Radionuclide methods and PET
These techniques are based on administering compounds with unstable isotopes into the body followed by the detection of ionizing radiation. Dynamic studies of cerebral blood flow help estimate the arrival rate of the labeled substance, 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 build a three-dimensional computer model.
Positron emission tomography (PET) is a high-precision modality of radionuclide diagnostics. Short-lived isotopes that decay with the emission of positrons are injected into the bloodstream. The signal is recorded within half an hour after injection. PET has a high spatial resolution (distinguishing structures from 0.5 cm³), with a single scan taking from a few seconds to minutes. The method is indispensable for visualizing lesions and identifying compensatory brain areas.
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 constant and alternating magnetic fields. The reorientation of their magnetic moments causes the emission of electromagnetic radiation. MRI provides detailed images of organ structures in seconds and enables angiography of large vessels without contrast agents.
Functional MRI (fMRI) records the intensity and decay of resonance waves, which depend on the inhomogeneity of the local magnetic field. 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 working, the ratio of these forms in the microvasculature changes, which alters the MRI signal. The 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 the impedance (electrical resistance) of head tissues 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 blood volume within the vessels. REG allows for the assessment of tone, blood supply to vascular beds, and the quality of venous outflow. Functional tests are used to clarify the diagnosis: if there is no response after administering vasodilators, this indicates atherosclerotic lesions of the walls.
Since neurons require a stable temperature regime and metabolism is accompanied by heat production, thermoencephaloscopy is used. It employs thermography techniques to record infrared radiation from the cortex. Digital data processing allows for the visualization of temperature distribution and the identification of the most actively functioning brain areas.
Advanced methods for studying metabolism
For an in-depth study of tissue chemical composition in vivo, microdialysis is used. A probe with a semipermeable membrane is implanted into the brain, through which metabolites diffuse from the intercellular space into the lumen of the device. This allows for long-term monitoring of metabolism in laboratory animals under free-moving conditions.
At the cellular level, early gene expression mapping is used. Memory formation and complex nervous activity require the synthesis of specific proteins, which is preceded by the activation of c-fos and c-jun genes. Oligonucleotides complementary to the mRNA of the target gene are synthesized and labeled with radioactive phosphorus-33. Hybridization takes place on brain slices, and binding sites are detected by autoradiography. Computer processing yields a three-dimensional image of areas where protein synthesis was actively occurring.
Briefly: answers to topical questions
Which specific radiopharmaceuticals are administered to patients during brain PET?
The provided sources do not specify the exact molecular names of radiopharmaceuticals for brain PET.
It is indicated that PET involves the administration of a radiopharmaceutical or short-lived radioactive isotopes emitting positrons, and recording their distribution in the brain. For brain PET/CT in metastatic lesions, tumorotropic radiopharmaceuticals are recommended, including contrast-enhanced options.
Most commonly, biomedical compounds labeled with the following are used:
- fluorine-18 ($^{18}$F),
- carbon-11 ($^{11}$C),
- oxygen-15 ($^{15}$O).
PET does not differentiate the original labeled molecule from its metabolites; contrast is created due to regional differences in tracer accumulation.
To memorize the fMRI principle: Oxyhemoglobin — Opposes the field (diamagnet), whereas Deoxyhemoglobin — Defines magnetization (paramagnet).
Class and exam questions
What is the physiological essence of functional MRI?
Neuronal activation increases blood inflow, which alters the local ratio of paramagnetic deoxyhemoglobin and diamagnetic oxyhemoglobin, thereby changing the signal from resonating hydrogen nuclei.
Why are vasodilator tests used in rheoencephalography?
Tests help distinguish functional spasm from organic lesions. The absence of a vascular response to the drug indicates vessel stiffness, frequently caused by atherosclerosis.
What does the early gene expression method allow to visualize?
It localizes areas of increased cellular activity with enhanced protein synthesis, which is particularly important for studying the subtle mechanisms of learning and long-term memory formation.
What else to study on this topic
- Evaluation of radiopharmaceutical transit time through the left and right hemispheres
- Physical principles of nuclear magnetic resonance (NMR) of hydrogen protons
- The role of c-fos and c-jun genes in intracellular neuronal signaling cascades
- Principles of autoradiography application on histological brain slices
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