MR-visible protons can
be thought of as belonging to two pools: freely mobile protons
(as in water), and motion-restricted protons (as in protons
bound on proteins, or membranes). The motion-restricted
protons will have a much shorter T2 than the freely mobile
protons (as discussed in Section 1.3.2). This means that the
"spectral width", or range of resonant frequencies for the
motion-restricted protons is larger than for the mobile
protons as shown in Figure 1.14. The spectral width is defined
by a reciprocal relationship with the T2 relaxation rate: If
T2 is long, then the spectral width is small, and if T2 is
short, the spectral width is larger. Under normal conditions,
magnetization is continually transferred between the two pools
both via purely magnetic interactions, and by actual chemical
exchange of protons. This property can be exploited to provide
a different type of contrast in an image that reflects the
rate of exchange between the free and bound proton pools in
each tissue voxel.
|
|
Figure 1.14
Simplified diagram representing
the free and bound protons in tissue. Both resonance
lines are centered at wo, but due to their shorter T2,
the bound protons have a much larger spectral width. It
is therefore possible to selectively excite and saturate
the bound protons using an off-resonance RF
pulse. |
An MTC "preparation"
can be added to the front end of a regular pulse sequence to
increase the amount of MTC in the image*. The MTC preparation
consists of an RF excitation pulse that is slightly
off-resonance for the narrow spectral peak corresponding to
the free protons. Because the bound proton peak is much wider,
it is possible to saturate the bound protons (i.e. to add the
maximum amount of RF energy that these protons can accept),
without having a significant effect on the magnetization in
the free pool. After the application of the off-resonance
saturation pulse, there is virtually no magnetization in the
bound pool that can be transferred to the free protons.
Magnetization in the free pool is gradually transferred to the
bound pool via the normal magnetization transfer interactions,
however, now the magnetization transfer flows only in one
direction. This means that magnetization from the larger free
pool is decreased in tissues where there is significant
magnetization transfer between the free and bound pools, and
the signal intensity in these tissues is decreased.
Even without using an MTC preparation, some
magnetization transfer contrast can also occur as a result of
off-resonant RF pulses from the imaging pulse sequence itself.
In multi-slice acquisitions, for example, RF excitation pulses
that are on-resonance for one particular slice are actually
off-resonance for the other slices. In FSE sequences, the
train of slice-selective 180 degree pulses act as off-resonant
pulses for other slices, causing decreased signal intensity in
tissues where the magnetization transfer rate is high between
free and bound pools. Muscle and cartilage are examples of
tissues whose signal intensity is decreased due to MTC in an
FSE sequence.
* A "preparation" is a
series of RF and gradient pulses that prepare the
magnietization in a desired state prior to the initiation of
the pulse sequence.
