Magnetic Resonance Imagery: Theory and Practical Applications

Introduction

Magnetic resonance imagery (MRI), is a non-invasive procedure that is used as a powerful diagnostic instrument. The principle of this procedure is to produce cross-sectional images of a patient’s body with magnetic fields. Strong electromagnetic fields are helpful for making the three-dimensional high-resolution image of the entire body, separate organ, or tissue.

The key aim of this paper is to offer a detailed review of the theoretical principles of MRI functioning, as well as a description of the standard operation for defining the overall diagnostic process. Additionally, MRI machine construction will be regarded for the detailed description of the physical process, as well as the diagnostic procedural details.

Theoretical Basis of Magnetic Resonance Imagery

The theory of the MRI process is based on the fact that the human body, similar to any other substance consists of atoms, each with its particular amount of particles: protons, neutrons, and electrons. These atoms, as well as isotops, are arranged in a particular order, creating molecules, and molecular structures. Therefore, the magnetic field “scans” the atomic structure of the tissues and organs, and defines the nature of structures. These atomic structures produce a particular magnetic field, which differs depending on the order of the structure (which is different for every material or substance), and the magnetic field of the MRI instrument “reads” the magnetic field of atoms, defining the magnetization of the atomic structures. 3-D images are produced by providing gradients in each direction. (Noll, Gomillion, et. Al., 2001, 2007).

Huda et.al. (, 2003123) provide further explanation of this principle: if atoms are featured with an uneven amount of neutrons, and protons, the electrical charge of this atom becomes positive or negative, and this causes the magnetic field to appear around the atom. The power of this field can be measured and affected, and this principle is used in the MRI machine.

In general, if an atom has a neutral magnetic field (the particles are in the neutral state), the magnetic orientation of each particle is randomized. The situation changes when an atom attains a magnetic field. The particles become oriented in accordance with the physical properties of each atom. Therefore, the initial alignment of the particles and atoms is essenttial for their positioning, as well as measuring the magnetic forces, and the magnetic field of the MRI machine creates the necessary force for adding magnetic power to particles, and measuring their allignment. As it is added by Gomillion, et al, (2007), when the studied object is put into the neutral magnetic environment, the particles reallign themselves, and this is measured by the secondary magnetic field that is introduced into the equation at a perpindicular orientation to the subject and the static magnetic field.

As Huda et. Al state (2003, pp. 143):

In the operation of the MRI device, after the patient body (or other material being studied) is entered into the static magnetic field, and the randomized, and naturally magnetized atomic particles have had the opportunity to realign themselves, a secondary magnetic field is introduced into the equation, and at a perpindicular orientation to the subject and the static magnetic field.

In the light of this fact, it shoudl be stated that the perpendicular magnetic fields provide the necessary alteration level for the atomic magnetic resonance. Nevertheless, it should be also stated that atoms generally prefer to exist in the most stabel state, that is featured with low energy of magnetic field. However, if an energy is gained, it is further released in a two decay process, that differs for each isotopic element. This process is also known as a relaxation phase (Kanal et.al, 2007). The alteration of the particles positioning between these two stages will cause the changes in the magnetic field of the atoms, which are measured and processed. (Noll, 2001). (Figure 1 represents the general outlook of the MRI machine). The accurate measurement of these changes, as well as the magnetic decay time create the particular resonant characteristics. The gyromagnetic ratio, magnitude of the applied magnetic field is helpful for defining the type of the isotope, and analysis of the resonant frequency which is unique for atoms of various materials. As it is emphasized by Noll (2001, p. 321):

In general practice, most MRI machines are operated at a magnetic magnitude of somewhere between 0.5 and 3.0T Once this resonant frequency is calculated, and the time it takes for the atoms to return back to their unexcited state, the computer can create a cross sectional image of the material being examined (in this case a patient) based upon the specific densities, and atomic compostion of the material.

Therefore, it is stated that each isotopic element has a specific decay pattern that is examined by the magnetic sensrs of the MRI machine. However, the simultaneous scanning of all isotopes available will cause the failure of the scannning program. that is why computer needs to get the data gradually. Focusing is used to resolve this problem. The scanning involves one particular type of isotopes. Hydrogen ion is used in most cases H1. It is one of the most plentiful isotopes in human body, and it is the most simple for measuring its magnetic field. Other isotopes that are used are C13, Na23, F19 (Gomillion, et al, 2007).

