• Chapter 6 • Contents
  Introduction Localization of Spins with Field Gradients Excitation of Selected Spins  Spin Echo  Gradient Echo Spatial Encoding  Frequency Encoding  Phase Encoding Two-Dimensional Imaging  Slice Selection  Slice Definition Multiple Slices The Complete Imaging Experiment  Frequency Encoding  2D Fourier Transform Partial Fourier Imaging  Three-Dimensional Fourier Imaging Parallel Imaging

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Chapter Six
Creation of MR Images

06-01 Introduction

n the preceding chapters we have discussed the magnetic resonance phe­no­me­non as such, the relaxation times, and the ap­pli­ca­tion of mag­ne­tic re­­so­­nan­­ce to che­mi­cal ana­ly­sis. However, the most important me­di­cal ap­­pli­­ca­­tion of mag­ne­tic re­so­nance is imaging (Figure 06-01).

Figure 06-01:
Peter A. Rinck and Robert N. Muller checking a Polaroid print of the first ECG-gated three-dimensional picture of the heart (in 1982). But where do such pictures come from? How are they made?

The manner in which the spatial information is obtained in magnetic re­so­nan­ce imaging is referred to as the reconstruction technique.

Images can be pro­du­ced point-by-point, line-by-line, in slices, in slices calculated from a whole volume, or as a complete 3D depiction (Figure 06-02).

Figure 06-02:
Excitation: point, line, slice, and entire volume.

Nearly all MR imaging techniques currently in use are either planar (slice) or volume techniques. In the former case, the MR experiment is restricted to a slice through the sample and is often referred to as a two-dimensional (2D) ex­pe­ri­ment since only two spatial dimensions have to be encoded. Volume techniques spatially encode the whole volume; therefore, they are referred to as three-dimensional (3D) techniques.

The formation of an image involves the following procedures:

localization of the spins of interest;
excitation of selected spins;
spatial encoding of their signal; and
signal detection and reconstruction.