Biomedical Engineering Reference
In-Depth Information
It is important to appreciate that the spatial information obtained in
MRI derives from a knowledge of the strength of the magnetic field at
each point in space at any given moment, a point to which we will
return shortly. In addition, a wide variety of artifacts can give rise to
spatial distortions of the images.
The “meaning” of the received signals depends on: the timing of the
exciting rf “pulse”; the timing of the signal received from proton de-
excitation; and the timing of the changes in magnetic field. One of the
charms of MRI is that, because there are so many variables, there is a
wide variety of information that can be obtained. Again, these are too
numerous, the process by which they are obtained too complex, and
the clinical implications far too broad for me to attempt to describe
them here. The three earliest forms of MRI produced so-called:
proton density-weighted, T1-weighted, and T2- weighted images. As
their names imply, none of these are pure measurements; rather, the
specified property is enhanced by technical means.
Proton-density images, as their name implies, give values dominated
simply by the density of free protons in each voxel. These images
tend to be of rather
low contrast. The
symbols T1 and T2
refer to different
relaxation times -
i.e., times for the
signal from de-
exciting protons to
decay away. T2 is
the time by which a
component of the
Figure 3.11. Sagittal sections of an MRI study:
(a) T1-weighted; (b) T2-weighted. Figure courtesy
of the Whole Brain Atlas.
emitted signal de-
cays away due to so-called spin-spin interactions - that is, inter-
actions between neighboring excited protons. T1 is the time by which
a component of the emitted signal decays away due to so-called
spin-lattice interactions - that is, interactions between an excited pro-
ton and the molecular structure of its environment. Typically, T2
relaxation times are quite a bit shorter than T1 times. Both types of
image tend to have good contrast.
To give a small appreciation of the quality and enormous wealth of
information in MRI, Figures 3.11a and 3.11b show typical T1-
weighted and T2-weighted sagittal sections through the head; and
Figure 3.12 shows a coronal image through the thorax.
