section i
.3
Measurement of pH
13
(MRI) are generated by measuring the relaxation time of
return to equilibrium for hydrogen nuclei in a constant
magnetic field following excitation by a radiofrequency
pulse. The time taken by the hydrogen nuclei to return
to their original position when the radiofrequency pulse
is terminated is known as the T1-relaxation time. The
time taken for the hydrogen nuclei to lose the energy that
they acquire during the radiofrequency pulse sequence is
known as the T2 relaxation time. T2 is always less than T1
and the quality of the MRI depends upon the concentration
of hydrogen nuclei (known as proton density or spin den-
sity) and the weight given to the T1 and T2 components.
In T1-weighted images, lipids have a characteristic short
T
1
relaxation time and are hyperintense (bright), whereas
water has a long T
1
relaxation time and is hypointense
(dark). Thus, tissues rich in fat appear bright and tissues
rich in free water appear dark. In T2 weighted images the
opposite is true; lipids appear dark and water bright.
The resolution of anatomical structures is achieved by
virtue of different relaxation times of hydrogen nuclei
in different tissues. Differences in relaxation time re-
flect gross chemical characteristics, including fat content,
degree of hydration, and presence of paramagnetic sub-
stances. Proton density is also an important parameter in
determining the intensity of an image, so that soft tissues
as well as bone can readily be visualized, making the tech-
nique superior to x-ray and other methods of imaging the
brain and other soft tissues.
Use of the intravascular contrast agent gadolinium-
diethylamine pentaacetic acid (GdDPTA) during the MRI
procedure enhances the T
1
relaxation time of hydrogen
nuclei. This alters the magnetic susceptibility of adjacent
tissue and provides information on the integrity of the
blood-brain barrier. MRI is the diagnostic procedure of
choice in several neurologic diseases. In one autoimmune
inflammatory demyelinating disorder of the central ner-
vous system,
multiple sclerosis
(MS), MRI is the preferred
imaging procedure both in diagnosis and as a prognos-
tic tool (Figure 1-11). MS is a progressive degenerative
disease and exhibits scattered focal lesions of the myelin
sheath of the axons. MS usually manifests during the third
or fourth decade of life and affects more women (60%)
than men (40%). No known risks are associated with MRI,
which is another advantage of the procedure.
Gibbs-Donnan Equilibrium
The bicarbonate-carbonic acid buffer system plays a major
role in regulating the pH of fluids in tissue spaces outside
blood vessels. This fluid, commonly referred to as inter-
stitial fluid and separated from plasma by the membrane
barrier known as the capillary endothelium, primarily
FIGURE 1-11
Magnetic resonance image (T1
weighted) of brain from a patient with
multiple sclerosis. The image obtained is a horizontal section at the level of
the head of the caudate nucleus showing characteristic marked increase of
signal as indicated by arrows. (Courtesy of Robert M. DiMauro and John
M. Hardman.)
contains the diffusible ions, Na+, K+, Cl” , and HC03.
Plasma contains proteins in addition to diffusible ions.
Membranes (Chapter 10) have a lipid-protein fluid mosaic
structure and the membrane proteins may occupy surface
positions or extend through the lipid bilayer (Figure 1-12).
Plasma proteins are polyionic at pH 7.4 and cannot dif-
fuse across membranes. The normal difference in concen-
trations of diffusible ions between the plasma and intersti-
tial compartments is due to the presence of nondiffusible
protein in plasma, shown in Table 1-5.
The difference is explained by Gibbs’ theory of equilib-
ria and was studied experimentally by Donnan; the overall
process is known as the
Gibbs-Donnan equilibrium.
Gibbs-Donnan equilibria can best be understood in a
two-compartment system. Compartment 1 contains the
sodium salt of an anionic protein (Na,|P"” ) at an ini-
tial concentration Q , with
n
representing the number
of charges; compartment 2 contains NaCl at an initial
