Positron emission tomography (PET) produces images that reflect the regional concentration of systemically administered radioactive compounds. Positron-emitting isotopes (mainly 11C, 18F, and 15O) are produced in a cyclotron or linear accelerator, injected into the patient, and incorporated into biologically active compounds in the body. The concentration of these tracers in various parts of the brain is determined by an array of radiation detectors and tomographic images are constructed by techniques similar to those used in CT and
MRI.
Local patterns of cerebral blood flow, oxygen uptake, and glucose utilization can also be measured by PET, and the procedure has proved to be of value in both detecting and grading brain tumors, distinguishing tumor tissue from radiation necrosis, localizing epileptic foci, and, in differentiating types of degenerative diseases. The technique has been applied to specially labeled ligands of beta-amyloid, producing images of the deposition of this protein in Alzheimer disease. This approach may become increasingly important in the study of degenerative diseases and their response to treatment. The ability of the technique to quantitate neurotransmitters and their receptors also promises to be of importance in the study of Parkinson disease and other degenerative conditions. However, this technology is costly and does not always add to the certainty of diagnosis. A representative PET of the brain is shown in Fig. 2-6.
Single-photon emission computed tomography (SPECT), a similar technique, uses isotopes that do not require a cyclotron for their production. Radioligands (usually containing iodine) are incorporated into biologically active compounds, which, as they decay, emit a single photon. This procedure allows the study of regional cerebral blood flow under conditions of cerebral ischemia and in degenerative diseases of the cortex, or during increased tissue metabolism (e.g., seizures and actively growing tumors). Once injected, the isotope localizes rapidly in the brain, with regional absorption proportional to blood flow, and is then stable for an hour or more. It is thus possible, for example, to inject the isotope at the time of a seizure, while the patient is undergoing video and electroencephalographic monitoring, and to scan the patient soon after. The limited anatomic resolution provided by SPECT limits its clinical usefulness, but it is more easily available than other functional imaging techniques. PET and SPECT techniques that use I123 labeled dopamine, have been introduced and offer the possibility of imaging striatal dopamine and assisting in the diagnosis of parkinsonian disorders (see Chap. 38).
An ultrasound technique may be used to insonate the cervical carotid and vertebral arteries, and the temporal arteries for the study of cerebrovascular disease. Their greatest use is in detecting and estimating the degree of stenosis of the origin of the internal carotid artery. In addition to providing an acoustic image of the vascular structures, the Doppler frequency shift caused by flowing red blood cells creates a display of velocities at each site in a vessel. The two techniques combined have been called “carotid duplex”; they allow an accurate localization of the locus of maximal stenosis as reflected by the highest rates of flow and turbulence. The display scale for the Doppler shift is color coded so as to make the insonated image and flow map easier to view and interpret.
This ultrasound technique using different sound frequencies and intensities, has also become a principal methodology for clinical study of the fetal and neonatal brain. Different tissues have specific acoustic impedances and send echoes back to the transducer, which displays them as waves of variable height or as points of light of varying intensity. In this way, one can obtain images in the neonate of choroid plexus, ventricles, and central nuclear masses. Usually several coronal and parasagittal views are obtained by placing the transducer over open fontanelles or the child’s thin calvarium. Intracerebral and subdural hemorrhages, mass lesions, and congenital defects can readily be visualized.