How MRI(Magnetic Resonance Imaging) works?

 This article aims to provide a brief overview and background information about magnetic resonance imaging (MRI) for readers who are interested in the topic. 

MRI, developed in the 1970s, became widely used in clinical settings during the 1980s. By the year 2000, there were over 20,000 MRI scanners worldwide, with more than 70 million magnetic resonance (MR) scans conducted annually [1]. Today, the number of MRI scanners has increased to 50,000 [6]. MRI stands out as a highly effective imaging technique for several reasons. Firstly, it offers exceptional differentiation between soft tissues and enables quantitative measurements. These unique characteristics distinguish MRI from other imaging methods, allowing researchers and doctors to explore its potential extensively. Current research areas, such as perfusion, diffusion, and functional MRI, leverage MRI's ability to provide insights into biophysical and biochemical properties of tissues. Secondly, MRI is versatile, capable of imaging any desired scan plane or three-dimensional volume. This versatility makes it applicable to a wide range of anatomical structures throughout the body. Lastly, MRI is noninvasive and does not involve ionizing radiation. This is crucial in clinical applications and has garnered increasing awareness as MRI progresses toward becoming a more frequently utilized imaging modality. Consequently, ergonomic ultraflexible MRI radio-frequency (RF) coils have become vital contenders for next-generation MRI scanners.

Over the years, modern MRI technology has made significant advancements and has become a highly advanced and versatile tool. Its capabilities in various applications are demonstrated through several examples. One such example involves the identification of specific types of high-risk carotid artery plaques, which are a leading cause of stroke. By utilizing multiple quantitative parameters and overlaying them with 3D images, an algorithm specifically designed to control magnetic field variations can pinpoint these plaques. Recent studies aim to avoid the use of contrast agents for this purpose. In areas of the carotid artery where a high signal-to-noise ratio is crucial, the imaging time can be reduced and image quality improved with the assistance of an ultraflexible RF coil.

Another application of MRI is its ability to assist in needle placement during motion using hydrostatic actuators. MRI is not limited to the diagnosis and treatment of diseases but also finds utility in surgical procedures. In such cases, the combination of an ultraflexible RF coil with other techniques can significantly enhance image quality, particularly when dealing with limited physical space.

MRI, which stands for magnetic resonance imaging, is a technique used in radiological imaging. It is based on the principles of nuclear magnetic resonance (NMR). The term "magnetic" refers to the utilization of magnetic fields, while "resonance" refers to the matching of frequencies between an oscillating magnetic field and the spin of a specific nucleus of interest. This matching is necessary to achieve the desired results in MRI.

NMR itself is derived from a phenomenon known as Zeeman splitting. When certain atomic nuclei are exposed to a magnetic field, they can exist in one of two states: a high energy state or a low energy state. As the strength of the magnetic field increases, the energy difference between these states also increases. The number of nuclei in the higher energy state is slightly lower than those in the lower energy state (for example, 1H, 13C, and 31P nuclei), with a ratio of approximately 1 in 106 for 1H. This difference in population between the two states enables MRI to work.

The detection of the magnetic signal in MRI occurs when the nuclei in the lower energy state absorb photons from the external environment and release photons as they return to a state of thermal equilibrium. This process allows for the visualization and analysis of internal structures in the body using MRI.

A thorough and precise explanation of the fundamental principles of magnetic resonance (MR) physics requires the framework of quantum mechanics. However, in many situations, a classical approach is sufficient to describe the behavior of macroscopic objects. Nuclei with an odd number of protons or neutrons possess a property called nuclear spin angular momentum, which leads to the observation of nuclear magnetic resonance (NMR) phenomenon. Conceptually, these nuclei can be thought of as spinning charged spheres, generating a small magnetic field. In the context of MR, we commonly refer to these nuclei as "spins." Among the various nuclei, hydrogen with a single proton is the most abundant (as the body is predominantly composed of water), the most responsive to MR signals, and extensively studied in biomedical MRI. The Larmor Equation below provides the angular frequency(\(w_r\)) of the electromagnetic fields involved in the MR process.

 \(w_r = \gamma B_0\)

where, \(B_0\) represents the intensity of the static magnetic field, while 𝛾 is referred to as the gyromagnetic ratio. Specifically for protons, the value of 𝛾/2𝜋 is equal to 42.58 megahertz per Tesla. Magnetic Resonance (MR) operates by the interaction between the spins of particles and three distinct types of magnetic fields: (i) the main field \(B_0\), (ii) the RF field B1, and (iii) the gradient fields G. Figure below depicts an MRI scanner, which reveals the internal structure. 

MRI scanner

It consists of (i) the primary magnet that generates the B0 field in the z-direction, (ii) the RF coil responsible for creating the B1 field in the xy-plane typically, and (iii) the gradient coils that introduce incremental variations in the strength of the B0 field along the z-direction. These gradient coils have a linear dependence on the x, y, or z directions, enabling the encoding of spatial information into the received signal.

When a magnetic field B0 is applied to the human body, the hydrogen nuclei start to spin along the z-direction in the laboratory frame. This causes the magnetization vector (M) to align with the z-direction. Next, a B1 field is applied, which rotates the magnetization vector from the z-direction to the xy-plane in a rotating frame of reference that matches the Larmor frequency. However, after the application of the B1 field, the magnetization gradually returns to its equilibrium position along the z-direction. During this relaxation process, the changing magnetic fields generated by the spinning nuclei induce voltage changes in the receiver coils. These RF (radiofrequency) magnetic signals are detected by the receiver coils and subsequently transformed into anatomical images.

To achieve this transformation, gradient coils play a crucial role in encoding spatial information during the data acquisition process. The precise design of the MR (magnetic resonance) pulse sequence is essential, considering factors such as the timing, duration, and waveform of the transmitted B1 field, gradient field, and received B1 field. The pulse sequence design involves several steps: using B1 and gradient fields to excite a specific imaging plane, reading out the signals using the receiver coils, and repeating the process to obtain data from different locations in the k-space (the spatial frequency domain in a 2D Fourier transform). The final image in the spatial domain is obtained by performing a Fourier transform on the data collected in the k-space.

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