In this blog post, we’ll start with nuclear magnetic resonance (NMR), which is used to analyze molecular structures, and explore the principles and evolution of magnetic resonance imaging (MRI), which is widely used in medical settings today.
MRI. It’s the name of a machine that everyone has heard of many times or has used at least once. When you lie inside the white, cylindrical machine, you hear strange noises intermittently for several tens of minutes before the scan ends. However, while undergoing the scan, some people may have wondered, “What exactly does this machine do?” Once you understand the principles behind how this machine works, any vague reservations or doubts you might have about MRI can be significantly reduced. In this post, we’ll explore what MRI (Magnetic Resonance Imaging) is and how it creates images of the inside of the human body.
MRI works on a slightly different principle than other imaging methods used to examine the inside of the human body, such as X-rays or CT scans. Of course, each imaging method has its own appropriate applications and its own set of advantages and disadvantages, but unlike X-rays or CT scans, MRI does not use ionizing radiation and is particularly effective at distinguishing soft tissues such as the brain, muscles, ligaments, and cartilage. Furthermore, it is not limited to images from a single direction; it allows for observation of the desired area from various angles and cross-sections. How, then, was MRI—which is so useful for observing the inside of our bodies—actually developed? In fact, MRI was not originally developed for medical imaging. The basic principle of MRI stems from nuclear magnetic resonance spectroscopy (NMR spectroscopy), an analytical method developed to determine molecular structures. Simply put, MRI can be understood as the application of nuclear magnetic resonance technology—originally used to analyze molecular structures—to the human body, utilizing magnetic fields, electromagnetic waves, and spatial information to create images of the distribution of human tissues. Therefore, to properly understand MRI, it is necessary to first understand the basic principles of NMR.
NMR is a spectroscopic technique that utilizes the fact that different signals are produced depending on the magnetic properties of atomic nuclei and their surrounding chemical environment. When NMR is performed on a substance, different signals appear depending on the types of atomic nuclei contained in that substance and their surrounding environment; this allows us to infer the elements composing the substance and the chemical environment within the molecule. The measurement principle of NMR is as follows. Some atomic nuclei possess spin—a quantum mechanical intrinsic angular momentum—and nuclei with spin exhibit a magnetic moment. Not all atomic nuclei are used in NMR; only those with a non-zero spin can generate an NMR signal. In particular, MRI relies primarily on hydrogen nuclei—that is, protons—which are abundant in the human body. Atomic nuclei with magnetic moments behave similarly to small magnets; when an external magnetic field B₀ is applied, their magnetic moments assume specific quantum states related to the external magnetic field.
Taking the hydrogen nucleus, which has a spin of 1/2, as an example, its states can be divided into a low-energy state aligned in the same direction as the external magnetic field and a high-energy state aligned in the opposite direction. There is an energy difference ΔE between these two states, and this energy difference increases as the strength of the external magnetic field increases. When radiofrequency (RF) waves at a frequency corresponding to this energy difference are applied, the hydrogen nucleus absorbs energy and transitions to the higher-energy state. This is generally referred to as a spin transition. Once the RF excitation ceases, the atomic nucleus’s magnetization returns to its equilibrium state, and the signal generated during this process is measured by a coil. In NMR, the frequency, amplitude, and time-dependent changes of this signal are analyzed. Since the resonance frequency and signal vary depending on the type of atomic nucleus and its surrounding chemical environment, this information can be used to determine the environment in which a specific element is located and the nature of its molecular structure. For example, ¹H NMR and ¹³C NMR provide information about the chemical environment surrounding hydrogen and carbon atomic nuclei, respectively. Even for atomic nuclei of the same element, the resonance frequency can vary depending on their bonding state with neighboring atoms and their chemical environment; therefore, analyzing NMR signals yields the information needed to predict molecular structure.
So, how can NMR be used to visualize the structure of the human body? The human body consists of a significant proportion of water, and water is composed of one oxygen atom and two hydrogen atoms. Therefore, wouldn’t measuring the NMR signals from hydrogen nuclei—which are abundant in the human body—allow us to obtain information about the body’s interior? It was precisely this idea that served as the crucial starting point for applying the principles of NMR to medical imaging. In an MRI scanner, the main magnet, which generates a strong magnetic field, plays a crucial role. When a person lies down inside this cylindrical magnet, the MRI scanner creates a strong magnetic field, and the hydrogen nuclei inside the human body are influenced by this field, assuming specific energy states. When an RF pulse is applied, the magnetization state of the hydrogen nuclei changes; after the RF pulse ends, a relaxation process occurs in which the magnetization returns to its original equilibrium state. The MR signal generated during this process is measured, and at the same time, gradient fields—which create minute spatial variations in the magnetic field—are used to determine the exact location within the body where the signal originated. The computer mathematically reconstructs the signals obtained in this way to generate cross-sectional images. Furthermore, since different tissues have distinct relaxation characteristics—such as T1 and T2—the signal intensity and image contrast vary depending on the tissue, even though the same hydrogen nuclei are used. By utilizing these differences, it is possible to distinguish between various soft tissues, including the brain, muscles, fat, and water.
You may now have a better understanding of that large cylinder you were told to lie inside during a hospital exam—and of that mysterious machine that emitted strange noises for several tens of minutes while you were lying inside it. In fact, MRI works by using a strong magnetic field, radio waves, and a computer to convert information from inside the human body into images. One advantage of MRI is that it does not use ionizing radiation, and it is particularly useful for detailed observation of soft tissues. However, you should not assume that “since there is no radiation, it is absolutely safe.” Because MRI uses a very strong magnetic field, metal objects can be drawn toward the magnet, potentially causing accidents, and some medical implants or electronic medical devices may be affected by the magnetic field or RF energy. Therefore, when undergoing an MRI scan, you must inform the medical staff of the type and model of any implanted medical devices, such as pacemakers, cochlear implants, or drug-infusion pumps, and confirm whether these devices are safe in an MR environment or are only usable under specific conditions. What began as a principle developed from NMR for analyzing molecular structures has now established itself as a vital medical imaging technology used in hospitals to diagnose diseases and visualize the interior of the human body. MRI will continue to be used to monitor our health as a technology that utilizes magnetic fields and electromagnetic signals to visualize subtle differences within the human body.
