Radionuclide Methods and PET
These techniques are based on the administration of compounds containing unstable isotopes into the body, followed by the detection of ionizing radiation. Dynamic studies of cerebral blood flow help evaluate the rate of radiotracer delivery, the time difference in reaching the right and left hemispheres, and the total transit time through the vascular bed. Tomographic gamma cameras are used for this purpose to construct a three-dimensional computer model.
Positron Emission Tomography (PET) is a high-precision modality of radionuclide diagnostics. Short-lived isotopes that decay with positron emission are introduced into the bloodstream, and the signal is recorded within half an hour post-injection. PET features high spatial resolution (distinguishing structures from 0.5 cm³), with a single scan taking anywhere from a few seconds to minutes. This method is indispensable for visualizing lesion foci and identifying compensatory brain zones.
Magnetic Resonance: MRI and fMRI
Magnetic Resonance Imaging (MRI) relies on the phenomenon of nuclear magnetic resonance (NMR). Hydrogen nuclei in tissues interact with strong static and alternating magnetic fields. The reorientation of their magnetic moments causes the emission of electromagnetic radiation. MRI provides detailed structural images of organs in seconds and enables angiography of major vessels without contrast agents.
Functional MRI (fMRI) records the intensity and decay of resonance waves, which depend on local magnetic field inhomogeneities. The basis of the method is the difference in the magnetic properties of hemoglobin: oxygen-saturated oxyhemoglobin is a weak diamagnet, while deoxygenated deoxyhemoglobin is a paramagnet. When neurons are actively firing, the ratio of these forms in the microvasculature shifts, altering the MRI signal. This method allows for non-invasive cortical mapping, identifying centers of speech, movement, and auditory/visual stimulation.
Blood Flow Assessment and Thermoencephaloscopy
The intensity of cerebral blood flow directly reflects metabolic demands. Rheoencephalography (REG) measures head tissue impedance (electrical resistance) while passing a weak alternating current (voltage 0.75–1.5 V, current 0.5–1.0 mA). Resistance fluctuations are synchronous with changes in vascular blood volume. REG assesses tone, blood filling in vascular beds, and venous outflow quality. To clarify the diagnosis, functional tests are used: if there is no reaction after administering vasodilators, it indicates atherosclerotic lesions of the vessel walls. Since neurons require a stable thermal regime and metabolism is accompanied by heat production, thermoencephaloscopy is applied. It utilizes thermographic techniques to record infrared radiation from the cortex. Digital data processing visualizes temperature distribution and identifies the most active areas of the brain.
Advanced Methods for Studying Metabolism
For in-depth in vivo analysis of tissue chemical composition, microdialysis is used. A probe with a semipermeable membrane is implanted into the brain, through which metabolites diffuse from the extracellular space into the device lumen. This allows for continuous metabolic monitoring in freely moving laboratory animals. At the cellular level, early gene expression mapping is applied. Memory formation and complex neural activity require the synthesis of specific proteins, preceded by the activation of c-fos and c-jun genes. Oligonucleotides complementary to the target gene mRNA are synthesized and labeled with radioactive phosphorus-33. Hybridization occurs on brain slices, and binding zones are captured via autoradiography. Computer processing yields a three-dimensional image of regions with active protein synthesis.