Page 248 - Biomedical Engineering and Design Handbook Volume 2, Applications

CHAPTER 8

DESIGN OF MAGNETIC

RESONANCE SYSTEMS

Daniel J. Schaefer
MR Systems Engineering, Milwaukee, Wisconsin

8.1 INTRODUCTION 227 8.5 OTHER MR SYSTEMS 240
8.2 MR MAGNET CHARACTERISTICS 229 8.6 SAFETY STANDARDS 241
8.3 GRADIENT CHARACTERISTICS 231 8.7 NEMA MR MEASUREMENT
8.4 RADIO-FREQUENCY MAGNETIC FIELD STANDARDS 241
AND COILS 235 REFERENCES 243

8.1 INTRODUCTION

Atomic nuclei containing odd numbers of nucleons (i.e., protons and neutrons) have magnetic
1
moments. Hydrogen ( H) nuclei (protons) have the highest magnetic moment of any nuclei and
are the most abundant nuclei in biological materials. To obtain high signal-to-noise ratios,
hydrogen nuclei are typically used in magnetic resonance imaging and spectroscopy. Note that
31
23
19
2
39
13
many other nuclei (e.g., H, C, F, Na, P, and K) may also be studied using magnetic
resonance.
In the absence of an external static magnetic field, magnetic moments of the various nuclei point
in random directions. So, without a static magnetic field, there is no net magnetization vector from
the ensemble of all the nuclei. However, in the presence of a static magnetic field, the magnetic
1
moments tend to align. For H nuclei, some nuclei align parallel with the static magnetic field, which
1
is the lowest energy state (and so the most populated state). Other H nuclei align antiparallel with

the static magnetic field. The energy of nuclei with a magnetic moment m in a static magnetic field

B 0 may be expressed as 1

•
W = m B 0 (8.1)
m
The difference in energy between protons aligned with the static magnetic field and those aligned
antiparallel is the energy available in magnetic resonance (MR) experiments. This energy is twice
that given in Eq. (8.1). Recall that the kinetic energy of the same nuclei at temperature T may be
expressed as 2
W = K T (8.2)
T
227