Brain SPECT involves the injection of a radiopharmaceutical that distributes in the brain according to CBF. For indications other than evaluation of suspected brain death, patients should abstain from large amounts of stimulants such as caffeine, alcohol, smoking, and other drugs that may affect cerebral perfusion. To avoid excessive sensory stimulation near the time of radiopharmaceutical injection, instructions should be given early and the intravenous cannula placed approximately 15 minutes prior to the injection.⁵ The patient should be injected in a quiet room with minimal stimuli. The patient should void prior to the scan. Patient comfort is critical to ensure that the patient will not move during the approximately 20 minutes required for scanning. The angiographic phase and planar anterior and lateral images are performed if the indication is suspected brain death. Dynamic images during the angiographic phase and SPECT images of the brain are obtained for most other indications. A low-energy, highor ultra-high-resolution collimator with a dual-head camera with 180-degree rotation with step-and-shoot mode and a matrix of 128 × 128 or higher is used. Dedicated brain cameras allow proximity to the head and facilitate less scatter and improved sensitivity and spatial resolution. Unprocessed data should be checked for motion or other artifacts prior to iterative reconstruction and the application of a low-pass filter. CT attenuation correction is superior to Chang attenuation correction.⁸ Images are reformatted to three orthogonal planes.⁵ With typically used techniques, HMPAO and ECD do not provide quantitative blood flow measurements. Quantitation would require an indwelling arterial line although alternative methods using the time-activity curves of the aorta or carotid artery and brain also exist.⁹,¹⁰,¹¹ and ¹²

This method will be explained in more detail in the section on PET quantitation. In most clinical settings, relative regional blood flow data are obtained by comparing the counts to the contralateral brain counts. In some cases, the assumption that the contralateral brain has normal CBF can be made. Semiquantitative blood flow can also be obtained by comparing the cortical CBF to that of the cerebellum. The cerebellum has greater flow than the cerebral cortex and basal ganglia. Normal rCBF to the cortex or basal ganglia is greater than or equal to 70% of cerebellar flow and to the medial temporal cortex is greater than or equal to 50% of cerebellar flow. In acute cerebrovascular disease, SPECT can provide information on complications, outcome, and treatment. SPECT can also detect hypoperfusion due to vasospasm following subarachnoid hemorrhage and can predict severity of future neurologic sequelae.¹³ An advantage of SPECT radiopharmaceuticals is the rapid uptake and prolonged retention in the brain, which allows injection during a seizure or other acute event and delayed imaging after the stabilization of the patient. The disadvantage is that the prepared radiopharmaceutical and a technologist need to be available at all times.

Patients fast for 4 to 6 hours, and the blood glucose levels are checked prior to FDG administration to confirm that they are not elevated. Excessive stimulation is to be avoided following tracer injection and thus it is recommended that the cannula for intravenous administration be placed 10 minutes or more before tracer injection. The patient is injected with 5 to 20 mCi (185 to 740 MBq) in a dimly lit room, and the patient is instructed to rest comfortably and not to speak, read, or do other activities. If an FDG study is performed for evaluation of epilepsy, EEG monitoring is necessary prior to and at least 20 minutes after injection. EEG leads are removed prior to image acquisition since most will cause attenuation artifacts. Imaging is performed 30 to 60 minutes following injection. Most current scanners are hybrid PET-computed tomography (CT). CT provides attenuation correction data for PET as well as anatomic correlation of the PET functional data. Basal ganglia, hippocampal area, thalami, cerebellum, posterior cingulate gyrus, and visual cortex are relatively metabolically stable with advancing age. Decreased cortical metabolism, especially in the frontal lobes, is reported with normal aging.¹⁵

Better image quality is possible with longer ¹⁸F-FDG imaging time (15 to 30 minutes) compared to shorter ¹⁵O-H₂O imaging time (1 to 2 minutes). The longer imaging time is possible because the ¹⁸F-FDG half-life is 110 minutes compared to 2 minutes for ¹⁵O-water. On the other hand, the advantage of the shorter ¹⁵O-H₂O half-life is that the study can be repeated immediately.

The various PET radiopharmaceuticals can provide hemodynamic (rCBF, oxygen extraction fraction [OEF], and regional cerebral metabolic rate of oxygen [rCMRO₂]), metabolic, cell proliferation, or neurotransmission data. PET regional CBF (rCBF) quantitation with ¹⁵O-H₂O PET water is regarded as the gold standard.¹⁶ Quantification of rCBF from ¹⁵O-H₂O PET requires the knowledge of the arterial input function (AIF) into the brain typically acquired by arterial blood sampling. PET can also provide quantification of the OEF and rCMRO₂.¹⁷ Due to the invasive nature of arterial sampling, research has focused on image-derived input function (IDIF). An example of this IDIF uses the common carotid arteries that can be localized by coregistered MRI.¹⁶ This is the same technique that can be used with SPECT as was shown in Figure 17.1. A region of interest (ROI) has been drawn over the common carotid arteries and used as an IDIF. Comparison of right and left hemisphere ROI time activity curves demonstrate relative perfusion differences. Deconvolution of the perfusion curves with the AIF can give an estimate of absolute perfusion.

PET can provide semiquantitative regional metabolic data. The brain depends almost exclusively on glucose metabolism. Unlike other organs, the brain cannot store oxygen or glucose, and rCBF is continuously changing to meet the changing metabolic demands. Thus, in most cases, regional metabolism correlates with rCBF. Standard uptake value is a semiquantitative parameter used in ¹⁸F-FDG PET.

SPECT can help determine which patients with symptoms of acute stroke may benefit from thrombolysis to limit neurologic sequela and which patients are at high risk for hemorrhage. If initial CT excludes hemorrhage in a patient with symptoms of acute stroke, the next consideration is whether thrombolysis might be necessary. Pretherapy semiquantitative SPECT provides important information that helps select the patients who would benefit from thrombolysis.² A retrospective study of 30 patients who had SPECT prior to endovascular thrombolysis revealed that ischemic tissue with CBF greater than 55% of cerebellar blood flow can be spared with thrombolysis even if administered more than 6 hours after the onset of symptoms and tissue with CBF greater than 35% of cerebellar flow can be spared with thrombolysis within 5 hours. Ischemic tissue with CBF less than 35% of cerebellar flow is at risk for hemorrhage despite prompt initiation of thrombolysis.²

Not only does CBF information help determine patient management, but it also provides prognostic information as listed in Table 17.2. Severe hypoperfusion in the first 6 hours after symptom onset is highly correlated with poor neurologic outcome.