Читать книгу Essential Concepts in MRI - Yang Xia - Страница 12

The description of NMR and MRI theory would become easier if we first briefly overview what is involved in an NMR experiment. In general, an NMR or MRI experiment consists of three sequential “stages”: preparation, excitation, and detection. In the first stage, a sample is placed in an externally applied magnetic field B₀, which allows the nuclear ensemble in the sample (e.g., water molecules in humans or animals or plants or test tubes) to reach the thermal equilibrium state. This preparation stage results in a net macroscopic magnetization in the sample. In the second stage, a perturbation is applied to the sample in order to force the net magnetization away from the thermal equilibrium into a non-equilibrium state. Finally, the response of the net magnetization to this perturbation is recorded via the detector, where the recording is termed as the NMR or MRI signal. Final post-acquisition signal processing generates an NMR spectrum or an MRI image. These three sequential stages in an NMR or MRI experiment are controlled by a list of individual commands, and each occurs at a different time. This list of commands is called a pulse sequence. Chapter 5, Chapter 6, and Chapter 13 will discuss the details of these instrumentational and experimental aspects.

A convention in NMR and MRI is that the externally applied magnetic field that is used to establish the net magnetization is always named as the B₀ field, which is a vector field and has a direction always along the z axis (Figure 1.2), that is, B₀ = B₀k, where k in this expression is the usual unit vector along the z direction in a 3-dimensional (3D) Cartesian coordinate system. The direction of this z axis in Cartesian coordinates, however, can be either in the vertical direction (for vertical-bore superconducting magnets, which are common in research labs, or “open” MRI scanners, which reduce claustrophobia for some patients) or in the horizontal direction (for the electromagnets in research labs, the “vertical donut” magnet MRI, or the horizontal-bore superconducting magnets in common clinical MRI scanners).

Figure 1.2 The B0 direction in NMR and MRI. (a) Vertical-bore superconducting magnet, which is common for NMR spectrometers in science and industry laboratories. (b) “Horizontal double-donut” magnet for “open” MRI. (c) Electromagnet or magnet in “vertical double-donut” MRI. (d) Horizontal-bore superconducting magnet, which is common for whole-body imagers for humans or animals.

In addition, this book adapts the convention that the clockwise rotation is positive when one looks into the arrowhead of any axis, shown in Figure 1.3. Among the NMR and MRI literature, this convention for rotation is not consistently adapted (i.e., some authors use the counterclockwise rotation as the positive rotation). This inconsistency can lead to either a + or – sign in some equations that describe the motion of the macroscopic magnetization. The notation used in this book is consistent with many books; for example, those by Fukushima and Roeder [1], Callaghan [2], Canet [3], and Haacke et al. [4]. We will comment on this issue at several places in Chapter 2.

Figure 1.3 The positive directions of rotations in a 3D Cartesian coordinate system, (a) when one looks into the +z axis, and (b) when one looks into the +x axis.