Combined SPECT Imaging of Regional Cerebral Blood Flow (99mTc-HexamethylPropyleneamine Oxime, HMPAO) and Blood Volume (99mTc-RBC) to Assess Regional Cerebral Perfusion Reserve in Patients with Cerebrovascular Disease

Combined diffusion-weighted and perfusion-weighted flow heterogeneity magnetic resonance imaging in acute stroke

BACKGROUND AND PURPOSE

The heterogeneity of microvascular flows is known to be an important determinant of the efficacy of oxygen delivery to tissue. Studies in animals have demonstrated decreased flow heterogeneity (FH) in states of decreased perfusion pressure. The purpose of the present study was to assess microvascular FH changes in acute stroke with use of a novel perfusion-weighted MRI technique and to evaluate the ability of combined diffusion-weighted MRI and FH measurements to predict final infarct size.

METHODS

Cerebral blood flow, FH, and plasma mean transit time (MTT) were measured in 11 patients who presented with acute (<12 hours after symptom onset) stroke. Final infarct size was determined with follow-up MRI or CT scanning.

RESULTS

In normal brain tissue, the distribution of relative flows was markedly skewed toward high capillary flow velocities. Within regions of decreased cerebral blood flow, plasma MTT was prolonged. Furthermore, subregions were identified with significant loss of the high-flow component of the flow distribution, thereby causing increased homogeneity of flow velocities. In parametric maps that quantify the acute deviation of FH from that of normal tissue, areas of extreme homogenization of capillary flows predicted final infarct size on follow-up scans of 10 of 11 patients.

CONCLUSIONS

Flow heterogeneity and MTT can be rapidly assessed as part of a routine clinical MR examination and may provide a tool for planning of individual stroke treatment, as well as in targeting and evaluation of emerging therapeutic strategies.

99mTc-HMPAO-SPECT with acetazolamide challenge to detect hemodynamic compromise in occlusive cerebrovascular disease

BACKGROUND AND PURPOSE

Insufficiency of collateral supply may lead to low-flow infarcts in severe occlusive cerebrovascular disease. The aim of this study was to evaluate the feasibility of technetium-99m-labeled hexamethylpropyleneamine oxime (99mTc-HMPAO) single-photon emission computed tomography (SPECT) to assess hemodynamic compromise in the anterior circulation.

METHODS

Cerebral blood flow before and after 1 g acetazolamide was analyzed by 99mTc-HMPAO-SPECT in 21 symptomatic patients with documented extracranial obstructions. SPECT findings were correlated with the results of angiography, transcranial Doppler sonography, and computed tomographic scan.

RESULTS

The acetazolamide-induced increase of cerebral blood flow could be reliably monitored by increase of cerebral 99mTc-HMPAO uptake, which varied between 11.4% and 47.6% in the less-affected hemisphere. Increment of hemispheric side-to-side asymmetry of tracer uptake after drug challenge revealed significant restriction of regional vasoreactivity in 11 patients. Agreement in assessing hemodynamic compromise was reached in 81% of patients with ophthalmic artery collaterals on angiography (p < 0.001), in 76% with low-flow infarcts on computed tomographic scan (p < 0.01), and in 91% with markedly reduced flow velocities on transcranial Doppler (p < 0.0001). One patient developed a low-flow infarct in the area predicted by SPECT during follow up.

CONCLUSIONS

We conclude that 99mTc-HMPAO-SPECT with acetazolamide challenge is a useful method for assessment of the adequacy of hemispheric collateral pathways in patients with severe occlusive cerebrovascular disease.

Clinical and hemodynamic aspects of low-flow infarcts

We used single-photon emission computed tomography to measure cerebral blood flow, cerebral blood volume, and cerebral perfusion reserve and transcranial Doppler sonography with CO2 stimulation to assess hemispheric vasomotor reactivity in 37 patients and in normal controls. Computed tomography and magnetic resonance imaging were performed to differentiate morphologically low-flow infarcts (n = 17) from territorial infarcts (n = 20). In patients with either type of infarct, blood flow was decreased and blood volume was increased in the infarcted areas compared with the same areas in the controls. Perfusion reserve and vasomotor reactivity were significantly reduced in patients with territorial infarcts and carotid artery occlusions (n = 12) and even more reduced in patients with low-flow infarcts (p less than 0.001). Both parameters were normal in patients with cardiac embolic territorial infarcts (n = 8). In patients with territorial infarcts, blood flow and perfusion reserve changes were restricted to the infarcted areas, whereas in patients with low-flow infarcts, regions of decreased perfusion reserve considerably exceeded the area of the infarct. Low-flow infarcts are related to the hemodynamic effects of severe extracranial carotid artery disease.

