, or active shimming, each method plays a critical role in optimizing the performance of small NMR systems.
Reference: "Experimental Course of Magnetic Resonance Imaging Technology" (Wang Hongzhi, Zhang Xuelong, Wu Jie)
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