Pain is recognized as a sensory and emotional experience in humans.
Unfortunately, there is no objective test for measuring pain. This
has hampered both the clinical management and the scientific understanding
of pain. In the clinical setting, physicians daily encounter difficulties
in diagnosing chronic pain conditions. The findings of commonly
used testing modalities (magnetic resonance imaging [MRI],
computed tomography, electromyography) are frequently normal. History
and physical examination are highly subjective tests and prone to
examiner bias. Complaints of chronic pain patients are frequently
labeled “psychogenic” in origin. The majority
of chronic pain patients suffer from depression; however, it is
difficult to determine if depression is a consequence of chronic
pain or vice versa. Indeed, emotional and pain brain networks share
similar anatomic structures.
Most of the limited knowledge of central nervous system (CNS)
pain processing is derived from animal research using electrophysiologic
recordings. The animal data do not provide adequate insight into
human aspects of pain processing, such as the affective component
of pain. There is need for a better understanding of the following
aspects of human pain networks: (1) chronic pain states in which
altered CNS processing is taking place; (2) effects of various drugs
on brain activation in acute and chronic pain; and (3) perception
of pain in altered states of conciousness.
The initial findings that provided an insight into human CNS
pain networks were accomplished by functional brain imaging using
positron emission tomography (PET) technology, followed by studies
using functional magnetic resonance imaging (fMRI). Although still
in their infancy, these tools are extremely valuable in bridging
the gap between clinical practice and animal research in understanding
Functional brain imaging techniques are based on similar methods
of measuring brain neuronal activity. Glucose is the main energy
source for human brain metabolism. The coupling between regional
cerebral blood flow and local cerebral glucose use has been established
and supported by experimental data. Glucose metabolism reflects
the brain neuronal synaptic and presynaptic activity needed for
maintenance of membrane potentials and restoration of ion gradients.
Functional brain imaging, by measuring the regional blood flow,
provides indirect data on regional brain neuronal activity, be it
local activation or inhibition.
PET is an instrument that allows accurate measurements of small
concentrations of radioactivity in living tissue. Positron-emitting
isotopes have been incorporated as tracers into a wide range of molecules
to provide information about various biologic processes after inhalation
or intravenous administration. By using this technique, the whole
brain metabolic activity in pain states can be measured. The ability
to overlap brain metabolic activity data acquired by PET with structural magnetic
images of brain (MRI) enhanced the effective spatial resolution
of PET. There are numerous studies using this technology in medicine.
The first brain mapping study focusing on pain used PET. This
study demonstrated that painful heat produced activation in multiple
brain areas, including cortical and subcortical structures.1 Since
then, PET with various experimental ...