Single-photon emission computed tomography (SPECT) scanning uses direct photon-emitting isotopes rather than radioisotopes. SPECT isotopes have an average half-life of 6-12 hours.
SPECT instrumentation is highly variable; therefore, use of a SPECT scanner with poor resolution can result in poor clinical performance. Positron-emission tomography (PET) scanning uses tracers that measure regional glucose metabolism (rCMRGlc). SPECT imaging is most commonly used for blood-flow measurement.
Early SPECT studies of blood flow replicated findings of functional reductions in the posterior temporal and parietal cortex. The severity of temporoparietal hypofunction has been correlated with the severity of dementia in a number of studies.
Reductions of blood flow and oxygen use can be found in the temporal and parietal neocortex in patients with Alzheimer disease and moderate to severe symptoms. Early reductions of glucose metabolism are seen in the posterior cingulate cortex.
SPECT scanning is not commonly used to assess Alzheimer disease. SPECT scanning is useful in the diagnostic assessment of Alzheimer disease if standardized and semiquantitative techniques are used.
Trollor et al examined 18 patients with early Alzheimer disease and 10 healthy, elderly control subjects with high-resolution SPECT scanning during their performance of a simple word-discrimination task and observed a gradation of regional cerebral blood flow (rCBF) values in both groups. The lowest values were in the hippocampus and the highest in the striatum, thalamus, and cerebellum. In the study, SPECT images were coregistered with individual MRI scans, allowing for the delineation of predetermined neuroanatomic regions of interest (ROI).
Compared with healthy control subjects, patients with Alzheimer disease had low relative rCBF in the parietal and prefrontal cortices. Analysis of individual the ROI demonstrated bilateral reduction of rCBF in the prefrontal poles and posterior temporal and anterior parietal cortex, with unilateral reduction of rCBF in the left dorsolateral prefrontal cortex, right posterior parietal cortex, and left cingulate body. No significant differences in hippocampal, occipital, or basal ganglia rCBF were found. Discriminant function analysis indicated that rCBF in the prefrontal polar regions permitted the best classification.
In class II studies, the sensitivity of SPECT scanning was lower than that of the clinical diagnosis. Sensitivity increased as the severity of dementia worsened, but the pretest probability of Alzheimer disease increased as well.
The added value of SPECT scanning was greatest for a positive test among patients with mild dementia in whom the diagnosis of Alzheimer disease was substantially doubted. In this situation, a positive SPECT scan result would have increased the posttest probability of Alzheimer disease by 30%, whereas a negative test result would have increased the likelihood of the absence of Alzheimer disease by only 10%.
Degree of confidence
Without surprise, clinically validated SPECT scan studies showing differences between patients with Alzheimer disease (Alzheimer’s disease) and control subjects reveal high sensitivities and specificities of 80-90%.
In one study, investigators compared patients from a dementia clinic with a community sample of control subjects using quantitative SPECT scanning and reported a 63% sensitivity and an 87% specificity. Alzheimer disease was defined in the study as temporal-lobe perfusion more than 2 standard deviations below control values.
Holman et al found that bilateral temporoparietal hypoperfusion had a positive predictive value of 82% for Alzheimer disease. Using inhaled xenon-133 (133 Xe) and injected technetium-99m [99m Tc]hexamethylpropyleneamine oxime, researchers reported a sensitivity of 76% and a specificity of 73%, with a positive predictive value of 92% and a negative predictive value of 57%. These studies may assist in the early and late diagnosis of Alzheimer disease and with the differential diagnosis of dementias.