Special characteristics and potential of single photon emission computed tomography in the brain

Evaluation of cerebral vasospasm in patients with subarachnoid hemorrhage using single photon emission computed tomography

Cerebral vasospasm (CVS) occurs as a result of the breakdown in cerebral autoregulation mechanisms. Because cerebral vasospasm can occur after subarachnoid hemorrhage (SAH), it is important to evaluate borderline perfusion. Evaluation of borderline vascular insufficiency is important to reduce ischemic complications. In this study 25 patients with SAH were investigated by somatosensory evoked potentials (SEP), computed tomography (CT), digital subtraction angiography (DSA) and single photon emission computed tomography (SPECT) in order to predict borderline ischemic areas. Clinical grades were also correlated with these investigations. Thirteen patients had symptomatic vasospasm and 15 patients had angiographic vasospasm. SPECT showed hypoperfusion in 22 out of 25 patients. CT predicted CVS in 8 of these 22 patients. Our study shows that brain perfusion SPECT is a non-traumatic, non-invasive, non-allergic, inexpensive method for the prediction of cerebral vasospasm. We conclude that brain SPECT with Tc-99m HM-PAO is an accessible technique that can demonstrate varying degrees of regional tissue hypoperfusion in patients with delayed ischemic deficits due to CVS following SAH.

Conventional radionuclide brain imaging in the era of transmission and emission tomography

The clinical applications for conventional radionuclide brain imaging have declined considerably since the introduction of newer imaging modalities (computerized cranial tomography [CCT], nuclear magnetic resonance [NMR]). Currently, conventional brain imaging primarily serves as a complementary test when CCT is negative or equivocal and strong clinical suspicion remains. Selected areas in which radionuclide imaging may be the preferred modality include evaluation of cerebral perfusion in assessment of brain death, detection of early viral encephalitis, evaluation of major venous sinus patency, external marking for localization of intracranial tumor, and in selected cases of suspected subdural hematoma, brain tumor, and cerebrovascular disease. The concept of conventional radionuclide brain imaging will likely undergo considerable change in the near future as newer radiopharmaceuticals are introduced and wider application is made of single photon emission computerized tomography (SPECT) and positron emission tomography (PET) imaging.

Contribution of radionuclide imaging to radiation oncology

Radionuclide imaging has been helpful to oncologists in the initial evaluation and follow-up of patients with primary and metastatic disease. With the introduction of ultrasound and computed tomography, there has been some reduction in the number of radionuclide imaging procedures. These imaging modalities should be used to complement one another. With the introduction of single-photon emission computed tomography in clinical nuclear medicine, there will be a significant improvement in the sensitivity, accuracy, and quantitative ability to detect abnormal lesions. There are intensive efforts in the development of tumor imaging using new agents. Specific antibodies for surface tumor antigens can be labeled with radioisotopes and used as tools for screening primary tumors and metastatic lesions. Therefore, for the present, radionuclide imaging will continue to contribute to clinical oncology.

Clinical experience with the use of xenon-enhanced CT blood flow mapping in cerebral vascular disease

Cerebral blood flow mapping with the xenon-enhanced/CT method has become a useful clinical tool in the management of patients with occlusive cerebral vascular disease. Studies involving 4-5 minutes of inhaling a xenon/oxygen mixture (less than or equal to 35%) can now be performed routinely with acceptable patient tolerance and compliance. Four cases with acute and chronic ischemic injuries are reported here to illustrate the manner in which this method has been used to characterize flow pattern in such patients and the relevance of this flow information to clinical patient management.

Progress in cerebrovascular disease: local cerebral blood flow by xenon enhanced CT

A noninvasive technique for measuring local cerebral blood flow (LCBF) by xenon enhanced x-ray transmission computed tomography (CT) has been developed an reported quite extensively in recent years. In this method, nonradioactive xenon gas in inhaled and the temporal changes in radiographic enhancement produced by the inhalation are measured by sequential computed tomography. Time dependent xenon concentrations within various tissue segments in the brain are used to derive both local partition coefficient (lambda) and LCBF. An assessment of this method reveals that although it provides functional mapping of blood flow with excellent anatomic specificity, there are distinct limitations. The assumptions underlying this methodology are examined and problems associated with various potential applications of this technique are discussed.