BioPHYSICAL chemistry

CHAPTER 16 MAGNETIC RESONANCE 361

magnetic field strength, as the tube on the right-hand side has a much
stronger magnetic field than the one on the left. The resonant condition
for each protein is proportional to the strength of the magnetic field B
(eqn 16.5). Due to the different strengths of the magnetic field, the
resonant frequencies are now different, and two peaks are measured in
the spectrum.

Eqn 16.5:

In general, if a large container of water is placed in a homogeneous
field only a single peak is measured. When the same container is placed
in a magnetic field that increases linearly with distance from one pole
face, then the spectrum effectively maps out an image of the water.
The container can be considered to consist of a series of planes, each at
a certain distance from the left pole face. As the magnetic field increases
from left to right, each plane will resonate at an increased frequency
that is proportional to the distance from the left pole face. The number
of protons in each plane will determine the amplitude of the signal, so
the resulting spectrum will provide a count of the number of protons at
every distance and the overall profile will match the image of the water
in the container.
In an MRI experiment, the strength of the magnetic field is not fixed.
Rather, the strength of the field increases steadily from one pole face to
the other. As a result the protons will resonate at different frequencies
depending upon their distance from the poles with an intensity pro-
portional to the number of protons. This produces an image that can be
made three-dimensional by rotation of the magnetic field by about the
same amount as the structure being imaged (for example, your head) as
shown in Figure 16.16. MRI is of great medical use as it is not invasive;
that is, the pictures can be obtained without the introduction of any drugs
or probes into the body.
In conventional MRI, protons from different tissues can be distinguished.
These images can be sharpened by use of contrast agents. These agents
sharpen the contrast between the protons of interest and those in their
proximity by changing the spin relaxation time of protons in the dif-
ferent tissues. For example, contrast agents can be injected through the
bloodstream and tissues to improve the contrast of the circulation system.
Work is underway to improve the quality of the contrast between dif-
ferent tissues. For example, the contrast agent can be made inactive by
attaching certain sugar groups to the agent until they are cleaved by specific
enzymes, allowing monitoring of gene delivery and expression.

Emh v

B

mIL LI=− ν =

γ

π

or

2