Self-Assembly of Synthetic and Biological Polymeric Systems of ...

are aligned by means of an external magnetic bias field (commonly in the range of 0.2-3 T) and
the precession of the spins is excited by transverse RF pulses at a proton resonance frequency
of about 42.58 MHz T -1 . After applying the pulse sequence, the induced magnetization decays
and the longitudinal (T1) and transverse (T2) relaxation times of the precessing nuclear
magnetic moments show tissue-specific differences that are used to generate the required
image contrast. Imaging is performed by controlling external field gradients so that the
resonance condition is fulfilled only in a restricted local region and, then, by scanning the
resonant volume to be imaged. Magnetic response signals are detected by pick-up coils. In this
way, the tissue-specific differences of the relaxation time T1 and/or T2 may be used for
construction of the T1 and T2 weighted images showing optimal contrast of special tissue
features. In practice, for optimization of tissue contrast a variety of different pulse sequences
(e.g., the widely applied spin-echo methods) may be used (60)(61).
Generally, a magnetic resonance scanner is essentially defined by three hardware groups and
their parameters: a) the main magnet with its homogeneity over imaging volume; b) the
magnetic field gradient system with its linearity over the imaging volume; and c) the
radiofrequency (RF) system with its RF signal homogeneity and signal sensitivity over the
imaging volume. Whole-body MRI imposes very special demands on these system components
(62)(63):
a) The main magnet. Magnetic resonance imaging requires a very strong magnetic field
that has precisely the same magnitude and direction everywhere in the region we want to
image. One of the key properties used to describe the quality of a MRI system is the
uniformity, or homogeneity, of the applied magnetic field. For example, high-quality MRI
systems made for clinical use in hospitals will have magnetic fields that vary less than 5 parts
per million (ppm) over a 40 cm diameter spherical volume in the region desired for imaging.
b) The magnetic field gradient. As mentioned earlier, a key property of the static
magnetic field of a MRI system is its homogeneity, but anything we place inside the magnetic
field tends to change the magnetic field slightly. To make the magnetic field as uniform as
possible and to compensate for changes caused by different objects in the field, we shim the
field. Shimming is typically handled by placing small amounts of iron at specific locations within
long trays that line the cylindrical magnetic field coil; or by several set of wire coils. Once the
shimming is complete, the magnetic field is highly uniform over a central region where the
imaging takes place.
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