Normal human body tissues have stated density, and, correspondingly, atomic structure. The structure and density of pathological elements is defined by the process that is described above, and meical experts use the MRI machins for locating tumours, foreign bodies, as well as abnormal lumps. (Kanal et.al, 2007)

Diagram of a Typical MRI Unit
Figure 1: Diagram of a Typical MRI Unit. Source: Huda, W. and Slone, R., 2003, Review of Radiologic Physics 2nd Edition Lippincott, Williams and Wilkins, Philadelphia.

Common Advantages and Disadvantages of the MRI

In general, the MRI is regarded as one of the most useful and effective diagnosis tools, the consideration of its advantages and disadvantages will help to increase the effectiveness of the diagnostic measures. Since the nature of this diagnostic tool is non-invasive, this may be regarded as the largest advantage. However, the operation is rather costly, the machine itself is enormously huge, and depending on the needs of the diagnostic procedure, it may require a patient stay immobilized for twenty minutes or even longer. Particular precautions should be also considered (Chugani, 1998). Since ferromagnetic materials are used for creating the magnetic field, any metal object may become the reason of a distorted analysis. Some tattoos are made with the use of metal-based ink; hence, patients with such tattoos should be warned of the possible skin pain caused by these inks. Additionally, all the objects that may be affected by a patient should be removed from the machine by the personnel, additionally, the personnel should examine a patient, and ask him/her leave all the metallic objects behind the MRI laboratory (Noll, 2001). A patient should not have implanted electronic or any other metal devices. Especially, it is forbidden to use MRI for diagnosing patents with a pacemaker implanted, as its work may be violated. (Huda, et al, 2003, Noll, 2001)

However, accurate, and reasoned usage of the MRI imaging may offer details, informative, and helpful images for arranging a proper diagnostic process. (Gomillion, et al, 2007). This is needed for accurate treatment of various deceases caused by tumours, pathologic growth of tissues, or foreign bodies.

Conclusions

Regardless of the fact that MRI procedure is regarded as a costly diagnosis tool, the number of afvantages offered by this technology makes MRI effective, and reliable procedure for gathering information for the reliable diagnostic procedure. It is helpful for medical experts for localizing things such as tumours, foreign bodies, and abnormal growth patterns in a human’s body. This is generally explained by the fact that tissues have different densities, and abnormal lumps differ by the density and magnetic characteristics. Therefore, regardless of the possible cautions and disadvantages, the principle of MRI technology is able to offer high quality three-dimensional images of any part of the human body. The magnetic forces that are used in this machine are the largest advantage of the technology, however, these are also the biggest disadvantage, as MRI can not be applied to patients with metal, or electronic implants.

Reference List

Chugani, T., 1998, Biological basis of emotions: brain systems and brain development. Paediatrics, 102, 1225–1229.

Gomillion, Matthew, Jung Hee Han “Chapter 61: Magnetic Resonance Imaging” Yao & Artusio’s Anaesthesiology: Problem Oriented Patient Management Sixth Edition Fun-Sun Yao, Vinod, Malhotra, Manuel, Fontes, (eds.) Lippincott, Williams and Wilkins, Publishers 2007 pp. 1232-1248.

Huda, W. and Slone, R., 2003, Review of Radiologic Physics 2nd Edition Lippincott, Williams and Wilkins, Philadelphia.

Kanal E, Barkovich AJ, Bell C, et al. (2007). “ACR Guidance Document for Safe MR Practices: 2007”. American Journal of Roentgenology. 188 (6): 1–27.

Noll, Douglas, 2001, A Primer on MRI and Functional MRI Departments of Biomedical Engineering and Radiology, University of Michigan, Michigan.

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NerdyRoo. (2022, May 5). Magnetic Resonance Imagery: Theory and Practical Applications. Retrieved from https://nerdyroo.com/magnetic-resonance-imagery-theory-and-practical-applications/

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