Magnetic resonance imaging (MRI ) (also known as NMR spectroscopy) is based on computer mapping of the response of magnetic fields to short-term application of radio waves. MRI is informative for studying the structure and function of biological tissues. The method uses the phenomenon of nuclear magnetic resonance (NMR).
NMR is the selective absorption of the energy of high-frequency radio waves by certain atomic nuclei placed in a strong constant magnetic field. Nuclei having at least one unpaired proton or neutron behave like small magnets, and a strong magnetic field causes their precession. When the precession frequency of such magnets corresponds to the frequency of the weak radio waves applied to the object under study, energy is absorbed from the radio waves, which is called resonance.
Most often with MRI, protons (nuclei of hydrogen atoms) are used to obtain an image. The examined person is placed inside a magnetic coil, creating a strong constant magnetic field. Magnetized protons (nuclei of hydrogen atoms) of body tissues, like small magnets, are oriented in a certain way in this field. Then, radio waves are applied to the object of study, which create an alternating magnetic field perpendicular to the main field. Protons absorb energy from this field and go into an excited state. When protons return from excited to equilibrium
state in the recording coil, a signal appears, which is transformed into an image using a certain algorithm. The NMR spectrum contains much information about the molecular structure of tissues, since organic molecules have many hydrogen atoms, absorbing the energy of waves of different lengths depending on the structure of the molecular bond. The method is used to assess pH, lactate, creatine, choline, and other substances .
Non-proton MRI is also used, for example, when examining the amount of phosphates. In this case, phosphorus is used to obtain the image, and carbon is used to evaluate glutamate (IJ Cox, 1996).
More recently, a technique has been developed for the quantitative imaging of local cerebral blood flow using MRI. In this method, a paramagnetic contrast agent is used intravenously to obtain maps of cerebral blood volume. Maps can be compared with an MRI anatomy and are characterized by good spatial and temporal resolution.
The method is safer than mapping using x-ray and gamma radiation. However, due to the presence of a strong magnetic field, MRI cannot be used in people with pacemakers or other metal objects in the body. This is also an expensive procedure.
Measurement of local cerebral blood flow (LMC) using isotopic clearance. Measurement of local cerebral blood flow became possible in the early 60s of the twentieth century. thanks to the development of methods for intraarterial and intracranial injection of xenon-133 using the method proposed by N. Lassen and D. Engwar. After injection of a liquid with a radioactive label using a multi-channel gamma camera, isotope clearance is recorded from one of the hemispheres. Measurement of cerebral blood flow by isotopic clearance begins a few seconds after the injection and lasts 40-50 s. The amount of blood flow is determined on a computer from the leaching curves for many areas (up to 254). The more intense the blood flow, the faster the label concentration will increase and then decrease. According to these data using computer procedures, the LMC values are determined.
The intra-arterial method of studying LMC allows you to measure blood flow only from the surface of the hemispheres due to the low energy level of emitted gamma rays. Other limitations of the method are related to the fact that not all areas of the brain are supplied through the carotid arteries.
For this reason, a non-invasive two-dimensional method for measuring LMK was developed, when the subject inhales a certain amount of xenon-133 for one minute. A physiologically passive label first enters the lungs and then enters the brain through the circulatory system, and then leaves the brain tissue and exhales through the lungs. A large set of detectors provides continuous registration of gamma radiation, which allows using a computer processing to build a LMK map.