A

Figure 3. Vasospasm post subarachnoid hemorrhage (SAH). Brain SPECT ECD scans were performed 2 and 4 days post-SAH. Only the transaxial images are shown. The initial study shows decreased left frontal perfusion secondary to intraparenchymal bleeding (arrow). The delayed study performed after the patient showed severe clinical deterioration reveals perfusion limited to the cerebellum and the lower brain stem (arrowheads), indicating bilateral cerebral hemispheric infarction with no possibility of regaining normal cerebral function.

(CT or MRI) relies on the detection of an anatomical lesion which may or may not be the actual ictal focus. A recent meta-analysis has confirmed the value of ictal SPECT (and PET) rCBF studies: focally increased perfusion has a sensitivity of 97% and a specificity of at least 98.5% for temporal lobe epilepsy localisation (Fig. 4). Although the most useful part of a SPECT evaluation is the ictal study, an interictal one (obtained if possible with EEG monitoring to exclude subclinical ictal activity at the time of injection) is warranted in order to facilitate interpretation of the ictal scan. Unlike the ictal scan, the one obtained interictally demonstrates decreased perfusion of the epileptogenic focus. Simple visual inspection of the two studies can be supplemented with statistical analysis that looks for regions of significantly increased perfusion in the ictal phase. The interictal study alone, however, cannot be relied upon to localise epileptic foci (sensitivity only 50% for temporal lobe epilepsy with false localisation rate of 10%).

For ictal studies, injection must be performed immediately after seizure onset, as rapid modification in the distribution of brain activation and perfusion can occur. For extra-temporal epilepsy, generalisation can be extremely rapid. Frontal lobe epilepsy requires an injection delay of no more than 10 seconds after clinical initiation of the seizure before epileptic activity spreads to other parts of the brain. In temporal lobe epilepsy the optimal interval for injection is during the first 30 seconds, although acceptable studies can be obtained for up to 60 seconds. If a first try is negative then repeat examinations must be obtained: the importance of precisely determining which area of the brain to remove can hardly be overemphasized. Its non-invasiveness, relatively low-cost, and remarkable performance amply justify the use of rCBF SPECT in the preoperative assessment of epilepsy.

To Support a Clinical Diagnosis of Alzheimer's Disease

Dementia can be defined as a deterioration in cognitive abilities that impairs the previously successful performance of activities of daily living. Ten percent of persons over age 70 and 20 to 40 percent of individuals over age 85 have clinically significant memory loss. Although there are many causes (Table 2), four types of dementia—

Figure 4. Epilepsy. Interictal and ictal transaxial, coronal and sagittal slices showing right temporal focus of decreased interictal perfusion and ictal hyperfusion. These images are 3D co-registered, allowing for precise subtraction (Ictal-Interictal) and superimposition over an MRI atlas of voxels with a subtraction value of more than 2 SD over the mean.

Figure 4. Epilepsy. Interictal and ictal transaxial, coronal and sagittal slices showing right temporal focus of decreased interictal perfusion and ictal hyperfusion. These images are 3D co-registered, allowing for precise subtraction (Ictal-Interictal) and superimposition over an MRI atlas of voxels with a subtraction value of more than 2 SD over the mean.

Alzheimer's disease, diffuse Lewy body dementia, frontotemporal dementia and vascular dementia—account for 90% of all cases.

SPECT rCBF imaging can be a valuable adjunct to clinical evaluation and conventional neuroimaging (CT and MRI) in patients with dementia. Certain scan patterns increase the likelihood of specific sub-types of dementia (Fig. 5), information that is valuable in establishing prognosis. This may also help in targeting drug therapies, a potentially useful application now that several medications have been approved for the treatment of Alzheimer's disease with additional agents undergoing clinical trials. However, SPECT interpretation must be integrated with all other information on the patient: on its own it can neither be used to prove nor disprove the presence of dementia, nor to establish a specific diagnosis of a sub-type of dementia, as patients with no clinical signs of dementia can have perfusion studies with a pattern suggestive of Alzheimer's disease (bilaterally decreased parietotemporal perfusion), and conversely some patients with documented dementia can retain a normal perfusion pattern. Special reconstruction techniques along the hippocampal long axis can reveal severe hypoperfusion of these structures. Some investigators believe that SPECT can help to differentiate nondementing illnesses (e.g., late-onset depression) from organic dementia, though others have questioned this.

Detection of Acute Ischemia

MRI and CT are both usually normal for several hours after interruption of flow to the brain as the tissue properties measured by these modalities take time to become abnormal. SPECT imaging of rCBF is a highly effective means for detecting acute

Table 2. SPECT patterns according to dementia subtype

0 0

Post a comment