LETTERS TO THE EDITORS
To the Editors:
I enjoyed reading the recent article entitled "The Structure of
the Human Brain" (May–June), by John S. Allen, Joel Bruss
and Hanna Damasio, in which the important role of magnetic resonance
imaging (MRI) for determining brain structure is emphasized.
Unfortunately, the brief paragraph describing the basic physics of
MRI is incorrect and incomplete.
Indeed, a powerful magnet is used to align the "spins," in
this case hydrogen protons. This magnet is, however, never
"turned off" in order to let the nuclei "fall back to
a normal state," as stated in the article. Rather, the spins
are manipulated by applying a very small radio-frequency (RF) field
oscillating in a plane perpendicular to the strong static field.
When a brief pulse of the small oscillating field is turned off, it
leaves the spins in an excited state, from which they return to
equilibrium, releasing energy in the form of RF waves.
A further stated misconception is that the "frequency of these
waves provides a measure of the local hydrogen concentration, which
varies according to tissue type, such as bone or fat." Although
there is a small frequency difference between protons within fat and
water molecules, this chemical shift is of no consequence in brain
imaging per se, where all the relevant signal arises from water protons.
Finally, it should be noted that the hydrogen concentration itself
is only one factor contributing to the rich image contrast exploited
by researchers using MRI to study the structure of the brain, other
critical factors being the water proton relaxation times.
Robert V. Mulkern