Yang Xia - Essential Concepts in MRI

This book is written ultimately for those who are interested in MRI. It contains the four essential components of MRI (Figure 1.6): the theory of physics that is the foundation of this fascinating phenomenon (Part I) [1, 2, 18, 19], the fundamental instrumentation and experimental techniques that facilitate the execution of this phenomenon (Part II) [1, 20], and two main applications – NMR spectroscopy (Part III) [21, 22, 23] and MRI (Part IV and Part V) [2, 4, 24]. Although each of these components could be taught in great detail in one or two semesters, the goal of this book is to cover the essential concepts in all four components in a typical one-semester course, hence the title of the book begins with Essential . I trust that you would be well prepared when you need to explore any topic deeper. In addition to the numerous equations, there are about 190 figures in the book that provide the graphical descriptions for the concepts.

Figure 1.6 Major conceptual components of NMR and MRI.

For the theory, I first give you the classical description of NMR, since it is easy to understand and visualize and provides a very useful first approximation. (If your goal is to do MRI on water-rich samples, the classical description is mostly sufficient.) I’ll then describe NMR in a compact (i.e., abbreviated) quantum mechanical form, so that you will be at least familiar with the basic approach and terminology of the mathematical treatment.

Sandwiched between the fundamental theory and practical applications are the NMR instrumentation and experimental techniques (Part II), which facilitate the execution of this phenomenon. For these techniques, I discuss the basic unit of the NMR system. (The additional hardware in MRI is discussed in Chapter 13.) These get-your-hands-dirty discussions on hardware and experimental techniques will let you see behind the equations and behind the black box, to understand how the experiments are carried out and what are the practical issues in spectroscopy and imaging. Although the hardware knowledge will be described in terms of NMR and MRI, it should be useful in other modern technologies involving electronics, computer applications, signal acquisition, and imaging.

The description of NMR spectroscopy aims to supply you with basic knowledge of the topic, which is more than what you can find from any of the MRI books. I truly believe that for any MRI researchers and technical personnel, the knowledge of NMR spectroscopy is critically important. The last two parts (IV and V) cover modern practice in MRI, with an emphasis on quantitative imaging, which is at the center of modern MRI research and diagnostics.

This book can be adapted for a one-semester course in several different formats. For the students who major in science (physics, chemistry, material science, engineering), a course should include all four components of MRI (theory, instrumentation and experiment, spectroscopy, imaging). For this format, one can teach either at the undergraduate senior level or the graduate level. If the students are mainly interested in imaging, a course can be tailored toward MRI, with just a brief introduction to NMR spectroscopy. One can teach this version of the course to students in medical school. Appendix 4has several sample syllabi for teaching.

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The states of atomic nuclei are quantum mechanical by nature. This means that the properties that we observe for a single nucleus belong to one in a discrete set of possibilities (i.e., quantum states). A deep understanding of NMR phenomenon therefore requires the assistance of quantum mechanics. In practical NMR and MRI experiments, however, we deal with an extremely large number of nuclei in any specimen (either a human or a tissue block or a drop of liquid). For example, if I give you a glass container that has 18.015 grams of liquid water, do you know how many water molecules are in the container? Well, we know precisely how many are in it: 6.022 × 10 23water molecules (Avogadro’s number, since one mole of water is 18.015 grams)! Consider taking just one gram of water from the container, which has a volume of one milliliter . If this one milliliter of water is further divided into 100 droplets, each tiny droplet still contains about 3.343 × 10 20water molecules, or about 6.686 × 10 20protons (i.e., hydrogen atoms ) since each water molecule has two hydrogen atoms. It is an enormous number.

It is fortunate that these protons in a small water droplet act largely independently, so that at the macroscopic level, the collection of these protons appears continuous. (If these protons were to act completely independently, NMR would not have much practical use at all. If, on the other hand, these protons were to couple or interact tightly with each other, NMR also would not have much practical usage since we simply do not know how to solve the complex interactions among the enormous number of particles in any practical system.)