Infarctions and non-invasive diagnosis in moyamoya disease: two case reports

An 31-year-old female and a 32-year-old male had clinical signs and angiographical confirmation of adult Moyamoya disease (MMD). Bilateral carotid siphon (C1), middle cerebral artery (M1), and anterior cerebral artery (A1) stenoses were diagnosed by means of transcranial Doppler sonography (TCD) and visualized during angio Magnetic Resonance Imaging (angio-MRI). In the woman, a 'rete mirabile' of dilated and tortuous lenticulostriate arteries could be visualized during proton weighted and T1-weighted MRI sequences. CO2-dependent vasomotor reactivity was bilaterally reduced and completely exhausted in the territory of right middle cerebral artery in both patients. Cerebral perfusion reserve, defined as the ratio of cerebral blood flow (CBF) to cerebral blood volume (CBV) was assessed by SPECT, and was found to be dramatically reduced in the anterior region of the male patient. Both patients had bilateral small subcortical infarctions in the corona radiata. TCD and MRI are important noninvasive techniques for a preliminary diagnosis of Moyamoya disease. Infarctions in Moyamoya disease may be hemodynamically produced low-flow infarctions.

Patterns of brain infarctions in internal carotid artery dissections

In 15 patients with internal carotid artery (ICA) dissections, patterns of brain infarctions visible on CT were categorized according to a pathogenetically oriented classification system. This differentiated ischemic brain damage due to low flow from thromboembolic infarctions. Simultaneously, cerebral hemodynamic reserve was evaluated by means of both CO2-dependent vasomotor reactivity and HMPAO- and 99mTc-RBC-SPECT. Six out of 11 patients with ischemic infarctions had the territorial type of brain lesion, suggesting distal embolism. Five patients revealed the pattern of hemodynamically induced low-flow infarctions. Cerebral hemodynamic reserve was significantly decreased only in these latter patients. In ICA dissections the frequencies of hemodynamically and thromboembolically induced brain infarctions is approximately equal. This finding suggests that only half of the patients with symptoms of symptomatic dissections had brain emboli, whereas in the other half the dissections lead to a low-flow effect in terminal supply areas which is so severe that non-embolic brain infarctions occur. This differentiation can help decide whether antithrombotic or hemodynamic treatment should be given.

Studies of central nervous system disorders with single photon emission computed tomography and positron emission tomography: evolution over the past 2 decades

Single photon emission computed tomography (SPECT) was introduced in the 1960s to detect breakdowns in the blood-brain barrier and was replaced by x-ray computed tomography in the mid-1970s. The development of the deoxyglucose (DG) technique to measure regional cerebral glucose metabolism by employing either autoradiography, using 14CDG, or positron emission tomography (PET), using 18FDG, added a major dimension to the investigation of brain function. In the late 1970s and early 1980s, the FDG-PET technique was widely used to examine a variety of neuropsychiatric disorders. It soon became apparent that functional imaging was more sensitive than anatomic imaging in detecting abnormalities of the brain related to aging, dementia, tumors, seizures, cerebral vascular accidents, and psychiatric problems. Because of its complexity and the cost involved, PET was used in a limited number of centers in the United States. However, the success of PET resulted in the resurgence of interest in SPECT as an alternative technology after almost a decade. This became possible because of the synthesis of iodine 123- and technetium 99m-labeled radiopharmaceuticals to determine regional cerebral blood flow. Since blood flow and metabolism are coupled in most pathological states, patterns of abnormality noted on SPECT were similar to those seen on PET in many disorders. Since the introduction of high resolution SPECT imaging instruments, the role of SPECT has been further enhanced. The successful synthesis of both positron and single emitting radioligands to image dopamine and other receptors has started a new era in neurosciences and will have a far-reaching impact on the day-to-day practice of neuropsychiatry.

Brain perfusion with intracarotid injection of 99mTc-HM-PAO in partial epilepsy during amobarbital testing

Technetium (99mTc) hexamethyl propylene amine oxime (HM-PAO) was injected into the internal carotid artery in ten epileptic patients after the end of amobarbital speech-memory tests. The cerebral perfusion as visualized from SPET was compared to cerebral angiographies, which showed unilateral filling of intracranial vessels in seven patients. SPET revealed cross-flow between the hemispheres in four of these seven patients. In three patients in whom the angiograms had shown bilateral contrast filling, SPET showed cross-flow in only two. It is concluded that 99mTc-HM-PAO SPET examinations provide valuable information for correct interpretation of amobarbital tests on cognitive hemisphere functions. The SPET technique may help to explain atypical speech and memory responses caused by unusual intracranial vascularization.