Functional Imaging Modalities
1) PET is
the gold standard of functional neuroimaging modalities.
can measure cerebral glucose metabolism.
3) A larger
number of radioligands, especially those used for neuroreceptor
characterization, are available for PET.
offers excellent spatial resolution ( 4
5) PET is
very expensive and requires relatively rapid access to a cyclotron, which
produces the positron-emitting radionuclides. Most PET centers have a cyclotron
on the premises. However, because of the high cost of obtaining and maintaining
a func-tional cyclotron, some centers do not have a cyclotron on the premises.
18F has a longer half-life (110 minutes) than other ra-dionuclides such as 11C
(20 minutes), 13N (10 minutes) and 15O (2 minutes), so 18FDG may be prepared
using a cyclotron at one site and then transported to other sites as long as
they are within hours (generally 1–3 half-lives) of the site of synthesis.
1) SPECT is
more affordable than PET and does not require a cyclotron for production of
SPECT provides inferior spatial resolution ( 6–8 mm) compared with PET.
SPECT spatial resolution worsens as one attempts to image deeper brain
structures, although this is becoming less of an issue with the introduction of
newer generation SPECT cameras.
provides superior spatial resolution, especially for deeper brain structures.
offers a broader array of radioligands for use in receptor studies and is the
only modality that allows for the measure-ment of metabolism.
3) SPECT is
less expensive than PET and is more widely available.
Although functional neuroimaging modalities include
PET, SPECT and functional MRI (to be discussed later), in clinical situ-ations
only PET and SPECT are generally used at this time. These modalities are now commonly
used as aids for diagnosis and monitoring treatment in cardiology and oncology
(including brain tumors). Also, the use of ligands for neuroreceptor
characteriza-tion is being increasingly used for the diagnosis and assessment
of basal ganglia diseases such as Parkinson’s disease. Still, most applications
of functional neuroimaging in psychiatry occur in the field of research.
However, a clinical role for functional neuroim-aging in dementia and seizures
is evolving and showing promise.
As characteristic functional neuroimaging profiles
emerge for var-ious forms of dementia, the role of PET and SPECT in the
evalua-tion of dementia is expanding. For example, Alzheimer’s disease is
associated with characteristic hypoperfusion in bilateral temporo-parietal
regions. Some studies have indicated that functional neu-roimaging can offer
better than 90% sensitivity and specificity in distinguishing Alzheimer’s
disease from other kinds of dementia (Silverman et al., 2001; Bonte et al.,
2001). Since the introduction of cholinesterase inhibitors for treatment of
dementia, early diagnosis of Alzheimer’s disease may have greater import than
in the past. Some PET studies of healthy older subjects with normal cognitive
function who are homozygous for the apolipoprotein E epsilon 4 allele (a gene
associated with the development of Alzheimer’s dis-ease) reveal that these
subjects have temporo-parietal hypoper-fusion before the onset of disease
(Small et al., 1995; Reiman et al.,
1996). Initiation of treatment, with cholinesterase inhibitors or other drugs being developed, in these
individuals prior to the onset of symptoms of dementia may prove to be of great
Some seizures, especially complex partial seizures,
are not al-ways detected by electroencephalogram (EEG). EEG measures cortical
surface electrical activity but is less efficacious if the sei-zure focus is
deeper in the brain. PET and SPECT images typi-cally demonstrate ictal
hypermetabolism and interictal hypome-tabolism (Krausz et al., 1996, Theodore and Gaillard 2000). This allows for the
detection of seizure foci during the predominant interictal period. To evaluate
a possible seizure disorder, func-tional neuroimaging is usually performed in
conjunction with EEG. PET is also useful for more precise localization of
seizure foci in a patient with a known seizure disorder, if neurosurgical
intervention is indicated.
Expanding upon the basic principles underlying
standard structural MRI used in clinical situations, functional magnetic
resonance imaging (fMRI) allows investigators to assess brain function. Using
either a standard or, preferably, a high-speed MR scanner with specific
image-acquisition parameters, indices of cerebral blood flow and blood volume
can be measured. While structural MRI studies rely on excitation and relaxation
of hydro-gen atoms in water, fMRI takes advantage of the paramagnetic
properties of hemoglobin to measure blood flow and volume.
fMRI has distinct advantages when compared with PET
and SPECT, and these include:
· fMRI does
not expose subjects to radiation and its relative safety has been documented.
spatial resolution of fMRI is at least equal to if not supe-rior to that of
The temporal resolution of fMRI is vastly superior
to that of PET or SPECT. While the temporal resolution of an 15O PET study is 1
to 2 min, multiple time points can be assessed per second with fMRI.
The major disadvantage of fMRI when compared with
PET or SPECT is that no satisfactory techniques for receptor neuroimaging have
yet been developed for fMRI.
While fMRI currently has little utility, it may be
use-ful in clinical situations in the future. One area of research that shows
particular promise is the use of intravenous paramagnetic contrast agents with
fMRI. This method, termed tracer kinetic technique, produces maps of cerebral
blood volume (CBV) (Bel-liveau et al.,
1991). These CBV maps are relatively well matched to PET images of FDG uptake
(Gonzalez et al., 1995) and HM-PAO
SPECT images of cerebral blood flow (Johnson et al., 1995). Because these fMRI CBV maps can be obtained after
only 1 to 2 min of imaging without exposure to radiation, this technique may be
used instead of PET or SPECT for evaluation of dementia or seizures in the