Fundamentals - Preventive Biomechanics - page 23

field represents a spatial volume element. Due to variation of the magnetic field in,
for instance, the y-direction, phase- incoherent precession of the protons' magnetic
moments is achieved, and each row and column of the matrix thus exhibits a
precessional motion with distinct phase-difference (phase-gradient field). After
switching off the phase-gradient field, the magnetic dipole moments in one slice
return to rotation with the same frequency. The phase-difference, however, is still
preserved. Ensuing variation of the magnetic field in the x-direction clearly links
the frequency with a spatial x-position. (Each row and column of the slice matrix
experiences the same magnetic field strength. Thus, these magnetic moments
exhibit the same resonance frequency). Phase difference and frequency difference
(together with slice position) allow clear spatial assignment of each volume ele-
ment, since each net magnetization vector belonging to a particular slice matrix
field is characterized by a unique phase angle and precessional frequency. The
resulting matrices thus contain specific frequency and phase-shift information,
which can be used to generate two-dimensional images. This reflects the spatial
distribution of excitable nuclei, such as the hydrogen nucleus.
The simplest pulse sequence thus contains a 90-pulse (for rotation into the
transverse plane), a slice-selection gradient pulse (z-position), a phase-encoding
gradient pulse (y-position), a frequency-encoding gradient pulse (x-position) and
the actual MR-signal induced in the coil's elements.
Image generation is accomplished using the frequency (and phase) content from
the net magnetization vector of the induced MR-signals. The Fourier transfor-
mation method is used to decompose the MR-signal (content) containing a fre-
quency composition into discrete frequency (and phase) information, to finally
spatially assign them to single slice matrix fields. The resulting MR-images are
generally shown in a grey-scale format where brighter regions indicate a higher
density of excitable elementary particles.
3.1.6 MRI-Assembly
A basic overview of the main parts of the MRI device is provided, with the
different coils and their functions being presented. Some information has previ-
ously been provided but redundancy is maintained for better understanding.
In Fig. 3.3 the coil assembly is schematically depicted. Most efficiently, a
superconductive magnet is used to generate the static magnetic field B 0
in the z-direction. This magnet type is composed of superconductive wires
(Niobium-Titan alloy) enclosed by liquid helium, which serves as a cooling liquid
at a temperature of about T = 4K.
The gradient coils generate (in addition to the homogenous static field B 0 ) the
magnetic field variations in the x-, y- and z-directions required for creating a
unique magnetic field (unique resonance frequency) at each spatial position,
allowing for frequency and phase encoding. The rf-coils generate the radio-
frequency field B 1 perpendicular to the static magnetic field B 0 to rotate the